CuInS2 quantum dot sensitized solar cells with high VOC ≈ 0.9 V achieved using microsphere-nanoparticulate TiO2 composite photoanode

Abstract

CuInS2 (CIS) based solar cell devices are fabricated by sensitizing TiO2 photoanodes with CIS quantum dots (CIS-QDs). Morphologically different TiO2, viz. Degussa P25 nanoparticles, smooth and fibrous microspheres (SμS and FμS respectively) are used to fabricate photoanodes. CIS-QDs are synthesized using dodecanethiol (DDT), CuI and In(OAc)3 precursors by solvothermal method. DDT surfactant present on the CIS QDs surface is replaced with 3-mercaptopropionic acid in a single phase one step procedure to enable efficient loading of QDs onto photoanode and as linker molecule for charge carrier extraction. The CIS QDs sensitized on SμS and FμS microsphere photoanode layers exhibit a photoconversion efficiency (η) of 3.2% and 1.6%, respectively, in comparison to η ≈ 2.1% for nanoparticulate TiO2 (Degussa P25). Further increase in efficiency is obtained (3.8% for SμS and 2.5% for FμS) when composite photoanode films made of porous microspheres filled with nanoparticulate P25 are used. A maximum efficiency of 3.8% (with JSC ≈ 6.2mA, VOC ≈ 926mV and FF ≈ 66 for cell area ≈ 0.25cm2 and thickness ≈ 20µm) is realized when 4.6nm CIS QDs sensitized on composite photoanode (consisting of 80wt. % SμS and 20wt. % P25) is used. High VOC observed is unprecedented and is possible due to combined effect of SμS+P25 composite photoanode properties such as fewer defects, good connectivity between particles, effective light scattering, minimum recombination, and effective electron transport and size optimized CuInS2 QDs. Electrochemical impedance spectroscopy studies reveal a low interfacial resistance and longer electron life time in SμS+P25 composite photoanodes.