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The Molecular Photonics Laboratories at UNSW Sydney. [Image: UNSW Sydney/Exciton Science]

A team of researchers from Australia and the U.S. have discovered a new way to convert near-infrared light into visible light from beyond the silicon band gap, an achievement that has the potential to boost solar cell efficiency (Nat. Photonics, doi: 10.1038/s41566-020-0664-3). The novel technique, a type of photochemical upconversion, allows light with sub-band-gap energies to be harvested by the solar cell instead of going to waste.

More work is still needed to increase the efficiency of the system, but the team has nevertheless reached a milestone by upconverting light with energy below the band gap of silicon—a challenge that has eluded the field up to this point.

Improving solar energy harvesting

Timothy Schmidt and his colleagues have been researching photochemical upconversion since 2007, driven by the tantalizing possibility of improving the energy harvesting of solar cells. Beyond photovoltaics, the technique also has applications for photocatalysis, photoelectrochemistry and biological imaging.

“Since silicon is the dominant solar technology, we were motivated to upconvert from beyond the silicon band gap,” said Schmidt, professor at the ARC Centre of Excellence in Exciton Science and UNSW Sydney, Australia.

In general, photochemical upconversion combines two types of molecules—a sensitizer and an emitter—to convert the energy of two photons into one photon of higher energy. The sensitizer absorbs the lower-energy photons, generating molecular triplet states. It then transfers the excitation energy to the lowest triplet state of the emitter, and these molecules interact to facilitate a process called triplet–triplet annihilation.

The result is one emitter molecule in an excited singlet state with the other quenched to its ground state. The excited emitter immediately fluoresces at a higher energy than that of the photons initially absorbed.

Successful upconversion of light

In the current study, lead sulphide nanocrystals served as the sensitizer molecules. They were placed in a solution that also contained oxygen and violanthrone, an organic compound known to photoluminesce in the presence of singlet oxygen molecules, as the emitter. Violanthrone was chosen for having a triplet state energy level around 0.98 eV, which is below the crystalline silicon band gap at 1.1 eV.

The researchers irradiated the solution with pulsed 1140-nm light filtered through a 200-micron-thick silicon wafer, which successfully resulted in an upconverted emission at 700 nm. Efficiencies are still low, limited by the harvesting of energy from the nanocrystals. However, Schmidt and his colleagues believe that this roadblock can be overcome by developing this technology in an optimized, solid-state device structure rather than a chemical solution.

“If we can make this efficient in a solid-state film, then we can apply this as a coating to the rear of silicon solar cells to harvest otherwise wasted light,” he said.