Photon recycling in lead iodide perovskite solar cells

Lead-halide perovskites have emerged as high-performance photovoltaic materials. We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites. We observed light emission at distances of >= 50 micrometers and found that the peak of the internal photon spectrum red-shifts from 765 to >= 800 nanometers. We used a lateral-contact solar cell with selective electron-and hole-collecting contacts and observed that charge extraction for photoexcitation >50 micrometers away from the contacts arose from repeated recycling between photons and electron-hole pairs. Thus, energy transport is not limited by diffusive charge transport but can occur over long distances through multiple absorption-diffusionemission events. This process creates high excitation densities within the perovskite layer and allows high open-circuit voltages.