Solution‐Processed TiO2 Nanoparticles as the Window Layer for CuIn(S,Se)2 Devices

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 CuInS_xSe_1?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%).