Does the band gap calculated from the photocurrent of Schottky devices lead to erroneous results? Analysis for CdTe

In recent years there has been an increased interest in flexible, lightweight photovoltaic modules based on thin metallic substrates. This paper reports some optoelectronic properties of electrodeposited CdTe thin films grown onto lightweight stainless steel (SS) foils. The optoelectronic properties were investigated with Schottky barriers of Au/CdTe/SS structure. The influence of the built-in potential of the Schottky junction on the bulk and the interface recombination of the photo-generated minority carriers is explained with the existing models. The voltage-dependent collection functions influence the photocurrent of the devices in both short- and long-wavelength regions of the spectrum. It is observed that in the photovoltaic mode the contribution due to the collection functions depends on the open-circuit voltage of the device. Au/CdTe Schottky devices, having higher open-circuit voltage, exhibit a better response in the long wavelength region. This is due to the efficient collection of the carriers generated in the bulk of the film and in such devices the contribution from the bulk collection function is higher. The enhancement in the bulk collection function causes a shift in the response of the device to higher wavelengths giving lower values for the calculated band gap. Due to this dependence of the long wavelength response on the open-circuit voltage of the devices, the band gap calculated from the photocurrent of different Schottky devices gives different values for the band gap of the material. Thus the method of calculating the band gap from the photocurrent of Schottky devices can lead to erroneous conclusions regarding the band gap of the material.