A weak donor-strong acceptor strategy to design ideal polymers for organic solar cells

Polymers to be used in bulk heterojunction (BHJ) solar cells should maintain a low highest occupied molecular orbital (HOMO) energy level as well as a narrow band gap in order to maximize the open circuit voltage (V(oc)) and the short circuit current (J(sc)). To concurrently lower the HOMO energy level and the band gap, we propose to modify the donor-acceptor low band gap polymer strategy by constructing alternating copolymers incorporating a "weak donor" and a "strong acceptor". As a result, the "weak donor" should help maintain a low HOMO energy level while the "strong acceptor" should reduce the band gap via internal charge transfer (ICT). This concept was examined by constructing a library of polymers employing the naphtho[2,1-b:3,4-b']dithiophene (NDT) unit as the weak donor, and benzothiadiazole (BT) as the strong acceptor. PNDT-BT, designed under the "weak donor-strong acceptor" strategy, demonstrated both a low HOMO energy level of -5.35 eV and a narrow band gap of 1.59 eV. As expected, a noticeably high V(oc) of 0.83 V was obtained from the BHJ device of PNDT-BT blended with PCBM. However, the J(sc) ( approximately 3 mA/cm(2)) was significantly lower than the maximum expected current from such a low band gap material, which limited the observed efficiency to 1.27% (with a 70 nm thin film). Further improvements in the efficiency are expected from these materials if new strategies can be identified to (a) increase the molecular weight and (b) improve the hole mobility while still maintaining a low HOMO energy level and a narrow band gap.