formation and influences of tbp-litfsi complexes in perovskite solar cells
Name: Sophia Pearl Valenzuela
Grad Year: 2022
Perovskite solar cells (PSCs) are next-generation photovoltaic devices with a p-i-n junction consisting of an electron transport layer (ETL), an intrinsic layer, and a hole transport layer (HTL). Lead halide perovskite materials function as the intrinsic layer, absorbing photons and generating free electron-hole pairs. The electrons are transferred to the ETL while the holes are extracted by the HTL to complete the circuit and generate electric current. PSCs have shown great promise in recent years, with efficiencies now rivaling those of traditional silicon solar panels. However, before commercialization can be feasible, the stability of PSCs must be further improved. One of the major bottlenecks hindering the enhancement of PSC stability is the interaction of hole transport layer additives LiTFSI and tBP. Although tBP improves layer uniformity and LiTFSI enhances the conductivity of the HTL, both of which lead to higher solar cell efficiencies, both additives also have negative effects on device stability. LiTFSI is a super-hygroscopic material that absorbs water, which in turn accelerates perovskite decomposition. And tBP, an easily-evaporated liquid component of PSCs, is corrosive to perovskite materials after extended interaction. In past PSC work, a 6:1 ratio of tBP:LiTFSI in the HTL has been widely used, but with little justification based on the interaction of tBP and LiTFSI. In this study, the formation of tBP-LiTFSI complexes at various ratios has been studied for the first time. These complexes can reduce the negative effects of tBP and LiTFSI on perovskite while still maintaining their positive effects on device efficiency. The formation of tBP-LiTFSI complexes not only slows degradation of perovskite from MAPbI3 to PbI2, but also limits the further decomposition of PbI2 to PbO. This work suggests that the best tBP:LiTFSI molar ratio for improved PSC performance is 4:1, as this ratio minimizes the hygroscopicity of LiTFSI as well as the evaporation speed and corrosive effect of tBP. In the future, this 4:1 ratio should be recognized as the standard ratio of tBP:LiTFSI in the hole transport layer of PSCs.
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