In-Situ Observation of High Deposition Rate Hydrogenated Polymorphous Silicon Cell Degradation through Variable Intensit
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1153-A07-09
In-Situ Observation of High Deposition Rate Hydrogenated Polymorphous Silicon Cell Degradation through Variable Intensity Method Measurements Erik V. Johnson, Ka-Hyun.Kim, and Pere Roca i Cabarrocas LPICM, CNRS, École Polytechnique, 91128 Palaiseau, France ABSTRACT The efficiencies of hydrogenated polymorphous silicon (pm-Si:H) solar cells have been previously demonstrated to show superior stability under light-soaking. This stability arises due to the fact that the decrease they show in fill factor (FF) is partially offset by an accompanying increase in open circuit voltage (VOC). Recently, high-deposition rate (9Å/s) pm-Si:H material deposited by standard RF-PECVD at 13.56MHz has been investigated as the intrinsic layer in photovoltaic modules as it has shown excellent electronic properties. The degradation behaviour of these high-deposition rate cells, however, differs significantly from that of lower deposition rate material. In particular, no beneficial increase in Voc is observed during light soaking. We investigate the degradation dynamics of solar cells made from this high growth rate material using a Variable Illumination Method (VIM) during light soaking to quantify the changes to these high-rate cells during light-soaking and directly contrast them with those of low-rate (1.5Å/s) cells. In particular, we discuss the importance of bulk recombination effects vs interface quality changes, as well as the dynamics of changes in VOC. INTRODUCTION In an industrial environment where equipment depreciation is a significant portion of the cost of thin-film photovoltaic devices, an effective strategy in reducing the cost-per watt of thinfilm solar cells is to increase the deposition rate while maintaining the same cell efficiency. The application of RF-PECVD at 13.56MHz to this strategy through the high-rate (HR) deposition of hydrogenated polymorphous silicon (pm-Si:H) has recently been demonstrated to result in device quality material [1]. In general, pm-Si:H is grown from hydrogen-diluted silane at relatively high (>1Torr) gas pressures during deposition – conditions that result in the presence of a small volume fraction of nanocrystallites embedded in the amorphous matrix. This material differs from protocrystalline or transition material [2] in that it does not undergo a structural change once a certain thickness has been reached, but rather maintains constant structural and electronic properties for layers of increasing thickness [3]. When pm-Si:H is applied as the i-layer in PIN structures, this has resulted in 10x10cm mini-modules with initial efficiencies up to 9%, even when the i-layer was deposited at 8-9Å/s [4,5]. Devices made from this high-deposition rate pmSi:H material, however, display significant differences under light-soaking (LS) when compared to devices made from lower deposition rate (1.5Å/s) pm-Si:H material. In this work, we develop a tool to examine these differences – an in-situ Light-Soaking and Variable Intensity Method (LS-VIM) setup – and to compare changes for two sets of
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