Researchers: “Upconversion” Creates “Super-efficient” Solar Cells

Researchers at the U. of Sydney say they have come up with a way to boost solar cell efficiency limit of c-Si and a-Si solar cells to 50% under the standard solar spectrum — an increase of roughly a third — using a process called “upconversion.”

The work, published in the journal Physical Chemistry Chemical Physics, aims to increase efficiency in crystalline and amorphous silicon solar cells through application of synthesized sensitizer and emitter molecules. Such “single-threshold” materials “produces voltage by promoting electrons above this threshold upon absorption of light,” explained paper co-author Tim Schmidt; photons with energy below that threshold can’t be harvested, and any energy above that threshold is lost through heat, he added.

Maximum efficiency of single-threshold PV converters is about 30%, the group notes. One proposed way to improve that is to place an upconverting material behind the cell, to convert low subthreshold photons into usable light; such a cell would have an efficiency limit of >50%, though best-recorded efficiency has been ∼ 10 -6. Their work specifically focuses on using “triplet-triplet annihilation” in organic molecules. When two triplet emitter molecules encounter each other, the result is either a singlet, triplet, or quintet spin state; if a singlet (1:9 chance), it converts to a lower energy state and fluoresces, yielding “upconverted” light.

Applying this “upconversion” method, the group says, results in striking improvements: efficiency limit “under the standard solar spectrum” of over 50%, and up to 63% under 100-fold solar concentration. From the paper abstract:

Emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.
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