CuSe/CuxS as a composite counter electrode based on P-N heterojunction for quantum dot sensitized solar cells

In this work, we present a new design for the development of quantum dot sensitized solar cells with a CuSe/CuxS substrate on F-doped tin oxide as the counter electrode. The best performing CuSe/Cu0.78S counter electrode notably achieves a photoelectric conversion efficiency of 5.70% with a significant improvement in the J(sc) for the CdS/CdSe co-sensitized solar cells, superior to a cell with CuSe (4.80%) or Cu0.78S (4.11%) as the counter electrodes. Two types of reverse barriers, including a positive reverse barrier and negative reverse barrier, at the interface between the CuxS and CuSe are investigated using solid-state measurements from a dark current t-voltage test. The transient photovoltage responses and Kelvin probe measurements further demonstrate reduction of the positive reverse barrier between the CuSe and CuxS interface with increasing Cu/S ratios of CuxS (x = 0.52-0.78), which is beneficial to the electron transfer from CuxS to CuSe. The electrochemical catalytic activity and stability of the CuSe/CuxS counter electrodes are verified with electrochemical impedance spectroscopy, tafel polarization and cyclic voltammetry results, which suggest that CuSe/CuxS counter electrodes have promising applications in solar cells.