Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells

Abstract

Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20%. This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance

“…PDT is in fact performed at a temperature approximately 100 °C below the deposition temperature, and the presence of alkali elements is known to further reduce group-III element diffusivities [19]. PDTs were reported to modify the surface and GB chemical composition at the surface [20] and grain boundaries [21] only in regions limited to a few nanometers. Therefore, it is justified to assume any micrometer-scale GGI inhomogeneities are present already at the end of the CIGS deposition before the PDT.…”

Intermixing and Formation of Cu-Rich Secondary Phases at Sputtered CdS/CuInGaSe2Heterojunctions

“…Recently it was shown [23] Cd is found in the CIGS surface layer at much lower concentration than in the PVD-CdS/CIGS junctions. Note that unlike in the PVD samples, the S signal persists deep into the CIGS ~5 at.…”

Native point defects in CuIn1−xGaxSe2: hybrid density functional calculations predict the origin of p- and n-type conductivity

“…CIGS is of particular interest as the absorber material in thin-film photovoltaic cells, as it has a very high optical absorption coefficient 1 . Moreover, photovoltaic cells based on polycrystalline CIGS hold record conversion efficiencies in the category of thin-film cells, currently already exceeding 20%, both on glass substrates and on flexible substrates 2,3 . Photoluminescence spectra of CIS and CGS, e.g.…”