The power conversion efficiency (PCE) of NiO based perovskite solar cells has recently hit a record 22.1% with a hybrid organic–inorganic perovskite composition and a PCE above 15% in a fully inorganic configuration was achieved. Moreover, NiO processing is a mature technology, with different industrially attractive processes demonstrated in the last few years. These considerations, along with the excellent stabilities reported, clearly point towards NiO as the most efficient inorganic hole selective layer for lead halide perovskite photovoltaics, which is the topic of this review. NiO optoelectronics is discussed by analysing the different doping mechanisms, with a focus on the case of alkaline and transition metal cation dopants. Doping allows tuning the conductivity and the energy levels of NiO, improving the overall performance and adapting the material to a variety of perovskite compositions. Furthermore, we summarise the main investigations on the NiO/perovskite interface stability. In fact, the surface of NiO is commonly oxidised and reactive with perovskite, also under the effect of light, thermal and electrical stress. Interface engineering strategies should be considered aiming at long term stability and the highest efficiency. Finally, we present the main achievements in flexible, fully printed and lead-free perovskite photovoltaics which employ NiO as a layer and provide our perspective to accelerate the improvement of these technologies. Overall, we show that adequately doped and passivated NiO might be an ideal hole selective layer in every possible application of perovskite solar cells.

中文翻译：

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NiO在钙钛矿光伏领域的进展、亮点和展望

基于氧化镍的钙钛矿太阳能电池的功率转换效率（PCE）最近在有机-无机杂化钙钛矿组合物中达到了创纪录的22.1％，并且在全无机配置中实现了15％以上的PCE。此外，氧化镍加工是一项成熟的技术，在过去几年中展示了不同的具有工业吸引力的工艺。这些考虑因素以及报告的优异稳定性，清楚地表明 NiO 是卤化铅钙钛矿光伏电池最有效的无机空穴选择性层，这也是本次综述的主题。通过分析不同的掺杂机制，重点讨论碱性和过渡金属阳离子掺杂剂的情况，讨论了 NiO 光电子学。掺杂可以调节 NiO 的电导率和能级，提高整体性能并使材料适应各种钙钛矿成分。此外，我们总结了 NiO/钙钛矿界面稳定性的主要研究。事实上，在光、热和电应力的作用下，氧化镍的表面通常会被氧化并与钙钛矿发生反应。应考虑以长期稳定性和最高效率为目标的接口工程策略。最后，我们介绍了采用 NiO 作为层的柔性、全印刷和无铅钙钛矿光伏电池的主要成就，并提供了加速这些技术改进的观点。总的来说，我们表明，充分掺杂和钝化的 NiO 可能是钙钛矿太阳能电池每种可能应用中理想的空穴选择性层。