Influence of Substrate Temperature during InxSy Sputtering on Cu(In,Ga)Se2/Buffer Interface Properties and Solar Cell Performance

Abstract

Indium sulfide (InxSy)—besides CdS and Zn(O,S)—is already used as a buffer layer in chalcopyrite-type thin-film solar cells and modules. We discuss the influence of the substrate temperature during very fast magnetron sputtering of InxSy buffer layers on the interface formation and the performance of Cu(In,Ga)Se2 solar cells. The substrate temperature was increased from room temperature up to 240 °C, and the highest power conversion efficiencies were obtained at a temperature plateau around 200 °C, with the best values around 15.3%. Industrially relevant in-line co-evaporated polycrystalline Cu(In,Ga)Se2 absorber layers were used, which yield solar cell efficiencies of up to 17.1% in combination with a solution-grown CdS buffer. The chemical composition of the InxSy buffer as well as of the Cu(In,Ga)Se2/InxSy interface was analyzed by time-of-flight secondary ion mass spectrometry. Changes from homogenous and stoichiometric In2S3 layers deposited at RT to inhomogenous and more sulfur-rich and indium-deficient compositions for higher temperatures were observed. This finding is accompanied with a pronounced copper depletion at the Cu(In,Ga)Se2 absorber surface, and a sodium accumulation in the InxSy buffer and at the absorber/buffer interface. These last two features seem to be the origin for achieving the highest conversion efficiencies at substrate temperatures around 200 °C.

Indium Chalcogenide Nanomaterials in the Forefront of Recent Technological Advancements

“…Similar to the research objectives in Ref [15], indium sulfide is yet again demonstrated as a promising alternative to the cadmium sulfide, in this case as a buffer layer in the Cu(In,Ga)Se 2 solar cell [19]. It was found that the indium sulfide-based solar cell achieved 15.3% efficiency compared to 17.1% recorded for the cadmium sulfide counterpart.…”

“…Elemental composition of In x S y and Cu(In,Ga)Se 2 layers deposited at different substrate temperatures.Reprinted with permission from Ref [19]…”

The Application of Sputtered Gallium Oxide as Buffer for Cu(In,Ga)Se2 Solar Cells

“…To reach decent CIGS solar cell efficiencies with sputtered buffer layers for various materials, typical substrate temperatures above 100 C were reported for Zn(O,S) [34] and In x S y, [32] whereas sputtering at RT resulted mostly in poor efficiencies. [30,35] Therefore, we checked temperature dependence during Ga 2 O 3 sputtering on the CIGS solar cell performance as shown in Figure 2.…”

“…The transition to crystalline structure was reported for temperatures above 325 C for layers grown by pulsed laser deposition [26] or even higher at 500 C for sputtered Ga 2 O 3 , [27,28] not suitable for the important and susceptible pn junction of a CIGS solar cell. [29,30] Sputtered indium-based binary counterparts like In 2 O 3 [31] and In 2 S 3 [30,32] exhibit crystalline structures if deposited at similar substrate temperatures, whereas gallium sulfide sputtered even at 500 C was reported to grow X-ray amorphous. [33]…”

Phototransistor Behavior in CIGS Solar Cells and the Effect of the Back Contact Barrier

“…The attractiveness of CIGS as a thin-film technology can be explained by the small amount of materials required to produce a solar panel, because the thickness of the device layers is typically in the range of a few microns. The layers that are normally present in thin-film solar cells are as follows: (i) a transparent front contact (TCO), which in CIGS devices is usually an n-type doped zinc oxide (ZnO) film of around 0.3 µm [3], although other alternatives are possible; (ii) a buffer layer as an interface between the front contact and the absorber-in CIGS, a thin cadmium sulfide (CdS) layer of 20 to 50 nm is used [4], although there is ongoing research to replace this toxic material with a more environmentally friendly one [1,5,6]; (iii) the absorber, in this case, is a Cu (In, Ga) (S, Se) 2 compound semiconductor with a thickness between 1-3 µm, and in some cases even lower thicknesses are reported, such as in [7,8]; (iv) the absorber is deposited on top of the back contact, which is usually a molybdenum (Mo) layer of 0.2 to 1 µm [9]; (v) all these materials are deposited on a soda-lime glass substrate that supplies sodium (Na) to the sample to enhance its performance [10,11]. However, excess Na was reported to degrade the performance of the solar cell [12].…”