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 PC 61 BM 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 .