A D–π–A1–π–A2 push–pull small molecule donor for solution processed bulk heterojunction organic solar cells

Herein, benzothiadiazole (BTD), as an acceptor A1, has been used as a backbone to link triphenylamine (TPA) as donor and naphthalimide (NPI) as acceptor (A2) moieties through ethylene linkers to design a small molecule. The donor–π–acceptor–π–acceptor (D–π–A1–π–A2) type small molecule denoted as TPA-BTD-NPI was synthesized. In order to use it as an electron donor for solution processed bulk heterojunction small molecule solar cells its photonic and electronic properties were explored. The small molecule organic solar cells based on the optimized blend of TPA-BTD-NPI with PC71BM processed in chloroform showed a power conversion efficiency (PCE) of 2.21%, which was significantly improved up to 6.67%, when a two-step annealing (TSA) treated blend was used as an active layer. The increase in the PCE was due to the enhancement in both Jsc and FF. The improvement in Jsc was related to the enhancement in the light harvesting efficiency of a TSA treated active layer relative to the as-cast layer, which is reflected in a better IPCE and better charge collection. The TSA treatment also leads to better nanoscale morphology for exciton dissociation into free charge carriers and improved crystallinity for balanced charge transport.