Electronic Defects and Device Performance in CuGaSe₂ Solar Cells

Abstract

The electronic and materials properties of two series of wide-bandgap solar cells with Cupoor CuGaSe 2 (CGS) absorbers have been studied, to better understand limitations on the device performance. One series of samples displayed distinct lateral non-uniformities in Cu/Ga ratio, Na content, and thickness. The C-V data on these samples suggested the presence of a distinct band of near-interface defect states. Although the second series of samples appeared uniform and C-V results did not indicate the presence of near-interface defect states, J-V-T results show V oc (0T) <E g in all samples, which typically is thought to indicate that a dominant recombination current at the interface limits V oc . The device performance did not appear to be correlated to defect densities in the bulk CGS film. EXPERIMENTAL METHODSThe samples were prepared on soda-lime glass, which was coated with a dc-sputtered, 1 µm Mo layer. The approximately 2 µm thick CGS layers were deposited by elemental evaporation. Samples were created in two different labs using slightly different techniques, namely a uniform single-step process and a three-step process, yielding sample series ìAî and ìBî, respectively. These deposition processes are described in more detail elsewhere [3,4]. All CGS films were grown Cu-poor. A thin CdS layer was deposited using a chemical bath, followed by rf sputtering of 50 nm of ZnO with a resistivity of 50 Ω-cm. The devices were completed by sputtering a 150 nm ITO layer with sheet resitistance 25 Ω /sq (series A) or a 500 nm ZnO:Al layer with sheet resistance 20 Ω/sq, (series B) and Ni-Al grids.

Metal-oxide broken-gap tunnel junction for copper indium gallium diselenide tandem solar cells

“…However, the best efficiency of CGS solar cells is only about 10%. This is due to a large bulk trap density [19] and poor band alignment between CGS and the buffer layer [20,21] which lead to a low open circuit voltage [22,23]. A second obstacle is thermal budget.…”