Quasi Fermi Level Scan of Band Gap Energy in Photojunction

Abstract

Photovoltage generation model results are compared with the correlated illumination intensity spectra of semiconductors photojunction. The moderate continuous increase of illumination intensity of semiconductor photojunction leads to remarkable increase of relative concentration of minority carriers and related to it quasi Fermi level scan along the energy band gap. The scanning energy region runs up from thermal equilibrium Fermi level for electrons and down for holes. For moderate illumination related changes of quasi Fermi levels energy of minority carriers dominate over the changes of majority carriers and they decide on measured open circuit voltage. Expected spectrum of quasi Fermi level scan on illumination intensity will strongly depend on interaction with electronic "defects" located in photojunction region (e.g. impurities, clusters, barriers, etc.) leading to the majority quasi Fermi level pinning. Measured region of quasi Fermi level energy pinning allows to estimate the defect states parameters (binding energy and concentration) in situ during the work of photojunction. The theoretical model of described effect will be presented and supported by experimental data measured for Si p/n junction and CdTe/ZnTe heterojunction.