The Voltage Boost Enabled by Luminescence Extraction in Solar Cells

Journal Article · Fri Jul 01 00:00:00 EDT 2016 · IEEE Journal of Photovoltaics

DOI:https://doi.org/10.1109/JPHOTOV.2016.2547580· OSTI ID:1328260

Ganapati, Vidya; Steiner, Myles A.; Yablonovitch, Eli

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}.

• Research Organization: National Renewable Energy Lab. (NREL), Golden, CO (United States); Energy Frontier Research Centers (EFRC) (United States). Light-Material Interactions in Energy Conversion (LMI)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-05CH11231; AC36-08-GO28308; AC36-08GO28308; SC0001293
• OSTI ID: 1328260
• Alternate ID(s): OSTI ID: 1266700; OSTI ID: 1328261
• Report Number(s): NREL/JA-5J00-66804; 7454706
• Journal Information: IEEE Journal of Photovoltaics, Journal Name: IEEE Journal of Photovoltaics Vol. 6 Journal Issue: 4; ISSN 2156-3381
• Publisher: Institute of Electrical and Electronics Engineers
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
36 MATERIALS SCIENCE
luminescence
photovoltaic cells
solar energy