Spectroscopic ellipsometric measurements of thin polycrystalline ͑4CuInSe 2 ͒ y ͑CuIn 5 Se 8 ͒ 1−y films reveal that there are important differences in optical properties and electronic structures between ␣-phase CuInSe 2 and Cu-poor CuInSe 2 . We report the optical functions of thin-film polycrystalline ͑4CuInSe 2 ͒ y ͑CuIn 5 Se 8 ͒ 1−y and describe how they change depending on the degree of Cu deficiency. We find a reduction in the absorption strength in the spectral region of 1 -3 eV for Cu-poor CuInSe 2 . This reduction can be explained in terms of density of the Cu 3d states in CuInSe 2 . Cu-poor CuInSe 2 samples show an increase in band gap due to reduced p-d interaction. We find that the reduction in the amplitudes of E 0 ͑A , B , C͒ transitions at the ⌫ point and E 1 ͑A , B͒ transitions at the N point are due to Cu deficiency. Local density approximation calculations with a modeled on-site self-interaction correction of the absorption coefficients of Cu 8 In 8 Se 16 ͑CuInSe 2 ͒, Cu 5 In 9 Se 16 ͑CuIn 3 Se 5 -like͒, and Cu 2 In 10 Se 16 ͑CuIn 5 Se 8 ͒ are in good agreement with those of thin-film polycrystalline CuInSe 2 samples with 24.1, 15.6, and 9.1 at. % Cu, respectively.

Previous work on CdS–CuInSe2 (CIS) solar cells, which reported distortions of their current-voltage (J–V) curves under red illumination, is expanded in this work to include CdS–CuIn1−xGaxSe2 cells with variable Ga and CIS cells with variable CdS thickness. Different amounts of J–V distortion were observed in these cells under red light. The details are in good agreement with predictions of a photodiode model, in which a secondary barrier caused by the positive conduction-band discontinuity (spike) at the buffer–absorber interface is responsible for the current limitation. The illumination of the cell with high-energy photons lowers the barrier due to buffer photoconductivity, and thus removes the J–V distortion.

The built-in electrical potential of Cu(In,Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. Profiles of the electrical potential along cross sections of the device demonstrate that the p–n junction is a buried homojunction, and the p/n boundary is located 30–80 nm from the CIGS/CdS interface in the CIGS film. The built-in electric field terminates at the CIGS/CdS interface, indicating that the CdS and ZnO layers of the device structure are inactive for the collection of photoexcited carriers.

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Spectroscopic ellipsometry measurements of CuInSe2 (CIS) and CuIn0.8Ga0.2Se2 (CIGS) reveal that there are important differences in electronic properties between stoichiometric CIS (CIGS) and Cu-poor CIS (CIGS). We find a reduction in the absorption strength in the spectral region of 1–3eV. This reduction can be explained in terms of the Cu 3d density of states. Cu-poor CIS (CIGS) materials show an increase in band gap due to the reduction in repulsion between Cu 3d and Se 4p states. The experimental results have important implications for the function of polycrystalline optoelectronic devices.