Device-quality CuInSe₂ produced by the hybrid process

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 × 10⁴ 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/(Cd_0.93Zn_0.07)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.