The Study of Deep Levels in CdS/CdTe Solar Cells Using Admittance Spectroscopy and its Modifications
- PDF / 346,902 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 21 Downloads / 254 Views
B9.6.1
The Study of Deep Levels in CdS/CdTe Solar Cells Using Admittance Spectroscopy and its Modifications A.S. Gilmore, V. Kaydanov, T.R. Ohno Colorado School of Mines, Dept. of Physics, Golden, CO 80401, U.S.A. ABSTRACT Measurements of an admittance over a wide frequency range were used to detect the defect electronic states and evaluate their properties in CdTe based solar cells. Cells prepared in various ways, from various facilities all exhibited a high defect state density (>1014cm-3, and often >1015cm-3). Two distinct energy levels or bands were observed at approximately 0.37eV and 0.61eV above the valence band. These were tentatively attributed to CuCd- and VCd-- respectively. Various post-CdTe deposition treatments, as well as stress tests, were applied to alter the defect state densities. The high defect concentration measured was not observed to inhibit cell performance in any way. INTRODUCTION Polycrystalline cadmium telluride thin film solar cells have demonstrated high efficiencies (record value of 16.4% [1]). Progress on these devices is limited partially due to an incomplete understanding of the degradation mechanisms diminishing their performance over time. These are largely thought to be dominated by defect mutation or migration over time. Therefore, understanding these defects is a critical aspect of continued commercialization. Admittance spectroscopy (AS) provides a tool to detect and characterize defect states [2, 3]. Defect properties such as emission rates, energy position in the band gap, and estimates of defect concentrations for a specific defect level or band can be obtained using admittance spectroscopy. Drive level capacitance profiling (DLCP) allows defect state densities as a function of depth through the cell’s absorber layer to be obtained [4, 5]. A large defect state density has been shown to exist in most CdTe based solar cells [6, 7]. This has historically been viewed as an impediment to device performance, as it is well known to hinder performance in silicon solar cells. The effect of a high defect state density in CdTe is not well understood, but these studies suggest that it may be not as deleterious as expected. EXPERIMENTAL DETAILS Completed solar cells from First Solar, LLC. (FS), the National Renewable Energy Laboratory (NREL) and the University of Toledo (UT) were obtained for these studies. These cells had all undergone the standard cadmium chloride treatment, and all had intentionally been doped with copper unless specifically stated. Various etching procedures were also applied to the cells (either bromine methanol or nitric phosphate were used depending upon the institution’s standard recipe) prior to the application of the back contact. These were all representative of devices having undergone the institutions standard processes. Incomplete cells, only processed through the CdTe deposition, were also obtained from FS. These samples were then given the standard back contact at Colorado School of Mines (FS + CSM) to compare the effects of different back contacts on t
Data Loading...
