Ellipsometric Spectroelectrochemistry: An In Situ Insight in the Doping of Conjugated Polymers

In this work we introduce in situ UV-Vis-NIR transmission spectroscopic ellipsometry as an alternative spectroelectrochemical method to examine the pi-electron orbital structure as well as intra- and inter-chain order in regioregular poly(3-hexylthiophene)(rr-P3HT) during electrochemical doping. With the suggested technique we measure the dielectric tensor of the P3HT film through a typical electrochemical cell consisting of a glass cuvette. The method provides a stable controlled electrochemical environment with full control on the applied potential during the ellipsometric measurement. P3HT, which is a model semi-transparent conducting polymer, undergoes a sequence of oxidation steps with increasing electrochemical potential. In the presented work the doping is monitored up to a level where finally almost every second thiophene ring is positively charged. The reference free determination of the real and imaginary part of the dielectric tensor components allows a quantitative analysis of resonance energies as well as the orientation depended oscillator strength of interchain exciton and polaron resonances. This parameters are directly linked with (i) the intrachain structure, (ii) the degree of electron localization (iii) the pi-pi stacking order, (iv) the average polymer orientation, and (v) the doping level. Additionally we could observe aggregation depending doping efficiency. The film optical properties appear fully reversible upon the cyclic electrochemical doping which proofs the stability of inter- as well as intramolecular structure while going through very high doping levels.