Revealing photoinduced bulk polarization and spin-orbit coupling effects in high-efficiency 2D/3D Pb–Sn alloyed perovskite solar cells

Abstract This paper reports that growth-orientated grains by introducing bulky organic ligands can essentially leads to enhanced spin-orbit coupling (SOC) and out-of-plane photoinduced bulk polarization, which are accountable for the enhanced photovoltaic performance in 2D/3D Pb–Sn alloyed perovskite solar cells (PSCs). Specifically, the enhanced SOC in the 2D/3D perovskites is evidenced by our photoexcitation-polarization dependent photocurrent studies where the photocurrent is modulated by switching photoexcitation between linear and circular polarizations, and confirmed by magnetic field effects of photocurrent. The enhanced SOC can improve the spin conversion from optically generated bright states (spin-allowed to recombine) to dark states (spin-forbidden to recombine), increasing the possibilities for charge dissociation towards better photovoltaic actions. On the other hand, the out-of-plane orientated grains in 2D/3D perovskite with reduced traps are favorable to align optical transition dipoles. This provides the precondition to realize photoinduced bulk polarizations, which is first suggested by our photoinduced impedance measurements and further verified through photoexcitation polarization-direction dependent photoluminescence (PL) characteristics. It is noted that the photoinduced bulk polarization in 2D/3D perovskite can enhance charge dissociation via dipole-dipole interaction between excitons, towards enhancing photovoltaic actions. Additionally, the carrier lifetime in 2D/3D perovskite is increased due to the decreased nonradiative loss from reduced defects and the lowered Coulomb scattering from orbital polarization alignment. Therefore, our work reveals that photoinduced bulk polarization and enhanced SOC can be realized to enhance photovoltaic performance with the 2D/3D design in Pb–Sn alloyed solar cell.