Interdigitated back contact silicon heterojunction solar cells: analysis with two-dimensional simulations
Date
2011
Authors
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Publisher
University of Delaware
Abstract
Interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cells using a-Si emitter and contact layers show significant potential advantages over standard hetero-junction devices: higher short-circuit current (Jsc) since there is no grid shading and higher open-circuit voltage (Voc) due to better surface passivation. However, they often suffer from low fill factor (FF). IBC-SHJ processing steps include two separate photoresist masks for doped amorphous silicon depositions, and all amorphous layers are deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD). The thicknesses of the front surface layers are optimized so that reflection and absorption are minimized. Measurement techniques, such as current-voltage, reflection, and quantum efficiency, were used to characterize the experimental devices. These techniques were also investigated in simulations to match modeled data to experimental results. Using two-dimensional simulations to model IBC-SHJ devices on Float Zone (FZ) n-Si, we found that the FF was nearly independent of the defect concentrations in contact and passivating i-layers but strongly dependent on the defects in emitter and the band gap in the rear i-layer. Voc and Jsc were nearly independent of defects in either doped layer. In a-Si doped layers it is well known that the number of defects increase with doping. We find that the FF is sensitive to either mid-gap or band tail states and that S-shaped JV curves responsible for low FF can be eliminated by a decrease in p-layer mid-gap or band tail defect levels, or by decreasing the rear i-layer’s band gap. The insensitivity of FF to defects in the n-layer or in the i-layer suggests the FF is dominated by minority carrier injection/collection from the p-type emitter layer. The dependence of FF on the rear i-layer band gap suggests that increasing the offset in the valence band impedes minority carrier collection. Rear-surface geometry, wafer resistivity, and wafer lifetime and thickness were also investigated in simulations, and their results are shown. With the advancement of IBC-SHJ technology, new device structures, such as larger cells with more interdigitated fingers, will be fabricated and simulated.