Impact of Thermal Annealing on Organic Photovoltaic Cells Using Regioisomeric Donor–Acceptor–Acceptor Molecules

We report a promising set of donor–acceptor–acceptor (D–A–A) electron-donor materials based on coplanar thieno­[3,2-b]/[2,3-b]­indole, benzo­[c]­[1,2,5]­thiadiazole, and dicyanovinylene, which are found to show broadband absorption with high extinction coefficients. The role of the regioisomeric electron-donating thienoindole moiety on the physical and structural properties is examined. Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on the thieno­[2,3-b]­indole-based electron donor NTU-2, using C₇₀ as an electron acceptor, show a champion power conversion efficiency of 5.2% under AM 1.5G solar simulated illumination. This efficiency is limited by a low fill factor (FF), as has previously been the case in D–A–A systems. In order to identify the origin of the limited FF, further insight into donor layer charge-transport behavior is realized by examining planar heterojunction OPVs, with emphasis on the evolution of film morphology with thermal annealing. Compared to as-deposited OPVs that exhibit insufficient donor crystallinity, crystalline OPVs based on annealed thin films show an increase in the short-circuit current density, FF, and power conversion efficiency. These results suggest that that the crystallization of D–A–A molecules might not be realized spontaneously at room temperature and that further processing is needed to realize efficient charge transport in these materials.