Synthesis and characterization of solution-processed indophenine derivatives for function as a hole transport layer for perovskite solar cells

A series of quinoidal molecules were synthesized via a new variant of the indophenine reaction. The previously reported synthetic route of indophenine dyes involves a one pot procedure starting with isatin and an appropriate 5-membered aromatic heterocycle in the presence of catalytic amount of sulfuric acid. However, the reaction conditions lead to a low yield and a poor selectivity with the formation of several by-products. Herein, we propose a modification of the standard reaction to increase the yield, using tertiary alcohol as a starting material instead of isatin. This modified protocol is highly selective and allows for a very clean reaction, i.e. high yields, isomer-free synthesis and avoids any complex purification issues. Further, this protocol allows access to quinoidal compounds with variable termini and π-conjugated core. The influence of the conjugation length of the π-bridge on the quinoidal electronic structure is investigated by spectroscopic and electrochemical measurements. Meanwhile, the charge carrier mobilities of selected compounds were determined by the space-charge-limited-current method. Their utility as a hole transport material for the perovskite solar cells is demonstrated.