Triphenylamine (TPA)-Functionalized Structural Isomeric Polythiophenes as Dopant Free Hole-Transporting Materials for Tin Perovskite Solar Cells

A new series of triphenylamine (TPA)-functionalized isomeric polythiophenes are developed as hole transporting materials (HTM) for inverted tin-based perovskite solar cells (TPSCs). Bithiophene (BT) is first functionalized with two TPA (electron donor; D) at 3 and 5 positions to give two structural isomeric compounds (3BT2D and 5BT2D). The functionalized BT2Ds are then coupled with 3,3′-bis(tetradecylthio)-2,2′-bithiophene (SBT-14)/3,3′-ditetradecyl-2,2′-bithiophene (BT-14) to produce structural isomeric polythiophenes (1-4), which are compared to conventional poly[N,N″-bis(4-butylphenyl)-N,N″-bis(phenyl)-benzidine] (poly-TPD) as HTMs for TPSCs. With the appropriate alignment of energy levels with regard to the perovskite layer, the TPA-functionalized polymers-based TPSCs exhibit enhanced operational stability and efficiency. Moreover, the long thiotetradecyl chain in SBT-14 with intramolecular S(alkyl)∙∙∙S(thio) interactions restricts the molecular rotation and has a strong impact on the molecular solubility and wettability of the film during device fabrication. Among all the polymers studied, TPSCs fabricated with 3-SBT-BT2D polymer exhibit the highest hole mobility as well as the slowest charge recombination and achieve the highest power conversion efficiency of 8.6%, with great long-term stability for the performance retaining ≈90% of its initial values for shelf storage over 4000 h, which is the best efficiency for non-PEDOT:PSS-based TPSCs ever reported.