Photon recycling in Lead-Iodide Perovskite solar cells (Conference Presentation)
We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites using a confocal microscopy setup with independent excitation and collection objectives. We observed regenerated PL emission at distances as far as 50 micrometers away from photoexcitation. We then made a scratch in the film to increase out-scattering and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. This is caused by the sharp decay of the absorption coefficient at the band tail, which allows longer wavelength photons to travel further between emission and absorption events, originating charges far from excitation. We then built a lateral-contact solar cell with selective electron- and hole-collecting contacts, using a combination of photolitography and electrodeposition. We used these devices as a platform to study photocurrent propagation and found that charge extraction can be achieved well beyond 50 micrometers away from the excitation. We connect these two observations by comparing the decay in intensity of the recycled component of the PL (which is around 765 nm) with the decay in photocurrent. Taking into account that PL is proportional to the square of charge density, whilst photocurrent is proportional to charge density. Photon recycling leads to an increase in internal photon densities, which leads to a build-up of excited charges. This increases the split of quasi-Fermi levels and enhances the achievable open circuit voltage in a solar cell.