Photon Recycling in Perovskite
Written by Xiwen Gong
Photon recycling describes the process whereby photons generated through radiative recombination of light absorbers are re-absorbed by themselves, instead of being emitted out of the solar cell. This process tends to happen when there is fast radiative recombination and slow nonradiative recombination. Recently, photo recycling has been observed in lead halide perovskite, and is drawing increasing attention from the photovoltaic field.
Johannes Richter from the University of Cambridge mentioned in his talk that photon recycling is crucial to boost open circuit voltage (Voc), leading to a promising pathway to approach the Shockley-Queisser limit in perovskite solar cell devices. The photoluminescence peak shows redshift when the excitation spot was moved farther away from the collection position. Meanwhile, the redshifted luminescence shows longer lifetime, compared to that with shorter wavelength. This phenomenon indicates photon recycling might happen so that photon intensity was maintained at large distance and long timescale.
As the photon recycling and normal carrier diffusion process occurs simultaneously, it is complicated to distinguish one from another by simply measuring transient photoluminescence from the sample surface. Takumi Yamada from Tokyo University proposed to solve this problem by exciting the perovskite confocal two-photon source: the excitation depth is dramatically enhanced and tunable by changing the focus length. By comparing the photoluminescent lifetimes at different excitation depths, the researchers were able to obtain the kinetics from phonon recycling and diffusion separately.