Development of Plasma-Assisted Processing for Selenization and Sulfurization of Absorber Layers

Abstract

In the synthesis of copper chalcopyrite solar absorbers the chalcogen source is always supplied in excess due to its low reactivity. This paper describes preliminary work aimed at addressing this issue through plasma processing. An inductively coupled plasma (ICP) was use to activate both sulfur and selenium vapors. First, the thermodynamic arguments for using activated chalcogens are presented. Next, this paper describes the experimental ICP setup and its characterization using optical emission spectroscopy (OES). Stable discharges have been achieved with both sulfur and selenium vapors using argon as a carrier gas. The potential of this approach was demonstrated by converting indium films into In2Se3 and InSx. The indium samples were inserted into chalcogen-containing ICP plasmas. Through X-ray diffraction it was observed that chalcogen conversion was achieved in a matter of minutes at room temperature by plasma processing.

Development of a Wide Bandgap Cell for Thin Film Tandem Solar Cells: Final Technical Report, 6 November 2003 - 5 January 2007

“…This model assumes that the S and Se incorporation in CIGSS is controlled by the kinetics of chemisorption of the chalcogen species and incorporation into the chalcopyrite lattice. This is justified by the following observations: (i) the evaporation pressure of S [11] and Se [12] is much greater than the vacuum during film growth so that the incorporation of S and Se into the film is controlled by its reaction with Cu, In, and Ga; (ii) since the reaction temperature is more than 300°C below the melting temperature of the four extremes of ternary chalcopyrites [13,14,15,16,17] the driving force to form the chalcopyrite phase from the vapor is sufficient with either S or Se [ 18,19,20]; and (iii) the growth rate is controlled by the fluxes of Cu, In, and Ga, and the relative compositions of the groups I and III species in the films match those of the fluxes. The solid lines in Figure 1 fitting the data for CIGSS samples were determined from the following simple equation derived from the kinetic model for chalcogen incorporation [10].…”

Plasma-Assisted Co-evaporation of S and Se for Wide Band Gap Chalcopyrite Photovoltaics: Final Subcontract Report, December 2001 -- April 2005

“…If flux per solid follows a cos n θ dependence and is constant with increasing R (i.e. neither Se-Se nor Se-Ar collisions cause significant spreading of the flux after it leaves the aperture), then it can be derived that (1) This expression includes the necessary factors for dependence of flux on θ, distance to slide, and angle between slide and direction of flux. A similar expression can be derived for a source axis that is not normal to the substrate plane.…”

“…This diagram shows the changes in Gibbs free energy for CuInSe 2 (left) and CuInS 2 (right) synthesis from various chalcogen sources. 1,2 The formation energy of copper and indium are zero, so the reactant energy simply reflects the chalcogen source. Sulfur and selenium both sublime as dimers that may readily oligomerize into ring structures of S n or Se n , where n varies from 2 < n < 8.…”