Characterization of CdS/CdTe Solar Cells Fabricated by Different Processes

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Characterization of CdS/CdTe Solar Cells Fabricated by Different Processes Potlog T. 1, Khrypunov G. 2, Kaelin M. 3, Zogg H. 3, and Tiwari A. N. 3 1 Physics Department, Moldova State University, 60, A.Mateevici str., Chisinau, MD 2009, Moldova 2 Kharkov Polytechnic Institute, National Technical University, Kharkov, Ukraine 3 Thin Film Physics Group, Laboratory for Solid State Physics, ETH Building-Technopark, Technoparkstr.1, Zurich, CH-8005, Switzerland ABSTRACT This paper analyses the photovoltaic parameters of CdS/CdTe solar cells prepared by close space sublimation (CSS) and high vacuum evaporation (HVE). CdS/CdTe solar cells that have an efficiency of ~10 % have been studied by current-voltage, capacitance-voltage and quantum efficiency measurements. The current-voltage characteristics show that the addition of small amounts of Cu or Te to back contacts by thermal evaporation improves contact properties by p+-doping the CdTe surface and creating a pseudo-ohmic contact. The cell deposited by CSS seriously suffers in FF compared to the cells prepared by HVE. The measurements reveal that the efficiency of CdS/CdTe solar cells fabricated by CSS is limited by a light-dependent shunt resistance and by a high series resistance, but concerning of the cells fabricated by HVE - by the formation of a thick layer of CdS1-xTex at the interface. INTRODUCTION Of the various alternatives for thin-film PV materials, CdTe is a leading candidate for several reasons: (1) It has a band gap of 1.5 eV, which is very close to optimal for the solar spectrum. (2) It is a simple binary compound that can be deposited as a thin-film by a variety of high deposition rate techniques [1-6]. (3) It is a low-cost and abundant material. (4) Thin-film CdTe has a relatively low density of extraneous grain-boundary states. (5) Native defects in CdTe generally make it p-type, and hence a good heterojunction match with n-type CdS. In the cell configuration that is now nearly universal, light enters through a glass support plate, and then passes through a transparent contact and the CdS window layer to be absorbed in the CdTe. The potential importance of CdS/CdTe solar cells for low-cost high performance applications makes it important to have a better understanding of the photoelectrical and electrical processes in these device obtained by different techniques. In this paper we will characterize CdS/CdTe solar cells obtained by (CSS) [7] and by (HVE) [8] using current-voltage, capacitance-voltage and quantum efficiency measurements. EXPERIMENTAL CdS/CdTe solar cells have been fabricated in a superstrate configuration as shown in figure 1 where the CdS and CdTe layers are grown sequentially onto a glass substrate covered with transparent conducting oxide. The CdS and CdTe layers were obtained by close space sublimation (CSS) [7] and high vacuum evaporation (HVE) [8]. Both types of the CdS/CdTe cells involve annealing the structure after CdTe layer is deposited onto CdS. In the case of cells fabricated by CSS the structures were held

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