Simply designed nonspiro fluorene-based hole-transporting materials for high performance perovskite solar cells

Abstract

Organic small molecule hole-transporting materials are considerably convenient to prepare and tailor for acquiring the superiorly optoelectronic behaviors. In this work, taking the prototypical spiro-bifluorene core in conventional spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) as reference, we present the simply structural design and synthesis of two simple hole-transporting materials, FH-0 and FH-3, which consist of nonspiro fluorene core with triphenylamine terminal. The evaluation of different core-system modifications on absorption/emission spectra, energy level positions, electronic molecule configuration and charge-carrier mobility are implemented systematically. Modifying by the aliphatic chain at the 9,9′-position of fluorene core, the FH-3 displays the moderate energy level, superior thermo-stability, proper structure packing trends and high hole mobility. Associated with excellent reproducibility, the optimal mixed-ion perovskite solar cell based on FH-3 obtains the impressive efficiency of 18.4%, which is comparable to that of spiro-OMeTAD (19.6%) under the same condition. Although the FH-0 possesses the same core backbone and similar photophysical, electrochemical properties with FH-3, the labile H atoms on 9,9′-fluorene suppress the photovoltaic performance of corresponding device. This work demonstrates that the core unit modification is indispensable for the hole-transporting materials toward high performance perovskite solar cells.