Pushing the boundaries of PV cell sensitivity

Some parts of the light spectrum go unused by many solar technologies, but scientists in Australia and the United States are pushing the boundaries of PV cell sensitivity, turning low energy light into more energetic, visible light that can excite silicon. The researchers have achieved this via photochemical ‘upconversion', with oxygen as the light conversion catalyst.

Although systems have been developed to upconvert light from photon energy in the near-infrared, upconversion from below the silicon bandgap has been out of reach. UNSW Sydney researchers – together with scientists from RMIT University and the University of Kentucky – recently explained in Nature Photonics that they have demonstrated an upconversion composition using semiconductor quantum dots to absorb low-energy light, and molecular oxygen to transfer light to organic molecules.

UNSW Professor Tim Schmidt said that one way to upconvert light is to capture multiple smaller energy photons of light and glue them together. “This can be done by interacting the excitons – bound states of electrons and electron holes that can transport energy without transporting net electric charge – in organic molecules,” Schmidt explained.

To extend the range of solar cell sensitivity, the researchers used oxygen, which is usually detrimental to molecular excitons. However, they demonstrated that oxygen can mediate energy transfer, allowing organic molecules to emit visible light, above the silicon bandgap.

“What’s interesting is that often without oxygen, lots of things work well. And as soon as you allow oxygen in, they stop working,” said Professor Jared Cole of RMIT University. “It was the Achilles heel that ruined all our plans but now, not only have we found a way around it, suddenly it helps us.”

The researchers have used PbS semiconductor nanocrystal sensitizers to absorb photons below the silicon bandgap and populate violanthrone triplet states below the singlet oxygen energy. Following energy delivery from two singlet oxygen molecules, the triplet-state violanthrone chromophores have luminesced in the visible spectrum.

Since the efficiencies are still very low, the scientists say there is quite a lot of materials development needed to see the technique used in commercial solar cells.

“Violanthrone doesn’t have the perfect photoluminescence quantum yield so the next step will be to look for an even better molecule. But I’m very hopeful and think that we can improve the efficiency quickly,” said lead author Elham Gholizadeh of UNSW Sydney.