Open-Circuit Voltage Physics in Amorphous Silicon Solar Cells
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Open-Circuit Voltage Physics in Amorphous Silicon Solar Cells L. Jiang,* J. H. Lyou†, S. Rane, E. A. Schiff,‡ Q. Wang§, Q. Yuan Department of Physics, Syracuse University, Syracuse, NY 13244-1130 ABSTRACT We have performed computer calculations to explore effects of the p/i interface on the opencircuit voltage in a-Si:H based pin solar cells. The principal conclusions are that interface limitation can occur for values of VOC significantly below the built-in potential VBI of a cell, and that the effects can be understood in terms of thermionic emission of electrons from the intrinsic layer into the p-layer. We compare measurements of VOC and electroabsorption estimates of VBI with the model calculations. We conclude that p/i interface limitation is important for current a-Si:H based cells, and that the conduction band offset between the p and i layers is as important as the built-in potential for future improvements to VOC. INTRODUCTION The open-circuit voltage Voc of amorphous silicon (a-Si:H) based pin solar cells remains the most ill-understood of its device parameters. This lack of insight is remarkable, since the experimental behavior of VOC is generally quite simple. VOC depends little upon the defect density or thickness of the intrinsic layer, and eVOC is roughly shifted down from the optical bandgap of this layer by about 0.8-0.9 eV. Furthermore, the value of VOC is mostly controlled by the fairly simple physics of the splitting of quasi-Fermi-levels in the intrinsic layer. Nonetheless, even the simplest question about it, which is whether Voc is reduced by non-ideal p/i or n/i interfaces, is not conclusively answered. Of course, this unsatisfactory state of affairs does leave open the tantalizing possibility of significant improvements in VOC – if only device-makers could be pointed in a better direction. In this paper, we first review the interrelation of the open-circuit voltage and quasi-Fermi levels in the device physics of a-Si:H based pin solar cells. In cells with ideal p and n layers, VOC reaches its “intrinsic limit,” which may be equated to the splitting of the electron and hole quasiFermi levels in bulk intrinsic material. In principle VOC measurements can become a very interesting alternative to photoconductivity measurements in intrinsic films. We next discuss the hypothesis that VOC achieves this intrinsic limit for optimized a-Si:H cells, and we conclude that this hypothesis is most likely incorrect. First, open-circuit voltages do not exhibit the dependence upon defect density which is known to apply to quasi-Fermi levels for thin films of intrinsic aSi:H. Second, we present electroabsorption measurements of the built-in potential in a-Si:H based solar cells which yield rather small values for Vbi (in the range 1.05 – 1.25 V). The built-in potential is generally recognized as the ultimate limit to VOC in any solar cell. We present computer calculations which strongly suggest that the measured values are small enough to be reducing VOC for the a-Si:H cells.
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Present address: Guidant Corpor
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