CdTe Solar Cell from Sputtering Method
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CdTe Solar Cell from Sputtering Method
Tara P. Dhakal1, Lakshmi K. Ganta1, Dino Ferizovic2, Surya Rajendran1, Daniel Vanhart1, Michael A. Seymour1, and Charles R. Westgate1
1
Center for Autonomous Solar Power, SUNY-Binghamton, Binghamton, NY, USA
2
Department of Physics, University of South Florida, Tampa, FL, USA
ABSTRACT We report CdTe/CdS solar cell with CdTe layer grown by sputtering method. A controlled etch and anneal process on the sputter-grown CdTe films was performed to increase the average grain size of the film. The process involved dipping the CdTe films in a saturated solution of cadmium chloride (CdCl2) in methanol (2.08 gram in 100 ml) followed by a 30 minute annealing at 400 °C. We performed various experiments on this process by varying the dipping times, drying process and annealing times and analyzed the resultant films using Scanning Electron Microscopy (SEM). We could see a clear increase in grain size from 200 nm to 5 µm after CdCl2 treatment. The process also increased the overall roughness of the sample so that more light is absorbed than reflected. We prepared solar cells using CdTe as p-type layer and CdS as n-type layer. The efficiency of the cell improved from 1.1% to 4.2% after air annealing. The effect of air-annealing is studied by means of quantum efficiency measurement. INTRODUCTION Cadmium telluride solar cells have been of research and commercial interest for a long time. CdTe has grown rapidly in acceptance and has been ranked second in usage, next to silicon, as solar material. CdTe is a p-type material with a direct band gap of 1.5 eV [1], which is in the range of optimal photo-energy conversion [2]. The light absorption coefficient of CdTe is greater than 105 cm-1 [1] for photons of greater energy than the band gap. The high absorption coefficient combined with its optimal band gap makes the quantum yield high from UV to the band gap wavelength of CdTe which is around 825 nm. CdTe/CdS cells show record efficiency of 16.5 % [1] and research shows different ways to improve it. While closed space sublimation has been a predominant way of getting cadmium telluride thin films [3], other methods like vacuum evaporation [4], electrodeposition [5] and sputtering [6] have been used. An efficiency of 11.6% has been reported in a CdTe/CdS solar cell in which both layers were grown by planar magnetron radio frequency sputtering [6]. The improved efficiency was attributed to the improved minority carrier life-time in the films grown by unbalanced magnetron. In this paper, we report the improvement of sputter-grown CdTe as an absorber layer by oxygen annealing and the consequent efficiency enhancement. The band gap derived from the transmittance spectrum
was 1.49 eV for the CdTe film grown using sputtering method for this study. The CdTe solar cell structure was made in a conventional superstrate configuration with CdS as an n-type window layer. The cell structure starting from the glass side was glass/FTO/CdS/CdTe/Ni. EXPERIMENT The CdTe absorber layer was grown by a commercial spu
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