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Optical Spintronics in Organic-Inorganic Perovskite Photovoltaics

Organic-inorganic halide CH$_3$NH$_3$PbI$_3$ solar cells have attracted enormous attention in recent years due to their remarkable power conversion efficiency. When inversion symmetry is broken, these materials should exhibit interesting spin-dependent properties as well, owing to their strong spin-orbit coupling. In this work, we consider the spin-dependent optical response of CH$_3$NH$_3$PbI$_3$. We first use density functional theory to compute the ballistic spin current generated by absorption of unpolarized light. We then consider diffusive transport of photogenerated charge and spin for a thin CH$_3$NH$_3$PbI$_3$ layer with a passivated surface and an Ohmic, non-selective contact. The spin density and spin current are evaluated by solving the drift-diffusion equations for a simplified 3-dimensional Rashba model of the electronic structure of the valence and conduction bands. We provide analytic expressions for the photon flux required to induce measurable spin densities, and propose that these spin densities can provide useful information about the role of grain boundaries in the photovoltaic behavior of these materials. We also discuss the prospects for measuring the optically generated spin current with the inverse spin Hall effect.