Ellipsometric studies for thin polymer layers of organic photovoltaic cells

Abstract

In this study, ellipsometry was used to study thin films on organic photovoltaic cells. The investigated cells contained a traditional donor-acceptor structure as the active layer in the form of a bulk heterojunction. The tested photovoltaic cells had the following structure: ITO/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/poly (3-hexylthiophene-2-diyl) (P3HT) + 1,3-phenyl-6-fluorine-1H-pyrazolo[3,4-b]quinoline (PQ)/Al. The active layer was a blend of P3HT and a compound from the pyrazoloquinoline group with a low molecular weight marked as PQ. The ITO film acted as the electrode collecting the holes. The PEDOT:PSS layer was used to smoothen the ITO film and facilitate the transport of holes. The layers were tested using spectroscopic ellipsometry (SE), and the spectral dispersion of optical constants was determined in a wavelength range of 300–1700 nm. The appropriate theoretical models were then fitted to the SE measurement results. This information was used to determine the thickness of the layers and the dispersion relation of refractive and extinction indices. The temperature dependence of refractive indices of polythiophene layers during the heating and cooling process is also presented for a temperature range of 25–110 °C. Additionally, the authors calculated thermo-optic and thermal expansion coefficients and characterized the conditions of thermal stability of the layers and reversibility issues in thermal processes.