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Title: The Voltage Boost Enabled by Luminescence Extraction in Solar Cells

Abstract

Over the past few years, the application of the physical principle, i.e., 'luminescence extraction,' has produced record voltages and efficiencies in photovoltaic cells. Luminescence extraction is the use of optical design, such as a back mirror or textured surfaces, to help internal photons escape out of the front surface of a solar cell. The principle of luminescence extraction is exemplified by the mantra 'a good solar cell should also be a good LED.' Basic thermodynamics says that the voltage boost should be related to concentration ratio C of a resource by ΔV = (kT/q) ln{C}. In light trapping (i.e., when the solar cell is textured and has a perfect back mirror), the concentration ratio of photons C = {4n2}; therefore, one would expect a voltage boost of ΔV = (kT/q) ln{4n2} over a solar cell with no texture and zero back reflectivity, where n is the refractive index. Nevertheless, there has been ambiguity over the voltage benefit to be expected from perfect luminescence extraction. Do we gain an open-circuit voltage boost of ΔV = (kT/q) ln{n2}, ΔV = (kT/q) ln{2 n2}, or ΔV = (kT/q) ln{4 n2}? What is responsible for this voltage ambiguity ΔV = (kT/q) ln{4} $${\asymp}$$ 36 mV? Finally, we show that different results come about, depending on whether the photovoltaic cell is optically thin or thick to its internal luminescence. In realistic intermediate cases of optical thickness, the voltage boost falls in between: ln{n2} < (qΔV/kT) < ln{4n 2}.

Authors:
; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States); Energy Frontier Research Centers (EFRC) (United States). Light-Material Interactions in Energy Conversion (LMI)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE); USDOE Office of Science (SC), Basic Energy Sciences (BES)
OSTI Identifier:
1328260
Alternate Identifier(s):
OSTI ID: 1266700; OSTI ID: 1328261
Report Number(s):
NREL/JA-5J00-66804
Journal ID: ISSN 2156-3381; 7454706
Grant/Contract Number:  
AC02-05CH11231; AC36-08-GO28308; AC36-08GO28308; SC0001293
Resource Type:
Published Article
Journal Name:
IEEE Journal of Photovoltaics
Additional Journal Information:
Journal Name: IEEE Journal of Photovoltaics Journal Volume: 6 Journal Issue: 4; Journal ID: ISSN 2156-3381
Publisher:
Institute of Electrical and Electronics Engineers
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; luminescence; photovoltaic cells; solar energy

Citation Formats

Ganapati, Vidya, Steiner, Myles A., and Yablonovitch, Eli. The Voltage Boost Enabled by Luminescence Extraction in Solar Cells. United States: N. p., 2016. Web. https://doi.org/10.1109/JPHOTOV.2016.2547580.
Ganapati, Vidya, Steiner, Myles A., & Yablonovitch, Eli. The Voltage Boost Enabled by Luminescence Extraction in Solar Cells. United States. https://doi.org/10.1109/JPHOTOV.2016.2547580
Ganapati, Vidya, Steiner, Myles A., and Yablonovitch, Eli. Fri . "The Voltage Boost Enabled by Luminescence Extraction in Solar Cells". United States. https://doi.org/10.1109/JPHOTOV.2016.2547580.
@article{osti_1328260,
title = {The Voltage Boost Enabled by Luminescence Extraction in Solar Cells},
author = {Ganapati, Vidya and Steiner, Myles A. and Yablonovitch, Eli},
abstractNote = {Over the past few years, the application of the physical principle, i.e., 'luminescence extraction,' has produced record voltages and efficiencies in photovoltaic cells. Luminescence extraction is the use of optical design, such as a back mirror or textured surfaces, to help internal photons escape out of the front surface of a solar cell. The principle of luminescence extraction is exemplified by the mantra 'a good solar cell should also be a good LED.' Basic thermodynamics says that the voltage boost should be related to concentration ratio C of a resource by ΔV = (kT/q) ln{C}. In light trapping (i.e., when the solar cell is textured and has a perfect back mirror), the concentration ratio of photons C = {4n2}; therefore, one would expect a voltage boost of ΔV = (kT/q) ln{4n2} over a solar cell with no texture and zero back reflectivity, where n is the refractive index. Nevertheless, there has been ambiguity over the voltage benefit to be expected from perfect luminescence extraction. Do we gain an open-circuit voltage boost of ΔV = (kT/q) ln{n2}, ΔV = (kT/q) ln{2 n2}, or ΔV = (kT/q) ln{4 n2}? What is responsible for this voltage ambiguity ΔV = (kT/q) ln{4} ${\asymp}$ 36 mV? Finally, we show that different results come about, depending on whether the photovoltaic cell is optically thin or thick to its internal luminescence. In realistic intermediate cases of optical thickness, the voltage boost falls in between: ln{n2} < (qΔV/kT) < ln{4n 2}.},
doi = {10.1109/JPHOTOV.2016.2547580},
journal = {IEEE Journal of Photovoltaics},
number = 4,
volume = 6,
place = {United States},
year = {2016},
month = {7}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record
https://doi.org/10.1109/JPHOTOV.2016.2547580

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