AVS 57th International Symposium & Exhibition
    Energy Frontiers Topical Conference Tuesday Sessions
       Session EN+TF-TuA

Paper EN+TF-TuA9
Optimizing Heterojunctions of ZnTe/ZnSe Solar Cells: Effect of Surface Treatment and Growth Conditions

Tuesday, October 19, 2010, 4:40 pm, Room Pecos

Session: Thin Films for Photovoltaics
Presenter: F. Fang, University of Delaware
Authors: F. Fang, University of Delaware
B. McCandless, University of Delaware
R. Opila, University of Delaware
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II-VI direct band gap semiconductors are attractive for thin film solar cell (TFSC) applications owing to their potential flexibility in tunable opto-electronic properties and possible application in tandem cells for being band gap materials (EG > 2 eV). For the n-ZnSe/p-ZnTe heterojunction solar cell, the defect states and electronic band alignment at the ZnSe/ZnTe interface are crucial for device performance. We have employed Al-Kα X-ray photoelectron spectroscopy as well as synchrotron source ultra-violet photoelectron spectroscopy to study the surface chemical composition and electronic structures at heterojunction interface. Scanning electron microscopy (SEM) was used to study observe the film microstructure morphology of the interface.

We used two different deposition techniques: Close Space Sublimation (CCS), a low-cost deposition method already demonstrated for high efficiency and commercial CdTe TFSC, and conventional thermal evaporation. Our preliminary results indicated that surface oxides on CCS-grown ZnSe film formed once open to air, and a significant valence band offset induced by this oxide is observed which acts like additional energy barrier for carrier transport, resulting in low open circuit voltage. Also, during sequential CSS deposition of the two stacking films, the covering ZnTe thin film layer growth damage the microstructure of the underlying ZnSe film, i.e., enlarged pores are observed in ZnSe films in the locations where partially covering ZnTe film was deposited. A degraded device performance is expected and low short circuit currents and fill factors of the cells are detected. By analogy to CdS/CdTe TFSC, we are aiming for close-packed column polycrystalline of ZnSe/ZnTe film growth. Therefore, we are exploring etching processes, annealing temperatures and ambient settings to optimize the growth conditions. Evaporation is under investigation, since we have the option of dual-sources in the self-designed chamber, sequential growth of ZnSe and ZnTe films without vacuum break is feasible. Film morphology as well as energy band alignment at the heterojunctions using evaporation growth is being studied.