Thiophene and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole based alternating copolymers for polymer solar cells

Two planar type photovoltaic polymers based on naphthobisthiadiazole, poly(2,5-bis(2-hexyldecyloxy)phenylene-alt-(5,10-dithiophen-2-yl)naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole) (h-PPDTNTz) and poly(2,5-bis(2-decyltetradecyloxy)phenylene-alt-(5,10-dithiophen-2-yl)naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole) (d-PPDTNTz) were synthesized by incorporating intrachain noncovalent Coulombic interactions in the molecular design. To achieve a delicate balance of molecular weight, solubility as well as bulk film morphology, hexyldecyloxy (h-) and decyltetradecyloxy (d-) side-chains were substituted, which played a decisive role in modulating morphology, film packing structure and macroscopic device properties. Both polymers showed a broad light absorption up to ~800 nm and d-PPDTNTz exhibited a deeper HOMO and preferentially face-on orientation in pristine and blended films with PC₇₁BM. The detailed optical, electrochemical, thermal, morphological and the resulting photovoltaic characteristics were studied. The best power conversion efficiency of ~6.7% was measured for d-PPDTNTz:PC₇₁BM, suggesting that the careful choice of side-chains is necessary for fully optimize the photovoltaic materials and devices.