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Off Grid Energy Independence
Posted on March 05, 2014

Exceeding solar cells efficiency limit by 'heating up' 'cold' photons

The quest for efficient low-cost solutions for solar energy conversion faces many obstacles, both, fundamental and technical. Fundamental limitations to the maximum achievable efficiency of solar cells stem from a combination of several factors. These include: (i) solar cells inability to use photons with the energies below the electronic bandgap of their material, (ii) thermalization of charge carriers generated by absorption of the photons with above-bandgap energies, and (iii) the losses caused by recombination of the light-generated charge carriers. As a result, even 'ideal' solar cells have maximum intrinsic efficiency - known as the Shockley-Queisser (S-Q) limit - of 33% for the illumination by the non-concentrated sunlight.
The S-Q limit can be exceeded by utilizing low-energy photons either via their electronic up-conversion or via thermophotovoltaic (TPV) conversion process. However, electronic up-conversion systems have extremely low efficiencies, and practical temperature considerations limit the operation of TPV converters to the low-quality narrow-gap solar cells. Recently, MIT researchers from the DOE-funded Solid State Solar-Thermal Energy Conversion Center (S3TEC) have proposed a new way to break the fundamental S-Q limit by using a mechanism of thermal up-conversion.
In the recent paper in Optics Communications ("Exceeding the Solar Cell Shockley-Queisser Limit via Thermal Up-Conversion of Low-Energy Photons"), researchers - via rigorous thermodynamic analysis - have shown that solar-to-electricity conversion efficiency higher than the S-Q limit can be achieved in a hybrid platform that combines a single-junction solar cell and a thermal up-converter. Within the proposed hybrid scheme, photons with energies below the bandgap of the solar cell ('cold' photons) are absorbed by the up-converter, which heats up and re-emits photons with higher energies ('hot' photons) towards the solar cell. To make this process possible, both front and back surfaces of the up-converter must have carefully designed angular- and frequency-selective emittance characteristics.
The researchers predict that the maximum efficiency of a hybrid device utilizing the thermal up-conversion scheme can reach 73% for the ideal spectrally- and angularly-selective emittance characteristics of the up-converter surfaces under illumination by the non-concentrated sunlight. Reaching high up-conversion efficiency requires raising the up-converter temperature, which for the commonly used PV materials (e.g., Si, GaAs, CdTe, and GaAsP) lies within practically achievable 900-1600K range.
Detailed analysis of non-ideal up-converter surfaces that allows for reasonable absorption and emission losses still yields limiting efficiency values exceeding 45% for moderate optical concentration of 300 suns. The proposed hybrid platform offers new opportunities for reaching high solar energy conversion efficiency under low levels of optical concentration achievable via simple low-cost technological solutions.
Top image: Svetlana V. Boriskina