Oxygenated CdS Window Layer for Sputtered CdS/CdTe Solar Cells
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Oxygenated CdS Window Layer for Sputtered CdS/CdTe Solar Cells Akhlesh Gupta, Karthikeya Allada, Sung Hyun Lee, and Alvin D. Compaan Department of Physics and Astronomy, The University of Toledo, Toledo, OH-43606 ABSTRACT It is known that carriers photogenerated in the polycrystalline CdS layer of a CdS/CdTe cell are not collected. Thus, the short-circuit current (JSC) of CdS/CdTe devices should be improved if the bandgap of CdS is increased to permit better blue response. Wu, et al, showed that alloying of CdS with oxygen can increase the absorption edge of the layer. We report here on studies of this “oxygenated” CdS and its use in sputtered cells. We find that at a deposition temperature of 250 oC the addition of O2 to the sputter gas results in a red shift of the absorption edge (from 2.35 eV to 1.94 eV), but that room temperature deposition gives a blue shift (from 2.36 eV to 3.28 eV). The Raman spectra of the room temperature deposited films show a considerable broadening of LO phonon peaks suggesting a micro- to nano- to amorphous transition as the O2 fraction increases. XRD measurements of these films confirm the formation of an amorphous structure at high O2 fractions. The quantum efficiency measurements of CdS/CdTe device with room temperature deposited oxy-CdS show an improvement in blue response and hence increased JSC, but are accompanied by poorer junction quality so that the overall efficiency is not increased. INTRODUCTION The conventional polycrystalline CdS used as a heterojunction partner/window layer in CdS/CdTe thin-film solar cells has a bandgap of ~2.4 eV which causes absorption in the shortwavelength region resulting in reduced short-circuit current density (JSC) in the devices. The JSC achieved from typical CdS/CdTe solar cells is much less than the theoretical expectation,[1] partly due to absorption of higher-energy photons in CdS films. This loss can be minimized by using thin CdS (50 nm or less) but this often causes reduced VOC generally thought to be due to shorts between the CdTe and TCO through pinholes in the thin CdS. One remedy for this is to use a high resistivity transparent (HRT) buffer layer between the conductive TCO and the CdS to prevent shorting but this requires very good quality HRT. The JSC can also be increased if CdS is made more transparent to higher energy photons. Recently, Wu, et. al. [2] used this approach and demonstrated improved current density using reactively sputtered CdS:O with close-spaced-sublimation (CSS) deposited CdTe solar cells. Since the CSS deposition of CdTe is done at high temperature (500-600 oC), it can cause CdS:O recrystallization and drive interdiffusion between CdS and CdTe, and therefore change the performance of the devices. Secondly, Wu et. al. also used very high quality conductive Cd2SnO4 as the TCO together with Zn2SnO4 as an HRT buffer in their solar cells. The main aim of the present work was to examine the effect of oxy-CdS in CdS/CdTe solar cells prepared by low temperature deposition techniques on standard SnO2:F which i
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