Sensitization of p-type NiO Using n-type Conducting Polymers

We report on the sensitization of a p-type inorganic semiconductor, NiO, by n-type conjugated polymers. NiO thin films were deposited using RF sputtering in pure Ar (NiO A) or in Ar + O2 (90% + 10%) (NiO B). XPS and Kelvin probe measurements indicate the incorporation of oxygen in NiO B leading to the formation of electron-accepting states. Photoconductance measurements using the time-resolved microwave conductance (TRMC) technique demonstrate that these electron-accepting states are required for the hopping-like charge transport within the metal oxide. Polymer/NiO hybrid bilayers were prepared by spin-coating poly(fluorene-bis(1-cyanovinylenethienylene)phenylene) (PF1CVTP) or poly(fluorene-bis(2-cyanovinylenethienylene)phenylene) (PF2CVTP) on top of NiO B. By determining the photoluminescence quenching, long exciton diffusion lengths were found amounting to 25 ± 2 nm and to 17 ± 2 nm, respectively. From I−V measurements on prototype n-type polymer/NiO hybrid solar cells carried out in the dark and under illumination, hole transfer from the polymer to the NiO is confirmed. The photovoltaic effect in cells based on PF2CVTP/NiO B bilayers was much larger than in PF1CVTP/NiO bilayers, which is related to the higher photoconductance observed in pristine PF2CVTP. It is proposed that only those photoexcitations leading to the formation of charge carriers within the bulk of the polymer film can be collected, while excitons reaching the NiO interface are lost by fast (surface) recombination.