Effects of a heteroatomic benzothienothiophenedione acceptor on the properties of a series of wide-bandgap photovoltaic polymers

A series of benzodithiophene (BDT) and benzothienothiophenedione (BTTDO) alternating wide-bandgap (WBG) copolymers, PBDT-O1, PBDT-S1 and PBDT-Se1, were designed and synthesized, in which heteroatoms (O, S and Se) were incorporated into the electron-deficient BTTDO motif. The effect of heteroatoms on the thermal stability, absorption spectra, energy level, charge carrier mobility, and photovoltaic properties of these WBG polymers was systematically studied. The results indicated that upon increasing the size of the heteroatoms, the maximum absorption peaks were red-shifted and the optical bandgap decreased. Solar cells with a conventional structure of ITO/PEDOT:PSS/polymers:PC₇₀BM (1:1, w/w)/Ca/Al were fabricated. Among the three polymers, PBDT-S1 achieved the best photovoltaic performance, with a high power conversion efficiency (PCE) of 9.0%, with an open-circuit voltage (V_oc) of 0.91 V, a short-circuit current (J_sc) of 12.99 mA cm^-2, and an unprecedented fill factor (FF) of 74.9%.