The role of oxygen doping on elemental intermixing at the PVD‐CdS/Cu (InGa)Se2 heterojunction

• Published in: Progress in photovoltaics Vol. 27; no. 3; pp. 255 - 263
• Main Authors: He, Xiaoqing, Ercius, Peter, Varley, Joel, Bailey, Jeff, Zapalac, Geordie, Nagle, Tim, Poplavskyy, Dmitry, Mackie, Neil, Bayman, Atiye, Lordi, Vincenzo, Rockett, Angus
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01.03.2019
• Subjects: Cadmium; Cadmium sulfide; CdS structure; Copper sulfides; Cu (In,Ga)Se2 photovoltaics; Cu diffusion; Doping; Energy conversion efficiency; Energy transmission; Heterojunctions; Mapping; Migration; Oxygen; Oxygen content; Photovoltaic cells; Physical vapor deposition; scanning transmission electron microscopy; Solar cells; STEM‐EDS mapping; Zinc oxide
• ISSN: 1062-7995; 1099-159X
• DOI: 10.1002/pip.3087

Elemental intermixing at the CdS/CuIn1−xGaxSe2 (CIGS) heterojunction in thin‐film photovoltaic devices plays a crucial role in carrier separation and thus device efficiency. Using scanning transmission electron microcopy in combination with energy dispersive X‐ray mapping, we find that by controlling the oxygen in the sputtering gas during physical vapor deposition (PVD) of the CdS, we can tailor the degree of elemental intermixing. More oxygen suppresses Cu migration from the CIGS into the CdS, while facilitating Zn doping in the CdS from the ZnO transparent contact. Very high oxygen levels induce nanocrystallinity in the CdS, while moderate or no oxygen content can promote complete CdS epitaxy on the CIGS grains. Regions of cubic Cu2S phase were observed in the Cu‐rich CdCuS when no oxygen is included in the CdS deposition process. In the process‐of‐record sample (moderate O2) that exhibits the highest solar conversion efficiency, we observe a ~26‐nm‐thick Cu‐deficient CIGS surface counter‐doped with the highest Cd concentration among all of the samples. Cd movement into the CIGS was found to be less than 10 nm deep for samples with either high or zero O2. The results are consistent with the expectation that Cd doping of the CIGS surface and lack of Zn diffusion into the buffer both enhance device performance. The oxygen content in the sputtering gas during the PVD deposition of CdS buffer layer can be used as a means to effectively control elemental intermixing between CdS and Cu (InGa)Se2 and the crystallinity of CdS in CIGS PV devices. More O suppresses Cu from CIGS and promotes Zn from ZnO into CdS and induces nanocrystalline CdS, while Cu2S phase was formed and epitaxially grown on Cu (InGa)Se2 when O was involved.