Metal chloride passivation treatments for CdTe solar cells
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Metal chloride passivation treatments for CdTe solar cells Jennifer Drayton, Russell Geisthardt, John Raguse, James R. Sites Colorado State University, Physics Department, Fort Collins, CO 80523 ABSTRACT The traditional CdCl2 passivation of CdTe is expanded by adding other chlorides such as MgCl2, NaCl, and MnCl2 into the process through a two-step passivation procedure that combines closed space sublimation step with a vapor process. This allows the possibility of forming a highly doped field at the back of the device that could act as an electron reflector that could boost device performance by directing electrons back into the absorber layer and increasing the voltage while limiting recombination at the back of the device. The effects the two-step passivation process on device performance are characterized by current-voltage measurements, and by electroluminescence and laser-beam induced current images to show the degree of device uniformity. Additionally, capacitance voltage measurements are used to study doping density, depletion width, and possible formation of a field at the back of the device. INTRODUCTION The use of CdCl2 to passivate CdTe-based photovoltaic devices is well known. The passivation process is multipurpose; it enhances the electronic properties of the p-n junction between the n-type CdS and p-type CdTe, it promotes recrystallization of the CdTe from small to larger grain polycrystalline film, and it modifies the grain boundaries of the polycrystalline film [1]. The best CdTe devices exhibit voltages of ~850 mV [2]. This is much less than the theoretical limit of 1.5 V for this type of device. The challenge is to increase the voltage of the device without decreasing other parameters. One way to do this is to limit or significantly decrease carrier recombination at the back contact by directing the carriers back into the absorber layer. Theoretically, an effective electron reflector (ER) to a CdTe device need only have a conduction band offset to the CdTe of 0.2 eV. One approach to this is to include an additional thin layer of material at the back of the device that would be an ER. Another method is to build a field in the CdTe layer that could act like an ER without adding a separate layer [3]. In our experiments we utilize a two-step passivation process in an effort to build a field at the back of the device that would act as an ER. The first passivation step is a CSS-type CdCl2 process immediately after the CdTe without any break in vacuum. We then move to a second system and utilize a vapor CdCl2 passivation step that incorporates other solid chloride materials with the CdCl2. In this way, we attempt to build a field at the back of the CdTe by doping with Mg, Mn, or Na. The results of our experiments are reflected in the Voc, measured with current – voltage (J-V), and changes in the carrier density of the CdTe demonstrated with capacitance voltage (CV) data. We will also show physical evidence of the changes to the semiconductor layers using the luminescence techniques of electroluminescenc
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