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

NiOx plays a vital role in fabricating efficient and stable inverted perovskite solar cells (PSCs). However, the relatively low conductivity and inefficient hole extraction of NiOx 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 NiOx HTL by simply introducing the small molecule 4-tert-butylpyridine (tBP) as additive in the NiOx precursor solution. It is revealed that the tBP efficient improves the charge selectivity of the NiOx 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: NiOx HTL exhibits significantly enhanced hole extraction and transport property. In addition, a thin layer of LiF is applied to passivate the surface of NiOx, 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: NiOx HTL delivers an enhanced efficiency of 20.2% relative to the control devices (17.5%). tBP: NiOx devices with a large area of 1 cm2 are also demonstrated, which achieve a decent efficiency of 16% with excellent long-term stability.