Nishinaga, J.; Abou-Ras, D.; Wargulski, D.R.; Thomas, S.; Kamikawa, Y.; Nishida, T.; Ishizuka, S.: Wide-bandgap Cu(In, Ga)Se2 solar cells with uniform Ga compositions fabricated by molecular beam epitaxy. Solar Energy Materials and Solar Cells 292 (2025), p. 113825/1-10
10.1016/j.solmat.2025.113825
Abstract:
Wide-bandgap, chalcopyrite-type materials are attractive candidates for top cells of tandem-type solar cells because of their high absorption coefficients and excellent chemical stability. Nevertheless, achieving high open-circuit voltages (VOC) and high conversion efficiencies using wide-bandgap Cu(In,Ga)Se2 (CIGS) absorbers remains challenging. Conventionally, wide-bandgap CIGS solar cells are fabricated by depositing polycrystalline CIGS (poly-CIGS) layers with high [Ga] on glass substrates, and these poly-CIGS layers feature high densities of grain boundaries at which enhanced nonradiative recombination occurs. Epitaxial CIGS (epi-CIGS) solar cells do not contain any grain boundaries and can potentially overcome the VOC limitation. In the present study, we fabricated epi-CIGS layers with high [Ga] by molecular beam epitaxy, and compared the microstructural, compositional, and optoelectronic characteristics of poly-CIGS and epi-CIGS solar cells by electron backscatter diffraction, energy dispersive X-ray spectroscopy, cathodoluminescence, and secondary ion mass spectrometry. For poly-CIGS, Ga segregation occurred during three-stage processing, and layers with high [In] decreased the bandgap energies and the VOC. In contrast, uniformly distributed [Ga] were confirmed in the epi-CIGS layers. For epi-CIGS solar cells, a conversion efficiency of 14.5 % and VOC of 0.93 V were achieved using absorbers with a bandgap energy of 1.6 eV. The external quantum efficiency spectra of the epi-CIGS solar cells showed a sharp absorption edge, and their radiative and nonradiative VOC losses were lower than those of the poly-CIGS solar cells. These results suggest that CIGS solar cells with uniformly distributed [Ga] are optimal for top cells of tandem-type solar cells.