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First, I will discuss the effects of photon reabsorption and surface scattering on the photoluminescence (PL) spectra and the extent of photon recycling in polycrystalline perovskite thin films. A novel curve-fitting model is introduced that, for the first time, allows the determination of the total photon escape probability from measurements of the external luminescence quantum efficiency and a PL spectrum using an integrating sphere setup [1]. Thereby, the large variety of reported externally observed PL spectra (asymmetric/red-shifted/broadened) for perovskite thin films can be explained simply by considering photon propagation and scattering. The model and assumptions are tested through various control experiments and Monte Carlo simulations. Applying the model to highly luminescent CH3NH3PbI3 films with external luminescence quantum efficiencies of up to 47.4% allows to correct previous estimates of the internal luminescence quantum efficiency of ~90% to a new benchmark of ~78% [1]. This emphasizes that there is still much scope for future material optimization of perovskite thin films, especially when integrated into a full solar cell stack where interface recombination still strongly limits the external luminescence quantum efficiency.
In addition, I will discuss the effect of parasitically absorbing layers in state-of-the-art perovskite solar cell stacks. It will be shown that parasitic absorption should be minimized not only to maximize light absorption and below-bandgap transmission for perovskite-based tandem solar cell, but also to maximize the extraction of photons initially-trapped in the perovskite thin film. Only this will allow the open-circuit voltage of perovskite solar cells to approach the radiative limit. What is more, luminescent-coupling effects in perovskite-based tandem solar cells crucially depend on the extent of parasitic absorption of the internal luminescence in the perovskite top solar cell. In that regard, I will present first experimental and modelling results on the extent of luminescent-coupling in highly efficient two-terminal perovskite/Si and perovskite/CIGS tandem solar cells.
[1] P. Fassl et al., Matter, 2021, 4, 1391–1412.
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