Surface energy band gap of polycrystalline CuInSe2 thin-films in tunneling spectroscopy

In this paper, the authors demonstrate a tunneling spectroscopic technique to determine energy band gaps of the CuInSe₂ thin-film solar cell surfaces. The I-V characteristics are measured with STM in ambient air. Using the well-known theory of tunneling spectroscopy, the normalized differential conductivity corresponding to surface density of states is both evaluated from the measured I-V characteristics and simulated with help of two gap models with and without additional localised state tails in the gap. The poorly defined gap edges are explained by additional localised states near the top of the valence band and the bottom of the conduction band. These additional localised states could be true surface defect states or originate from deep atomic layers underneath the surfaces due to band bending