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Polymer solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) were fabricated using two different solvents. P3HT:PCBM films casted from chlorobenzene solution absorb more red light than the films casted from chloroform solution. After thermal annealing, the films casted from chloroform show higher absorption than the films casted from chlorobenzene. Solar cells made from P3HT:PCBM chlorobenzene solution show no change in the white light power conversion efficiency (2.2%) after annealing. Solar cells processed from P3HT:PCBM chloroform solution show a white light power conversion efficiency of 1.5% without thermal annealing and 3.4% after the thermal annealing. The stated efficiencies are not corrected for the spectral mismatch.

Summary: Low‐bandgap π‐conjugated polymers that consist of alkyl thiophene/alkoxy phenylene and 2,3‐diphenylthieno[3,4‐b]pyrazine units have been prepared in high yields by a Sonogashira polycondensation. The copolymers are characterized by NMR, IR, UV, GPC, and elemental analysis. Thin films of the polymers P1, P2, and P3 exhibit an optical bandgap of ≈1.57–1.60 eV. Under simulated AM 1.5 conditions P2/PCBM devices on polyester foil provide a short circuit current of ISC = 10.72 mA · cm−2, an open circuit voltage of Voc = 0.67 V, and a power conversion efficiency of 2.37%.