Sputtering as a viable route for In₂S₃ buffer layer deposition in high efficiency Cu(In,Ga)Se₂ solar cells

Abstract

Alternative buffer layers in CIGSe are deposited mainly using chemical bath deposition because of its benefits like simplicity, good film quality and surface/step coverage. All the layers in CIGSe cell stack such as back contact, absorber and window layers are deposited by vacuum-deposition methods such as coevaporation, sputtering, and sometimes thermal evaporation, except for the buffer layer. Therefore, in the present work we demonstrate the feasibility to deposit In 2 S 3 by RF magnetron sputtering reaching cell efficiencies of 13.6%, which is the highest value available for sputtered In 2 S 3 in literature to date. Absorber surface damage and nonuniform buffer layer thickness are the primary limitations when using sputtering, and hence need to be eliminated for reaching reasonable cell efficiencies. We studied the extent of sputter induced damage on CIGSe absorber as well as the sputtering-and annealing-induced intermixing phenomenon at the In 2 S 3 /Cu(In,Ga)Se 2 interface at the subnanometer level using atom probe tomography. We have also shown that a post deposition annealing not only significantly improves the crystallinity of In 2 S 3 , but also recovers the surface damage caused by sputter-induced intermixing resulting in an improved p-n Junction quality (as shown by the electron beam induced current investigations), and substantially improves cell efficiency. The present work opens a new way for designing efficient and industry-compatible CIGSe cells using sputter-deposited Cdfree buffer layers. Moreover, this work clearly demonstrates that this novel and fully vacuum-deposited CIGSe cell meets the standard requirements, in terms of chemistry, structure, and electrical performance of a working cell for the PV industry.

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

“…For our sputtered buffer layers, we applied high sputtering deposition rates around 0.8 nm/s; compared to slower deposition processes such as ALD, CBD, ILGAR, and thermal evaporation. Nevertheless, there is still an efficiency gap compared to the CdS-buffered reference cells, which was also observed in our previous works with sputtered In x S y buffers [10,17] and reported by other groups for CIGS cells with sputtered In x S y buffers [12,18,19], evaporated In x S y buffers [20,21], and our own cells with In x S y layers grown by metal-organic chemical vapor deposition [22].…”

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. As anticipated, the cells with Ga 2 O 3 sputtered at RT result in very poor efficiencies around 3%, due to strongly reduced open-circuit voltage (V OC ), fill factor (FF), and short-circuit current density ( J SC ).…”

A Thin In2S3 Interfacial Layer for Reducing Defects and Roughness of Cu2ZnSn(S,Se)4 Thin‐Film Solar Cells

“…In this study, to improve the morphology and reduce interfacial recombination, we introduced a thin In 2 S 3 passivation layer between the CZTSSe and CdS layers. In 2 S 3 has a wide bandgap, stable chemical composition under ambient conditions, and dielectric properties that can impede charge transfer from the buffer layer to the back contact directly [36,38,39] . In addition, In 2 S 3 exists in the defect spinel structure, which can help decrease nanoscale surface defects, improving V OC and FF [36] .…”

“…In 2 S 3 has a wide bandgap, stable chemical composition under ambient conditions, and dielectric properties that can impede charge transfer from the buffer layer to the back contact directly. [36,38,39] In addition, In 2 S 3 exists in the defect spinel structure, which can help decrease nanoscale surface defects, improving V OC and FF. [36] Therefore, physical defect passivation by the In 2 S 3 layer can effectively reduce the shunt path without adverse effects on the band alignment.…”