Structural and optical characterization of polycrystalline CuInSe2

In this study device quality CuInSe2 films were grown using a relatively simple, tolerant and reproducible two-stage technique. This process involved the selenization of various CuIn precursors in a H2Se/Ar atmosphere. The variation in the microstructure (grain size and lattice defects) and optical properties of the CuInSe2 thin films on gradually changing the stoichiometry from Cu rich to In rich was investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed that Cu-rich and In-rich films were clearly distinct in their defect structure. In general, In-rich material exhibited relatively small grains (0.2-0.8 mu m), which were highly defected. In contrast to In-rich films, Cu-rich films were characterized by the presence of large faceted grains (1-4 mu m) with relatively low defect density. These material properties were clearly reflected in low temperature photoluminescence (PL) studies, indicating sharp transitions for Cu-rich films compared to broad emission lines for In-rich materials. Cu-rich materials were characterized by four relatively sharp emission lines at 1.036 eV, 0.993 eV, 0.971 eV and 0.942 eV at 6 K. As the composition was gradually changed from Cu rich to stoichiometric compositions only one broad emission line could be observed at 0.964 eV, which corresponds to a donor-acceptor pair transition. In the case of In-rich material (Cu/In atomic ratio = 0.3-0.6), three dominant transitions were observed at 1.10 eV, 0.975 eV, and 0.89 eV. The observed spectra are explained by considering the formation energies of the defects and the composition of the specific film.