Phase-Engineering of Layered Nickel Hydroxide for Synthesizing High-Quality NiO x Nanocrystals for Efficient Inverted Flexible Perovskite Solar Cells.

Nickel oxide (NiO x ) nanocrystals have been widely used in inverted (p-i-n) flexible perovskite solar cells (fPSCs) due to their remarkable advantages of low cost and outstanding stability. However, anion and cation impurities such as NO 3 - widely exist in the NiO x nanocrystals obtained from calcinated nickel hydroxide (Ni(OH) 2 ). The impurities impair the photovoltaic performance of fPSCs. In this work, we report a facile but effective way to reduce the impurities within the NiO x nanocrystals by regulating the Ni(OH) 2 crystal phase. We add different alkalis, such as organic ammonium hydroxide and alkali metal hydroxides, to nickel nitrate solutions to precipitate layered Ni(OH) 2 with different crystalline phase compositions (α and β mixtures). Especially, Ni(OH) 2 with a high β-phase content (such as from KOH) has a narrower crystal plane spacing, resulting in fewer residual impurity ions. Thus, the NiO x nanocrystals, by calcinating the Ni( OH ) x with excess β phase from KOH, show improved performance in inverted fPSCs. A champion power conversion efficiency (PCE) of 20.42% has been achieved, which is among the state-of-art inverted fPSCs based on the NiO x hole transport material. Moreover, the reduced impurities are beneficial for enhancing the fPSCs' stability. This work provides an essential but facile strategy for developing high-performance inverted fPSCs.