Boosting inverted perovskite solar cell performance by using 9,9-bis(4-diphenylaminophenyl)fluorene functionalized with triphenylamine as a dopant-free hole transporting material

In this study, two newly developed small molecules based on 9,9-bis(4-diphenylaminophenyl)fluorene functionalized with triphenylamine moieties, namely TPA-2,7-FLTPA-TPA and TPA-3,6-FLTPA-TPA, are designed, synthesized and characterized. The electrochemical, optical and thermal properties of both materials are investigated using various techniques. Afterwards, these materials are employed as dopant-free hole transporting materials (HTMs) in planar inverted perovskite solar cell devices with the aim of determining the device performance and studying their stability in comparison with reference N-4,N-4,N-4,N-4-tetra([1,10-biphenyl]-4-yl)-[1,1:4,1-terphenyl]-4,4-diamine (TaTm)-based devices. Under 1 sun conditions, TPA-3,6-FLTPA-TPA-based devices achieve a power conversion efficiency (PCE) of 13.9% whereas TPA-2,7-FLTPA-TPA-based devices exhibit the highest PCE of 17.1% mainly due to an improvement in the fill factor (FF). Meanwhile, the devices prepared using TaTm as the reference HTM exhibit an overall efficiency of 15.9%. In addition to the higher efficiency, our newly developed HTM TPA-2,7-FLTPA-TPA-based devices demonstrate good stability which is comparable to those with TaTm under similar aging test conditions.