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High Open-Circuit Voltage in Silicon Heterojunction Solar Cells

Published online by Cambridge University Press:  01 February 2011

Qi Wang
Affiliation:
qi_wang@nrel.gov, NREL, EDMD, 1617 Cole Blvd, Golden, CO, 80410, United States
Matt R. Page
Affiliation:
Matt_page@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Eugene Iwancizko
Affiliation:
eugence_iwancizko@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Yueqin Xu
Affiliation:
yueqin_xu@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Lorenzo Roybal
Affiliation:
Lorenzo_Roybal@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Russell Bauer
Affiliation:
russell_bauer@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Dean Levi
Affiliation:
Dean_levi@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Yanfa Yan
Affiliation:
yanfa_yan@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
Tihu Wang
Affiliation:
tihu_wang@nrel.gov, Suntech Power, Wuxi, N/A, China, People's Republic of
Howard M. Branz
Affiliation:
howard_branz@nrel.gov, National Renewable Eenergy Laboratory, EDMD, Golden, CO, 80410, United States
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Abstract

High open-circuit voltage (Voc ) silicon heterojunction (SHJ) solar cells are fabricated in double-heterojunction a-Si:H/c-Si/a-Si:H structures using low temperature (<225°C) hydrogenated amorphous silicon (a-Si:H) contacts deposited by hot-wire chemical vapor deposition (HWCVD). On p-type c-Si float-zone wafers, we used an amorphous n/i contact to the top surface and an i/p contact to the back surface to obtain a Voc of 667 mV in a 1 cm2 cell with an efficiency of 18.2%. This is the best reported p-type SHJ voltage. In our labs, it improves over the 652 mV cell obtained with a front amorphous n/i heterojunction emitter and a high-temperature alloyed Al back-surface-field contact. On n-type c-Si float-zone wafers, we used an a Si:H (p/i) front emitter and an a-Si:H (i/n) back contact to achieve a Voc of 691 mV on 1 cm2 cell. Though not as high as the 730 mV reported by Sanyo on n-wafers, this is the highest reported Voc for SHJ c-Si cells processed by the HWCVD technique. We found that effective c-Si surface cleaning and a double-heterojunction are keys to obtaining high Voc. Transmission electron microscopy reveals that high Voc cells require an abrupt interface from c-Si to a-Si:H. If the transition from the base wafer to the a-Si:H incorporates either microcrystalline or epitaxial Si at c Si interface, a low Voc will result. Lifetime measurement shows that the back-surface-recombination velocity (BSRV) can be reduced to ~15 cm/s through a-Si:H passivation. Amorphous silicon heterojunction layers on crystalline wafers thus combine low-surface recombination velocity with excellent carrier extraction.

Type
Research Article
Copyright
Copyright © Materials Research Society 2007

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References

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