Photoconductive CdS: how does it Affect CdTe/CdS Solar Cell Performance?
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Photoconductive CdS: how does it affect CdTe/CdS solar cell performance? S. Hegedus, D. Ryan, K. Dobson, B. McCandless, D. Desai Institute of Energy Conversion, University of Delaware, Newark, DE 19716 USA ABSTRACT Photoconductive CdS (PC-CdS) in CdS/CdTe solar cells from five different sources is investigated using spectral sensitization of apparent quantum efficiency (AQE) and J-V analysis. Red bias light significantly enhances the blue AQE, commonly leading to AQE>1 below 550 nm, and blue bias light enhances the red AQE, but to a much smaller extent. These enhancements are more pronounced with increasing forward bias, after stress and in devices with intentionally Cu-doped CdS. This behavior is observed to some degree in all devices with CdS, but is absent in cells without CdS. These effects are consistent with blue light, either ac monochromatic or dc bias, increasing the CdS conductivity. This causes an increase in the field and depletion width in the CdTe to maintain balanced space charge, leading to increased collection of carriers from the CdTe. The CdS conductivity modulation can also change the AQE due to a change in equivalent circuit resistance. Analysis of J-V data measured with white, blue, red or no light indicates little dependence of series resistance or diode quality factor on the illumination spectrum. Thus, the PC-CdS resistance has little effect on the solar cell J-V performance, but does influence AQE. INTRODUCTION CdS is the common n-type wide band gap heterojunction window layer for all present day high efficiency or commercial CdTe based solar cells. There is little direct control over the CdS electrical properties during deposition or subsequent processing. Its nominal resistivity is in the range of 0.110 Ohms-cm [1, 2] but this can increase substantially due to compensating impurities. CdS photoconductivity (PC) was investigated extensively with regard to Cu-related acceptor states in CdS of CdS/Cu2 S solar cells created by the unintentional diffusion of Cu from the Cu2S [1,2]. The enhanced PC was often correlated with a large light-to-dark crossover (LDXO) in the J-V curves, and of special interest to this work, with a large enhancement of long wavelength AQE by short wavelength light. Cu creates deep hole traps in the CdS, greatly increasing the dark resistivity, but this could be recovered with illumination. Cu is presently used to make nearly all CdTe based devices to form a low resistance back contact and to dope the CdTe. It has been shown that Cu is present in CdS of CdS/CdTe solar cells, even in special devices made without any intentional Cu layer or Cu doping [3,4]. The Cu concentration in the CdS also increases after accelerated thermal and voltage bias stressing [3,4]. Several recent papers have investigated the PC in unstressed CdS/CdTe devices using AQE at far forward bias ~1.5V [5-7]. In this work, we use the bias light spectrum to characterize the AQE in CdS/CdTe devices from several groups. The term AQE is used instead of QE since effects are observed which violate
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