Investigation of AgGaSe₂ as a Wide Gap Solar Cell Absorber

Abstract

The compound AgGaSe 2 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 AgGaSe 2 has a very low tolerance to off-stoichiometry, reference materials of possible secondary phases in the Ag 2 Se−Ga 2 Se 3 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 Ag 9 GaSe 6 secondary phase was present along with AgGaSe 2 , which resulted in devices being shunted under illumination. When absorbers were grown under Ag-deficient conditions, the AgGa 5 Se 8 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 AgGa 5 Se 8 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 AgGa 5 Se 8 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 AgGaSe 2 solar cells.

Performance Limitations of Wide‐Gap (Ag,Cu)(In,Ga)Se2 Thin‐Film Solar Cells

“…This phase is not very conductive but has a low E g and is likely defect rich. [43] Summarizing all findings, it can be concluded that Ag alloying reduces recombination at the buffer interface in chalcopyrite solar cells with GGI > 0.5. [24] This allows producing such low V OC deficits with CdS as only possible for pure CIGS when using alternative buffers (see Figure S4, Supporting Information).…”

“…Obviously, the OVC surface fraction decreases toward stoichiometric composition until eventually no OVCs can be measured for [I]/[III] > 0.97. For larger [I]/[III] values, the I-rich Ag 9 GaSe 6 phase starts to segregate, [28,43] possibly accompanied by a minor fraction of (Ag y ,Cu 1Ày ) 2Àx Se. It is unclear if for common absorber deposition routines, a phase-pure 1:1:2 chalcopyrite film can be formed at perfect integral stoichiometry or if minor OVC and Ag 9 GaSe 6 segregations coexist.…”

“…98 is likely caused by parasitic absorption in defect-rich Ag 9 GaSe 6 (E g % 0.6 eV), which forms for ACIGS with high Ag concentrations, instead of the binary and highly conductive (Ag y ,Cu 1Ày ) 2Àx Se phase commonly observed for low-or no-Ag-containing absorbers. [28,37,43] For I-deficient samples, a long-wavelength reduction in EQE is observed that becomes more pronounced with decreasing [I]/[III] value. However, very low [I]/[III] values < 0.8 seem to produce more squared spectra again.…”

“…Figure 11 sketches the corresponding trends found in this study or reported in earlier works. [24,[28][29][30]43] The [I]/[III] range in which the transition between the different features occurs (e.g., OVCs at the back contact or not) decreases with increasing Ag and Ga content, due to the narrowing of the 1:1:2 single-phase region.…”

“…The OVC patches exhibit a larger bandgap (ΔE g % 0.2-0.3 eV) as compared with the surrounding 1:1:2 phase. [28,43,[69][70][71][72][73][74] Although they do not cover the whole surface, a somewhat beneficial effect on V OC cannot be excluded just by the larger E g . In addition, a very pronounced CBO ("spike" > 0.4 eV) at the OVC/1:1:2 and OVC/CdS interfaces is expected for the measured absorber and OVC compositions.…”