Optimization of Processing and Modeling Issues for Thin Film Solar Cell Devices Including Concepts for the Development of Polycrystalline Multijunctions Annual Subcontract Report, 24 August 1999 - 23 August 2000

Abstract

SUMMARYThe overall mission of the Institute of Energy Conversion (IEC) 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 12 month NREL subcontract are to advance the state of the art and the acceptance of thin film PV solar cells in the areas of improved technology for thin film depositions, device fabrication, and material and device characterization and modeling, relating to solar cells based on CuInSe2 and its alloys, on doped and intrinsic microcrystalline Si films, and on CdTe. CuInSe 2 -BASED SOLAR CELLS In-line Evaporation of Cu(InGa)Se 2In-line evaporation is a potentially effective means to achieve the high rate uniform deposition necessary for commercial-scale manufacture of Cu(InGa)Se 2 modules. In this process, the substrate is linearly translated over thermal sources from which the elemental materials are evaporated.An in-line evaporation system for the deposition of Cu(InGa)Se 2 films has been put into operation at IEC. The system's performance in terms of uniform deposition over large areas at relatively high rates and in terms of device performance and reproducibility has been successfully demonstrated. The compositional uniformity across the 6-inch wide deposition zone is within the uncertainty limits of EDS measurements. A set of depositions with translation speeds ranging from 1 to 2.5 inches/min produced films with thicknesses from 2.1 to 0.9 µm. Devices from these runs had efficiencies from 13.2-14.5 %, for thickness > 1 µm, demonstrating the run-to-run reproducibility of the system. The best cell produced with Cu(InGa)Se 2 from this in-line system had 14.9% efficiency. Effect of Deposition Temperature on Cu(InGa)Se 2 Films and DevicesOne means to reduce manufacturing costs for thin film Cu(InGa)Se 2 is by reducing the substrate temperature at which the Cu(InGa)Se 2 layer is deposited. The effect of substrate temperature on the film grain size, morphology, and compositional uniformity and device performance of Cu(InGa)Se 2 deposited at 480°C was characterized and compared to previous results at 400°C and 550°C. At 480°C, the soda lime glass substrate is below the strain point of the glass, reducing handling problems.Cu(InGa)Se 2 films were deposited by multisource elemental evaporation with different deposition sequences to determine the effect of a Cu-rich growth step, i.e., deposition with the Cu molar flux greater than the sum of the In and Ga fluxes. The grain size was determined from atomic force microscopy images. The films deposited at 480°C have comparable grain size and morphology to those deposited at 550°C. However, V oc , FF, and efficiency of devices with films deposited at 550°C were greater. A Cu-rich growth step during the deposition does not affect the measured grain size or device performance at these temperatures. The films deposited at 400°Ciii have smaller grains and poorer device performance. At this lower temperature the Cu-...