The photovoltaic effect in poly(alkylthiophene) films on a silicon substrate

A study of photovoltage was made for a series of sandwich structures on the basis of poly(3-dode-cylthiophene) films having characteristic thicknesses 100 and 500 nm and being deposited on n-Si and p-Si substrates from a solution. Semitransparent Al and Au electrodes were obtained on the surfaces of these films by thermal evaporation. A clear photoresponse was obtained in films on an n-Si substrate. Two distinct spectral components of the photovoltage were observed in the 1.3-to 3.6-eV (900–300 nm) energy range for incident quanta. The first component corresponds to the absorption edge of the Si substrate (1.4–1.6 eV). The other corresponds to the π-π* absorption of the polythiophene films (1.7–2.1 eV). The dependences of the photovoltage upon radiation intensity are different for these two spectral components. The relaxation time of the photoresponse for the second component, corresponding to the absorption in the film, is 10–20 min. This is 3–4 orders of magnitude higher than the relaxation time for the first component. A model of the potential barrier at the polythiophene/n-Si interface, allowing one to explain the main experimental results, is proposed. This barrier is formed as a result of the chemical interaction of the polythiophene molecules with the substrate.