The role of Dawson Polyoxometalates as interfacial layers on the energy band alignment between indium tin oxide and poly(3-hexylthiophene) films

Abstract In this work a series of Dawson Polyoxometalates (POMs) with different cations and addenda atoms were grown as interfacial layers between indium tin oxide (ITO) and regio-regular poly(3-hexylthiophene) (rr-P3HT) in order determine the effect of different POMs films on the energy band alignment at the heterojunction rr-P3HT/ITO. The POMs/ITO and rr-P3HT/POMs/ITO interfaces were investigated by X-ray Photoelectron and Ultra-Violet Photoelectron Spectroscopies (XPS, UPS) to gain insight into their chemical morphology, the energetics of the formed interfaces and the charge carrier barriers. We show that POMs are reduced upon deposition on ITO and the extent of their reduction is controlled by their cation and addenda atom. The deposition of POMs on ITO results to the formation of interface dipoles and the work function of ITO is increased up to 5.9 eV. When, a thin layer of rr-P3HT is formed on top of POMs modified ITO, electron transfer from rr-P3HT to the substrate occurs when the work function of POM/ITO substrates is >4.5 eV, resulting to a pinning of the highest occupied molecular orbital (HOMO) of rr-P3HT and the formation of ohmic contacts. The results were interpreted by the integer charge transfer model (ICT).