Modulating the Symmetry of Benzodithiophene by Molecular Tailoring for the Application in Naphthalene Diimide-Based N-Type Photovoltaic Polymers

To investigate the effect of symmetry of copolymerized unit on the properties of naphthalene diimide (NDI)-based photovoltaic polymers, three n-type semiconductor polymers, PNDI-BDT, PNDI-T-BTh, and PNDI-T-P-T are synthesized by copolymerizing NDI with benzodithiophene (BDT), thienylene-benzothiophene (T-BTh), and thienylene-phenylene-thienylene (T-P-T) units, respectively. The latter two building blocks could be regarded as the derivatives from BDT by molecular tailoring and only T-BTh is an asymmetric segment. The light absorption, carrier mobility, film morphology, crystallinity, and photovoltaic properties of three PNDIs are systematically characterized and compared. Although PNDI-T-BTh adopts a face-on molecular orientation, it shows a high field-effect electron mobility (mu(e)) of 1.06 x 10(-2)cm(2) V-1 s(-1), which is comparable to the mu e of PNDI-BDT (1.37 x 10(-2)cm(2) V-1 s(-1)) with edge-on molecular orientation. The photovoltaic devices with PNDI-T-BTh as the acceptor show the highest power conversion efficiency (PCE) of 5.99% with V-OC of 0.87 V, J(SC) of 10.28mAcm(-2), and FF of 0.67, while PNDI-BDT and PNDI-T-P-T containing the symmetric copolymerized units give lower PCE of 2.06 and 4.12%, respectively. The higher PCE with PNDI-T-BTh could be attributed to the favorable face-on molecular orientation, fiber-like blend film morphology and relatively balanced hole and electron mobilities. Our results indicate that the introduction of the asymmetric unit could efficiently improve the photovoltaic performance, which provides a feasible and effective strategy to design novel NDI-based and even other type rylene diimide-based photovoltaic polymers.