Naphthalene diimide-based random terpolymer for efficient all-polymer solar cells with high open circuit voltage

A series of naphthalene diimide (NDI)-based random terpolymers x-NDI as polymer acceptors were synthesized with variation of T-NDI/Se-NDI ratios in polymers, which x-NDI were composed of naphthalene diimide with an octyldodecyl branched chain acceptor units and randomly distributed thiophene (T) and selenophene (Se) donor units. Meanwhile, the reference alternating copolymers (P(NDI2OD-T) and P(NDI2OD-Se)) were synthesized. The structure-property relationships of the random terpolymers were systematically studied. The effects of T-NDI/Se-NDI ratios on the absorption, energy levels, crystallinity, film morphology, charge transport and photovoltaic performance of the fabricated polymer solar cells were investigated. The acceptor 0.5-NDI with proper crystallinity was fully miscible with the chosen donor polymer PBDT(T)TPD, and formed the nanostructured blend. Compared to that of PBDT(T)TPD/P(NDI2OD-T) or PBDT(T)TPD/P(NDI2OD-Se) solar cells, the PBDT(T)TPD/0.5-NDI solar cells exhibited an improved short-circuit current density (Jsc) of 9.1 mA/cm2, a high open circuit voltage (Voc) of 1.05 V and enhanced power conversion efficiency (PCE) of 5.4%. Our work demonstrates that the development of random terpolymers is a simple and effective method to obtain polymer acceptor matching with the chosen polymer donor leading to efficient PSCs.