Large-area chemical bath deposition of CdS on Cu(In,Ga)Se 2
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1165-M05-05
Large-area chemical bath deposition of CdS on Cu(In,Ga)Se2 Nirav Vora1, Ingrid Repins1, Steve Robbins1, Jonathan Mann1, Hector Castenada2, Brian Armstrong2, Dominic Weber1 1 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 2 Heateflex Corporation, 405 E. Santa Clara Street, Arcadia, CA 91006 ABSTRACT Chemical bath deposition (CBD) is a commonly used method of depositing cadmium sulfide (CdS) films for photovoltaic application. The method is based on decomposition of a sulfur source in an alkaline solution of a cadmium source on the surface of the Cu(In,Ga)Se2 (CIGS) substrate. On the lab scale the CdS film is deposited by submerging a 1” square CIGS substrate in a heated beaker containing the chemical bath. This batch processing method is the one used for record-performing devices. There is an ongoing effort at the National Renewable Energy Laboratory to scale-up the CBD process to deposit CdS films on 6” square substrate. Efforts are focused at designing both batch and flow reactors for depositing uniform, device quality CdS films on larger substrates. Batch reactor designs involve reproducing the deposition process in the beaker on a bigger scale with minimal chemical waste, while flow reactors are designed for continuous processing, such as encountered in roll-to-roll manufacturing lines. INTRODUCTION CdS thin films, used as a buffer layer in both CdTe and CIGS thin film photovoltaic devices, have been deposited using various processes like sputtering [1], closed space sublimation [2], chemical vapor deposition [3], spray pyrolysis [4], and chemical bath deposition [5, 6]. The method is based on decomposition of a sulfur source in an alkaline solution of a cadmium source. Various recipes involving different cadmium and sulfur sources have been used for the CBD of CdS. The recipe used in the presented work involves 1.5mM of CdSO4,(50 ml) 0.15 M thiourea (SC(NH)2) (25 ml), and 28-30% NH4OH (62.5 ml) in DI water (362.5 ml) [7]. CdS film is deposited on smaller CIGS substrate (1” square) by submerging it in a stirred chemical bath with the above-mentioned recipe in a beaker maintained at 65°C. The CdS film in this beaker process, is deposited on the substrate via a surface reaction, the mechanism [8] of which is as shown below: i.
Reversible adsorption of dihydroxodiammino-cadmium complex:
Cd(NH3)42+ + 2OH- + site [Cd(OH)2(NH3)2]ads + 2NH3 ii.
Adsorption of thiourea by formation of a metastable complex:
[Cd(OH)2(NH3)2]ads + SC(NH2)2 → [Cd(OH)2(NH3)2 SC(NH2)2]ads iii.
(1)
(2)
Formation of CdS and site regeneration by the metastable complex decomposition;
[Cd(OH)2(NH3)2 SC(NH2)2]ads → CdS + CN3H5 + NH3 + 2H2O + site (Rate limiting step)
(3)
This “ion by ion” reaction mechanism on the surface of the substrate results in a uniform CdS film. The quality of the film can be affected by a side reaction taking place in the liquid phase with subsequent precipitation of CdS particles. The mechanism of this side reaction is as shown below: Cd(NH3)n2+ Cd2+ + nNH3
(4)
SC(NH2)2 +
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