Zhonglin Du

Zn–Cu–In–Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%

The enhancement of power conversion efficiency (PCE) and the development of toxic Cd-, Pb-free quantum dots (QDs) are critical for the prosperity of QD-based solar cells. It is known that the properties (such as light harvesting range, band gap alignment, density of trap state defects, etc.) of QD light harvesters play a crucial effect on the photovoltaic performance of QD based solar cells. Herein, high quality ∼4 nm Cd-, Pb-free Zn-Cu-In-Se alloyed QDs with an absorption onset extending to ∼1000 nm were developed as effective light harvesters to construct quantum dot sensitized solar cells (QDSCs). Due to the small particle size, the developed QD sensitizer can be efficiently immobilized on TiO2 film electrode in less than 0.5 h. An average PCE of 11.66% and a certified PCE of 11.61% have been demonstrated in the QDSCs based on these Zn-Cu-In-Se QDs. The remarkably improved photovoltaic performance for Zn-Cu-In-Se QDSCs vs Cu-In-Se QDSCs (11.66% vs 9.54% in PCE) is mainly derived from the higher conduction band edge, which favors the photogenerated electron extraction and results in higher photocurrent, and the alloyed structure of Zn-Cu-In-Se QD light harvester, which benefits the suppression of charge recombination at photoanode/electrolyte interfaces and thus improves the photovoltage.

Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%

The mean power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of the counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with submillimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm(2), Voc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs.

Nitrogen-Doped Mesoporous Carbons as Counter Electrodes in Quantum Dot Sensitized Solar Cells with a Conversion Efficiency Exceeding 12%

The exploration of catalyst materials for counter electrodes (CEs) in quantum dot sensitized solar cells (QDSCs) that have both high electrocatalytic activity and low charge transfer resistance is always significant yet challenging. In this work, we report the incorporation of nitrogen heteroatoms into carbon lattices leading to nitrogen-doped mesoporous carbon (N-MC) materials with superior catalytic activity when used as CEs in Zn-Cu-In-Se QDSCs. A series of N-MC materials with different nitrogen contents were synthesized by a colloidal silica nanocasting method. Electrochemical measurements revealed that the N-MC with a nitrogen content of 8.58 wt % exhibited the strongest activity in catalyzing the reduction of a polysulfide redox couple (Sn2-/S2-), and therefore, the corresponding QDSC device showed the best photovoltaic performance with an average power conversion efficiency (PCE) of 12.23% and a certified PCE of 12.07% under one full sun illumination, which is a new PCE record for quantum dot based solar cells.

Optimization of TiO2 photoanode films for highly efficient quantum dot-sensitized solar cells

As a fundamental part of quantum dot-sensitized solar cells, the composition and configuration of the TiO2 photoanode film plays an important role in photovoltaic performance. In this work, the preparation and optimization of films have been systematically studied, including the TiCl4 treatment technique, transparent layer and light-scattering layer thickness and composition. Experimental results show that the sole TiCl4 treatment on fluorine doped SnO2 (FTO) glass is sufficient for achieving a high efficiency in the resultant cell devices when compared with the simultaneous treatment on both FTO glass and TiO2 mesoporous films. The thickness and porosity of the transparent layer have been optimized by tuning the number of transparent layers and the ethyl cellulose contents in the paste. Moreover, the influence of the light-scattering layer pastes with different contents of the large-sized TiO2 particles on the performance of the cells has also been explored. The CdSe-sensitized solar cells based on the optimized TiO2 film photoanode exhibits a power conversion efficiency of 5.53% under 1 full sun illumination, which is among the best efficiencies for plain CdSe QD-based solar cells.

Graphene hydrogel-based counter electrode for high efficiency quantum dot-sensitized solar cells

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.

Cuprous sulfide on Ni foam as a counter electrode for flexible quantum dot sensitized solar cells

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.

Metal–organic framework derived Co,N-bidoped carbons as superior electrode catalysts for quantum dot sensitized solar cells

An efficient electrocatalyst for the reduction of polysulfide electrolytes is vital to the construction of quantum dot sensitized solar cells (QDSCs). Herein, Co,N-bidoped carbon nanomaterials, prepared simply via the pyrolysis of bimetallic (Zn and Co) zeolite-type-metal organic frameworks, were for the first time attempted as electrocatalysts to develop counter electrodes (CEs) for QDSCs. The CEs developed exhibit superior catalytic activity for polysulfide reduction in QDSCs, resulting in a low charge transfer resistance at the interface of the CE/electrolyte, an improved fill factor (FF) and a high short circuit current density (Jsc). The outstanding performances of the CEs can be ascribed to the inherent characteristics of Co,N-bidoped carbons with homogeneously dispersed active dopants of Co and N atoms, large hydrophilic surface area, and good conductivity. Moreover, density functional theory (DFT) indicated that the Co–Nx sites would be active sites for polysulfide reduction. When Co,N-bidoped carbons deposited on F-doped tin oxide glass were used as CEs, an impressive power conversion efficiency of 9.12% (Voc = 0.635 V, Jsc = 26.15 mA cm−2, FF = 0.549) under one sun illumination with 100 mW cm−2 intensity was observed on QDSCs using Zn–Cu–In–Se QDs as sensitizers. Consequently, there is a good chance to fabricate QDSCs of high efficiency with the CEs derived from MOFs.

Titanium mesh based fully flexible highly efficient quantum dots sensitized solar cells

The development of highly efficient flexible quantum dot sensitized solar cells (QDSCs) has still been challenging. Herein, fully flexible titanium (Ti) mesh based QDSCs with desirable photovoltaic performance are proposed and fabricated successfully. ZnO/ZnSe/CdSe nanocrystals grown on a Ti mesh substrate as photoanodes were prepared by surface selenization of ZnO nanosheets to form ZnO/ZnSe core/shell nanostructures, followed by partial conversion of ZnSe to CdSe through an ion replacement process. Meanwhile, flexible Ti mesh based counter electrodes (CEs) were prepared with the use of structure controllable mesoporous carbon microspheres as catalytic materials. Benefiting from the in situ growth of ZnO/ZnSe/CdSe heterojunction photoanodes with effective light harvesting capacity and the highly catalytic activity of MC/Ti CEs, a new power conversion efficiency (PCE) record with an average value over 5% was achieved for fully flexible QDSCs under one full sun illumination.

Solar Paint from TiO2 Particles Supported Quantum Dots for Photoanodes in Quantum Dot–Sensitized Solar Cells

The preparation of quantum dot (QD)–sensitized photoanodes, especially the deposition of QDs on TiO2 matrix, is usually a time-extensive and performance-determinant step in the construction of QD-sensitized solar cells (QDSCs). Herein, a transformative approach for immobilizing QD on the TiO2 matrix was developed by simply mixing the as-prepared oil-soluble QDs with TiO2 P25 particles suspension for a period as short as half a minute. The solar paint was prepared by adding the TiO2/QD composite in a binder solution under ultrasonication. The QD-sensitized photoanodes were then obtained by simply brushing the solar paint on a fluorine-doped tin oxide substrate followed by a low-temperature annealing at ambient atmosphere. Sandwich-structured complete QDSCs were assembled with the use of Cu2S/brass as counter electrode and polysulfide redox couple as an electrolyte. The photovoltaic performance of the resulting Zn–Cu–In–Se (ZCISe) QDSCs was evaluated after primary optimization of the QD/TiO2 ratio as well as the thicknesses of photoanode films. In this proof of concept with a simple solar paint approach for photoanode films, an average power conversion efficiency of 4.13% (Jsc = 11.11 mA/cm2, Voc = 0.590 V, fill factor = 0.631) was obtained under standard irradiation condition. This facile solar paint approach offers a simple and convenient approach for QD-sensitized photoanodes in the construction of QDSCs.