Optical spectroscopy of photovoltaic systems based on low-bandgap polymers

We report on the optical characterization of a low-bandgap thieno[3,4-b]pyrazine phenylenevinylene copolymer. Individual layers on quartz and Si substrates as well as layered structures on indium tin oxide covered glass with and without a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) layer were examined using spectroscopic ellipsometry, transmission, reflectance, electroreflectance and external quantum efficiency measurements. For comparison, current-voltage and capacitance-voltage characteristics were also measured. Changes in optical spectra of the pristine polymer due to an applied external voltage are analyzed quantitatively in terms of the Stark effect which allows the determination of the exciton radius, binding energy and ionization field strength. For structures with an inserted PEDOT:PSS layer, a significant charge trapping in the polymer layer is observed that considerably modifies both optical and electrical properties.