Introducing optically polarizable molecules into perovskite solar cells by simultaneously enhanced spin–orbital coupling, suppressed non-radiative recombination and improved transport balance towards enhancing photovoltaic actions

Abstract

By considering facile molecular diffusion and optically generated electrical polarization, this article reports, for the first time, that directly introducing optically polarizable non-fullerene electron acceptor ITIC molecules into organic–inorganic hybrid perovskite (MAPbI_3−xCl_x) films with uniform and non-uniform gradient distributions can surprisingly enhance spin–orbital coupling (SOC) with the consequence of increasing the conversion from photogenerated spin-singlets to spin-triplets, boosting the spin states available for photovoltaic actions and enhancing the power conversion efficiency (PCE) from 15.3% to 17.2% with a J_sc of 20.2 mA cm⁻², a V_oc of 1.08 V, and a FF of 79% in perovskite solar cells [ITO/PEDOT:PSS/MAPbI_3−xCl_x/PCBM/PEI/Ag]. Specifically, we find that dispersing ITIC molecules can largely decrease the ΔJ_sc (photocurrent difference) generated by switching the photoexcitation from linear to circular polarization with the same intensity. This phenomenon provides direct evidence that dispersed optically polarizable ITIC molecules can increase the SOC in organic–inorganic hybrid perovskites through interaction between introduced electrical polarization and SOC. Furthermore, impedance studies indicate that dispersing ITIC molecules can decrease the density of trap states by passivating the grain boundary defects and simultaneously increases the carrier mobilities. Consequently, non-radiative recombination is reduced upon decreasing the density of trap states through dispersing the ITIC molecules, according to transient photoluminescence studies. The UV-Vis and FTIR spectral studies indicate formation of a bond between Pb, N and O, showing a direct interaction between the hybrid perovskite MAPbI_3−xCl_x and dispersed ITIC molecules. Clearly, introducing optically polarizable non-fullerene electron acceptor molecules presents a new opportunity to enhance photovoltaic actions in perovskite solar cells through SOC and grain boundary passivation effects.