An alkylthio side chain tuned the PM6 structure and elevated photovoltaic performance of ternary donor polymers†

Abstract

Modification of conjugated polymer skeletons using side chain engineering is important for the development of efficient conjugated donor polymers. In this work, alkylthio-substituted BDD units (SBDD) were introduced into the high-efficiency donor PM6 to construct a series of conjugated polymers (PBDB-TF-S5, PBDB-TF-S10 and PBDB-TF-S20, with the molar ratios of SBDD of 5%, 10%, and 20%, respectively), and the effect of the third component SBDD on the photovoltaic performance of organic solar cells (OSCs) was systematically investigated. First, we demonstrated that the highest occupied molecular orbital energy level (E_HOMO) of polymers gradually decreases when the content of SBDD increases, which facilitates the obtainment of progressively higher open-circuit voltages (V_OC) for the corresponding devices. Second, the detailed experimental results proved that OSCs based on PBDB-TF-S5:Y6 revealed a lower energy loss (E_loss), suitable degree of crystallinity, good miscibility with Y6, more balanced carrier mobilities and weaker charge recombination. Eventually, the power conversion efficiency (PCE) of the device based on PM6:Y6 (15.47%) was increased to 16.46% with a J_SC of 25.68 mA cm⁻², V_OC of 0.861 V and fill factor (FF) of 74.45% with the help of an alkylthio side chain. This work provides a sufficient reference for optimizing the efficient donor polymer PM6 and confirms that PBDB-TF-S5 is a promising and efficient donor polymer for OSCs.