Electronic transitions in the bandgap of copper indium gallium diselenide polycrystalline thin films

The electronic properties of polycrystalline copper indium gallium diselenide thin films have been investigated, with emphasis on understanding the distribution and origin of electronic states in the bandgap. The samples studied were working photovoltaic devices with the structure ZnO/CdS/CuIn_1−xGa _xSe₂/Mo, and photovoltaic efficiencies ranging from 8 to 16%. The CdS layer and the p-type CuIn_1−xGa _xSe₂ film create the n+- p junction at the heart of these devices. The samples were investigated using four techniques based on the electrical response of the junction: admittance spectroscopy, drive level capacitance profiling, transient photocapacitance spectroscopy, and transient photocurrent spectroscopy. From these measurements the free carrier densities, defect densities within the bandgap, spatial uniformity, and minority carrier mobilities have been deduced. The sub-bandgap response from the CuIn_1−xGa_xSe₂ film was dominated by two defects. One exhibited a thermal transition to the valence band with an activation energy ranging between 0.1 and 0.3 eV and thermal emission prefactors obeying the Meyer-Neldel rule. The second was detected as an optical transition 0.8 eV from the valence band edge. Neither of these defects exhibited densities that varied systematically with gallium content, implying that they are not directly connected with the group III elements in these alloys. The defect densities also do not clearly correlate with the photovoltaic device performance; however, the position of the 0.8 eV defect lies nearer to mid-gap in the higher gallium, and hence higher band gap, material. This implies that it may be a more important recombination center in these devices and may be partially responsible for the reduced photovoltaic efficiencies observed when Ga/(In + Ga) > 0.4. An additional defect response was observed near mid-gap in films grown by processes known to produce lower quality devices. The influence of defects located at grain boundaries was also investigated by comparisons with a device based on an epitaxial single crystal CuIn_1−xGa _xSe₂ film. The grain boundaries do not appear to contain significant quantities of additional defects with sub-bandgap electronic transitions. Finally, metastabilities in the defect distributions resulting from light exposure were also explored. Understanding these metastable changes is likely to lead to a better understanding of the role of the defects in the bandgap of CuIn_1−xGa_xSe₂ films.