Strategies for tapping the energy from hot carriers

For photovoltaics and solar fuels, some electrons, holes or electron-hole pairs are created upon the absorption of photons with energies greater than the bandgap of the material. These carriers, known as “hot” carriers, possess excess energy that is usually not utilized for the final power output, which limits the power conversion efficiency (PCE) of solar cells.

Arthur Nozik provides an overview of the different strategies for making use of the energies in hot carriers for maximizing PCE for photovoltaics and solar fuels.

The PCE can be increased by different mechanisms either by extracting the energy from the hot carriers before they cool to their bandgap energy, or by using the excess energy to create additional carriers through carrier multiplication.

Nozik categorized the approaches according to one of three potential operating principles. The first utilizes quantization effects in semiconductor nanostructures to slow the cooling of hot carriers by the introduction of phonon bottlenecks. The second explores the unique properties of certain bulk semiconductors, such as those semiconductors with wide phononic gaps. The final approach takes advantage of carrier multiplication, which is enabled in quantized semiconductors by multiple exciton generation or by singlet fission that occurs in certain molecular chromophores.

According to Nozik, although the progress in the development of photovoltaics, for example in silicon-based solar panels, has been dramatic over the past decades, the development of solar fuels technology has been much slower. Solar fuels, which are combustible fuels generated using sunlight by, for example, splitting water into hydrogen and oxygen, have advantages over photovoltaics for critical applications.

“It is essential to make PCE as high as possible,” said Nozik.

Source: “Quantization effects in semiconductor nanostructures and singlet fission in molecular chromophores for photovoltaics and solar fuels,” by Arthur J. Nozik, Chemical Physics Reviews (2021). The article can be accessed at https://doi.org/10.1063/5.0028982 .