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Perovskites with bandgaps between 1.7 and 1.8 eV are optimal for tandem configurations with crystalline silicon (c-Si) because they facilitate efficient harvest of solar energy. In that respect, achieving a high open-circuit voltage (V OC ) in such wide-bandgap perovskite solar cells is crucial for a high overall power conversion efficiency (PCE). Here, we provide key insights into the factors affecting the V OC in wide-bandgap perovskite solar cells. We show that the influence of the hole transport layer (HTL) on V OC is not simply through its ionization potential but mainly through the quality of the perovskite−HTL interface. With effective interface passivation, we demonstrate perovskite solar cells with a bandgap of 1.72 eV that exhibit a V OC of 1.22 V. Furthermore, by combining the high-V OC perovskite solar cell with a c-Si solar cell, we demonstrate a perovskite−Si four-terminal tandem solar cell with a PCE of 27.1%, exceeding the record PCE of single-junction Si solar cells.

Low-cost solar technologies such as perovskite solar cells are not only required to be efficient, but durable too, exhibiting chemical, thermal and mechanical stability. To determine the mechanical stability of perovskite solar cells, the fracture resistance of a multitude of solution-processed organometal trihalide perovskite films and cells utilizing these films were studied. The influence of stoichiometry, precursor chemistry, deposition techniques, and processing conditions on the fracture resistance of perovskite layers was investigated. In all cases the perovskites offered negligible resistance to fracture failing cohesively below 1.5 J/m 2 . The solar cells studied featured these perovskites and a variety of organic and inorganic charge transporting layers and carrier-selective contacts. These ancillary layers were found to significantly influence the overall mechanical stability of the perovskite solar cells and were repeatedly the primary source of mechanical failure, failing at values below those measured for the isolated fragile perovskite films. A detailed insight into the nature of perovskite and perovskite solar cell fracture is presented and the influence of grain size, device architecture, deposition techniques, environmental variables, and molecular additives on these fracture processes is reported. Understanding the influence of materials selection, deposition techniques and processing variables on the mechanical stability of perovskite solar cells is a crucial step in their development.

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The fabrication of high-quality cesium (Cs)/formamidinium (FA) doublecation perovskite films through a two-step interdiffusion method is reported. Cs x FA 1-x PbI 3-y(1-x) Br y(1-x) films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs + and Bron the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm 2 ) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs 0.1 F A 0.9 PbI 2.865 Br 0.135 ) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).