Formation of Ga 2 O 3 barrier layer in Cu(InGa)Se 2 superstrate devices with ZnO buffer layer
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Formation of Ga2O3 barrier layer in Cu(InGa)Se2 superstrate devices with ZnO buffer layer Jes K. Larsen, Peipei Xin, William N. Shafarman Institute of Energy Conversion, University of Delaware, Newark, DE, 19716, USA ABSTRACT The junction formation when Cu(InGa)Se2 is deposited onto ZnO in a superstrate configuration (glass/window/buffer/Cu(InGa)Se2/contact) is investigated by x-ray photoelectron spectroscopy and analysis of device behavior. When Cu(InGa)Se2 is deposited on ZnO, a Ga2O3 layer is formed at the interface. Approaches to avoid the formation of this unfavorable interlayer are investigated. This includes modifications of the process to reduce the thermal load during deposition and improvement of the thermal stability of the ZnO buffer layer. It was demonstrated that both lowering of the substrate deposition temperature and deposition of the ZnO buffer layer at elevated temperature limits the Ga2O3 formation. The presence of Ga2O3 at the junction does affect the device behavior, resulting in a kink in JV curves measured under illumination. This behavior is absent in devices with limited Ga2O3 formation. INTRODUCTION Cu(InGa)Se2 devices with superstrate configuration are interesting for several reasons. The superstrate configuration opens the possibility to deposit the window layer at elevated temperature to improve its optoelectronic properties. This device structure, furthermore, makes it easier to texture the window layer for light trapping and engineer the reflectance of the back contact as needed for cells with thin, < 1 μm, absorber layers. At the same time this approach eliminates the need for a transparent back cover, which potentially leads to cost reduction. A critical issue for the superstrate device structure is to control the junction formation when Cu(InGa)Se2 is deposited onto the buffer layer. Since Cu(InGa)Se2 is typically deposited at elevated temperature, inter-diffusion and reactions at the interface play an important role. In the present study, Cu(InGa)Se2 is deposited by co-evaporation onto a ZnO buffer layer under various conditions. The chemical interactions are investigated by x-ray photoelectron spectroscopy (XPS) depth profiling and the relationship to device behavior is investigated. Cu(InGa)Se2 devices with a superstrate structure have been studied previously. Early studies using CdS buffer layers, typically used in substrate Cu(InGa)Se2 devices, demonstrated CdS/Cu(InGa)Se2 interdiffusion at elevated deposition temperatures1,2. To eliminate the problem of interdiffusion, CdS was replaced by a ZnO buffer layer, which did improve device performance2. The best reported superstrate device, reaching 12.8 % efficiency, utilized ZnO as the buffer layer3. Two groups have reported on interactions at the interface between Cu(InGa)Se2 and ZnO in superstrate devices4,5. In both studies it was demonstrated that ZnO reacts with Cu(InGa)Se2 during absorber deposition, leading to the formation of a Ga2O3 layer at the interface. Considering the difference in electron affinity between Ga2O36 and
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