Photocurrent Spectral Distribution and Relaxation in CdS/CdTe Heterojunctions
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1012-Y03-31
Photocurrent Spectral Distribution and Relaxation in CdS/CdTe Heterojunctions Sergiu A. Vatavu1, Petru A. Gasin1, Chris S. Ferekides2, and Iuliana M. Caraman3 1 Physics, Moldova State University, 60 A. Mateevici str., Chisinau, MD 2009, Moldova 2 Electrical Engineering, University of South Florida, 4202 East Fowler Ave., Tampa, FL, 33620 3 Engineering, University of Bacau, 157 Calea Marasesti, Bacau, 600115, Romania
ABSTRACT The analysis of the photoelectrical processes in CdS/CdTe heterojunction is carried out by means of photocurrent spectral distribution studies for samples, having CdTe thicknesses in the 2.3-6.6 µm range (short-circuit and biased regime). The measurement have been performed in the 78-293K temperature range. The charge carriers lifetime is determined (varying from 12 µs to 280 µs at 293K). INTRODUCTION Thin film CdS/CdTe solar cells have proved their importance in use as solar energy converters achieving 16.5 % efficiency [1]. The photovoltaic properties of CdS/CdTe heterojunctions a mainly determined by the energetic diagram of the localized states in heterojunctionís band gap. Lifetime, and the energetic position of the recombination levels are one of the factors determining the solar cellís efficiency. These parameters can be analyzed by several methods such as quantum efficiency and photoconductivity relaxation plus their temperature and bias dependence. By analyzing the photosensitivity along with the absorption coefficient spectral distribution close to the edge of fundamental absorption and CdS and CdTe thin films one can determine the interface recombination velocity. The spectral response of the open circuit voltage [2], short circuit current decay [3] and diffusion length measurements [4] have prooved to be important characterisation tools for solar cells. EXPERIMENT Thin film CdS/CdTe heterojunctions have been deposited by close spaced sublimation method (CSS), onto SnO2 (ρ∼10-3Ω⋅cm, 80% transparency − visible spectral range) covered glass plates (2х2 cm2). The deposition temperatures for substrate and source were 445∫C and 640∫C for CdS films and 445∫C and 550∫C for CdTe layer respectively. The thickness of CdS and CdTe thin films was 0.3-1.6 µm and 2.3-6.6 µm respectively. To enhance CdS/CdTe heterojunctions photoelectrical parameters and photosensitivity, the deposition procedure was followed by annealing in presence of CdCl2 at 420°C for 15-60 min. Ni, deposited by thermal evaporation or magnetron sputtering, has been used as back contact to CdTe. After annealing, the photoelectrical parameters at 100 mW/сm2 are: Uoc=0.73 V, Isc=21.7 mA/cm2, ff=0.43. The photoconductivity spectral distribution has been measured by using a spectrophotometric device based on a monochromator with a diffraction grid (600 mm-1), with a spectral resolution of ∼0.5 meV in the 1.3-2.5 eV spectral range. The incandescent linear filament was used and its
temperature was stabilized at 2850K. The heterojunctionís short-circuit current kinetics was investigated with a complex device which includ
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