Three-dimensional nanostructured electrodes for efficient quantum-dot-sensitized solar cells

Author’s Accepted Manuscript Three-dimensional Nanostructured Electrodes for Efficient Quantum-Dot-Sensitized Solar Cells Jian-Kun Sun, Yan Jiang, Xinhua Zhong, Jin-Song Hu, Li-Jun Wan www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(16)30587-0 http://dx.doi.org/10.1016/j.nanoen.2016.12.022 NANOEN1673

To appear in: Nano Energy Received date: 15 October 2016 Revised date: 12 December 2016 Accepted date: 13 December 2016 Cite this article as: Jian-Kun Sun, Yan Jiang, Xinhua Zhong, Jin-Song Hu and LiJun Wan, Three-dimensional Nanostructured Electrodes for Efficient QuantumDot-Sensitized Solar Cells, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.12.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Three-dimensional Nanostructured Electrodes for Efficient Quantum-Dot-Sensitized Solar Cells

Jian-Kun Suna,1, Yan Jianga,1, Xinhua Zhongb*, Jin-Song Hua*,Li-Jun Wana

a

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of

Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Science, 2 North 1st Street, Zhongguancun, Beijing 100190, China; University of Chinese Academy of Science, Beijing 100049, China b

College of Materials and Energy, South China Agricultural University, 483 Wushan

Road, Guangzhou 510642, China

Abstract Quantum dot sensitized solar (QDSSC) has been considered as a promising candidate for the low-cost third-generation photovoltaics due to the unique optoelectronic properties of quantum dot light absorbers. Over the past years, QDSSCs have witnessed tremendous progress with a rapid rising of the power conversion efficiency from sub-5 % in 2010 to 11.6 % in 2016. Herein, we present a comprehensively review on the recent progresses in QDSSCs with an emphasis on the design and fabrication of three-dimensional (3D) nanostructured electrodes for efficient photoanodes and counter electrodes (CEs). By increasing QD loading at photoanode and catalyst loading at CEs, enlarging solid-liquid interface to reduce charge transfer resistance, facilitating charger transport and mass transfer, and enhancing the light harvesting, 3D nanostructured electrodes have demonstrated their promising potentials for the construction of efficient photoanodes and CEs. Together with the efforts on the surface engineering to inhibit the interfacial charge recombination and the exploration of efficient quantum dot absorbers and electrolytes, such 3D nanostructured photoanodes and CEs will open up great opportunities to achieve 1

These authors contributed equally. 1

high-performance QDSSCs with industrially appealing PCEs and stability for practical applications. Graphical Abstract Quantum dot sensitized solar (QDSSC) has been considered as a promising candidate for the low-cost third-generation photovoltaics due to the unique optoelectronic properties of quantum dot light absorbers. Over the past years, QDSSCs have witnessed tremendous progress with a rapid rising of the power conversion efficiency from sub-5 % in 2010 to 11.6 % in 2016. Herein, we present a comprehensively review on the recent progresses in QDSSCs with an emphasis on the design and fabrication of three-dimensional (3D) nanostructured electrodes for efficient photoanodes and counter electrodes (CEs). By increasing QD loading at photoanode and catalyst loading at CEs, enlarging solid-liquid interface to reduce charge transfer resistance, facilitating charger transport and mass transfer, and enhancing the light harvesting, 3D nanostructured electrodes have demonstrated their promising potentials for the construction of efficient photoanodes and CEs. Together with the efforts on the surface engineering to inhibit the interfacial charge recombination and the exploration of efficient quantum dot absorbers and electrolytes, such 3D nanostructured photoanodes and CEs will open up great opportunities to achieve high-performance QDSSCs with industrially appealing PCEs and stability for practical applications. Keywords: QDSSCs; QDSCs; nanostructures; quantum dots; photoanodes; counter electrodes

1 Introduction With continuous increase of global energy consumption, new initiatives that can advance the existing energy infrastructure and offer sustainable and clean energy are essential ways to meet the huge energy demand and address the environmental crisis caused by the consumption of conventional carbon-based energy sources. Solar energy is one of the most abundant, sustainable and green energy on the planet. In the form of optic-electric or optic-thermal conversion, solar energy is considered to be a major energy source for mankind in the near future. Solar cell is the device that use photovoltaic effect to directly convert solar light into electricity. As the third 2

generation solar cells, quantum dot solar cells have attracted enormous attention due to the potential of boosting the energy conversion efficiency beyond the Schockley-Quesisser limit of 31 % for single p-n junction solar cells [1]. Quantum dots-based photovoltaics have two types of cells configurations: (i) solid-state semiconductor heterojunction solar cells (i.e. all-solid-state quantum dot solar cells) and (ii) liquid junction solar cells (i.e. colloidal quantum-dot-sensitized solar cells (QDSSCs)) [2]. So far the world record power conversion efficiency (PCE) of all-solid-state quantum dot solar cells is 11.3 %, validated by the National Renewable Energy Laboratory [3]. The colloidal quantum–dot-sensitized solar cells have achieved 11.6 % of PCE on a basis of the best laboratory devices in Zhong’s group in 2016 [4]. QDSSC is a straightforward extension of dye-sensitized solar cell (DSSC) by inheriting its sandwiched device structure consisting a photoanode, infiltrating liquid electrolyte and a counter electrode. Inorganic semiconductor quantum dots instead of conventional organic dyes are used as photosensitizers in QDSSCs and are expected to achieve a higher PCE by taking advantage of the inherent outstanding properties of semiconductor quantum dots, for example, i) tunable bandgap and light absorption to better match solar spectrum [9-11]. In semiconductor nanocrystals, the quantum confinement effect gives rise to discrete electron and hole states, which can be precisely tuned by varying particle size, as shown in Figure 1a. ii) multiple exciton generation (MEG) phenomenon [8,12-14]. As indicated in Figure 1b, the process of MEG in QDs can convert a high-energy photon into multiple electron-hole pairs instead of single pair in bulk material, which is expected to generate higher photocurrents in device. iii) high extinction coefficient, resulting in high light absorbing efficiency. iv) large dipole moments to enhance charge separation. v) hot carrier collection to minimize the thermalization loss [15], etc., which makes it possible for QDSSCs to break through the theoretical PCE of 31 % for single-junction solar cells. Thanks to these merits, QDSSC is considered an excellent candidate for the third generation solar cell.

3

Fig 1. (a) QDs match the sunlight spectrum absorption by varying size. The discreteness of electon and hole states precisely control the size of QDs [5,6]. (b) Schematic diagram of multiple exciton generation. The top image in left, an electron can transfer energy to more than one exciton. The down image show an electron promotes to a high energy state (blue) plus the “hole” vacated by the electron (red), generating the original exciton (now dark green/red) and a new exciton (light green/orange) after MEG [7]. The right image shows the specific process of multiple electron-hole pair (exciton) generation (MEG) in quantum dots [8].

1.1 Working mechanism of QDSSCs A classical QDSSC is composed of quantum-dot-sensitized photoanode, electrolyte and counter electrode. As shown in Figure 2a, photoanode is commonly fabricated by printing a mesoporous semiconductor oxide layer (TiO2, ZnO or SnO2) on a transparent conductive oxide (TCO) substrate, followed by annealing to yield a sintered film in a thickness of around 15-25 μm with 50-60 % porosity. The mesoporous semiconductor oxide film is then sensitized by semiconductor quantum dots. Polysulfide solution containing S2-/Sn2- redox couple is widely used as the liquid electrolyte. Conventional counter electrode materials include noble metals, e.g. Pt and Au, carbon-based materials, polymer-based materials and metallic compound materials, etc.

4

Fig 2. (a) Shematic illustration of the typical structure of QDSSCs. (b) Schematic charge transfer processes in a QDSSC [16].

When QDSSCs are under light irradiation, QDs deposited on the semiconductor oxide film can absorb photons of certain energy and generate electrons from QD valence band (VB) to conduction band (CB). The carrier generation leads to a series of charge transfer processes afterwards, as shown in Figure 2b, i) The excited electrons on the CB of QDs inject into the CB of semiconductor oxide (i.e. TiO2) (Reaction 1). ii) Photogenerated electrons then transfer from TiO2 nanoparticle network to TCO layer and flow to the external circuit. (Reaction 2). iii) QDs are regenerated to their original state by transferring photogenerated holes to the redox couple (Reaction 3). iv) Concurrently, electrons from the external circuit reach the oxidized Sn2- ions on CE surface, reducing redox couple in the electrolyte from Sn2- to nS2- (Reaction 4). The above electron transfer flows are the favorable processes for QDSSCs, however, additional unfavorable processes may also take place in the form of carrier recombination. For example, photogenerated electrons can reduce Sn2- from the electrolyte due to the direct contact of electrolyte and QDs (Reaction 5) or semiconductor oxide (Reaction 6). During the whole working processes, the driving force of the electron circulation is the energy-level differences between the aligned Fermi levels of QDs/semiconductor oxide and the redox potential of electrolyte [17]. QDs +h  QDs (electron + hole)  electron (TiO2) + hole (QDs)

(1)

Electron (TiO2) + TCO  TiO2 + electron (TCO)

(2)

Hole (QDs) + S2-  QDs + Sn2-

(3)

Sn2- + electron (CE)  S2-

(4) 5

Sn2- + electron (QDs)  S2-

(5)

Sn2- + electron (TiO2)  S2-

(6)

1.2 Recent progress in QDSSCs On the 1st International Workshop on Nanostructures in Photovoltaics in 2001, Grätzel’s

group

brought

forward

the

concept

of

quantum-dot-sensitized

nanocrystalline TiO2 solar cells [18]. Since then, continuously increasing attention has been paid in the field. As demonstrated by the publications on QDSSCs (Figure 3), more and more researches on the liquid-junction QDSSCs have been reported, accompanying with the progressive increase in PCE. Especially in the past few years, benefiting from the integration of high-performance photoanode, CE and electrolyte, PCEs increased from sub-5 % in 2010 to 11.6 % in 2016 [19-21]. Recent advances on developing new QD sensitizers, enhancing surface coverage of QD on semiconductor oxide, suppressing interfacial charge recombination, and improving electrolyte will be briefly reviewed in the next sections.

Fig 3. Number of publications per year on QDSSCs (data source: ISI Web of Knowledge, topic research with “quantum dot sensitized solar cell”.

1.2.1 Quantum dot photosensitizers To achieve a high device PCE, QD photosensitizer should possess a high absorption coefficient and an appropriate bandgap to efficiently harvest solar light. Besides, an ideal QD sensitizer should show a conduction band edge that is slightly higher than 6

that of semiconductor oxide, e.g. TiO2 or ZnO, so as to accelerate the electron injection However, single-component QDs, such as CdS, CdSe, CdTe, Sb2S3, PbS, PbSe, Ag2S, etc, usually do not possess simultaneously above properities of excellent photosensitizers. Hence QDSSCs based on the single-component QDs show relatively low PCEs. To address this issue, various co-sensitized QDs or binary, ternary or quaternary QDs, e.g., CdS/CdSe [21-24], CdSe/ZnxCd1-xS [25], CdTe/ZnxCd1-xS [25], CdSxSe1-x [26], CdSexTe1-x [26], CuInSe2 [27], CuInSexS2-x [28], etc, have been designed and synthesized. These QDs systems are considered as promising options to achieve wider light harvesting ranges and higher electron injection rates. For example, when CdS/CdSe hybrid QDs with a stepwise structure are used, Fermi level alignment of the two QD components redistributes the electrons in the system. This is supposed to trigger band edge downward shift in CdS and upward shift in CdSe (Figure 4). The electron redistribution not only enhances the driving force for the electron injection from CdSe to CdS QDs then to TiO2, but also benefits the hole-recovery in both inner and outer QD layers. Therefore, the PCE of CdS/CdSe co-sensitized QDSSCs was boosted up to 4.22 %, which is 3 times that of CdS-sensitized QDSSCs [21].

Fig 4. Schematic diagram of Fermi level alignment [21]. (a) Relative band edges of TiO2, CdS, and CdSe in bulk. (b) A proposed band edge structure for TiO2/CdS/CdSe photoanode after the electron redistribution between CdS and CdSe interface (termed as Fermi level alignment).

Moreover, core/shell QD systems can provide fast charge separation channels, thus reduce charge recombination rate. For instance, QDSSCs based on CdTe/CdSe QDs with type-II core/shell structure achieved a record PCE of 6.67 % in 2013 [29]. ZnTe/CdSe QD-sensitized QDSSCs exhibited an even higher PCE of 7.17 %, benefiting from the much larger CB offset compared to that of CdTe/CdSe (1.22 vs 0.27eV) as shown in Figure 5. The augment of band offset increases the charge accumulation across the QD/TiO2 interface under illumination and induces the 7

stronger dipole effects, which upward shifts TiO2 CB edge after sensitization and thus results in the enhancement of the photovoltage of device [30].

Fig 5. (a) Schematic diagram of the bandgap and band offsets at the interfaces between bulk ZnTe/CdSe and CdTe/CdSe. (b) J-V curves of QDSSCs [30].

Among photosensitizer materials, Cd- and Pb-based QDs are most widely used, owing to their unique optical properties, strong and tunable photo-luminescence [2]. However, high toxicity of elements Cd or Pb strongly restricts their practical applications. To overcome the obstacle, “green” alternatives that based on Cd- or Pb-free QDs, such as CuInS2/ZnS [27,31,32], CuInSexS2-x [28,33] are developed in recent years. PCEs of 6 ~ 8.1 % has been realized with the “green” QDs [4,32]. Hyeon et al. reported copper-indium-selenide (CISe) QDSSCs with controlled ZnS overlayers yielded a conversion efficiency of 8.1 % [32]. Very recently, Zhong et al. developed Cd or Pb-free quaternary Zn–Cu–In–Se alloyed QDs in a size of ∼4 nm with an absorption onset extending to ∼1000 nm. Benefiting from such small particle size, these quaternary QD sensitizer could be efficiently and quickly immobilized on TiO2 film electrode. Compared with ternary Cu–In–Se alloyed QDs, the incorporation of Zn in QDs raised the CB edge, which favored the photogenerated electron extraction and thus resulted in higher photocurrent. Together with the suppression of charge recombination at photoanode/electrolyte interfaces by the alloyed structure which improved the photovoltage, QDSSC with these eco-friendly Zn–Cu–In–Se QDs achieved the certified record PCE of 11.61 % [4].

1.2.2 Surface coverage of QD on semiconductor oxide Efficiently loading QDs on semiconductor oxide film to achieve a high surface 8

coverage is essential to enhance photocurrent and PCE of QDSSC. Generally, techniques to load QDs on semiconductor oxide include successive ionic layer adsorption and reaction (SILAR), chemical bath deposition (CBD), electrophoresis deposition and linker-assisted binding, etc [34-38]. SILAR and CBD are in-situ growth methods. Electrophoresis deposition and linker-assisted binding are usually used as ex-situ methods to load pre-synthesized QDs. In-situ growth of QDs on semiconductor oxide film can ensure the intimate contact, thus facilitate the electron injection. However, these approaches commonly suffer from non-tailored QD sizes and poor crystallinity, causing the relatively low PCE of QDSSCs. In contrast, QDs with uniform and controllable sizes and high crystallinity can be easily synthesized via hot injection method. The pre-synthesized QDs can be subsequently loaded via ex-situ methods. Parkinson et al. demonstrated that the ex-situ QDs loading route, e.g. linker assisted binding method was effective to efficiently load QDs on TiO2 film in 2010 [39]. However, it is still challenging for ex-situ approaches to form a homogeneous QD layer on semiconductor oxides owing to the large QD size relative to the pore size in TiO2 mesoporous film and the long alkyl ligands capped on QD surface [37,40]. Recently, Zhong’s group developed a method to increase the surface coverage of QDs on TiO2 film by exchanging long alkyl ligands into short ligands [37]. The initial long alkyl ligands, such as alkylphosphines, alkylphosphine oxide, alkylamines, etc [41], are first replaced by bi-functional short linker, including 3-mercaptopropionic acid (MPA),

2-mercaptoacetic

acid

(TGA),

mercaptoalkanoic

acid

(MAA),

methoxybenzoic acid (MBA) or cysteine (CYS) [42-44]. The thiol group (-SH) of these bi-functional linker can effectively bind to QD surface while carboxylic acid groups (-COOH) of the linker can selectively attach onto TiO2 surface, leading to the efficient loading of QDs on TiO2. The application of ligand exchange technique significantly improved the PCEs of QDSSCs from 5.32 % to 6.76 %, and to 7.17 % in CdS/CdSe [45], CdTe/CdSe [29], and ZnTe/CdSe sensitized QDSSCs, respectively [30].

1.2.3 Interfacial charge recombination Charge recombination occurring at photoanode/QD/electrolyte interface will deteriorate the performance of QDSSCs and should be inhibited during the device fabrication [46-48]. The continuous operation of QDSSCs depends largely on the 9

efficient transport of charge carriers (electrons and holes). An ideal charge transfer process is that photo-excited electrons are injected to the CB of TiO2 from QDs, followed by the transportation to CE through the external circuit. Simultaneously, holes are reduced by redox couple in electrolyte and the redox pairs are regenerated by the electrons from CE. The quick separation/transport of electrons/holes and regeneration of redox couple are necessary to boost the PCE. But charge recombination inevitably takes place at the interface of TiO2/QD/electrolyte during the process, which hinder and neutralize effective electron transport, thus downgrades the device PCE [49,50]. The unfavorable charge recombination mainly happens through the following ways. i) The surface trap defects of QD, such as lattice strain and dangling bonds, can capture the excited electron. The development of high-quality QD sensitizers is an effective way to reduce such surface trap defects-related recombination in QDSSCs [49]. As mentioned above, high-temperature synthetic method can not only prepare high-quality QDs with less trap state density but also tune the electron properties of QDs. ii) The excited electron on the CB of TiO2 or QDs can recombine with oxidized couple of electrolyte. It was reported that such recombination could be inhibited by overcoating a thin layer of inorganic materials, including ZnS, Al2O3, MgO or SiO2, etc [42,51-54]. The inorganic coating layer not only plays a physical barrier to preventt the direct contact of the excited electron and the oxidized electrolyte but also passivates the surface trap state of QD, causing the significant increase of PCE. ZnS are most widely used in the past few years. As shown in Figure 6, ZnS shell on QDs apparently increases their photoluminescence and facilitates the electron injection from the competitive radiative recombination, which is ascribed to the reduction of trap state density in the presence of ZnS shell [55]. Moreover, ZnS shell plays a potential barrier owing to its large bandgap which gives the so-called “type-I” band alignment to push up the Fermi level, and thus appreciably increases open circuit voltage (Voc). Very recently, Zhong et al. designed a novel sequential inorganic ZnS/SiO2 double barrier coating on QD-sensitized photoanode [56]. The quantitative analysis on the distribution of empty states evidenced that SiO2 coating on ZnS shell further reduced the density of surface states by one additional order of magnitude, compared to the single inorganic barrier coating layer, such as ZnS or SiO2, thus significantly reduced electron dissipation at outermost surface. Moreover, ZnS/SiO2 double inorganic coating improved the recombination resistance and electron 10

diffusion length, which strongly inhibited interfacial recombination process in QDSSCs. Simultaneously, back recombination at the oxide surface was strongly suppressed and cell stability was improved. As a result, CdSexTe1−x QD sensitized QDSSCs exhibited a certified record efficiency of 8.21 %, corresponding to a rise of 20 % from the previous record efficiency of 6.8 % [26].

Fig 6. Radiative recombination and trapping in surface/interfacial states (SS) for photogenerated carriers in QDs: (A) in colloidal dispersion, (B) after attachment to TiO2, (C) followed by ZnS treatment. The arrow width qualitatively indicates the number of carriers involved in the corresponding pathway [55].

1.2.4 Electrolyte Since the I-/I3- redox couple which is commonly used in DSSCs is corrosive to QDs and thus causes the rapid decrease in photocurrent and PCE, a majority of QDSSCs are currently using polysulfide solution containing S2/Sn2- redox couple as electrolyte instead [42]. Although several new electrolytes were tried and gave higher Voc in comparison with S2-/Sn2- redox couple, the overall PCEs were still very poor. For example, [Co(bpy)3]2+/[Co(bpy)3]3+ redox system as electrolyte instead of polysulfide increased Voc and fill factor (FF) due to the lower redox level, but short-circuit current density (Jsc) and PCE of QDSSCs is less than 3 mA cm-2 and 1 %, respectively [57]. Fe2+/Fe3+ and Fe(CN)65-/Fe(CN)64- redox systems were also studied in CdS sensitized QDSCs, but PCE was also very low [58]. Although S2-/Sn2- redox system has been using as a preferred choice to achieve the highest PCE, the exploration of new electrolyte is highly desirable to push device PCE to a new stage. Very recently, Meng et al. introduced fumed SiO2 into polysulfide electrolyte to create an energy barrier for electron injection from the QDs or TiO2 into the electrolyte. As a result, CdSexTe1-x QDSSCs based on SiO2 modified electrolyte exhibited higher electron collection efficiency (98 %) and longer electron lifetime, and achieved a certified PCE of 11.3 % [59]. 11

2. Overview of three-dimensional nanostructured materials for QDSSCs Distinct from other reviews in QDSSC field, this review will focus on the application of three-dimensional (3D) nanostructured materials in QDSSCs. The term of “three-dimensional nanostructured” was used in this review to cover the materials in two aspects: 1) the materials are assembled by one- or two-dimensional (2D) nanostructures, or their combination as building blocks to offer three-dimensional structures; 2) the materials possess both an electron transportation network and a mass transfer network which are inter-penetrated or combined in three dimensions. To simultaneously achieve efficient electron transportation and mass transfer is essential for a high-efficiency QDSSC. Compared with bulk materials, 3D nanostructured materials have shown various superior properties, including high surface area, improved carrier transport and mass transfer, etc. Numerous successful applications using 3D nanostructured materials have been demonstrated in many research fields, e.g. heterojunction catalysis, piezoelectrics, chemical and biological sensing, lithium-ion batteries, microbial fuel cells, capture of circulating tumor cells, solar cells, etc [60-69]. QDSSC is a complicated system which combines a series of processes including light harvesting, carrier separation and recombination, charge transport and catalytic redox reaction. 3D nanostructured materials can play significant role in improving each process or resolving issues therein. As illustrated in Figure 7, 3D nanostructured materials for photoanodes and counter electrodes (CEs) in QDSSCs commonly originate from the assembly of one-dimensional building blocks (nanorods, nanowires, nanotubes, nanobelts, etc.), two-dimensional

building

blocks (nanosheets, nanoflakes, nanoplates etc.), or their combinations at different levels. Besides inheriting the features from the building blocks, 3D nanostructure architectures allow for increasing QD loading at photoanode and catalyst loading at CEs, enlarging solid-liquid interface, facilitating charger transport and mass transfer, and enhancing the light trapping, opening up opportunities for fabricating efficient photoanodes and CEs to boost PCEs of QDSSCs.

12

Fig 7. Schematic diagram of the assembly of 3D nanostructured materials from building blocks at different levels and the desirable features for the application in QDSSCs.

Different from the existing review papers on QDSSCs, the benefits from 3D nanostructured materials as building blocks for electrodes will be briefly summarized here. Recent progresses in photoanodes and counter electrodes based on 3D nanostructured materials will be comprehensively reviewed in the Part 3 and 4, respectively. Potential structural design and materials functionalization towards highly efficient QDSSCs will be also discussed later.

2.1 Improving QD loading at photoanode Different from bulk, QDs are able to show continuously varied bandgaps by adjusting the crystal sizes in several nanometer scales with potential to best match solar light harvesting. However, such small size renders QDs larger surface to volume ratio, leading to a large amount of surface defects. These surface defects are likely to act as the non-radiative recombination centers, limiting carrier diffusion length to between tens to hundreds of nanometers [70]. Such film thickness is insufficient for solar light absorption. Thus mesoporous semiconductor oxide (e.g. TiO2) nanocrystal film with a thickness of 10-25 μm are introduced as the electron acceptor, QD loading is much increased. Although inherited from dye-sensitized solar cells, QDSSCs with 13

mesoporous TiO2 layer still hold the record of device PCE [4]. Other 3D nanostructured materials, such as nanowires, nanotubes, nanorods and hierarchical structures can not only facilitate QD loading compared to mesoporous thin film due to larger interspaces for mass transfer and achieve relatively high loading, but also provide additional merits in light absorption, carrier transport and mass transfer as discussed in details later.

2.2 Improving catalyst loading at CE Similar to QD loading at photoanode, the catalysts loading and its activity at CE also significantly influence the device performance. Since the catalytic performance is highly related to the number of catalytically active sites, 3D conductive nanostructured electrodes can not only provide conductive network with higher surface area to load more catalysts, but also facilitate mass transfer due to 3D structure, showing great potentials as CEs of QDSSCs. Generally, 3D nanostructured CEs can either be achieved by the self-assembly of low dimensional nanostructured catalysts, e.g. Cu2S, PbS or loading them on 3D conductive scaffolds, e.g. 3D graphene, carbon nanotubes, and other 3D conductive nanostructure arrays, etc [71-73]. Sufficient catalytic sites exposed to the electrolyte on 3D nanostructured CE can efficiently regenerate the oxidized redox pairs by offering the electrons from the external circuit so as to improve the device performance of QDSSC.

2.3 Enlarging solid-liquid interface Two processes are involved during the photo-excited holes transfer from photoanode to CE: i) oxidization of S2- to nS2- at photoanode and ii) reduction of nS2- to S2- at counter electrode. Kinetics of the two processes are highly related to the contact area of the solid-liquid interface. 3D nanostructured materials offer enlarged solid-liquid interface than thin film structure, thus benefit the holes transfer. For example, the widely used 3D meso-structured TiO2 photoanode shows excellent hole trapping and scavenging ability by S2- [16]. And the result from our group shows that 3D nanostructured counter electrode, can significantly reduce the charge transfer resistance between solid-liquid interfaces, as discussed late [74].

14

2.4 Enhancing light harvesting 3D nanostructured electrodes, such as nanowire and nanotube arrays-based electrodes, have been proved to strongly promote light trapping property. Localized light performs multiple scattering within 3D nanostructures, leading to the significant increase in light absorption by photoactive materials. The light trapping and absorption efficiency are closely dependent on the geometrical dimensions and architectures of 3D nanostructures [75]. For example, enhanced short-circuit current has been demonstrated in silicon nanowire array solar cells [76]. QD-coated wide bandgap semiconductor nanowires and nanotubes, e.g. TiO2 and ZnO etc., have also been deliberately designed to construct QDSSC photoanodes and achieved high photocurrents due to the enhanced light harvesting [44-46,49,77-79].

2.5 Facilitating charge transport The QDSSC performance is highly dependent on charge transfer dynamics in photoanode and CE. On one hand, since QDs on the photoanode commonly show large charge transfer resistance due to the existence of a great amount of grain boundaries and surface states, a special expressway for photo-excited electrons transport is extraordinary important and essential to boost the PCEs of QDSSCs. On the other hand, metal sulfide catalysts frequently used on CE suffer from relative larger resistance that will retard the electrolyte regeneration by extracting the holes. 3D nanostructured electrodes with high conductivity, e.g. single-crystalline TiO2 or ITO nanowires arrays and carbon nanotubes arrays, can effectively act as current collectors to efficiently transfer the photo-generated carriers to the electrodes.

2.6 Facilitating mass transfer As mentioned, 3D nanostructured electrodes, especially for those composed of 1D nanostructure (e.g. nanowire, nanotube, nanorod, and their hierarchical composite) arrays usually possess larger interspaces compared to planar or mesoporous thin film electrodes, which will facilitate mass transfer to benefit not only QD loading at photoanode and catalysts loading at CE, but also electrolyte transfer for accelerating carrier transportation.

15

3 Three-dimensional QDSSCs

nanostructured

photoanodes

for

One of the major differences between QDSSCs and thin film solar cells is the separation of the roles of photo-excited carrier generation and transport in different materials [80]. For QDSSCs, the carrier generation happens at QDs under illumination. The subsequent carrier transport takes place at the semiconductor oxide for electrons and the polysulfide electrolyte for holes. All these processes occur at the photoanode, making it the key component of a QDSSC. A highly performed photoanode should possess several features: (i) Sufficient loading of QDs with a high absorption coefficient and a wide light harvesting range; (ii) The semiconductor oxide scaffold with preferred features, e.g. (1) little absorption in visible range and good light scattering capability to maximize solar light utilization, (2) large specific surface areas for QD sensitizer anchoring, (3) appropriate conduction band edge position relative to that of QDs for effective electron injection, (4) high electron mobility for fast electron transport, (5) open structure to favor the electrolyte infiltration, etc.; (iii) High quality interfaces between QDs and semiconductor oxide to facilitate the carrier separation and suppress the carrier recombination. Photoanode composed of 3D nanostructure materials will well match these requirements and thus show excellent device performance. In this section, we will briefly mention recent progresses on the photoanodes of QDSSCs with the emphasis on the morphology and structure design, especially 3D nanostructured photoanodes, to meet the above-mentioned requirements thus enhance the device performance. In the QDSSC field, a great deal of effort has been made on the development highly performed sensitizers to enhance light harvest ability [26,32]. However, designing photoanode semiconductor film with preferred configuration and composition for photoanode is also important to enhance the photovoltaic performance by increasing QD loading, shortening charge transfer pathway and inhibiting charge recombination [81-83]. To date, TiO2 mesoporous films are widely used as the electron acceptor and scaffolds of QDs, Although QDSSCs inherit the structure of DSSCs, the optimal photoanode structure for DSSCs is not suitable for QDSSCs due to the significant difference between molecular dyes and QD sensitizers. In terms of TiO2 mesoporous films, disputes exist on the thickness of photoanode film [78,79,84-86], and TiCl4 treatment [87,88], etc. Therefore, Zhong et al. systematically 16

investigated the influence of the configurations and structures of mesoporous TiO2 film on the device performances [89]. (i) TiCl4 treatments were performed either on a bare FTO glass or on a printed mesoporous TiO2 film via the hydrolysis of a TiCl4 aqueous solution [56,90]. The results showed that the TiCl4 treatment has no obvious effect on Voc, but can enhance Jsc and FF. (ii) The TiO2 layer deposited on FTO substrate can act as a blocking layer to reduce electron back transfer from FTO to electrolyte; and that on TiO2 mesoporous films can passivate the surface defects of nanocrystalline TiO2 to reduce recombination rate at the TiO2/electrolyte interface. (iii) Increasing the thickness of TiO2 mesoporous layer in a certain range can enhance the loading amount of QDs and thus solar light harvesting ability, but the excessively thicker TiO2 layer unfavorably extends the electron transfer pathway and increases the number of grain boundaries, which increases the charge recombination. Therefore, the appropriate thickness of photoanode film is the balanced result between light harvesting and effectively electron transfer. CdSe-sensitized QDSSCs with an optimized mesoporous film thickness of 9 μm achieved the highest PCE of 5.55 % [89]. (iv) The porosity and pore size of TiO2 film have a crucial influence on the loading amount of QD sensitizer, which is mainly determined by the content of ethyl cellulose (EC) in the precursor paste [84,91]. It was found that the highest PCE was observed on QDSSCs using TiO2 mesoporous films with a porosity of 0.59 achieved by the weight ratio of EC: P25 of 0.5:1. (v) Besides, the scattering layer composed of a mixture of TiO2 P25 and large TiO2 nanoparticles in hundreds of nanometers exhibited better photovoltaic performance than that with all large TiO2 particles. Other materials or structures were also studied to fabricate mesoporous photoanodes with reasonable device performance. For example, ZnO microsphere assembled by the oriented ZnO nanosheets were used as scattering layer to enhance the light diffuse reflection and harvesting , and thus the photogenerated current [92]. Benefiting from the improved electron transfer, QDSSC with graphene frameworks (GFs) incorporated TiO2 photoanode exhibited 33 % higher PCE. Graphene frameworks was able to remarkably enhance the continuity of charge transfer and provide multiple electron transport channels, leading to efficiently promote electron transport in TiO2-based QDSSCs [93]. Although mesoporous semiconductor nanoparticles films are mostly used in photoanodes, they have some unsatisfactory aspects. For example, (i) The random-walking and particle-to-particle hopping electron transport pathway from 17

nanoparticle film may cause the extended pathway and retard the electron transport [94-96]. (ii) A large number of defects or surface states usually exist on the nanoparticle surfaces, causing unfavorable electron trapping [97]. (iii) Numerous interparticle boundaries are involved when electrons transport in nanoparticles film, significantly increasing the probability of carrier recombination. In contrast, photoanodes composed of 3D ordered nanostructures, such as nanowire or nanotube arrays can overcome these drawbacks by offering directional carrier transport pathways and separating carrier transport pathways to efficiently facilitate charge transport and suppress charge recombination [77,98]. Especially, 3D nanostructured photoanodes assembled by the conductive building blocks, such as single-crystalline TiO2, ZnO nanorod, nanowire, or nanotube arrays, carbon nanotube arrays, and their combinations etc. can not only promote light trapping property, also effectively collect and transfer photo-generated carriers to the electrodes. [34,98-104]. Furthermore, compared with mesoporous nanoparticle films, such highly-crystallized building blocks deliver much less defects or surface trap states, thus significantly reduced charge recombination. Together with the suitable energy levels of these materials relative to that of QDs and the improved QD loading and mass transfer from 3D structures, these advantages from such 3D nanostructured photoanodes contribute to the enhancement of the device performance of QDSSCs.

3.1 3D nanostructured TiO2 photoanodes 3D nanostructured TiO2 photoanodes, including nanorod, nanowire or nanotube arrays provide straight electron transport pathways along the oriented direction instead of random particle-to-particle electron hopping likely happening on mesoporous TiO2 nanocrystal film photoanodes. Currently, the ordered TiO2 nanostructure arrays mainly contains closely packed TiO2 nanotube arrays and appropriately-interstitial TiO2 nanorod arrays [34,99,105-113]. Chen et al. reported a single-crystalline TiO2 nanorod array photoanode prepared by hydrothermal approach [111]. The configuration of TiO2 nanorod array, especially the diameter and spacing of nanorods, appreciably affects the light trapping of photoanode and QD loading. Nanorods in smaller diameter show a larger specific surface area, increasing QD loading to enhance light absorption. A suitable density of TiO2 nanorodsnot only facilitates the light trapping and improves the utilization of incident illumination but also preserves 18

enough space for QD deposition. Figure 8a-b are SEM images of TiO2 nanorod photoanode with optimal device performance. The cross-section image reveals the ordered TiO2 nanorods uniformly covered the entire surface of FTO substrate. Distinct from disordered TiO2 nanoparticle anode, the TiO2 nanorod array was highly oriented and grown along [001] direction. Consequently, the electron transport pathway in the single-crystalline TiO2 nanorods is discovered to be five timers shorter than that in TiO2 nanoparticle films [114-116]. Another type of nanostructured photoanode is TiO2 nanotube arrays, which is usually fabricated by the anodic electrooxidation of metallic Ti film to achieve tightly packed crystalline nanotubes with aligned pores [105,106,108-110]. Figure 8c-d present the typical morphology of TiO2 nanorod arrays in top and cross-sectional view, respectively. The TiO2 nanotube arrays showed a vertically oriented structure without any bundles as well as a highly uniform porous morphology with the average nanotube inner diameter of around 120 nm. The unique structure of nanotube array photoanode significantly increased the surface area for efficient QD loading compared to TiO2 nanorod arrays, since QDs could deposit on both inner and outer nanotubes surfaces [117,118]. Huang et al. converted TiO2 nanorod arrays into nanotube arrays by hydrothermally selective etching and found that PCEs of QDSSCs using TiO2 nanotube array photoanodes increased by 60 % than those using TiO2 nanorod photoanodes [117].

Fig 8. SEM images of typical TiO2 nanorod and nanotube photoanode. (a) Top view of TiO2 nanorod array. (b) Cross-sectional view of TiO2 nanorod array. (c) Top view of TiO2 nanotubes array. (d) Cross-sectional view of TiO2 nanotubes array [110,111]. 19

In QDSSCs, Jsc is determined jointly by light harvesting efficiency (ηlh), electron injection efficiency (ηinj) and charge collection efficiency (ηcc) according to the equation as Jsc= qηlhηinjηccl0, where q and l0 are constant under one-sun AM 1.5 G illumination. Commonly, light harvesting efficiency depends on two factors: one is QD intrinsic bandgap and its loading amount; the other is the light scattering ability of photoanode. ηinj and ηcc are related to the electron transport process. Although ordered TiO2 nanorod or nanotube arrays provide rapid charge transfer pathways, the overall light-to-electric energy conversion yield is still low, which could be ascribed to their relative lower specific surface areas for QD loading compared to mesoporous TiO2 nanoparticle film. Therefore, the structural modifications on a basis of these ordered nanostructures were further investigated. For example, hierarchical branched TiO2 consisted of long nanowire trunks and short nanorod branches arrays were designed and fabricated by Kuang et al. via a simple surfactant-free hydrothermal route (Figure 9) [119]. Four main advantages were claimed by such a design. (i) Specific surface area of TiO2 scaffold was significantly enlarged, which offered much more sites to load QDs and thus improved the light absorption. (ii) The hierarchical architectures led to multiple light scattering within photoanode, increasing the utilization of solar light. (iii) The TiO2 backbones and branches provided 3D interconnected electron transport pathways for enhancing the electron transport rate. (iv) A large number of TiO2 branches grown on the backbones enhanced the contact between the electrolyte and QDs, contributing to the rapid hole scavenge by redox electrolyte. Charge recombination kinetics of QDSSCs using both smooth and hierarchical branched TiO2 nanowire photoanodes were investigated by intensity-modulated photocurrent spectroscopy (IMVS) and photovoltage spectroscopy (IMVS). The QDSSC based on hierarchical branched TiO2 nanowire photoanode showed longer electron lifetime (τr) and transport time (τd) than smooth TiO2 nanowire arrays without branches (Figure 9e). The results indicated 3D hierarchical structure could effectively suppress electrons-holes recombination. The charge collection efficiency of QDSSCs were calculated according to the equation ηcc=1-(τd/τr). The hierarchically branched 20

nanowire photoanode showed a higher charge collection efficiency than smooth TiO2 nanowire array photoanode (Figure 9f). Consequently, QDSSCs with such 3D hierarchical nanostructured photoanodes achieved a higher Jsc and a total PCE [119]. Similar results were reported by Park et al, where assembling TiO2 nanotube arrays on a thin TiO2 hollow nanofiber backbones as photoanode achieved 3 times higher device PCE compared to TiO2 nanotube arrays on FTO substrate [120].

Fig 9. (a) Schematic diagram showing the preparation process of CdS/CdSe co-sensitized hierarchical TiO2 nanowire photoanode, as well as the pathways of electron injection (green arrow) and electron transport (yellow arrow). (b-d) SEM images of hierarchical TiO2 nanowire array photoanode in different magnification. (e) Electron transport time and electron lifetime. (f) Charge collection efficiency of QDSSCs based on different TiO2 photoanodes as a function of various light intensities [119].

3.2. 3D nanostructured ZnO photoanode ZnO is another widely used semiconductor oxide. Compared to TiO2, ZnO possesses similar physical properties, e.g. bandgap (~ 3.2 eV) and conductive band edge (~ -0.3 eV), but ZnO has a much higher electron mobility than TiO2 (205-1000 cm2 V-1 S-1 for ZnO vs. 0.1-4 cm2 v-1 s-1 for TiO2). Such a high electron mobility can significantly facilitate the electron transport, thus 3D nanostructured ZnO photoanodes have attracted extensive attention in QDSSCs [121,122]. 3D ZnO nanowire arrays can be obtained through several approaches [123-125]. 21

Seed-assisted

hydrothermal

synthesis

is

commonly

used

due

to

its

substrate-independence. Avdil et al. demonstrated QDSSCs with CdSe-sensitized ZnO nanowires array as photoanode [101]. As schematically shown in Figure 10, ZnO nanowires were vertically grown on FTO substrates, sensitized with CdSe QDs, and served as 3D nanostructured photoanode. Although rapid electron transfer pathways were created, QDSSCs with such a photoanode exhibited low PCE, especially the short-circuit currents in a range of 1 to 2 mA cm-2. Possible reasons are the insufficient QD loading and high carrier recombination rates at ZnO nanowire surfaces. Hong et al. improved the loading amount of CdS QDs on ZnO nanowire arrays by multiple SILAR cycles, but Jsc was still only 5.61 mA cm-2, leading to a very low PCE of 1.61 % [126].

Fig 10. Schematic diagram of QDSSCs with ZnO nanowire array phothanode [101].

The complex 3D nanostructured ZnO arrays with multiple branches were further studied for the similar reasons to those of the hierarchical branched TiO2 arrays as mentioned above. Park et al. reported a hierarchical branched ZnO nanowire array photoanode by a two-step growth (Figure 11a-b) [127]. The density of ZnO branches was regulated by tuning 1-h thioacetamide (TAA) treatment time, which determined the thickness of seed shell on ZnO nanowire backbone. The longer treatment formed a thicker seed shell, leading to denser ZnO branches. Absorbance and diffused reflectance spectra showed that the denser ZnO branches resulted in the higher light 22

scattering property for photon wavelengths between 400-800 nm by increasing optical-path lengths (Figure 11c). Diffused transmittance spectra evidenced that the denser ZnO branches favored the light harvesting due to the lower diffused transmittance (Figure 11d). Besides, the hierarchical ZnO branches provided more surface for extra QD loading as confirmed by inductively coupled plasma atomic emission spectrum (ICP-AES). Moreover, Zhu et al. demonstrated another type of 3D hierarchical ZnO array photoanode, which was composed of multi-layered and close-packed ZnO nanorod bundle (Figure 11e-f) [128]. Specifically, ZnO nanoparticles were deposited on ITO substrate as seeds to induce the growth of ZnO nanorods along different angles. After that, additional ZnO nanoparticles were absorbed on the freshly prepared ZnO nanorods for the second-round growth. With several rounds, hierarchical multi-layered ZnO nanorod photoanodes on ITO substrates were obtained. The ZnO nanorod layer grown along the vertical direction not only provided larger surface to increase QD loading, but also improved light scattering, leading to the improvement of device performance.

Fig 11. (a) Schematic preparation process of 3D hierarchical branched ZnO nanowires photoanode. (b) SEM image of branched ZnO nanowire. (c) Absorptance and diffused reflectance of hierarchical ZnO nanowire photoanodes before CdS sensitization. (d) Diffused transmittance 23

spectra of hierarchical ZnO nanowire photoanodes after CdS sensitization. The time in (c-d) refers to TAA treatment time. The longer treatment can form thicker seed shell, leading to denser ZnO branches. (e) Schematic fabrication process of multi-layered and close-packed ZnO nanorod bundles on ITO substrate (f) SEM image of 3D nanostructured ZnO nanorods at the bottom [127,128].

Despite 3D nanonstructured ZnO nanowire arrays as photoanodes hold the above-mentioned merits, they still commonly suffered from the high charge recombination due to the increased contact area with polysulfide electrolyte. Therefore, surface modification of ZnO photoanode is essential to reduce the charge recombination during the electron transport [101,129,130]. Many research results showed the charge recombination can be reduced by forming core-shell structured photoanode. Yong et al. reported a CdSe QDs sensitized ZnO/CdS core/shell nanowire array photoanode, as shown in Figure 12 [103]. Due to Fermi level alignment, the excited electron efficiently transferred from CdSe to CdS when CdSe and CdS contacted as a cascade band structure, resulting the rapid separation and transportation and thus the reduced recombination of the photogenerated charge carriers. Moreover, CdS interlayer overcoated on ZnO nanowires acted as a passivation layer, further inhibiting the recombination of excited electrons and oxidizing species in the electrolyte. As a result, PCE of QDSSCs based on such a CdSe/CdS/ZnO nanowire array photoanode increased from 1.61 % to 4.15 % with a significantly enhanced Jsc of 17.3 mA cm-2. The similar research using double-shelled CdS and CdSe cosensitized ZnO porous nanowire arrays proved that 3D CdSe/CdS/ZnO array photoanode contributed to the high Jsc and PCE [131]. Moreover, Park et al. coated a TiO2 passivation layer on ZnO nanowires to suppress recombination [127]. The impedance analysis confirmed that the recombination resistance at the interface of ZnO/polysulfide electrolyte increased after the coating. The open-circuit voltage decay measurement corroborated that the slower decay response and extended carrier lifetime were achieved on TiO2 passivated hierarchical ZnO nanowire photoanode.

24

Fig 12. Schematic diagram showing the charge-transfer processes of the QDSSC based on a CdSe/CdS/ZnO heterostructure nanowire photoanode [103].

3.3 3D nanostructured TiO2/ZnO hybrid photoanodes Besides 3D nanostructured photoanodes with single component, hybrid hierarchal nanostructured ones exhibit the potential for efficient QDSSCs by combining the advantages from each component. TiO2/ZnO hybrid nanostructured photoanodes were mostly investigated in this aspect. Kuang et al. reported two TiO2/ZnO hybrid photoanodes:TiO2

nanowire/ZnO

nanorod

(TNW/ZNR)

arrays

and

TiO2

nanowire/ZnO nanosheet (TNW/ZNS) arrays [69]. After the growth of vertically aligned TiO2 nanowire arrays, ZnO nanosheet and nanorod arrays were subsequently grown on the nanowires via a seed-assisted hydrothermal methods with different precursors. As shown in Figure 13, TiO2 nanowire showed smooth surface while TNW/ZNS contained lamellar nanosheets on the surface of TiO2 nanowire backbone, which interconnected to form a porous network. TNW/ZNR showed low-density ZnO branches grown on the TiO2 backbones (Figure 13h-i). Both TNW/ZNS and TNW/ZNR exhibited higher reflectance than the smooth TiO2 nanowire arrays between photon wavelength of 380 to 800 nm, indicating that the hierarchical nanosheets and nanorod branches improved light scattering. Comparing TNW/ZNS and TNW/ZNR, TNW/ZNS arrays showed better light-scattering capability due to the 25

higher density of hierarchical nanostructures. QDSSCs based on the hybrid 3D hierarchical TNW/ZNS photoanodes came out a significantly higher PCE than that with single-component TiO2 nanowire phototanode (4.57 % vs. 2.75 %). EIS measurements revealed the Rs of hierarchical nanostructured photoanodes were smaller than that TNW arrays due to the tighter contact with the front electrode, which enhanced FF. However, the hierarchical TiO2/ZnO electrodes gave a lower recombination resistance (R2), indicating a rapid charge recombination at the hierarchical hybrid photoanode. IMPS and IMVS measurements confirmed that the hierarchical branches increased the charge recombination rate, which hindered electron transport and shortened electron lifetime. It was anticipated that the interfacial engineering or surface passivation could be performed to relieve these issues.

Fig 13. Schematic illustrations and SEM images of TNW arrays (a−c), TNW/ZNS (d−f), and TNW/ZNR hybrid arrays (g−i) [69].

3.4 3D aligned carbon nanotube network photoanode Carbon nanotube (CNTs) is another candidate for conducting scaffold in QDSSCs due to its unique properties, including large surface area, high electron mobility and 26

superior chemical stability etc [132,133]. For photoanodes, Chen et al. designed a vertically aligned CNT (VACNT) hexagonal network on FTO substrate [134]. SEM images showed that VACNTs grew perpendicularly to the substrate with a morphology of honeycomb-shaped network consisting of uniform interconnected hexagonal microchannels in 6 μm. (Figure 14a-b). The length of the VACNTs was ~ 5 μm. This 3D aligned CNT network offered plenty of interspaces for facile electrolyte permeation to suppress the electron recombination. Although the short circuit current of a QDSSC based on VCNAT photoanode was higher than screen-printed CNT photoanode, the PCE was only 1.1 %, which could be ascribed to the high light transmittance of hexagonal network and the insufficient QD loading caused by inadequate surface coverage and film thickness. Lei et al. introduced an additional ZnO nanorod layer on VACNT arrays (Figure 14c) [135]. The ZnO nanorods grown on the hexagonal CNT channels not only largely reduced the light transmittance but also improved the QD loading amount, increasing Jsc and PCE compared with photoanode without ZnO. Moreover, compared with ZnO/CdSe nanorod photoanode without VACNT, 3D VACNT/ZnO/CdSe photoanode showed an extended electron lifetime which could be contributed by the high intrinsic electron mobility of carbon nanotubes, leading to the increase in FF and thus 40 % increase of device PCE. It should be mentioned that even if carbon nanotubes exhibited the favorable features as photoanode materials like large surface area, excellent chemical stability, and high electron mobility, the strong light absorption and significantly insufficient QD loading of carbon nanotube based photoanodes still hindered them to be efficient photoanodes compared with mesoporous TiO2 photoanodes.

Fig 14. (a-b) SEM images of 3D vertically aligned CNT (VACNT) hexagonal network [69]. (c) SEM image of ZnO nanorod branched VACNT (VACNT/ZnO) hexagonal network [135]. 27

Although the carbon nanotubes and other conductive materials were attempted to fabricate QDSSC photoanodes, TiO2 and ZnO nanostructures are still the mostly investigated electron acceptors for QDSSC photoanode because of their appropriate optical bandgap as well as CB and VB energy levels relative to the widely used QDs. Looking at the recently reported QDSSCs (see Table 1), it could be found that the device PCEs are quite different between QDSSCs with nanostructured TiO 2 and ZnO photoanodes. Such difference could be caused by the following possible reasons: i) ZnO and TiO2 have different intrinsic physical and chemical properties. ZnO shows much higher electron mobility than TiO2 (205-1000 cm2 V-1 s-1 for ZnO vs. 0.1-4 cm2 V-1 s-1 for TiO2), leading to more efficient electron transport for ZnO based photoanodes. TiO2 is more chemical inert than ZnO in both acid and basic environment, which can influence QDSSC performance. ZnO-based photoanodes usually have more surface defects, thus causing higher surface charge recombination. ii) Different structures and morphologies were used in ZnO and TiO2 nanostructured photoanodes. iii) Different QDs and CEs were used to fabricate QDSSCs. Different QDs show different light absorption coefficient, bandgap, energy band levels and chemical stability in the electrolyte. Different CEs give different catalytic activities. QDSSCs performance with different components can be significantly different. iv) Different preparation strategies for photoanode electrode, QD loading, and CEs, as well as the different experiences in cell assembly will also cause the apparent deviation in the device performances of QDSSCs.

4 Three-dimensional nanostructured counter electrodes for QDSSCs In QDSSCs, the kinetics of charge transfer can significantly affect the device performance. Because charge transfer rate at CE is 2~3 orders of magnitude lower than that at photoanode, it is considered as the bottleneck of the whole system [16]. The unbalanced charge transfer rates lead to the accumulation of holes at the QD valance band, increasing the charge recombination rate on the photoanode [90]. Therefore, accelerating the reaction kinetics between CE and polysulfide electrolyte is one of the key factors to improve the performance of QDSSCs. The deliberately 28

designed 3D CE is capable of enhancing the charge transfer at both solid-liquid (catalyst and electrolyte) and solid-solid (catalyst and charge collector) interfaces, as well as the charge transport from charge collector to external circuit. Moreover, some 3D CEs have shown much improved chemical and mechanical stability, thus the cell lifetime. The recent achievement in 3D nanostructured CEs for QDSSCs will be reviewed hereinafter.

4.1 3D nanostructured chalcogenide counter electrodes Due to the superior intrinsic catalytic activity of metal chalcogenides for reducing polysulfides which was commonly used as the liquid electrolyte in QDSSCs, a variety of 3D metal chalcogenide nanostructures, including Cu2S [51,99,136], Cu1.8S [137], CuSe [138,139], FeS2 [140], CoS [141,142], Co9S8 [143], PbS [144-146], NiS [147-149], have been investigated to improve device performance by taking advantages of 3D nanostructured CEs.

4.1.1 3D flake-like nanostructured Cu2S/FTO counter electrodes

Fig 15. (a-b) SEM images of Cu2S/FTO CEs. (a) top-view and (b) cross-view. (c-d) Stability test of QDSSC with CdSe QD sensitized photoanode and Cu2S/FTO CE. (c) PCE curve. The inset is the J-V curve and (d) Temporal evolution of Jsc, Voc curve [150].

Among metal-chalcogenides, brass-based Cu2S is most widely used due to its simple preparation and superior electrocatalytic activity to reduce Sn2- to nS2-. Cu2S/brass 29

electrode can be simply prepared by immersing pre-cleaned brass foil into polysulfide electrolyte for only a few seconds to minutes [74,150]. However, QDSSCs using brass-based Cu2S CE usually suffered from poor stability due to the chemical instability of brass in the polysulfide electrolyte. To solve this issue, Zhong et al. successfully developed a Cu2S/FTO CE by sulfurating Cu film, which is pre-electrodeposited on FTO substrate [150]. As shown in Figure 15a-b, the Cu2S film is composed of 2D flake-like structures in a thickness of nanometers. It was found that this nanostructured Cu2S/FTO CE not only preserved a large specific surface area for high catalytic activity, but also exhibited improved chemically stability in electrolyte due to the inert nature of FTO substrate instead of Cu foil. After optimizing the electrodeposition conditions, the Cu2S film can be grown securely on FTO substrate, ensuring the long-term stability of the device (Figure 15c-d).

4.1.2 Hierarchical nanoplates-microspherical chalcogenides/FTO counter electrodes

structured

Besides Cu2S/FTO CE, hierarchically nanostructured CuS/FTO (h-CuS/FTO) CEs have also been developed to fabricate QDSSCs with good PCE and stability. For example, the hierarchical CuS film composed of microsphere-like CuS nanostructures with concaved cavities and nanosheets was prepared on the FTO glass by one-step electrochemical deposition (Figure 16a) [151]. This unique structure provided plenty of active sites for efficient electrocatalytic reduction of polysulfide electrolyte. Importantly, this electrodeposited CuS film was reported to intimately adhere to FTO substrate without detach issue, benefiting for charge transfer and device stability (Figure 16b). As a result, QDSSCs with CdS/CdSe sensitized photoanodes and such h-CuS/FTO CEs exhibited PCE higher than that with Cu2S/brass CE and Pt CE and, more interestingly, a negligible PCE decay for 10 days under continuous illumination. (Figure 16c) To further reduce the overpotential and improve the electrocatalytic activity of CE, Lee et al. further synthesized tetragonal Cu2SnS3 hierarchical nanoflowers on FTO substrate with enriched nanoflakes for exposing more active sites compared with rhombohedral Cu1.8S (Figure 16d-e) [152]. Electrochemical impedance spectroscopy (EIS) measurements evidenced that Cu2SnS3/FTO CE exhibited appreciably decreased Rct value compared with Cu1.8S/FTO CE (11.4 vs. 6.2 Ω cm2) (Figure 16f), leading to an improved PCE. Moreover, nanoplates micropherical structured 30

hierarchically nanostructured Cu2ZnSnS4 CE with a high roughness factor and electrocatalytic activity has also been reported [153].

Fig 16. (a) SEM image of the integral pattern morphology of hierarchical CuS nanoplates-structured CE. The inset is zoom-in SEM image. (b) Cross-sectional SEM image of CuS nanoplates structured CE, showing they are adhered intimately on FTO. (c) Stability measurement of QDSSC with h-CuS/FTO CE. (d) SEM image of Cu2SnS3 hierarchical nanoplates-structured microsphere CE. (e) SEM image of hierarchical Cu1.8S nanoplate CE. (f) Nyquist plots of FTO, Cu1.8S/FTO, Cu2SnS3/FTO symmetric cells with polysulfide redox electrolyte [151,152].

4.1.3 Chalcogenide 1D nanostructure arrays as 3D ordered Counter Electrodes One of the effective routes to fabricate 3D nanostructured CEs is assembling 1D nanostructures, such as nanorods, nanowires, nanotubes, etc. Since these 1D nanostructure building blocks are usually single crystalline, 3D metal chalcogenide nanostructure arrays hold the merits: i) the superior catalytic activity from metal chalcogenides; ii) significantly enlarged solid-liquid interfaces and facile mass transfer from 3D ordered nanostructures; and iii) fast charge transportation from single-crystalline building blocks. These features make them promising for efficient CEs of QDSSCs. Zhong et al. fabricated a bismuth sulfide (Bi2S3) 3D network which was composed of interconnected 2D sheets assembled by Bi2S3 single-crystalline nanorods [154]. The interconnected Bi2S3 nanorods perpendicularly interlaced together (Figure 17). It was found that the morphology of Bi2S3 nanorods could be tailored by changing the concentration of thiourea during the preparation process, which significantly 31

influenced the charge transfer resistances at both solid-liquid and solid-solid interfaces when the film was used as CE in QDSSCs. As a result, QDSSCs with such interconnected Bi2S3 single-crystalline nanorods as 3D CEs exhibited enhanced PCE compared with Pt CEs, especially after the additional decoration of Pt nanoparticles on Bi2S3 nanorods.

Fig 17. SEM image of Bi2S3 nanorod network CE. (a) Cross-sectional image. (b) Top-view image [154].

However, owing to the relatively low conductivity and high charge transfer resistance of Bi2S3 compared with Cu2S, the device PCE was still not very high [150,154]. Cobalt chalcogenide nanostructure arrays directly grown on conductive substrate are other promising candidates due to their corrosion resistance to electrolyte solution and good catalytic activity. Lin et al. reported a template method for directly growing Co9S8 hollow nanoneedle array films on FTO substrate [143]. With this method, the binding between Co9S8 arrays and FTO was enhanced compared with those by doctor blade method. Electrochemical results showed that both Co9S8 CEs with or without annealing exhibited nearly an order of magnitude higher current density (J0) than that of Pt CE, implying their better electrocatalytic abilities. Comparing the two Co 9S8 CEs, the annealed one gave slightly higher J0 (Figure 18a). The observation was consistent with their Rct in EIS measurements (2213 Ω cm2 for Pt CE, 4.59 Ω cm2 for unannealed Co9S8 CE, and 2.29 Ω cm2 for annealed Co9S8 CE (Figure 18b). Together with the increase of FF, QDSSC with annealed Co9S8 CE exhibited a PCE of 3.72 %, higher than 3.13 % for that with unannealed one and 2.12 % with Pt CE. Moreover, Sun et al. prepared CoS nanorod arrays in different lengths on ITO and used them as CE in QDSSCs with CdS sensitized ZnO photoanodes. Although the CoS nanorod arrays showed good crystallinity and uniformity, the photovoltaic performances of 32

QDSSCs were still poor, possibly due to the use of I3-/I- redox rather than polysufides as electrolyte [155].

Fig 18. Characterization and performance of Pt, unannealed Co9S8 and annealed Co9S8 CEs. (a) Tafel polarization curves, (b) Electrochemical impedance spectra, and (c) J-V curves of QDSSCs [143].

Self-standing NiCo2S4 single crystalline hollow nanorod arrays was explored by Xiao et al., which were fabricated via vulcanization, calcination and etching processes (Figure 19) [156]. The single-crystalline nature of NiCo2S4 nanorod building blocks provided fast electron transport pathways and alleviated interfacial carrier recombination. The 3D hollow nanostructures offered a high catalytically active area. Therefore, NiCo2S4 CE showed a smaller Rct and larger double-layer capacitance (Cdl) than a Pt CE in EIS measurements and a higher J0 and limited current density (Jlim) in Tafel polarization measurements, resulting in a higher overall PCE of QDSSC.

Fig 19. The Schematic illustration of the preparation of hollow metal sulfide (MSx) nanorod arrays via the vulcanization and subsequent etching process [156].

To further explore the potential of 3D nanostructure arrays as CEs for QDSSCs, more 33

complex nanostructures with an attempt to inherit merits from building blocks at different scales have also been investigated. Qian et al. designed Cu7S4 nanotubes each of which was assembled by hexagonal nanoplates (Cu7S4-HNT) (Figure 20a-b) [157]. The nanotubes was fabricated initially on a copper substrate and then transferred onto FTO to prevent the copper corrosion in polysulfide electrolyte. This Cu7S4-HNT combined the advantages from building blocks at nanoscale and 3D hollow structure arrays at microscale, such as a large specific surface area, porosity and accessible inner surface, contributing to the abundant active sites and fast mass/electron transfer. Results from EIS measurements confirmed that Cu7S4-HNT CE had a lower Rct than the Cu2S/brass or Pt CE. As a result, PCE of QDSSC based Cu7S4-HNT CE significantly increased by 27.1 % and 60.5 % compared with that with Cu2S/brass CE and Pt CE, respectively. What’s more, Cu7S4-HNT CE showed excellent long-term stability. After 1000 CV cycles in the polysulfide electrolyte, the current density remained 94.3 % of its initial value, indicating its promising application as QDSSC CE.

Fig 20. (a) SEM and (b) TEM images of Cu7S4-HNT. (c) The first 50 cycles of cyclic voltammogram (C-V) curves of Cu7S4-HNT and brass/Cu2S. (d) Current densities variation vs. C-V cycles at 1.0 V. The insert is 1000 cycles of C-V [157].

4.2 3D nanostructured carbon based counter electrodes Apart from the metal sulfide CEs, carbon materials are also explored as QDSSC CEs owing to their high carrier mobility, high corrosion resistance in the liquid electrolyte and low cost. By designing a series of 3D nanostructured carbon CEs, high catalytic 34

activity towards reducing Sn2-/S2- redox has been witnessed in QDSSCs.

4.2.1 3D Hierarchical porous carbon counter electrodes Recently, various hierarchical porous carbon nanostructures have shown the potentials as CEs for efficient QDSSCs. For instance, carbon dots grafted 3D graphene networks improved the photovoltaic performance of QDSSCs.[158] Mesocellular carbon foams (MSU-F-Cs) with a high surface area (911 m2 g-1) and large pores (~ 25 nm) were synthesized by Lee et al. using a template assisted method [159]. As shown in Figure 21a, such porous architecture provided a larger effective surface area for Sn2reduction. The interconnected pore channels accelerated the electrolyte diffusion. Consequently, mesocellular carbon foam CE improved PCEs of CdS/CdSe cosensitized QDSSCs compared with the conventional Pt and commercial carbon CE. By printing the pre-synthesized hollow core-mesoporous shell carbon (HCMSC) on FTO, Yu et al. developed a hierarchical porous carbon network as QDSSC CE [160]. As shown in Figure 21b, the HCMSC CE exhibited a hollow macroporous core in a diameter of approximately 300 nm and a mesoporous shell in 25~30 nm thick. The unique structure offered both a large specific surface area and a high pore volume, which increased the contact area between CE and electrolyte and facilitated the mass transport. Results from EIS measurements showed that HCMSC CE had the serial resistance (Rh) similar to Pt (3.96 vs. 3.76 Ω cm2), while its Rct was much smaller than Pt (8.25 vs. 49.86 Ω cm2), leading to a faster electron transfer from CE to redox couple and enhanced device performance.

Fig 21. (a) SEM images of mesocellular carbon foam CE. The inset is the zoom-in SEM image 35

[159]. (b) Schematic illustration of hierarchical porous network of HCMSC CE [160]. Typical SEM (c) and TEM (d) image of OMPC [161].

Yu et al. reported another ordered multimodal porous carbon (OMPC) CE, which was composed of porous-skeleton nanostructures with pores in both meso and macro scales (Figure 21c-d) [161]. The three-dimensional interconnected porous structure contributed to a large number of catalytic sites and fast electron/mass transfer. Therefore, QDSSC with such an OMPC CE presented an enhanced PCE of 4.36 %, while that with Pt CE and activated carbon CE only gave a CPE of 2.29 % and 3.30 %, respectively. Very recently, Zhong et al. developed a novelty mesoporous carbon (MC) CE supported by Ti mesh supported [162]. MC/Ti CE can not only provide an efficient 3D electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE, but also demonstrate high catalytic capacity due to the robust and submillimeter-thick carbon film. Herein, MC/Ti CE promote FF of CdSe0.65Te0.35 QDSSCs improve 12 % than that of Cu2S/FTO CE the due to the lower internal series resistance of cell. More importantly, MC/Ti CE show a capacity of downshifting the redox potential of polysulfide to boost a higher Voc (~ 0.1 V). Based on the above effects, MC/Ti CE contribute a certified efficiency exceeding 11 %, which opened a new strategy to further improve the efficiency of QDSSCs using MC/Ti CE or the similar CE concept. Moreover, doped carbon materials were also used for QDSSC CE. As mentioned, the additional electrons contributed by heteroatoms chemically doped into carbon network can improve the electrocatalytic properties of the intrinsic carbon [80]. Nitrogen-doped hollow carbon nanoparticles (N-HCNPs) has been reported to reduce the overpotential in the reduction of polysulfide electrolyte, resulting in a higher Voc and PCE when they were used as CE [163]. CV measurements indicated that the N-HCNPs CE exhibited strikingly stable electrochemical behavior compared to un-doped HCNPs, CNTs or Pt based CEs.

4.2.2 3D mesoporous carbon nanofiber counter electrode Besides 3D hierarchical interconnected porous structure, an open mesoporous carbon nanofiber (MCNF) arrays with tailored nanostructures were developed through a nanocasting technology by using a commercially available anodic aluminium oxide (AAO) membrane, colloidal silica as pore-making hard templates, and phenolic resin as a carbon source [164]. As shown in Figure 22, the obtained MCNF arrays was 36

composed of uniform discrete porous carbon nanofibers in an adjustable lengths of ca. 5-60 μm and diameters of ca. 180-250 nm. Interconnected open spherical mesopores of ~30 nm in diameter were well distributed in the entire CNF. The sizes of MCNFs could be tailored by the pore diameters of AAO template. The size of mesopores could be adjusted by the size of silica bead templates. Consequently, the high surface area of 921 m2 g-1 and a large pore volume of 1.50 cm3 g-1 (mesopore volume: 1.14 cm3 g-1, accounting for ca. 76 % of the total pore volume) were achieved with this MCNF arrays. Such large mesopores and pore volume were expected to facilitate the electrolyte to access the catalytic active sites, as evidenced by the small R ct and the Nernst diffusion impedance (Zw). Benefited from the 3D nanostructures combining 1D nanofiber arrays and interconnected open mesopores, QDSSC with such MCNF arrays as CE revealed a FF of 0.60 and a PCE of 4.81 %.

Fig 22. SEM images of MCNFs at low (a) and high (b) magnification. (c) TEM image of a single MCNF. (d) J-V curves of QDSSCs based on various CEs [164].

4.2.3 3D interconnected conducting polymer counter electrode 3D interconnected porous network nanostructures not only possess large specific surface area but also provide multi-direction channels to facilitate electron transport and ion diffusion. These nanostructures have shown the potential as CEs to improve QDSSC performance. Ho et al. reported three kinds of conducting polymer network based CEs, i.e. poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene (PT) and 37

polypyrrole (PPy) [165]. These 3D conducting polymer networks were prepared by directly electrochemical polymerization on ITO substrates. PEDOT films exhibited an interconnected porous structure with net-like fibers in various dimensions. PT and PPy films showed smooth surfaces and less porosities (Figure 23). Similarly, the open porous structure enabled the electrolyte penetration into the film and made more catalytic active sites accessible for Sn2-/S2- redox reaction. Therefore, QDSSCs with PEDOT porous film CE exhibited higher Jsc and PCE than that with PT or PPy CE. Although the PCEs of QDSSCs based on these CEs are still not satisfactory, this work demonstrated the possible application of conducting polymer networks as interesting CE candidates.

Fig 23. SEM images of conducting polymer CEs: (a) PEDOT, (b) PT. (c) PPy [165].

4.3 3D conducting nanostructures as scaffolds for efficient counter electrodes Different from the above-mentioned 3D nanostructured active materials as CEs, fabricating 3D conducting nanostructures as electrode scaffolds followed by integrating selected highly active nanocatalysts is an alternative and promising way to fabricate CEs with sufficiently low series resistance and high catalytic activity for electrolyte reaction. Several conducting materials including ITO nanostructure arrays, carbon nanotubes, and 3D carbon network have been investigated as such scaffolds towards efficient QDSSC CEs.

4.3.1 3D hierarchically assembled ITO nanowire arrays as scaffolds for counter electrode Brass/Cu2S CEs exhibit excellent performance in QDSSCs but usually suffer from 38

chemical and mechanical instability. Other Cu2S-based CEs prepared on FTO substrates show substantially improved device stability, but the device PCEs still lag behind those using brass/Cu2S CEs mainly due to the unwanted increase in device series resistance. To achieve both high PCE and stability, Hu et al. designed and fabricated a series of ITO@Cu2S nanowire arrays directly on FTO substrate as efficient QDSSC CEs (Figure 24) [74]. Such ITO@Cu2S nanowires consisted of tin-doped indium oxide (ITO) nanowire cores as conductive scaffold and Cu2S nanocrystal shells for efficient catalytic reaction. This intentionally designed 3D hybrid system can act as efficient CEs of QDSSCs with the following features: (i) ITO nanowire array cores provide a three-dimensional conductive network as an efficient carrier highway. (ii) The chemically inert nature of ITO nanowire and FTO substrate prevents the potential corrosion from the liquid electrolyte. (iii) Degenerated n-type ITO and degenerated p-type Cu2S can form tunnel junction with carrier transport path shorter than 100 nm, effectively reducing charge transfer resistance at solid-solid interfaces. (iv) The intimate and defect-free interface between ITO and Cu2S nanocrystals inhibited the charge recombination and benefited the charge transfer at solid-solid interfaces. (v) A large number of Cu2S nanocrystals uniformly distributed on ITO nanowire arrays without aggregation, offering plenty of accessible catalytically active sites and facilitating charge transfer at solid-liquid interfaces. As a result, PCE of QDSSC with such ITO@Cu2S nanowire CE increased by 84.5 % and 33.5 % compared with that with planar Au and Cu2S CEs, respectively. To further understand the influence of ITO/Cu2S tunnel junctions on the device performance, a set of ITO@Cu2S nanowire array CEs were fabricated by varying the deposition methods and thus the morphologies of Cu2S shells. The deposition methods include SILAR, physical vapor deposition (PVD), and different cation-exchange (EX) methods with varying procedures [166]. As shown in Figure 25, it was found that the deposition methods significantly affected the morphologies, the surface coverage of Cu2S shells on ITO nanowire cores, the interfaces of ITO and Cu2S tunnel junction, and thus the PCEs of QDSSCs with these CEs. PVD via the magnetron sputtering delivered a homogeneous Cu2S shell with a smooth surface on ITO nanowire cores, leading to the relatively small surface area of Cu2S catalysts (Figure 25d). Close TEM observations revealed that the interface between ITO and Cu2S was very poor with enormous defects. These features made QDSSC with such 39

Fig 24. Scheme for the direct preparation of ITO@Cu2S nanowire arrays on FTO glass. (a) Sputtering Au catalysts on FTO substrate; (b) CVD synthesis of ITO nanowire arrays; (c) Chemical bath deposition of CdS shell on ITO nanowire arrays; (d) Cation exchange to form ITO@Cu2S nanowire arrays; (e) Calcination for improving the interfaces between ITO nanowire core and Cu2S nanocrystal shell; (f) Typical low-magnification SEM image of ITO nanowires; (g) SEM image of ITO@Cu2S nanowires. (h) HRTEM image showing the interface of Cu2S nanocrystal shell and ITO nanowire core. The red line marked the intimate and defect-free interface [74].

CE exhibit the worst performance with the lowest FF of only 31.67 % and the lowest of Jsc, as well as the poor stability. The SILAR method could realize a full coverage of Cu2S nanocrystals on ITO nanowire cores, but Cu2S nanocrystals were not grown in a well-defined way, causing an inevitable agglomeration (Figure 25c). Large aggregates in micrometers could be easily found on the surface of ITO@Cu2S, which was unfavorable for the charge transfer and thus degraded the device performance. Two cation exchange methods gave two kinds of Cu2S nanocrystal shells with different morphologies: one with discrete Cu2S nanocrystals decorated on ITO nanowire cores and the other with continuous Cu2S nanocrystal shells on ITO nanowire cores. Both cation exchange methods achieved good interface between ITO and Cu2S. The QDSSC measurements showed that ITO@Cu2S nanowire array CEs with a continuous Cu2S nanocrystal shell in optimal 55 nm thick exhibited the best QDSSC 40

performance. PCE was increased by 11.6 % and 16.5 % compared with that with the discrete Cu2S nanocrystal shell and the classic brass/Cu2S CE, respectively. This work highlighted that although the same ITO@Cu2S system was used as CEs, device performances could be significantly influenced by the morphologies of Cu2S shells and the interface between Cu2S shells and ITO nanowire cores. The delicate control on CE morphology, interfaces and the catalyst loading should be paid more attention to develop a highly efficient CE for QDSSCs.

Fig 25. (a−e) Typical SEM images of ITO and ITO@Cu2S nanowire arrays prepared with different methods: (a) ITO nanowires, (b) EX1-ITO@Cu2S, (c) SILAR-ITO@Cu2S, (d) PVD-ITO@Cu2S, and (e) EX2-ITO@Cu2S. (f) TEM image of EX2-ITO@Cu2S [166].

Beyond the progress achieved on the above ITO@Cu2S CEs, it is noted that the FFs of QDSSCs are still much lower than that with traditional brass/Cu2S CEs, which retards the further enhancement of device PCE. Because the FF is determined by the series resistance (Rs) and the shunt resistance (Rsh) of the device, optimizing ITO@Cu2S CEs to reduce Rs and raise Rsh would be an effective way to increase FF and further boost the device PCE. For this purpose, hierarchically assembled ITO nanowire arrays were designed and fabricated via simply repeating the CVD processes, followed by the growth of Cu2S nanocrystal shells. Different configurations were achieved: ITO@Cu2S-I for ITO@Cu2S nanowire arrays without branches, ITO@Cu2S-II for that with second-generation branches (Figure 26a), ITO@Cu2S-III for that with third-generation hierarchically assembled branches (Figure 26b)) [90]. The hierarchically assembled ITO nanowire arrays not only provide an efficient 3D charge transport network but also allow for depositing more 41

catalytically active Cu2S nanocrystals. Interestingly, we found that Rs decreased progressively while Rsh increased appreciably from ITO@Cu2S-I, to ITO@Cu2S-II, and to ITO@Cu2S-III (Figure 27a). The electrochemical analyses indicated that both the exchange current density and the limited current density increased, while the charge transfer resistance decreased as the increase of ITO@Cu2S branches from ITO@Cu2S-I, to ITO@Cu2S-II, and to ITO@Cu2S-III, suggesting the catalytic activities of CEs were increased in the order. (Figure 27b-d) The photovoltaic measurements draw out the consistent conclusion that the hierarchically branched ITO@Cu2S nanowire array CE exhibited the appreciably decreased Rs and increased Rsh, which contributed to the enhancement of Voc and FF in QDSSC. As a result, QDSSCs with such 3D hierarchically branched ITO@Cu2S-III CE exhibited an extraordinarily high Voc of 0.688 V, an enhanced fill factor of 58.39 % and an improved PCE of 6.12 %, which is 21.2 % higher than that with conventional brass/Cu2S CE. It was expected that the further improvement of device PCE could be achieved by using the state-of-the-art photoanodes to fabricate QDSSCs.

Fig 26. SEM images of hierarchical ITO nanaowire arrays before and after the coaxial growth of the Cu2S shell. SEM image of (a) ITO-II nanowire arrays, (b) ITO@Cu2S-II nanowire arrays, (c) ITO-III nanowire arrays, and (d) ITO@Cu2S-III nanowire arrays [90]. 42

Fig 27. (a) Resistance−Voltage (R−V) curves of QDSSCs with various ITO@Cu2S CEs. (b) Cyclic voltammograms of various ITO@Cu2S CEs. (c) Tafel polarization curves and (d) Nyquist plots from EIS measurements on symmetrical cells with various ITO@Cu2S CEs [90].

4.3.2 3D porous graphene framework as a scaffold for counter electrode Graphene has been widely used as substrate to load functional nanomaterials for a variety of applications including QDSSCs. The composites of graphene and metal sulfides as QDSSC CEs are expected to outperform the pristine metal sulfide due to the faster electron/ion transport rates and lower inner energy losses, thus leading to the enhanced electrocatalytic activities and photovoltaic performances [72,167]. However, 2D graphene sheets are subject to irreversible agglomeration due to the strong - stacking interaction among graphene sheets, which results in the aggregation of the catalysts and the loss of catalytic sites. Furthermore, the fast electron transport is expected along the graphene planes but not the stacking direction, which may cause an inefficient electron transport in 2D graphene based counter electrode. Zhu et al. interestingly reported a 3D graphene framework (3D GF) with uniformly dispersed CuS nanocrystals as shown in (Figure 28a) [72]. This 3D GF was 43

composed of the interconnected network of conductive graphene sheets, which not only prevented the - stacking of graphene sheets to achieve the uniform and high loading of active CuS nanocrystals, but also provided multi-channel and continuous electron/ion transfer pathways, leading to the improvement of the electrical conductivity and catalytic activity of CE. EIS measurements demonstrated that the GF-CuS composite CE showed the smallest Rct and Zw among all used CEs, indicating its best catalytic activity for Sn2- reduction and fastest electrolyte diffusion rate. Therefore, the CdS/CdSe co-sensitized QDSSCs based on such GF-CuS CE achieved a relatively high PCE of 5.04 % ,which was higher than those of the conventional Pt CE (3.18 %), GF-only CE (3.60 %), CuS-only CE (3.75 %) and 2D graphene-CuS composite CE (4.17 %). Moreover, it was noted that such 3D GF could securely adhere on FTO substrate, so that QDSSC based on GF-CuS CE exhibited much improved device stability than Pt or CuS CE in terms of 90 % of the initial PCE remained after 600 h test.

Fig 28. (a) The scheme illustrating the reduction of Sn2- on 3D GF-CuS CE in QDSSCs. (b) SEM image of GF-CuS composite. (c) Nyquist plots of symmetrical cells with various CEs. (d) Stability of QDSSCs with various CEs [72].

4.3.3 3D interpenetrated carbon nanotube/graphene network as a scaffold for counter electrode Hierarchical interpenetrated conductive networks as scaffolds to fabricate 3D 44

nanostructured CEs with catalytically active nanomaterials can not only combine their individual properties but also substantially increase electron and mass transfer channels due to the unique structure. Yong et al. reported a 3D interpenetrated composite as QDSSC CE, which was composed of CNT-graphene hybrid scaffold and TiN nanoparticles coated on the scaffold (Figure 29) [168]. CNT-graphene hybrid scaffold acted as a highly conductive interpenetrated network, which prevented the bundling of CNTs and the stacking graphene sheets, as well as the aggregation of TiN nanoparticles. AFM measurements confirmed that the resulting TiN/CNT-graphene CE showed a higher root mean square roughness (Rq) compared with pure TiN, TiN/graphene and TiN/CNT CEs, demonstrating the rougher surface for exposing more active sites. EIS measurements verified that TiN/CNT-graphene exhibited a 15 times higher capacitance than that of bare TiN, indicating much enlarged contact of CE and electrolyte. Benefiting from the increased active sites, improved electron pathways and electrolyte diffusion, QDSSC with TiN/CNT-graphene CE showed an improved PCE of 4.13 % vs. 3.35 % that with Au CE. The lowest Rct and Zw of TiN/CNT-graphene CE should be responsible for the enhancement of FF and Jsc. Moreover, similar 3D nanostructured CEs composed of molybdenum-containing compounds (Mo2N, Mo2C or MoS2) on the CNT-reduced graphene oxide (rGO) hybrid scaffold have also been investigated and shown their possibility as efficient CEs for QDSSCs.

Fig 29. The schematic working diagram of QDSSC with TiN/CNT-graphene CE and CdSe/CdS/ZnO-nanowire photoanode [168].

As mentioned above, 3D conductive interconnected porous networks as scaffolds of CEs can not only offer large specific surface area for loading more active catalysts, but also provide multi-direction channels to facilitate electron transport and mass transfer. Therefore, the composites of such 3D networks and metal sulfide will hold 45

the merits from such 3D scaffolds and the selected metal sulfides with high catalytic activity. As a result of the low Rct, Rs, Zw and high J0, Jlim, these 3D nanostructured CEs usually exhibit excellent performance and thus boost the device PCEs. Besides the mentioned composites of CNT/graphene and metal sulfide nanostructures, a variety of other CEs with similar structure have been explored. For example, FeS/nickel foam has been reported as a stable and efficient CE for QDSSCs with enhanced PCEs [169]. 3D carbon dot grafted graphene networks improved the photovoltaic performance of CdS/CdSe co-sensitized QDSSCs [158]. Various porous ITO film-supported metal sulfides (CuS, CoS, NiS and PbS) were also used as CEs for QDSSCs [170]. Likewise, these 3D conductive scaffolds enhanced the performance of CEs and thus photovoltaic performance of QDSSCs. In short, tailoring the structure of QDSSC CEs and improving the interfaces by means of fabricating 3D nanostructures, such as 3D hollow structures, hierarchical porous structures, or interconnected conductive network scaffolds, have been proved as effective routes to augment the accessible catalytically active sites, decrease the sheet resistance of CEs and the charge transfer resistance at both solid/solid and solid/liquid interface, accelerate the charge transportation and mass transfer, reduce the series resistance and raise the shunt resistance of QDSSC, as well as improve the stability of CEs. All these effects will contribute to the enhancement of photovoltage, photocurrent and FF, and thus PCE of a QDSSC.

5 Summary and Outlook Based on thorough understanding of the working principles and bottlenecks of a QDSSC, systematic optimization on each of its components has been carried out in the last decades, including designing and synthesizing appropriate QD absorbers, designing nanostructured semiconductor oxide scaffolds to increase QD loading amount and facilitate charge separation and transportation, developing QDs compatible electrolytes, and fabricating highly-performed CEs. Significantly improved device performances have been achieved with a current record PCE of 11.61 %, which is comparable to that of state-of-the-art OPVs and approaching that of DSCs[3]. However, there are still several limitations on QDSSCs, causing that the present device PCE of QDSSC is much lower than thin-film Si or perovskite solar 46

cells. 1) The surface coverage of QDs on semiconductor oxide film is still very low. Although Zhong’s group improved the PCEs of QDSSCs from 5.32 % to 6.76 %, and to 7.17 % by increasing the surface coverage of QDs, the highest surface coverage of QDs on semiconductor oxide is just only 34 %.[8-10] Further increasing QD loading and achieving the uniform coverage of QDs on semiconductor oxide film would be effective to further boost Jsc and device PCE. 2) QDSSC is such a complex system that involves various processes including light harvesting, carrier separation and recombination, carrier transport, ion diffusion and catalytic reaction. Breakthrough on each single process will boost the device performance. Besides, the interfaces between TCO, semiconductor oxide film, QD, electrolyte, catalytic materials or electrode on CE will also appreciably influence the device PCE. Intensively understanding and improving these interfaces to significantly suppress the charge recombination will effectively enhance the carrier collection efficiency and thus the device performance. Progressively updating on the world-record PCEs for QDSSCs in the past few years indicates the field is far from well discovered (Table 1). Mesoporous films in photoanodes of QDSSCs are inherited from DSSCs and demonstrated much success in both device structures. Different from QDSSCs or DSSCs, thin-film Si and OPVs do not use such mesoporous films while perovskite solar cells (PSCs) use much thinner mesoporous films in sub-micrometer scale. Taking MAPbI3 PSC for example, it has quite similar device structure and working mechanism to solid-state QDSSCs. It is found that a thicker TiO2 mesoporous film is not favorable for the PSC performance. This implies that such a thick mesoporous film (10 ~ 25 μm) used in the state-of-the-art QDSSC may possess several drawbacks, e.g. insufficient charge transfer ability, large number of defects or surface trap states. Considering the liquid electrolyte used in the QDSSC, the mass transfer may also be a problem in such a thick film. The absorption coefficients of inorganic QDs are on the same level to that of MAPbI3 perovskite, which means that QD films in sub-micrometer will be enough for light absorption. However, due to the much shorter carrier diffusion length of QDs (tens to hundreds of nanometers for QDs vs. tens of micrometers for MAPbI3 perovskite), the electron acceptor, TiO2 mesoporous film, 47

can effectively collected the electrons from QDs and absorb sufficient solar light by increasing the film thickness. Although great progress has been achieved with mesoporous film, it may act as a limitation factor in the further development of QDSSCs. And it is time to reconsider its functionality and design new material or structures with no issues of mesoporous film. Tailoring material morphology and structure has been proved to be able to make big breakthroughs or even innovations in research fields. Owing to the unique structures and electronic properties, 3D nanostructured materials are increasingly used in the photoelectric devices to improve the device performances. Advantages in using 3D nanostructured electrodes for QDSSCs are briefly reviewed in this paper, including (i) enhancing the loading amount of photoactive materials or catalysts, (ii) enlarging solid-liquid interface for efficient hole scavenging, (iii) promoting light harvesting ability, and (iv) improving charge transport property. It is reasonable to believe that by carefully designing and functionally-directed modifying 3D nanostructured electrodes, further breakthroughs could be achieved. Possible strategies are proposed here and illustrated in Figure 30. 1) Constructing functionally designed 3D core/shell ordered nanostructure arrays as photoanode electrode, which are composed of highly conductive 3D nanostructure cores and ultrathin but densely-coated insulating shells with a similar conduction band and an appreciably lower valence band relative to that of QDs. The 3D ordered nanostructure cores will efficiently collect and transport electrons. The insulating shells will effectively block holes and allow for electron tunneling to the conductive cores, thus eliminate carrier recombination from the unwanted back electron transfer. 2) Preparing super hydrophilic 3D nanostructured electrodes by surface modification with self-assembled monolayer. The surface tension induced electrolyte penetration issue will be thus expected to be solved. 3) Integrating specially designed 3D nanostructured photoanodes and CEs by modeling optical property of the entire system to maximize light trapping efficiency, which will enhance the light harvesting ability of QDs to the greatest extent. Besides the electrode design, the development of QDs with optimal size and absorption range to better match the solar spectrum, the synthesis of Cd-, Pb-free QDs to achieve 48

environmental friendly energy conversion devices, the application of appropriate surface passivation techniques on QDs to diminish the density of trap states, the exploration of new electrolytes to substantially increase the redox potential (vs. NHE), thus significantly enhance Voc, as well as the fabrication of highly active catalysts with suitable size or thickness at CEs to facilitate the electron transfer will be also important and necessary to further boost PCEs of QDSSCs.

Fig 30. Schematic diagram illustrating the prospective strategies for designing 3D nanostructured electrodes to boost the device performance of QDSSCs. (a) 3D core/shell ordered nanostructure arrays photoanodes with functional construction to achieve efficient electron collection and transport. (b) Surface engineering of 3D nanostructured electrodes (photoanodes and CEs) to induce electrolyte infiltration. (c) Integrating specially designed 3D nanostructured photoanodes and CEs to maximize light trapping efficiency.

In addition to lift PCEs of QDSSCs, much attention should also be paid on the device stability before carefully considering its commercialization. 3D nanostructured electrodes have shown their advantages on this aspect and are believed to further figure out the stability related issues. It is expected that by carefully designing nanostructured electrodes for photoanodes and CEs together with the efforts on the development of the stable, green, and efficient QDs and electrolytes, highly stable QDSSCs with industrially appealing PCEs will be achieved in the near future.

49

Table 1. Summary of recent QDSSCs with different configurations. Counter electrodes

Photoanodes

PCE

Ref.

Year

Au Au Au/FTO Au sputtered Au/RGO Au/Pt/RGO Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe ZnO NWs/CdS/CdSe CdS/ZnO NWs CdS/ZnO NWs CdS/ZnO NWs TiO2 NPs /CdS TiO2 NPs/CdSe TiO2 NPs/CdSe TiO2 NPs/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/CdSe0.45Te0.57 TiO2 NT arrays/CdSe 3D TiO2 NT-branched TiO2 NR arrays/CdS RGO/TiO2 NPs/CdS/CdSe SnO2 NPs/CdS/CdSe ZnO NRs/CdS/CdSe Zn2SnO4-ZnO NPs/CdS/CdSe Graphene framework/TiO2 NPs/CdS/CdSe MWCNTs/CdSe TiO2 NWs/CdS/CdSe hierarchical TiO2 NWs/CdS/CdSe ZnO/ZnSe/CdSe/ZnSe NCbs ZnO/ZnSe/CdSe/ZnSe NCbs TiO2 NPs/CuInSexS2-x TiO2 NPs/CdS/CdSe ZnO NPs/CdS/CdSe ZnO/ZnSe arrays/CdSe passivated ZnONPs/CdS/CdSe TNW/ZnO NR (ZNR)/CdS/CdSe TNW/ZnO sheet (ZNS)/CdS/CdSe SnO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs/Mn-doped CdS

2.98 2.94 4.15 3.60 2.70 4.50 1.05 2.33 1.36 1.68 3.17 1.24 1.17 1.99 2.14 2.01 3.00 3.01

[171] [138] [74,172] [173] [173] [173] [160] [164] [154] [150] [137] [174] [175] [137] [176] [168] [177] [178]

2015 2015 2010 2015 2015 2015 2012 2011 2014 2014 2014 2013 2015 2015 2014 2013 2015 2011

1.04 3.58 3.43 0.52 2.08

[179] [93] [23] [180] [181]

2015 2014 2011 2014 2013

4.20 4.17 3.23 4.20 2.27 0.33 5.13 3.39 2.38 4.54 4.68 3.20 3.57 3.68 4.19 2.53

[93] [182] [119] [119] [153] [153] [183] [176] [184] [185] [184] [69] [69] [23] [186] [186]

2014 2015 2014 2014 2012 2012 2013 2014 2013 2012 2013 2015 2015 2011 2012 2012

Pt Pt Pt Pt Pt Pt Pt Pt/FTO Pt/FTO Pt/FTO FTO CuxS Cu2S Cu2S Cu2S Cu2S Cu2S Cu2S Cu2S Cu2S/GO Cu2S/GO

50

Cu2S/GO Cu2S /FTO Cu2S/FTO Cu2S/FTO Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass Cu2S/brass ITO@Cu2S-I NW arrays ITO@Cu2S-II NW arrays ITO@Cu2S-III NW arrays Cu1.8S nanosheets arrays Cu1.8S hierarchical microspheres/FTO Cu7S4 NTs by hexagonal NPLs 3D opening skeketal Cu7S4 NCs CuS nanosheets arrays CuS NPLlet CuS knit coir mat Cu2-xS NTs Cu2-xSe NTs Cu2-xSe NPs Cu2-xSe NWs Cu2Se NPs CuSySe1−y NPLs Cu2−xSySe1−y NPLs Cu2SnS3 hierarchical microspheres/FTO Cu2ZnSnS4 Cu2ZnSnSe4 Cu2ZnSn4 hierarchical microspheres Cu2ZnSn(S1-xSex)4

TiO2 NPs/Mn-doped CdS/CdSe TiO2 NPs /CdSe graphene networks/TiO2 NPs/CdS/CdSe Carbon dot grafted graphene/TiO2 NPs/CdS/CdSe TiO2 NPs/PbS:Hg TiO2 NPs/ core/shell (CdTe/CdSe) TiO2 NPs/CuInS2 TiO2 NPs/CuIn1.5Se3 ZnO NPs/CdS/CdSe ZnO NPs/CdS/CdSe ZnO NPs/microspheres/CdS/CdSe ZnO NRs/CdS/CdSe ZnO NRs-nanosheets/CdS/CdSe ZnO branched NR-TP/CdS/CdSe ZnO NR-TP/CdS/CdSe TiO2 NPs/CdSeTe TiO2 NPs/CdSeTe TiO2 NPs/CdSeTe TiO2 NPs/CdS/CdSe

5.42 5.21

[186] [150]

2012 2014

4.37

[158]

2016

4.69 5.60 6.76 7.04 8.10 3.32 1.66 2.89 1.37 3.28 5.24 4.43 5.31 5.62 6.12 3.30

[158] [187] [29] [188] [32] [143] [92] [92] [156] [156] [143] [143] [90] [90] [90] [137]

2016 2013 2013 2014 2015 2013 2014 2015 2014 2014 2013 2013 2015 2015 2015 2015

ZnO/ZnSe/CdSe NCb arrays

3.65

[152]

2014

TiO2 NPs/CdS/CdSe

4.53

[157]

2015

TiO2 NPs/CdS/CdSe TiO2 NPs/CdS/CdSe TiO2 NPs /CdS/CdSe/ZnS TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe ZnO NRs/CdS/CdSe ZnO/ZnSe/CdSe/ZnSe NCbs ZnO/ZnSe/CdSe/ZnSe NCbs

4.43 3.95 4.06 4.53 5.02 6.25 6.50 5.93 2.28 5.01 4.63

[189] [137] [190] [191] [138] [138] [171] [171] [192] [172] [172]

2014 2015 2015 2014 2015 2015 2015 2015 2015 2014 2014

ZnO/ZnSe/CdSe NCbs TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe

4.06 1.72 1.31

[152] [174] [174]

2014 2013 2013

ZnO/ZnSe/CdSe/ZnSe NCbs TiO2 NPs /CdS/CdSe

3.73 1.31

[153] [174]

2012 2013 51

CoS2 film CoS nanosheets 3D Cu-doped CoS nanosheets CoS NR arrays CoS NR arrays/graphite paper amorphous NiCo2S4 NR arrays hollow NiCo2S4 single crystalline NR arrays Bi2S3 NWs NiS nanosheets

TiO2 NPs /CdS/CdSe TiO2 NPs /CdSe0.45Te0.55

4.10 4.60

[193] [177]

2013 2015

TiO2 NPs /CdS/CdSe ZnO NPs /CdS

3.72 0.64

[143] [155]

2015 2016

ZnO NRs/CdS/CdSe

2.70

[180]

2014

TiO2 NPs /CdSe

4.22

[156]

2013

TiO2 NPs /CdSe TiO2 NPs /CdS/CdSe TiO2 NPs /CdSe0.45Te0.56

2.20 3.47 6.10

2014 2015 2015

FeS/FTO FeS/C

TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe

2.76 4.58

[156] [154] [175] [169, 194] [194]

FeS/nickel foam PbS/Carbon black ZnO/PbS core/shell NR arrays nanocarbon black

TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe

4.39 3.91

[169] [146]

2015 2012

TiO2 NPs /CdSe TiO2 NTs/CdS/CdSe hierarchical 3D hollow TiO2 NFs/CdS/CdSe TiO2 NPs /CdSe

3.06 0.90

[195] [120]

2014 2013

2.80 3.34

[120] [164]

2013 2011

TiO2 NPs /CdSe

3.90

[164]

2011

TiO2 NPs /CdSe

4.36

[164]

2011

TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe

1.45 2.67

[163] [163]

2012 2012

TiO2 NPs/CdSe0.65Te0.35

11.16

[162]

2016

TiO2 NPs/Zn-Cu-In-Se ZnO NRs/CdS/CdSe

11.61 0.71

[4] [180]

2016 2014

TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe

1.75 1.62

[159] [73]

2011 2014

TiO2 NPs /CdSe TiO2 NPs /CdS/CdSe ZnO NWs/CdSe TiO2 NPs /CdS/CdSe TiO2 NPs /CdS

4.81 3.86 1.51 2.09 2.47

[164] [73] [168] [163] [196]

2011 2014 2013 2012 2014

nanocarbon black actived carbon AC hollow mesoporous core/shell carbon ordered multimodal porous carbon (OMPC) hollow carbon NPs (HCNPs) Nitrogen-doped HCNPs Mesoporous carbon/Ti (MC/Ti) Mesoporous carbon/Ti (MC/Ti) graphite paper carbon foam carbon fiber MCNF open mesoporous carbon NFs CuS/carbon fiber CNT CNTs MWCNTs/FTO

2015 2015

52

MWCNTs/FTO MWCNTs/FTO MWCNTs/FTO Pt/CNT-RGO Au/CNT-RGO RGO-Cu2S Cu2S/RGO graphene 3D graphene frameworks/CuS 3D graphene frameworks Pd/CNT-RGO CNT-graphene TiN TiN/CNT-Graphene TiN/CNT TiN/graphene

TiO2 NPs /PbS TiO2 NPs /PbS/CdS TiO2/ graphite platelet/PbS/CdS hierarchical ZnO NW/CdS/CdSe hierarchical ZnO NW/CdS/CdSe TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe ZnO NWs/CdSe

1.95 3.01 3.82 5.90 5.69 4.72 4.40 1.44

[196] [196] [196] [197] [197] [193] [167] [168]

2014 2014 2014 2014 2014 2013 2011 2013

TiO2 NPs /CdS/CdSe TiO2 NPs /CdS/CdSe hierarchical ZnO NW/CdS/CdSe ZnO NWs/CdSe ZnO NWs/CdSe ZnO NWs/CdSe ZnO NWs/CdSe ZnO NWs/CdSe

5.04 3.60 5.42 1.94 4.13 0.80 3.89 3.47

[72] [72] [197] [168] [168] [168] [168] [168]

2016 2016 2014 2013 2013 2013 2013 2013

Wherein, Nanoparticle=NP; nanorod=NR; nanowire=NW; nanoplatelet=NPL; nanotube=NT; nanofibe=NF; nanocage=NC; nanocable= NCb; graphene oxide=GO

Acknowledgements This work was supported by the National Key Project on Basic Research (2015CB932302), the National Natural Science Foundation of China (21573249), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100).

References [1] P. G. Hoertz, A. Staniszewski, A. Marton, G. T. Higgins, C. D. Incarvito, A. L. Rheingold, G. J. Meyer, J. Am. Chem. Soc. 128 (2006) 8234-8245. [2] P. V. Kamat, J. Phys. Chem. Lett. 4 (2013) 908-918. [3] Best Research-Cell Efficiencies (NREL, accessed on 14 August 2016). http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. [4] J. Du, Z. Du, J. S. Hu, Z. Pan, Q. Shen, J. Sun, D. Long, H. Dong, L. Sun, X. Zhong, L. J. Wan, J. Am. Chem. Soc. 138 (2016) 4201-4209. [5] X. Lan, S. Masala, E. H. Sargent, Nat. Mater 13 (2014) 233-240. [6] P. Zrazhevskiy, M. Sena, X. Gao, Chem. Soc. Rev. 39 (2010) 4326-4354. [7] Size matters: Quantum dots could make solar panels more efficient. 53

http://phys.org/news/2011-03-size-quantum-dots-solar-panels.html. [8] A. J. Nozik, M. C. Beard, J. M. Luther, M. Law, R. J. Ellingson, J. C. Johnson, Chem. Rev. 110 (2010) 6873-6890. [9] S. Baskoutas, A. F. Terzis, J. Appl. Phys. 99 (2006) 013708. [10] L. Li, J. Hu, W. Yang, A. P. Alivisatos, Nano Lett. 1 (2001) 349-351. [11] J. M. Pietryga, R. D. Schaller, D. Werder, M. H. Stewart, V. I. Klimov, J. A. Hollingsworth, J. Am. Chem. Soc. 126 (2004) 11752-11753. [12] J. B. Sambur, T. Novet, B. A. Parkinson, Science 330 (2010) 63-66. [13] R. J. Ellingson, M. C. Beard, J. C. Johnson, P. Yu, O. I. Micic, A. J. Nozik, A. Shabaev, A. L. Efros, Nano Lett. 5 (2005) 865-871. [14]M. C. Beard, J. M. Luther, O. E. Semonin, A. J. Nozik, Acc. Chem. Res. 46 (2013) 1252-1260. [15] A. Polman, H. A. Atwater, Nat. Mater. 11 (2012) 174-177. [16] P. V. Kamat, Acc. Chem. Res. 45 (2012) 1906-1915. [17] G. Chen, J. Seo, C. Yang, P. N. Prasad, Chem. Soc. Rev. 42 (2013) 8304-8338. [18] A. Nozik, Physica E 14 (2002) 115-120. [19] V. González-Pedro, X. Xu, I. Mora-Seró, J. Bisquert, ACS Nano 4 (2010) 5783-5790. [20] J. H. Bang, P. V. Kamat, ACS Nano 3 (2009) 1467-1476. [21] Y. L. Lee, Y. S. Lo, Adv. Funct. Mater. 19 (2009) 604-609. [22] Z. Yang, C. Y. Chen, C. W. Liu, H. T. Chang, Chem. Commun. 46 (2010) 5485-5487. [23] M. A. Hossain, J. R. Jennings, Z. Y. Koh, Q. Wang, ACS Nano 5 (2011) 3172-3181. [24] H. J. Lee, J. Bang, J. Park, S. Kim, S. M. Park, Chem. Mater. 22 (2010) 5636-5643. [25] W. J. Zhang, H. Zhang, Y. Y. Feng, X. H. Zhong, ACS Nano 6 (2012) 11066-11073. [26] Z. Pan, K. Zhao, J. Wang, H. Zhang, Y. Feng, X. Zhong, ACS Nano 7 (2013) 5215-5222. [27] J. S. Niezgoda, E. Yap, J. D. Keene, J. R. McBride, S. J. Rosenthal, Nano Lett. 14 (2014) 3262-3269. [28] H. McDaniel, N. Fuke, J. M. Pietryga, V. I. Klimov, J. Phys. Chem. Lett. 4 (2013) 355-361. [29] J. Wang, I. Mora-Seró, Z. Pan, K. Zhao, H. Zhang, Y. Feng, G. Yang, X. Zhong, J. Bisquert, J. Am. Chem. Soc. 135 (2013) 15913-15922. [30] S. Jiao, Q. Shen, I. n. Mora-Seró, J. Wang, Z. Pan, K. Zhao, Y. Kuga, X. Zhong, J. Bisquert, ACS Nano 9 (2015) 908-915. [31] K. L. Ou, J. C. Fan, J. K. Chen, C. C. Huang, L. Y. Chen, J. H. Ho, J. Y. Chang, J. Mater. Chem. 22 (2012) 14667. [32] J. Y. Kim, J. Yang, J. H. Yu, W. Baek, C. H. Lee, H. J. Son, T. Hyeon, M. J. Ko, ACS Nano 9 (2015) 11286-11295. [33] H. McDaniel, A. Y. Koposov, S. Draguta, N. S. Makarov, J. M. Pietryga, V. I. Klimov, J. Phys. Chem. C 118 (2014) 16987-16994. 54

[34] Y. Lai, Z. Lin, D. Zheng, L. Chi, R. Du, C. Lin, Electrochim. Acta 79 (2012) 175-181. [35] D. Liu, P. V. Kamat, J. Phys. Chem. 97 (1993) 10769-10773. [36] X. Y. Yu, J. Y. Liao, K. Q. Qiu, D. B. Kuang, C. Y. Su, ACS Nano 5 (2011) 9494-9500. [37] H. Zhang, K. Cheng, Y. Hou, Z. Fang, Z. Pan, W. Wu, J. Hua, X. Zhong, Chem. Commun. 48 (2012) 11235-11237. [38] A. Salant, M. Shalom, I. Hod, A. Faust, A. Zaban, U. Banin, ACS Nano 4 (2010) 5962-5968. [39] J. B. Sambur, S. C. Riha, D. Choi, B. A. Parkinson, Langmuir 26 (2010) 4839-4847. [40]J. Chen, J. Song, X. Sun, W. Deng, C. Jiang, W. Lei, J. Huang, R. Liu, Appl. Phys. Lett. 94 (2009) 153115. [41] D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano Lett. 1 (2001) 207-211. [42] L. Li, X. Yang, J. Gao, H. Tian, J. Zhao, A. Hagfeldt, L. Sun, J. Am. Chem. Soc. 133 (2011) 8458-8460. [43] R. S. Selinsky, Q. Ding, M. S. Faber, J. C. Wright, S. Jin, Chem. Soc. Rev. 42 (2013) 2963-2985. [44] D. Lawless, S. Kapoor, D. Meisel, J. Phys.Chem. 99 (1995) 10329-10335. [45] Z. Pan, H. Zhang, K. Cheng, Y. Hou, J. Hua, X. Zhong, ACS Nano 6 (2012) 3982-3991. [46]I. Mora-Seró, S. Giménez, F. Fabregat-Santiago, R. Gómez, Q. Shen, T. Toyoda, J. Bisquert, Acc. Chem. Res. 42 (2009) 1848-1857. [47] I. Hod, A. Zaban, Langmuir 30 (2014) 7264-7273. [48] S. Rühle, S. Yahav, S. Greenwald, A. Zaban, J. Phys. Chem. C 116 (2012) 17473-17478. [49] K. Zhao, Z. Pan, X. Zhong, J. Phys. Chem. Lett. 7 (2016) 406-417. [50] D. A. Hines, R. P. Forrest, S. A. Corcelli, P. V. Kamat, J. Phys. Chem. B 119 (2015) 7439-7446. [51] X. Y. Yu, B. X. Lei, D. B. Kuang, C. Y. Su, J. Mater. Chem. 22 (2012) 12058-12063. [52] Z. Liu, M. Miyauchi, Y. Uemura, Y. Cui, K. Hara, Z. Zhao, K. Sunahara, A. Furube, Appl. Phys. Lett 96 (2010) 233107. [53] Z. Tachan, I. Hod, M. Shalom, L. Grinis, A. Zaban, Phys. Chem. Chem. Phys. 15 (2013) 3841-3845. [54] K. E. Roelofs, T. P. Brennan, J. C. Dominguez, C. D. Bailie, G. Y. Margulis, E. T. Hoke, M. D. McGehee, S. F. Bent, J. Phys. Chem. C 117 (2013) 5584-5592. [55] N. Guijarro, J. M. Campiña, Q. Shen, T. Toyoda, T. Lana-Villarreal, R. Gómez, Phys. Chem. Chem. Phys. 13 (2011) 12024-12032. [56] K. Zhao, Z. Pan, I. Mora-Seró, E. Cánovas, H. Wang, Y. Song, X. Gong, J. Wang, M. Bonn, J. Bisquert, X. Zhong, J. Am. Chem. Soc. 137 (2015) 5602-5609. [57] S. Y. Chae, Y. J. Hwang, O. S. Joo, RSC Adv. 4 (2014) 26907-26911. [58] Y. Tachibana, H. Y. Akiyama, Y. Ohtsuka, T. Torimoto, S. Kuwabata, Chem. Lett. 55

36 (2007) 88-89. [59] H. Wei, G. Wang, J. Shi, H. Wu, Y. Luo, D. Li, Q. Meng, J. Mater. Chem. A (2016). [60] C. Gao, Z. Zhang, X. Li, L. Chen, Y. Wang, Y. He, F. Teng, J. Zhou, W. Han, E. Xie, Sol. Energy Mater. Sol. Cells 141 (2015) 101-107. [61] Z. Cai, F. Li, W. Xu, Y. Jiang, F. Luo, Y. Wang, X. Chen, Nano Energy 26 (2016) 257-266. [62]Y. Qi, N. T. Jafferis, K. Lyons, C. M. Lee, H. Ahmad, M. C. McAlpine, Nano Lett. 10 (2010) 524-528. [63] Z. Hu, B. J. Deibert, J. Li, Chem. Soc. Rev. 43 (2014) 5815-5840. [64] B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou, G. Shen, Nano Lett. 12 (2012) 3005-3011. [65] X. Li, A. Dhanabalan, L. Gu, C. Wang, Adv. Energy Mater. 2 (2012) 238-244. [66]S. Chen, H. Hou, F. Harnisch, S. A. Patil, A. A. Carmona-Martinez, S. Agarwal, Y. Zhang, S. Sinha-Ray, A. L. Yarin, A. Greiner, U. Schroder, Energy Environ. Sci. 4 (2011) 1417-1421. [67] X. Xie, L. Hu, M. Pasta, G. F. Wells, D. Kong, C. S. Criddle, Y. Cui, Nano Lett. 11 (2011) 291-296. [68] V. M. Weaver, S. Lelièvre, J. N. Lakins, M. A. Chrenek, J. C. Jones, F. Giancotti, Z. Werb, M. J. Bissell, Cancer cell 2 (2002) 205-216. [69] H. L. Feng, W. Q. Wu, H. S. Rao, Q. Wan, L. B. Li, D. B. Kuang, C. Y. Su, ACS Appl. Mat. Interfaces 7 (2015) 5199-5205. [70] G. H. Carey, L. Levina, R. Comin, O. Voznyy, E. H. Sargent, Adv. Mater. 27 (2015) 3325-3330. [71] M. Seol, D. H. Youn, J. Y. Kim, J. W. Jang, M. Choi, J. S. Lee, K. Yong, Adv. Energy Mater. 4 (2014). [72] Y. Zhu, H. Cui, S. Jia, J. Zheng, P. Yang, Z. Wang, Z. Zhu, Electrochim. Acta 208 (2016) 288-295. [73] L. Li, P. Zhu, S. Peng, M. Srinivasan, Q. Yan, A. S. Nair, B. Liu, S. Samakrishna, J. Phys. Chem. C 118 (2014) 16526-16535. [74] Y. Jiang, X. Zhang, Q. Q. Ge, B. B. Yu, Y. G. Zou, W. J. Jiang, W. G. Song, L. J. Wan, J. S. Hu, Nano Lett. 14 (2013) 365-372. [75] J. Michallon, D. Bucci, A. Morand, M. Zanuccoli, V. Consonni, A. Kaminski-Cachopo, Optics express 22 (2014) A1174-A1189. [76] E. Garnett, P. Yang, Nano Lett. 10 (2010) 1082-1087. [77] X. Qi, G. She, Y. Liu, L. Mu, W. Shi, Chem. Commun. 48 (2012) 242-244. [78] I. Robel, V. Subramanian, M. Kuno, P. V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385-2393. [79] L. J. Diguna, Q. Shen, J. Kobayashi, T. Toyoda, Appl. Phys. Lett. 91 (2007) 3116. [80] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (1999) 692-698. [81] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P. V. Kamat, J. Am. Chem. Soc. 130 (2008) 4007-4015. [82] W.-Q. Wu, H.-L. Feng, H.-S. Rao, Y.-F. Xu, D.-B. Kuang, C.-Y. Su, Nat. 56

Commun. 5 (2014). [83] I. J. Kramer, E. H. Sargent, Chem. Rev. 114 (2014) 863-882. [84] Q. Zhang, X. Guo, X. Huang, S. Huang, D. Li, Y. Luo, Q. Shen, T. Toyoda, Q. Meng, Phys. Chem. Chem. Phys. 13 (2011) 4659-4667. [85] J. Tian, R. Gao, Q. Zhang, S. Zhang, Y. Li, J. Lan, X. Qu, G. Cao, J. Phys. Chem. C 116 (2012) 18655-18662. [86] Y. L. Lee, B. M. Huang, H. T. Chien, Chem. Mater. 20 (2008) 6903-6905. [87] M. Samadpour, P. P. Boix, S. Giménez, A. Iraji Zad, N. Taghavinia, I. Mora-Seró, J. Bisquert, J. Phys. Chem. C 115 (2011) 14400-14407. [88] F. Zhao, G. Tang, J. Zhang, Y. Lin, Electrochim. Acta 62 (2012) 396-401. [89] Z. Du, H. Zhang, H. Bao, X. Zhong, J. Mater. Chem. A 2 (2014) 13033-13040. [90] Y. Jiang, B. B. Yu, J. Liu, Z. H. Li, J. K. Sun, X. H. Zhong, J. S. Hu, W. G. Song, L. J. Wan, Nano Lett. 15 (2015) 3088-3095. [91] H. S. Kim, S. B. Ko, I. H. Jang, N. G. Park, Chem. Commun. 47 (2011) 12637-12639. [92] J. Tian, L. Lv, X. Wang, C. Fei, X. Liu, Z. Zhao, Y. Wang, G. Cao, J. Phys. Chem. C 118 (2014) 16611-16617. [93] Y. Zhu, X. Meng, H. Cui, S. Jia, J. Dong, J. Zheng, J. Zhao, Z. Wang, L. Li, L. Zhang, ACS Appl. Mat. Interfaces 6 (2014) 13833-13840. [94] J. Nelson, R. E. Chandler, Coord. Chem. Rev. 248 (2004) 1181-1194. [95] K. D. Benkstein, N. Kopidakis, J. Van de Lagemaat, A. Frank, J. Phys. Chem. B 107 (2003) 7759-7767. [96] H. Scher, E. W. Montroll, Phys. Rev. B 12 (1975) 2455. [97] T. Toyoda, Q. Shen, J. Phys. Chem. Lett. 3 (2012) 1885-1893. [98] L. Li, T. Zhai, Y. Bando, D. Golberg, Nano Energy 1 (2012) 91-106. [99] Q. Shen, A. Yamada, S. Tamura, T. Toyoda, Appl. Phys. Lett. 97 (2010) 123107. [100] L. Tao, Y. Xiong, H. Liu, W. Shen, Nanoscale 6 (2014) 931-938. [101] K. S. Leschkies, R. Divakar, J. Basu, E. Enache-Pommer, J. E. Boercker, C. B. Carter, U. R. Kortshagen, D. J. Norris, E. S. Aydil, Nano Lett. 7 (2007) 1793-1798. [102] M. Seol, E. Ramasamy, J. Lee, K. Yong, J. Phys. Chem. C 115 (2011) 22018-22024. [103] M. Seol, H. Kim, Y. Tak, K. Yong, Chem. Commun. 46 (2010) 5521-5523. [104] H. L. Feng, W. Q. Wu, H. S. Rao, Q. Wan, L. B. Li, D. B. Kuang, C. Y. Su, ACS Appl Mater Interfaces 7 (2015) 5199-5205. [105] S. Gao, J. Yang, M. Liu, H. Yan, W. Li, J. Zhang, Y. Luo, J. Power Sources 250 (2014) 174-180. [106] S. W. Jung, J.-H. Park, W. Lee, J. H. Kim, H. Kim, C. J. Choi, K. S. Ahn, J. Appl. Phys. 110 (2011) 054301. [107] J. Song, X. Zhang, C. Zhou, Y. Lan, Q. Pang, L. Zhou, J. Electron. Mater. 44 (2015) 22-27. [108] W. T. Sun, Y. Yu, H. Y. Pan, X. F. Gao, Q. Chen, L. M. Peng, J. Am. Chem. Soc. 130 (2008) 1124-1125. [109] H. Yang, W. Fan, A. Vaneski, A. S. Susha, W. Y. Teoh, A. L. Rogach, Adv. Funct. Mater. 22 (2012) 2821-2829. 57

[110] Z. X. Li, Y. L. Xie, H. Xu, T. M. Wang, Z. G. Xu, H. L. Zhang, J. Photochem. Photobiolo. A 224 (2011) 25-30. [111] C. Wang, Z. Jiang, L. Wei, Y. Chen, J. Jiao, M. Eastman, H. Liu, Nano Energy 1 (2012) 440-447. [112] H. Wang, Y. Bai, H. Zhang, Z. Zhang, J. Li, L. Guo, J. Phys. Chem. C 114 (2010) 16451-16455. [113] J. Jiao, Z. J. Zhou, W. H. Zhou, S. X. Wu, Mater. Sci. Semicond. Process. 16 (2013) 435-440. [114] A. I. Hochbaum, P. Yang, Chem. Rev. 110 (2009) 527-546. [115] H. Sun, J. Deng, L. Qiu, X. Fang, H. Peng, Energy Environ. Sci. 8 (2015) 1139-1159. [116] X. Feng, K. Zhu, A. J. Frank, C. A. Grimes, T. E. Mallouk, Angew. Chem. Int. Ed. 124 (2012) 2781-2784. [117] H. Huang, L. Pan, C. K. Lim, H. Gong, J. Guo, M. S. Tse, O. K. Tan, Small 9 (2013) 3153-3160. [118] J. H. Bang, P. V. Kamat, Adv. Funct. Mater. 20 (2010) 1970-1976. [119] H. S. Rao, W. Q. Wu, Y. Liu, Y. F. Xu, B. X. Chen, H. Y. Chen, D. B. Kuang, C. Y. Su, Nano Energy 8 (2014) 1-8. [120] H. Han, P. Sudhagar, T. Song, Y. Jeon, I. Mora-Seró, F. Fabregat-Santiago, J. Bisquert, Y. S. Kang, U. Paik, Chem. Commun. 49 (2013) 2810-2812. [121] I. Gonzalez-Valls, M. Lira-Cantu, Energy Environ. Sci. 2 (2009) 19-34. [122] Q. Zhang, C. S. Dandeneau, X. Zhou, G. Cao, Adv. Mater. 21 (2009) 4087-4108. [123] L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson, Y. Zhang, R. J. Saykally, P. Yang, Angew. Chem. Int. Ed. 115 (2003) 3139-3142. [124] L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai, P. Yang, Nano Lett. 5 (2005) 1231-1236. [125] M. Zheng, L. Zhang, G. Li, W. Shen, Chem. Phys. Lett. 363 (2002) 123-128. [126] S. S. Mali, H. Kim, P. S. Patil, C. K. Hong, Dalton T. 42 (2013) 16961-16967. [127] W. Lee, S. Kang, T. Hwang, K. Kim, H. Woo, B. Lee, J. Kim, J. Kim, B. Park, Electrochim. Acta 167 (2015) 194-200. [128] H. Chen, W. Li, Q. Hou, H. Liu, L. Zhu, Electrochim. Acta 56 (2011) 8358-8364. [129] P. Ruankham, L. Macaraig, T. Sagawa, H. Nakazumi, S. Yoshikawa, J. Phys. Chem. C 115 (2011) 23809-23816. [130] F. Xu, L. Sun, Energy Environ. Sci. 4 (2011) 818-841. [131] P. Y. Kuang, Y. Z. Su, K. Xiao, Z. Q. Liu, N. Li, H. J. Wang, J. Zhang, ACS Appl. Mat. Interfaces 7 (2015) 16387-16394. [132] W. J. Lee, E. Ramasamy, D. Y. Lee, J. S. Song, ACS Appl. Mat. Interfaces 1 (2009) 1145-1149. [133] H. Anwar, A. E. George, I. G. Hill, Solar Energy 88 (2013) 129-136. [134] C. Li, J. Xia, Q. Wang, J. Chen, C. Li, W. Lei, X. Zhang, ACS Appl. Mat. Interfaces 5 (2013) 7400-7404. 58

[135] J. Chen, C. Li, D. Zhao, W. Lei, Y. Zhang, M. Cole, D. Chu, B. Wang, Y. Cui, X. Sun, Electrochem. Commun. 12 (2010) 1432-1435. [136] K. Meng, P. K. Surolia, O. Byrne, K. R. Thampi, J. Power Sources 248 (2014) 218-223. [137] M. Ye, X. Wen, N. Zhang, W. Guo, X. Liu, C. Lin, J. Mater. Chem. A 3 (2015) 9595-9600. [138] X. Q. Chen, Z. Li, Y. Bai, Q. Sun, L. Z. Wang, S. X. Dou, Chem. Eur. J. 21 (2015) 1055-1063. [139] H. Zhang, C. Wang, W. Peng, C. Yang, X. Zhong, Nano Energy 23 (2016) 60-69. [140] J. Xu, H. Xue, X. Yang, H. Wei, W. Li, Z. Li, W. Zhang, C. S. Lee, Small 10 (2014) 4754-4759. [141] Z. Yang, C. Y. Chen, C. W. Liu, C. L. Li, H. T. Chang, Adv. Energy Mater. 1 (2011) 259-264. [142] Y. Shengyuan, A. S. Nair, Z. Peining, S. Ramakrishna, Mater. Lett. 76 (2012) 43-46. [143] C. Chen, M. Ye, N. Zhang, X. Wen, D. Zheng, C. Lin, J. Mater. Chem. A 3 (2015) 6311-6314. [144] C. Y. Lin, C. Y. Teng, T. L. Li, Y. L. Lee, H. Teng, J. Mater. Chem. A 1 (2013) 1155-1162. [145] Z. Tachan, M. Shalom, I. Hod, S. Rühle, S. Tirosh, A. Zaban, J. Phys. Chem. C 115 (2011) 6162-6166. [146] Y. Yang, L. Zhu, H. Sun, X. Huang, Y. Luo, D. Li, Q. Meng, ACS Appl. Mat. Interfaces 4 (2012) 6162-6168. [147] H. J. Kim, D. J. Kim, S. S. Rao, A. D. Savariraj, K. Soo-Kyoung, M. K. Son, C. V. Gopi, K. Prabakar, Electrochim. Acta 127 (2014) 427-432. [148] H. J. Kim, T. B. Yeo, S. K. Kim, S. S. Rao, A. D. Savariraj, K. Prabakar, C. V. Gopi, Eur. J. Inorg. Chem. 2014 (2014) 4281-4286. [149] C. V. Gopi, S. S. Rao, S.-K. Kim, D. Punnoose, H. J. Kim, J. Power Sources 275 (2015) 547-556. [150] K. Zhao, H. Yu, H. Zhang, X. Zhong, J. Phys. Chem. C 118 (2014) 5683-5690. [151] F. Wang, H. Dong, J. Pan, J. Li, Q. Li, D. Xu, J. Phys. Chem. C 118 (2014) 19589-19598. [152] J. Xu, X. Yang, T. L. Wong, C. S. Lee, Nanoscale 4 (2012) 6537-6542. [153] J. Xu, X. Yang, Q. D. Yang, T. L. Wong, C. S. Lee, J. Phys. Chem. C 116 (2012) 19718-19723. [154] H. Yu, H. Bao, K. Zhao, Z. Du, H. Zhang, X. Zhong, J. Phys. Chem. C 118 (2014) 16602-16610. [155] Y. Sun, L. Yang, M. Wei, G. Chen, G. Che, J. Yang, Z. Wang, Z. Gao, J. Mater. Sci.: Mater. Electron. 27 (2016) 1457-1462. [156] J. Xiao, X. Zeng, W. Chen, F. Xiao, S. Wang, Chem. Commun. 49 (2013) 11734-11736. [157] M. Wang, W. Chen, J. Zai, S. Huang, Q. He, W. Zhang, Q. Qiao, X. Qian, J. 59

Power Sources 299 (2015) 212-220. [158] R. Zhang, B. Zhang, Y. Liu, S. Sun, Journal of Materials Chemistry C 4 (2016) 1633-1644. [159] P. Sudhagar, E. Ramasamy, W.-H. Cho, J. Lee, Y. S. Kang, Electrochem. Commun. 13 (2011) 34-37. [160] G. S. Paul, J. H. Kim, M.-S. Kim, K. Do, J. Ko, J. S. Yu, ACS Appl. Mat. Interfaces 4 (2011) 375-381. [161] S. Q. Fan, B. Fang, J. H. Kim, B. Jeong, C. Kim, J. S. Yu, J. Ko, Langmuir 26 (2010) 13644-13649. [162] Z. Du, Z. Pan, F. Fabregat-Santiago, K. Zhao, D. Long, H. Zhang, Y. Zhao, X. Zhong, J. S. Yu, J. Bisquert, J. Phys. Chem. Lett. 7 (2016) 3103-3111. [163] J. Dong, S. Jia, J. Chen, B. Li, J. Zheng, J. Zhao, Z. Wang, Z. Zhu, J. Mater. Chem. 22 (2012) 9745-9750. [164] B. Fang, M. Kim, S. Q. Fan, J. H. Kim, D. P. Wilkinson, J. Ko, J. S. Yu, J. Mater. Chem. 21 (2011) 8742-8748. [165] M. H. Yeh, C. P. Lee, C. Y. Chou, L. Y. Lin, H. Y. Wei, C. W. Chu, R. Vittal, K. C. Ho, Electrochim. Acta 57 (2011) 277-284. [166] Y. Jiang, X. Zhang, Q. Q. Ge, B. B. Yu, Y. G. Zou, W. J. Jiang, J. S. Hu, W. G. Song, L. J. Wan, ACS Appl. Mat. Interfaces 6 (2014) 15448-15455. [167] J. G. Radich, R. Dwyer, P. V. Kamat, J. Phys. Chem. Lett. 2 (2011) 2453-2460. [168] D. H. Youn, M. Seol, J. Y. Kim, J. W. Jang, Y. Choi, K. Yong, J. S. Lee, ChemSusChem 6 (2013) 261-267. [169] H. Geng, L. Zhu, W. Li, H. Liu, L. Quan, F. Xi, X. Su, J. Power Sources 281 (2015) 204-210. [170] H. Chen, L. Zhu, H. Liu, W. Li, J. Phys. Chem. C 117 (2013) 3739-3746. [171] Y. Bai, C. Han, X. Chen, H. Yu, X. Zong, Z. Li, L. Wang, Nano Energy 13 (2015) 609-619. [172] J. Xu, X. Yang, Q. Yang, W. Zhang, C.-S. Lee, ACS Appl. Mat. Interfaces 6 (2014) 16352-16359. [173] V.-D. Dao, Y. Choi, K. Yong, L. L. Larina, H.-S. Choi, Carbon 84 (2015) 383-389. [174] Y. Cao, Y. Xiao, J. Y. Jung, H. D. Um, S. W. Jee, H. M. Choi, J. H. Bang, J. H. Lee, ACS Appl. Mat. Interfaces 5 (2013) 479-484. [175] C. V. Thulasi-Varma, C. V. V. M. Gopi, S. S. Rao, D. Punnoose, S.-K. Kim, H.-J. Kim, J. Phys. Chem. C 119 (2015) 11419-11429. [176] M. Ye, C. Chen, N. Zhang, X. Wen, W. Guo, C. Lin, Adv. Energy Mater. 4 (2014). [177] S. Peng, T. Zhang, L. Li, C. Shen, F. Cheng, M. Srinivasan, Q. Yan, S. Ramakrishna, J. Chen, Nano Energy 16 (2015) 163-172. [178] X. F. Guan, S. Q. Huang, Q. X. Zhang, X. Shen, H. C. Sun, D. M. Li, Y. H. Luo, R. C. Yu, Q.B. Meng, Nanotechnology 22 (2011) 465402. [179] B. Liu, Y. Sun, X. Wang, L. Zhang, D. Wang, Z. Fu, Y. Lin, T. Xie, J. Mater. Chem. A 3 (2015) 4445-4452. 60

[180] M. Que, W. Guo, X. Zhang, X. Li, Q. Hua, L. Dong, C. Pan, J. Mater. Chem. A 2 (2014) 13661-13666. [181] L. B. Li, Y. F. Wang, H. S. Rao, W. Q. Wu, K. N. Li, C. Y. Su, D. B. Kuang, ACS Appl. Mat. Interfaces 5 (2013) 11865-11871. [182] S. S. Bhande, R. B. Ambade, D. V. Shinde, S. B. Ambade, S. A. Patil, M. Naushad, R. S. Mane, Z. A. Alothman, S.-H. Lee, S.-H. Han, ACS Appl. Mat. Interfaces 7 (2015) 25094-25104. [183] H. McDaniel, N. Fuke, N. S. Makarov, J. M. Pietryga, V. I. Klimov, Nat. Commun. 4 (2013). [184] J. Tian, Q. Zhang, E. Uchaker, R. Gao, X. Qu, S. Zhang, G. Cao, Energy Environ. Sci. 6 (2013) 3542-3547. [185] J. Xu, X. Yang, Q. D. Yang, T. L. Wong, S. T. Lee, W. J. Zhang, C. S. Lee, J. Mater. Chem. 22 (2012) 13374-13379. [186] P. K. Santra, P. V. Kamat, J. Am. Chem. Soc. 134 (2012) 2508-2511. [187] J. W. Lee, D. Y. Son, T. K. Ahn, H. W. Shin, I. Y. Kim, S. J. Hwang, M. J. Ko, S. Sul, H. Han, N. G. Park, Scientific reports 3 (2013). [188] Z. Pan, I. Mora-Seró, Q. Shen, H. Zhang, Y. Li, K. Zhao, J. Wang, X. Zhong, J. Bisquert, J. Am. Chem. Soc. 136 (2014) 9203-9210. [189] W. Chen, M. Wang, T. Qian, H. Cao, S. Huang, Q. He, N. Liang, C. Wang, J. Zai, Nano Energy 12 (2015) 186-196. [190] C. V. Thulasi-Varma, S. S. Rao, C. S. S. P. Kumar, C. V. Gopi, I. K. Durga, S.-K. Kim, D. Punnoose, H.-J. Kim, Dalton T. 44 (2015) 19330-19343. [191] A. D. Savariraj, K. K. Viswanathan, K. Prabakar, ACS Appl. Mat. Interfaces 6 (2014) 19702-19709. [192] M. Eskandari, V. Ahmadi, Mater. Lett. 142 (2015) 308-311. [193] M. S. Faber, K. Park, M. Cabán-Acevedo, P. K. Santra, S. Jin, J. Phys. Chem. Lett. 4 (2013) 1843-1849. [194] H. Geng, L. Zhu, W. Li, H. Liu, X. Su, F. Xi, X. Chang, Electrochim. Acta 182 (2015) 1093-1100. [195] X. Song, M. Wang, J. Deng, Y. Ju, T. Xing, J. Ding, Z. Yang, J. Shao, J. Power Sources 269 (2014) 661-670. [196] P. N. Kumar, S. Mandal, M. Deepa, A. K. Srivastava, A. G. Joshi, J. Phys. Chem. C 118 (2014) 18924-18937. [197] M. Seol, M. Choi, Y. Choi, K. Yong, J. Electronchem. Soc. 161 (2014) H809-H815.

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Author information Jian-Kun Sun received her B.S. degree in Polymer Materials and Engineering in 2011 and M.S. degree in Material Science in 2014 from Qingdao University. She is currently a Ph. D candidate at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). Her research interests focus on design and synthesis of nanostructured materials for photovoltaic application, e.g. the design and synthesis of quantum dots and nanostructured electrodes.

Yan Jiang received his B.S. in Material Chemistry from Sun Yat-Sen Unievrsity, China in 2010. After that he joined the Key Laboratory of Molecular Nanostructure and Nanotechnology at ICCAS, and obtained his Ph.D. in 2015. He is currently a postdoctoral scholar in Prof. Yabing Qi’s unit at OIST, Japan. His research focuses on design and synthesis of nanostructured materials for photovoltaic applications, e.g. QDSSCs and organic-inorganic hybrid perovskite solar cells.

Xinhua Zhong received his B.E. degree from East China University of Science and Technology in 1992, M.S. degree from Nankai University in 1995, and PhD degree from National University of Singapore (NUS) in 2002, respectively. He then became a postdoctoral fellow at NUS, Institute of Materials Research and Engineering, and Max Planck Institute for Polymer Research in sequence. In 2006 He served as a full professor at East China University of Science and Technology. In 2016 he moved to South China Agricultural University. His research mainly involves synthetic and optoelectronic applications of nanoscaled materials, and quantum dot based solar cells.

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Li-Jun Wan received his B.S. and M.S. degrees in Materials Science from Dalian University of Technology in 1982 and 1987, respectively, and Ph.D. in Materials Chemistry from Tohoku University of Japan in 1996. He served as a full Professor at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) since 1999, and the Director of ICCAS in 2004-2013. He was appointed the President of University of Science and Technology of China in 2015. His research focuses on the physical chemistry of single molecules and molecular assemblies, nanomaterials for applications in energy and environmental science, and scanning probe microscopy. In recognition of his research achievements, he is an elected academician of CAS and the Academy of Sciences for the Developing World (TWAS), and the Fellow of Royal Society of Chemistry, UK.

Jin-Song Hu received his Ph.D. degree in Physical Chemistry at ICCAS in 2005, then joined in ICCAS as an Assistant Professor and was promoted as an Associated Professor in 2007. He worked in Professor Charles M. Lieber's group at Harvard University in 2008-2011, then moved back to ICCAS as a Full Professor. His research currently focuses on developing new functional nanomaterials for efficient solar energy conversion and electrochemical energy conversion.

Highlights 

A comprehensive review on the recent advances and new insights in QDSSC field.

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Detailed progress in three-dimensional nanostructured electrodes for efficient photoanodes and counter electrodes of QDSSCs

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Possible strategies about the design and function-directed modification of 3D nanostructured electrodes towards highly efficient QDSSCs

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Graphical Abstract

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