Critical Role of Pendant Group Substitution on the Performance of Efficient All-Polymer Solar Cells

Most high-performance all-polymer solar cell systems employ donor polymers with side groups containing bulky aromatic units. The rationale behind the use of bulky side groups in efficient all-polymer systems, however, is not well-understood. In this study, we investigate the doubling of power conversion efficiency in all-polymer solar cells that occurs when substituting the pendant oxygen group in polymer donor PTB7 for thiophene. Specifically, polymer blends using either PTB7 or PTB7-Th as donor with P(NDI2OD-T2) as acceptor are compared. We comprehensively examine the photophysics, morphology, and device physics of these two systems and find that PTB7-Th:P(NDI2OD-T2) blends have suppressed geminate recombination and improved charge collection efficiencies compared to PTB7:P(NDI2OD-T2) blends. While the switching of oxygen for thiophene does not have a dramatic effect on blend morphology, the bulky side group in PTB7-Th helps to destabilize the interfacial charge transfer state, with 5-fold higher hole mobility of PTB7-Th also resulting in improved charge collection.