Synthesis of Eco-Friendly CuInS₂ Quantum Dot-Sensitized Solar Cells by a Combined Ex Situ/in Situ Growth Approach

A cadmium-free CuInS₂ quantum dot (QD)-sensitized solar cell (QDSC) has been fabricated by taking advantage of the ex situ synthesis approach for fabricating highly crystalline QDs and the in situ successive ionic-layer adsorption and reaction (SILAR) approach for achieving high surface coverage of QDs. The ex situ synthesized CuInS₂ QDs can be rendered water soluble through a simple and rapid two-step method under the assistance of ultrasonication. This approach allows a stepwise ligand change from the insertion of a foreign ligand to ligand replacement, which preserves the long-term stability of colloidal solutions for more than 1 month. Furthermore, the resulting QDs can be utilized as sensitizers in QDSCs, and such a QDSC can deliver a power conversion efficiency (PCE) of 0.64%. Using the SILAR process, in situ CuInS₂ QDs could be preferentially grown epitaxially on the pre-existing seeds of ex situ synthesized CuInS₂ QDs. The results indicated that the CuInS₂ QDSC fabricated by the combined ex situ/in situ growth process exhibited a PCE of 1.84% (short-circuit current density = 7.72 mA cm^–2, open-circuit voltage = 570 mV, and fill factor = 41.8%), which is higher than the PCEs of CuInS₂ QDSCs fabricated by ex situ and in situ growth processes, respectively. The relative efficiencies of electrons injected by the combined ex situ/in situ growth approach were higher than those of ex situ synthesized CuInS₂ QDs deposited on TiO₂ films, as determined by emission-decay kinetic measurements. The incident photon-to-current conversion efficiency has been determined, and electrochemical impedance spectroscopy has been carried out to investigate the photovoltaic behavior and charge-transfer resistance of the QDSCs. The results suggest that the combined synergetic effects of in situ and ex situ CuInS₂ QD growth facilitate more electron injection from the QD sensitizers into TiO₂