Inverted perovskite solar cells employing doped NiO hole transport layers: A review

Highlights: • Doping strategies of NiO_x hole transport layers in perovskite solar cells are reviewed. • Modulation of the properties of NiO_x induced by extrinsic doping is highlighted. • Current challenges and perspectives for future progress are given. -- Abstract: Perovskite solar cells (PSCs) have shown unprecedented efficiency progress from 3.8% in 2009 to 24.2% in 2019. Up to now, the highest device efficiencies were recently achieved by employing n-type SnO₂ on the transparent front electrode with conventional structure (n-i-p structure), while TiO₂ remains the most used electron transport layer in PSCs. However, the comparably large J-V hysteresis in planar PSCs and the high temperature process required in mesoporous TiO₂ structures severely limit the further commercial application. Therefore, inverted PSCs (p-i-n structure) employing p-type NiO_x as the hole transport layer (HTL) on the front electrode have attracted massive attention in recent years. This is mainly due to their lower processing temperature for large scale and flexible devices, negligible J−V hysteresis effects, and furthermore, better stability as compared to organic HTLs. In spite of all these merits of NiO_x based HTLs, the reported efficiencies of inverted PSCs are still lower than that of conventional PSCs. The main reasons can be assigned to limitations arising from the low conductivity and a mismatched band position of NiO_x. Doping has been considered to be an effective way to adjust the electrical and optical properties of semiconductor oxides in a large extent and has already shown promising results in improving the photovoltaic performance of NiO_x based inverted PSCs. In this review, recent investigations about the influence of doping on the structural, electrical, and optical properties of NiO_x HTLs are summarized. We also discuss the advantages and current challenges of utilizing NiO_x HTLs in PSCs and attempt to give prognoses on future progress exploiting them in high-efficiency inverted PSCs.