PHOTOELECTROCHEMICAL PROPERTIES OF POLY(3-ALKYLTHIOPHENE) FILMS IN AQUEOUS-SOLUTION

The photoelectrochemical properties of pi-conjugated poly(3-hexylthiop hene) films in aqueous solution are presented. Application of potentia l biases more negative than +0.3 V (SCE) to the polymer film immersed in an aqueous electrolyte gives rise to a depletion layer at the polym er/solution interface. Photoexcitation of the polymer leads to photore duction of water. The photovoltage observed is linearly related to pH of the solution in accordance with the variation in redox potentials o f the H+/H-2 couple, indicating the absence of Fermi level pinning. Qu antum efficiencies are dependent on the pH of the solution, irradiatio n power, excitation wavelength, and film thickness. The capture cross section of irradiation was varied by controlling the film thickness. W ith an irradiation wavelength of 435 nm, >90% of the incident light wa s absorbed within the first 280 nm. Using this wavelength of incident light, quantum efficiencies reached a maximum for 220 nm thick films. Quantum efficiencies decreased with increasing incident power.