Abstract
This paper presents a three-dimensional numerical analysis of homojunction/heterojunction hybrid silicon wafer solar cells, featuring front-side full-area diffused homojunction contacts and rear-side heterojunction point contacts. Their device performance is compared with conventional full-area heterojunction solar cells as well as conventional diffused solar cells featuring locally diffused rear point contacts, for both front-emitter and rear-emitter configurations. A consistent set of simulation input parameters is obtained by calibrating the simulation program with intensity dependent lifetime measurements of the passivated regions and the contact regions of the various types of solar cells. We show that the best efficiency is obtained when a-Si:H is used for rear-side heterojunction point-contact formation. An optimization of the rear contact area fraction is required to balance between the gains in current and voltage and the loss in fill factor with shrinking rear contact area fraction. However, the corresponding optimal range for the rear-contact area fraction is found to be quite large (e.g. 20-60 % for hybrid front-emitter cells). Hybrid rear-emitter cells show a faster drop in the fill factor with decreasing rear contact area fraction compared to front-emitter cells, stemming from a higher series resistance contribution of the rear-side a-Si:H(p+) emitter compared to the rear-side a-Si:H(n+) back surface field layer. Overall, we show that hybrid silicon solar cells in a front-emitter configuration can outperform conventional heterojunction silicon solar cells as well as diffused solar cells with rear-side locally diffused point contacts.
Highlights
To increase the cost effectiveness of high-efficiency silicon wafer based solar cells, a reduction of the silicon wafer thickness coupled with a cell efficiency improvement is widely pursued
Hybrid rear-emitter cells show a faster drop in the fill factor with decreasing rear contact area fraction compared to front-emitter cells, stemming from a higher series resistance contribution of the rear-side amorphous silicon (a-Si):H(p+) emitter compared to the rear-side a-Si:H(n+) back surface field layer
We show that hybrid silicon solar cells in a front-emitter configuration can outperform conventional heterojunction silicon solar cells as well as diffused solar cells with rear-side locally diffused point contacts
Summary
To increase the cost effectiveness of high-efficiency silicon wafer based solar cells, a reduction of the silicon wafer thickness coupled with a cell efficiency improvement is widely pursued. In order to obtain this ultra-high efficiency, locally diffused contacts are typically deployed in combination with an excellent passivation of the non-contacted regions (i.e. the passivated emitter, rear locally-diffused PERL solar cell concept used by UNSW). The optimization of the local rear contact geometry, quality of passivation layers, and the diffusion profiles is important to balance between charge collection and the bulk / surface recombination rates which can degrade the cell efficiency. Solar cells utilizing local contacts benefit from a reduced contact recombination due to a reduction of the contact area, thereby enhancing the open-circuit voltage VOC. The remaining high surface recombination at the contacts limits the VOC improvement.
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