Optimization of Processing and Modeling Issues for Thin Film Solar Cell Devices: Final Report, 24 August 1998-23 October 2001

Abstract

The overall mission of the Institute of Energy Conversion is the development of thin film photovoltaic cells, modules, and related manufacturing technology and the education of students and professionals in photovoltaic technology. The objectives of this 20 month NREL subcontract are to advance the state of the art and the acceptance of thin film PV modules in the areas of improved technology for thin film deposition, device fabrication, and material and device characterization and modeling, relating to solar cells based on CuInSe 2 and its alloys, on a-Si and its alloys, and on CdTe. CuInSe 2 -BASED SOLAR CELLS Effect of Deposition Temperature on Cu(InGa)Se 2 Films and DevicesLowering the Cu(InGa)Se 2 substrate temperature from 550 -600°C, used for the highest efficiency devices can reduce processing costs associated with glass handling and thermal stress on the deposition system, and potentially enable polyimide substrates to be used. The effects of substrate temperature and evaporation sequence, with regard to Cu-rich film growth on Cu(InGa)Se 2 , films and devices and to quantitatively determine the effect of grain size. Cu(InGa)Se 2 films were deposited by elemental evaporation of Cu, In, Ga, and Se using three flux versus time profiles to give depositions with: (1) Cu-rich flux, Cu/(In+Ga) > 1, at the start of the run, (2) Cu-rich flux in the middle of the run, and (3) uniform fluxes so that the films composition is never Cu-rich. The final composition of all devices was the same at the completion of the deposition. These depositions were each done with substrate temperatures (T SS ) = 400, 480, and 550°C. The grain size distribution and surface morphology of each Cu(InGa)Se 2 film was determined using atomic force microscopy and compared to the device behavior. PUBLICATIONSAs a result of research performed under this subcontract reporting period, IEC published 34 papers, as shown in Appendix 1. ORGANIZATION OF THE REPORTThis report is organized into three technical sections: CuInSe 2 -based solar cells, Si-based solar cells, and CdTe-based solar cells. Each section describes the progress made at IEC in addressing the critical issues discussed above during the period of this subcontract.

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

“…The angular variation of density field (ρr 2 ) estimated from data using method of characteristics calculations is only a function of θ. The approximate relation for density variation is; ) 2 ( cos 2 2 ϕ πθ α ρ r (6) where ρ is the density of expanding gas, r -distance from jet source, θ -angle from the normal to jet source. The constant φ depends on specific heats ratio.…”

“…Along a given gas stream line the speed of the gas (u) is essentially constant. The flux is give by the equation; u ρ = dA dM chalcogen (7) Combining equations (6) and 7 The thickness comparison that was shown schematically in Figure 33 was evaluated quantitatively. Figure 34 shows the measured thickness as a function of radial position across the substrate.…”

Tolerance of Three-Stage CIGS Deposition to Variations Imposed by Roll-to-Roll Processing: Phase I Annual Report, May 2002--May 2003

“…This cos 3 θ dependence of effusion source distributions has also been measured at the Institute for Energy Conversion (IEC). 16 IEC reports no dependence of the profile on source metal, and no dependence on deposition rate over a 100× range of deposition rates. Thus -when combined with information on effusion source placement, source-to-substrate distance, substrate speed, and rate of material leaving the source -the cos 3 θ distribution is an adequate starting point to define the roll-coater metals flux profiles for the bell jar experiments.…”