Buffer Layers for Narrow Bandgap A-SIGE Solar Cells
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X. B. LIAO, J, WALKER and X. DENG Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606
ABSTRACT
In high efficiency narrow bandgap (NBG) a-SiGe solar cells, thin buffer layers of unalloyed hydrogenated amorphous silicon (a-Si) are usually used at the interfaces between the aSiGe intrinsic layer and the doped layers. We investigated the effect of inserting additional a-SiGe interface layers between these a-Si buffer layers and the a-SiGe absorber layer. We found that such additional interface layers increase solar cell V., and FF sizably, most likely due to the reduction or elimination of the abrupt bandgap discontinuity between the a-SiGe absorber layer and the a-Si buffer layers. With these improved narrow bandgap solar cells incorporated into the fabrication of triple-junction a-Si based solar cells, we obtained triple cells with initial efficiency of 10.6%. INTRODUCTION
Narrow bandgap a-SiGe materials and solar cell devices have been studied extensively for their use in the spectrum splitting, multiple-junction a-Si based solar cells [1-3]. To achieve high conversion efficiency solar cells, an a-SiGe absorber layer is usually sandwiched between two thin a-Si buffer layers which are in direct contact with the p- and n- doped layers [3,4]. These a-Si buffer layers were found to enhance the performance of a-SiGe solar cells. However, even with these a-Si buffer layers, there are still abrupt discontinuity in the bandgap at the interfaces between these buffer layers and the a-SiGe absorber layer. In this paper, we report our study on the insertion of additional a-SiGe interface layers (with less Ge compared with the absorber) which reduces the bandgap offset. EXPERIMENTAL DETAILS The a-SiGe and a-Si materials used in this study were deposited using a ultra high vacuum plasma enhanced chemical vapor deposition (PECVD) system at the University of Toledo (UT). The intrinsic and doped layers were deposited in separated chambers of this multi-chamber, loadlocked deposition system, having a base vacuum of 2x×10. Torr. A gas mixture of GeH 4, Si 2H( and H2 were used for the a-SiGe deposition and Si 2H 6 and H2 for a-Si deposition. The deposition conditions include a substrate temperature range of 300-400 TC, a chamber pressure of 0.5-0.6 Torr and an rf power of 2.5-3.0 W for the a-SiGe and a-Si layer depositions. Different GeH 4 to Si 2H6 ratios were used to achieve different bandgaps for a-SiGe material. Graded bandgaps for the a-SiGe absorber layer and the a-SiGe buffer layers were achieved by adjusting GeH 4 flows during growth. The GeHJ/Si 2H6 gas flow ratio for the a-SiGe absorber layers studied here was 0.88. All of the intrinsic absorber and buffer layers were deposited in a deposition chamber designated for intrinsic layer growth. Boron doped microcrystalline silicon (ltc-Si) p-layer and phosphorus doped a-Si n-layer were deposited in another chamber designated for the growth of doped layers The device structure used in this study was: SS/Ag/ZnO/n(a-Si)/b(a-Si)/i(NBG a-SiGe)/b(a-Si)/p(ýtc-S
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