Toward High Performance Epitaxial and Low-temperature Cu ( In , Ga ) Se 2 Solar Cells
The CIGS research effort at the University of Illinois represents a three-pronged approach to understanding and solving some of the most critical issues in CIGS device. These three prongs are: (1) development of a basic understanding of the issues limiting performance in CIGS devices, (2) advancing the performance of the devices through single crystal epitaxial layers for integration into high-performance cells, and (3) developing novel growth processes that will allow lower deposition temperatures necessary to multijunction devices. This paper presents an approach for CIGS/GaAs and CIGS/Ge heterojunction solar cells for multijunction high-efficiency devices. In addition, application of ionized physical vapor deposition to lowtemperature deposition of CIGS is described. The two projects will be coupled and results from one used to enhance progress in the other as part of the Beyond the Horizon and High Performance PV programs now starting.
DOE/NREL fundamental and exploratory research into solar cells
The U.S. Department of Energy/National Renewable Energy Laboratory supports fundamental and exploratory research into solar cells. The purpose of the following set of research projects is twofold: (1) to provide fundamental experimental and theoretical foundations to existing photovoltaic (PV)/solar cell technologies, and (2) to explore whole new possibilities in PV/solar cell technologies, however revolutionary or nonconventional they might be. As surely as a number of revolutionary new solar cell technologies will most likely involve high-risk, long-term R&D to bring them to fruition, and may also well be needed as a complement to a future energy portfolio, so we also need timely results. As such, one of the primary driving motives for this effort is the possibility of developing one or more "leapfrog" technologies. These are technologies that have the distinct possibility of "leaping ahead" of existing technologies rather than taking the more characteristic development time of 10 to 20 years.
Device-quality CuInSe2 produced by the hybrid process
Abstract Solar cells with conversion efficiencies exceeding 7% have been fabricated based on CuInSe2 produced by a hybrid process combining sputtering of copper and indium with evaporation of selenium. This process has been shown to be highly controllable, with linear relationships observed between the target ion currents and the metal-constituent content of the resulting films. Furthermore, below saturation at 50 at.% the selenium content in the films has also been shown to vary linearly with incident selenium flux. To demonstrate the quality of the resulting material, heterostructure solar cells have been fabricated by the authors, the Institute of Energy Conversion at the University of Delaware (IEC), and International Solar Energy Technology (ISET). As a first step, Cu/In alloy layers deposited by codeposition of the metals in the hybrid system were selenized and fabricated into devices at ISET. These had V oc values as high as 0.34 V, which demonstrated that acceptable control of the copper and indium fluxes could be achieved. Following calibration of the deposition rates and film compositions as a function of substrate temperature and constituent fluxes, single-layer films were produced which had chalcopyrite structure and resistivities in agreement with previous observations (0.797 μ cm for copper-rich films and 2.094 × 10 4 μ cm for indium-rich layers). Transmission electron microscope analysis of the film structures showed no detectable differences between the hybrid process material and device-quality CuInSe2 produced by three-source evaporation. All films had substantial numbers of defects, including stacking faults, microtwins, dislocations and intergranular pores. The density of each defect is a function of film composition. No second-phase material was observed in any near-stoichiometric ( ±2 at .% in any constituent) film — even for copper-rich material. Stable defect structures were observed, with recognizable features in the electron diffraction pattern, probably by forming vacancies on one or more of the structure sublattices. Based on these results, bilayers were produced which consisted of copper-rich regions, 3 μm thick, capped with indium-rich layers of varying thickness. For the initial series of films these layers had nominal atomic per cent compositions of 26% Cu, 22% In, and 52% Se for the copper-rich layers.and 19% Cu, 29% In, and 52% Se for the indium-rich layers. Point-contact CuInSe2/CdS:In devices based on this material had diode-like behavior which improved significantly with air annealing. Under illumination, these devices produced V oc values as high as 0.34 V. Glass/Mo/CuInSe2/(Cd0.93Zn0.07)S:In/ITO/Ni devices with an area of 0.08 cm2 produced and tested at IEC using similar films had a best performance after air annealing of 7.2% efficiency, V oc = 0.348 V , J sc = 0.32.9 mA , and 55% fill factor under simulated 87.5 mW cm−2 solar illumination. Although all of these parameters can be improved by further optimization of the CuInSe2 layer, they are excellent for the first devices tested. The results to date clearly demonstrate the potential of the hybrid process.