As a wide-bandgap semiconductor, titanium dioxide (TiO2) with a porous structure has proven useful in dye-sensitized solar cells, but its application in low-cost, high-efficiency inorganic photovoltaic devices based on materials such as Cu(InGa)Se2 or Cu2ZnSnS4 is limited. Here, a thin film made from solution-processed TiO2 nanocrystals is demonstrated as an alternative to intrinsic zinc oxide (i-ZnO) as the window layer of CuInSxSe1−x solar cells. The as-synthesized, well-dispersed, 6 nm TiO2 nanocrystals are assembled into thin films with controllable thicknesses of 40, 80, and 160 nm. The TiO2 nanocrystal films with thicknesses of 40 and 80 nm exhibit conversion efficiencies (6.2% and 6.33%, respectively) that are comparable to that of a layer of the typical sputtered i-ZnO (6.42%). The conversion efficiency of the devices with a TiO2 thickness of 160 nm decreases to 2.2%, owing to the large series resistance. A 9-hour reaction time leads to aggregated nanoparticles with a much-lower efficiency (2%) than that of the well-dispersed TiO2 nanoparticles prepared using a 15-hour reaction time. Under optimized conditions, the champion TiO2 nanocrystal-film-based device shows even higher efficiency (9.2%) than a control device employing a typical i-ZnO film (8.6%).
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