Dopant-Free Triazatruxene-Based Hole Transporting Materials with Three Different End-Capped Acceptor Units for Perovskite Solar Cells

A series of dopant-free D-π-A structural hole-transporting materials (HTMs), named as SGT-460, SGT-461, and SGT-462, incorporating a planner-type triazatruxene (TAT) core, thieno[3,2-b]indole (TI) π-bridge and three different acceptors, 3-ethylthiazolidine-2,4-dione (ED), 3-(dicyano methylidene)indan-1-one (DI), and malononitrile (MN), were designed and synthesized for application in perovskite solar cells (PrSCs). The effect of three acceptor units in star-shaped D-π-A structured dopant-free HTMs on the photophysical and electrochemical properties and the photovoltaic performance were investigated compared to the reference HTM of 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD). Their highest occupied molecular orbital (HOMO) energy levels were positioned for efficient hole extraction from a MAPbCl3−xIx layer (5.43 eV). The hole mobility values of the HTMs without dopants were determined to be 7.59 × 10−5 cm2 V−1 s−1, 5.13 × 10−4 cm2 V−1 s−1, and 7.61 × 10−4 cm2 V−1 s−1 for SGT-460-, SGT-461-, and SGT-462-based films. The glass transition temperature of all HTMs showed higher than that of the spiro-OMeTAD. As a result, the molecular engineering of a planer donor core, π-bridge, and end-capped acceptor led to good hole mobility, yielding 11.76% efficiency from SGT-462-based PrSCs, and it provides a useful insight into the synthesis of the next-generation of HTMs for PrSC application.