Significantly enhanced energy conversion efficiency of CuInS2 quantum dot sensitized solar cells by controlling surface defects

Aiming at improving the cell performance of colloidal CuInS2 (CIS) quantum dot sensitized solar cells (QDSCs), a modified synthetic method has been developed to prepare CIS quantum dots (QDs), and Cu/In non-stoichiometric ratios of CIS QDs have been carefully controlled for the first time. It is found that, with the amount of In element increasing, the short-circuit photocurrent density (J(sc)), open-circuit voltage (V-oc) and fill factor (FF) of CIS QDSCs will gradually increase, leading to the cell performance enhanced. Up to 8.54% PCE has been achieved when the Cu/In precursor molar ratio is 1/4, which is a new record for the CIS-based solar cells. Electrochemical impedance analysis, open-circuit voltage-decay (OCVD) and time-resolved photoluminescence analyses further confirm that In-rich CIS QDs can bring about surface defect states significantly reduced, thus leading to the charge recombination at TiO2/CIS/electrolyte interfaces efficiently inhibited. Interfacial electron recombination mechanism of the solar cells is proposed that photo-generated carrier recombination in the cell is mainly dominated by the electron transfer process from the conduction band of TiO2 to unoccupied defect states of CIS, which has a great influence on the FF of the device. This work provides a new and simple way to reduce the loss of photo-generated carriers, improve the interfacial carrier collection and achieve highly efficient QDSCs.