Although copper sulfide and/or carbon materials have been utilized in counter electrodes (CEs) due to their good catalytic activity and conductivity, the efficiency of the assembled quantum dot-sensitized solar cells (QDSCs) is still unsatisfactory because of the relatively low photovoltage (Voc), which is commonly less than 0.7 V. In this study, graphene hydrogels (GHs) compressed onto titanium mesh served as the CE and the assembled CdSeTe QDSCs exhibited a photovoltaic conversion efficiency (PCE) of 9.85% and a Voc as high as 0.756 V, which increased by 19.0% and 14.9%, respectively, and are higher than those of the conventional CuS on FTO. By incorporating CuS nanoparticles into GH during gelation, the as-prepared GH–CuS CEs show further improved performance and the maximum PCE and Voc obtained were 10.71% and 0.786 V, respectively. The fill factor of the cells was also continuously increased. The excellent performance of the devices could be attributed to the synergistic effects of the water-rich GH (having a 3D porous structure accompanied by good conductivity) and highly catalytic CuS, reflected from the small series resistance, high catalytic activity, small electron transfer resistance, and stability, which have been confirmed by EIS, Tafel polarization, and CV curves.

A facile one-pot noninjection synthesis of CdTe magic-sized clusters (MSCs) and their use as building blocks for assembling two-dimensional (2D) quantum confined nanoplatelets (NPLs) are reported. Four distinct MSC families, with the first exciton absorption peaks at 447 nm (F447), 485 nm (F485), 535 nm (F535), and 555 nm (F555), are synthesized by the reaction between cadmium oleate and trioctylphosphine tellurium (TOP-Te) in octadecene media containing primary amine and TOP at appropriate intermediate temperatures. Especially, F447 is obtained in pure form and can self-assemble in situ into 2D NPLs in the reaction solution. The formation, growth, and transformation of CdTe MSCs are monitored mainly by UV–Vis absorption spectroscopy. The pure F447 and its assembled 2D NPLs are further characterized using transmission electron microscopy. The influence of various experimental variables, including reaction temperature, the nature, and amount of capping ligands, on the stability and growth kinetics of the obtained MSC families has been systematically investigated. Experimental results indicate that the appropriate reaction temperature and the presence of long hydrocarbon chain primary amines play a crucial role in the formation of MSCs and the subsequent assembly into 2D NPLs. Primary amines can also promote ultra-small sized CdTe regular nanocrystals to transform into MSCs, and therefore, CdTe MSCs can be obtained indirectly from regularly sized nanocrystals.

The development of a highly efficient stretchable counter electrode (CE) for quantum dot sensitized solar cells (QDSCs) is still challenging. In this work, a flexible Cu/Ni film has been pre-prepared via a novel redox reaction between Ni foam and Cu ions. Further, a flexible Cu2S/Ni CE was fabricated for the first time by sulfidation of the Cu/Ni film. A high photovoltaic conversion efficiency (PCE) of 8.94% for a model CdSeTe QDSC composed of the flexible Cu2S/Ni CE and a glass based photoanode was obtained, with the performance being strongly attributed to the excellent catalytic activity, high conductivity and good adhesion between Cu2S and Ni foam. Furthermore, flexible photovoltaic devices constructed using the as-prepared bendable Cu2S/Ni CE as well as a TiO2 based plastic photoanode have also been assembled with the highest PCE of 3.55%. Satisfactory mechanical properties and stability after repeated bending have also been achieved.

•Copper selenides with heavily deficient copper are used as counter electrodes.•The champion efficiencies of 6.49% and 8.72% for CdSe and CdSeTe QDSCs are achieved, respectively.•Copper selenide counter electrodes show high conductivity, stability and activity.Semiconductors such as sulfides have been commonly used as counter electrodes (CEs) in quantum dot sensitized solar cells (QDSCs) with high stability and catalytic activity. But the intrinsic unsatisfactory conductivity has been making researchers find alternative materials for further improving the efficiency. Here nanometer sized copper selenides substrated on F-doped SnO2 (CuxSe/FTO) are used as CE in the construction of QDSCs. Through optimizing the composition and structure variables of the CE materials, including Cu/Se ratio, film thickness, sintering temperature and time, we achieve the power conversion efficiencies up to 6.49% and 8.72% for CdSe and CdSeTe based QDSCs, respectively. Our results show that the excellent photovoltaic performance is strongly associated with the low Cu/Se molar ratio in the range of 1.20–1.38, suggesting the heavily deficient copper in CuxSe. The resultant good conductivity and electrochemical catalytic activity of the CuxSe/FTO CE have been verified by the electrochemical impedance spectroscopy, Tafel polarization and four-probe measurement results.

Because of their good conductivities and high catalytic activities, carbon materials and copper sulfides have been individually and jointly used as counter electrodes in quantum-dot-sensitized solar cells (QDSCs). However, obtaining a combination of high conversion efficiency and stability is still challenging. In this work, we present a facile method for fabricating Cu1.8S–C hybrid counter electrodes through the sulfidation of a copper–carbon composite synthesized by grinding a mixture of organic binder, commercial copper powder, and carbon material containing activated carbon and carbon black in a designed mass ratio. The assembled CdSeTe-sensitized QDSCs achieved a high PCE of 8.40%, larger than that of pure carbon (5.25%) and comparable to that of conventional CuxS/brass-based QDSCs (8.44%). Significantly, the devices based on Cu1.8S–C showed excellent stability. The improved performance is mainly attributed to the good conductivity and stability of carbon and the high catalytic activity of Cu1.8S.

Bi2S3 films consisting of two-dimensional interconnected Bi2S3 single-crystalline nanorod networks have been fabricated on a F:SnO2 (FTO) glass substrate through the formation of intermediate BiOI nanosheets from layer-structured BiI3 by chemical vapor deposition and subsequent hydrothermal transformation into Bi2S3 networks. A continuous lattice and structure-directed topotactic transformation mechanism is supposed for the formation of Bi2S3 network film. The prepared Bi2S3/FTO films were employed as counter electrode (CE) for CdSe quantum dot-sensitized solar cells for the first time and showed better photovoltaic performance than that from the convenient Pt CE. The influence of the preparation conditions for Bi2S3/FTO films on the resulting solar cell performance was systematically investigated and optimized with use of J–V curves, scanning electron microscopy (SEM), UV–vis absorption, and electrochemical impedance spectroscopy. To further improve the cell device efficiency, the modification of the Bi2S3 network CE with metal particles was also studied.

CdS nanocrystals with a wide variety of shapes including spheres, tetrahedra, and branched and flower-like structures have been conveniently tuned by only changing the amount of trioctylphosphine (TOP) used in a noninjection synthetic approach, in which CdO and S powder were directly used as reactants in paraffin media. The noninjection approach could be facilely scaled up to tens of grams scale in one batch reaction. The shape-controlled growth mechanism could be explained by the TOP-concentration-dependent nuclei structure and monomer concentration. This is further confirmed by the effects of the amount of oleic acid and the Cd/S ratio on the morphology of the obtained CdS nanocrystals. All the CdS nanocrystal samples with different morphologies exhibit good photocatalytic activity for degradation of dyes. The observed lower photocatalytic activity of the sphere-shaped CdS nanocrystals could be ascribed to the higher PL emission efficiency relative to those with other morphologies, which results in low electron–hole separation efficiency. Our reported preparation approach can satisfy the requirements of industrial production bearing the advantage of low-cost, reproducibility and scalability.

Nano-scaled semiconductors serving as photocatalysts in wastewater treatment are an important research area and the synergistic effect of adsorption and photocatalytic reaction should be considered in degradation. Bi2S3 nanostructures with different dimensionalities including dots, rods, and sheets have been synthesized for the study on their photocatalytic activities in degradation of organic dyes. X-ray diffraction and transmission electron microscopy results show that the orthorhombic structured samples have the dominant surface facets of (001), (010), and (100) for dots, rods, and sheets, respectively. The nanocrystals could selectively enhance the photodegradation of three typical dyes including rhodamine B, methylene blue, methyl orange, and the efficiencies are dependent on dimensionalities. Langmuir–Hinshelwood mode simulation studies reveal that the photodegradation satisfies the first-order dynamics for rods and sheets, while first-order exponential decay for dots. Investigations on the adsorption efficiency, visible-light irradiation, and activities on several cycles exhibit that the improvement in photodegradation efficiency of dots on rhodamine B is resulted from the synergistic effect of strong adsorption and photocatalytic reaction.Highlights► The degradation efficiencies of Bi2S3 depend on dimensionality. ► Synergistic effect of adsorption and photocatalytic reaction is evaluated. ► Rods and sheets as catalysts satisfy the first-order dynamics.

A surfactant-assisted colloidal chemistry technique for the controllable growth of various PbS architectural nanostructures catalyzed by Ag nanocrystal seeds was reported. The sizes and morphologies of the obtained PbS nanostructures were dependent on the amount of Ag nanocrystal seeds and the ratio of Pb to S precursors. By varying the amount of Ag seed, sphere-, cube- and rod-shaped PbS nanostructures were obtained; while the morphologies changed from nano-scaled sphere to small cube and then to large truncated-cube when decreasing the precursors’ ratio of Pb to S from 5:1 to 2:1 and then to 1:2 with the fixation of the amount of Ag seed. Control of morphology under different conditions has been systematically studied and the mechanism of morphology formation was also primarily discussed.

Inverted type-I CdS/CdSe hetero-nanostructures with various dimensionalities were controlled and synthesized through successive ionic layer adsorption and reaction method by tuning the reaction temperature, core size, and shell thickness. The morphologies and crystal structures were characterized by transmission electron microscopy and X-ray diffraction analyses. Hetero-nanospheres and nanotetrapods were obtained at relative high temperature and low temperature, respectively. The rod- and cone-shaped nanostructures were controlled via using different sized cores at medial growth temperature. All the hetero-nanostructures showed fine absorption and photoluminescence properties attributed to CdSe shell. The morphology evolution mechanism was also discussed.

CdSe quantum dot sensitized solar cells (QDSCs) modified with graphene quantum dots (GQDs) have been successfully achieved in this work for the first time. Satisfactorily, the optimized photovoltage (Voc) of the modified QDSCs was approximately 0.04 V higher than that of plain CdSe QDSCs, consequently improving the photovoltaic performance of the resulting QDSCs. Served as a novel coating on the CdSe QD sensitized photoanode, GQDs played a vital role in improving Voc due to the suppressed charge recombination which has been confirmed by electron impedance spectroscopy as well as transient photovoltage decay measurements. Moreover, different adsorption sequences, concentration and deposition time of GQDs have also been systematically investigated to boost the power conversion efficiency (PCE) of CdSe QDSCs. After the coating of CdSe with GQDs, the resulting champion CdSe QDSCs exhibited an improved PCE of 6.59% under AM 1.5G full one sun illumination.Graphene quantum dots (GQDs) were coated on CdSe to fabricate CdSe-GQDs QDSCs and the improved PCE of 6.59% was achieved. GQD coating effectively suppressed charge recombination.Download high-res image (138KB)Download full-size image

Although copper sulfide and/or carbon materials have been utilized in counter electrodes (CEs) due to their good catalytic activity and conductivity, the efficiency of the assembled quantum dot-sensitized solar cells (QDSCs) is still unsatisfactory because of the relatively low photovoltage (Voc), which is commonly less than 0.7 V. In this study, graphene hydrogels (GHs) compressed onto titanium mesh served as the CE and the assembled CdSeTe QDSCs exhibited a photovoltaic conversion efficiency (PCE) of 9.85% and a Voc as high as 0.756 V, which increased by 19.0% and 14.9%, respectively, and are higher than those of the conventional CuS on FTO. By incorporating CuS nanoparticles into GH during gelation, the as-prepared GH–CuS CEs show further improved performance and the maximum PCE and Voc obtained were 10.71% and 0.786 V, respectively. The fill factor of the cells was also continuously increased. The excellent performance of the devices could be attributed to the synergistic effects of the water-rich GH (having a 3D porous structure accompanied by good conductivity) and highly catalytic CuS, reflected from the small series resistance, high catalytic activity, small electron transfer resistance, and stability, which have been confirmed by EIS, Tafel polarization, and CV curves.

The development of a highly efficient stretchable counter electrode (CE) for quantum dot sensitized solar cells (QDSCs) is still challenging. In this work, a flexible Cu/Ni film has been pre-prepared via a novel redox reaction between Ni foam and Cu ions. Further, a flexible Cu2S/Ni CE was fabricated for the first time by sulfidation of the Cu/Ni film. A high photovoltaic conversion efficiency (PCE) of 8.94% for a model CdSeTe QDSC composed of the flexible Cu2S/Ni CE and a glass based photoanode was obtained, with the performance being strongly attributed to the excellent catalytic activity, high conductivity and good adhesion between Cu2S and Ni foam. Furthermore, flexible photovoltaic devices constructed using the as-prepared bendable Cu2S/Ni CE as well as a TiO2 based plastic photoanode have also been assembled with the highest PCE of 3.55%. Satisfactory mechanical properties and stability after repeated bending have also been achieved.