“Upconversion” creates “super-efficient” solar cells

Researcher Tim Schmidt from the U. of Sydney explains the results from work on “upconversion” as a way to boost solar cell efficiency limit of c-Si and a-Si solar cells by one-third.

November 11, 2009 – 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, 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” material “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. Their work specifically focuses on using “triplet-triplet annihilation” (TTA) 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.

More explanation 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%.

Though the TTA-upconversion process is “very much still at the ‘in vitro’ stage,” it “has great potential, and it is very versatile,” Schmidt told PV World in an e-mail exchange. Fully efficient upconversion can improve cell efficiency by about one-third — i.e., a 15% cell becomes 22%, Schmidt explained. “If we can sustain 20% efficiency for TTA-UC then the 15% cell would get to maybe 17%,” he said, though currently this requires solar concentration.

The main point of this work is to demonstrate that the process “is not fundamentally limited to an 11% ceiling efficiency, as widely thought,” he said, and hopes it will spur “fresh impetus” to develop upconverting units for placement behind bifacial cells. Efficiencies need to be higher with TTA-UC before it’s implemented in a solar cell; “it would be better to have a cell with a threshold of 2eV, rather than 1.12 eV of silicon,” he said, and applying it to silicon will require further work on the chromophores and emitters. (And, he added, “there is going to be the problem with O2, but at least the cell will shield the molecular material from the UV.”)

Click to Enlarge 
Photo of red to blue upconversion, a jump of 1eV, in a 2-photon photochemical upconversion. (Source: U. of Sydney/Tim Schmidt)

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