Charge transport analysis of poly(3-hexylthiophene) by electroreflectance spectroscopy

The charge transport in organic semiconductors is still not completely understood. We use electroreflectance spectroscopy to investigate charge transport in organic field effect devices in order to obtain optical information on the charges within the charge accumulation layer at the organic semiconductor-insulator interface. Here, the reflectance geometry allows an analysis of devices prepared on opaque substrates. The recorded spectra were analyzed based on optical layer stack simulations to extract quantitatively the change of the dielectric function due to the charge injection of the accumulation layer in poly(3-hexylthiophene) (P3HT). For the simulation the anisotropic optical response of spin-coated P3HT layers is experimentally determined by spectroscopic ellipsometry. Using the developed theoretical approach the characteristic change of three different preparations of the organic semiconductor-insulator interface is analyzed. Laterally resolved measurements close to an injecting contact have revealed characteristic spectral changes in dependence of the preparation conditions, and significant spectral changes within the first $90\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the contact were found, which were attributed to an energetic relaxation of charges during the charge transport process.