Donor Polymers Containing Benzothiadiazole and Four Thiophene Rings in Their Repeating Units with Improved Photovoltaic Performance

A series of internal donor-acceptor type of copolymers containing benzothiadiazole and four thiophene rings (bis(2,2'-dithienyl)benzothiadiazole) in their repeating units were synthesized and characterized. The effect of the position of alkyl groups attached to different thiophene rings of the new polymers on their optical, electrochemical, and photovoltaic properties was investigated and compared with poly(2,7-(9,9-dioctofluorene)-alt-5,5-(4',7'-di-2-thienylbenzo[c][1,2,5]thiadiazole)) (PFO-DBT). One of the new polymers, poly(2.7-(9,9-dicotylfluorene)-alt-5',5'-(4,7-bis(3'-hexyl-2,2'-bithiophen-5-yl)benzol[c][1,2,5] thiadiazole)) (PFO-M3), showed the best device performance with a power-conversion efficiency of 2.63%, an open-circuit voltage of 0.86 V, a short-circuit current density of 5.86 mA cm(-2) and a fill factor of 0.52, which is better as compared with PFO-DBT. This work demonstrated that increasing the number of thiphene rings ill the repeating units of the donor-acceptor polymers is a simple and effective method to red-shift their absorption spectra and also improve their solar cell performance.