Molecular Engineering of Polytriarylamine-Based Hole-Transport Materials for p–i–n Perovskite Solar Cells: Methyl Groups Matter

In this work, we performed a comparative study of two structurally similar polytriarylamines as hole-transport layer materials for p–i–n perovskite solar cells: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) bearing three methyl groups and poly[bis(4-phenyl)(4-methylphenyl)amine] (PTA) loaded with just a single methyl substituent per repeating unit. Our theoretical DFT calculations revealed that introducing two methyl groups at ortho positions of the phenyl ring in PTAA leads to steric hindrance effects, which are compensated for by the out-of-plane rotation of this substituent with respect to the polymer backbone. On the contrary, the p-methylphenyl substituent is sterically less hindered, which makes the PTA backbone more planar as compared to that of PTAA. We explained this effect as being a consequence of a T-shaped interaction between (PTA)n chains. Consequently, PTA could produce more compact films with better charge-transport characteristics, leading to superior solar cell characteristics. This hypothesis was proved experimentally by the demonstration of >20% efficient p–i–n perovskite solar cells using PTA as hole-transport layer material, whereas the reference cells with PTAA delivered the best efficiency of 18.7%. It has been also shown that PTA could be processed by thermal evaporation, which is the key approach to form compact, uniform, and defectless coatings atop the ITO electrodes as revealed by IR s-SNOM microscopy.