The Application Of Sputtered Gallium Oxide As Buffer For Cu(In,Ga)Se-2 Solar Cells

Crystalline gallium oxide is a promising wide-bandgap semiconductor material, especially for applications in high-frequency and high-power devices. With an optical bandgap energy well above 4 eV, which implies no visible light absorption, it is also a candidate for one of the front-side layers in thin-film solar cells. X-ray amorphous gallium oxide (a-Ga2O3) deposited by RF magnetron sputtering is applied as an n-type buffer layer in substrate-type configuration solar cells based on industry-relevant inline coevaporated Cu(In,Ga)Se-2 (CIGS) absorbers, which include an i-ZnO/ZnO:Al bilayer as the front electrode. The cells exhibit an efficiency peak at Ga2O3 deposition temperatures in the range of 140-180 degrees C and show mostly a gain in short-circuit current density compared with the CdS-buffered reference cells, as a result of the high optical bandgap of a-Ga2O3 in the range of 4.6-4.8 eV. The CIGS solar cells with sputtered a-Ga2O3 buffers reach efficiencies of almost 14%, lacking in open-circuit voltage and fill factor, whereas the CdS-buffered cells are on a 16-17% level. Light soaking of the cells leads to a slight improvement of the fill factor, but the gap in open-circuit voltage of 80-100 mV in contrast to the CdS-buffered reference cells remains unaffected.