4-tert-butylpyridine induced Ni3+/Ni2+ ratio modulation in NiOx 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 Ni 3+ /Ni 2+ 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 ( t BP) as additive in the NiO x precursor solution. It is revealed that the t BP 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 Ni 2+ to Ni 3+ conversion, and the resulting t BP: 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 cm 2 ) fabricated with t BP: NiO x HTL delivers an enhanced efficiency of 20.2% relative to the control devices (17.5%). t BP: NiO x devices with a large area of 1 cm 2 are also demonstrated, which achieve a decent efficiency of 16% with excellent long-term stability.