Development and Application of Electroluminescence Imaging for CdS/CdTe Characterization
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Development and Application of Electroluminescence Imaging for CdS/CdTe Characterization Scott Feldman, Fred Seymour, Tim Ohno, Victor Kaydanov, and Reuben Collins Physics Department, Colorado School of Mines Golden, CO 80401 ABSTRACT A technique for spatially resolved optical characterization of CdS/CdTe thin film solar cells has been developed using electroluminescence (EL). In EL, excess minority carriers are injected via forward biasing. Light produced in radiative carrier recombination is collected with a CCD camera. Because EL intensity depends upon radiative vs. non-radiative recombination lifetimes, EL provides insight into material quality. Spatial resolution is a key benefit of EL as it provides insight into the non-uniformities of polycrystalline CdTe. At high magnification the resolution is diffraction limited, but coarser measurements of up to several millimeters in range may also be made. Non-uniformities in emission have been observed throughout this range. Further benefits of EL as a characterization technique are as follows: EL probes the region of most interest, namely the CdTe near the main junction. Also, it is observable at room temperature and data acquisition is fast. Finally, EL is observable at very low carrier injection rates, comparable to short circuit current. (Though more structure is often revealed at higher injection rates.) This low injection means that EL can be a non-destructive probe. This fact, along with the aforementioned ease of observation, means that EL could possibly be used for quality control and in situ testing of modules. Data gathered from CdS/CdTe cells from various institutions deposited using different methods such as close spaced sublimation, vapor transport, and sputtering are presented. In addition to changes in deposition technique, changes in processing parameters were observed to affect EL emission. Furthermore, overall EL emission decreased noticeably with stress at various biases and elevated temperature, with non-uniformity increasing in many cases. Changes in EL become apparent before changes in parameters acquired with standard currentvoltage measurements, suggesting that this technique can be used as an early indicator for degrading cells. Finally, some dramatic changes in EL with stress suggest highly non-uniform degradation of the back contact. INTRODUCTION Advances in the performance and stability of thin film CdTe based solar cells have often relied heavily on the empirical approaches. To continue to make advances in the technology it is becoming more important to develop a better understanding of the fundamental processes and material properties behind cell performance and stability. One inherent property in any device based on polycrystalline thin film is some degree of non-uniformity of the photo-electronic properties. Standard photovoltaic characterization techniques such as current-voltage, capacitance-voltage, and quantum efficiency measurements average properties over the area of a solar cell. Contrasting with these methods is a new w
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