Investigation of AgGaSe₂ as a Wide Gap Solar Cell Absorber

The compound AgGaSe2 has received limited attention as a potential wide gap solar cell material for tandem applications, despite its suitable band gap. This study aims to investigate the potential of this material by deposition of thin films by co-evaporation and production of solar cell devices. Since AgGaSe2 has a very low tolerance to off-stoichiometry, reference materials of possible secondary phases in the Ag2Se-Ga2Se3 system were also produced. Based on these samples, it was concluded that X-ray diffraction is suited to distinguish the phases in this material system. An attempt to use Raman spectroscopy to identify secondary phases was less successful. Devices were produced using absorbers containing the secondary phases likely formed during co-evaporation. When grown under slightly Ag-rich conditions, the Ag9GaSe6 secondary phase was present along with AgGaSe2, which resulted in devices being shunted under illumination. When absorbers were grown under Ag-deficient conditions, the AgGa5Se8 secondary phase was observed, making the device behavior dependent on the processing route. Deposition with a three-stage evaporation (Ag-poor, Ag-rich, and Ag-poor) resulted in AgGa5Se8 layers at both front and back surfaces, leading to charge carrier blocking in devices. Deposition of the absorber with a one-stage process, on the other hand, caused the formation of AgGa5Se8 locally extended through the entire film, but no continuous layer was found. As a consequence, these devices were not blocking and achieved an efficiency of up to 5.8%, which is the highest reported to date for AgGaSe2 solar cells.