Study explores tunneling and hot electron collection to improve efficiency

Hot-carrier solar cells, a concept introduced decades ago, have long been considered a potential technological breakthrough in solar energy. These cells could surpass the Shockley-Queisser efficiency limit, which is a theoretical maximum efficiency for single-junction solar cells. Despite their promise, their practical implementation has encountered significant challenges, particularly in managing the rapid extraction of hot electrons across material interfaces.

Recent research has focused on using satellite valleys in the conduction band to temporarily store hot electrons before collection. However, experiments have revealed a parasitic barrier at the heterostructural interface between the absorber and extraction layers. This barrier complicates the transfer process, which occurs in real space rather than momentum space. When the energy bands of the two materials are not perfectly aligned, electrons can bypass this barrier by tunneling, a process influenced by complex band structures.

In a new study published in the Journal of Photonics for Energy The researchers studied these evanescent states and their impact on electron tunneling using an empirical pseudopotential method. This approach calculates energy bands in momentum space and aligns them with experimental data on critical points, providing insights into the physics that enables hot carrier extraction between carrier valley states and across heterointerfaces.

These results provide insight into the tunneling process and could pave the way for more efficient hot-carrier solar cells, bringing us closer to breaking the efficiency limits of current solar technology.

Specifically, the study showed that the tunneling coefficient, which measures how easily electrons can pass through the barrier, is exponentially high in indium aluminum arsenide (InAlAs) and indium gallium arsenide (InGaAs) structures due to the mismatch of the energy bands of these two materials. This problem is compounded by a slight roughness at the interface, only a few atoms thick, which severely hinders electron transfer. These results are consistent with observations of poor performance of experimental devices using this material system.

Interestingly, the situation improves considerably in a system comprising AlGaAs and gallium arsenide (GaAs) materials in which the aluminum composition of the barrier creates degeneracy in the lower energy satellite valleys. Such a system benefits from better alignment of the energy bands and the ability to grow with atomic precision.

For example, the tunneling coefficient for electron transfer between AlGaAs and GaAs can be as high as 0.5 or even 0.88, depending on the specific AlGaAs composition used. This suggests a much more efficient transfer process and the potential to exploit valley photovoltaics and realize solar cells beyond the current single band gap limits.

In high electron mobility transistors made from AlGaAs/GaAs, electrons typically move from the AlGaAs to the GaAs. However, hot carriers in the GaAs can gain enough energy to be transferred back to the AlGaAs, a process known as real-space transfer. While this is generally undesirable in transistors, it is beneficial for valley photovoltaics, where efficient transfer and storage of hot carriers is essential.