Donor and acceptor behavior in a polyfluorene for photovoltaics

We investigate photovoltaic devices based on a red-absorbing conjugated polymer poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) (APFO-3). We show that the polymer acts as an electron donor when blended with ZnO nanoparticles, giving a short-circuit quantum efficiency of 28%. When blended with poly(3-hexylthiophene) (P3HT), however, the APFO-3 acts as an electron acceptor, giving a short-circuit quantum efficiency of 12%. We study this charge-transfer process by comparing photoinduced absorption spectra of the hybrid blends with the absorption spectra of chemically doped APFO-3, which allows us to distinguish features due to positive and negative polarons. We also present dark current measurements of single-carrier devices which demonstrate that APFO-3 has similar mobilities for electrons and holes, consistent with ambipolar behavior in photovoltaic devices.