Combining hot-carrier and multijunction solar cells increases efficiency, lowers cost

Conventional single-junction solar cells have an efficiency limit near 33 percent. Their fixed bandgap introduces a trade-off: a small bandgap material absorbs most photons but only harnesses a small amount of their energy, while a large bandgap material generates a large amount of energy per photon but cannot absorb low-energy radiation.

Multijunction solar cells overcome this problem using multiple subcells with decreasing bandgaps, allowing them to extract more energy per photon. These cells are the most efficient manufactured today, but they are expensive and have limited material combinations.

In contrast, hot-carrier solar cells attempt to absorb photons with a wide range of energies in a single junction, storing and recovering energy above the bandgap as temperature. This requires minimizing the thermalization in a material, a difficult task.

Giteau et al. introduced a solar cell with a thin top hot-carrier layer that absorbs high energy photons, while lower energy photons are transmitted to another subcell.

The team compared each individual technology to the combined cell using a detailed balance model.

“The model has one big adjustable parameter: the strength of the thermalization,” said author Maxime Giteau. “In a hot-carrier solar cell, the thermalization must be about three orders of magnitude lower than in conventional materials to achieve efficiencies similar to multijunctions. In the hot-carrier, multijunction solar cell, we only need two orders of magnitude reduction.”

Although reducing thermalization remains an open problem, this technology makes it more manageable. The synergistic design also allows for more flexible material combinations and has the potential to reduce costs of multi-junction solar cells.

The researchers are currently working on the development of prototype hot-carrier, multijunction solar cell designs.

Source: “Hot-carrier multi-junction solar cells: A synergistic approach,” by Maxime Giteau, Samy Almosni, and Jean-François Guillemoles, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0073274 .