Serendipitous Doping in Nickel Oxide upon Microwave-Induced Low-Temperature Crystallization Enhances Efficiency of Perovskite Solar Cells

Solution-processed nickel oxide (NiOx) requires high temperatures (approximate to 300 degrees C) and a long time (up to 1 h) to crystallize from precursors. Herein, thin films of NiOx are successfully crystallized at significantly low processing temperatures of approximate to 130 degrees C within a few minutes utilizing microwave irradiation. Microwave-annealed NiOx (MWA-NiOx) shows high electrical conductivity and transmittance comparable with those of conventional thermally annealed NiOx (CTA-NiOx). NiOx crystallization occurs by ohmic heating mechanisms in the indium tin oxide layer, which simultaneously facilitates the diffusion of In and Sn metal cations into the NiOx layer. Consequently, the chemical composition in the MWA-NiOx layer shows that both Ni2+ and Ni>3+ species are reduced, which is infeasible with CTA processes, where a decrease in one species necessitates an increase in another. MWA-NiOx is incorporated as a hole transport layer in triple-cation perovskite (Cs-0.05(MA(0.17)FA(0.83))(0.95)Pb(I0.83Br0.17)(3))-based solar cells. The short-circuit current and open-circuit voltage are enhanced compared with those of CTA-NiOx devices owing to the combined effects of enhanced conductivity, reduced Ni>3+ composition, and better energy-level alignment. This proposed crystallization technique of using microwave irradiation could be an effective alternative to conventional processes in improving the suitability of NiOx for optoelectronic applications.