Low band gap polymer bulk heterojunction solar cells

Bulk heterojunction solar cells comprising a new low band gap polymer (PBEHTB) based on alternating electron-donating 3,4,3′,4′-tetrakis[2-ethylhexyloxy]-2,2′-bithiophene and electron-deficient 2,1,3-benzothidiazole units along the chain as electron donor and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as electron acceptor have been prepared. The polymer absorbs light up to 800 nm and thus covers a substantial part of the solar spectrum. Solar cells based on PBEHTB:PCBM exhibit excellent behavior giving an external quantum efficiency in the near infrared of 13%. Under simulated AM1.5 conditions an open circuit voltage of 0.77 V, a short circuit current of 2.8 mA/cm2, and a power conversion efficiency of 0.9% have been achieved, which is among the best achieved for low band gap polymer solar cells.