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Perovskite solar cells have demonstrated low non‐radiative voltage losses and open‐circuit voltages (VOCs) that often match the internal voltage in the perovskite layer, i.e. the quasi‐Femi level splitting (QFLS). However, in many cases, the VOC differs remarkably from the internal voltage, for example in devices without perfect energy alignment. In terms of recombination losses, this loss often outweighs all non‐radiative recombination losses observed in photoluminescence quantum efficiency measurements by many orders of magnitude. As such, understanding this phenomenon is of great importance for further perovskite solar cell development and tackling stability issues. The classical theory developed for Si solar cells explains the QFLS‐VOC mismatch by considering the partial resistances/conductivities for majority and minority carriers. Here, the authors demonstrate that this generic theory applies to a variety of physical mechanisms that give rise to such a mismatch. Additionally, it is found that mobile ions can contribute to a QFLS‐VOC mismatch in realistic perovskite cells, and it is demonstrated that this can explain various key observations about light soaking and aging‐induced VOC losses. The findings in this paper shine a light on well‐debated topics in the community, identify a new degradation loss, and highlight important design principles to maximize the VOC for improved perovskite solar cells. [ABSTRACT FROM AUTHOR]

Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic–inorganic perovskites. This is mainly caused by lower open‐circuit voltages (VOCs). Herein, the reasons for the low VOC in inorganic CsPbI2Br perovskite solar cells are investigated. Intensity‐dependent photoluminescence measurements for different layer stacks reveal that n–i–p and p–i–n CsPbI2Br solar cells exhibit a strong mismatch between quasi‐Fermi level splitting (QFLS) and VOC. Specifically, the CsPbI2Br p–i–n perovskite solar cell has a QFLS–e ·VOC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic–inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI2Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3% with a MeO–2PACz hole‐transporting layer and 20.8% on compact TiO2. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS–e ·VOC mismatch and strategies for overcoming this VOC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved. [ABSTRACT FROM AUTHOR]

Efficient mixed metal lead‐tin halide perovskites are essential for the development of all‐perovskite tandem solar cells, however they are currently limited by significant short‐circuit current losses despite their near optimal bandgap (≈1.25 eV). Herein, the origin of these losses is investigated, using a combination of voltage dependent photoluminescence (PL) timeseries and various charge extraction measurements. It is demonstrated that the Pb/Sn‐perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted PL from the device rises on the exact same timescales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping‐induced electronic charges but by the movement of mobile ions toward the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. Finally, these findings are generalized to lead‐based perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism. [ABSTRACT FROM AUTHOR]