Cyclopentadiene‐Based Hole‐Transport Material for Cost‐Reduced Stabilized Perovskite Solar Cells with Power Conversion Efficiencies Over 23

Hole transport materials (HTM) are an important component in perovskite solar cells (PSC). Despite a multitude of HTMs developed in recent years, only few of them lead to solar cells with efficiencies over 20%. Therefore, it is still a challenge to develop high‐performing HTMs, which have ideal energy levels of the frontier orbitals, are highly efficient in transporting charges, and stabilize the solar cell at the same time. In this work, the development of a structurally novel molecular HTM, CPDA 1, is described which is based on a common cyclopentadiene core and can be efficiently and inexpensively synthesized from readily available starting materials, which is important for future realization of low‐cost photovoltaics on larger scale. Due to excellent optoelectronic, thermal, and transport properties, CPDA 1 not only meets the envisioned properties by reaching high efficiencies of 23.1%, which is among the highest reported to date, but also contributes to a respectable long‐term stability of the PSCs. Novel hole transport material CPDA 1 can be efficiently and inexpensively synthesized from readily available starting materials. The cyclopentadiene acetal core is surrounded by four triarylamine arms in a star‐shaped fashion. Excellent optoelectronic, thermal, and transport properties lead to high power conversion efficiencies of 23.1% in perovskite solar cells with respectable long‐term stability.