Enhanced photovoltaic performance of quantum-dot-sensitized solar cells using Graphene/Cu2-xSe composite counter electrode

The counter electrode (CE) plays an important role in determining the overall performance of quantum dots (QDs) sensitized solar cells (QDSCs) by collecting the electrons from the external circuit and catalyzing the reduction of oxidized electrolyte. Metal sulfides/selenides have been commonly used as CE in QDSCs. Among these CEs, the copper selenides are considered to be a superior CE for realizing highly efficient QDSCs due to its excellent catalytic activity. However, there has always been a trade-off between catalytic activity and conductivity in copper selenides CEs, such as the CE based on copper selenide nanoparticles has highest specific surface areas, leading to best catalytic activity compared with the CEs based on other copper selenide nanostructures (nanowires, nanorods, nanosheets etc.). But it also faces the worst conductivity due to a large number of grain boundaries existed in the copper selenide film based on nanoparticles. In this work, graphene (GR)/Cu2-xSe composite material has been prepared via a facile hot injection method, and their photovoltaic and electrochemical properties are studied in detail. Through optimizing the ratio of GR: Cu2-xSe and sintering temperature, the QDSCs based on the optimized GR/Cu2-xSe composite CE shows an obvious improvement in cell performances and yield a PCE of 6.66% compared with that of the cells based on bare Cu2-xSe (5.95%), bare Graphene (2.82%) and bare CuxS (4.85%) CEs. The improved performance of the composite CE was attributed to the combining effect of high catalytic activity of Cu2-xSe nanoparticles and good conductivity of graphene. This work demonstrates that GR/Cu2-xSe composite CE could be potential candidate for realizing high efficiency QDSCs.