Metal halide perovskites combine strong optical absorption with high luminescence efficiencies, making the interaction between photon transport and charge-carrier recombination particularly important for their photovoltaic performance. Photon recycling – the reabsorption and subsequent re-emission of internally generated photons – has therefore been proposed as an important mechanism for reducing non-radiative voltage losses and approaching the radiative efficiency limit. However, accurately determining the internal luminescence quantum efficiency from external photoluminescence measurements requires a quantitative understanding of photon escape, reabsorption, and recycling within the perovskite layer.

Therefore, we investigated photon propagation, reabsorption, scattering, and recycling in highly luminescent polycrystalline metal halide perovskite thin films. We demonstrate that photoluminescence (PL) spectra strongly depend on the experimental collection geometry and show that commonly observed asymmetric, broadened and red-shifted PL spectra can be explained by the propagation and scattering of initially trapped photons. In particular, photons confined by total internal reflection can propagate laterally over substantial distances and become outcoupled through scattering before being reabsorbed. [1]

Based on these observations, we developed a quantitative optical model that decomposes externally emitted PL into directly escaping and scattering-induced contributions and determines the effective photon escape probability from experimentally measured PL spectra. The model was validated by Monte Carlo simulations and applied to highly luminescent CH₃NH₃PbI₃ thin films exhibiting external luminescence quantum efficiencies of up to 47.4%. We find effective photon escape probabilities exceeding 25%, more than twice the values commonly assumed for planar perovskite films. Consequently, previously reported estimates of internal luminescence quantum efficiencies approaching 90% are revised to a benchmark value of 78.0 ± 0.5%. [1, 2] These results demonstrate that scattering and photon outcoupling need to be considered explicitly when evaluating photon recycling and reveal substantially greater scope for reducing non-radiative recombination losses than previously anticipated.

More generally, these findings illustrate the intimate interplay between optoelectronic material quality and optical photon management in perovskite photovoltaics. While photon recycling becomes increasingly relevant as perovskite absorbers approach the radiative limit, controlling photon propagation, parasitic absorption, reflection, and spectral utilization becomes equally important in complex multi-junction architectures.

Building on this perspective, I will finally provide an outlook on recent work at KIT toward light management in monolithic perovskite/perovskite/silicon triple-junction solar cells. [3,4] Optical simulations and experimental optimization of absorber bandgaps and thicknesses enable improved current matching between the individual sub-cells, contributing to triple-junction devices with power conversion efficiencies exceeding 24%, [3] with recent advances in interfacial engineering pushing efficiencies beyond 30%. These developments highlight how understanding photon transport at the material level can inform optical design strategies for next-generation multi-junction photovoltaics approaching their fundamental efficiency limits.