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

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 (S mu S and F mu S 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 S mu S and F mu S microsphere photoanode layers exhibit a photoconversion efficiency (eta) of 3.2% and 1.6%, respectively, in comparison to eta approximate to 2.1% for nanoparticulate TiO2 (Degussa P25). Further increase in efficiency is obtained (3.8% for S mu S and 2.5% for F mu S) when composite photoanode films made of porous microspheres filled with nanoparticulate P25 are used. A maximum efficiency of 3.8% (with J(SC) approximate to 6.2 mA, V-OC approximate to 926 mV and FF approximate to 66 for cell area approximate to 0.25 cm(2) and thickness approximate to 20 mu m) is realized when 4.6 nm CIS QDs sensitized on composite photoanode (consisting of 80 wt. % S mu S and 20 wt. % P25) is used. High V-OC observed is unprecedented and is possible due to combined effect of S mu S + 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 S mu S + P25 composite photoanodes.