Local Current Flow in Mixed-Phase Silicon Solar Cells and Correlation to Light-Induced Open-Circuit Voltage Enhancement
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0910-A23-06
Local Current Flow in Mixed-Phase Silicon Solar Cells and Correlation to Light-Induced Open-Circuit Voltage Enhancement Baojie Yan1, C.-S. Jiang2, H. R. Moutinho2, M. M. Al-Jassim2, Jeffrey Yang1, and Subhendu Guha1 1 United Solar Ovonic Corporation, 1100 West Maple Road, Troy, Michigan, 48084 2 National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado, 80401
ABSTRACT We use conductive atomic force microscopy (C-AFM) to measure the local current flow in the mixed-phase hydrogenated silicon n-i-p solar cell structure without the top ITO contact. The forward biased C-AFM images reveal that for the fully amorphous region the current is very low on the entire surface. However, high current spikes appear in the mixed-phase region, where the current spikes are correlated to the formation of nanocrystallite aggregations with a diameter of ~500 nm. Furthermore, the density of the current spikes increases from the mixed-phase to the substantially nanocrystalline regions. The nanocrystallite aggregation supports our previously proposed parallel-connected two-diode model for Voc drops with crystalline volume fraction and light-induced Voc increase in the mixed-phase solar cells. Adding a 50-nm thick aSi:H buffer layer between the p and i layers significantly reduces the magnitude of the high current spikes, even though the top morphology appears unaffected. This result is also consistent with the previously proposed two-diode model for explaining the carrier transport in the mixedphase solar cells. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) solar cells made close to the transition from amorphous to nanocrystalline phases, but still in the amorphous regime, show an improved initial performance and stability [1]. On the other hand, hydrogenated nanocrystalline silicon (nc-Si:H) solar cells made close to the transition, but in the nanocrystalline regime, also show superior performance [2]. The devices made in the transition region are called mixed-phase solar cells, which exhibit an open-circuit voltage (Voc) distribution between 1.0 and 0.5 V. The mixed-phase solar cells show a very interesting phenomenon that the Voc increases after prolonged light soaking [3,4], which is opposite to the conventional light-induced degradation in a-Si:H solar cells caused by the Staebler-Wronski effect [5]. An explanation for this unexpected observation based on light-induced structural changes from crystalline phase to amorphous phase was proposed [3,4]. Subsequently, a complementary model with two parallel-connected diodes offered explanation for both the decrease of Voc with the increase of crystalline phase, and the Voc increase after light soaking [6]. The two-diode model is based on the assumption that the forward-current density in the crystalline region is much higher than the amorphous region, and there is no significant lateral transport between the amorphous and nanocrystalline regions. The low resistance of the nanocrystalline diodes provides a high current path resulting in a reduction of Vo
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