Breakthrough: Semiconductor Materials Under Bending Can Generate Bigger Photovoltages Than the Theoretical Limit of a Standard Solar Cell

by Arlet Lozano Arenas

The discovery, made by a collaboration between scientists at ICN2 and the Universities of Nanchang, Fuzhou and Xi’an in China, opens up flexoelectric engineering as a promising pathway to enhance the efficiency of solar cells and other light-harvesting devices.

In the past two decades, halide perovskites have emerged as promising semiconductor materials for generating electricity when exposed to light, via the photovoltaic effect. The standard photovoltaic effect appears when charges are generated by light near semiconductor interfaces: positive charges go to one side of the interface and negative charges to the other, thereby generating a photocurrent and a photovoltage. This interfacial photovoltaic effect is behind all solar cells, but it has inherent physical limits. constraints and limitations in the way these semiconductors function in solar cells, and their properties are approaching these limits. In particular, the maximum voltage that can be achieved cannot exceed the so-called “forbidden band gap” of the semiconductor, which is the energy required to excite an electron from the valence band to the conduction band. Semiconductors must have small band-gaps to ensure that the photon energy of visible light can move electrons from the valence band to the conduction band, but these small band gaps mean that maximum photovoltages will also be small.

Therefore, further improvements in efficiency or performance are increasingly difficult to achieve through traditional physics, prompting the exploration of alternative physical effects, such as the bulk photovoltaic effect. This effect allows certain materials to generate photocurrents in their interior instead of at their interfaces, unlike in traditional solar cells. Crucially, the bulk photovoltaic effect is not limited by the band-gap, and can therefore generate high photovoltages. Unfortunately, the bulk photovoltaic effect can only exist in materials with an asymmetric crystal structure, which is not the case for most photovoltaic semiconductors. On the other hand, a symmetric crystal structure can be made asymmetric by bending, and this is the basis for the flexophotovoltaic effect, first discovered 6 years ago in oxide materials. The original flexophotovoltaic effect, however, was rather modest, generating photovoltages still smaller than the band-gap of the material and therefore no better than what can be achieved in standard solar cell technology.

Gustau Catalán, ICREA Research Professor and leader of the Oxide Nanophysics Group at Institut Català de Nanociència i Nanotecnologia (ICN2), in collaboration with a team of collaborators in China led by Prof. Longlong Shu of Nanchang University, has now unveiled that halide perovskites display a flexo-photovoltaic effect a thousand times higher than the previously studied oxide materials, and can reach photovoltages bigger than the band-gap and therefore bigger than those that can be achieved with the traditional photovoltaic effect.

The work has been published in Physical Review Letters, where it has been highlighted as an Editor’s Choice, and has further been selected for a Viewpoint feature in Physics Magazine. This distinction is usually reserved for articles that are considered important landmarks, which go on to receive on average more citations than papers published in such well-known journals as Nature or Science. Additionally, Physics Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. In this case, the Viewpoint article was written by Prof. Mingming Yang, the scientist who discovered the original flexophotovoltaic effect.

Future implications

The above-band-gap flexo-photovoltage suggests that adding flexoelectricity to solar cells can increase their open-circuit voltage and consequently the overall efficiency of the device. In halide perovskites, known for their already high photovoltaic efficiency, the flexo-photovoltaic effect may thus be able to raise the efficiency of perovskite solar cells to new heights. Moreover, while the bigger-than-bandgap effect has been demonstrated in halide perovskites, there is no a priori reason why it should be limited to these materials. Therefore, two parallel pursuits emerge from the present discovery: to engineer strain gradients into perovskite solar cells to break through the current efficiency ceiling, and to explore other semiconductors in search for large flexophotovoltaic effects.

All in all, both the flexo-photovoltaic effect and above-band-gap photovoltage offer promising opportunities for advancing both the field of halide perovskite optoelectronics, and the larger field of photovoltaics in general, leading to more efficient, flexible, and tuneable devices for applications in solar energy harvesting.

References:

Wang, Zhinguo; Shu, Shengwen; Wei, Xiaoyong; Liang, renhong; Ke, Shanming; Shu, Longlong & Catalán, Gustau (2024). Flexo-Photovoltaic Effect and Above-Band-Gap Photovoltage Induced by Strain Gradients in Halide Perovskites. Physical Review Letters. 132(8), 086902. DOI: https://doi.org/10.1103/PhysRevLett.132.086902

Yang, Mingming (2024, Feb. 20). Harness Strain to Harvest Solar Energy. Physics, 17(27). https://physics.aps.org/articles/v17/27