Light Soaking and Thermal Annealing Effects on the Micro-Electrical Properties of Amorphous and Nanocrystalline Mixed-ph
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0989-A02-05
Light Soaking and Thermal Annealing Effects on the Micro-Electrical Properties of Amorphous and Nanocrystalline Mixed-Phase Silicon Solar Cells C.-S. Jiang1, B. Yan2, H. R. Moutinho1, M. M. Al-Jassim1, J. Yang2, and S. Guha2 1 National Renewable Energy Laboratory, Golden, CO, 80401 2 United Solar Ovonic LLC, Troy, MI, 48084
ABSTRACT We report on the measurement of local current flow in hydrogenated amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells in the initial, light-soaked, and annealed states using conductive atomic force microscopy (C-AFM). The C-AFM measurement shows that the nanometer-size grains aggregate and the local current densities in the nanocrystalline aggregation areas significantly decreased after light soaking and recovered to similar values as the initial state after annealing at a high temperature in vacuum. This result supports the twoparallel-connected-diode model for explaining the light-induced open-circuit voltage increase in the mixed-phase solar cells. INTRODUCTION Amorphous and nanocrystalline mixed-phase hydrogenated silicon (Si:H) n-i-p solar cells have been fabricated using glow-discharge chemical vapor deposition [1, 2]. This was achieved by optimizing the deposition parameters in the amorphous to nanocrystalline transition regime. Measurement of micro-electrical properties of the mixed-phase cell is not only useful for the device optimization of both amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) solar cells, but also helpful for the understanding of device physics with complex structures. The mixed-phase solar cells show an open-circuit voltage (Voc) ranging from 0.5 to 1.0 V, which is between the Voc values of typical a-Si:H and nc-Si:H cells. One interesting phenomenon for the mixed-phase solar cells is that the Voc increases after light soaking [1,2], opposite to commonly observed for a-Si:H and nc-Si:H solar cells where the Voc decreases after light soaking due to the Staebler-Wronski effect [3]. The original explanation for the light-induced Voc increase in the mixed-phase solar cell was based on light-induced structural changes from crystalline to amorphous phase [1, 2]. Subsequently, a complementary model of two parallel-connected diodes (two-diode model) was proposed [4]. There are two key points made in the two-diode model. First, the amorphous phase and nanocrystalline phase can be considered as two separate diodes with significantly different characteristics. Second, the current versus voltage (I-V) characteristics of nc-Si:H diode in the mixed-phase cells degrade by light soaking. The degradation of nc-Si:H causes a decrease in the electric current, when a forward voltage larger than the Voc of the nc-Si:H cell is applied, resulting in an increase in the Voc of the mixed-phase solar cells [4]. However, the size of the nanocrystallites observed by X-ray diffraction and Raman spectroscopy is very small, ranging from a few nm to 30 nm. It is difficult to believe that such small grains can form complete diodes through the ent
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