Study highlights improved efficiency for hot carrier solar cells

Study highlights improved efficiency for hot carrier solar cells

by Clarence Oxford

Los Angeles CA (SPX) Sep 25, 2024

Hot carrier solar cells, first proposed decades ago, have been a key focus for surpassing the Shockley – Queisser efficiency limit in single-junction solar cells. Despite their theoretical advantages, these cells have faced significant challenges, particularly in rapidly extracting hot electrons across material interfaces.

Recent studies have investigated using satellite valleys in the conduction band to temporarily store hot electrons before extraction. However, a parasitic barrier at the interface between the absorber and extraction layers has posed a major obstacle. This barrier complicates electron transfer, which occurs in real space rather than momentum space. When the energy bands between materials aren’t perfectly aligned, electrons bypass the barrier through a tunneling process, which is affected by complex band structures.

In a new study published in the ‘Journal of Photonics for Energy (JPE)’, researchers explored evanescent states and their impact on electron tunneling using an empirical pseudopotential method. This approach helped calculate energy bands in momentum space and aligned them with experimental data, offering valuable insights into the mechanics of hot carrier extraction between valley states and across material interfaces.

The research provides a deeper understanding of tunneling processes and could lead to more efficient hot carrier solar cells, potentially breaking the efficiency limits of current solar technologies.

The study specifically highlighted that the tunneling coefficient, which measures how easily electrons move through the barrier, is exponentially large in indium-aluminum-arsenide (InAlAs) and indium-gallium-arsenide (InGaAs) structures due to mismatched energy bands. Even slight roughness at the interface-just a few atoms thick-can severely hamper electron transfer, which aligns with observed performance issues in experimental devices using these materials.

However, the situation improves with aluminum-gallium-arsenide (AlGaAs) and gallium-arsenide (GaAs) structures. In these systems, aluminum in the barrier creates degeneracy in lower energy satellite valleys, leading to better energy band alignment and more efficient electron transfer. The tunneling coefficient in AlGaAs/GaAs structures can be as high as 0.5 to 0.88, depending on the aluminum composition, suggesting a significantly more efficient transfer process.

These findings hint at the potential for valley photovoltaics, which could allow for solar cells that exceed current single bandgap efficiency limits. In high-electron mobility transistors made from AlGaAs/GaAs, electron transfer from AlGaAs to GaAs is common, but hot carriers in GaAs can also gain enough energy to transfer back to AlGaAs-a process known as real-space transfer. While usually undesirable in transistors, this process is beneficial for valley photovoltaics, where efficient hot carrier storage and transfer are crucial.

Research Report:On the use of complex band structure to study valley photovoltaics: toward efficient hot carrier extraction