Photon recycling in lead iodide perovskite solar cells
Perovskite solar cells recycle photons Inorganic-organic perovskite solar cells are very efficient in part because the charge carriers exhibit very long path lengths. Pazos-Outón et al. show that photon recycling, as seen previously in highly efficient gallium arsenide solar cells, contributes to this effect (see the Perspective by Yablonovitch). In most solar cells, the recombination of photogenerated charge carriers (electrons and holes) wastes all of the energy. In these lead tri-iodide cells, recombination emits a photon that can be reabsorbed and create more charge carriers. Science, this issue p. 1430; see also p. 1401 Exceptionally long charge-extraction lengths are enabled by multiple cycles of photon absorption and emission. [Also see Perspective by Yablonovitch] 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-diffusion-emission events. This process creates high excitation densities within the perovskite layer and allows high open-circuit voltages.