Method to determine the absorptance of thin films for photovoltaic technology

We have demonstrated a novel method to determine optical properties of opaque or semi-transparent films for photovoltaic (PV) applications. Such films may be the basis of transparent conductors or photoconductive material. As an example, we measure the absolute absorptance (at visible and near infrared wavelengths) of an optically thick single-wall carbon nanotube (SWCNT) film by using a pyroelectric detector. This novel method obviates the need for analysis with respect to polarization and associated difficulties of ellipsometry. The Kramers-Kronig relation is used to determine the thick film index of refraction, which we use to calculate the optical properties of thin films as a function of thickness. A transmittance measurement obtained from a thin SWCNT film shows excellent agreement with results from our model.