Nature Commun. 7, 11585 (2016)

Organic solar cells (OSCs) can be solution-processed and are thus a potentially low-cost alternative to silicon-based photovoltaic devices. However, the highest-performing OSCs use materials, such as fullerene acceptors, that are costly, difficult to produce on an industrial scale and that have limited stability. Poly(3-hexylthiophene) (P3HT) is a relatively stable donor polymer that can be synthesized in large quantities at low cost. Sarah Holliday, Iain McCulloch and colleagues at Imperial College London and elsewhere have now designed a non-fullerene small molecule acceptor called IDTBR that, when partnered with P3HT in a bulk heterojunction solar cell, achieves a power conversion efficiency (PCE) of 6.4% — a record value for fullerene-free solar cells with a P3HT donor.

The IDTBR acceptor has an indacenodithiophene unit at its core, and benzothiadiazole and rhodanine flanking groups. Its planar structure improves crystallization and morphological properties and red-shifts the optical absorption to make it complementary to that of P3HT. Its properties can be further tuned by side-chain engineering with either n-octyl or 2-ethylhexyl chains. The solar cells have an inverted architecture of glass/ITO/ZnO/P3HT:IDTBR/MoO3/Ag and their PCE is much higher than that of fullerene-based control samples (reaching only 3.7%). Moreover, the IDTBR-based devices retain 73% of the original PCE after 1,200 hours in air, which is significantly higher than in control samples.