A Tale of Current and Voltage Interplay of Band Gap and Energy Levels of Conjugated Polymers in Bulk Heterojunction Solar Cells

We recently proposed a design strategy of weak donor strong acceptor copolymer to approach ideal polymers with both a low HOMO energy level and a small band gap for bulk heterojunction (BHJ) polymer solar cells in order to achieve both a high open circuit voltage (V-oc) and a high short circuit current (J(sc)) [ACS Appl Mater Interfaces 2010, 2, 1377] To further demonstrate the delicate interplay of the V-oc and J(sc) through molecular design of conjugated polymers, two such "weak donors, naphtho[2,1-b 3,4-b'] dithiophene (NDT) and dithieno [3 2-f 2' 3'-h]quinoxaline (QDT), which differ only by two atoms, were copolymerized with a common acceptor, 4,7-di(2-thienyl) 2,1,3-benzothiadiazole (DTBT) The BHJ devices based on PNDT-4DTBT and PQDT-4DTBT as the donor polymer (with PC61BM as the acceptor) exhibit overall efficiencies of 5 1% and 4 3%, respectively Through these two structurally related polymers, we demonstrate that incorporating electron-withdrawing atoms in the donor unit would lead to a lower HOMO energy level This lower HOMO energy level translates Into a higher open circuit voltage (V-oc) of 0 83 V in PQDT-4DTBT-based BHJ devices In contrast, the slightly higher HOMO energy level (-5 34 eV) of PNDT-4DTBT limits the V-oc to 0 67 V Since the LUMO levels of both polymers are similar, the higher HOMO energy level of PNDT-4DTBT leads to a smaller band gap of 1 61 eV (vs 1 70 eV of PQDT-4DTBT), which contributes to a noticeably higher J(sc) of 14 20 mA/cm(2) (vs 11 38 mA/cm(2) of PQDT-4DTBT based BHJ devices) Future research needs to focus on the employment of even weaker donor and stronger acceptors via innovative structural modification in order to concurrently achieve a deeper HOMO and a lower band gap, thereby enhancing both V-oc and J(sc).