Low-temperature Photoluminescence Studies of CdTe Thin Films Deposited on CdS/ZnO/Glass Substrates
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Low-temperature Photoluminescence Studies of CdTe Thin Films Deposited on CdS/ZnO/Glass Substrates Corneliu Rotaru1, Sergiu Vatavu1,2,3, Christoph Merschjann2, Chris Ferekides3, Vladimir Fedorov1, Tobias Tyborski2, Mihail Caraman1, Petru Ga in1, Martha Ch. Lux-Steiner2 and Marin Rusu1,2 1
Faculty of Physics and Engineering, Moldova State University, 60 A. Mateevici str., MD-2009 Chisinau, Republic of Moldova
2
Institut für Heterogene Materialsysteme, Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner-Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
3
Department of Electrical Engineering, University of South Florida, 4202 East Fowler Ave, Tampa, FL 33647, USA ABSTRACT
The CdTe photoluminescence spectra of CdTe/CdS/ZnO heterojunctions annealed in the presence of CdCl2 have been analyzed in the 4.7-100K temperature range. The analysis has been performed for laser excitation power between 0.01 mW and 30 mW. The analysis showed that the photoluminescence spectrum in the 1.1-1.6 eV region consists of a defect band (1.437 eV) having complex structure and revealing well contoured LO phonon replicas and bound exciton annihilation in the 1.587-1.593 eV region. The band analysis has been carried out by deconvoluting the spectra. It has been shown that the defect band consists of two elementary bands and their phonon replica. An “unusual” temperature dependence of the defect band has been found. INTRODUCTION CdTe-based thin film heterojunctions used for PV device fabrication continue to be a promising means of direct solar to electricity conversion. Defects and dopants are important to further enhancement of PV parameters. One of the possibilities to explore the direct consequences of technology influence on the development of CdTe solar cells is thorough study of the low-T photoluminescence (PL). This technique can reveal photo-active levels and can be used to relate their influence to solar cell performance. In particular, PL analysis is suitable for investigating the influence of the technology variations during the fabrication of CdS/CdTe heterojunctions [1] for photovoltaic use. The analysis publications related to the topic of investigation shows that the results and the interpretation of the physical processes in CdTe related to luminescence differ and are sometimes ambiguous. Such situations often occur when authors cross-reference to results measured in different experimental set-ups (i.e. different excitation lasers, etc.) or to different samples (annealed/unannealed, thin film or single crystal etc.).
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An attempt to add more information to the analysis of the radiative transitions in CdTe thin film is made. This work is focused on the investigation of the evolution of the PL of CdTe thin films deposited onto CdS/ZnO/Glass substrate in device configuration. EXPERIMENT CdS/CdTe heterojunctions have been manufactured by a modified CSS system as described in a previous work [2]. The analysis has been carried out for heterojunctions deposited onto iZnO/ZnO:Al/Glass substrates. The deposition
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