My research

My research effort is tackling the problem of the limited efficiency of light-to-electricity converters, with the aim of developing a new class of efficient and cost effective devices.

Actually, the conversion of light into electricity is one of the prominent and diffuse way in which our society can harvest energy in a renewable way, and it is at the base of many everyday technologies. However, because of some fundamental limitations, the efficiency of these technology is bound to low values, preventing their wider diffusion. New solutions are urgently needed to overcome these limitations in order to step into a new generation of light harvesters.

It is clear that at the core of the problem there is the not optimal design of nowadays devices, based on p-n junctions, in which carrier thermalization dominates energy losses. A new device architecture based on different effects has to be outline. My research goal is to find a solution to this need.

Strain induced photovoltaic effects represent one of newest and exciting way to overcome most of the issues related to p-n junctions. My intention I want to explore the potential of this new class of effects in the field of light harvesting.

Recently, in the framework of my Marie Curie Individual Fellowship I tackled the problem of heat losses in solar cells, by their hybridization with thermoelectric generators (see the project outcomes). However, this is in some extent a very rude way of tackling the problem of heat losses in solar cells. Essentially it is not a way to solve the problem, but more a strategy in which is possible to reconvert part of the losses into useful power. The question is, there is a more elegant and effective way in which is possible to avoid heat losses in solar cells?

This leads to the need of basically re-thinking the solar cell architecture from the very basis. The surface photovoltaic effect, based on p-n junctions, on which the major part of the commercial solar cells are designed has inherently some limitations. Actually, the conversion of light into electricity requires the presence of an internal electric field able to separate the carries generated within the absorbing material. In nowadays solar cells the different concentration of free carriers in the n and p region leads to a carrier diffusion, inducing the generation of built in potential across the material interface. This potential can separate photo-generated carriers and drive them towards the device electrodes. The width of the interface region (the so called depletion region) can vary among different materials but is normally in the order of the micron. However photo-generated carriers, while driven towards the electrodes, tend to loose most of their energy in a much shorter length scale (around 70-100 nm). This carrier thermalization, which is at the origin of heat losses in solar cells cannot be avoided in p-n junction solar cells, and sets their maximum efficiency to a value around 30%.

Thus the question becomes, it is possible to think effective photovoltaic beyond p-n junctions? This is the big question that I want to face and possibly solve in my future research carrier.

One possible way comes from the so-called bulk photovoltaic effect, normally happening in non-centrosymmetric materials, in which a spontaneous electric polarization can be able of separating photo-generated carriers. The advantage of this approach is the fact that a proper material engineering can lead to the generation of a nanoscale built-in potential, able to collect photo-generated carriers before thermalization. This can potentially lead to higher efficiencies than p-n junction solar cells, and the demonstration of its principle was already reported in literature. However, non-centrosymmetric materials have the problem to have wide energy gaps, meaning a small absorption matching with the solar spectrum. Consequently the achievable efficiencies are very small, and without any potential applicability.

Recently a new kind of bulk photovoltaic effect based on strain-induced photovoltaic effect was reported. Strain induced PV effect is a symmetry-breaking phenomena for which an electrical polarization is developed within a material subjected to a strain. This polarization can be deployed to separate photo-generated carriers and can be in principle designed to overcome carrier thermalization. In addition because this effect is present in all materials, without the need of a p-n junction, the deployment of a strain induced PV effect can potentially overcome all the problem of nowadays technologies and simplify the device architecture. Nevertheless, even if the application of flexoelectricty to photovoltaic has been already proven, the real potential of this solution, along with a complete theoretical understanding is still mostly unexplored.

To my perspectives strain applied to photovoltaic has a great potential and can in principle be a future breakthrough technology. The universality of this effect is its great advantage. Strain induced polarization has already been shown to happen even in proteins, and to contribute in bones and hair repair mechanisms. This ubiquity promises the possibility of new kind of efficient and cost-effective light harvesters.

In this context my intention to explore the potential of strain induced polarization, applied in conversion of light to electricity in the spectral range from near-infrared to near ultra-violet. I will mainly do it designing and developing devices and system deploying strain induced polarization, and characterize them experimentally. I also want to study this subject form a theoretical point of you, contributing to the understanding of this phenomena still to be fully comprised.