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Hybrid Material May Smash the Ceiling of Solar Cell Efficiency

hybrid material solar

Image credit: When light is absorbed in pentacene, the generated singlet excitons rapidly undergo fission into paris of triplets that can be efficiently transferred onto inorganic nanocrystals. Credit: Maxim Tabachnyk

In the process of photosynthesis, only 10% of the light plants receive from the sun is turned into usable hydrogen to help the reaction. Last summer, some scientists succeeded in reaching world-record 44.7% with a new cell in regard to laboratory efficiency, and their final goal would be 50%. However such record could be overtaken by a new hybrid material developed by Maxim Tabachnyk’s team from the University of Cambridge, was capable propelling solar cells with an efficiency of over 95%, by combination of both organic and inorganic materials. Their research result was published in the latest edition of Nature Materials.

As photons absorbed by solar cells, they would produce particles called excitons, which helped light and matter interac. There were two different kinds of excitons, spin-singlet excitons were visually bright and easy to capture by solar cells, while spin-triplets were visually ‘dark’ and difficult to harvest. In this case, spin-triplets could generate two electrons for every photon for the maximum of efficiency.

An inorganic material, silicon was more likely to be used within

solar cells, even if it might not be the one which was most efficient. However researchers got a lot of experience in making circuits with it, which would push forward the development process much easier. As silicon cells could just absorb spin-singlet excitons, they would obtain one electron per photon absorbed.

Pentacene, as an organic molecule incorporated with five fused benzene rings being discovered in leaves, was easily capable of absorbing photons to go for efficient spin-triplet excitons. But it was not very good at capturing the electrons when they were used. The solution found by Tabachnyk’s was the combination of the two materials for best possible result.

According to Tabachnyk, the key to producing a better solar cell was attributed to extraction of the electrons from these dark triplet excitons. If they succeeded in combining pentacene with traditional semiconductors such as silicon, they would be able to make a break-through in great enhancement of the solar cells’ efficiency.

In the experiments of Tabachnyk’s team, such hybrid material could ensure the organic pentacene to absorb the dark triplets and then turn them soon to an inorganic semiconductor. By application of laser spectroscopy generating a pulse of light every few femtoseconds (10-15 seconds), the team discovered that the material with efficiency of 95% could transfer the triplets to the semiconductor, where the electrons could be captured.

The aim of the team’s further research was targeted on finding the approach to apply their work to produce the hybrid systems more effectively. For the time being, they were making efforts to develop a cheap organic coating, which could absorb the dark triplets and get them along to a silicon solar cell that would collect the energy so as to increase the system’s efficiency.

As Akshay Rao, the project’s principal investigator said, the combination of organic semiconductors characteristic of low cost and easy processing, with more efficient inorganic semiconductors would allow scientists to much improve inorganic solar cells’ efficiency, such as those made of silicon.

Source: University of Cambridge

Journal reference: Tabachnyk, Maxim, et al. “Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals.” Nature materials (2014).