In the past decade, organic–inorganic hybrid perovskite materials have attracted increasing attention due to their high absorption coefficient, efficient ambipolar charge transport, long exciton diffusion length and lifetime, solution processability, and low cost. The power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have rapidly increased from 3.8% to 23.3%. However, some key issues need to be addressed, such as toxicity and device stability. Interfacial modification or doping in perovskite films, using organic semiconductors with functional groups, can suppress hysteresis effect, improve the quality of perovskite films, reduce defects and trap states in bulk perovskite and at interface, as well as enhance device efficiency and stability.
In order to passivate oxygen-vacancy-related defects, and suppress charge recombination at SnO2/perovskite interface, Xiaowei Zhan’s group in the College of Engineering used a fullerene derivative C9 with a long side chain to anchor the surface of SnO2. The C9 modifying layer enhances the extraction of photogenerated charge carriers due to negligible charge injection barrier and strong electron affinity. C9 suppresses charge recombination due to efficient passivation of oxygen-vacancy-related defects on the SnO2 surface through the formation of chemical bonds between the under-coordinated Sn in SnO2 and hydroxyl terminal groups in C9. Moreover, the long alkyl chain in C9 facilitates ordered self-assembly. C9 also improves the quality of the perovskite films with enlarged grain size, reduced grain boundary and improved crystallinity. As a result, PSCs based on C9-modified SnO2 exhibited a maximum PCE up to 21.3% with negligible hysteresis. More importantly, the ambient, photo and electric-field stability of the modified devices were also enhanced (Energy Environ. Sci., DOI: 10.1039/c8ee02172d).
Zhan’s group proposed fused ring electron acceptor (FREA)-perovskite hybrid as a promising platform to fabricate PSCs. A FREA molecule (INIC2) with fluorinated electron-withdrawing end-groups was introduced into perovskite precursor solution to control the morphology of perovskite films, leading to larger grain sizes and stronger crystallinity. Furthermore, INIC2 owns lone pair electrons, which can passivate the undercoordinated Pb atoms and reduce charge recombination. INIC2 can promote charge transport in PSCs due to suitable energy levels and high mobility. Consequently, the PSCs based on INIC2-perovskite hybrid films achieved a champion PCE of 21.7% and enhanced ambient stability (J. Am. Chem. Soc., DOI: 10.1021/jacs.8b09300).
These research works were supported by the National Nature Science Foundation of China, and benefited from the cooperation with Prof. Qingbo Meng at the Institute of Physics of Chinese Academy of Sciences, Yingfeng Tu at Soochow University, Huanping Zhou at Peking University, Xinhui Lu at the Chinese University of Hong Kong, and Justin M. Hodgkiss at Victoria University of Wellington.
Molecule structure and device structure