Bending for better photovoltaics

When light is absorbed by a material, the energy of the photons is typically transferred to electrons. This excites them to the conduction band, where they are free to move around. If there is any asymmetry in the system (for example, near an interface), the electrons will drift in the direction dictated by the asymmetry, thus establishing an electrical current. This is the basis of the photovoltaic effect utilized in solar cells.Some materials, such as ferroelectrics, are intrinsically asymmetric, and interfaces are not needed to generate a photocurrent. The photovoltaic effect in these materials is said to be a “bulk photovoltaic effect”. Unfortunately, most ferroelectrics are poor conductors of electricity, so their photocurrents are small. There is a third type of photovoltaic effect, however: the flexophotovoltaic effect. It consists in bending a material, thereby breaking its symmetry (one face is compressed while the other is stretched), thus allowing a bulk-like photovoltaic effect, even in materials that were centrosymmetric before bending.The flexophotovoltaic effect was discovered in 2018 in dielectric (insulating) materials. For it to be useful, however, it should exist in semiconductors, as they absorb more light and generate bigger photocurrents. This year, we have discovered that semiconductor halide perovskites display a flexophotovoltaic effect orders of magnitude bigger than reported for any other material so far, generating photovoltages even bigger than the semiconductor band gap, something impossible with the standard photovoltaic effect1. The work has been highlighted by Physics2, the wide-audience journal of the American Physics Society.The consequences of this discovery cannot be overstated: it means that strain gradients (such as those generated by bending) can be used to push the efficiency of solar cells beyond their current values, a possibility we have demonstrated in a follow-up paper3.