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ECN publication
Evaluation of bottom cell concepts for perovskite / crystalline silicon tandems
Published by: Publication date:
ECN Solar Energy 28-8-2016
ECN report number: Document type:
ECN-M--16-060 Conference Paper
Number of pages: Full text:
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In this paper, we investigate the potential performance of 4-terminal tandem cell structures based on different types of crystalline silicon (cSi) bottom cells, with a methylammonium-lead-triiodide perovskite solar cell (PSC) on top. We evaluate industrial high-efficiency 6 inch bottom cells, including several back-contact technologies, as developed at ECN. The cell types are: n-type PERT, MWT and IBC, all with diffused boron emitter and phosphorus BSF; MWT silicon heterojunction (MWT-SHJ); and cSi solar cells with polysilicon passivating contact layers on both back and front. We perform approximate analysis of the feasible tandem efficiency. Since the semitransparent PSC is much smaller size than the 6 inch cSi cells, we use a nearly full-area IR longpass filter, which mimicks the presence of a PSC top cell, to measure the cSi bottom cells in tandem configuration. This procedure is calibrated through a measurement of a small size version of the n-type MWT cell in actual tandem stack with the PSC layer stack as filter. Based on the best bottom cell (a 6 inch MWT-SHJ cell of 21.9% efficiency) and our best PSC top cell of 14.6% efficiency and Voc=1.03 V as measured in backward I-V trace, a “virtual” tandem cell efficiency of approx. 24.7% is obtained. The tandems based on the other cSi bottom cells mostly perform “virtually” around 24.1%. We analyse in particular the potential of using cSi cells with front and rear polysilicon passivating contact layers as bottom cells. The double sided use of polysilicon allows for very high Voc. When used on a standalone cSi cell, a front polysilicon layer causes a large optical loss at short wavelengths, which however falls practically completely within the absorption band of the PSC. Therefore we observe that our preliminary double-sided polysilicon cells in tandem configuration already perform quite similar to the nMWT cell (“virtual” tandem efficiency of 23.1%). By reducing thickness and doping level of the polysilicon, tandem performance exceeding 25% should become possible.

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