Solvent Engineering for Layer Formation Control with Cost-Effective Hole Transport Layer in High-Efficiency Perovskite Solar Cell

Among hole transport materials (HTMs), 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) is the most frequently adopted, due to its suitable energy band level in conventional-type perovskite solar cells (PSCs). However, the high price of spiro-OMeTAD is an obstacle faced in its research and commercialization. In our previous work, we introduced a low-cost HTM, (E,E,E,E)-4,4′,4″,4‴-[Benzene-1,2,4,5-tetrayltetrakis(ethene-2,1-diyl)]tetrakis[N,N-bis(4-methoxyphenyl)aniline] (α2); however, it was immiscible in the conventional solvent chlorobenzene, leading to the adoption of dichloromethane (DCM) as an alternative. Nevertheless, its high vapor pressure led to poor reproducibility, limiting its practical applicability. To address this issue, we investigated alternative solvents to DCM to facilitate the application of α2 to dichloride alkane materials, from 1,2-dichloroethane (DCE) to 1,4-dichlorobutane. In these materials, DCE exhibits the most superior properties in terms of layer formation control, due to its vapor pressure in spin-coating. Accordingly, a PSC containing α2-DCE HTL showed high performance, with 1.15V of open-circuit voltage and a 22.7% power conversion efficiency.