A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability

Journal Pre-proof A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability Xiaolong Yang, Jun Xi, Yuanhui Sun, Yindi Zhang, Guijiang Zhou, Wai-Yeung Wong PII:

S2211-2855(19)30653-6

DOI:

https://doi.org/10.1016/j.nanoen.2019.103946

Reference:

NANOEN 103946

To appear in:

Nano Energy

Received Date: 11 June 2019 Revised Date:

22 July 2019

Accepted Date: 27 July 2019

Please cite this article as: X. Yang, J. Xi, Y. Sun, Y. Zhang, G. Zhou, W.-Y. Wong, A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability, Nano Energy (2019), doi: https://doi.org/10.1016/ j.nanoen.2019.103946. 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 Published by Elsevier Ltd.

Graphical Abstract

A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability

Dopant-Free HTM

Voc = 1.11 V Jsc = 22.21 mA cm−2 FF = 76.18% PCE = 18.78%

XY1

1.11

The twisted organic small-molecule hole transport material XY1 exhibits advantages such as good film morphology, strong absorption in the UV range but high transmittance in the visible and near-infrared range, high hole mobility, and perfect energy level alignment with the perovskite layer. Consequently, both small- and large-area inverted perovskite solar cells based on the dopant-free XY1 show state-of-the-art photovoltaic performance.

1

A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability

Xiaolong Yang‡a,b, Jun Xi‡c, Yuanhui Sunb, Yindi Zhangb, Guijiang Zhou*b and Wai-Yeung Wong*a

a

Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China

b

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China

c

Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 08826, Republic of Korea

*Corresponding Authors: G. J. Zhou ([email protected]) W.-Y. Wong ([email protected]) ‡ Xiaolong Yang and Jun Xi contributed equally to this work.

2

Abstract Low-cost solution-processable inverted perovskite solar cells (PSCs) demonstrate great potential toward future photovoltaic market. Unfortunately, general hole transport materials (HTMs) within inverted structure make the performance and stability far uncompetitive compared to the normal structure. Interrogating the fundamentals of these materials, moderate carrier mobility and susceptible environmental stability of the undoped molecules are the main causes. Herein, a twisted molecule XY1 is developed as a potential robust dopant-free HTM

for

inverted

PSCs.

Compared

with

traditional

poly(3,4-

ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), XY1 exhibits much higher hole transport ability and peculiar ultraviolet absorption. Hysteresis-free inverted PSCs based on XY1 exhibits the power conversion efficiency (PCE) of 18.78%, which is among the highest values for inverted PSCs based on dopant-free HTMs. After light soaking for 200 h, the original PCE of XY1-based device is still maintained at the 95% level, indicating the substantially improved operational stability. Besides, large-area (1.00 cm2) inverted PSC based on XY1 shows a competitive PCE of 17.82%.

Keywords dopant-free hole transport material, p-i-n perovskite solar cell, large-area solar cell, inverted structure, twisted molecule

1. Introduction Within only ca. 10 years, the power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PSCs) has been dramatically improved from the initial 3.81% to 24.2%, which makes PSCs the most competitive candidate for the next-generation solar cells [1-5]. The rapid development of PSCs owes to the tremendous dedications of distinct field researches, such as device architecture design, methods for growing high quality perovskite

3

film, regulation of the perovskite composition, and charge extraction/transport material engineering [2, 6-14]. The device architecture of PSCs can be divided into two classes, namely the regular (n-i-p) device with the conducting glass/electron transport layer/light absorbing perovskite layer/hole transport layer/metal configuration and the inverted (p-i-n) device with the conducting glass/hole transport layer/light absorbing perovskite layer/electron transport layer/metal configuration [15]. To date, the best performed PSCs with PCEs above 20% are mainly based on n-i-p architecture [2, 9, 10, 16, 17]. However, the fabrication of highly efficient n-i-p devices often requires considerably high temperature (≥ 450 °C) treatment to sinter TiO2 electron transport layers, which is not cost-efficient and also cannot be compatible with flexible substrates in the development of large size flexible PSCs [9, 17]. Moreover, other serious issues, such as the notable current density–voltage (J–V) curve hysteresis, the relatively low device stability induced by the heavily doped HTMs and the degeneration of TiO2 under UV illumination, still need to be carefully handled for n-i-p PSCs [11, 18-20]. In contrast, p-i-n PSCs can be facilely fabricated through low temperature (< 150 °C) solution-processable approaches at a low cost, demonstrating the potential for developing large size flexible solar cells [19, 21, 22]. Furthermore, p-i-n PSCs usually exhibit hysteresis-less/-free characteristics [23-25]. As is known, n-i-p PSCs often require hole transport layers with the thickness of 100-300 nm to sufficiently collect and transport holes to the metal electrode [2, 26, 27]. On the contrary, the direct illumination on the p-side of p-i-n PSCs can facilitate efficient hole extraction processes, and thus the thickness of HTMs in p-i-n PSCs can be reduced to only around 10 nm to tremendously reduce the production cost [10, 28, 29]. Therefore, the low-temperature-processed p-i-n PSC technology is much more attractive for future applications. Besides relying on high quality perovskite layers, the performance of p-i-n PSCs is also very dependent on the properties of HTMs. Since the first p-i-n PSC reported in 2013, PEDOT:PSS has been commonly used as the HTM in inverted PSCs [19, 30-32]. However, the 4

acidity and hygroscopic nature of PEDOT:PSS are detrimental to the stability of the corresponding PSCs. Meanwhile, its deep-lying lowest-unoccupied molecular orbital (LUMO) energy level cannot efficiently block the electron from the perovskite layer to be directly transferred to the metal anode, leading to charge recombination losses and thus inferior overall efficiency [33, 34]. Therefore, to reduce these drawbacks, great efforts have been dedicated to either tailor the property of PEDOT:PSS by doping/modification approaches or replace PEDOT:PSS directly with other HTMs [33, 35-38]. Inorganic p-type semiconducting materials, such as NiOx, CuxO, and CuSCN, are good replacements for PEDOT:PSS due to their high hole mobility, thermal stability, and high transparency in the visible and near-infrared range [39, 40]. However, the preparation of inorganic HTM layers maybe involve high temperature processes and can be very complicated [41-43]. Conjugated polymers, such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and poly-3hexylthiophene (P3HT), are popular for PSCs due to their good film forming properties and the easy tuning of energy levels [44-46]. Unfortunately, the inevitably wide molecular weight distributions and batch-to-batch molecular weight variations of polymeric HTMs limit the reproducibility and consistency of device performance [47]. Importantly, due to the special device configuration of p-i-n PSCs, the incident light needs to pass through the HTM to reach the light absorbing perovskite layer. In this scenario, the HTM should have strong absorptions in the UV range to protect the perovskite layer from the UV damage, and exhibit weak or no absorption in the visible and near-infrared range to ensure the light absorption of the perovskite layer [48]. However, although polymeric HTMs usually possess strong ultraviolet light absorption capacity, they may also cause intense absorption in the visible range to impair the light absorption of the perovskite layer [49-51]. With the advantages of clear chemical structure information, precise molecular weight with high purity, convenient regulation of molecular structure, matched energy level, organic small molecules are considered to be ideal HTMs for PSCs [52]. 5

To

date,

2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene

(Spiro-OMeTAD) and other organic small molecules have been used as HTMs for PSCs [53, 54]. However, the synthesis procedures of these HTMs are tedious and complicated with high cost. Additionally, these organic small-molecule HTMs often exhibit low hole transport ability, which need to be doped with additives to improve their hole extraction/transport capability [36, 55]. The doping process not only complicates the device preparation process but also is harmful to device stability due to the hygroscopic and volatile characters of the widely used additives like lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and tertbutylpyridine (t-BP) [56]. Therefore, organic small-molecule HTMs with high hole transport abilities to avoid the doping process as well as simple and efficient synthesis procedures to reduce the time and cost are still highly desirable. Herein, we report a twisted organic small molecule XY1 synthesized by only one step as the dopant-free HTM for high performance pi-n PSCs. The high hole mobility of the thiophene ring is very attractive for developing effective HTMs [55, 57]. Besides, it has reported that the S−Pb2+ interaction (−116.8 kcal mol−1) is quite strong where the sulfur atom in the sulfide compound as a soft Lewis base may passivate perovskite trap states to enhance the device performance [58]. However, compared to that of sulfide or thiol, the coordination ability of the sulfur atom in thiophene ring with aromaticity should be quite small. Therefore, we chose the thieno[2,3-b]thiophene core with two sulfur atoms in the same side to enhance the interaction between the HTM and the perovskite layer. In addition, due to the small core size, we can deliberately attach four triphenylamine substituents to highly twist the molecular structure to increase the solubility and glass transition temperature for obtaining good film morphology. The highly twisted structure can also limit the conjugation length to adjust the optical property. Compared with PEDOT:PSS, XY1 showed strong absorption in the UV range, which could help to filter the high-energy photons to protect the perovskite layer from the UV-induced deterioration, and simultaneously keep high transparency in the visible and near-infrared range to ensure the 6

incoming light to be fully absorbed by the perovskite layer. With a small core, the size of the whole molecule will be minimized, thus the “density” of electron-rich triphenylamine moieties can be increased to benefit the hole transport ability of the XY1 film. As a consequence, XY1 exhibited higher hole mobility (3.76 × 10−4 cm2 V−1 s−1) than PEDOT:PSS (2.26 × 10−4 cm2 V−1 s−1). Together with the well-aligned energy levels, XY1 possessed more efficient hole extraction/transport ability as indicated by the steady-state photoluminescence (PL) and time-resolved PL (TRPL) decay measurements. Encouragingly, hysteresis-free inverted PSCs using XY1 as the dopant-free HTM demonstrate outstanding photovoltaic performance with the open-circuit voltage (Voc) and PCE up to 1.11 V and 18.78%, which are not only much higher than those of the PEDOTP:PSS-based control device (1.03 V and 16.31%) but are also among the highest values reported for inverted PSCs based on dopantfree organic small molecule HTMs. Moreover, under continuous AM 1.5G one sun illumination for 200 h, the PCE measured at the maximum power point for the PEDOT:PSSbased device quickly dropped to ca. 66% of its initial value, whereas that of the XY1-based device still remained at ca. 95%, indicating the substantially improved operational stability of the XY1-based device. Impressively, the large-area (aperture 1.00 cm2) inverted PSC based on XY1 without the doping process also showed excellent performance with the high Voc and PCE of 1.10 V and 17.82%, respectively, comparable to those of state-of-the-art large-area (≥ 1.00 cm2) inverted PSCs based on organic HTMs. This work here demonstrates the one-step synthesized compound XY1 is a very promising dopant-free organic small-molecule HTM in the practical application of developing high performance inverted PSCs.

2. Results and Discussion 2.1. Materials Synthesis, Theoretical Calculation and Characterization As illustrated in Fig. 1a, the neutral organic HTM XY1 was synthesized through a simple onestep Suzuki-Miyaura coupling reaction from the readily available commercial starting 7

materials, which was much more effective and inexpensive compared with the complex and expensive procedure of the widely used spiro-OMeTAD. The detailed synthetic procedure of XY1 is provided in the Supplementary data. The target product XY1 was fully characterized by 1H NMR,

13

C NMR, and matrix assisted laser desorption ionization time of flight

(MALDI-TOF) mass spectrometry (Figure S1-S3). As depicted in Fig. 1b, the density functional theory (DFT) calculation indicated that XY1 possessed a low molecular symmetry configuration due to the uneven highly twisted behavior of triphenylamine moieties linked to the thieno[2,3-b]thiophene core. As revealed by the calculated electrostatic potential surface (EPS) result (Fig. 1c), methoxy groups on the surface of the molecule showed strong negativity and pointed outwards in different directions. Besides, XY1 also showed substantial negative potential around the thieno[2,3-b]thiophene core because of the two electron-rich sulfur atoms in the same side. Thus, strong negativity on the surface of XY1 molecule may facilitate the interaction between the Lewis basic heteroatoms (O and S) and Pb2+ on the perovskite layer surface to passivate the perovskite surface defects, and hence benefit the device performance [29, 59-61]. The highest occupied molecular orbital (HOMO) was mainly contributed by the two triphenylamine moieties in the same side of the XY1 molecule, and the LUMO was mainly localized on the three phenyl rings linked to the thieno[2,3-b]thiophene core (Fig. 1d and 1e). Therefore, the EPS calculation result as well as the HOMO and LUMO distributions indicated the low molecular symmetry of XY1 both in the chemical structure and the electronic structure. Partially due to the low molecular symmetry, XY1 could show good solubility in common organic solvents such as tetrahydrofuran, chloroform, dichloromethane, and toluene. Compared with the above mentioned organic solvents, dimethyl sulfoxide and dimethyl formamide showed relatively lower solubility for XY1, which was beneficial to device fabrication with the solution-processable mothed. More importantly, with a low molecular symmetry configuration, the HTM can show good morphological properties in the film state, which will benefit the device performance [62]. We investigated the XY1 film 8

morphology with the field emission scanning electron microscopy (SEM) and atomic force microscopy (AFM). As shown in Fig. 2a and 2d, the bare ITO showed relatively high roughness with a root mean square (RMS) value of 1.3 nm indicated by the AFM. By spincoating a thin PEDOT:PSS layer on ITO (ITO/PEDOT:PSS), the surface was much smoother (Fig. 2b and 2e) with a smaller RMS value of 0.6 nm. However, with XY1 as the HTM (ITO/XY1), the uniformity and morphology was substantially improved (Fig. 2c and 2f) since almost no crease could be observed and the RMS value was significantly reduced to 0.4 nm. Thus, XY1 could densely cover the ITO surface to form a smoother film with no phase separation, which would lead to a better suppression of the direct charge recombination between the perovskite layer and the ITO surface to improve the device performance [63]. O O

(a) O Br Br

S

O O B

Br S

Br

O N

N

O

O

Pd(PPh3)4, K2CO3 (2 M)

+

Toluene, 110 °C, 16 h

N

N O

S

O

XY1

O

(b)

S

N

O

(c)

124.3° °

62.4° ° 43.3° °

133.9° ° −0.03 a.u.

(d)

0.03 a.u.

(e)

HOMO

LUMO

9

Fig. 1. (a) The synthetic route of XY1. (b) Optimized molecular configuration of XY1 showing twisted angles. (c) Calculated ESP map of XY1. (d) HOMO and (e) LUMO distribution of XY1. (Hydrogen atoms are omitted for clarity)

(a)

ITO

ITO/PEDOT:PSS

(b)

1 µm (d)

(c)

ITO/XY1

1 µm

1 µm (e)

(f)

RMS: 1.3 nm

RMS: 0.6 nm

RMS: 0.4 nm

ITO/PEDOT:PSS

ITO

ITO/XY1

Fig. 2. SEM surface images of (a) bare ITO, (b) ITO/PEDOT:PSS, and (c) ITO/XY1. AFM images (5 µm × 5 µm) of (d) bare ITO, (e) ITO/PEDOT:PSS, and (f) ITO/XY1.

The good film morphology of XY1 may also be related to its thermal behavior. Thermogravimetric

analysis

(TGA)

and

differential

scanning

calorimetry

(DSC)

measurements were performed under inert atmosphere (Figure S4). The TGA result showed that XY1 possessed excellent thermal stability with the decomposition temperature (Td) observed up to 435 °C. The DSC plot indicated a glass transition temperature (Tg) for XY1 as high as 223 °C, which could be partially attributed to the significantly twisted molecular structure [64]. The impressively high Tg of XY1 would effectively avoid crystallization to keep XY1 in an amorphous phase, thus improving the XY1 film morphology as well as the corresponding device performance [64]. 10

2.2. Photophysical Properties and Molecular Energy Levels The UV-vis absorption (Abs.) and PL spectra of XY1 in CH2Cl2 are depicted in Fig. 3a. XY1 displayed almost no absorption in the visible and near-infrared range but strong absorptions below 400 nm in the UV range, which could be ascribed to the restricted conjugation length because of the highly twisted structure. The UV-vis absorption and PL spectra of the XY1 film were tested for comparison. As expected, both the UV-vis absorption and PL spectra of the XY1 film were red-shifted. However, most part of the absorption spectrum still remained within 425 nm (as indicated by the blue dashed line in Fig. 3a). The UV-vis absorption and transmittance (Tra.) property of XY1 on the ITO surface (ITO/XY1) (Fig. 3b) were also recorded. For comparison, the UV-vis absorptions and transmittances of bare ITO and ITO spin-coated with PEDOT:PSS (ITO/PEDOT:PSS) were measured under the same condition. As shown in Fig. 3b, PEDOT:PSS had essentially no influence on the optical properties of ITO since the UV-vis absorption and transmittance spectra of the ITO/PEDOT:PSS film were identical to those of the bare ITO. However, the ITO/XY1 film displayed much stronger absorption in the UV range than the ITO/PEDOT:PSS film. The strong absorption in the UV range of HTMs can effectively filter high-energy photons and thereby help to prevent the degradation of perovskite layers and ultimately to enhance the stability of the related devices under continuous illumination [11, 48, 63]. Despite its relatively lower transparency in the UV range, the ITO/XY1 film showed high transparency in the visible and near-infrared range, which ensured the incoming visible and near-infrared light to reach the perovskite layer. More importantly, by absorbing the high-energy photons, XY1 could emit intense light in the visible range (Fig. 3a) with a high photoluminescent quantum yield of 0.66 to enhance the light harvesting of the perovskite layer. Therefore, due to the strong absorption in the UV range and intense re-emission in the visible range, XY1 could expectedly improve the corresponding device performance by not only limiting the photodegradation but also increasing the light harvesting efficiency of the perovskite layer [48]. 11

1.0 0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.2

0.0

0.0

300 350 400 450 500 550 600 650 Wavelength (nm)

Abs.

0.6 0.4

XY1 Repeat Current (A)

Tra. ITO ITO/PEDOT:PSS ITO/XY1

60 40 20 0

400

500 600 Wavelength (nm)

700

(d)

100

Current (a.u.)

(c)

80

0.8

Transmittance (%)

PL intensity (a.u.)

1.0 Absorbance (a.u.)

Absorbance (a.u.)

0.0

100

(b)

XY1 solution XY1 film 1.0

(a)

10-1

10-2

PEDOT:PSS (2.26 × 10−4 cm2 V−1 s−1) XY1 (3.76 × 10−4 cm2 V−1 s−1)

0.0 0.5 Voltage vs Fc/Fc+ (V)

2.0x105

2 3 4 Applied Bias (V)

5

100

1.5x105 1.0x105 5.0x104 0.0

1

(f) Perovskite PEDOT:PSS/Perovskite XY1/Perovskite

PL Intensity (a.u.)

PL Intensity (a.u.)

2.5x105 (e)

0

1.0

700

750 800 Wavelength (nm)

10-1

Perovskite PEDOT:PSS/Perovskite XY1/Perovskite

10-2

850

0

200

400 time (ns)

600

Fig. 3. (a) UV-vis absorption and PL spectra of of XY1 in CH2Cl2 solution and film. (b) UV-vis absorption and transmittance (Tra.) property of bare ITO, the ITO/PEDOT:PSS film, and the ITO/XY1 film. (c) CV curve of XY1 in CH2Cl2. (d) Current-voltage characteristics of PEDOT:PSS and XY1 measured by the SCLC method. (e) Steady-state PL spectra and (f) Time-resolved PL decay properties for perovskite layers on the surfaces of glass, PEDOT:PSS, and XY1, respectively.

The electrochemical property of XY1 was investigated by the cyclic voltammetry (CV) in CH2Cl2 under an inert atmosphere (Fig. 3c). During the CV measurement, no reduction wave was observed, and XY1 only showed well reversible and repeatable oxidative behavior with the oxidation potential (Eox) against the ferrocene couple (Fc/Fc+) estimated to be ca. 0.31 V,

12

indicating the good electrochemical stability as well as hole capture ability. The hole mobility (µh) of the XY1 film was determined by the space-charge limited current (SCLC) method under dark condition (Fig. 3d), which revealed that XY1 possessed a much higher µh of 3.76 × 10−4 cm2 V−1 s−1 than that of PEDOT:PSS (2.26 × 10−4 cm2 V−1 s−1) under the same condition, which could be attributed to the high “density” of electron-rich triphenylamine moieties on a small thieno[2,3-b]thiophene core. Therefore, there was no need to dope the HTM. The good hole capture ability and higher hole transport ability of XY1 would facilitate a better hole extraction process from the perovskite layer to the ITO anode. This conclusion was further demonstrated by the steady-state PL and TRPL decay measurements of the perovskite layers on the surfaces of glass (Perovskite), the PEDOT:PSS film (PEDOT:PSS/Perovskite) and the XY1 film (XY1/Perovskite). The perovskite layers were prepared by properly mixing three cation salts, i.e., cesium iodide (CsI), formamidinium iodide (FAI), and methylammonium bromide (MABr) and two lead salts, i.e., PbI2 and PbBr2, to form the film with a composition of (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (abbreviated as CsFAMA) [65]. As shown in Fig. 3e, the perovskite layer solo displayed an intense emission at ca. 796 nm. The emission intensity of the perovskite layer atop of PEDOT:PSS was considerably reduced. This emission could be dramatically quenched by the XY1 film, indicating the stronger hole extraction ability of XY1 compared with that of PEDOT:PSS. Fig. 3f depicts the TRPL decay behaviors for the perovskite layers respectively on the surfaces of glass, PEDOT:PSS, and XY1 measured at the peak emission wavelengths. By fitting these curves with the bi-exponential decay function of I = I0 + A1exp(-t/τ1) + A2exp(-t/τ2) [66], the averaged decay times (τave) of ca. 806, 221, and 74 ns were deduced for Perovskite, PEDOT:PSS/Perovskite, and XY1/Perovskite layers, respectively, indicating a much faster hole extraction from the perovskite layer to XY1, which was in good agreement with the strongly quenched PL observed for the XY1/Perovskite layer [29, 67, 68].

13

Besides the hole capture and transport abilities, the relative energy levels of the HOMOs for HTMs compared to the valence band edge of perovskite layers also affect the hole extraction efficiency. Therefore, the HOMO level of the XY1 film was determined by the ultraviolet photoelectron spectroscopy (UPS). As shown in Figure S5, the high binding energy cut-off (Ecut-off) and low binding energy onset (Eonset) of the XY1 film were 16.92 and 1.14 eV, respectively, corresponding to a deep HOMO level of −5.44 eV. From the absorption onset at ca. 410 nm, the optical energy gap of XY1 was estimated to be 3.02 eV, and thus the LUMO level of XY1 was deduced as −2.42 eV. The valence band maximum (VBM) and conduction band minimum (CBM) of the triple cation perovskite were reported to be −5.85 and −4.22 eV, respectively [65]. Therefore, compared with PEDOT:PSS, XY1 possessed a deeper HOMO and thus a smaller offset of its HOMO energy with respect to VBM of the perovskite layer, which could not only facilitate the hole extraction and transport processes but also reduce the energy loss. In addition, the high lying LUMO of XY1 could effectively block electrons from the perovskite layer to be directly transferred to the ITO anode, and thereby further enhance the performance of the resultant devices. 2.3. Photovoltaic Performance Encouraged by the impressive properties of XY1, planar inverted PSCs with the device structure of ITO/XY1 (ca. 8 nm)/light absorption layer (ca. 400 nm)/C60 (20 nm)/bathocuproine (BCP 6 nm)/Cu (80 nm) were fabricated. Except for the depositions of C60, BCP, and Cu electrode, other functional layers were all prepared by the solution process approach at low temperature (not higher than 100 °C). For comparison, planar inverted PSCs based on PEDOT:PSS with the thickness of ca. 8 nm were fabricated. However, these devices showed quite low PCEs of around 11% (Figure S6). Then, the thickness of PEDOT:PSS was increased to ca. 30 nm to optimize the performance of PEDOT:PSS-based devices [32], such that we would know whether the XY1-based devices could even outperform the optimized PEDOT:PSS-based devices. The classical triple cation perovskite CsFAMA was used as the 14

light absorption layer because of its high efficiency and stability [69, 70]. Due to the high µh, there was no need of an extra doping process for the XY1 film. The detailed fabrication procedure is provided in the Supporting Information. Fig. 4a illustrates the energy levels of materials used in the corresponding devices, showing a perfect energy band alignment with the perovskite layer and a smaller HOMO energy level offset for the device based on XY1 [65, 71, 72]. Cross-sectional SEM image of a full device using XY1 as the HTM is depicted in Fig. 4b, which clearly shows the planar layered structure. For comparison, PEDOT:PSS-based devices were also fabricated with the same device structure. Photovoltaic properties of these devices with an aperture area of 0.0729 cm−2 were tested under AM 1.5G illumination at an intensity of 100 mW cm−2. Fig. 4c shows the distribution of photovoltaic parameters obtained from the reverse scanning direction for 12 identical devices based on PEDOT:PSS and 19 identical devices based on XY1. The related statistical data are summarized in Table 1. Apparently, compared with those of devices based on PEDOT:PSS, the three photovoltaic parameters, open voltage (Voc), short-circuit current (Jsc), and fill factor (FF), of the devices based on XY1 were all improved. Especially, a significant increase of the average Voc (1.10 ± 0.01 V) was achieved by XY1-based devices due to the much deeper HOMO level and the enhanced hole extraction/transport ability of the dopant-free HTM XY1. Therefore, devices based on XY1 could exhibit an average PCE of 18.30 ± 0.30%, which was ca. 20% higher than that of devices based on PEDOT:PSS (average PCE = 15.47 ± 0.18%). It is worth mentioning that the performance of the devices based on PEDOT:PSS reported here was optimized, and yet was still inferior to those of the devices based on XY1. In addition, the narrow distributions of the photovoltaic parameters for XY1-based devices indicated the good reproducibility of using XY1 as a dopant-free HTM to fabricate high performance inverted PSCs.

15

Fig. 4. (a) Energy levels of the materials used in the devices. (b) Cross-sectional SEM image of a device based on XY1. (c) Box charts of photovoltaic parameters for devices based on PEDOT:PSS (orange box) and XY1 (red box). Table 1. Photovoltaic performance for devices based on PEDOT:PSS and XY1. HTM PEDOT:PSS

reverse forward

XY1

reverse forward

a)

Voc [V]

Jsc [mA cm−2]

FF [%]

PCE [%]

a)

1.03 (1.02 ± 0.01) b) 1.03 c)

21.39 (21.24 ± 0.13) 21.36

74.03 (71.36 ± 3.18) 73.91

16.31 (15.47 ± 0.18) 16.26

1.11 a) (1.10 ± 0.01) b) 1.11 c)

22.21 (21.95 ± 0.23) 21.95

76.18 (75.70 ± 0.86) 76.83

18.78 (18.30 ± 0.30) 18.72

Photovoltaic parameters obtained from the reverse scanning direction for the best devices based on

PEDOT:PSS and XY1, respectively.

b)

Average photovoltaic parameters obtained from the reverse

direction for devices based on PEDOT:PSS and XY1, respectively.

c)

Photovoltaic parameters obtained

from the forward scanning direction for the best devices based on PEDOT:PSS and XY1, respectively.

16

The current density−voltage (J−V) curves of the two champion devices utilizing the PEDOT:PSS and XY1 as HTMs are compared in Fig. 5a. To estimate the hysteresis effect of the devices, J−V curves were also scanned in the forward direction. Both PEDOT:PSS-based and XY1-based devices exhibited almost completely coincident J−V curves scanning from reverse and forward directions. The hysteresis index (HI) calculated from the formula HI = [(PCEreverse − PCEforward)/PCEreverse] × 100% was only 0.31% and 0.32% for devices based on PEDOT:PSS and XY1, respectively, suggesting XY1 was as good as PEDOT:PSS for fabricating the hysteresis-free inverted PSCs. More importantly, the device based on XY1 displayed much better performance than the devices based on PEDOT:PSS. As revealed by the J−V curves, a conspicuous advantage of the XY1-based devices was the considerably raised Voc, which had been credited to the low-lying HOMO level as well as the higher hole extraction/transport capability of XY1. The best device based on PEDOT:PSS showed the Voc of 1.03 V, Jsc of 21.39 mA cm−2, and FF of 74.03%, corresponding to a PCE of 16.31%, whereas the best device based on XY1 displayed a more impressive performance with the Voc, Jsc, FF, and PCE increased to 1.11 V, 22.21 mA cm−2, 76.18%, and 18.78%, respectively. To the best of our knowledge, this efficiency is one of the highest PCEs reported for inverted PSCs based on dopant-free organic small-molecule HTMs [24, 28, 29, 37, 58, 61, 72-82]. Fig. 5b summarizes several representative inverted PSCs showing outstanding performance based on dopant-free organic small-molecule HTMs reported from 2017 to the present [24, 28, 58, 61, 72-82], clearly confirming the superiority of using the dopant-free HTM XY1 to fabricate inverted PSCs with significantly improved Voc and thereby high PCE. Actually, some parallel aligned grain boundaries (Fig. 4b) can impair the charge transport along the vertical direction [83, 84], leading to the relatively lower Jsc of XY1-based device compared with those of the most efficient devices reported recently [2]. This result indicates that the PCE of the device based on XY1 can be further increased by improving the quality of the perovskite layer. We measured the UV-vis absorption spectra for the perovskite layers atop of PEDOT:PSS and 17

XY1 separately (Fig. 5c), which showed that the XY1/Perovskite layer displayed the enhanced light absorption ability across the whole UV and visible range (300 – 750 nm). Considering the higher hole transport efficiency and enhanced light absorption property of the XY1/Perovskite layer, the increase of Jsc observed for the XY1-based device was reasonable [85]. In addition, as shown in the external quantum efficiency (EQE) spectra (Fig. 5d), the XY1-based devices showed higher EQE values than the PEDOT:PSS-based devices in the range from ca. 350 nm to 760 nm. The integrated current densities for devices based on PEDOT:PSS and XY1 were 21.08 and 22.00 mA cm−2, respectively, showing a good agreement with the J−V measurements. To confirm the efficiencies obtained from the J−V measurements, the stable PCE and photocurrent outputs at the maximum power point (MPP) were measured for 200 s (Fig. 5e). Devices based on PEDOT:PSS and XY1 showed the steady-state PCE of ca. 15.4% and 18.3%, and steady-state current densities of ca. 18.8 and 20.1 mA cm−2, respectively, which further confirmed the superior performance of dopant-free XY1 over PEDOT:PSS as the HTM for inverted PSCs.

18

(b)

-10

# #

18

-15

16 Published in 2017 Published in 2018 Published in 2019

-20

14 0.0

0.2

1.0

1.2

0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 Voc (V) 100 (d)

PEDOT:PSS/Perovskite XY1/Perovskite

EQE (%)

0.8 0.6 0.4 0.2

25

80

20

60

15

40

10 PEDOT:PSS XY1

20

400

(e)

21

500 600 Wavelength (nm)

700

~18.3 %

14

0 300

800

~15.4 %

7 PEDOT:PSS @ 0.82 V XY1 @ 0.91 V

0 -7

0 -7

−2

-14

-14

~18.8 mA cm

-21

-21

−2

~20.1 mA cm

0

50

100 Time (s)

150

500 600 700 Wavelength (nm)

800

0 900

95%

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5

(f)

21 Normalized PCE

0.0 300

J (mA cm− 2)

Absorbance (a.u.)

1.0 (c)

0.4 0.6 0.8 Voltage (V)

Integrated Jsc (mA cm−2)

-25

PCE (%)

This Work

20

PEDOT:PSS reverse PEDOT:PSS forward XY1 reverse XY1 forward

-5

PCE

Current density (mA cm−2)

(a)

0

0.8

0.4 0.2 0.0

200

66%

0.6

PEDOT:PSS XY1 100

200 300 Time (h)

400

Fig. 5. (a) J–V curves of the best performing devices based on PEDOT:PSS and XY1. (b) PCEs and Vocs of several representative inverted PSCs based on dopant-free organic small-molecule HTMs. (# The double hole transport layer of NiOx/organic HTM was used in these two PSCs). (c) UV-vis absorption spectra of PEDOT:PSS/Perovskite and XY1/Perovskite layers. (d) EQE spectra and integrated current densities for devices based on PEDOT:PSS and XY1. (e) Steady-state PCEs and current densities of devices based on PEDOT:PSS and XY1 measured at the MPP. (f) Normalized PCEs stability tracked at the MPP in a glovebox without encapsulation under constant illumination.

In addition to the efficiency, another crucial issue in the future commercial application of PSCs is the stability, especially the stability of PSCs at the work state under constant

19

illumination similar to the realistic operational conditions. Therefore, the aging tests on the best performed devices without encapsulation were carried out under constant AM 1.5G one sun illumination in a glovebox at room temperature. Normalized PCEs recorded at the MPP for devices based on PEDOT:PSS and XY1 are presented in Fig. 5f. The PCE of the PEDOT:PSS-based device dropped rapidly to ca. 66% of its initial efficiency in 200 h, whereas the PCE of the XY1-based device was maintained at ca. 95%. This result unambiguously demonstrated the superior operational stability of the device based on XY1 over that based on PEDOT:PSS. The PCE of the XY1-based device was still as high as ca. 86% of its initial efficiency after constant AM 1.5G one sun illumination for 480 h. The device stability at 85 °C was also tested in a glovebox without encapsulation (Figure S7). After 168 h, the PCEs of XY1-based devices slightly decreased to ca. 97% of its initial efficiency. In contrast, the PCEs of PEDOT:PSS-based devices dropped to less than 75%. This result demonstrated that the XY1-based device could still show enhanced stability over the device based on PEDOT:PSS at the higher temperature. It is worth noting that this stability was tested under constant illumination with full power outputs, and thus this result manifested the outstanding operational stability. Two obvious advantages of XY1 should have made positive contribution to the significantly improved stability. The acidic and chemical active nature of PEDOT:PSS may corrode the ITO and perovskite layer to impair the device stability [34, 35]. However, such drawbacks were completely absent from the stable and neutral XY1. Besides, compared with that of PEDOT:PSS, the enhanced UV absorption ability of XY1 could better filter high energy photons to protect the perovskite layers from UV-light-induced degradation, leading to the high stability of the XY1-based device [11, 48]. Actually, we also compared the structural properties of the perovskite layers atop of PEDOT:PSS and XY1. As shown in Figure S8, the perovskite layers prepared on the surfaces of PEDOT:PSS and XY1 displayed essentially identical X-ray diffraction (XRD) spectra, in which the typical perovskite peaks with strong 20

intensity at ca. 14° as well as the weak PbI2 peaks at ca. 12.7° were observed. Thus we turned our attention to the morphology property of the perovskite layers due to its significant influence on the device performance in terms of both efficiency and stability [86-88]. Fig. 6a and 6b depict the SEM surface images of the perovskite layers atop of PEDOT:PSS and XY1, respectively. The PEDOT:PSS/Perovskite layer exhibited the grain size similar to that of the XY1/Perovskite layer. However, obvious defects such as creases and pinholes were observed for the PEDOT:PSS/Perovskite layer which would result in current leakage to wreck the corresponding device. Xu et al. also observed that the perovskite layer atop of PEDOT:PSS showed pinholes between the crystalline domains [33]. Fortunately, the XY1/Perovskite layer possessed the compact and homogeneous pinhole-free morphology. Therefore, the much better morphology of the perovskite layer atop of XY1 was an additional reason for the significantly improved stability of XY1-based device. PEDOT:PSS/Perovskite

1.5 µm (b)

XY1/Perovskite

1.5 µm

Current density (mA cm− 2)

(a)

(c)

0

Voc [V]

Jsc [mA cm−2]

R

1.10

21.52

75.28 17.82

F

1.10

21.41

74.94 17.65

-10 -15

1 cm2

reverse (R) forward (F)

-5

FF [%]

PCE [%]

-20 -25

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Fig. 6. SEM surface images of perovskite layers atop of (a) PEDOT:PSS and (b) XY1. (c) Photovoltaic performance of the large-area inverted PSC based on dopant-free XY1.

With regard to the practical application, it is more important to develop high performance large-area PSCs at low cost. However, enlarging the active area of functional layers is more likely to cause defects in terms of coverage, uniformity, and traps, resulting in significant decreases of the Voc, Jsc, and FF, and thereby impairing efficiencies of the corresponding 21

PSCs [89, 90]. Due to the advantages of the good film morphology, strong absorption in the UV range but high transmittance in the visible and near-infrared range, high hole mobility, and perfect energy level alignment with the perovskite layer, XY1 had demonstrated its great potential as the dopant-free HTM to construct small-area inverted PSCs with the excellent device performance in terms of both efficiency and stability. Encouraged by this attractive result, we further fabricated the large-area (aperture 1.00 cm2) inverted PSC to estimate the potential of using XY1 as the dopant-free HTM in upscaling the inverted PSCs. The fabrication procedure was similar to that for the above investigated small-area inverted PSCs, and thus they had the same device structure but different device area. Fig. 6c shows the J−V curves measured from both reverse and forward scanning directions under AM 1.5G illumination at an intensity of 100 mW cm−2. Scanned in the reverse direction, the device exhibited an outstanding performance with the Voc, Jsc, FF, and PCE of 1.10 V, 21.52 mA cm−2, 75.28%, and 17.82%, respectively. During the scan in the forward direction, the device showed the J−V curve essentially identical to that obtained from the reverse scanning direction, indicating the hysteresis-free character. Generally, upscaling the device area will notably reduce the FF, leading to a significant decrease in the PCE [62, 91, 92]. However, it seems that the attractive properties of XY1 exhibited in the small-area inverted PSC still benefited the large-area inverted PSC, since the FF only slightly dropped from 76.18% to 75.28%. Therefore, the PCE of the large-area inverted PSC remained as high as 17.82%, which is among the highest PCEs reported for large-area (≥ 1.0 cm2) inverted PSCs based on organic HTMs [63, 89-91, 93-95]. This result demonstrated the promising potential of using XY1 as the dopant-free HTM to develop highly efficient large-area PSCs with the inverted structure.

3. Conclusion In summary, we had successfully attached four methoxy substituted triphenylamine groups 22

on the small thieno[2,3-b]thiophene core through a one-step reaction to purposely construct a highly twisted molecule XY1. Due to the restricted conjugation, the resultant compound showed strong absorptions in the UV range while keeping high transparency in the visible and near-infrared range. Compared with PEDOT:PSS, XY1 could form a uniform and smoother film to grow a pinhole-free perovskite layer with the compact and homogeneous morphology. Furthermore, because of the much higher hole mobility and well-aligned energy level to the valence band edge of the perovskite layer, XY1 could facilitate more efficient hole extraction/transport processes, resulting in superior photovoltaic performance of the XY1based device (Voc, Jsc, FF, and PCE of 1.11 V, 22.21 mA cm−2, 76.18%, and 18.78%, respectively) over the PEDOT:PSS-based device (Voc, Jsc, FF, and PCE of 1.03 V, 21.39 mA cm−2, 74.03%, and 16.31%, respectively). The Voc and PCE of this hysteresis-free XY1-based device are among the highest values reported for inverted PSCs based on dopant-free organic small-molecule HTMs. More importantly, under continuous AM 1.5G one sun illumination, the XY1-based device exhibited substantially improved operational stability compared with the PEDOT:PSS-based device. Finally, large-area (aperture 1.00 cm2) inverted PSCs based on XY1 without the doping process were fabricated to show the best performance with Voc, Jsc, FF, and PCE of 1.10 V, 21.52 mA cm−2, 75.28%, and 17.82%, respectively, which also acted as one of the most efficient large-area inverted PSCs based on organic HTMs. In brief, we developed a simple dopant-free organic small molecule HTM with a highly twisted structure, which has a great potential in the practical application of fabricating high performance inverted PSCs. The material design strategies, i.e., increasing the “density” of electron-rich triphenylamine moiety on a small central core as well as the degree of molecular distortion, together with the structure-property relationship investigated can provide important information for the future development of simple dopant-free HTMs to improve efficiency and operational stability of perovskite solar cells.

23

Acknowledgements W.-Y. Wong thanks the Hong Kong Research Grants Council (C5037-18G), the Hong Kong Polytechnic University (1-ZE1C) and the Endowment Fund from Ms Clarea Au (847S) for financial support. X. Yang, Y. Sun, and G. Zhou would like to thank the National Natural Science Foundation of China (21602170, 51803163, and 21875179), the China Postdoctoral Science Foundation (2015M580831 and 2016M600778), the Shaanxi Province Postdoctoral Science Foundation (2017BSHYDZZ02 and 2017BSHEDZZ03), the Fundamental Research Funds for the Central Universities (xjj2016061, xjj2017099, and cxtd2015003), Key Laboratory Construction Program of Xi'an Municipal Bureau of Science and Technology (201805056ZD7CG40). The characterization assistance from the Instrument Analysis Center of Xi’an Jiaotong University is also acknowledged. J. Xi would like to acknowledge the Global Frontier R&D Program of the Center for Multiscale Energy System, Seoul National University (2012M3A6A7054855), supported by Prof. Mansoo Choi.

Notes The authors declare no competing financial interest.

Appendix A. Supplementary data Supplementary data to this article can be found online at

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Dr. Xiaolong Yang is an associate professor at Xi’an Jiaotong University (XJTU). He received his B.S. degree in Applied Chemistry in 2010 and Ph.D. degree in Materials Science and Engineering in 2015 from XJTU. After that, he worked as a postdoctoral fellow in Prof. Wai-Yeung Wong's group at The Hong Kong Polytechnic University for one year. His research interest focuses on organic functional small molecule materials for high performance optoelectronic devices.

Dr. Jun Xi received his Ph.D. degree from Xi’an Jiaotong University (XJTU) in 2017. Now he is working as a postdoctoral researcher at Global Frontier Center for Multiscale Energy System, Seoul National University. His research interest mainly focuses on the study of multiscale micro-nano materials, stable and efficient optoelectronic devices.

Dr. Yuanhui Sun received her Ph.D. degree in Chemistry from Institute of Chemistry, Chinese Academy of Sciences in 2016 and from then she works as a lecturer at Xi’an Jiaotong 32

University (XJTU). Now, she is also working as a visiting scholar in Prof. Henning Sirringhaus’s group in Cavendish Laboratory at the University of Cambridge. Her current research interest concentrates upon the design, synthesis and characterization of organic semiconductor materials for a range of optoelectronic applications.

Yindi Zhang received her B.S. degree in Material Chemistry from Northwest University in 2017. She is currently a graduate student at Department of Chemistry, School of Science, Xi’an Jiaotong University. Her research interests include functionalized phosphorescent organometallic materials for electroluminescence.

Prof. Guijiang Zhou is Tengfei Professor at Xi’an Jiaotong University (XJTU). He received his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences in 2003. After a year of postdoctoral fellow in the National Creative Research Center in Korea, he worked with Prof. Wai-Yeung Wong at Hong Kong Baptist University. From April 2007 to September 2008, he was a postdoctoral fellow at the University of Murcia. In November 2008, he joined the Department of Applied Chemistry, XJTU as a Professor. His current research interests

include

functionalized

phosphorescent

electroluminescence and optical power limiting.

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organometallic

materials

for

Prof. Wai-Yeung Wong obtained his Ph.D. degrees from the University of Hong Kong. After postdoctoral works at Texas A&M University and University of Cambridge, he joined Hong Kong Baptist University from 1998 to 2016 and he now works at the Hong Kong Polytechnic University as Chair Professor of Chemical Technology. He is currently Associate Editor of Journal of Materials Chemistry C and Editor of Journal of Organometallic Chemistry and Topics in Current Chemistry. He was awarded numerous awards such as RSC Chemistry of the Transition Metals Award and State Natural Science Award from China. His research focuses on synthetic inorganic/organometallic chemistry, especially aiming at developing metal-based molecular materials for optoelectronics.

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Highlights: Dopant-free hole transport material XY1 is developed for inverted perovskite solar cells (PSCs) XY1 has advantages of strong UV absorption, high hole mobility and perfect energy level alignment Both small- and large-area inverted PSCs show state-of-the-art efficiencies Operational stability of the PSCs has been substantially enhanced