Fluorene-based enamines as low-cost and dopant-free hole transporting materials for high performance and stable perovskite solar cells

The power conversion efficiency of perovskite solar cells is approaching the Shockley-Queisser limit, and therefore this technology is next to the commercialization stage. Inexpensive and stable hole transporting materials are highly desirable for the successful scale-up. Most high performing devices generally employ expensive hole conductors that are synthesized via cross-coupling reactions which require expensive catalysts, inert reaction conditions and time-consuming sophisticated product purification. In a quest to employ cost-effective chemistry to combine the building blocks, we explore enamine-based small molecules that can be synthesized in a simple condensation reaction from commercially available materials leading to an estimated material cost of a few euros per gram. The synthesized fluorene-based enamines exhibit a very high hole mobility up to 3.3 x 10(-4) cm(2) V-1 s(-1) and enable the fabrication of perovskite solar cells with a maximum power conversion efficiency of 19.3% in a doped configuration and 17.1% without doping. In addition, both PSC systems demonstrate superior long-term stability compared to spiro-OMeTAD. This work shows that hole transporting materials prepared via a simple condensation protocol have the potential to compete in terms of performance with materials obtained via expensive cross-coupling methods at a fraction of their cost and deliver exceptional stability of the final device. This work provides a design strategy for the further development of novel, low-cost semiconductors.