Double acceptor D-A copolymers containing benzotriazole and benzothiadiazole units: chemical tailoring towards efficient photovoltaic properties

This study reports the design and synthesis of a series of D-A copolymers alternating benzothiadiazole and benzotriazole acceptors to a donor co-unit, and investigates their photovoltaic properties in bulk heterojunction solar cells with PC71BM. Successive modifications to the copolymers are carried out, passing from thiophene to benzodithiophene (BDT) donor co-units and from regular to random alternation of the accepting units. A copolymer containing a thiophene co-unit has reached a power conversion efficiency (PCE) of 1.88% in optimised devices. Moving from thiophene to BDT leads to some advantages, such as a lower optical energy gap and a lower confinement of the photo-generated charges, that positively affect the spectral coverage to solar radiation and the fill factor (FF) parameter in the devices. Passing from regular to random distribution of the accepting units, the BDT copolymer solar cells have reached a PCE of 5%. Such encouraging photovoltaic performances are combined with high solubility, high molecular weight and with an easy preparation procedure. These characteristics are necessary requirements to envisage the scale-up to industrial applications.