Low Temperature Photoluminescence Spectroscopy of Defect and Interband Transitions in CdSe x Te 1-x Thin Films
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.516
Low Temperature Photoluminescence Spectroscopy of Defect and Interband Transitions in CdSexTe1-x Thin Films Niraj Shrestha, Corey R. Grice, Ebin Bastola, Geethika K. Liyanage, Adam B. Phillips, Michael J. Heben, Yanfa Yan, and Randy J. Ellingson Department of Physics and Astronomy, Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo, OH, USA 43606
We present the defect analysis by photoluminescence (PL) spectroscopy of CdSexTe1-x thin films, grown with varying Se content by a co-sputtered deposition method. We observe a peak at 1.203 eV in the CdSexTe1-x film for x = 0.21, which shifts towards higher energies with increase in laser power. This peak was assigned to a donor-to-acceptor (DAP) transition, with a measured j-shift of ~4.7 meV/decade. Temperature dependent PL intensity measurements confirm that the observed DAP peak involves a shallow defect state of binding energy ~34.7 meV. In contrast, a free-to-bound (FB) peak at 1.294 eV involving a shallow defect of binding energy ~18.3 meV was observed in the CdSexTe1-x film for x = 0.14. Additionally, we observe band edge emission at 1.452 eV and 1.448 eV in CdSexTe1-x films for x = 0.14 and x = 0.21 respectively. Our analysis shows that the Se concentration not only changes the band gap energy of the resulting CdSexTe1-x alloy thin film, but also modifies the nature of the dominant observed defect emission.
INTRODUCTION Initial interest in CdSe as a heterojunction partner for the well-established CdTe absorber layer has led to the development of CdSe xTe1-x as a graded layer at the front side of CdTe solar cells [1-5]. The presence of such an alloy in CdTe solar cells increases the current densities in resulting devices by increasing the cell photo-response for both short- and long-wavelength photons [1, 2]. For the longer wavelengths (> ~850nm), the optical absorption is enhanced from the decrease in the band gap of the CdSexTe1-x alloy as a result of a band gap bowing effect [6-8]. For shorter wavelengths (< ~500nm), the design yields a substantial optical improvement over the use of the more conventional CdS window layer, for which parasitic optical absorption results in relatively little contribution to the photocurrent. However, the reported open circuit voltage (V oc) and fill factor (FF)
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are lower than for CdS/CdTe solar cell designs. Photo-generated carrier lifetime is also lower than that in non-alloyed CdTe thin films [3]. This reduction in photogenerated charge carrier dynamics are an indicator of an increase in recombination site density in the CdSexTe1-x alloy region. Thus, understanding of the recombination mechanisms in CdSexTe1-x is warranted. There have been many studies reported on identification
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