Five-source PVD for the deposition of Cu(In1−xGax)(Se1−ySy)2 absorber layers
Introduction
The multinary compound semiconductor Cu(In1−xGax)(Se1−ySy)2 is a viable option as an absorber material for thin film solar devices. Typically In-rich deposited Cu(In1−xGax)Se2 films with band gaps of 1.2 eV [1], or Cu-rich deposited Cu(In1−xGax)S2 films with band gaps of 1.5 eV are investigated in solar cell deposition [2]. Both of these processes use only one of the chalcogen species, either selenium, or sulfur, respectively. With respect to higher band gaps, which should lead to improved module performance and could enable the development of thin film based tandem solar cells, the use of sulfur and selenium together should be an interesting option. For thin film tandem cell photovoltaics, a top cell with a band gap in the range 1.6 eV≤Eg≤1.9 eV and a small subband gap absorption is needed. While the selenium system is limited to 1.7 eV, the use of both selenium and sulfur offers a band gap range from 1.0 up to 2.4 eV.
For the deposition of homogeneous CIGSS, sequential processes, in which metal or binary layers were deposited at low temperatures and then reacted in mixed hydride gases or elemental vapors, were ruled out due to difficulties in attaining uniform incorporation of Ga and In [3]. For complete control of the incorporation of sulfur and selenium, an independent deposition of both species would be favorable. Therefore, in the first part of this paper, we report a double-layer deposition process with the sulfur and the selenium deposited in different layers and investigate the intermixing of the chalcogen species. The objective of these processes was to separate the S and Se depositions so the precise control of the chalcogen fluxes was not critical. In the second part of the paper, we will present results of solar cells with band gaps of 1.5 eV, deposited In-rich in a single step process varying the Ga, and S content of the absorber layers.
Section snippets
Experimental details
A new deposition system has been designed and built to deposit CIGSS films by five-source elemental thermal evaporation. An illustration showing the layout is given in Fig. 1. Boron nitride crucibles are used as sources for Cu, In, and Ga. These crucibles are heated in a boron nitride furnace using tantalum wire resistive heaters and multiple layers of thermal shielding. The sources are mounted in stainless steel jackets. While these metal sources typically operate at temperatures between 1100
Double-layer depositions
Two different processes for a double-layer deposition were investigated. The goal was to deposit an approximately 2.5-μm-thick Cu-rich CIGSS film with uniform composition through the film. The films were deposited on Mo-coated soda lime glass. In process P1, a pure In–Ga–Se film was deposited first at 350 °C with Ga/(In+Ga) ratio of 0.29 and a Se/(In+Ga) ratio of 3. On top of this layer, a Cu–S film was deposited at a temperature of 550 °C, with a S/Cu ratio of 18. In process P2, first an
Conclusions
A new five-source PVD system was constructed. The main new feature of the system is its water-cooled, well controllable chalcogen sources which allow the controlled deposition of the penternary chalcopyrite semiconductor Cu(In1−xGax)(Se1−ySy)2. Depositions of CIGSS using double-layer processes in which the chalcogens were deposited separately lead to layered structures with two separate layers of CIGSS with different stoichiometry. To overcome the problem of the incomplete intermixing between
Acknowledgements
We would like to thank K. Hart and T. Lampros for support in the device production. J. Pankow from NREL for the Auger depth profiling, and Th. Hahn from the University of Jena for the RBS measurements. This work was supported by NREL under the High Performance Photovoltaics Initiative.
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Cu(In<inf>1-x</inf>Ga<inf>x</inf>)Se<inf>2</inf> and CuIn(Se<inf>1-x</inf>S<inf>x</inf>)<inf>2</inf> Thin Film Solar Cells
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