Characterization of Deep Defects in CdS/CdTe Thin Film Solar Cells Using Deep Level Transient Spectroscopy
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Characterization of deep defects in CdS/CdTe thin film solar cells using Deep Level Transient Spectroscopy Jorg Versluys and Paul Clauws Department of Solid State Sciences, Ghent University Krijgslaan 281 – S1, B-9000 Gent, Belgium ABSTRACT The presence of deep defect levels in thin film solar cells can highly affect the characteristics of the photovoltaic energy conversion. Therefore, knowledge of the origin and nature of these defects is desirable. Deep level transient spectroscopy (DLTS) was performed on a series of CdS/CdTe thin film solar cells which were activated in vacuum or air ambient. Temperature scans between 5 and 320K revealed semi-shallow and mid-gap majority traps. These mid-gap traps were also investigated using isothermal DLTS (region 250 to 330K) where the temperature is kept constant and the rate window is varied. This way the mid-gap traps can be characterized completely. Using electrical injection DLTS and optical DLTS minority traps could be detected. Electrical injection DLTS showed a single defect in the bulk of the air activated cells, while optical DLTS revealed the presence of defects close to the CdS/CdTe interface in both types of samples. The nature and origin of these defects are unknown. INTRODUCTION CdS/CdTe is a promising candidate for the application in low cost photovoltaic modules. [1] Although theoretically CdTe thin film solar cells can have efficiencies up to 27% [2], values up to 16% were achieved in laboratory samples [3], and only 9% in large commercial modules (compare with 15% for silicon based commercial photovoltaic modules). One major reason for this relatively low efficiency is the presence of deep defects in the CdTe layer. These deep defects capture the charge carriers generated in the solar cells by the photovoltaic process. This results in a decrease in output current, a loss in VOC and a lowering of the cell’s efficiency. This paper deals with the detection of both minority and majority levels in CdS/CdTe thin film solar cells, activated in vacuum and air ambient using Deep Level Transient Spectroscopy. EXPERIMENTAL DETAILS Complete CdS/CdTe thin film solar cells were supplied by Antec Technology GmbH. The cells have a standard configuration: soda-lime glass is used as substrate with sputtered ITO/SnO2 as transparent conducting front contact. CdS was deposited by close space sublimation (CSS) with a thickness of 80nm. The absorber layer CdTe (also deposited by CSS) has a thickness of about 9µm. The active area of the cell measures 0.25 cm2. Prior to the deposition of the back contact, the cells received a CdCl2 activation (20 minutes at 400°C). Two types of samples were investigated: the first type had their CdCl2 activation in vacuum (pressure below 2x10-5 mbar) and was etched with bromine-methanol (Br:CH3OH). The second series received their activation in an air ambient and was etched with an aqueous mixture of nitric and phospheric acid (so-called NP-etch). Gold was used as the back contact in both types
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