Multi-Functional MoO₃ Doping of Carbon-Nanotube Top Electrodes for Highly Transparent and Efficient Semi-Transparent Perovskite Solar Cells

MoO₃ doping of carbon-nanotube top electrodes in perovskite solar cells is multi-functional and facilitates p-doping, favorable energy-level alignment, and enhanced hole transport. The optimal layer thickness of MoO₃ (8 nm) is determined for decreasing the sheet resistance of carbon-nanotube electrodes without damaging the perovskite film. The sheet resistance decreases by approximately one-third from its original value, which is a substantially better result than that previously reported for acid doping of carbon-nanotube top electrodes. MoO₃ deposition lowers the Fermi level of the carbon-nanotube electrode, improving its energy-level alignment and hole-transfer performance. When coated with 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD), MoO₃ crystallizes on the carbon nanotubes and further enhances hole collection. Semi-transparent perovskite solar cells with MoO₃-doped carbon-nanotube electrodes have a power conversion efficiency of 17.3% with a transmittance of approximately 60% (at a wavelength of 1000 nm). Because of their favorable transparency in the infrared region, these perovskite solar cells are evaluated for use in a tandem structure with silicon solar cells via computational simulations. The predicted device efficiency (23.7%) exceeds that of conventional indium-tin-oxide-based tandem solar cells (23.0%).