4-tert-butylpyridine induced Ni³⁺/Ni²⁺ ratio modulation in NiO_x hole transport layer towards efficient and stable inverted perovskite solar cells

NiO_x plays a vital role in fabricating efficient and stable inverted perovskite solar cells (PSCs). However, the relatively low conductivity and inefficient hole extraction of NiO_x hole transport layer (HTL) due to the low Ni³⁺/Ni²⁺ ratio limit its use in high-efficiency PSCs. Here, we report a facile strategy to remarkably enhance the electrical conductivity and hole extraction capability of NiO_x HTL by simply introducing the small molecule 4-tert-butylpyridine (tBP) as additive in the NiO_x precursor solution. It is revealed that the tBP efficient improves the charge selectivity of the NiO_x interface, inducing the preferable p-type carrier concentration on the surface. Consequently, more oxygen is involved in the process of Ni²⁺ to Ni³⁺ conversion, and the resulting tBP: NiO_x HTL exhibits significantly enhanced hole extraction and transport property. In addition, a thin layer of LiF is applied to passivate the surface of NiO_x, which enables improved hole migration ability by the dipole effect of LiF and reduced trap state density of the surface. The inverted PSCs (0.16 cm²) fabricated with tBP: NiO_x HTL delivers an enhanced efficiency of 20.2% relative to the control devices (17.5%). tBP: NiO_x devices with a large area of 1 cm² are also demonstrated, which achieve a decent efficiency of 16% with excellent long-term stability.