Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

Abstract

The following article appeared in Journal of Applied Physics 111.3 (2012): 034903 and may be found at http://scitation.aip.org/content/aip/journal/jap/111/3/10.1063/1.3679604Na has deliberately been incorporated into Cu(In,Ga)Se2 (CIGSe) chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Na surface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and - for very high Na contents - indications for the formation of a (Cu,Na)-(In,Ga)-Se like compound. With increasing Na surface content, also a shift of the photoemission lines to lower binding energies could be identified, which we interpret as a reduction of the downward band bending toward the CIGSe surface explained by the Na-induced elimination of In Cu defects.X.S., R.F., D.G., R.G.W., and M.B. are grateful to the Helmholtz-Association for financial support (VH-NG-423). R.F. also acknowledges the support by the German Academic Exchange Agency (DAAD; 331 4 04 002)

Incorporation of alkali metals in chalcogenide solar cells

“…For a NaF thickness higher than 20 nm contradictory reports appear: cell degradation; peeling off of the CIGSe layer during the chemical bath deposition of CdS; or no effects. This approach has been used in several CIGSe growth processes: the traditional three-stage [8,121], low temperature co-evaporation processes [118,119], in-line co-evaporation processes [16,122,123], rapid thermal annealing [124] and both on flat and flexible substrates like steel [125] and polyimides [126,127]. For CZTSSe the same thickness range has been reported [78,80] but no detailed studies about the effects of thick or thin layers are yet known.…”

NaF/KF post-deposition treatments and their influence on the structure of Cu(In, Ga)Se<inf>2</inf> absorber surfaces

“…Sodium -incorporated into the absorber by (uncontrolled) diffusion from the underlying soda lime glass substrate -is known to modify the CIGSe properties and improve the cell efficiency. [2]- [5] A controlled incorporation of alkali elements in the absorber by means of a post-deposition treatment (PDT) employing alkali-fluorides helps to further enhance performance. [6]- [8] While it was shown that a combined NaF/KF-PDT significantly enhances the device efficiency [7], its impact on the chemical, electronic, and topographical absorber structure is poorly understood.…”

Impact of Mo density on Mo/CIGSe interfaces: An XPS study

“…Two different chemical environments of Na are observed for the CIGSe_2. The first contribution (α 1 * = 2061.2 eV) is detected for all CIGSe samples and is in same energy region as α*(Na) values of Na 2 O, Na 2 SeO 3 and Na 2 CO 3 [1]. It corresponds to Na in an oxidized environment.…”

“…Beneficial effects of alkali and especially of Na onto CIGSe based solar cell performance is well known for a long time and has been extensively discussed [1]- [4]. In fact high efficiencies are only reached when appropriate amount of Na is available prior, during or after CIGSe growth process [5]- [6].…”