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Atomic layer deposition of Zn1−xMgxO buffer layers for Cu(In,Ga)Se2 solar cells
- Source :
- Progress in Photovoltaics: Research and Applications. 15:225-235
- Publication Year :
- 2007
- Publisher :
- Wiley, 2007.
-
Abstract
- Thin-film solar cells based on Cu(In,Ga)Se2 contain a thin buffer layer of CdS in their standard configuration. In order to avoid cadmium in the device for environmental reasons, Cd-free alternatives are investigated. In this thesis, ZnO-based films, containing Mg or S, grown by atomic layer deposition (ALD), are shown to be viable alternatives to CdS. The CdS is an n-type semiconductor, which together with the n-type ZnO top-contact layers form the pn-junction with the p-type Cu(In,Ga)Se2. From device modeling it is known that a buffer layer conduction band (CB) position of 0-0.4 eV above that of the Cu(In,Ga)Se2 layer is consistent with high photovoltaic performance. For the Cu(In,Ga)Se2/ZnO interface this position is measured by photoelectron spectroscopy and optical methods to –0.2 eV, resulting in increased interface recombination. By including sulfur into ZnO, a favorable CB position to Cu(In,Ga)Se2 can be obtained for appropriate sulfur contents, and device efficiencies of up to 16.4% are demonstrated in this work. From theoretical calculations and photoelectron spectroscopy measurements, the shift in the valence and conduction bands of Zn(O,S) are shown to be non-linear with respect to the sulfur content, resulting in a large band gap bowing. ALD is a suitable technique for buffer layer deposition since conformal coverage can be obtained even for very thin films and at low deposition temperatures. However, deposition of Zn(O,S) is shown to deviate from an ideal ALD process with much larger sulfur content in the films than expected from the precursor pulsing ratios and with a clear increase of sulfur towards the Cu(In,Ga)Se2 layer. For (Zn,Mg)O, single-phase ZnO-type films are obtained for Mg/(Zn+Mg) < 0.2. In this region, the band gap increases almost linearly with the Mg content resulting in an improved CB alignment at the heterojunction interface with Cu(In,Ga)Se2 and high device efficiencies of up to 14.1%.
- Subjects :
- Valence (chemistry)
Renewable Energy, Sustainability and the Environment
Band gap
business.industry
Chemistry
Inorganic chemistry
Analytical chemistry
Heterojunction
Condensed Matter Physics
Electronic, Optical and Magnetic Materials
law.invention
Atomic layer deposition
Semiconductor
X-ray photoelectron spectroscopy
law
Solar cell
Electrical and Electronic Engineering
Thin film
business
Subjects
Details
- ISSN :
- 1099159X and 10627995
- Volume :
- 15
- Database :
- OpenAIRE
- Journal :
- Progress in Photovoltaics: Research and Applications
- Accession number :
- edsair.doi...........90f3b544a213114aada578d296def31d