Modified P3HT materials as hole transport layers for flexible perovskite solar cells

Flexible perovskite solar cells (f-PSCs) are light-weight, conformal and thus ideal for seamless integration of photovoltaics onto wearable and portable electronics. Nevertheless, the spread of f-PSCs is limited by both the lower efficiency compared to the conventional rigid counterparts and the employment of costly materials. Among them, hole-transporting materials (HTM) represent the most expensive component and also a weak spot for long-term stability, due to poor resistance against heat and moisture. In this context, we propose poly-3-hexylthiophene (P3HT)-derivated HTMs embodying benzothiadiazole (BDT) moieties as electron-poor host. BDT is inserted along P3HT polymeric backbone, creating a donor-acceptor system able to promote the charge mobility throughout the HTM. A first series of copolymers, synthetized by Stille coupling, shows an increase of benzothiadiazole/thiophene ratio (namely, 1:2, 1:4, 1:6), allowing to modulate both their electronic and optical properties. Additionally, a greener approach (Kumada polycondensation) is employed to synthetize a homologous copolymer (VI-LM-027) that, used as HTM in f-PSCs, leads to power conversion efficiency comparable to commercially available P3HT and shows improved stability under continuous illumination. Finally, VI-LM-027 is also employed in 6x6 cm2 modules, delivering 6.9% efficiency on 16 cm2 of active area and demonstrating the feasibility of the proposed HTMs for large area manufacture.