Abstract
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.
Highlights
Thin-film solar cells based on Cu(In,Ga)(Se,S)[2] (CIGS(e)) absorbers have reached efficiencies up to 23.4% for laboratoryscale devices,[1] demonstrating the great potential of chalcopyrite absorber materials
The highest efficiency of 4.5% based on a AgGaSe2/CdS heterojunction was published by Murthy et al already in 1990.6 The same efficiency was reached by Yamada et al in 2006.7 To the best of our knowledge, better performing AgGaSe2 solar cells have not been reported since 2006
The X-ray diffraction (XRD) patterns of all samples are compared to literature patterns in the Supporting Information (Figures S1−S5)
Summary
Thin-film solar cells based on Cu(In,Ga)(Se,S)[2] (CIGS(e)) absorbers have reached efficiencies up to 23.4% for laboratoryscale devices,[1] demonstrating the great potential of chalcopyrite absorber materials. Nakada et al achieved a conversion efficiency of 9.3% for devices with [Ga]/([Ga] + [In]) = 0.8.8 An efficiency of 10.7% was reported for Ag(In,Ga)Se2 with [Ga]/([Ga] + [In]) = 0.75 after air-annealing the completed solar cell for 5 min at 200 °C.9. It should be noted, that these results are not directly comparable with AgGaSe2 because In alloying decreases the material band gap, thereby increasing its Shockley−Queisser (SQ) limit. Aspects that have been investigated for Ag(In,Ga)Se2 solar cells include the formation of non-Ohmic back contact due to the absence of MoSe2,10,11 segregation of Ag during three-stage growth,[12,13] and peculiarities of the effect of Na.[14,15]
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