Role of NiO in wide-bandgap perovskite solar cells based on self-assembled monolayers

NiO/self-assembled monolayer (SAM) double hole transport layers (HTLs) has become the mainstream choice in high-efficiency single-junction and tandem perovskite solar cells (PSCs). However, the underlying role of NiO in double HTLs from the microscale is not systematically revealed currently. Herein, we reveal that NiO plays an important role in inducing a more uniform and thinner SAM from three aspects. Firstly, compared with indium tin oxide (ITO), the NiO surface exposes less crystal facet orientations and a higher metal atom density, suggesting that sufficient chemical binding sites for SAMs are provided to facilitate the close-packed assembly of SAMs. Secondly, NiO modified ITO exhibits smaller surficial roughness and strongly bonded-OH, which is beneficial to the uniform distribution of SAMs and strong adhesion to the NiO surface. Utilizing the well- constructed SAM, we fabricate wide-bandgap (>1.75 eV) PSCs using vacuum-assisted technology and achieve an impressive photoelectric conversion efficiency of 19.55 %. These findings not only deepen the understanding of SAM assembly on substrates but also provide essential guidance for the future industrialization of PSCs.