Infrared photocurrent response of charge-transfer exciton in polymer bulk heterojunction

We study the charge-transfer exciton absorption and photocurrent response in solution-processed bulk heterojunction based on poly(3-hexylthiophene) donor and (6,6)-phenyl-C61-butyric acid methyl ester acceptor in the near-infrared wavelength region. While the exciton absorption exists only for wavelength below 650nm, direct generation of charge-transfer exciton formed between the donor and acceptor extends the absorption wavelength to 950nm. For films with micrometer thickness, the photon-to-electron conversion efficiency is about 60% at 750nm wavelength under reverse voltage bias and the photocurrent to dark current ratio is about 8.6 at 900nm and remains 3.6 even at 1000nm. Photodetector with high sensitivity covering exclusively the 650–1000nm near infrared region can therefore be made without a low bandgap material. The charge-transfer exciton absorption coefficient and photocurrent sensitivity depend on the annealing condition which controls the donor-acceptor morphology.