Recent advances in quantum dot-sensitized solar cells: insights into photoanodes, sensitizers, electrolytes and counter electrodes

Abstract

Quantum dot-sensitized solar cells (QDSCs), as promising candidates for cost-effective photoelectrochemical solar cells, have attracted much attention due to their characteristic properties such as processability at low cost, feasibility to control light absorption spectrum in a wide region, and possibility of multiple electron generation with a theoretical conversion efficiency up to 44%. QDSCs have analogous structures to dye-sensitized solar cells typically consisting of semiconductor photoanodes sensitized with quantum dots (QDs), redox electrolytes, and counter electrodes (CEs). Much effort has been dedicated to optimizing each component of QDSCs for higher device performance. In this review, recent advances of photoanodes with various architectures, QDs with tunable band gaps, electrolytes in liquid, quasi-solid or solid state, and CEs with great electrocatalytic activity for QDSCs will be highlighted. We aim to elaborate the rational strategies in material design for QDSC applications. Finally, the conclusion and future prospects emphasize the key developments and remaining challenges for QDSCs.

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Introduction

Solar energy is sustainable and particularly abundant as the most promising energy source for humanity's future. Several technologies have been exploited to convert solar photons into heat, electricity, fuel gas and biomass for practical use. Among these, solar cells are photovoltaic devices that directly convert sunlight into electricity via the photoelectric effect (e.g., silicon/thin film solar cells) or photochemical effect (e.g., dye-/quantum dot-sensitized solar cells (DSCs/QDSCs)).^1,2 Of the various kinds of solar cells, QDSCs possess some distinct advantages, such as ease in fabrication, size-dependence of quantum dots (QDs) with tunable band gaps and high extinction coefficients, and potential of high power conversion efficiency (PCE) based on the generation of multiple excitons by the impact-ionization effect in QDs.^3–7 Thus, QDSCs have been considered as promising candidates of the third-generation solar cells, and thus the number of relevant research articles is increasing nearly year by year (Fig. 1).

Fig. 1 Evolution of the number of publications for quantum dot-sensitized solar cells. Inset is the share distribution of publications for each component of QDSCs in 2014. (Source: ISI Web of Science, Thomson Reuters. Date: January 1, 2017. Key words: quantum dot-sensitized solar cells.)

Essentially, QDSCs have a similar structure and working mechanism to DSCs. As shown in Fig. 2, a typical QDSC has a sandwich structure consisting of a several micron thick semiconductor photoanode (e.g., TiO₂ or ZnO) sensitized with a moderate amount of QDs (e.g., CdS, CdSe or CdTe), an electrolyte containing a redox couple (e.g., S²⁻/S_x²⁻), and a counter electrode (CE; e.g., Pt, Cu₂S or CoS).⁸ Under light illumination, the absorbed QDs on the photoanodes harvest some light within a certain region of wavelengths according to their band gaps to generate photo-excited electrons, which are subsequently injected from the conduction band (CB) of the QDs into the CB of the semiconductor and then quickly migrated to the external circuit through the conductive substrate, thereby creating an electric current. Meanwhile, holes are synchronously produced by the photo-excitation process in the valence band (VB) of the QDs, which are immediately transferred to the redox electrolyte to oxidize it. The oxidized electrolyte is restored by the electrons in the CE from the external circuit back to the cycling circuit in the cell.⁹ Briefly, QDs show a photovoltaic phenomenon when exposed to light, consequently producing photovoltage and photocurrent. The photovoltage (V_OC) corresponds to the difference between the Fermi level (E_F) of the QD-sensitized photoanode and the redox potential of the electrolyte. The photocurrent is strongly associated with the light harvesting efficiency as well as the electron separation, transport and collection efficiency of the cell devices.¹⁰

Fig. 2 Operating principle and schematic diagram of the interfacial charge-transfer processes in a quantum dot-sensitized solar cell.

Many research experiments have been made for QDSCs in order to achieve high-performance devices.^11,12 Each component of the cells vitally determines the cost and cell efficiency of QDSCs. Therefore, in past years, all the materials used in QDSCs have experienced rational design and careful modification (inset in Fig. 1 and 3), including the fabrication of nanostructured semiconductor photoanodes with effective dye loading and fast electron transport,^13–15 the synthesis of versatile QDs with high absorption coefficients and appropriate band gaps to maximize light harvesting,^16–18 the preparation of redox electrolytes with strong capacity for efficient hole transport and long-term stability,^19–21 and the formation of various CEs with high electrocatalytic ability in reduction of the oxidized electrolyte.^22,23 After a series of optimization processes, the best PCE of QDSCs up to 12% has been achieved.²⁴

Fig. 3 A summarized energy level diagram of some representative materials applied in QDSCs.

Hence, this review highlights recent advances with insights into the photoanodes, QDs, electrolytes and CEs of QDSCs. A general overview of these aspects in QDSCs will be given. Detailed information and comprehensive discussion of different issues for QDSCs can be found in some special review articles.^25–29

Recent advances in photoanodes of QDSCs

Photoanodes composed of semiconductor films with various nanostructures tremendously influence the PCE of QDSCs.³⁰ The functions of photoanodes in QDSCs can be summarized into two main aspects: (1) offer smooth electrical channels to facilitate the transfer of photo-excited electrons from photoanodes to conductive substrates and finally to the external circuit (Fig. 2); (2) provide sufficient surface area to load enough QDs for light absorption.³¹ Accordingly, a preferred photoanode should possess a fast electron transport rate to guarantee a high electron collection efficiency and a large surface area to ensure a high QD loading capacity. Therefore, intensive studies have been concentrated on these two issues.^15,32–35 In general, TiO₂ nanoparticle (NP) and ZnO nanowire (NW) films are the most widely used semiconductor photoanodes in QDSCs because of their attractive advantages, such as high electron mobility, large surface area, easy fabrication, high stability and low cost.^33,36–38 During the past few years, considerable effort has been dedicated to designing and optimizing the preparation routes to form TiO₂/ZnO photoanodes with increased QD loading, shortened electron transport pathways, and reduced charge recombination.^39–42 These rational strategies of photoanodes can be roughly divided into several aspects: nanostructure design,^43–46 additional treatment (e.g., TiCl₄ immersion, etching, etc.),^36,47,48 doping with ions (e.g., Zn,⁴⁹ Nb,⁵⁰ B/S,⁵¹etc.), and decoration with other materials (e.g., Au or Ag NPs,^52,53 g-C₃N₄,⁵⁴ graphene,⁵⁵ carbon nanotubes (CNTs),⁵⁶ Al₂O₃/MgO,⁵⁷ up/down conversion crystals,^58,59etc.). Next, the recent progress in the photoanodes of QDSCs will be discussed in detail according to the above mentioned aspects.

Nanostructure design is a popular way to obtain effective TiO₂/ZnO photoanodes for QDSCs. As listed in Table 1, in addition to NPs and NWs, other morphologies, such as nanosheets (NSs),^46,60,61 nanorods (NRs),^62–64 microspheres (MSs),^43,65–67 nanotubes (NTs),^68,69 nanofibers (NFs),^70,71 mesoporous (MPs),^14,72 tetrapods (TPs),^42,73 and nanodisks (NDs),⁷⁴ have also been developed for TiO₂/ZnO photoanodes. For example, “oriented attachment” TiO₂ NRs prepared from a hydrothermal process showed a high QD loading capacity and efficient electrolyte diffusion ability due to the porous structures of the resultant films, in combination with a high electron diffusion coefficient and long electron lifetime due to their quasi-one-dimensional feature. Therefore, a best efficiency of 5.96% (Table 1) was achieved for the TiO₂ NR-based QDSCs.³⁵ Recently, complex hierarchical architectures constructed of two or more kinds of single structures together have also been intensively investigated since they are especially favourable to combine different merits of their building blocks. Several hierarchical structures, particularly branched or dendrite structures (denoted as “backbone-branch”), such as NRs–NTs,⁷⁵ NRs–NSs,⁷⁶ NWs–NWs,⁷⁷ NFs–NTs⁷⁸ and MPs–NWs¹⁵ of TiO₂, NRs–NSs of ZnO,^32,79 and TiO₂ NFs–ZnO NSs,⁸⁰ ZnO NRs–TiO₂ NSs,⁴⁰ TiO₂ NWs–TiO₂ NSs–ZnO NRs,³⁴ and TiO₂ NWs–ZnO NSs³⁹ of hybrid materials, have been rationally designed for high-performance QDSCs. For instance, TiO₂ NWs–TiO₂ NSs–ZnO NRs hyperbranched heterostructured arrays were vertically grown on FTO substrates via a multi-step route (Fig. 4A). Compared to pristine TiO₂ NWs, double-branched photoanodes are beneficial to enhance light harvesting due to the improved ability of light scattering and absorption (Fig. 4B) and strengthened charge collection capability due to the larger surface area for a thinner QD layer (Fig. 4C), ultimately leading to a peak efficiency of 5.38% for the optimized QDSCs with Cu₂S CEs.³⁴

Table 1 Photovoltaic parameters and photoactive materials of QDSCs based on different photoanodes

Photoanode	Quantum dot	Electrolyte	Counter electrode	J SC (mA cm^−2)	V OC (mV)	FF	PCE (%)	Ref.
TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	Cu2S	15.54	563	0.61	5.53	36
TiO2 NSs	CdSe/ZnS/SiO2	S^2−/Sx^2−	Cu2S	16.95	591	0.50	5.01	60
TiO2 NRs	CdSe/ZnS	S^2−/Sx^2−	Cu2ZnSnS(Se)4	17.5	563	0.605	5.96	35
TiO2 MSs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	13.89	540	0.54	4.05	66
TiO2 NTs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	10.81	689	0.62	4.61	68
TiO2 MPs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	14.05	571	0.517	4.15	14
ZnO NWs	CdS/CdSe	S^2−/Sx^2−	Au	17.3	627	0.383	4.15	33
ZnO NRs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	9.93	0.61	0.52	3.14	64
ZnO TPs	CdS/CdSe	S^2−/Sx^2−	Pt	13.85	722	0.424	4.24	42
ZnO NDs	CdS/CdSe	S^2−/Sx^2−	Pt	16.0	620	0.49	4.86	74
TiO2 NWs–NWs	CdS/CdSe	S^2−/Sx^2−	Pt	17.98	465	0.502	4.20	77
TiO2 NFs–NTs	CdS/CdSe/ZnS	S^2−/Sx^2−	Carbon black	8.8	610	0.503	2.8	78
TiO2 MPs–NWs	CdS/CdSe	S^2−/Sx^2−	Cu2S	19.32	531	0.586	6.011	15
ZnO NRs–NSs	CdS/CdSe	S^2−/Sx^2−	Au	14.2	631	0.48	4.4	79
ZnO NRs–NSs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	10.74	610	0.50	3.28	32
TiO2 NFs–ZnO NSs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	13.64	511	0.44	3.05	80
TiO2 NWs–TiO2 NSs–ZnO NRs	CdS/CdSe	S^2−/Sx^2−	Cu2S	19.19	517	0.54	5.38	34
TiO2 NWs–ZnO NSs	CdS/CdSe	S^2−/Sx^2−	Cu2S	16.11	512	0.55	4.57	39
TiO2 NFs/TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	Cu2S	11.92	503	0.54	3.24	71
TiO2 MSs/TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	Cu2S	14.06	608	0.55	4.70	81
ZnO NDs/TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	PbSe	15.34	659	0.53	5.36	46
ZnO NPs + ZnO MSs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	17.13	560	0.53	5.08	61
ZnO NWs + ZnO MSs	CdS	S^2−/Sx^2−	Pt	9.01	511	0.51	2.35	82
ZnO NRs + ZnO TPs	CdS/CdSe/ZnS	S^2−/Sx^2−	Cu2S	16.56	703	0.45	5.24	73
ZnO NRs/Ag NPs/TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	Cu2S	15.65	744	0.508	5.92	52
ZnO NRs/g-C3N4	CdS	S^2−/Sx^2−	Pt	11.1	650	0.34	2.43	54
ZnO NR/Zn2SnO4 MSs	CdS/CdSe	S^2−/Sx^2−	Pt	11.32	492	0.37	2.08	88
TiO2 NPs/MgO/Al2O3	CdS/CdSe	S^2−/Sx^2−	CuS	11.4	630	0.56	3.25	57
SnO2 MSs	CdS/CdSe/ZnS	S^2−/Sx^2−	Carbon NFs	7.5	587	0.562	2.5	93
BiTiO3 NPs	CdS/ZnS	S^2−/Sx^2−	Cu2S	3.74	608	0.554	1.26	89
NiO MPs	CuInSxSe2−x	S^2−/Sx^2−	CuxS	9.13	350	0.39	1.25	91

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Fig. 4 (A) Schematic diagram of the growth procedures and (B) diffuse reflectance spectra of hyperbranched TiO₂ NW–TiO₂ NS–ZnO NR heterostructured arrays, and (C) IMPS/IMVS tests of QDSCs based on those photoanodes (reproduced from ref. 34 with permission from the Royal Society of Chemistry). (D) Schematic diagram of the preparation routes and (E) IPCE plots of QDSCs based on double-layer photoanodes, and (F) time resolved PL spectra of different CdS-sensitized photoanodes (reproduced from ref. 82 with permission from the Royal Society of Chemistry).

Moreover, photoanode films prepared by simply mixing two different structures (denoted as “one structure/another structure”), such as TiO₂ NFs/TiO₂ NPs,⁷¹ TiO₂ MSs/TiO₂ NPs,⁸¹ and ZnO NDs/TiO₂ NPs,⁴⁶ are also available to enhance the performance of QDSCs. For example, an appropriate amount of ZnO NDs with higher conductivity was embedded into TiO₂ NP films to reduce the resistance at the interface for effective electron transport, thus yielding an enhanced cell efficiency up to 5.36% (Table 1).⁴⁶ Furthermore, photoanodes consisting of double layers (denoted as “under layer + upper layer”), such as ZnO NPs + ZnO MSs,⁶¹ ZnO NRs + ZnO TPs,⁷³ and ZnO NWs + ZnO MSs,⁸² are well accepted to combine the different advantages of the two layers. For instance, a layer of hollow mesoporous ZnO MSs was covered onto another layer of vertical ZnO NW arrays to form a double-layered photoanode (Fig. 4D) with large surface area attributed to the upper layer (Fig. 4E), and fast electron transport ability for high charge collection associated with the under layer (Fig. 4F).⁸²

The post-treatment of photoanodes via a TiCl₄ solution is a common way to modify TiO₂ films for DSCs and QDSCs.^36,47 Such TiCl₄ treatment can form a compact TiO₂ layer with a thickness of hundreds of nanometers to improve the photovoltaic performance of devices. Recently, Zhong et al. studied the effect of TiCl₄ treatment on FTO substrates and TiO₂ mesoporous films with different modes (i.e., FTO/TiO₂/TiCl₄, FTO/TiCl₄/TiO₂, and FTO/TiCl₄/TiO₂/TiCl₄). It was indicated that all the TiCl₄ treatments were helpful to improve the performance of QDSCs with enhanced photocurrent due to the lower charge recombination rate. In particular, the sole TiCl₄ treatment on FTO substrates is more efficient than any other modes on the TiO₂ films.³⁶

Ion doping is a common approach to tune the position of either the CBs or VBs of semiconductor materials for photocatalysis applications. For QDSCs, ion doping (e.g. B/S,⁵¹ N,⁸³ Nb,⁵⁰ Zn⁴⁹ and Y⁸⁴) has recently been applied to retard charge recombination and accelerate electron transfer in photoanodes. For example, Nb doping in TiO₂ photoanodes could obviously increase the photovoltaic performance of the resultant QDSCs, primarily due to enhanced electron injection and electron transfer efficiency resulting from the negative shift in CB and the increased electron conductivity.⁵⁰

Decoration with some useful materials is authentically beneficial to boost the photovoltaic performance of TiO₂/ZnO photoanodes in DSCs and QDSCs. One kind of such decoration materials is novel metals, such as Au and Ag NPs, mainly owing to their surface plasmon resonance effect to localize incident light and extend the optical path length for increased light harvesting of QDSCs.^52,53 Significantly, the incorporation of Ag NPs into TiO₂/ZnO NRs not only enhanced light absorption and increased electron injection rate, but also reduced charge recombination and prolonged electron lifetime (Fig. 5A–C), consequently resulting in improved photocurrent (Table 1). More impressively, an enhanced photovoltage was also achieved due to the direct contact of Ag NPs with TiO₂ NPs leading to Fermi level alignment with an upward shift of more negative potential in TiO₂ NPs/Ag NPs/ZnO NRs (Fig. 5A).⁵² Another type of decoration materials is carbon-based materials, including CNTs,⁵⁶ graphene,⁵⁵ carbon dots,⁸⁵ and graphitic carbon nitride (g-C₃N₄),⁵⁴ largely due to their positive effects on the electron transport rate of QDSCs. Remarkably, the insertion of g-C₃N₄ sheets into ZnO NR photoanodes maximized the light harvesting efficiency of cell devices primarily because of the lower rate of photoinduced electron interception, the optimum loading of sensitizer QDs, and the stronger electronic interactions between g-C₃N₄ sheets and ZnO NRs (Table 1 and Fig. 5D–F).⁵⁴ Finally, the introduction of some metal oxides (e.g., Al₂O₃/MgO,⁵⁷ MoO₃,⁸⁶ BaTiO₃ (ref. 87) and Zn₂SnO₄ (ref. 88)) into TiO₂/ZnO photoanodes is also feasible to enhance the photovoltaic performance of QDSCs. For example, photoanodes of TiO₂ NPs were coated with two different insulating metal oxides of MgO and Al₂O₃ to optimize the passivation effect and reduce the charge recombination losses for higher performance QDSCs as compared to the bare TiO₂ films.⁵⁷

Fig. 5 (A) Schematic of the charge behavior, (B) IPCE plots, and (C) EIS spectra of QDSCs based on photoanodes of TiO₂ NPs/Ag NPs/ZnO NRs (reproduced from ref. 52 with permission from the American Chemical Society). (D) Schematic illustration of the fabrication process and charge transport of g-C₃N₄–ZnO NR composites, and (E) steady-state photoluminescence (PL) spectra of the composites with varying g-C₃N₄ to ZnO NR weight ratios and (F) I–V curves of the corresponding QDSCs (reproduced from ref. 54 with permission from the Royal Society of Chemistry).

Apart from TiO₂ and ZnO, a series of other materials including BaTiO₃,⁸⁹ BiVO₄,⁹⁰ NiO,⁹¹ CoO,⁹² SnO₂,⁹³ SrTiO₃,⁹⁴ Zn₂GaO₄,⁹⁵ Zn₂SnO₄,^96–98 ZnTiO₃ ⁹⁹ and ZrO₂ ¹⁰⁰ have been exploited to act as photoanodes in QDSCs. For example, cauliflower-like SnO₂ hollow MSs were sandwiched with carbon nanofibers as CEs to form QDSCs with a best PCE of 2.5%.¹⁰¹ NiO is a typical p-type semiconductor and has been used in p-type sensitized solar cells.^91,102–104 The application of mesoporous NiO electrodes combined with CuInS_xSe_2−x QDs in p-type QDSCs could give a cell efficiency of 1.25% by optimizing the materials.⁹¹

Recent advances in quantum dots of QDSCs

QDs, as an essential part in a QDSC, should possess a high absorption coefficient and an appropriate band-gap energy to maximize the harvesting efficiency of the incident light.^105,106 The energy levels of QDs applied in QDSCs must match those of the sensitized wide band-gap semiconductors. If the band-gap energy of the QDs is high, although a large V_OC can be achieved, the wavelength range of light absorption will be narrowed concurrently. Contrarily, a low band-gap energy of QDs may contribute to a wide wavelength region of light absorption, but inevitably may lead to a low V_OC.¹⁰⁵ Accordingly, numerous research attempts have been carried out to develop different QDs with suitable band gaps as light harvesters in QDSCs, including Ag₂S,¹⁰⁷ Bi₂S₃,¹⁰⁸ CdS,¹⁰⁹ CdSe,¹¹⁰ CdTe,¹¹¹ CuInS₂,^112,113 Cu₂ZnSnS₄,¹¹⁴ InAs,¹¹⁵ In₂S₃,¹¹⁶ InP,¹¹⁷ PbS,^118,119 PbSe,¹²⁰ Si,¹²¹ graphene,¹²² perovskite CH₃NH₃PbI₃ (ref. 123) and so on (Fig. 3 and Table 2). The strategies for fabrication of high-performance QDs can be generally summarized into several types: QD type (e.g., co-sensitization,¹²⁴ and ternary alloyed QDs¹²⁵), synthesis methods (e.g., chemical bath deposition (CBD),¹²⁶ successive ionic layer adsorption reaction (SILAR),¹²⁷ and linker-assisted assembly (LAA)¹²⁸), parameter optimization (e.g., size,¹²⁹ layer,¹³⁰ and coverage¹³¹), and modification treatment (e.g., doping,¹³² post-annealing,¹³³ and deposition of passivation layers¹⁰⁹).

Table 2 Photovoltaic parameters and photoactive materials of QDSCs based on different QDs

Quantum dot	Method	Photoanode	Electrolyte	Counter electrode	J SC (mA cm^−2)	V OC (mV)	FF	PCE (%)	Ref.
Ag2S/ZnS	SILAR	TiO2 NPs	I^−/I3^−	Pt	28.5	270	0.238	1.76	107
CdS/Bi2S3/ZnS	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	9.3	502	0.537	2.52	108
CdS/CdSe	ED	TiO2 MSs	S^2−/Sx^2−	Pt	18.23	489	0.54	4.81	136
CdSe/ZnS	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	13.9	538	0.51	3.84	124
CdSe/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	16.96	561	0.566	5.42	174
CdSe/ZnS	EPD	TiO2 NTs	S^2−/Sx^2−	Cu2S	9.0	554	0.35	1.7	177
CuInS2/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	20.65	586	0.581	7.04	112
CuInSe/ZnS	LAA/SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	26.93	528	0.57	8.10	16
CuInSexS2−x/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	16.8	560	0.59	5.51	151
Cu2ZnSnS4/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	18.86	510	0.494	4.70	114
CdS/CdSe/ZnS	CBD	TiO2 NPs	S^2−/Sx^2−	Au	16.8	514	0.49	4.22	126
CdS/CdSe/CdS/ZnS	CBD/SILAR	TiO2 NTs	S^2−/Sx^2−	Cu2S	17.7	555	0.557	5.47	109
CdS/CdSe/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	18.02	527	0.56	5.32	128
CdS/CdSe/ZnSe	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	20.11	580	0.55	6.39	166
ZnCuInSe/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Carbon	25.25	739	0.622	11.61	18
Mn–CdS/CdSe/ZnS	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	20.7	558	0.47	5.42	132
Ni-doped CdS	SILAR	TiO2 NPs	S^2−/Sx^2−	CuS	8.91	643	0.543	3.11	178
CdS/CdSe/Mn–ZnSe	SILAR	TiO2 NPs	S^2−/Sx^2−	CuS	17.59	584	0.551	5.67	185
CdSexTe1−x/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	19.35	571	0.575	6.36	125
CdSexTe1−x/ZnS/SiO2	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	20.78	653	0.605	8.21	189
CdSexTe1−x/CdS/ZnS/SiO2	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	20.82	712	0.639	9.48	155
CdSxSe1−x/CdSe/ZnS	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	14.22	557	0.553	4.46	148
CdTe/CdSexTe1−x/ZnS	LAA/SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	16.58	629	0.694	7.24	111
CdS/CdSxSe1−x/CdSe/ZnS	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	14.65	640	0.54	5.06	147
CdTe/CdSe/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	19.59	606	0.569	6.76	139
CdTe/CdS/CdS	LAA/SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	20.19	610	0.51	6.32	143
PbxCd1−xS/CdS/ZnS	SILAR	TiO2 NPs	S^2−/Sx^2−	Cu2S	24.47	421	0.52	5.33	140
PbS/CdS/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	18.81	595	0.642	7.19	141
ZnTe/CdSe/ZnS	LAA	TiO2 NPs	S^2−/Sx^2−	Cu2S	19.29	640	0.552	6.82	146
CH3NH3PbI3	CBD	TiO2 NPs	I^−/I3^−	Pt	15.82	706	0.586	6.54	123

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Among various QDs, Cd chalcogenide QDs (Fig. 3; e.g., CdS,¹³⁴ CdSe¹³⁵ and CdTe¹¹¹) are preferred because of their relatively high stability in QDSCs even though they maybe unstable under visible light. Nevertheless, other kinds of QDs with narrow band gaps (Fig. 3), such as PbS,¹¹⁸ PbSe,¹²⁰ Ag₂S¹⁰⁷ and InAs,¹¹⁵ have also been considered due to their abilities to widen the light absorption into the infrared (IR) region. In addition, co-sensitization of two or three types of QDs, for example, CdS/CdSe,¹³⁶ CdS/CdTe,¹³⁷ CdSe/CdTe,^138,139 CdS/PbS,^140,141 CdS/CdSe/PbS,¹⁴² CdTe/CdS/CdS,¹⁴³ CdS/CuInS₂,¹⁷ CuInS₂/In₂S₃,¹⁴⁴ In₂S₃/PbS,¹⁴⁵ and ZnTe/CdSe,¹⁴⁶ is well accepted because of the complementary effect of extending the wavelength range of light harvesting, and/or facilitating the electron transfer process. More significantly, the design of alloy materials, such as CdSe_xS_1−x,^147,148 CdSe_xTe_1−x,^149,150 CuInSe_xS_2−x,¹⁵¹ and Zn–Cu–In–Se,^18,152 has recently attracted much attention since it is feasible to engineer the physical characteristics (e.g., band gaps, energy band positions and photoelectrical properties) of QDs through controlling the compositional ratio between each chalcogenide and metal element.¹²⁵ For example, Zn–Cu–In–Se QDs with an absorption onset extending to ∼1000 nm were successfully fabricated for high-efficiency QDSCs with a champion PCE of 11.91% and a certified PCE of 11.61% (Table 2). The remarkably photovoltaic performance was supposed to be associated with the high loading capacity of QDs, broad light harvesting range stretching to near-IR light (Fig. 6A and B), and reduced charge recombination rate and fast electron extraction from the special alloyed structure of Zn–Cu–In–Se QDs (Fig. 6C).¹⁸ CdSe_xTe_1−x alloyed QDs are another promising alternative due to their intrinsic superior optoelectronic properties, such as an absorption edge extending to the near-IR region (Fig. 6D), a high conduction band edge, and high chemical stability.¹²⁵ Accordingly, the application of CdSe_xTe_1−x alloyed QDs with good crystalline quality and high surface coverage in QDSCs showed a stable cell efficiency of 6.36% (Table 2 and Fig. 6E and F).¹²⁵ By controlling the semiconductor photoanodes, a higher efficiency up to 7.55% was obtained.¹⁵³ After introducing Mn-doped CdSe and CdS coating layers onto the CdSe_xTe_1−x alloyed QDs, the corresponding efficiencies were further improved to 8.14% (ref. 154) and 9.48%,¹⁵⁵ respectively. In addition, QDSCs employing a sensitizer with a structure of Mn doped CdSe_xTe_1−x/Mn doped ZnS/SiO₂ also showed a best efficiency of 9.40%.¹⁵⁶ Moreover, when adopting Ti mesh supported mesoporous carbon (MC/Ti) films as CEs, the CdSe_0.65Te_0.35-based QDSCs exhibited a certified efficiency of 11.16%.¹⁵⁷

Fig. 6 (A) Schematic energy level diagrams of TiO₂, Cu–In–Se (CISe), and Zn–Cu–In–Se (ZCISe) QDs, (B) IPCE spectra and (C) Nyquist plots of the corresponding QDSCs (reproduced from ref. 18 with permission from the American Chemical Society). (D) Absorption spectra of CdSe_xTe_1−x, CdTe, and CdSe QD aqueous dispersions, (E) J−V curves of the corresponding QDSCs, and (F) temporal evolution of J−V curves of the QDSCs based on CdSe_xTe_1−x QDs (reproduced from ref. 125 with permission from the American Chemical Society).

To date, a series of routes have been developed to prepare QDs with high crystalline quality and well-defined particle sizes. Among them, CBD, SILAR and LAA are the three common techniques to synthesize and incorporate QDs into photoanodes as sensitizers (Table 2). CBD is one simple and cost-effective way to directly grow QDs onto nanostructured photoanodes by immersing photoactive electrodes into a solution containing the cationic (e.g., Cd) and anionic (S and Se) precursors for the slow precipitation of the desired QDs.^110,158 Intriguingly, some additional procedures, such as hydrothermal treatment,¹³³ microwave,^159,160in situ electric field,¹⁶¹ and linker seeding,^162,163 recently were adopted to assist the preparation of QDs via CBD for a facile, fast and effective deposition. SILAR is another simple and reproducible method involving dipping the nanostructured electrodes into different solutions in succession.^107,135,164 For one deposition cycle of QDs, the electrodes are firstly dipped into a solution with cationic precursor (e.g., Cd, Ag and Pb) to strongly adsorb the desired ions, subsequently water rinsing is performed to remove the excess solution on the electrodes, and finally a second dipping of the above electrodes into a new solution containing the corresponding anionic precursor (e.g., S and Se) for chemical reaction with the pre-adsorbed cations.^165,166 The particle sizes and quality of QDs prepared by SILAR can be readily optimized by tuning the reaction conditions, including deposition cycle,¹²⁴ concentration and type of precursors,^134,167 solution acidity,¹⁶⁸ some additives,¹⁶⁹ and so on. LAA involves the deposition of ex situ grown QDs onto nanostructured photoanodes.^111,170,171 In general, monodisperse QDs are pre-synthesized by using capping agents to adjust the nanocrystal size, shape and light absorption properties, and then they are cleaned and dissolved in an organic solvent for use. Meanwhile, the metal oxide photoanodes are soaked in a solution with a bifunctional molecular linker (e.g., (COOH)-R-SH, like mercapto-carboxylic acid). The carboxyl groups of this bifunctional molecular are able to link with the metal oxide, while the mercapto groups wait for the attachment with QDs. After immersing the modified metal oxide electrodes into the above mentioned solution of QDs, the mercapto groups assist the adsorption of QDs onto the surface of the metal oxide films typically by partial ligand exchange.^172–174 Additionally, other methods, such as pulsed laser deposition,¹⁷⁵ electrodeposition (ED),¹³⁶ spray pyrolysis deposition,¹⁷⁶ and electrophoretic deposition (EPD),^120,177 also can be found in some of the literature.

Metal ion dopants are usually employed to manipulate the optoelectronic properties of QDs in QDSCs. A variety of transition metal ions, such as Mn²⁺,¹³² Ni²⁺,¹⁷⁸ Co²⁺,¹⁷⁹ and Cu⁺,¹⁸⁰ have been reported recently for doping into QDs. Particularly, the doping of Mn²⁺ is most commonly used in QDs (e.g., CdS,¹⁸¹ CdSe,¹⁸² CdSe_xTe_1−x,¹⁵⁶ and PbS¹⁸³), which may produce new sub-band-gap electronic states due to the Mn d–d transition with a long emission lifetime of a few hundred microseconds and then significantly affect the charge transfer and recombination kinetics of QDs (Fig. 7A–C).¹³² Moreover, coating a passivation layer (e.g., ZnS,¹⁰⁹ ZnSe,^184,185 Cu_xS,¹⁸⁶ TiO₂ ¹⁸⁷ and Al₂O₃ ¹⁸⁸) onto QDs is also helpful to enhance the photovoltaic performance of QDSCs. For example, a thin ZnS overlayer onto CuInSe₂ QD-sensitized TiO₂ electrodes did not influence the energetic characteristics of the photoanodes; on the contrary, it was capable of reducing the interfacial electron recombination with electrolyte and nonradiative recombination associated with QDs, therefore yielding a best PCE of 8.10% (Fig. 7D–F).¹⁶ Recently, a combined ZnS/SiO₂ treatment on CdSe_xTe_1−x QDs was revealed to retard the interfacial recombination and improve the charge collection efficiency as compared with the conventional ZnS treatment alone.¹⁸⁹ In addition, an Al₂O₃ overlayer prepared by atomic layer deposition can remarkably boost the photoelectrochemical performance of PbS/CdS QD-sensitized TiO₂ nanotube arrays, primarily due to surface recombination passivation, catalytic improvement in interfacial charge transfer kinetics, and chemical stabilization.¹⁸⁸

Fig. 7 (A) Schematic diagram illustrating electron transfer (k_et), (B) J–V characteristics, and (C) photocurrent stability of Mn-doped CdS/CdSe (reproduced from ref. 132 with permission from the American Chemical Society). (D) TEM image of CuInSe₂-sensitized TiO₂ after 7 ZnS SILAR cycles, (E) J–V characteristics and (F) recombination resistance of the CuInSe₂ QDSCs with respect to the number of ZnS SILAR cycles (reproduced from ref. 16 with permission from the American Chemical Society).

Recent advances in electrolytes of QDSCs

Redox electrolytes, as the mediums to transfer charge between counter electrodes and photoanodes for the regeneration of oxidized QDs, significantly influence both the efficiency and stability of QDSCs.²⁵ Although the iodide/triiodide (I⁻/I₃⁻) couple serves as the most efficient electrolyte in DSCs, cadmium chalcogenide QD-based QDSCs are unstable in the iodide/triiodide electrolyte because of photoanodic dissolution and the formation of cadmium iodide. Thus, a polysulfide (S²⁻/S_n²⁻) redox couple aqueous solution electrolyte is commonly applied in QDSCs due to its ability to stabilize the cadmium chalcogenide QDs with a fairly decent performance of the corresponding QDSCs.¹⁹⁰ However, there are still some disadvantages of the polysulfide electrolyte; for example, the V_OC of the resultant QDSCs is usually lower than 0.65 V due to the relatively high redox potential of the polysulfide redox couple.⁸ Hence, many works have been done to optimize the polysulfide electrolyte, including controlling the concentration of the redox mediator,²¹ introducing additives (e.g., SiO₂,¹⁹¹ polyethylene glycol (PEG),¹⁹ poly(vinyl pyrrolidone) (PVP),¹⁹² and guanidine thiocyanate¹⁹³), and using a modifying solvent.¹⁹⁴ For example, it was reported that the addition of SiO₂ NPs into the polysulfide electrolyte enabled the formation of an energy barrier for the recombination at the photoanode/electrolyte interface (Fig. 8A), thereby contributing a higher electron collection efficiency (98%) (Fig. 8B) and a longer electron lifetime (Fig. 8C) for the resultant CdSe_xTe_1−x-based QDSCs with an impressive PCE of 11.23%.¹⁹¹ In addition, alternative redox couples with relatively low redox potentials, such as [Co(o-phen)₃]^2+/3+, Fe(CN)₆^3−/4−, Mn poly(pyrazolyl)borate and some organic redox couples,^195,196 have been exploited to improve the photovoltaic performance, especially the V_OC values of QDSCs. Unfortunately, the stability of the most popular cadmium chalcogenide QDs in these electrolytes is still a big problem.

Fig. 8 (A) Schematic illustration of the role of SiO₂ NPs in the polysulfide electrolyte as well as the electron transport and charge recombination processes, (B) charge collection efficiency (η_cc), and (C) plots of electron lifetime in CdSe_xTe_1−x QDSCs (reproduced from ref. 191 with permission from the Royal Society of Chemistry). (D) SEM cross-sectional image, (E) illustration of the proposed working mechanism, and (F) I–V curves of solid-sate QDSCs (reproduced from ref. 212 with permission from the American Chemical Society).

Moreover, for liquid electrolytes, sealing problems and long-term durability related to the leakage and volatilization of the liquid solution substantially hinder the practical applications of QDSCs. Thus, alternatives to liquid electrolytes, that is, quasi-solid-state and solid-state electrolytes have been developed as well. Ionic liquids,¹⁹⁷ gel electrolytes,^198–202 and hydrogel electrolytes^203,204 containing redox couples are commonly used as quasi-solid-state electrolytes to overcome the stability challenge of liquid electrolytes. For example, a gel electrolyte containing sodium carboxymethylcellulose (CMC-Na) to solidify a polysulfide aqueous solution showed superabsorbent and water-holding capability with a high ion mobility in the three dimensional (3D) porous network, and then a notable PCE of 9.21% was achieved when it was applied in CdSe_xTe_1−x-sensitized QDSCs.²⁰⁵ Solid-state electrolytes, including various hole-transporting materials, are also popular due to their relatively high stability. Organic polymers (e.g. 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD),^206–209 poly(3-hexylthiophene) (P3HT),^210–212 3,3′′′-didodecylquaterthiophene (QT12),²¹³ PVP,²¹⁴ and poly(ethylene oxide)-poly(vinylidene fluoride) (PEO-PVDF)²¹⁵) have been successfully employed in solid-state QDSCs. For instance, solid-state QDSCs with a structure of FTO/blocking TiO₂/mesoscopic TiO₂/PbS/CuS QDs/P3HT/Au presented an impressive PCE of 8.07% (Fig. 8D–F; V_OC: 0.6 V, J_SC: 20.7 mA cm⁻², and FF: 0.65).²¹²

Recent advances in counter electrodes of QDSCs

CEs in QDSCs have the important task of transferring electrons from the external circuit and catalyzing the reduction reaction of the oxidized electrolytes at the electrolyte/CE interface. To ensure a superior electrocatalytic performance, CEs should possess high electrical conductivity, excellent electrocatalytic activity and great stability.^10,216 Noble metals, especially Pt films, are the most common CE materials in DSCs due to their good electrocatalytic ability for the reduction of iodide/triiodide electrolyte. However, Pt CEs are poorly active in polysulfide electrolyte since sulfur atoms would easily adsorb onto the Pt surface to lower the conductivity and then cause a considerable overpotential for electrolyte reduction, consequently reducing the device efficiency of QDSCs.²³ Accordingly, alternative CE materials in QDSCs have been extensively investigated, which can be generally summarized into several main types (Table 3): metal chalcogenides,^217,218 carbon-based materials,^219,220 and various composites.^22,221,222

Table 3 Photovoltaic parameters and photoactive materials of QDSCs based on different CEs

Counter electrode	Photoanode	Quantum dot	Electrolyte	J SC (mA cm^−2)	V OC (mV)	FF	PCE (%)	Ref.
Cu2S/brass	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	14.54	634	0.667	6.15	226
Cu2−xSe NPs	TiO2 NPs	CdSeTe/ZnS/SiO2	S^2−/Sx^2−	21.09	673	0.614	8.72	24
Cu7S4 NTs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	19.09	480	0.494	4.53	234
Cu1.8S NSs	TiO2 MSs	CdS/CdSe	S^2−/Sx^2−	16.07	500	0.41	3.30	237
CuS NSs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	17.43	596	0.495	5.15	238
CuS NPs	TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	17.7	551	0.491	4.78	239
CuS MSs	TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	16.05	550	0.489	4.32	240
Mn-doped CuS NPs	TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	17.58	583	0.532	5.46	241
Cu1.8Se NSs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	20.5	540	0.50	5.01	242
Cu2−xSe NTs	TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	18.85	559	0.593	6.25	243
Cu2−xSe NPs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	21.39	567	0.586	7.11	244
Cu2SnSe3 NPs	TiO2 NPs	CdSe	S^2−/Sx^2−	16.29	563	0.54	4.93	218
Co9S8 NRs	TiO2 NRs	CdS/CdSe	S^2−/Sx^2−	16.80	480	0.462	3.72	217
Cu-doped CoS NSs	TiO2 NPs	CdSe0.45Te0.55/ZnS	S^2−/Sx^2−	18.2	565	0.593	6.1	246
CoS2 NPs	TiO2 MSs	CdS/CdSe	S^2−/Sx^2−	14.44	510	0.565	4.16	247
FeS2 MSs	ZnO NWs	ZnSe/CdSe/ZnS	S^2−/Sx^2−	13.58	743	0.387	3.90	248
FeS NSs/Ni foam	TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	18.70	410	0.58	4.39	250
MoS2 NSs	TiO2 NPs	CdS/ZnS	S^2−/Sx^2−	6.22	477	0.41	1.21	251
NiS NPs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	11.67	493	0.527	3.03	253
NiCo2S4 NRs	TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	15.58	550	0.489	4.22	254
Cu2ZnSnS4 MSs	ZnO NWs	ZnSe/CdSe	S^2−/Sx^2−	11.06	822	0.41	3.73	255
Cu2ZnSn(S1−xSex)4	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	12.71	550	0.43	3.01	258
PbS NPs	TiO2 NPs	CuInS2/CdS/ZnS	S^2−/Sx^2−	18.3	580	0.45	4.7	261
PbSe NPs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	16.91	570	0.48	4.67	262
CuAgSe NTs	TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	17.57	581	0.55	5.61	265
CoS MSs/NiS NPs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	14.76	590	0.53	4.70	268
NiS NPs/PbS NPs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	14.52	596	0.539	4.52	271
CNTs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	16.09	586	0.495	4.67	219
Carbon NFs	TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	11.99	620	0.60	4.81	274
Carbon NPs	TiO2 MPs	CdS/CdSe/ZnS	S^2−/Sx^2−	13.53	510	0.40	2.67	276
Carbon MSs	TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	12.41	600	0.52	3.90	277
Carbon foam	ZnO NWs	CdS/CdSe	S^2−/Sx^2−	12.6	685	0.42	3.60	278
C60	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	12.6	546	0.60	4.18	279
AuPt NPs/graphene	ZnO NWs	CdS/CdSe	S^2−/Sx^2−	15.2	720	0.409	4.5	280
Pt/CuS	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	15.31	600	0.46	4.22	281
Au/CuS NSs	TiO2 NPs	PbS/ZnS	S^2−/Sx^2−	24.17	420	0.564	5.73	282
Cu2S MSs/RGO	TiO2 MSs	CdS/CdSe	S^2−/Sx^2−	15.85	556	0.44	3.85	23
CuS NPs/carbon NFs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	14.46	521	0.507	3.86	286
CuS NPs/3D graphene	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	16.19	590	0.528	5.04	287
Cu1.18S/GO nanoribbon	TiO2 NPs	CdTe/CdS/CdS	S^2−/Sx^2−	20.55	626	0.53	6.81	288
FeS/carbon NPs	TiO2 NPs	CdS/CdSe	S^2−/Sx^2−	20.33	440	0.51	4.58	289
PbS NPs/graphene	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	11	559	0.42	2.63	290
ITO NWs/Cu2S NPs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	14.31	540	0.525	4.06	291
CuS NPs/ZnO NRs	ZnO NRs	CdS/CdSe	S^2−/Sx^2−	14.48	760	0.38	4.18	292
PbS NPs/ZnO NRs	TiO2 NPs	CdS/CdSe/ZnS	S^2−/Sx^2−	13.28	633	0.566	4.76	293
NiS NPs/PANI NFs	TiO2 NPs	CdSe	S^2−/Sx^2−	13.62	610	0.50	4.15	294
Mo2N/CNT–RGO	ZnO NWs	CdS/CdSe	S^2−/Sx^2−	16.93	680	0.47	5.41	295
Carbon MPs/Ti mesh	TiO2 NPs	CdSe0.65Te0.35/ZnS/SiO2	S^2−/Sx^2−	20.68	798	0.68	11.16	157
CNT/Cu2ZnSnSe4 NPs	TiO2 NPs	CdSe/ZnS	S^2−/Sx^2−	17.04	530	0.51	4.60	256
TiN NPs/CNTs–RGO	ZnO NWs	CdS/CdSe	S^2−/Sx^2−	14.0	642	0.46	4.13	222
TiC NPs	TiO2 NPs	CdS	S^2−/Sx^2−	8.51	530	0.47	2.11	297

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Metal chalcogenides, including a large variety of compounds, have been much exploited to replace the conventional Pt or Au CEs for DSCs and QDSCs.^223,224 Among these materials, Cu₂S CEs are the most popular choice due to their relatively high electrocatalytic activity in the polysulfide redox system. Normally, Cu₂S CEs are obtained by placing brass sheets in a sulfide solution to form Cu₂S layers on the metal surface.^225,226 Nevertheless, the problem of mechanical instability is inevitable for these Cu₂S CEs when they are used in polysulfide electrolyte. Accordingly, copper chalcogenides (i.e., Cu_xS and Cu_xSe) with different stoichiometric forms and various morphologies (Table 3) have been successfully developed to function as effective CE materials in QDSCs with high electrocatalytic activity as well as excellent stability.^{218,226–244} For example, Cu_xS films with various stoichiometries were prepared to study the Cu/S ratio effect on the electrocatalytic activity toward polysulfide reduction (Fig. 9A). It was indicated that Cu-deficient films (CuS and Cu_1.12S) were superior to Cu-rich films (Cu_1.75S and Cu_1.8S) as CEs in QDSCs (Fig. 9B and C). Meanwhile, the stability of CuS and Cu_1.12S CEs was also higher than that of Cu_1.75S and Cu_1.8S.²³⁶ In addition, many other metal chalcogenides with satisfactory electrocatalytic properties have been fabricated (Table 3), including cobalt sulfides (e.g., CoS,^245,246 Co₉S₈,²¹⁷ and CoS₂ ²⁴⁷), iron sulfides (e.g., FeS₂,²⁴⁸ and FeS^249,250), molybdenum sulfides (e.g., MoS₂),²⁵¹ nickel sulfides (e.g., Ni₂S₃,²⁵² and NiS²⁵³), nickel cobalt sulfides (e.g., NiCo₂S₄),²⁵⁴ copper zinc tin sulfides/selenides (e.g., Cu₂ZnSnS₄ (CZTS)^255–257 and Cu₂ZnSn(S_1−xSe_x)₄ ²⁵⁸), lead sulfides/selenides (e.g., PbS,^259–261 and PbSe^242,262), and others (e.g., AgBiS₂,²⁶³ Bi₂S₃,²⁶⁴ CuAgSe,²⁶⁵ Cu₂SnS₃,²⁶⁶ and Cu₂SnSe₃ ²¹⁸). For example, Cu–CoS porous nanosheet films grown on FTO substrates via a hydrothermal process followed by a cation exchange reaction exhibited excellent electrocatalytic activity toward polysulfide electrolyte with a best PCE of 6.1% and high stability for the resultant QDSCs.²⁴⁶ Besides, combining two kinds of metal chalcogenides, such as CoS/CuS,²⁶⁷ CoS/NiS,²⁶⁸ CuS/NiS,²⁶⁹ CuS/PbS,²⁷⁰ and NiS/PbS,²⁷¹ is usually adopted to further boost the electrocatalytic performance of CEs in QDSCs. For instance, NiS/PbS composites (4.52%) prepared by the CBD method showed high electrocatalytic activity and stability in the corresponding QDSCs as compared with NiS (3.26%), PbS (3.06%) and Pt (1.29%) CEs (Fig. 9D–F).²⁷¹

Fig. 9 (A) Photographs and (B) EIS plots of different copper sulfides, and (C) J–V curves of QDSCs with various Cu_xS CEs (reproduced from ref. 236 with permission from the American Chemical Society). (D) SEM image of NiS/PbS electrode, (E) cyclic voltammetry measurements of NiS/PbS and Pt electrodes in a polysulfide redox couple, and (F) stability test of QDSCs based on different CEs (reproduced from ref. 271 with permission from the Royal Society of Chemistry). (G) SEM image of ITO/Cu₂S film, (H) J–V curves of QDSCs and (I) stability test of a symmetrical cell based on ITO/Cu₂S CEs (reproduced from ref. 291 with permission from the American Chemical Society).

Carbon-based materials are widely applied as CEs both in DCSs and QDSCs because of their high electrical conductivity, great electrocatalytic activity, and excellent thermal and chemical stability.^216,272,273 A series of carbon-based materials, including carbon films with various structures (e.g., NTs,²¹⁹ NFs,²⁷⁴ dots,²⁷⁵ NPs,²⁷⁶ hollow spheres²⁷⁷ and foam²⁷⁸), C₆₀,²⁷⁹ and graphene,²⁸⁰ are well confirmed as highly efficient CEs in QDSCs. For example, Ti mesh supported mesoporous carbon (MC/Ti) with a 3D interconnected framework not only provided high electrocatalytic activity, but also reduced the redox potential of polysulfide electrolyte for a high photovoltage, thereby yielding an unprecedented PCE of 11.16% with a remarkable enhancement of over 26% as compared to Cu₂S CEs (8.79%) in QDSCs.¹⁵⁷

Composite films usually consisting of two or more components that combine the advantages of each component into one have been widely accepted as effective CEs in QDSCs. Significantly, these composite CEs involve a variety of functional materials, such as novel metals combined with metal sulfides (e.g., Pt/CuS,²⁸¹ Au/CoS,²² and Au/CuS²⁸²), metal sulfides (e.g., Cu_xS,^23,283–288 FeS,²⁸⁹ and PbS²⁹⁰) combined with carbon-based materials, ITO/Cu₂S,²⁹¹ CuS/ZnO,²⁹² PbS/ZnO,²⁹³ CNTs/PANI,²²¹ NiS/PANI,²⁹⁴ Mo-compound/CNT/graphene composites,²⁹⁵ and so on. For example, Cu₂S nanocrystals were selectively grown on conductive single-crystalline ITO nanowire arrays to assemble 3D hierarchically nanostructured composites (Fig. 9G), which exhibited a superior electrocatalytic performance in QDSCs and thus a highest PCE of 6.12% with a 21.2% improvement as compared with a conventional brass/Cu₂S CE (Fig. 9H and I).²⁹¹

Moreover, other special materials, such as metal nitrides (e.g., NiN,²⁹⁶ and TiN²²²), metal carbides (e.g., TiC),²⁹⁷ conductive polymers (e.g., poly(3,4-ethylenedioxythiophene) (PEDOT)²⁹⁸ and PANI²⁹⁴), and so on, have also been researched as CEs in QDSCs. For example, Ti-based ceramic materials of TiN and TiC NPs were employed as CEs in CdS QDSCs, and the TiC NPs showed the best efficiency (2.11%) as compared with TiN NPs (1.97%) and Au films (1.64%).²⁹⁷

Conclusion and future prospects

Over the past few years, QDSCs have been the subject of extensive studies for the optimization of all the active materials relating to cell devices. Significantly, the conversion efficiency of QDSCs has greatly improved from less than 1% up to 12% in no more than ten years. In this feature article, we roughly summarize the recent developments of photoanodes, QD sensitizers, electrolytes and CEs in QDSCs. Many attempts have been carried out to obtain effective photoanode materials for fast electron extraction, optimum QDs to maximize light harvesting, stable electrolyte mediums for efficient charge transport, and versatile CE materials with excellent electrocatalytic ability. Remarkably, it is inspiring that the best PCE of QDSCs is now comparable to that of DSCs. However, many challenges still hinder the practical applications of QDSCs, such as achieving higher efficiency, long-term durability, and large-scale production. Therefore, the future big leaps in QDSCs will involve rational strategies to increase the V_OC and FF values of cell devices by optimizing the relevant active materials, developing more stable sensitizer layers and electrolytes or hole-transporting materials for solid-state QDSCs, and exploiting some techniques available for large-scale manufacture.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Nature Science Foundation of China (no. 21503177), the Natural Science Foundation of Fujian Province of China (no. 2017J01026), the Fundamental Research Funds for the Central Universities of China (no. 20720150031), and the “111” Project (B16029).

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