The Flexo-Photovoltaic Effect

An artists impression of squeezing more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells. (Source: M. Garlick, Univ. Warwick)

Physicists at the Univer­sity of Warwick have found a way to literally squeeze more power out of solar cells by physically deforming each of the crystals in the semi­conductors used by photo­voltaic cells. The Warwick researchers Marin Alexe, Ming-Min Yang, and Dong Jik Kim looked at the physical constraints on the current design of most commer­cial solar cells which place an absolute limit on their effi­ciency. Most commer­cial solar cells are formed of two layers creating at their boundary a junction between two kinds of semi­conductors, p-type with positive charge carriers and n-type with negative charge carriers. When light is absorbed, the junction of the two semi­conductors sustains an internal field splitting the photo-excited carriers in oppo­site direc­tions, generating a current and voltage across the junction. Without such junctions the energy cannot be har­vested and the photo-exited carriers will simply quickly recombine elimi­nating any electrical charge.

That junction between the two semi­conductors is funda­mental to getting power out of such a solar cell but it comes with an effi­ciency limit. This Shockley-Queisser limit means that of all the power contained in sunlight falling on an ideal solar cell in ideal condi­tions only a maximum of 33.7% can ever be turned into elec­tricity. There is however another way that some materials can collect charges produced by the photons of the sun or from else­where. The bulk photo­voltaic effect occurs in certain semi­conductors and insu­lators where their lack of perfect symmetry around their central point – their non-centro­symmetric structure – allows gene­ration of voltage that can be actually larger than the band gap of that material. Unfor­tunately the materials that are known to exhibit the anomalous photo­voltaic effect have very low power gene­ration effi­ciencies, and are never used in practical power-gene­ration systems.

The Warwick team wondered if it was possible to take the semi­conductors that are effective in commercial solar cells and mani­pulate or push them in some way so that they too could be forced into a non-centro­symmetric structure and possibly there­fore also benefit from the bulk photo­voltaic effect. They decided to try literally pushing such semi­conductors into shape using conductive tips from atomic force micro­scopy devices to a nano-in­denter which they then used to squeeze and deform indi­vidual crystals of strontium­titanate, titanium­dioxide and silicon. They found that all three could be deformed in this way to also give them a non-centro­symmetric structure and that they were indeed then able to give the bulk photo­voltaic effect.

“Extending the range of materials that can benefit from the bulk photo­voltaic effect has several advantages: it is not necessary to form any kind of junction; any semi­conductor with better light absorption can be selected for solar cells, and finally, the ultimate thermo­dynamic Shockley-Queisser limit of the power conver­sion effi­ciency can be overcome. There are engi­neering challenges but it should be possible to create solar cells where a field of simple glass based tips could be held in tension to sufficiently deform each semi­conductor crystal. If such future engi­neering could add even a single percen­tage point of effi­ciency it would be of immense commercial value to solar cell manu­facturers and power suppliers”, Marin Alexe said. (Source: U. Warwick)

Reference: M.-M. Yang et al.: Flexo-photovoltaic effect, Science, online 19 April 2018; DOI: 10.1126/science.aan3256

Link: Functional Electronic Materials (M. Alexe), University of Warwick, Coventry, UK

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