One of the main obstacles in making high-efficiency photovoltaic devices comes from inherent losses in a material that reduce its solar power-conversion efficiency (defined by the amount of solar radiation falling on the material divided by the delivered power that comes out). The power output of a solar cell is determined by how much current and voltage the cell can deliver to a load and choosing the right materials is key to achieving higher efficiency.

To do this, however, researchers need to use materials that both strongly absorb photons from sunlight and efficiently collect the charge carriers (electrons and holes) produced when photons are absorbed. The first requirement means that a typically thick layer (1 µm or more) containing quantum-well structures must be employed while the second requires a much thinner layer (0.2–0.3 µm) material if quantum wells are included in the design.

The key to the new work is an approach that reconciles these conflicting needs.

Slab waveguide
"We recognized that a quantum-well region located above an InP substrate creates a so-called slab waveguide thanks to the region being bounded on top and on the bottom by materials that have a lower index of refraction," Yu told "Photons can then propagate in a direction parallel to the solar cell surface, within the slab waveguide structure, and photons travelling along this path have a high probability of being absorbed due to the long optical path length within the quantum-well region."

Yu says that the approach works for even relatively thin multiple quantum-well layers, from which photogenerated carriers can be efficiently extracted. The trick then is to direct photons incident in a direction perpendicular to the waveguide. "We have shown that this can be accomplished by scattering from metal or dielectric nanoparticles that can very easily be deposited on the solar cell surface," he explained.

The more light that a cell can collect, the higher the efficiency and current the solar cell device will typically deliver. The nanoparticles aid in collecting more light, which leads to higher current densities and power conversion in Yu and colleagues' devices.

According to the team, the concept of light scattering into waveguide modes within a device could be applied to thin film photovoltaics and any such devices that do not allow for conventional anti-reflection layers to be deposited, like polymer-based solar cells.

Space applications
High-efficiency quantum-well solar cells would be ideal for space applications, where high efficiency and small size/low weight are crucial, or in terrestrial concentrator systems. Compared with the more well established multi-junction tandem cells, quantum-well solar cells and related devices are single-junction structures and are thus not subject to the current-matching conditions that plague multi-junction cells. This would be an advantage in applications in which the amount of solar radiation falling on a device varies with location, time and atmospheric conditions, for example.

The team is now looking into using even lower refractive index substrates to further improve light trapping efficiency.

The work was published in Applied Physics Letters.