Influence of the Annealing Temperature on the Photovoltaic Performance and Film Morphology Applying Novel Thermocleavable Materials

Di-2-thienyl-2,1,3-benzothiadiazole (DTBT) bearing thermally cleavable ester groups in different positions were prepared and copolymerized with alkylsubstituted cyclopentadithiophene (CPDT). The polymers were found to have band gaps in the range of 1.66−2.03 eV and were explored in polymer photovoltaic devices as mixtures with soluble methanofullerenes. The positioning of the ester groups proved to be very significant despite the identical conjugated backbone of 2-methyl-2-hexyl 5-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-2-(7-(3-(((2-methylhexan-2-yl)oxy)-carbonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T1) and 2-methyl-2-hexyl 2-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-5-(7-(4-(((2-methylhexan-2-yl)oxy)carbonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T2). Power conversion efficiencies of up to 1.92% were observed for polymers bearing ester groups on the 4-positions of the thienyl groups (T2), but shifting them to the 3-positions (T1) reduced the efficiency significantly to 0.18%. The thermal behavior of the polymers was studied with thermogravimetric analysis (TGA) that showed a weight loss around 200 °C corresponding to elimination of the ester side chains followed by a second weight loss around 300 °C corresponding to loss of CO2 via decarboxylation. The temperature of thermocleavage of the active layer films was optimized to 265 °C whereby the T2:PCBM solar cells maintained a significant performance giving efficiencies up to 1.49%.