Quantifying Extraction Losses in Perovskite Solar Cells Using Voltage-Dependent Photoluminescence

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Original abstract

The widespread belief that diffusion lengths directly correlate with efficient charge carrier collection has led to their frequent citation as a crucial factor in the superior performance of lead-halide perovskite solar cells (PSCs). However, while long diffusion lengths are essential for highly efficient photovoltaics, they prove inadequate as sole indicators of effective charge collection in the presence of undoped and low-conductivity transport layers (TLs). Notably, the use of low mobility organic TLs, common in inverted PSCs, can lead to substantial accumulations of excess charge carriers within the absorber layer, as evidenced by short-circuit photoluminescence (PL) measurements. This phenomenon can result in a paradoxical situation where charge collection at short circuit is inefficient, despite the diffusion length significantly exceeding the perovskite thickness. Furthermore, charge collection efficiencies in halide PSCs depend on absorber layer thickness, with thicker perovskite layers exhibiting reduced collection efficiencies. While traditionally attributed to insufficient diffusion lengths within semiconducting layers, my research indicates that in the presence of low conductivity contact layers, charge collection can be thickness dependent even when the perovskite absorber has an infinite charge-carrier mobility. This investigation formulates analytical equations elucidating the influence of electronic properties of TLs employed in inverted PSCs on charge collection losses. The equations demonstrate how poor TL mobility leads to diminished collection efficiencies. Additionally, this research develops a model characterizing thickness-dependent charge collection losses arising from the slow exchange of charge carriers between the perovskite absorber and contact layers in bulk or interface-limited systems. The thesis also presents experimental findings, based on voltage-dependent photoluminescence, that align with analytical predictions. These results reveal that at short circuit, PSCs with three distinct TL types exhibited varying extracted photocurrents. Moreover, PSCs featuring three different absorber layer thicknesses displayed disparate collection efficiencies. The experimental results from triple-cation perovskite devices with different absorber layer thicknesses approximate the analytical scenario in which the impact of interfacial recombination is suppressed, allowing bulk recombination to emerge as the dominant mechanism.

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