Novel inorganic electron transport layers for planar perovskite solar cells: Progress and prospective

Novel inorganic electron transport layers for planar perovskite solar cells: Progress and prospective

Journal Pre-proof Novel Inorganic Electron Transport Layers for Planar Perovskite Solar Cells: Progress and Prospective Kai Wang, Selina Olthof, Waqas...

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Journal Pre-proof Novel Inorganic Electron Transport Layers for Planar Perovskite Solar Cells: Progress and Prospective Kai Wang, Selina Olthof, Waqas Siddique Subhani, Xiao Jiang, Yuexian Cao, Lianjie Duan, Hui Wang, Minyong Du, Shengzhong (Frank) Liu PII:

S2211-2855(19)30996-6

DOI:

https://doi.org/10.1016/j.nanoen.2019.104289

Reference:

NANOEN 104289

To appear in:

Nano Energy

Received Date: 16 September 2019 Revised Date:

31 October 2019

Accepted Date: 8 November 2019

Please cite this article as: K. Wang, S. Olthof, W.S. Subhani, X. Jiang, Y. Cao, L. Duan, H. Wang, M. Du, S.(F.) Liu, Novel Inorganic Electron Transport Layers for Planar Perovskite Solar Cells: Progress and Prospective, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104289. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

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Novel Inorganic Electron Transport Layers for Planar Perovskite

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Solar Cells: Progress and Prospective

3

Kai Wang a, Selina Olthof c, Waqas Siddique Subhani a, d, Xiao Jiang a, Yuexian Cao a, Lianjie Duan a,

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Hui Wang a, Minyong Du a, Shengzhong (Frank) Liu a, b *

5

a

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Academy of Sciences, Dalian, 116023, China.

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b

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Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology;

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Institute for Advanced Energy Materials; School of Materials Science and Engineering, Shaanxi

Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical Physics, Chinese

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi Key

10

Normal University, Xi’an 710119, P. R. China.

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c

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Germany.

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d

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E-mail: [email protected]

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Keywords: perovskite solar cell; electron transport layer; inorganic n-type material; planar structure;

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flexible device

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Abstract

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Perovskite solar cells (PSCs) have emerged as a promising class of photovoltaic devices since

19

they combine the benefits of high efficiency beyond 20%, low material cost, as well as easy

20

and scalable processing. The appropriate choice of the electron transport layer (ETL) in these

21

devices is one crucial aspect for achieving high efficient PSCs. The conventional ETL TiO2 is

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not the best choice due to its relatively low conductivity and problematic photocatalytic

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activity. Therefore, novel ETLs have attained increasing attention and are making rapid

24

progress and with it the further development and optimization of planar PSCs has been

25

promoted. In this review, we start by introducing the essential functions of ETLs in planar

26

PSCs. Next, we give an extensive description of novel ETL materials, looking at both

Department of Chemistry, University of Cologne, Luxemburger Straße 116, 50939 Cologne,

University of the Chinese Academy of Sciences, Beijing 100039, China.

1

1

crystalline and amorphous systems. Their emergence, development, and accompanying

2

optimization strategies will be discussed. Additionally, we provide a brief discussion about

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the correlation between materials, fabrication methods, and interface related issues. In the end,

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we propose some prospective research subjects that will be relevant for the further

5

development of novel ETLs.

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1. Introduction

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The clean and inexhaustible solar energy is regarded as an outstanding substitution for fossil

8

fuels, able to alleviate the serious global energy crisis and counter environmental pollution.[1]

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Among the various kinds of solar cells, thin film devices are of particular interest due to the

10

unique combination of simple manufacturing and low-cost preparation. Various material

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systems have been studied in the last decades in order to improve their efficiency and

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eventually enable large-scale industrialization. Candidates included CdTe solar cells,[2]

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copper indium gallium selenide solar cells,[3, 4] dye-sensitized solar cells (DSSCs),[5, 6]

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quantum dot solar cells,[7, 8] and organic solar cells,[9] which all attracted considerable

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attention from both academic and industrial communities.

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In the last years, perovskite solar cells (PSCs) have appeared as one of the most promising

17

types of photovoltaic devices and gained momentum swiftly since 2012.[10, 11] The light

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absorbing layer employed in PSCs is, in the simplest case, an organometallic halide

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perovskite composed of CH3NH3PbI3 (MAPbI3), which possesses a remarkable combination

20

of advantageous properties for solar cells. This includes having a direct band gap,[12] high

21

optical absorption coefficient,[13] long carrier lifetimes,[14, 15] and the capability of

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ambipolar charge transport.[16] Furthermore, a variety of analogous alternatives to MAPbI3 2

1

(ABX3 structure, Figure 1a, A=CH3NH3+, NH2CHNH2+, Cs+, Rb+, B=Pb2+, Sn2+, X= Br-, I-)

2

have been introduced to tune the optical gap[17], enhance photovoltaic performance, and

3

increase film as well as device stability. The combined work of numerous research groups

4

around the world helped to steadily increase the maximum power conversion efficiency (PCE)

5

of PSCs, reaching a record value of 25.2% in 2019.[18]

6

Initially, PSCs evolved as a variant of DSSCs, which is the reason why similar device

7

structures are used, as seen in Figure 1b.[19] A typical PSC is constituted of a transparent

8

conductive oxide (TCO)-based substrate (e. g. fluorine-doped tin oxide, FTO), an electron

9

transport layer usually consisting of an n-type metal oxide , a p-type organic hole transport

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layer (HTL), and a metal top electrode (although there are many variations on this common

11

structure).[20] The electron transport layers (ETLs) are often composed of, both, a compact

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layer and a mesoporous one. Together, they are responsible for the efficient extraction of

13

photo-generated electrons and their transport into the TCO. The wide gap ETL furthermore

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acts as a hole blocking layer, due to a large energy level offset between the respective VB

15

positions of ETL and perovskite. Furthermore, the mesoporous ETL works as a scaffold for

16

the perovskite which increases the effective interface to enhance charge transfer, especially in

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the case of short or unbalanced hole and electron diffusion lengths. Later, planar PSCs

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without mesoporous layers were proposed, as shown in Figure 1c, which is possible due to the

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excellent charge transport properties of the perovskite itself for both electrons and holes.[21]

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In this case the ETL is only composed of a compact metal oxide layer which enables a

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simpler fabrication process. In addition, such a planar structure is better for flexible, i.e. foil

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based, devices as mesoporous layers generally require high temperature (HT) annealing 3

1

processes (500 oC) to remove the organic templates; this also effects the cost of production.

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Thus, ever-increasing attention is given to the planar PSCs, and efficiencies skyrocketed from

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4.9% to greater than 23.3%, which is close to the highest reported efficiency (25.2%) of

4

PSCs.[21-23] Finally, with the aim of simplifying the device structure and further reducing

5

the cost, also the compact ETL layer has been removed, forming the so-called ETL-free

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structure (Figure 1d). These will be referred to as TCO-ETL PSCs throughout this review

7

because strictly speaking the TCOs are themselves also electron transport layers, even though

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their properties are not optimal for charge extraction from the perovskite, as will be discussed

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in detail later.

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The progress of planar PSCs is intimately correlated with the booming development of

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novel ETLs. Ideal ETLs must combine good electrical conductivity (at least under

12

illumination), suitable energy levels, low surface defect density, high transmittance,

13

pinhole-free morphology, as well as good optical and thermal stability. These properties will

14

ensure that charges are efficiently generated in and extracted from the perovskite with

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minimal energy loss or recombination; furthermore, no degradation will take place within the

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ETL or at the interface to the absorber layer. In general, ETLs can be classified into two

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categories: organic and inorganic materials. Organic electron transport materials, such as

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[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and C60, have been employed as ETLs in

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efficient inverted perovskite solar cell (IPSCs, Figure 1e), achieving high PCEs exceeding

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20%.[24, 25] In addition, our group demonstrated that a good ionic conductor with

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antireflection properties, high electron mobility, and a suitable work function can also act as

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efficient ETL.[26] However, organic ETLs are generally expensive and unstable, which is not 4

1

suitable for mass production. Therefore, low-cost and stable inorganic electron transport

2

materials are considered more promising ETLs in PSCs. TiO2 is a representative and often

3

used electron transport material whose proper band alignment and high transmittance enable

4

the preparation of high performance of solar cells. However, while the electron mobility in

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lead based perovskites is fairly high with values in the range of 10~100 cm2 V-1 s-1,[27, 28]

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the electron mobility of 0.1~4 cm2 V-1 s-1 [29] in TiO2 is significantly lower which thereby

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limits the efficiency of devices. More importantly, TiO2 is sensitive to ultraviolet irradiance,

8

which can accelerate the degradation of perovskite.[30] Accordingly, devices based on TiO2

9

often exhibit pronounced hysteresis in their current-voltage (J-V) curves[31, 32] along with

10

unexpected photo-lability[30] even if high performance PCE is achieved.

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Hence, investigations in efficient ETLs with good electron mobility and improved stability

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generate enormous interest in the PSCs research community. As such, a multitude of novel

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ETLs have been developed and investigated in devices over the past few years. Some reviews

14

concerning ETLs have been published, but most of them focused on TiO2, mesoporous

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structures, and low temperature (LT) fabricated ETLs.[33-41] Nowadays, TiO2 is not the most

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best electron transport material for PSCs any more as numerous alternative electron transport

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materials are emerging that also exhibit excellent performance in PSCs. Besides, planar ETL

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films have clear advantages when it comes to large-scale manufacturing. Therefore, it is

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essential to present a comprehensive and systematic review focusing on the novel ETLs in

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planar PSCs. We first discuss the basic functions of ETLs in planar PSCs. Next, we present an

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overview over the different materials and their development as well as optimization strategy,

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separating them into crystalline and amorphous layers. Finally, we discuss the overall 5

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correlation between current ETLs, their preparation methods, and remaining interface issues.

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This review aims at presenting a timely update on the recent development regarding novel

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ETL based PSCs and provides some guidelines for further optimization and design of PSCs

4

based on novel ETLs.

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2. The functions of ETLs in PSCs

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We would like to start the discussion on the role of electron transport layers in perovskite

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solar cells, by first introducing device architectures that do not contain specialized ETLs.

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Here, the perovskite is directly coated on a TCO which, as mentioned earlier, also acts as an

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ETL in the so-called TCO-ETL PSCs.[31, 42-49] Degenerately doped TCO possesses high

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transmittance and high conductivity which meets the basic requirements of ETLs. As an

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example, Liu et al. described TCO-ETL PSCs where a high efficiency of 13.5% was attained

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with the structure of ITO/MAPbI3/Spiro-OMeTAD/Ag.[50] Our group also prepared this type

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of PSCs with different TCOs, including AZO, ITO and FTO, and obtained a PCE exceeding

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10%.[46] However, the TCO-ETL PSCs usually possess comparatively low PCEs compared

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to conventional PSCs architectures. This is due to the severe charge carrier recombination, as

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both electrons and holes reach the TCO/perovskite interface where they recombine rapidly.

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Moreover, in case the perovskite film exhibits pinholes, shunt paths will form between the

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HTL and the TCO. To alleviate this issue in TCO-ETL PSCs, the TCO/perovskite interface

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was optimized using interface engineering such as surface chemical modification,[42, 49, 51]

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UV-ozone treatment[43], and electrochemical etching.[44] In addition, modifications of the

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perovskite film were implemented by tuning the perovskite from p-type to n-type,[52]

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blending the perovskite with fullerene or its derivatives,[53, 54] and increasing the perovskite 6

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grain size to efficiently prevent a shunting between the HTL (or metal contact) and TCO.[48]

2

The highest efficiency for TCO-ETL PSCs have been reported by Huang et al., using a

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tetramethylammonium hydroxide modified FTO/perovskite interface, reaching PCE values of

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20.1% and 18.5% under the reverse and forward scan, respectively.[51]

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Initially, Zhang et al. suggested that TCO-ETL PSCs yield PCE values that are “inflated”

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because of the accumulation of migrating ions under forward bias which give rise to a

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temporary electric field in the perovskite that supports charge separation.[31] They claimed

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further that in the absence of a suitable bias, this field reduces or disappears due to a revision

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of this ion migration, re-establishing a field-free layer. Accordingly, a TCO-ETL PSC should

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display no or very low measurable steady-state power output despite showing high efficiency

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in the swept J-V curves. Currently, it is widely agreed that the TCO-ETL PSCs shows a

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negligible built-in potential across the perovskite/ITO heterojunctions and the charge transfer

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from perovskite to TCO is promoted by the carrier concentration gradient because many

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charge carriers accumulated in perovskite close to the perovskite/ITO heterojunctions that can

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be demonstrated by the large capacitance in devices.[55] Overall, it seems that TCO-ETL

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PSCs can still output steady power, even though this power is generally lower than the PCE

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extracted from J-V curves due to obvious hysteresis effects and capacitive current.[56] For

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instance, Yu et al. prepared highly efficient TCO-ETL PSCs with a PCE of 18.2% by

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increasing the perovskite grain size to 700 nm.[48] However, the steady output efficiency of

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15.59% is inferior to the attained PCE from J-V results, and severe hysteresis was observed.

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After summarizing the severe limitations in TCO-ETL PSCs, it is worthwhile to discuss

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theoretically why additional dedicated ETLs are important in highly-efficient PSCs. In the 7

1

typical PSCs, any work function (WF) difference between ETL and perovskite will lead to a

2

flow of electrons from the low WF to the high WF material; in thermal equilibrium a space

3

charge layer will form across the interface.[57] The resulting built-in electric field in the

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depletion region can help to extract electrons into the ETL, suppress the back transfer of

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electrons from ETLs to the perovskite, and further assist in the charge separation close to the

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interface.[58, 59] The ideal energy level landscape would be one in which the space charge

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layer extended across the entire, few hundred nanometers wide perovskite layer resulting in a

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p-i-n structure; this has been shown to occur for perovskite on TiO2.[60] In theory, the

9

depletion width (x0) at the ETL/perovskite heterojunction can be expressed as:[61, 62] 2

=

2

=

=

+

+

=

2

+

(1)

(2) +

+

(3)

10

where xn and xp are the depletion widths in ETL and perovskite, respectively. V0 is the

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difference in Fermi levels, εn and εp stand for the static permittivity of ETL and perovskite,

12

respectively, e is the elementary charge, nn and np refer to the doping density of ETL and

13

perovskite, respectively. When looking at the TCO/perovskite heterojunction it can be

14

assumed that

15

be simplified to:



, because of the high charge carrier density in the TCO. Then eq. 3 can

= 16

=

2

(4)

Due to, both, the increase in the magnitude of the space charge layers field and the somewhat 8

1

wider space charge layer width in the ETL/perovskite system, better charge separation is

2

expected from this interface compared to the TCO/perovskite system, assuming that all other

3

properties remain the same. Therefore, we can conclude that the capability of ETLs to

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suppress charge recombination originates not only from the physical isolation of the TCO

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from other cell components but also from the advantageous changes to the space charge layer.

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In summary, ETLs are important components in highly-efficient PSCs for several reasons.

7

1.

promotes charge separation and therefore reduces recombination.

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They normally form an advantageous space charge layer in the perovskite which

2.

The recombination of photogenerated holes near the perovskite interface with electrons

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in the ETL will be reduced because of the much lower concentration of electrons in the ETL

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compared with that in the TCO. In addition, the deep valence band of ETL can contribute to

12

the blocking of holes.

13

3.

It reduces shorting of the HTL or back metal contact directly to the TCO.

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Note that only 2 is an intrinsic property; both 1 and 3 could be prevented by other means.

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For example, for point 1 a TCO with lower WF could result in a large space charge layer or it

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could be induced by the HTM side of the perovskite; for 3, better perovskite coverage can

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help, ensuring a lack of shorting paths through the cells (as done, to a large extent, in

18

ref.[48]).

9

1 2 3 4 5

Figure 1. (a) The crystal structure of perovskites. Structure diagrams of (b) n-i-p mesoscopic PSCs, (c) n-i-p planar PSCs, (d) TCO-ETL PSCs and (e) p-i-n inverted PSCs. (f) Electron mobility and band structure of various electron transport materials. The references are listed as follows: ZnO[63], SnO2[64], WOx[65, 66], Fe2O3[67], Nb2O5[68-70], In2O3[71, 72], CdS[73, 74], CdSe[73, 75], Zn2SnO4 (ZSO)[76], BaSnO3 (BSO)[77], SrTiO3 (STO)[78], ZnSe[79], In2S3[80], TiS2[81, 82], SnS2[83].

6

3. Novel inorganic ETLs

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Inorganic, wide bandgap, n-type semiconductors are excellent ETL candidates and have been

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utilized extensively to fabricate highly-efficient PSCs thanks to their low cost and

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considerable charge mobility. So far, almost all the electron transport materials which have

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been employed in PSCs are binary and ternary metallic oxides and some chalcogenides. Their

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band structures and electron mobilities listed in Figure 1f; here, the values have been obtained

12

from papers focusing on thin films and solar cells. It should be noted that these hold some

13

uncertainty since values reported in different papers show significant spread due to

14

differences in material preparation as well as measurement method.

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According to their crystallinity, ETLs can be classified into either crystalline or amorphous

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films. There are two main approaches, both consisting of two steps, for the preparation of

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crystalline ETLs. (i) For the first approach, the primary step is preparing a thin film either via

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spin-coating the precursor sols,[84, 85] spray pyrolysis,[86, 87] chemical bath deposition 10

1

(CBD),[88] or electro-deposition.[89] The following step is annealing at HT to eliminate the

2

organic components and form a (usually) nanocrystalline film. (ii) For the second approach,

3

the first step is synthesizing nanoparticles via a solvent thermal method,[76] precipitation,[90]

4

etc. followed by a spin-coating of the nanoparticle dispersion and annealing, usually at LT;

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this strategy is called nanoparticles+spin coating (NSC) in this review. Amorphous ETLs

6

have been demonstrated in PSCs, which can in general be prepared at LT. There are also two

7

approaches to prepare amorphous ETLs. On the one hand, vacuum deposition process are

8

used, such as atomic layer deposition (ALD), magnetron sputtering (MS), electron-beam

9

evaporation (EB), or thermal evaporation (TE). On the other hand, the precursor solution can

10

be spin-coated and merely heating at LT, which we refer to as solution+spin coating (SSC) in

11

this review; obviously, the SSC route is similar to the preparation of crystal ETLs, where

12

heating at HT is used.

13

Further optimization of the ETLs is an important research topic, since there can be a high

14

density of trap states present on the surface as well as inside the semiconductors, which

15

negatively influences the charge transport, recombination reaction, and hysteresis.[91] Apart

16

from ETL-inherent issues, the ETL/perovskite interface will also effect the charge carrier

17

dynamics and the crystal growth behavior of perovskite.[92] Thus, researcher are following

18

two strategies for the optimization of ETLs. One aims to enhance the properties of ETLs via

19

doping[93-96] and functional material composition (introducing materials such as conductive

20

polymers, organic electron transport materials, or metallic nanoparticles.[97-100]) with the

21

purpose of reducing trap states, increasing conductivity, and tailoring band structure. The

22

focus of the other approach is to modify the interface towards the perovskite by inserting 11

1

self-assembled monolayers, ionic liquids, inorganic blocking or buffer layers, and organic

2

molecules (e.g., fullerene and its derivatives). The aim is to optimize charge transfer, suppress

3

interface recombination, and enhance the crystallization of the perovskite. In the following

4

sections we will systematically discuss the emergence, development, and optimization of

5

different ETL, first for crystalline and then for amorphous material systems.

6

3.1 Crystalline ETLs

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3.1.1 ZnO based ETLs

8

ZnO was first proposed as a promising alternative to TiO2 since it shows excellent

9

performance in DSSCs[101] due to its charge carrier mobility that is orders of magnitude

10

higher than that of TiO2.[63] The valence and conduction band positions of ZnO are similar to

11

that of TiO2, which ensures the same beneficial energy level alignment with many commonly

12

used perovskites. Strikingly, ZnO is capable of crystallizing at a LT,[102] which potentially

13

enables a lower cost of production and an utilization in flexible devices. In the inception

14

phase of PSCs, pioneers have prepared ZnO thin films as ETLS in planar PSCs by using NSC

15

strategy[102, 103] and electrodepositon[104]. With the former method, promising efficiencies

16

as high as 15.7% and 10.2% were achieved in rigid and flexible devices, respectively.[103]

17

These results were significant because they demonstrated the potential of ZnO and

18

encouraged the exploration of other novel ETLs in PSCs. Further, owing to the possibility of

19

LT crystallization, Heo et al. fabricated ZnO thin films via a sol-gel route at 150 oC and

20

systematically compared the advantages of ZnO over TiO2.[105] It is worth noting that, with

21

the exception of ZnO, it is very challenging to obtain crystalline oxides at 150 oC via a sol-gel

22

route. In this research, ZnO-based PSCs exhibited superb efficiency and mitigated J-V 12

1

hysteresis in comparison to the TiO2 controls due to the longer carrier lifetime and higher

2

electron diffusion coefficient in ZnO-based ETLs.

3

However, the chemical instability induced at the interface to ZnO emerged quickly as a

4

major challenge in these solar cell designs. Extended heat-treatment turns the perovskite from

5

dark brown to bright yellow. Yang et al. revealed that ZnO can induce proton-transfer

6

reactions at the ZnO/perovskite interface, leading to eventual decomposition of the perovskite

7

film into PbI2.[106] In this reaction, the surface oxygenic functional groups such as the

8

hydroxyl on the wet-method-processed ZnO ETLs can accelerate the degradation of

9

perovskite. Therefore, a range of approaches including optimizing preparation methods,

10

heteroatom doping, and interface modification have been successfully established to enhance

11

the stability of the ZnO-perovskite interface and with it the photovoltaic performance. To

12

reduce the density of oxygenic groups on the surface, vacuum-processed ZnO films without

13

organic ligands were applied as ETLs. ALD was adopted early on to prepare ZnO ETLs and

14

the corresponding PSCs achieved a PCE of 13.1%.[107] It has been suggested that the higher

15

density of oxygen vacancies in ALD prepared ZnO films can foster the formation of

16

perovskite at room temperature (RT). Furthermore, MS-processed ZnO films were also

17

utilized as ETLs in PSCs. This high energy preparation method not only enhances the

18

adhesion of ZnO to the substrate, but by controlling the working atmosphere also the work

19

function and the oxygen defect density can be adjusted.[108] Our group demonstrated that

20

ZnO films obtained with a 1:4 Ar/O2 ratio possesses large-size grains, low defect state density,

21

and good optical crystalline quality, with PSCs achieving a PCE of 16.6%.[109]

22

Although vacuum-processing of ZnO has positive effects on the device stability, it still 13

1

suffers from the reaction between oxygen in ZnO and available protons in CH3NH3+. As such,

2

heteroatom doping and interface modification were employed with the purpose of inhibiting

3

the reaction at ZnO/perovskite interface. Here, Mg2+,[110] Al3+,[111-113] and alkali

4

metals[114, 115] have been employed as dopants in ZnO. For example, Tseng et al.

5

successfully prepared Al-doped ZnO (Al:ZnO) film by MS.[116] Compared to the undoped

6

ZnO devices, PSCs constructed with Al:ZnO exhibited higher PCE (17.6%) and superior

7

stability owing to their higher conductivity, together with better band alignment (lower work

8

function) and acid resistance. Additionally, Azmi et al. incorporated the alkali cations Li+, Na+,

9

and K+ into ZnO, which improved electron mobility, raised the Fermi energy level, and

10

passivated interfacial defects of ZnO-ETLs.[114] In this research, PSCs fabricated with

11

K:ZnO achieved the highest PCE of 19.9% among these alkali metal-incorporated devices. In

12

addition, more than 90% of the initial PCE was retained after 800 h of storage in air (40-50%

13

RH), compared to 36% for the undoped ZnO cells, indicating the improved stability.

14

Apart from heteroatom doping, direct modification of the interface can prevent or reduce

15

the (photo) chemical interaction between ZnO and perovskite and affect other properties such

16

as film formation and charge extraction. Until now, many interfacial materials have been

17

introduced at the ZnO/perovskite interface, including self-assembled monolayers (e.g.,

18

3-aminopropanoic acid and 1,2-ethanedithiol)[115, 117, 118], small molecules (e.g. fullerene

19

derivatives), polymers (e.g., poly(ethylenimine)),[113, 119] Au nanorods,[120], monolayer

20

graphene,[121] and ZnS[122, 123]. All these materials are able to prevent the aforementioned

21

decomposition of the perovskite by thermal annealing. Moreover, they can also promote the

22

charge extraction and reduce charge carrier recombination. For example, the positive effects 14

1

from small molecules and polymers are attributed to the enhanced electronic coupling

2

between ZnO and perovskite, improved band alignment, and surface passivation of ZnO. The

3

sulfur compounds are of great interest for future research, since the sulfur not only passivates

4

oxygen defects in ZnO[118] but also forms strong coordination with the Pb in perovskite,

5

thereby enhancing the electronic coupling at the interface.[124] In 2018, a composited layer

6

consisting of MgO and protonated ethanolamine (MgO-EA+) was employed at the

7

ZnO/perovskite interface as shown in Figure 2.[125] In this research, the ZnO compact layer

8

was immersed in 2-methoxyethanol solution of magnesium acetate (MgAc2) and EA, and

9

annealed at 450 °C for 30 min. In addition to the hole-blocking property of MgO, Mg can

10

11 12 13 14

remove the proton on the

Figure 2. (a) Schematic illustration and (b) cross-sectional SEM of a planar PSC modified with ZnO–MgO–EA+; (c) the J–V data, and (d) stability of the PSC measured under AM 1.5G illumination with a relative humidity of 70%.[125] Copyright 2018 Wiley-VCH.

15

hydroxyl end group of EA. The resulting EA+ can create preferable electrostatic interaction

16

with perovskite. As a result, PSCs reached a PCE of up to 18.3% along with negligible

17

hysteresis; device stability under AM 1.5G illumination at a relative humidity of 70% was 15

1

greatly enhanced compared to the control device.

2

3.1.2 SnO2 Based ETLs

3

SnO2 is a stable semiconducting material which is extensively used in electronic devices such

4

as transistors[126] and gas sensors.[127] Regarding its application in PSCs, SnO2 is receiving

5

ever-increasing attention because various of its properties are surpassing the ones of TiO2. (i)

6

The deeper conduction band (CB) favors the electron injection from perovskite and reduces

7

the potential Schottky barrier at FTO/SnO2 interface; although, in theory, it could also lead to

8

additional loss in open-circuit voltage (VOC) because of the expected increased conduction

9

band offsets. (ii) Next, the high electron mobility of around 240 cm2 V-1 s-1 supports fast

10

electron transport in devices;[63] and (iii) the wider band gap (3.5-3.6 eV) reduces the effect

11

of photoexcitation due to ultraviolet illumination. SnO2 was first applied using a mesoscopic

12

PSCs architecture, pointing out its competitiveness but also revealing that severe charge

13

recombination occurs in mesoporous SnO2 layers (a thin TiO2 layer was deposited on the

14

mesoporous SnO2 to reduce the high recombination found on bare SnO2).[128, 129] Since

15

then, SnO2 is usually utilized in planar PSCs rather than in mesoscopic configurations. Here,

16

Ke et al. fabricated nanocrystalline SnO2-based thin films as ETLs via the LT-SSC route for

17

use in efficient planar PSCs.[130] By showing a very high efficiency of 16.0%, this study not

18

only displayed the feasibility and simplicity of the SSC route but also pushed the performance

19

of planar PSCs to a higher level using a novel ETL However, LT-SSC-processed SnO2 films

20

also encounter some problems, e.g. when the annealing temperature is not high enough, the

21

SnO2 films crystallize incompletely and exhibit relatively low electron mobility, leading to

22

poorer charge dynamics and limiting the efficiency. Moreover, additional HT- post-treating 16

1

processes cannot be included in the SSC route since this usually leads to the emergence of

2

pinholes and cracks.[131]

3

To resolve this problem, facile post-treatment methods such as UV-sintering,[132] plasma

4

treatment,[133, 134] and pulsed photonic annealing[135] were employed, which do not show

5

the negative effects from HT annealing processes. It is suggested that the UV light or plasma

6

treated SSC-processed SnO2 thin films are activated by breaking the alkoxy and hydroxy

7

groups, which enables the formation of a metal−oxide−metal network followed by film

8

densification. Therefore, this type of facile process can induce high-quality films with

9

excellent coverage compared with that of thermal treatment. For example, Yu et al. proposed

10

a superfast RT activation of SnO2 thin films via atmospheric pressure O2/Ar plasma oxidation,

11

by which the

12 13 14

Figure 3. SEMs of SnO2 layers deposited by (a) ALD (b) SSC and (c) SSC+CBD; (d) Statistical parameters of PCE for the

15

ETLs can be prepared in only 5 minutes (note, this ETL is amorphous; we mention it in this

16

section nonetheless since it fits into the logic of the storyline). As a result, a champion PCE of

17

19.6% was attained due to effective electron extraction and reduced non-radiative

corresponding devices.[64] Copyright 2016 The Royal Society of Chemistry.

17

1

recombination at the SnO2/perovskite interface.[133]

2

In addition to the special post-treatment, Anaraki et al. combined SSC with CBD to prepare

3

efficient SnO2 films as shown in Figure 3.[64] They investigated the influence of fabrication

4

techniques on the efficiency of PSCs wherein SSC, CBD, and ALD were used to prepare

5

SnO2 films. The obtained order in device efficiency is: SSC+CBD (combined method) >

6

CBD > SSC > ALD. They emphasized that SSC yields thin films that may contain a high

7

density of pinholes and give rise to fast recombination dynamics; thus combining CBD as a

8

post-treating process with SSC can improve the film coverage on the substrates. The

9

combined SSC+CBD method yielded impressive PCEs as high as 20.8% with reduced

10

hysteresis and promising long-term stability. The ALD-PSCs are inferior to SSC+CBD

11

controls, which is attributed to high interface resistance generated by such a planar and

12

smooth SnO2 surface. In contrast to that, CBD has the advantage of resulting in a rougher

13

surface and making an excellent electronic contact. Accordingly, CBD-processed SnO2 is

14

widely utilized in highly-efficient planar PSCs today.[136, 137]

15

Given that special post-treatment is required in the SSC route, the NSC method was

16

proposed as another solution based method; here, highly-crystalline SnO2 nanoparticles are

17

synthesized and then spin-coated on a conductive substrate. Song et al. initially fabricated

18

SnO2-based ETLs in this manner.[138] In combination with optimizing the perovskite layer,

19

the obtained planar PSCs not only achieved a high PCE of 13.0% at that time, but also

20

exhibited high durability to ambient atmosphere for 30 days. Further, this NSC approach was

21

extended to quantum dot SnO2.[139] The quantum dot (3-5 nm diameter) SnO2 possesses an

22

higher carrier density (but still low in the range of 1012 cm-3) than larger nanocrystalline SnO2 18

1

(too insulating to measure by Hall). PSCs employing SnO2 quantum dot films treated at

2

100 °C or 200 °C showed efficiencies of 18.7% and 20.8% respectively, which indicates the

3

strong potential of this SnO2 in low cost and flexible solar applications. Based on the

4

excellent properties of SnO2, commercial nanoparticles were utilized to prepare efficient

5

ETLs for PSCs.[140, 141] Currently, high PCEs of 21.6% for a small area (0.074 cm2) and

6

20.1% for a large area (1 cm2) have been achieved by the commercial SnO2 based PSCs in

7

combination with a two-step fabrication method of perovskite in which the PbI2 content was

8

precisely controlled.[141] If long-term stability can be assured, these results are an important

9

step towards commercialization of SnO2-based PSCs.

10

Besides the solution based approaches, vacuum deposition methods such as MS[142-145]

11

and EB[146] were also utilized to prepare crystalline SnO2 as ETLs. The as-deposited SnO2

12

films generally exhibit low crystallinity but nonetheless they still possess a high electron

13

mobility exceeding 30 cm2 V-1 s-1, which is sufficient for the demands of electron transport in

14

PSCs.[142] In these vacuum-based approaches not only the commonly controlled properties

15

like temperature and layer thickness are important, but also oxygen content can be controlled

16

(e.g. via the Ar/O2 ratio) to achieve changes in high quality SnO2 formation. Qiu et al. found

17

that a highly oxidizing environment is essential for the formation of high quality SnO2 films.

18

The sole O2 atmosphere, without Ar content, can reduce gap state density and increase the

19

carrier mobility. PSCs based on such sputtered SnO2 ETLs show an efficiency up to 20.2%

20

and a T80 operational lifetime of 625 h.[143] In a similar study, Bai et al found that the Ar/O2

21

ratio of 2/3 is the optimal condition for sputtering SnO2 and also attained a high PCE of

22

18.2%.[144] 19

1

Apart from the investigations regarding the preparation techniques, optimization strategies

2

using interface modification and heteroatom doping are also applicable to SnO2-based ETLs.

3

TiO2[147], MgO[148], PCBM[136], C60[149], 3-aminopropyltriethoxysilane (APTES) [150]

4

and triphenylphosphine oxide[151] were employed to modify SnO2-ETLs. In general,

5

interface modifications by inorganic wide band gap semiconductors is mainly used for

6

reducing charge recombination, whereas organic materials can usually enhance the interface

7

charge extraction. For instance, Rao et al. significantly boosted the PCE of SnO2-based PSCs

8

from 6.5% to 14.7% by coating SnO2 with thin layers of TiO2. They explained this

9

observation by an improved perovskite coverage brought about by the decreased contact

10

angle as well as reduced charge recombination.[147] In the case of SnO2 passivated by

11

APTES [150] an enhancement in PCE from 14.7% to 17.0% was shown. The APTES offers

12

multiple functions, including: (i) inducing the formation of high quality perovskite films with

13

a favorable morphology and enhanced crystallinity; (ii) forming dipoles on the ETL surface,

14

decreasing work function of ETLs, and enlarging built-in potential; (iii) passivating the trap

15

states at the perovskite surface via hydrogen bonding; (iv) inhibiting the recombination

16

process at the interface.

17

In terms of heteroatom doping, Mg2+[152], Li+[153], Nb5+[154, 155], Sb3+[156], Zn2+[157],

18

Cl-[158], F-[159], La3+[160], Ga3+[161], and Al3+[162] have been applied to tailor the properties

19

of SnO2 and improve the performance of PSCs. To compare the effects from various dopants,

20

we summarize the parameter variations from the published reports regarding heteroatom

21

doping in Table 1. The mechanism(s) by which extrinsic doping influences SnO2 are complex,

22

despite the common observation that the overall PCE is improved. On the one hand, all 20

1

introduced dopants showed increased electron mobility and reduced recombination of the

2

SnO2-ETL, leading to higher FF and lower hysteresis index (HI). On the other hand, there are

3

trends that are less easily explained; the observed variations in CB position have almost no

4

correlation to the VOC of the PSCs, even though injection or extraction barriers should lead to

5

changes here.[163] This observation supports the statement that VOC is rather related to the

6

splitting of the quasi-Fermi energy level of electrons and holes in the perovskite itself.[50,

7

164] Another controversy is related to the effects on charge carrier density. It has been

8

indicated that a lower carrier density favors the suppression of charge recombination in the

9

cases of Mg2+ and Y3+ doping, whereas the higher carrier density observed for Sb3+ and Ga3+

10

doping also leads to a decrease in recombination rate. According to equation (3) in section 2,

11

it is seen that the depletion width x0, is inversely proportional to the square root of the carrier

12

density. There is an optimum value for x0 for absorbers in general: if the carrier density is too

13

high, x0 may be much less than the semiconductor absorber thickness and part of the absorber

14

will be field free, increasing the chances of recombination. If the carrier density is too low

15

and the (full) space charge layer width is larger than the absorber thickness, the full electric

16

field will not develop across the absorber, again increasing the chances of recombination.

17

Therefore the optimal width will be a function of the dielectric constant of the perovskite, its

18

(unintentional) doping density, and its thickness. Successful control of the properties of ETLs

19

via doping can be utilized to design multilayer ETLs with gradient changes. Based on

20

precisely-controlled properties of F:SnO2, Gong et al. fabricated bilayer ETLs with a

21

significant difference in carrier concentration between the two layers (higher in the inner layer)

22

but only a small difference in the CB positions. The resulting gradient concentration within 21

1

the bilayer ETLs induces a space charge region and thus enhances the built-in electric field.

2

PSCs based on the bilayer ETLs achieved a PCE as high as 20.2% along with VOC of 1.13

3

V.[159]

4

Table 1. The effects of extrinsic doping on SnO2 Element

Ne

µe

σ

CB

Rec

Mg2+











τave

VOC

JSC

FF

HI









PCE variation

Ref

15.5% to 17.9%

[152, 165]

Li+ Nb Sb

↑ 5+

3+

3+

Y

Ga

3+

-

F



-







15.3% to 18.2%

[153]



-

-





15.1% to 17.6%

[154]













-



-





15.7% to 17.2%

[156]













-







13.4% to 17.3%

[166]







-





-





12.7% to 16.4%

[167]















16.2% to 20.2%

[159]









-



15.8% to 17.0%

[162]

18.3% to 20.0%

[158]

Al Cl





3+

-















-





-

5 6 7

Annotation: ↑, ↓ and – represent an increased, decreased, and unchanged material or device property, represent. For the blank

8

Aside from heteroatom doping, carbon materials e.g., graphene,[168] graphene quantum

9

dots (GQDs)[97], and carbon nanodots[169] were also utilized to optimize SnO2 ETLs. For

10

instance, Xie et al. added a small amount of GQDs in SnO2 and found that the electrons

11

transferred from GQDs can effectively fill the trap states and improve the conductivity of

12

SnO2, which is

boxed the changes are not reported. Rec represents charge recombination rate, τave is calculated from photoluminescence decay curves, TD is trap density, HI hysteresis index, σ conductivity, Ne carrier density, and µe electron mobility.

22

1 2 3 4

Figure 4. (a) Schematic illustration of Fermi level of EDTA, SnO2, and EDTA-SnO2; (b) Electron mobility for EDTA, SnO2, and

5

beneficial for improving the electron extraction efficiency and reducing the recombination at

6

the ETL/perovskite interface.[97] The device fabricated with SnO2:GQDs reached an average

7

PCE of 19.2 ± 1.0% with negligible hysteresis and a maximum stabilized value of 20.2%.

8

Very recently, our group reported that ethylene diamine tetraacetic acid (EDTA) modification

9

can shift the CB of SnO2 upwards and also increase its electron mobility as shown in Figure

10

4a-b. Eventually, a record PCE of 21.6% was attained for rigid devices and 18.3% for flexible

11

devices (Figure 4c).[170] The device based on modified SnO2 maintains 92% of its initial

12

efficiency after 2880 h of exposure to ambient atmosphere in the dark; in contrast, the device

13

using untreated SnO2 only kept 74% of its initial efficiency under the same storage condition

14

(Figure 4d).

15

3.1.3 Other binary ETLs

16

In this section, we will introduce some less common binary electron transport materials

17

including WOx, Nb2O5, In2O3, and Fe2O3. These materials exhibit some drawbacks regarding

EDTA-SnO2 using the SCLC model; (c) the J–V curves and (d) Stability test under a ambient condition.[170] Copyright 2018 Nature.

23

1

their application as ETLs, but intriguing properties have been shown when integrated in

2

PSCs.

3

WOx is a chemically stable semiconductor that can, however, be present in various

4

oxidation states with varying electronic properties. The complex polymorphism and defect

5

chemistry enables its survival in harsh and corrosive environments such as strong acids.[171,

6

172] WOx also has superior carrier mobility (10–20 cm2 ·V-1·s-1)[65] compared to TiO2 which

7

is the reason why in various fields researchers are interested in this material.[173-176] In

8

view of these appealing properties, WOx was used as ETL in mesoscopic PSCs in 2014;

9

however, the devices needed a TiO2 surface modification to suppress the fast charge

10

recombination observed for the untreated oxide.[34] Our group obtained highly efficient planar

11

PSCs based on amorphous WOx which surpasses the properties of the crystalline material

12

[177] as will be discussed in more detail in the amorphous ETL section. NSC is the common

13

route for the preparation of crystalline WOx-based PSCs. As illustrated in Figure 5, mixed

14

fullerene/functionalized self-assembled monolayers (SAMs) were incorporated into

15

nanocrystalline WOx based-ETLs.[178] The SAMs increase the surface roughness of WOx

16

layer from 3.2 nm to 3.9 nm, likely because WOx was partly etched by the overnight dipping

17

into the SAMs solution. Combining the morphology regulation and interface interaction,

18

hysteresis-free PSCs with a maximum efficiency of 14.9% were fabricated. This moderate

19

PCE remains one of the highest ones in WOx nanoparticle-based PSC which indicates that

20

there is still room for improvement in this area. The stability of WOx based PSCs is also lower

21

than expected at the present time. Gheno et al. prepared planar PSCs employing commercial

22

WO3 nanoparticles and observed an acceptable stability in the dark, but the performance 24

1

under continuous standard illumination rapidly decreased. They conjectured that the

2

undesired stability is associated with the high density of oxygen vacancies that can be

3 4 5

Figure 5. (a) Schematics of the PSC architecture and SAMs molecules; AFM topography of the surface of a WOx film (b) and a

6

present in the various nonstoichiometric WOx structures.[179-181] From our own perspective,

7

the photovoltaic performance and stability of WOx-based PSCs could still be further improved.

8

Some crucial issues regarding oxygen vacancies, photoelectric properties, photocatalysis, and

9

electrochromism have not been unraveled in this system. WOx was also employed

10

successfully as HTL and hole buffer layer in many studies,[182-187] although it is not able to

11

block electrons due to the strong n-type nature of the material.

mixed SAMs on top of WOx (c); (d) The corresponding J-V characteristics.[178] Copyright 2015 Wiley-VCH.

12

In2O3 is an n-type semiconductor that possesses a wide band gap (∼3.75 eV), a high

13

electron mobility (∼20 cm2 V-1 s-1), and good thermal stability. Qin et al. fabricated

14

In2O3-based planar PSCs with a PCE of 13.0% via a sol-gel route in 2016.[72] In their study,

15

PCBM was also employed to modify the In2O3 surface. The corresponding PCE was

16

improved to 14.8%, but the hysteresis was in both cases rather severe. These unfavorable

17

results were believed to be due to the high density of pinholes in In2O3 films because of the 25

1

vigorous hydrolysis of In3+. To eliminate pinholes, Chen et al. optimized the components of

2

the precursors by introducing acetylacetone as a chelator; the roughness of In2O3 films was

3

decreased (Figure 6a-b), and

4 5 6 7

Figure 6. SEM of In2O3 derived from precursor without (a) and with (b) acetylacetone; (c) Photoelectrochemical oxidation of

8

afterwards PSCs based on In2O3 without PCBM achieved a PCE of 15.3%.[188] Despite the

9

undesired morphology caused by pinholes, In2O3 possesses an advantage as illustrated in

10

Figure 6c-d. The photocatalytic activity of In2O3 is small compared with that of ZnO and TiO2

11

due to its wider bandgap; this leads to negligible ultraviolet absorption and therefore high

12

light stability. PSCs based on In2O3 exhibited the best stability under illumination when

13

compared with those based on ZnO and TiO2. To further improve the PCE of In2O3 based

14

PSCs, Yoon found that the pinhole density could be limited when the precursor was

15

spin-coated in a N2 atmosphere. They also revealed that increasing the annealing temperature

16

of In2O3 from 200 °C to 300 °C can increase the oxygen vacancy density and

17

conductivity.[189] In their research, an ultrathin TiOx interlayer was also introduced between

18

the substrate and In2O3 films to form bilayer ETLs and offset the negative effect from

CH3NH2 by ZnO, TiO2 and In2O3 under illumination; (d) Normalized PCE of PSCs based on ZnO, TiO2 and In2O3 plotted as a function of exposure time under illumination.[188] Copyright 2017 The Royal Society of Chemistry.

26

1

pinholes, resulting in an impressive PCE of 16.4%.[190]

2

Nb2O5 possesses a similar or somewhat higher optical band gap (3.2 – 4.0 eV) as TiO2 with

3

comparable electronic properties and chemical stability.[70] Stoichiometric Nb2O5 is an

4

insulator that becomes an n-type semiconductor at lower oxygen content such as Nb2O4.978.

5

Previous reports on DSSCs and polymer solar cells showed that Nb2O5 is an effective

6

blocking layer, suppressing surface charge recombination.[191, 192] Kogo et al. used

7

SSC-processed Nb2O5 films as ETLs on an Al2O3 scaffold layer to fabricate PSCs.[193]

8

Nb2O5 exhibited a strong hole blocking effect and as a result, PSCs achieved a larger VOC up

9

to 1.13 V. However, they also reported that the conductivity of Nb2O5 is rather low. Following

10

this work, Fernandes et al. prepared Nb2O5 thin films via MS, which was applied in

11

mesoscopic PSCs with the architecture FTO/Nb2O5/mp-TiO2/MAPbI3/Spiro-OMeTAD/Au. In

12

this structure, the device stability under illumination was improved and the hysteresis effect

13

was diminished compared with a TiO2 ETL.[68] After this, Ling et al. successfully utilized

14

sputtered Nb2O5 as ETLs in planar PSCs.[70] In their research, amorphous Nb2O5 (a-Nb2O5)

15

exhibited less pinholes than crystal-Nb2O5 (c-Nb2O5). PSCs based on c-Nb2O5 achieved a

16

PCE as high as 17.2%; the results on amorphous Nb2O5 will be discussed in the appropriate

17

section below. This publication illustrated the great potential of Nb2O5 in PSCs. In addition,

18

Nb2O5-ETLs for planar PSCs were also prepared via the SSC[194] and NSC[195] routes and

19

the devices based on the latter method yielded a high PCE of 20.2% along with superior UV

20

stability compared with that based on TiO2.

21

α-Fe2O3 (2.2 eV) is the most stable iron oxide with n-type semiconducting properties. In

22

general, the VB and CB of a transition metal oxide mainly consist of the 2p band of the 27

1

oxygen anion and the ns band of the transition metal anion, respectively. However, the Fe 3d

2

band energies are located between the 4s and 2p bands as illustrated in Figure 7a.[196]

3

Therefore, the band gap of Fe2O3 is the lowest among the metallic oxide ETLs. This complex

4

energy band structure causes unexpected results in PSCs. Fe2O3 and Ni:Fe2O3 based ETLs

5

have been

6 7 8 9

Figure 7. (a) Energy-level diagram of iron oxide.[196] Copyright 1996 Elsevier B.V. (b) SEM of the α-Fe2O3 nano-island film; (c) J–V curves of the PSCs based on nano-island and planar Fe2O3; (d) Photoelectrochemical oxidation of CH3NH2 using α-Fe2O3 nano-islands and mesoporous TiO2.[197] Copyright 2017 Wiley-VCH.

10

prepared via the SSC route. Fe2O3 can provide strong electron extraction capability and a

11

significant reduction in the charge accumulation at the perovskite/Fe2O3 interface thereby

12

leading to less hysteresis. The fabricated PSCs merely reached PCEs in the range of 14%.[198,

13

199] A further problem with Fe2O3 is its low bandgap (~2.2 eV) which can result in parasitic

14

light absorption and therefore loss in photocurrent. To overcome this dilemma, small α-Fe2O3

15

nano-islands were grown on a compact α-Fe2O3 underlayer, as shown in Figure 7b, to provide

16

high transmittance and at the same time good charge extraction.[197] PSCs with this bilayer

17

structure achieved an impressive PCE of 18.2% with an alleviated hysteresis (Figure 7c). 28

1

Meanwhile, Fe2O3 was confirmed to be insensitive to illumination (Figure 7d, similar to

2

In2O3), which makes it possible to achieve photo-stable PSCs.

3

3.1.4 Metal sulfide and selenide based ETLs

4

In addition to oxides, many metallic sulfides and selenides including CdS, In2S3, ZnSe, TiS2,

5

and SnS2 were also employed as ETLs in PSCs. In the past, CdS has been widely

6

implemented as n-type buffer-layer material in polymer, CIGS, and CdTe solar cells due to its

7

high electron mobility and tunable band gap.[200] Moreover, CdS quantum dots are also

8

utilized as photoactive material in quantum dot solar cells.[201] In the field of PSCs,

9

sputtered CdS ETLs were initially reported in 2014 by Mora-Sero et al. with the purpose of

10

analyzing the effect of ETLs on the charge transfer process in PSCs; the PCE was extremely

11

low (1.53%).[56] Despite this finding, the higher CB of CdS can lead to higher photovoltage

12

compared with TiO2. Liu et al. used CBD-CdS thin films in planar PSCs and achieved a PCE

13

of 11.2%.[202] These CdS film exhibited low crystallinity, which suggests that this PCE can

14

be improved by optimizing the CdS crystallization. Furthermore, another issue should be

15

noted, that is, Cd2+ may diffuse into the perovskite during heating, which can influence the

16

photovoltaic performance and stability. Shohl et al. suggested that Cd2+, driven by heating

17

during the preparation of perovskite (120 °C), may diffuse from the CdS-ETL into the

18

overlying perovskite films, thus forming CdI2. An insulating barrier of MA2CdI4 is then

19

generated in combination with MAI, which accounts for the major decline in device

20

performance.[203] On the other hand, Tong et al. argued that 110 °C annealing of the

21

perovskite on CdS leads to in-situ Cd-doping and that this dopant may improve the device

22

performance.[204] Overall, the heating temperature should be tailored systematically for each 29

1

system during optimization. Peng et al. fabricated CdS-based PSCs via CBD with a

2

processing temperature below 100 °C.[205] In this research, which resulted in a high PCE of

3

16.1% , the authors suggested that the efficient carrier extraction and reduced surface defects

4

at the CdS/perovskite interface helped to mitigate the hysteresis and improve the long-term

5

stability. Apart from the commonly-used CBD method, Dong et al. synthesized CdS

6

nanoparticles via a one-step solvothermal reaction. PSCs based on the highly-crystalline

7

nanoparticles achieved a highest PCE of 16.5%.[206] This high PCE is contributed to the

8

higher transmittance in the short wavelength region of size-quantized CdS than the films

9

prepared via CBD and sputtering methods.

10

The successful application of CdS films may be extend to other analogous inorganic

11

semiconducting materials. It should be noted that the toxicity of Cd presents a significant

12

hurdle for application. Thus, more environmentally-friendly analogous compounds have been

13

considered. Based on its adequate carrier mobility (17.6 cm2 V-1 s-1) and moderately-high

14

band gap (2.0-2.8 eV),[80] In2S3 nanoflake-based films were prepared via CBD and applied

15

as ETLs in PSCs.[80, 207] These In2S3-based devices exhibited enhanced light trapping and

16

low recombination rates, achieving a high efficiency of 18.2% (5.6%) for MAPbI3 (CsPbIBr2)

17

-based PSCs. In addition, Xu et al. successfully prepared In2S3-ETLs via a NSC route and

18

reached an impressive PCE of 18.8%. Compared with TiO2, In2S3-ETLs display low trap-state

19

density, superior electron mobility, and high electron extraction capability.[208]

20

ZnS was also utilized in PSCs; however, the energy level mismatch due to the very high

21

CB resulted in a PCE lower than 1%.[202] ZnSe was then proposed due to the direct band gap

22

of 2.8 eV, a much more suitable band alignment with the perovskite, and a high mobility 30

1

(200–280 cm2 V-1 s-1).[79] Li et al. first fabricated ZnSe films via CBD as ETLs and achieved

2

an impressive PCE of 17.8%.[209] The stability was improved because ZnSe, with its 2.8 eV

3

band gap, acts as an ultraviolet blocking layer thereby retarding the degradation of the

4

perovskite under illumination.

5

2D layered materials are also promising electron transport materials. Due to their inherent

6

confinement in the out-of-plane direction, 2D materials can produce a perfect surface

7

accompanied by low defect density and high in-plane conduction, enabling the preparation of

8

ultrathin photovoltaics. For example, our group demonstrated that 2D TiS2 can act as an ETL

9

in PSCs. The NSC processed TiS2 film exhibits good electron transport and electron

10

collection ability, while its higher CB is favorably aligned with the one of the perovskite. The

11

poor photocatalytic properties of TiS2 ensure its excellent UV stability. As a result, PSCs

12

based on TiS2 exhibited a PCE as high as 17.4% and can retain approximately 90% of the

13

time-zero PCE after 50 h UV light soaking.[82] Huang et al. emphasized that UV-ozone

14

treatment can partially oxidize the TiS2 film surface, passivate the sulfur vacancies, and

15

increase the work function. PSCs constructed with this treated TiS2 can achieve a high PCE of

16

18.8% and maintain over 95% of its initial value after 816 h aging without encapsulation.[210]

17

We would therefore like to note that TiS2, similar to WOx, is also a promising hole transport

18

material.[211] In addition, 2D SnS2 was also used as ETL in PSCs.[83, 212] As shown in

19

Figure 8a-b, Zhao et al prepared a highly dense 2D SnS2 ETL, possessing a dominant size of

20

300 nm and a film thickness of 2.4 nm containing six-SnS2 layers. They also found that the

21

large-scaled multilayered 2D SnS2 sheet structure triggers a heterogeneous nucleation over

22

the perovskite precursor film (Figure 8c).[212] The intermolecular Pb⋅⋅⋅S interactions (Figure 31

1

8d) between perovskite and SnS2 could passivate interfacial trap states, which would suppress

2

charge recombination and thus more electrons can be extracted, leading to a more balanced

3

charge distribution throughout the device. Therefore, in Figure 8e, a PCE of 20.1% was

4

realized for the 2D SnS2 ETL based PSCs. Lastly, it should be noted that with respect to the

5

metal sulfide and selenide based ETLs, more attention should be given to the interaction

6

between Pb and S(Se), since it can enhance the interface interaction significantly.

7 8 9 10 11

Figure 8. (a) Atomic force microscope (AFM) images of 2D multilayer SnS2 film. (b) AFM image of 2D monolayer SnS2 film

12

3.1.5 Ternary ETLs

13

Material engineering plays a key role in the field of functional electrodes. The interest in

14

ternary complex oxides has recently grown in the quest to further improve their physical and

15

chemical properties. Ternary oxides possess excellent electron mobility and a high density of

16

chemical sites; they have tunable band structures and are chemically stable even in extreme

17

conditions. However, the complex synthesis of multi-component oxides presents a challenge

18

in comparison with that of binary oxides.

19

and the film thickness. (c) The forming process of perovskite onto SnO2 and SnS2 substrates. (d) Pb 4f core level for perovskite, perovskite/SnS2, and perovskite/SnO2 films. (e) J–V curves of devices based on SnO2 and SnS2.[212] Copyright 2018 WILEY-VCH.

Zn2SnO4 is recognized for its high electron mobility and appealing optical properties that 32

1

make it suitable for a wide range of applications. Zn2SnO4 immediately showed excellent

2

performance when it was used for the first time. In 2015, Shin et al. presented a facile route to

3

synthesize highly-dispersed Zn2SnO4 nanoparticles (~11 nm, Figure 9a) from a Zn-hydrazine

4

complex precursor at LT (≤ 100 °C) and used them to prepare highly-efficient flexible

5

PSCs.[76] A Zn2SnO4 nanoparticle-based film can significantly improve transmittance (Figure

6

9b) of the substrate due to the low refractive index (1.37) and low extinction coefficient of

7

nearly zero in

8 9 10 11 12 13

Figure 9. (a) Plane view and cross-sectional SEM image of Zn2SnO4 thin film; (b) Transmittance and reflectance spectra of PEN/ITO/Zn2SnO4, PEN/ITO/TiO2 and PEN/ITO substrate; (c) photograph of the ZSO-based flexible perovskite solar cell; (d) J– V curve flexible PSCs based on Zn2SnO4 nanoparticles. [76] Copyright 2015 Nature. (e) Band gaps of Zn2SnO4 nanoparticles with different sizes. (f) Schematic illustration of ESLs designed by incorporating Zn2SnO4 QD and nanoparticles. (g) J−V curve of flexible PSCs based on designed ESLs.[213] Copyright 2016 American Chemical Society.

14

the relevant spectral range. By combining high electron mobility and suitable band structure,

15

the best-performing flexible PSC based on the Zn2SnO4 exhibited a high PCE of 15.7%

16

(Figure 9c-d). This work presents a breakthrough regarding the fabrication of ternary

17

metal-oxide semiconductors on flexible substrate. The energy level positions and electron

18

transporting capability of Zn2SnO4 were tailored by decreasing the particle size down to

19

quantum dot dimension as shown in Figure 9e. The authors found that the large Zn2SnO4

20

nanoparticles are beneficial for electron transport as well as electron transfer at the 33

1

substrate/ETL interface which is due to relatively fewer grain boundaries. On the other hand,

2

Zn2SnO4 quantum dots can lead to a large built-in potential at ETL/perovskite interface,

3

which is beneficial for the electron transfer from the perovskite to the ETL. Thus, in Figure 9f,

4

a bilayer architecture was proposed in which nanoparticles are employed as an underlayer and

5

quantum dots as an overlayer; when both advantages are combined, an efficiency over 16%

6

was achieved in flexible PSCs (Figure 9g).[213] Given the complex preparation route for

7

Zn2SnO4 nanoparticles, Wu et al. proposed a simple SSC route with a HT annealing process in

8

which an efficiency of 16.4% was achieved.[214] Apart from planar structures, mesoscopic

9

PSCs based on Zn2SnO4 nanoparticles[215] and nanofibers[216] have also been widely

10

reported. However, the efficiencies are not satisfactory due to severe charge recombination.

11

This finding indicates that Zn2SnO4 is not suitable in a mesoporous structure, similar to SnO2.

12

Inorganic perovskite oxides are also suitable electron transport materials for PSCs. BaSnO3

13

was first proposed as a substitute for mesoporous TiO2 in mesoscopic PSCs (with TiO2

14

compact layers) due to the identical perovskite crystal structure which may favor the growth

15

of the halide perovskite layer. Although a promising PCE of 12.3% was achieved, the PSCs

16

exhibited extremely serious hysteresis behavior.[217] BaSnO3 compact layers for PSCs must

17

be fabricated to achieve the full potential of high conductivity for BaSnO3. Also, La doped

18

BaSnO3 was prepared as an ETL using a superoxide colloidal solution route with

19

temperatures below 300 °C to prepare planar PSCs.[218] La:BaSnO3 exhibits an

20

exceptionally high electron mobility of 320 cm2 V-1 s-1 and its high electron concentration

21

causes a large built-in potential at the ETL/perovskite junction. The PSCs constructed with

22

La:BaSnO3 exhibited a high steady-state PCE of 21.2%. As additional advantage, in Figure 34

1

10a-b, La:BaSnO3 exhibits very low UV photocatalytic capability. As presented, the

2

encapsulated La:BaSnO3-based PSCs can retain 93% of its time-zero performance after 1000

3

hours of constant illumination (AM 1.5G), indicating a high photostability. However, La3+

4

dopant induce almost no change in energy levels of BaSnO3 therefore Sr2+ was doped into

5

BaSnO3 in order to tailor its energy levels with the aim of improving VOC. The smaller ionic

6

radius of Sr2+ compared to Ba2+ leads to a large octahedral tilting distortion whereby larger

7

amounts of Sr2+ dopants (Sr/Ba > 0.5) can change the perovskite structure from cubic phase to

8

orthorhombic phase (Figure 10c-d).[219] As a

9 10 11 12 13 14

Figure 10. (a-b) Photostability test under constant solar illumination for encapsulated and unencapsulated PSCs based on La:BaSnO3.[218] Copyright 2016 by the American Association for the Advancement of Science. (c) Crystal structures of BaSnO3, (Ba,Sr)SnO3 and SrSnO3; (d) Unit cell parameters of the Ba1-xSrxSnO3; (e) J–V curves and (f) Photostability test of Ba0.8Sr0.2SnO3 and TiO2 based PSCs under constant simulated solar illumination.[219] Copyright 2019 The Royal Society of Chemistry. (g) The energy band diagram of SrSnO3 and Y:SrSnO3.[220] Copyright 2019 Elsevier B.V.

15

result, Sr2+ incorporation led to a decrease in work function of BaSnO3, thereby minimizing

16

the voltage loss at the interfaces of ETL/perovskite and leading to an increase in VOC from

17

1.07 to 1.13 V in PSCs (Figure 10e). Meanwhile, the Ba0.8Sr0.2SnO3 cell, similar to the 35

1

La-doped BaSnO3 device, shows much better photostability than the TiO2 cell; again, one

2

reason for this must be the lower photocatalytic activity of BaSnO3 (Figure 10f). It is worth

3

noting that SrSnO3 is also a potential electron transport material for PSCs but its band

4

structure, conductivity, and charge carrier mobility are not as good. Therefore, Guo et al

5

incorporated Y3+ in SrSnO3 and used this in PSCs. In this research, the Y3+ incorporation can

6

shift the CB down and thereby narrow the band gap (Figure 10g), leading to better band

7

alignment along with superior electron transport capability which boosted the PCE from 16.9%

8

to 19.0%.[220] Motivated by a similar perovskite crystal structure, SrTiO3 (µ = 5-8 cm2 V-1 s-1,

9

Eg = 3.2 eV) was also considered a suitable replacement for TiO2.[78] Bera et al. introduced

10

SrTiO3 as mesoporous layer in PSCs for the first time and obtained a high voltage but low

11

current.[78] Also, graphene-modified mesoporous SrTiO3 was incorporated in PSCs to

12

enhance the photocurrent, and a PCE of 10.5% was obtained.[221] Unfortunately, SrTiO3 has

13

not yet been applied in planar structures.

14

Along with common ternary oxides, tunable composite ternary oxides composed of two

15

binary oxides have been presented as novel electron transport materials. The advantage of

16

such ternary oxides is the possibility to tune photoelectric properties by altering the

17

components ratios. In the TiO2-ZnO system, three compounds - TiZn2O4, TiZnO3, and

18

Ti3Zn2O8 - have been reported to produce stable compounds. Among them, TiZn2O4 with a

19

spinel-type structure was identified as a suitable electron transport material and planar PSCs

20

based on this compound delivered a PCE of 15.1%.[222] Compared with pure ZnO, the

21

composited oxide exhibited superior stability, indicating the advantages of composite ternary

22

oxides. In the TiO2-FeOx system, Ti0.67Fe0.33Ox, Ti0.5Fe0.5Ox, and Ti0.33Fe0.67Ox are considered 36

1

stable oxides and have been utilized as ETLs in planar PSCs. These ternary composite oxides

2

show superior properties compared to Fe2O3, including good coverage on substrates, suitable

3

energy levels, and superior transmittance. Among these oxides, Ti0.5Fe0.5Ox-based PSCs

4

attained the highest PCE of 14.7%.[223] From our point of view, these studies on ternary

5

composite oxides are important because precise regulation of photoelectric properties is

6

achieved by tailoring the components, which is beneficial for designing novel electron

7

transport materials; this is a feasible strategy to avoid the various shortcomings that can

8

accompany imperfect materials.

9

With the development of novel ETLs, some notable unusual materials were also used in

10

PSCs that demonstrated excellent performance. Li4SiW12O40, a polyoxometallate, possesses a

11

framework structure composed of tungsten oxide octahedral, with a Si4+ located at the center.

12

Choi et al. prepared Li4SiW12O40 as ETLs for PSCs via a LT solution process, obtaining a

13

PCE of 14.3%.[224] It is noteworthy that Li4SiW12O40 possesses excellent solubility in many

14

solvents, favoring a wide range of thin film preparation methods. We assume that Li4SiW12O40

15

can work successfully in PSCs due to the structural units of W3O102− clusters, which are

16

similar to the crystal structure of WOx. Hence, we conjecture that other tungstates and

17

molybdates may be also promising for PSCs.

18

3.2 Amorphous ETLs

19

Flexible PSCs are potentially important for large scale integration owing to their wide

20

adaptability and light weight. Preparing high quality ETLs at LT, which is required for foil

21

based substrates, is the bottleneck problem for flexible PSCs. According to the above

22

discussions, ETLs in flexible PSCs are crystalline and usually prepared via the NSC route. 37

1

However, the energy consumed in the crystallization process will increase the energy payback

2

time of devices and may also destroy flexible substrates. By contrast, amorphous materials

3

can completely solve the thermodynamic limitations, achieve a real LT preparation, and

4

reduce the energy repayment time. Hence, amorphous ETLs were proposed for LT-processed

5

ETLs and flexible PSCs. Commonly, charge transport properties of amorphous materials are

6

inferior to those of crystalline ones due to the high density of trap states resulting in low

7

carrier mobilities. However, our group found that amorphous TiOx can possess comparable or

8

even better properties than crystalline TiO2 in PSCs,[96, 225] thus indicating that amorphous

9

ETLs can also exhibit superior performance in PSCs. To date, amorphous SnO2, WOx, Nb2O5,

10

CdS, and CeOx have been reported as ETLs in PSCs.

11

SSC and vacuum deposition are the two main approaches for the preparation of amorphous

12

ETLs. As already mentioned in section 3.1.2, the SSC route for SnO2 was conducted in a

13

temperature range of 180 – 200 °C, which is slightly beyond the estimated critical

14

temperature of most plastic substrates. However, the efficiency decreased with decreasing

15

annealing temperature below 150 °C.[226] Hence, attention was turned to vacuum deposition

16

strategies wherein ALD is the dominant approach for amorphous SnO2 ETLs. Generally, the

17

ALD-processed SnO2 film is amorphous if the post treatment temperature is lower than

18

250°C.[227] Correa-Baena et al. fabricated amorphous SnO2 via ALD and found that the

19

deeper CB of SnO2 compared to TiO2 could improve band alignment and achieve a

20

barrier-free energetic alignment to mixed halide perovskite. PSCs based on amorphous SnO2

21

achieved a PCE exceeding 18% accompanied with negligible hysteresis.[228] This work

22

shows the significance of amorphous SnO2 for the use in mixed halide/cation perovskite solar 38

1

cells. The solvent free and LT process at 120 °C shown here enables the application of

2

amorphous SnO2 in tandem solar cells where this preparation step can avoid heat-induced

3

damage to the bottom cell and still provide good photovoltaic performance. With this in mind,

4

Albrecht et al. fabricated a monolithic tandem solar cell composed of a silicon heterojunction

5

bottom- and an ALD-SnO2-based perovskite top-cell, which yielded a high voltage of 1.78 V

6

and a high PCE of 19.9%.[229]

7

Despite the high efficiency achieved, further reduction in the deposition temperature is

8

difficult because temperatures lower than 120 °C impact the reaction of the metal precursor in

9

the thermal ALD process negatively. Therefore, plasma-enhanced atomic layer deposition

10

(PEALD) was proposed in order to reduce the preparation temperature further to 100 °C. In

11

combination with the surface passivation by C60-SAMs, PEALD-SnO2 based PSCs have

12

achieved PCEs as high as 19.0% (rigid) and 16.8% (flexible) with negligible hysteresis and

13

good stability.[230] In successive studies, the presence of impurities in amorphous SnO2 were

14

addressed which result in high resistivity.[231, 232] Water vapor post-treatment was

15

introduced to improve the conductivity, and high PCEs of 20.0% and 18.4% were achieved in

16

rigid and flexible devices, respectively.[233] These results reveal that amorphous SnO2 is an

17

effective electron transport material exhibiting comparable performance with crystalline

18

SnO2.

19

Besides SnO2, Ling et al. applied RT-sputtered amorphous Nb2O5 films as ETLs for planar

20

PSCs.[70] In this work the advantages and disadvantages of amorphous materials are clearly

21

established: (i) the CB of amorphous materials is usually lower than that of crystalline

22

materials due to the random atomic arrangement, leading to improved charge extraction 39

1

capability (Figure 11a-b); (ii) amorphous materials generally possess lower electron mobility

2

but higher carrier concentration, which helps to maintain a fair conductivity; (iii) PSCs based

3

on amorphous materials suffer from severe charge recombination rates due to the high density

4

of trap states (e.g., oxygen vacancies). As a result, the PCE of PSCs based on a-Nb2O5

5

reached 17.1% (Figure 11c), which is comparable to that of HT-heated crystalline Nb2O5

6

controls. Recently, in a device based on the sputtered Nb2O5 discussed above, remaining

7

surface states were passivated by the simultaneous introduction of PCBM and ionic liquids

8

which enhanced the charge transfer dynamics, leading to a high PCE of 18.8%.[234] In

9

addition to MS, our group utilized EB-processed Nb2O5 films as ETLs to fabricate flexible

10

PSCs.[235, 236] Such Nb2O5 films are extremely uniform and are thus capable of being used

11

in large-scale flexible modules. After further optimization of the perovskite layer, such

12

flexible PSCs based on amorphous Nb2O5 could achieve a PCE up to 18.4%. Moreover,

13

large-area (1.2 cm2) flexible

14 15 16 17 18

Figure 11. (a) Energy-level diagram of Nb2O5 and in PSCs; (b) Time-resolved PL decay of the perovskite on a-Nb2O5 and c-Nb2O5; (c) J-V curves of PSCs based on a-Nb2O5 and c-Nb2O5.[70] Copyright 2017 American Chemical Society. (d) J–V curves of different active areas for the Nb2O5-based flexible PSCs; (e) The PCEs of flexible device at different bending curvature radii after 5000 flexing cycles.[235] Copyright 2018 Wiley-VCH.

40

1

PSCs also showed excellent performance (13.4%, Figure 11d). The flexible PSCs exhibited

2

excellent bending resistance, even when the bending curvature radius was as small as 4 mm

3

(Figure 11e) showing that amorphous Nb2O5 can be effective ETLs in large-scale PSCs.

4

Vacuum preparation strategies possess the advantages of high reproducibility and precise

5

control over layer thickness and composition. ETLs prepared through this method usually

6

benefit from high uniformity, purity, and stability. However, they require more complex

7

instrumentation as well as higher energy input, increasing the overall cost of PSCs. Therefore,

8

the simple and low-cost SSC route should be conducted for amorphous ETLs. Nevertheless,

9

owing to insufficient annealing, the existence of residual impurities and the lack of

10

crystallization decrease the conductivity of these ETLs. Hence, up to now, there are only few

11

reports concerning solution-processed amorphous ETLs.

12

In 2015, our group prepared amorphous WOx ETLs (Figure 12a) through a simple SSC

13

process (WCl6 in n-propanol) at 150 °C. Compared with crystalline TiO2-ETLs, WOx-ETLs

14

exhibited comparable light transmittance but higher electrical conductivity. PSCs based on

15

WOx-ETLs exhibited a comparable PCE (9.0%) but lower VOC and FF than that of TiO2, due

16

to increased charge recombination through the high density of trap states.[177] Based on this

17

work, heteroatom doping and interface engineering were adopted to optimize the amorphous

18

WOx. As shown in Figure 12b-c, an titanium salt was introduced into the precursor solution

19

for fabricating WOx–TiOx composite ETLs at 70 °C or even at RT.[95] The composite with

20

TiOx decreased the work function, trap state density, and charge recombination. As a result,

21

the PCE was notably improved from 9.0% to 14.5%. In a similar approach, PSCs with

22

composite ETLs fabricated at 70 °C and RT were also efficient, with maximum PCEs of 13.5% 41

1

and 11.6%, respectively. Besides, Eze et al. also utilized C60 to modify amorphous WOx ETLs,

2

which led to more efficient charge transfer at the interface and achieved a high PCE of

3

16.07%.[237] However, follow up research revealed that HCl, released from the reaction

4

while employing WCl6 as precursors, can corrode the TCO. Therefore, W(C2H5O)5 was used

5

to prepare WOx–ETLs for efficient flexible PSCs. Eliminating HCl can reduce the corrosivity

6

and give a smoother film surface. Moreover, in Figure 12 d, NbOx was employed to modify

7

the amorphous WOx–ETLs, which improved the electrical conductivity of WOx-based ETLs

8

by enhancing donor density, reducing interfacial depletion width, and minimizing trap

9

states.[94] The PCE of the resulting flexible PSCs was consequently improved, and high

10

PCEs of 15.7% and 13.1% were achieved when ETLs were fabricated at 120 °C and RT,

11

respectively. Motivated by this work, solution-processed amorphous NbOx films were

12

employed as ETLs as well and a PCE of 19.1% was obtained in rigid devices.[238]

13 14 15 16 17

Figure 12. (a) X-ray diffraction result of amorphous WOx;[177] Copyright 2015 American Chemical Society. (b) Mott–Schottky curves of ESLs based on WOx and WOx-TiOx; (c) J–V curves of PSCs based on WOx-TiOx fabricated at various temperatures.[95] Copyright 2015 Wiley-VCH. (d) J−V curves of champion PSCs based on WOx-TiOx ESLs.[94] Copyright 2017 Elsevier B.V.

Aside from the above result, some other works regarding amorphous ETLs were also 42

1

applied in PSCs, including CBD-processed SnO2,[239] SSC-processed CeOx[240], and

2

thermally evaporated CdS[241]. We will not review them systematically due to various

3

reasons such as inferior PCE or high production cost.

4

3.3 Inorganic ETLs in IPSCs

5

PSCs can be separated into two main geometries – normal cells, where the cell is illuminated

6

from the ETL side, and IPSC where illumination is occurring from the HTL side. Since in

7

IPSCs the ETL is deposited onto the perovskite, it is clear that high temperature processes

8

cannot be applied, especially for hybrid perovskites which contain a small organic cation on

9

the A site. This means that many, if not most, of the common ETL materials cannot be used in

10

IPSCs, at least not in the way they are presently made. Here, the molecule PCBM is the most

11

commonly-used electron transport material due to its adequate mobility and easy processing;

12

it was also shown that it is able to passivate trap states in the perovskite and reduce the

13

hysteresis.[242] IPSCs based on PCBM have achieved a PCE of 21.0%.[24] However, several

14

critical issues will play a role when employing PCBM as an ETL:

15

(i) The limited solubility of CBM in the commonly employed solvents like

16

chlorobenzene results in non-uniform coverage which in turn leads to uncovered perovskite

17

surface and shunting pathways.

18

(ii) PCBM is sensitive to, both, water and oxygen from ambient air.

19

(iii) Ions from the metal electrode and the perovskite can diffuse through the amorphous

20

PCBM and react with each other.

21

(iv) The lowest unoccupied molecular orbital level of PCBM (∼4.2 eV) does not match

22

well with the high work function of stable metal electrodes such as Au. Metal electrodes with 43

1

low work function, such as Al and Ca, are important to ensure efficient electron extraction,

2

which reduces the long term stability of the devices.[243]

3

Taking the current situation of PCBM into account, more work focusing on ETLs of IPSCs

4

should be conducted to circumvent the above dilemmas. Inserting inorganic ETLs between

5

PCBM and metal electrode is an effective approach since they can optimize the energy level

6

alignment and act as protective layers against elemental diffusion. Inorganic ETLs in IPSCs

7

should be prepared at LT without introducing detrimental solvents to perovskite.

8

Docampo et al. first incorporated TiOx layers between PCBM and metal electrode in IPSCs

9

to improve the coverage and fill the pinholes in the PCBM.[244] However, the corresponding

10

PCE was only ca. 10% due to the poor crystallinity and low conductivity of the TiOx. Further,

11

ZnO was introduced in IPSCs due to its superior conductivity and easier crystallization;

12

numerous researchers inserted ZnO nanocrystalline films between fullerenes and the metal

13

electrode using an annealing-free NSC.[245-253] In these studies, the ZnO interlayer

14

dramatically improved the hole-blocking capability due to the deep VB level, enhanced the

15

charge selectivity of electron-accepting contacts by preventing the reaction between

16

electrodes and perovskite layers, and improved the ambient stability of the devices by acting

17

as a protection that inhibited the penetration of moisture into perovskite in parallel with

18

minimizing the diffusion of metal ions from the electrode into the PCBM layer. Beside these

19

enhancements, ZnO is popular in semitransparent IPSCs since it provides a protection effect

20

during the process of sputtering a top transparent electrode.[252] Notably, Najaf et al. have

21

improved the PCE of IPSCs to 18.6% by introducing ZnO nanocrystalline layers and over 85%

22

of the maximum stabilized output efficiency was retained after 1,000 h of light soaking in 44

1

N2.[253] Using ZnO, the nanoparticles aggregate spontaneously because of their high surface

2

energy, which leads to the formation of rough layers and a high density of trap states.

3

Therefore,

4

poly[(9,9-bis(3′-(N,N-dimethyla-mion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]

5

(PFN) and polyethylenimine (PEI) were blended in ZnO nanoparticles. This approach can

6

reduce the unwanted aggregation and enhance the conductivity, resulting in excellent

7

photovoltaic

8

highly-crystalline SnO2[256, 257], Zn2SnO4,[258] and CeOx[259], were also used in IPSCs.

9

PCEs in the region of 17%–20% were achieved due to the optimized electron transport and

10

decreased recombination loss. Moreover, these results indicate the evident function of

11

inorganic oxides on improving the long-term stability of IPSCs.

polymers

performance

and

such

long-term

stability.[254,

255]

as

Apart

from

ZnO,

12

Vacuum deposition routes for ELTs are more widespread in IPSCs which avoid possible

13

negative effects of solvents on the underlying perovskite layer. The application of ALD-ZnO

14

films in IPSCs displays several advantages such as fine-tunability of the work function, low

15

deposition temperature, high charge selectivity, good electron transport ability, and excellent

16

film coverage.[243] For example, Seo reported that the conductivities of ALD-processed ZnO

17

and Al:ZnO are 0.02 and 47.3 mS cm-1, respectively. In comparison, they merely found a

18

conductivity of 0.01 mS cm-1 for PCBM.[260] The inserted- highly conductive Al:ZnO films

19

act as efficient electron-transporting layers and dense passivation layers. As a result, IPSCs

20

based on ALD-ZnO can attain an enhanced PCE of 18.5% and retain 86.7% of the initial

21

efficiency for 500 h under continuous 1 Sun illumination at 85 °C in ambient air. However,

22

the commonly used precursor Zn(C2H5)2 in combination with H2O will induce the 45

1

decomposition of perovskite films so that the ALD process must be modulated mildly and a

2

blocking layer such as PCBM between ZnO and perovskite seems to be very crucial.[261]

3

This technique is similar to the MS process where the C60 interlayer must be placed between

4

the perovskite and sputtered ZnO layers to prevent damage to perovskite layer during

5

sputtering.[262]

6

As mentioned before, the use of organic materials in IPSCs will likely negatively influence

7

the stability. Correspondingly, replacing all the organic charge transport materials with

8

inorganic ones, i.e. fabricating all-inorganic devices, can greatly enhance the device stability

9

while at the same time the cost of fabrication is reduced. Therefore, given that ZnO exhibits

10

excellent performance in IPSCs, You et al. introduced all-inorganic IPSCs without organic

11

charge transport materials by utilizing NiO as HTL and ZnO nanoparticles as the top ETL.

12

The device with an initial PCE of 16.1%, decreased by less than 15% in efficiency after 60

13

days of storage in ambient air with room light soaking, while the reference device without

14

ZnO dropped to zero efficiency after 5 days.[263] Likewise, ETLs based on In-doped ZnO

15

nanoparticles were also fabricated, resulting in IPSCs with an excellent stability and improved

16

PCE of 16.2%.[264]

17

Inverted PSCs that employ nanocrystalline inorganic semiconductors as ETLs have in the

18

past also shown decent PCE values and good stability, however disadvantages exist. For

19

example, the grain boundaries between nanoparticles will decrease the conductivity and favor

20

the diffusion of ambient oxygen and water into the device. Hence, minimizing the grain

21

boundaries and diffusion channels is a paramount subject for ETLs in IPSCs. As discussed

22

above, amorphous semiconductors have already emerged as promising electron transport 46

1

materials in regular PSCs. The amorphous structure allows the connection of atoms inside

2

amorphous metallic oxides by ionic bonds to form a dense, continuous, and

3

grain-boundary-free random network structure, which allows the complete self-encapsulation

4

of photovoltaic devices.

5

With this consideration, Brinkmann et al. proposed the cooperative effect of dual inorganic

6

functional layers in IPSCs. Al:ZnO nanoparticle films and ALD-processed amorphous SnO2

7

films were separately deposited on the surface of PCBM. As shown in Figure 13a-f, the

8

nanoparticles could collect and transport electrons effectively but the protective capability of

9

nanoparticle-based films is insufficient because the structural defects in Al:ZnO nanoparticle

10

layers gives rise to water permeation. However, in combination with an amorphous SnO2

11

layer not only the external water and oxygen ingress can be stopped but this combination also

12

hinders thermal decomposition of the perovskite by forming a very dense gas permeation

13

barrier. Therefore, in Figure 13g-h, a device stability of longer than 400 h was achieved when

14

the device is aged in ambient air (50% rH) at RT and the lifetime was increased to 1,000 h

15

when the device is aged at 60 oC in nitrogen atmosphere.[265] Similarly, Li et al. prepared

16

amorphous Bi2S3 thin

17 18

Figure 13. (a, b) Schemes of aging for the Al:ZnO and the Al:ZnO/SnOx samples; corresponding SEM of the 10 nm Ag layer in

47

1 2 3 4

case of the Al:ZnO sample (c, d), and for the Al:ZnO/SnOx sample (e); (f) Schemes of aging and decomposition of the perovskite

5

films on perovskites by vacuum evaporation. The amorphous thin film was composed of

6

stacked, one-dimensional nanoribbons of (Bi4S6)n held together by Van der Waals forces. The

7

grain-boundary-free structure reduced the diffusion channel for water and oxygen. In addition,

8

the amorphous film showed reasonably high electron concentration and mobility. The

9

Bi2S3-based IPSC attained a PCE of 13%, and retained about 75% of its initial efficiency after

10

storing in ambient at RT and >50% RH for 30 days compared to the PCBM control which

11

totally degraded after 10 days.[266]

12

4. Assessment of various ETLs

13

4.1 Materials and methods

14

Tables S1-S3 summarize the information of the planar PSCs based on various ETLs including

15

the device structures, precursor for preparing ETLs, thickness of ETLs, and device

16

performances. Various discussion points arise from this listing.

in case of the Al:ZnO and the Al:ZnO/SnOx samples; Characteristics of IPSCs with varied cathode electron-extraction assemblies; Stability upon storage (g) in ambient air (at 23 oC and 50% rH) and (h) at 60 oC in nitrogen atmosphere.[265] Copyright 2017 Nature.

17

1. Regarding the materials, ZnO possesses the advantage of easy crystallization as

18

reflected by the LT sol-gel route and the fact that the co-precipitation method synthesizing

19

ZnO nanoparticles at LT is very mature. ZnO can become a popular electron transport

20

material if the stability issue can be addressed. The combination of the advantageous optical,

21

electrical, and chemical properties of SnO2 fuels its popularity in PSCs. Most PSCs based on

22

SnO2 can achieve high PCEs, exceeding 18%, independent of preparation routes which

23

illustrates the outstanding advantages of SnO2 in highly-efficient planar PSCs. It is

24

noteworthy that currently the highest PCE of flexible and rigid PSCs are 19.1%[267] and 48

1

23.3%[268], respectively, and both of them are constructed by using SnO2 ETLs. Hence,

2

SnO2 with its excellent electron transport properties has emerged as promising alternative to

3

TiO2. The PCEs of PSCs based on the uncommon ETLs including WOx, In2O3, Nb2O5 and

4

Fe2O3 are still inferior to those using ZnO and SnO2. Further optimization approaches should

5

be applied on these materials. Ternary oxides usually exhibit excellent and tunable properties

6

that favor high PCE for PSCs along with outstanding photostability. Finally, metal sulfide and

7

selenide may be suitable ETLs in IPSCs due to their strong interplay with the perovskite layer

8

which can alleviate insufficient electron extraction issue.

9

The elements employed in ETLs for PSCs are summed up in the periodic table of

10

elements in Figure 14. The elements in the grey field are usually p-type semiconductors used

11

as HTLs[269-274], and those in the purple field are used as scaffold or interface modification

12

layers.[275, 276] The elements in yellow fields are suitable for ETLs. The metallic elements

13

employed here are mainly transition metals and a few p-block main group elements. Although

14

these materials possess diverse properties (stability, transmittance, mobility), they show a

15

suitable energy band structure regarding the energetic alignment to perovskites. In most

16

oxides (sulfides), the VB position is mainly determined by the O-2p (S-3p) atomic states. The

17

CB

26 Fe 3d

Orbit 21 Sc 39 Y

22 Ti 3d 40 Zr

La-Lu

72 Hf

23 V

24 Cr

25 Mn

41 Nb 4d 73 Ta 5d

42 Mo 74 W 5d

5 B

6 C

7 N

13 Al

14 Si

15 P

31 Ga

32 Ge

33 As

49 In 5s 81 Tl 6s

50 Sn 5s 82 Pb 6s

51 Sb 5p 83 Bi 6p

Atomic number

27 Co

28 Ni

29 Cu

43 Tc

26 Fe 3d 44 Ru

45 Rh

46 Pd

47 Ag

75 Re

76 Os

77 Ir

78 Pt

79 Au

30 Zn 4s 48 Cd 5s 80 Hg

8 O 2p 16 S 3p 34 Se 52 Te 84 Po

18 19

Figure 14. (a) Partial periodic table of elements with electronic orbitals contributing to the CB of metal oxides. Reference:

20

Tl2O3[277], CdO[278], Ta2O5[279], PbO2[280], Sb2O3[281], Bi2O3[282].

49

1

position on the other hand mainly arises from the outermost d or s orbitals of the metal ions as

2

marked in Figure 14. The electronic orbital energy decreases with increasing atomic number

3

due to the change in electronegativity. As such, either in the main group or subgroup, the

4

energy bands from the same orbit shift to lower positions with increasing atomic number. .In

5

accordance with previous results,[283] we can further summarize as follows: (i) The CB of

6

the sub-oxide is higher than that of high-valent oxide for the same metal; and (ii) both the CB

7

and VB of these sulfides are generally higher than those of the oxides. By combining the

8

above statements, we speculate that Tl2O3, CdO, Ta2O5, PbO2, Sb2O3, and Bi2O3 may possess

9

an appropriate energy level values and are potential ETL materials or interfacial layers.

10

General considerations like these should be helpful for the further material development and

11

extensive optimization, for example by doping.

12

2. In terms of methodology, fabricating PSCs at low temperature is an important and

13

growing trend since it will be ensure compatibility with mass production and large-scale

14

flexible devices. The vacuum approaches include MS, EB, and ALD, which bring about some

15

exciting advantages including large-scale roll-to-roll fabrication on plastic substrates, high

16

degree of purity, absence of batch-to-batch variation, and outstanding stability. However, this

17

is accompanied by relatively large power consumption and the need for sophisticated

18

instruments and high-cost precursors, which increases the manufacturing cost. Therefore,

19

attention has shifted towards solution-processing which is also compatible with some mass

20

production processes such as spray coating, slot-die coating, ink-jet printing, and doctor

21

blading. For example, ZnO-based PSCs have been successfully manufactured with a 3D

22

printer-based slot-die coater by using pre-synthesized ZnO nanoparticles.[284, 285] Except 50

1

for the evaporated metal electrodes, the PSCs were fully printed and achieved a PCE of

2

12.0%. CBD is a widely accepted approach, especially for metal sulfides, but this wet

3

chemical method wastes material due to the large amount of precursor solution involved in

4

preparation. NSC is a widely used strategy to prepare ETLs at RT or LT whereas complex

5

procedures such as long-time solvothermal at 200 oC are required for the synthesis of

6

nanoparticles. Therefore, synthesizing nanoparticles via a simple and LT route is a promising

7

subject for the NSC strategy. Moreover, SSC-processed crystalline ETLs usually possess

8

pinholes and cracks due to the heating process. By contrast, the SSC-processed amorphous

9

ETLs exhibit a uniform morphology along with excellent self-encapsulation function,

10

favoring the stability of IPSCs. However, the amorphous films are limited by the high density

11

of trap states so more optimization strategies for amorphous ETLs should be explored and

12

there is still much room to improve the photovoltaic performance when using amorphous

13

ETL-based PSCs.

14

4.2 Interface reaction and lattice matching

15

Anomalous photocurrent hysteresis as a common phenomenon in PSCs, especially in planar

16

structures, can lead to significant overestimation of the PCE and instability of the PSCs.

17

Hysteresis has been related to a number of different explanations involving the unbalanced

18

charge extraction, ion migration/accumulation, trapping process, interfacial capacitive effect

19

and recombination process. These problems exist independently and affect each other. For

20

instance, Calado demonstrated that hysteresis requires the combination of both ion migration

21

and interface recombination; therefore, passivating contact recombination leads to higher

22

photogenerated charge concentrations which screen the ionic charge and reduce 51

1

hysteresis.[286] Obviously, besides the preconditions, hysteresis is strongly affected by the

2

quality of perovskite, the interface properties and charge transport layers. For the perovskite

3

layers, various strategies including solvent engineering, additive engineering and surface

4

modifications have been implemented to passivate trap states and inhibit ion migration,

5

thereby alleviating hysteresis, which have been discussed in many articles.[287-291] In this

6

paper, we also elaborated on the applications of various ETLs and optimizing strategies on

7

eliminating hysteresis, such as using high-mobility ETLs and dopants. Until now, interface

8

modifications strategies are always discussed but most people only focus on how to modify

9

interface without paying attention to the root cause of interface problems. As such, the

10

hysteresis in the PSCs based on bare inorganic semiconductor ETLs, even on SnO2, is

11

challenging to completely eliminate and still present to some degree in many reports.[159,

12

292, 293] Here, we will provide two reasons to explain the root cause of defective interface.

13

The first reason is that the unwanted interfacial reaction, e.g. at the SnO2/perovskite

14

interface, during the preparation of perovskite layers. In the co-evaporation process

15

(MAI+PbI2), Xu et al and Zhou et al found PbI2 phase inevitably formed at the very initial

16

growth stage due to its high absorption energy on various substrates, including ITO,

17

PEDOT:PSS, Si, glass and ZnO, even under the conditions of a MAI-rich environment. Then

18

the resulting PbI2 could block carrier transport into the electrode and thus deteriorate solar cell

19

performance.[294, 295] By contrast, Olthof et al demonstrated a different interface reaction in

20

this co-evaporation process where volatile compounds are generated firstly from the organic

21

component in the initial deposition phase under the catalytic effect of substrates.[296] Their

22

similarity is that this resulting impurity layer will break the band alignment and also hinder 52

1

charge extraction as well as lead to hysteresis. In addition to the vacuum-processed

2

perovskites, Hu et al further demonstrated the spontaneous formation of a PbI2 interfacial

3

layer between ALD-processed SnOx and the solution-processed MAPbI3 perovskite. Moreover,

4

an interface dipole between SnOx and PbI2 layer was found, which acts as an electron

5

extraction barrier and brings down the device performance.[297]

6

The other reason is the lattice mismatch, which results in uncoordinated electrons, thermal

7

vibrations, and defect formation at the interface, that can act as trap states or recombination

8

centers.[298] A closely-matched lattice at the interface can reduce interface defects, favoring

9

the charge transfer and reducing recombination[299] while a lattice mismatch at the

10

ETL/perovskite interface can lead to interface capacitances along with serious hysteresis[300]

11

and rapid device degradation.[301] Herein, the 2D quasipotential, V/V0, was proposed to

12

estimate the lattice-matching degree and θ was proposed to represent the azimuthal angle of

13

the overlayer with respect to the substrate (calculation method was shown in supporting

14

information). V/V0 varies continuously with θ. The smaller the value of V/V0 (in the region of

15

0~1), the higher the commensurate degree.[302] Accordingly, the possible contacting

16

interfaces of substrate (ETLs) and overlayer (perovskite) lattices were considered. The crystal

17

plane (110) of perovskite (MAPbI3) was selected to simplify the calculations because

18

perovskites usually grows along the preferable orientation of <110>. The crystalline structure

19

and the three corresponding high-diffraction planes of each ETL are selected according to the

20

XRD results in published papers. The lattice parameters and the calculated results are

21

summarized in Table S4. The order of the three crystal planes corresponds to the intensities of

22

the diffraction peaks. 53

1

Table S4 shows that the lowest V/V0 value of 0.5131 was obtained in the ZnO (100)

2

plane, which indicates its partial matching towards perovskite. Conversely, the highest value

3

of 0.9655 was obtained in the case of the In2O3 (222) plane, which may account for the

4

serious hysteresis in In2O3-based PSCs. For a better overview, the results corresponding to the

5

main crystal planes of ETLs are illustrated in Figure 15. CdS, SrTiO3, SnO2, anatase TiO2,

6

ZnO, CdSe, BaSnO3, and hexagonal-phase WO3 show partial matching with the perovskite.

7

Among them, SnO2, ZnO, and hexagonal-phase WO3 exhibit lower θ, which indicates that the

8

perovskite can grow almost parallel to the crystal lattice of the substrate. These results are

9

potentially very important because they can provide directions for the regulation and

10

manipulation of crystal planes and thereby another handle on interface modifications. We

11

stress that, while important, lattice-matching is not the only parameter behind the

12

crystallization and electron kinetics of perovskite on ETLs. Other parameters such as

13

interfacial adsorption energy, surface wettability and surface passivation are also important

14

factors which are certainly beyond the scope to discuss here. 110

001 1

101

110

110

311

101

101

111

110

111

200

1 001

104

110

311

002 2

222

70

1.0

60 0.8

V/V0

40 30

0.4

20

θ / degree

50 0.6

0.2 10 0

0.0 MAPbI3 TiS tis2 MAPbI 3 2

15 16 17

CdS SrTiO SrTiO3 SnO SnO2 In2s3 ATiO2 ZnO CdSe BaSnO3 znse WO3 sns2 Fe2O3 R-TiO2 Zn2SnO4 WO3 In2O3 CdS 3 2 In2S3 A-TiO2 ZnO CdSe BaSnO3 ZnSe H-WO3 SnS2 Fe2O3 R-TiO2 ZSO M-WO3 In2O3

Electron transport materials

Figure 15. V/V0 and θ between the interfaces of MAPbI3 and ETLs (A-TiO2, R-TiO2, H-WO3 and M-WO3 represent anatase TiO2, rutile TiO2, hexagonal phase WO3, and monoclinic phase WO3, respectively)

18 19

5 Conclusions and outlook

20

ETLs are essential components in PSCs that facilitate selective charge extraction and have a 54

1

significant influence on charge recombination. In this review, we have reported the important

2

steps undertaken in literature to develop alternative ETLs for PSCs. Compared to the early

3

days, TiO2 is no longer the most important material and certainly not the best candidate ETL;

4

SnO2, ZnO, and some more unconventional oxides such as Zn2SnO4 have been developed and

5

exhibit excellent performance in PSCs. These novel materials possess certain advantageous

6

properties, but also all have their own drawbacks. For example, the highly conductive ZnO

7

suffers from instability, the highly transmissive Zn2SnO4 has a rather complex synthesis, and

8

Fe2O3 (which shows exceptionally low photocatalytic activity) is adversely affected from low

9

transmittance. To alleviate some of these shortcomings, elemental doping, functional material

10

design, and interface modifications have been developed as optimization strategies. Thereby,

11

a precise control over the photoelectric properties of ETLs can be achieved and the charge

12

dynamics at the interface can be enhanced. By combining this with optimized fabrication

13

methods, PSCs based on novel materials can achieve high PCE exceeding 20%. SnO2 is still

14

the most prospective candidate for ETLs due to its high conductivity and wide band gap.

15

Despite the encouraging performances, some issues should be addressed that could

16

accelerate the progress of PSCs in future studies. Although regular PSCs have high

17

efficiencies, they suffer from anomalous hysteresis and photocatalysis at the ETL interface.

18

Conversely, IPSCs that use inorganic ETLs suffer from a lower efficiency despite their

19

superior stability. Some potential and excellent electron transport materials still exist,

20

especially multicomponent oxides, which so far have not been implemented in PSCs.

21

Meanwhile, most optimization strategies regarding ETLs focus on introducing additional

22

components, such as doping and interface modification. A few reports concentrate on 55

1

micro-level adjustments including crystal planes and oxygen vacancies. For crystalline ETLs,

2

optimized fabrication strategies have been rather well established. However, the properties of

3

amorphous ETLs should be better studied in future, focusing on the microstructure, band

4

structure, trap states and the stability. In addition, based on the development and optimization

5

of novel materials, multilayer ETLs should be rationally designed to enhance the built-in

6

electric field and reduce unwanted barriers at different interfaces, such as the cascade ETLs

7

with different CBs or carrier concentrations.[159, 303, 304]

8

In summary, the booming investigations on novel ETLs for PSCs, especially flexible PSCs

9

and IPSCs, have led to a wide range of new developments accompanied by significant device

10

improvements. Future studies on ETLs, including the development and optimization of novel

11

materials, remain highly important in order to continue to reduce the energy loss from ETLs

12

in general and for the further advancement of large-scale and flexible devices.

13

Acknowledgements

14

Thanks to Prof. Gary Hodes for his modification. This work was supported by Strategic

15

Priority Research Program of Chinese Academy of Sciences (Grant No. XDA17040506),

16

National Nature Science Foundation of China (21805274, 61674098), the Doctor Startup

17

Foundation of Liaoning Province (20180540099), National Key Research Program of China

18

(2016YFA0202403), the 111 Project (B1404), and Chinese National 1000-Talent-Plan

19

program (Grant No. 111001034).

20 21 22 23 24 25

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30 31

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Highlight 1. The essential functions of ETLs in planar PSCs were discussed in terms of ETL-free PSCs. 2. We give an extensive description of novel ETL materials, looking at both crystalline and amorphous systems. 3. We provide a brief discussion about the correlation between materials, fabrication methods, and interface related issues.

Kai Wang received his bachelor degree in 2012 at Dalian University of Technology, China. After five years, he received his Ph.D. degree under supervisions of Professor Tingli Ma and Yantao Shi at the same university. Now he is a research assistant in Professor Shengzhong Liu’s group at Dalian Institute of Chemical Physics, China. His research interests focus on functional photo-anode and inorganic perovskite for dye-sensitized solar cell and perovskite solar cells.

Waqas Siddique Subhani received his MSc and MPhil Degree in Applied Physics from University of engineering and Technology Lahore, Pakistan in 2012 and 2015. He is currently pursuing his Ph.D. under the supervision of Prof. Shengzhong Liu at Dalian Institute of chemical physics, China. His research interest is mainly focused on hybrid and all-inorganic perovskite solar cells.

Xiao Jiang was born in Gansu Province and got his Bachelor’s degree at Dalian Polytechnic University (DLPU) in 2015. Afterwards, he obtained his Master’s degree at Dalian Polytechnic University in 2019. In his Master period, he aimed at fabrication and modification of biomass materials. After graduated from DLPU, he worked in Professor Shengzhong Liu’s group at Dalian Institute of Chemical Physics and carried out researches on perovskite solar cells.

Lianjie Duan was born in Liaoning Province and got her Bachelor’s degree at Shenyang Normal University in 2015, and then she studied at Liaoning Normal University for her master’s degree. She majored in nanomaterials during the study for a master degree. After graduation, she worked in Professor Shengzhong Liu’s group at Dalian Institute of Chemical Physics and started the research of thin film solar cell, including perovskite solar cells and amorphous silicon solar cells.

Minyong Du was born in Shandong Province and got his Bachelor’s degree at Ludong University in 2013, and then he studied at Beijing University of Technology for his master’s degree. He majored in amorphous silicon solar cell during the study for a master degree. After graduation, he worked in Professor Shengzhong Liu’s group at Dalian Institute of Chemical Physics and continued the research of thin film solar cell, including perovskite solar cells and amorphous silicon solar cells.

Shengzhong (Frank) Liu received his Ph.D. degree from Northwestern University (Evanston, Illinois, USA) in 1992. Upon his postdoctoral research at Argonne National Laboratory (Argonne, Illinois, USA), he joined high-tech companies in US for research including nanoscale materials, thin film solar cells, laser processing, diamond thin films, etc. His invention at BP Solar in semi-transparent photovoltaic module won R & D 100 award in 2002. In 2011, he was selected into China's top talent recruitment program and now he is a professor at Shaanxi Normal University and Dalian Institute of Chemical Physics, Chinese Academy of Sciences.

Supporting information Novel Inorganic Electron Transport Layers for Planar Perovskite Solar Cells: Progress and Prospective Kai Wang a, Selina Olthof c, Waqas Siddique Subhani a, d, Xiao Jiang a, Yuexian Cao a, Lianjie Duan a, Hui Wang a, Minyong Du a, Shengzhong (Frank) Liu a, b * a

Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China.

b

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for

Advanced Energy Technology; Institute for Advanced Energy Materials; School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China. c

Department of Chemistry, University of Cologne, Luxemburger Straße 116, 50939 Cologne, Germany.

d

University of the Chinese Academy of Sciences, Beijing 100039, China.

E-mail: [email protected]

Table S1. A summary of the published performances of planar PSCs with different ETLs T Method

PCE Material

Structure

o

( C)

Thickness Ref.

Precursor

(%)

Note (nm)

ALD

70

ZnO

FTO/ZnO/MAPbI3/HTL/Au

13.1

2014[1]

Diethylzinc, H2O

30

-

MS

100

ZnO

ITO/ZnO/MAPbI3/HTL/Au

15.9

2015[2]

ZnO target, pure Ar

40

-

MS

RT

ZnO

PDMS/ITO/ZnO/MAPbI3/HTL/Au

13.1

2015[3]

-

50

-

MS

RT

ZnO

ITO/Al:ZnO/MAPbI3/HTL/Ag

17.6

2016[4]

2 wt% Al2O3 in ZnO target, pure Ar

20

-

MS

RT

ZnO

FTO/ZnO/MAPbI3/HTL/Au

16.6

2018[5]

ZnO target, Ar/O2=1/4

80

-

NSC

RT

ZnO

ITO/ZnO/MAPbI3/HTL/Ag

15.7

2014[6]

25

NSC

120

ZnO

FTO/ZnO/MAPbI3/C

8.1

2015[7]

NSC

200

ZnO

ITO/ZnO/PEI/MAPbI3/HTL/Au

10.2

2015[8]

NSC

150

ZnO

ITO/ZnO-SnO2/MAPbI3/HTL/Ag

15.2

2016[9]

Sumitomo Osaka Cement Co.

-

-

NSC

150

Al:ZnO

ITO/Al:ZnO/PCBM/MAPbI3/HTL/MoO3/Al

16.9

2019[10]

Sigma-Aldrich

40

-

Sol-gel

150

ZnO

ITO/ZnO/MAPbI3/PTAA/Au

17.7

2016[11]

50

Stirring at 60 oC

Sol-gel

290

ZnO

ITO/ZnO/PCBM/MAPBI3/PTB7/Th-MoOx/Ag

12.2

2014[12]

30

-

Sol-gel

160

ZnO

ITO/ZnO/SAM/MAPbI3/HTL/MoO3/Ag

15.7

2015[13]

40

-

Zn(AC)2, KOH, methanol

55

Precipitation at 65 oC

30

Zn(AC)2, HO(CH2)2NH2, CH3O(CH2)2OH Sol-gel

150

Mg:ZnO

FTO/Mg:ZnO/PCBM/MAPbI3/HTL/Au

17.8

2018[14]

-

-

Sol-gel

200

Li:ZnO

ITO/Li(Cs):ZnO/PCBA/MAPbI3/HTL/Au

18.0

2017[15]

-

Stirring at 70 oC

Sol-gel

130

K:ZnO

ITO/K(Na, Li):ZnO/MAPbI3/HTL/Ag

19.9

2018[16]

40

-

Electro-deposition

150

ZnO

ITO-PEN/ZnO/MAPbI3/HTL/Ag

10.9

2015[17]

Zn(NO3)2, H2O

780

-

120

ZnO

ITO/ZnO/MAPbI3/P3HT/Ag

12.0

2015[18]

Zn(AC)2, NaOH, n-heptane, ethanol

25

Slot-die

Using nanoparticles

Coating

Coprecipitation

SSC

180

SnO2

FTO/SnO2/MAPbI3/HTL/Au

17.2

2015[19]

SSC

185

SnO2

FTO/SnO2/MAPbI3/HTL/Au

15.1

2015[20]

60

-

40

-

SnCl2, ethanol SSC

70

SnO2

FTO/SnO2/MAPbI3/HTL/Ag

16.2

2017[21]

40

-

SSC

RT

SnO2

FTO/SnO2/Cs0.056FA0.76MA0.15PbI2.42Br0.48/HTL/Au

19.6

2018[22]

15

-

SSC

RT

SnO2

FTO/SnO2/Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3/HTL/Au

20.3

2018[23]

SnCl4, isopropanol

-

-

NSC

200

SnO2

FTO/SnO2/MAPbI3/HTL/Ag

13.0

2015[24]

Wako Chemicals

-

-

NSC

200

SnO2

20.8

2018[25]

SnCl2, H2O, thiourea

30

FTO/SnO2/Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 /HTL/Au

hydrolysis precipitation Quantum dot

FTO/SnO2/K0.03Cs0.05(FA0.85MA0.15)0.92Pb(I0.85Br0.15)3 NSC

180

SnO2

hydrolysis precipitation, 19.2

2018[26]

SnCl2, H2O

90 oC

/HTL/Au NSC

150

SnO2

ITO/SnO2/(FAPbI3)0.97(MAPbBr3)0.03/HTL/Au

20.5

2016[27]

Alfa Aesar

25

-

NSC

130

SnO2

FTO/SnO2/MAPbI3/HTL/Ag

20.5

2019[28]

SnCl2, H2O, butanol

40

refluxed at 110 oC

30

CBD at 70 oC

-

refluxed at 60 °C

-

SSC: SnCl4, isopropanol SSC+CBD

180

SnO2

FTO/SnO2/CsxFAyMA1-x-yPbIzBr3-z/HTL/Au

20.7

2016[29]

CBD: urea, water, mercaptoacetic acid, HCl, SnCl2 SnCl4, acetylacetone, n-butanol,

Sol-gel

500

SnO2

FTO/SnO2/TiO2/MAPbI3/HTL/Au

14.7

2015[30] para-toluene-sulfonic acid, water

SSC

150

SnO2

CBD

80

SnO2

FTO/SnO2/MgO/MAPbI3/HTL/Au

19.0

2018[31]

SnCl2, ethanol

24

17.1

2016[32]

SnCl4, water

-

18.8

2017[33]

FTO/SnO2/PCBM/FA0.83Cs0.17Pb(I0.6Br0.4)3 /HTL/Ag FTO/SnO2/FA0.83MA0.17Pb(I0.87Br0.17)3 CBD

180

SnO2

SnCl2, water, urea, HCl,

/HTL(SWCNT)/Ag

-

CBD at 70 °C

3-mercaptopropionic acid

MS

RT

SnO2

FTO/SnO2/MAPbI3/HTL/Au

13.7

2018[34]

SnO2 target, Ar/O2= 4/6

19

-

MS

RT

SnO2

FTO/SnO2/Cs0.06MA0.27FA0.67PbI2.7Br0.3/HTL/Au

20.2

2018[35]

SnO2 target, Ar/O2= 1/9

17

-

MS

RT

SnO2

FTO/SnO2/FA0.85MA0.15Pb(I0.85Br0.15)3/HTL/Au

18.2

2019[36]

SnO2 target, Ar/O2= 4/6

20

-

EB

180

SnO2

18.2

2017[37]

SnO2 powders

40

0.2 Pa of oxygen pressure

18.9

2016[38]

-

-

-

-

FTO/SnO2/Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 /HTL/Au FTO/SnO2/C60/Cs0.05(FA0.83MA0.17)0.95Pb(l0.9Br0.1)3 SSC

185

SnO2 /HTL/MoO3/ITO

SSC

180

SnO2

FTO/SnO2/APTES/MAPbI3/HTL/Au

SnCl2, ethanol. 18.3

2017[39]

SSC

185

SnO2

FTO/SnO2/PCBM/MAPbI3/HTL/Au

19.1

2016[40]

SSC

60

SnO2

FTO/SnO2:EDTA/FA0.95Cs0.05PbI3/HTL/Au

21.6

2018[41]

40

-

-

-

-

-

-

refluxed at 170 oC

Alfa Aesar NSC

180

SnO2

FTO/SnO2/TPPO/(CsFAMA)Pb(IBr)3/HTL/Au

20.7

2019[42]

NSC

100

Sb:SnO2

ITO/Sb:SnO2/MAPbI3/HTL/Au

17.9

2016[43]

SnCl4, SbCl3,·ethylene glycol, HAC, (CH3)4NOH NSC

200

Al:SnO2

ITO/Al:SnO2/MAPbI3/HTL/Ag

18.2

2019[44]

SnCl2, CH4N2S, water

Sol-gel

550

Mg:SnO2

FTO/Mg:SnO2/MAPbI3/HTL/Au

15.2

2016[45]

SnCl2, Mg(AC)2, ethanol

50

-

SSC

185

Li:SnO2

FTO/Li:SnO2/MAPbI3/HTL/Au

18.2

2016[46]

SnCl2, LiTFSI, ethanol

40

-

SSC

190

Nb:SnO2

FTO/Nb:SnO2/MAPbI3/HTL/Au

17.6

2017[47]

SnCl2, ethanol, Nb(C2H5O)5

-

-

Sol-gel

380

F:SnO2

FTO/F:SnO2/(FA0.85MA0.15)Pb(I0.85Br0.15)3/HTL/Au

20.2

2018[48]

SnCl2, HAC, (CH3)4NOH, NH4F, ethanol

50

-

CBD

180

Nb:SnO2

20.5

2018[49]

30

CBD at 70 °C

40

-

-

-

FTO/Nb:SnO2/Cs0.05(FA0.83MA0.17)0.95Pb(l0.9Br0.1)3

Urea, water, mercaptoacetic acid, HCl,

/HTL/Au SSC

180

SnO2

FTO/SnO2-GQDs/MAPbI3/HTL/Au

Quantum dot

SnCl2 20.2

2017[50] SnCl2, ethanol

SSC

185

SnO2

FTO/SnO2/PCBM/MAPbI3/HTL/Au

19.5

2016[51]

NSC

140

WOx

ITO/WOx/SAM/MAPbI3/HTL/Ag

14.9

2015[52]

Nano grade,

-

-

SSC

500

WO3

FTO/WO3/MAPbI3/HTL/Au

10.1

2016[53]

Tungsten isopropoxid, isopropanol

15

-

NSC

100

WO3

ITO/WO3/MAPbI3/HTL/gold

9.5

2017[54]

Nano grade

50

-

Sol-gel

450

WO3

FTO/WO3/Cs2CO3/PCBM/MAPbI3/P3HT/Au

10.5

2016[55]

H2WO4, H2O2, water

50

-

Sol-gel

200

In2O3

FTO/In2O3/MAPbI3/HTL/Au

13.0

2016[56]

In(NO3)3, ethanol

120

-

Sol-gel

200

In2O3

FTO/In2O3/MAPbI3/HTL/Ag

15.3

2017[57]

In(NO3)3, ethanol, acetylacetone

40

SSC

300

In2O3

ITO/In2O3/MAPbI3/PTAA/Au

14.6

2017[58]

In(NO3)3, ethanol,

50

Better than In(AcAc)3 or InCl3

Sol-gel

200

In2O3

ITO/TiOx/In2O3/MAPbI3/ HTL /Ag

16.4

2018[59]

In(NO3)3, ethanol, acetylacetone

40

MS

500

Nb2O5

FTO/Nb2O5/MAPbI3/HTL/Au

17.2

2017[60]

Nb2O5 target, Ar gas

100

SSC

500

Nb2O5

FTO/Nb2O5/MAPbI3/HTL/Ag

14.8

2018[61]

NbCl5, ethanol

-

NSC

150

Nb2O5

FTO/Nb2O5/MAPbI3/HTL/Au

8.9

2018[62]

-

-

NSC

RT

Nb2O5

FTO/Nb2O5/(CsFAMA)Pb(IBr)3/HTL/Au

20.2

2019[63]

NbCl5, ethanol, C6H5CH2OH

30

SSC

500

Fe2O3

FTO/Fe2O3/MAPbI3/HTL/Au

14.2

2017[64]

Fe(NO3)3, ethanol

30

SSC

500

Ni: Fe2O3

FTO/Ni:α-Fe2O3/MAPbI3/HTL/Au

14.2

2017[65]

Ni(COOH)2, Fe(NO3)3, ethanol

14

SSC

500

Fe2O3

FTO/Fe2O3/MAPbI3/HTL/Au

18.2

2017[66]

Fe(NO3)3, ethanol, polysorbate-80

100

nanoisland

CBD

100

CdS

FTO/CdS/MAPbI3/HTL/Au

11.2

2015[67]

NH4Cl, CdCl2, thiourea, NH4OH, H2O

30

CBD at 60 oC

CBD

65

CdS

FTO/CdS/MAPbI3/HTL/Ag

16.1

2016[68]

-

60 H2O, NH4OH, CdSO4, thiourea

CBD

72

CdS

FTO/CdS/MAPbI3/HTL/Au

15.1

2016[69]

37

CBD

85

CdS

FTO/CdS/MAPbI3/HTL/Ag

14.7

2017[70]

CdCl2, thiourea, NH4OH, NH4Cl, H2O

50

CBD

60

CdS

ITO/CdS/MAPbI3/HTL/Au

10.7

2017[71]

H2O, CdSO4, NH4OH, thiourea

60

CBD

90

CdS

FTO/CdS/MAPbI3/HTL/Ag

2.3

2017[72]

CdCl2, thiourea

200

MS

150

CdS

ITO/CdS/MAPbI3/HTL/Au

13.2

2018[73]

CdS target, pure Ar

20

NSC

RT

CdS

16.5

2017[74]

NaOH, oleic acid, CdCl2, Na2S

116

17.8

2018[75]

FTO/CdS/Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 Hydrothermal at 180 oC

/HTL/Au ZnSO4, N2H4·H2O, NH4OH, H2O, CBD

80

ZnSe

FTO/ZnSe/MAPbI3/HTL/Au

selenourea

CBD

80

In2S3

FTO/In2S3/MAPbI3/HTL/Ag

18.2

2017[76]

CH3CSNH2, citric acid, InCl3, H2O

CBD

70

In2S3

FTO/In2S3/CsPbIBr2/HTL/Ag

5.6

2019[77]

NSC

80

In2S3

FTO/In2S3/MAPbI3/HTL/Au

18.8

2018[78]

129 InCl3, oleylamine, cyclohexane, S,

40

Hydrothermal at 180 oC

N-dodecyl mercaptan SSC

300

In2S3

FTO/In2S3/MAPbI3/HTL/Au

15.5

2019[79]

In(OH)3, CS2, n-butylamine

-

-

Hydrothermal

190

SnS2

FTO/SnS2/MAPbI3/HTL/Au

13.6

2018[80]

SnCl4, CH3CSNH2, H2O

80

Hydrothermal at 190 oC

NSC

180

SnS2

FTO/SnS2/FA0.75MA0.15Cs0.1PbI2.65Br0.35/HTL/Au

20.1

2019[81]

Commercial

-

-

NSC

RT

TiS2

FTO/TiS2/MAPbI3/HTL/Au

17.4

2018[82]

TiS2 powder, isopropanol

-

NSC

RT

TiS2

FTO/TiS2/FAxMA1-xPbIyBr3-y/HTL/Ag

18.8

2018[83]

exfoliate by sonication

TiS2 powder, Butyllithium in hexane, ethanol NSC

100

Zn2SnO4

ITO/Zn2SnO4/MAPbI3/PTAA/Au

15.7

2015[84]

100

heated at 90 oC

110

Hydrothermal

ZnCl2, SnCl4, water, N2H4 NSC

100

Zn2SnO4

ITO/Zn2SnO4/MAPb(I0.9Br0.1)3/PTAA/Au

17.6

2016[85]

SSC

500

Zn2SnO4

FTO/Zn2SnO4/MAPbI3/HTL/Au

16.4

2018[86]

ZnCl2, SnCl4, ethanol

50

-

NSC

500

La:BaSnO3

FTO/La:BaSnO3/MAPbI3/PTAA/Au

21

2017[87]

BaCl2, SnCl4, citric acid, H2O2, ammonia,

120

-

NSC

500

Sr:BaSnO3

FTO/Ba0.8Sr0.2SnO3/MAPbI3/PTAA/Au

21

2019[88]

La(NO3)3 or Sr(NO3)2

-

-

NSC

100

Y:SrSnO3

FTO/Y:SrSnO3/FAxMA1-xPbIyBr3-y/HTL/Au

19.0

2019[89]

30

-

SrCl2, SnCl4, citric acid, H2O2, Y(NO3)3, NH4OH Spray pyrolysis

350

TiZn2O4

FTO/TiZn2O4/MAPbI3/HTL/Au

15.1

2016[90]

Zn(OAc)2, Ti(i-OPr)2(acac)2

60

-

SSC

500

Ti0.5Fe0.5Ox

FTO/Ti0.5Fe0.5Ox/MAPbI3/HTL/Au

14.7

2017[91]

Ethanol, Ti(OCH(CH3)2)4, Fe(NO3)3

40

-

SSC

150

SiW12O404-

FTO/SiW12O404-/MAPbI3/HTL/Au

14.3

2017[92]

H4SiW12O40, water, Li2CO3

15

-

Note: HTL refers to Spiro-OMeTAD, EA represents ethanolamine.

Table S2. A summary of the published performances of planar PSCs with amorphous oxides as ETLs Method

Temperature (oC)

ETLs

Structure

PCE (%)

Ref.

Precursor

Thickness (nm)

ALD

118

SnO2

FTO/SnO2/(FAPbI3)0.85(MAPbBr3)0.15/HTL/Au

18.4

2015[93]

-

ALD

120

SnO2

FTO/SnO2/FAPbI3/HTL/Au

19.5

2016[94]

ALD

180

SnO2

FTO/SnO2/MAPbI3/HTL/Ag

18.3

2019[95]

PEALD

100

SnO2

FTO/SnO2/MAPbI3/HTL/Au

19.0

2016[96]

TDMASn, O2

15

PEALD

100

SnO2

FTO/SnO2/MAFAPbI3/HTL/Au

20.4

2017[97]

-

40

PEALD

100

SnO2

ITO/SnO2/MA0.7FA0.3PbI3/HTL/Au

18.4

2017[98]

TDMASn, O3

12

TDMASn, O2

PEALD

100

SnO2

FTO/C60/SnO2/MAFAPbI3/HTL/Au

20.1

2017[99]

-

CBD

55

SnO2

ITO/SnO2/MAPbI3/HTL/Au

14.8

2017[100]

SnCl4 in water

20

EB

RT

Nb2O5

FTO/Nb2O5/(FAPbI3)0.85(MAPbBr3)0.15/HTL/Au

18.6

2017[101]

Nb2O5 powder

60

MS

RT

Nb2O5

FTO/Nb2O5/MAPbI3/HTL/Au

17.1

2016[60]

Nb2O5 target, Ar gas

110

MS

RT

Nb2O5

FTO/Nb2O5/PCBM/IL/MAPbI3/HTL/Au

18.8

2018[102]

Nb2O5 target, Ar/O2 = 20/1

100

SSC

RT

NbOx

FTO/NbOx/Cs0.05[(FAPbI3)0.85(MAPbBr3)0.15]0.95/HTL/Ag

19.1

2018[103]

Nb(C2H5O)5, ethanol

25

TE

RT

CdS

FTO/CdS/MAPbI3/HTL/Ag

12.2

2016[104]

CdS powder

20

SSC

150

WOx

FTO/WOx/MAPbI3/HTL/Ag

9.0

2015[105]

WCl6, n-propanol

200

SSC

150

WOx-TiOx

FTO/WOx/MAPbI3/HTL/Ag

14.5

2016[106]

WCl6, TiAcAc, n-propanol

200

SSC

150

WOx

ITO/WOx/C60/MAPbI3/HTL/Au

16.1

2017[107]

WCl6, ethanol

50

SSC

120

WOx-NbOx

ITO/WOx/MAPbI3/HTL/Ag

15.6

2017[108]

W(C2H5O)5, Nb(C2H5O)5, ethanol

50

SSC

150

CeOx

FTO/CeOx/PCBM/MAPbI3/HTL/Ag

17.0

2017[109]

Ce(acac)3, ethanol

60

Note: HTL is spiro-OMeTAD, EE is E-beam evaporation, TE is Thermal evaporation, TDMASn is tetrakis(dimethylamino)-tin(IV)

Table S3. A summary of photovoltaic performance of IPSCs involing inorganic ETLs PCE

Temperature Method

o

ETLs

Structure

( C) NSC

RT

Thickness Ref.

Precursor

(%) ZnO

ITO/PEDOT:PSS/MAPbI3/PCBM/ZnO/Al

15.9

Note (nm)

2014[110]

Zn(AC)2, DMSO, (CH3)4NOH, ethanol

-

-

NSC

100

ZnO

Glass/H:In2O3/PTAA/MAPbI3/PCBM/ZnO/Al:ZnO/Al

16.1

2016[111]

NSC

RT

ZnO

ITO/PEDOT:PSS/C3-SAM/MAPbI3/PC61BM/ZnO/Ag.

11.6

2015[112]

SSC

RT

ZnO

ITO/PEDOT:PSS/MAPbI3/PCBM/ZnO/Al

16.8

2015[113]

NSC

RT

ZnO

ITO/NiOx/MAPbI3/ZnO/Al

16.1

2016[114]

ALD

100

ZnO

ITO/PEDOT:PSS/MAPbI3/ZnO/AgNWs/Al2O3/PET

10.8

2015[115]

ALD

100

Al:ZnO

ITO/NiOx/Cs0.05MA0.95PbI3/PCBM/Al:ZnO/Al

18.5

2018[116]

NSC

RT

ZnO

ITO/PEDOT:PSS/PSS-Na/MAPbI3/PCBM/ZnO:PEI/Al

11.7

2016[117]

NSC

RT

ZnO

ITO/PEDOT:PSS/PSS-Na/PVSK/PC61BM/ZnO:PFN/Ag

12.8

2016[118]

MS

RT

ZnO

ITO/PEDOT:PSS/MAPbI3/C60/ZnO/Al

10.9

2015[119]

NSC

RT

ZnO

ITO/GO/MAPbI3/PCBM/ZnO/Al

12.4

2014[120]

Sigma-Aldrich

-

-

-

-

110

-

70

-

ZnEt2 and H2O

70

-

tri(methyl)aluminum, ZnEt2 and H2O

40

KOH, Zn(OAc)2, methanol

150

Zn(OAc)2, methanol, KOH

-

10

-

20

-

25

-

Zn(OAc)2, methanol, KOH NSC

RT

ZnO

ITO/NiO/Cs0.05(FA0.83MA0.17Pb(I0.83Br0.17))0.95/PCBM/ZnO/Al

18.6

2018[121]

TE

RT

Bi2S3

ITO/NiO/MAPbI3/Bi2S3/Au

13.1

2016[122]

Bi2S3 powder

50

NSC

RT

In:ZnO

FTO/NiOx/Cs0.05(FA0.83MA0.17Pb(I0.83Br0.17))0.95/IZO/Al

16.2

2017[123]

Zn(AC)3, InCl3, benzyl alcohol

150

Microwave heating at 160 ALD

RT

SnO2

ITO/PEDOT/MAPbI3/PCBM/AZO/SnOx/Ag

18.1

AZO: Nanograde

AZO: 100

SnOx: TDMASn, water

SnOx: 20

SnCl2, benzyl alcohol, ethanol

-

-

25

Reacting at 260 oC

2017[124]

SSC

300

SnO2

FTO/NiO/MAPbI3/SnO2/CSCNT

14.3

2017[125]

NSC

RT

CdSe

ITO/PEDOT:PSS/MAPBI3/CdSe/LiF/Ag

15.1

2017[126]

-

CdO, oleic acid, trioctylphosphine oxide, Se, tri-n-octylphosphine, acetone (NH4)2CO3, water, ethanol, Zn(NO3)2, SnCl4, NSC

RT

Zn2SnO4

ITO/NiOx/MAPbI3/PCBM/Zn2SnO4/Ag

17.1

2016[127]

Hydrothermal at 180 100

(CH3)4NOH NSC

RT

SnO2

FTO/NiO/MAPbI3/C60/SnO2/Ag

18.8

C

120

2016[128] SnCl4, (CH3)4NOH, ethanol

NSC

RT

SnO2

FTO/P3CT-K/MAPbI3/PCBM/SnO2/Al

19.7

2018[129]

NSC

RT

CeOx

FTO/NiMgLiO/MAPbI3/PCBM/CeOx/Ag

18.7

2018[130]

15 Ce(NO3)3, tert-butylamine, oleylamine, H2O,

Hydrothermal at 200 o

C

solvothermal at 180 40

methylbenzene

o

o

C

NSC

RT

CdSe

ITO/PEDOT:PSS/MAPbI3/CdSe:PCBM/LiF/Ag

13.7

2017[131]

Mesolight Inc

-

NSC

RT

CdS

ITO/Cu:NiO/MAPBI3/CdS/Au

13.4

2018[132]

CdCl2, Na2S,·water

54

Hydrothermal at 180 °C

Note: TE is Thermal evaporation.

Calculation method In the field of crystallization, nucleation and growth of crystals on substrates are defined in terms of the degree of epitaxy by comparing the 2D lattice parameters of the substrate and overlayer crystals.[133] The 2D quasipotential, V/V0, was proposed and calculated using the EpiCalc 5.0 software to estimate the lattice-matching degree. This parameter represents the fit between the substrate and overlayer lattices as a dimensionless potential energy parameter. In this calculation, the 2D lattice parameters are described by two vectors. Figure S1 shows the lattice parameters of the substrate described as a1 and a2. Similarly, b1 and b2 were used for the overlayer. θ was proposed to represent the angle between vectors a1 and b1, which defines the azimuthal angle of the overlayer with respect to the substrate. V/V0 varies regularly with θ. The smaller the value of V/V0 (in the region of 0~1), the higher the commensurate degree.[134] The crystal structure was obtained from the Inorganic Crystal Structure Database, and the lattice parameters of each crystal plane were obtained using the material studio software. In each calculation, the value used for the overlayer size was 25b1×25b2. The minimum V/V0 values of perovskite on ETLs were calculated using the parameters of the corresponding unit cell and the supercell.

β b2 b1 a2

θ α

a1

Figure S1. The scheme of lattice matching between substrate (black) and overlayer (red), lattice parameters of a1, a2 and α belong to substrate; b1, b2 and β belong to overlayer.

Table S4. Lattice parameters and calculation results for potential lattice matching between the contacting surface of perovskite and ETLs

Material

crystal face

MAPbI3

Parameter

Orientation angle

V/V0

b1(Å)

b2(Å)

Angle(degree)

110

6.37409

8.850264

89.10535

Anatas-TiO2

101

5.491746

3.7971

110.22897

56.75

0.5725

21-1272*

103

7.680119

3.7971

104.31207

35.00

0.6931

9854**

004

3.7971

3.7971

90.000

7.00

0.6370

R-TiO2[135]

110

2.9588

6.49619

90.000

43.00

0.7803

21-1276*

211

7.13826

5.463949

108.45331

55.00

0.6113

88624**

101

5.463949

4.5935

90.000

47.00

0.8877

SnO2[20, 27]

110

3.1865

6.700544

90.000

1.00

0.5531

41-1445*

101

5.709853

4.738

90.000

26.25

0.6483

0.00

Data

Average

0.0000

0.0000 0.6342

0.7598

0.6140

9163**

211

7.419641

5.709853

106.87067

50.25

0.6406

ZnO[7, 17]

101

6.137495

3.249

105.34821

14.25

0.5943

36-1451*

100

3.249

5.207

90.000

54.75

0.5131

41488**

002

3.249

3.249

60.000

18.75

0.7928

M-WO3[136]

002

7.306

7.540

90.000

55.50

0.8846

43-1035*

020

7.692

7.306

90.881

35.00

0.5226

16080**

200

7.540

7.692

90.000

55.50

0.9357

H-WO3[55]

200

7.298

3.899

90.000

8.75

0.6136

33-1387*

001

7.298

7.298

60.000

19.25

0.7896

32001**

201

10.680337

7.298

70.02221

15.50

0.6819

CdS[137]

101

7.88485

4.136

105.2051

38.75

0.5212

41-1049*

100

4.136

6.713

90.000

14.25

0.8815

60629**

103

12.837829

4.316

80.73004

44.50

0.9554

CdSe[138]

111

4.297088

4.297088

120.000

43.00

0.6025

19-0191*

220

6.077

4.297088

90.000

44.25

0.5563

41528**

311

7.442775

4.297088

106.77865

40.00

0.6467

In2O3[56, 57]

222

14.307599

14.307599

120.000

51.50

0.9655

71-2194*

400

10.117

10.117

90.000

32.00

0.5387

14387**

440

8.761579

8.761579

70.52878

51.50

0.6694

Fe2O3[65]

104

7.394498

5.0285

90.000

39.75

0.7618

79-0007*

110

7.394498

5.421512

95.88002

25.00

0.5367

64599**

116

8.709617

7.394498

66.88256

42.25

0.5655

BaSnO3[87]

110

4.1239

4.1239

90.000

40.00

0.6125

03-0675*

211

7.142804

5.832075

90.000

55.75

0.8142

43138**

200

4.1239

4.1239

90.000

40.00

0.6125

Zn2SnO4[84]

311

10.594043

6.116474

106.77865

53.00

0.8791

0.6334

0.7810

0.6950

0.7860

0.6018

0.7245

0.6213

0.6797 0.8800

24-1470*

220

8.650

6.116474

90.000

48.00

0.8231

28235**

511

16.182668

6.116474

100.89339

48.5

0.9379

SrTiO3[139] 73-0661* 23076** In2S3[78, 79] 84-1385 or 32-456* 202353** TiS2[82] 88-1967* 52195** SnS2[80] 23-0677* 43004** ZnSe[75] 03-065-9602* 652227**

110

3.905

5.22504

90.000

54.75

0.5313

200

3.905

3.905

90.000

54.75

0.6970

211

6.763658

5.522504

90.000

39.75

0.9529

311

13.195401

7.618368

106.77865

35.00

0.5563

440

10.774

7.618368

90.000

55.50

0.6955

400

7.618368

7.618368

90.000

26.25

0.5566

001

3.4073

3.4073

60.000

60.00

0.5024

011

3.4073

6.636726

104.87436

45.50

0.5247

102

8.201556

3.4073

90.000

48.50

0.6077

001

3.645

3.645

60.000

42.75

0.6648

100

3.645

5.901

90.000

0.00

0.5010

101

6.935981

3.645

74.76609

8.00

0.7126

111

3.995153

3.995153

120.000

42.25

0.6129

220

5.650

3.995153

90.000

38.75

0.5338

311

6.919809

3.995153

106.77865

26.25

0.9451

0.7271

0.6028

0.5449

0.6261

0.6973

* Powder diffraction file (PDF) card number, ** ICSD number, (A-TiO2, R-TiO2, H-WO3 and M-WO3 represent anatase TiO2, rutile TiO2, hexagonal phase WO3 and monoclinic phase WO3, respectively)

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: