Capturing more of the Sun’s energy to improve photovoltaic cells

October 29, 2012

SEM image of a four-layer antireflective coating on a silicon substrate (credit: Martin F. Schubert et al./Appl. Phys. Express)

Photovoltaic cell efficiency may soon get a big boost from new materials that capture more of the Sun’s energy.

Professor E. Fred Schubert, of Rensselaer Polytechnic Institute‘s Department of Electrical, Computer, and Systems Engineering, is investigating new ways to achieve four-layer antireflection transparent thin-film materials that capture more of the Sun’s energy by achieving a low refractive index (how much light is bent).

These tunable-refractive-index materials are based on nanoporous silicon dioxide (SiO2), indium-tin oxide (ITO), and titanium dioxide (TiO2), and we can precisely control porosity by using oblique-angle deposition – a technique in which the substrate is at non-normal angle of incidence with respect to the deposition source,” says Schubert.

Meanwhile, scientists from the National Renewable Energy Laboratory (NREL) have demonstrated the first solar cell with external quantum efficiency (EQE) exceeding 100 percent for photons with energies in the solar range. (The EQE is the percentage of photons that get converted into electrons within the device.)

While traditional semiconductors only produce one electron from each photon, nanometer-sized crystalline materials such as quantum dots avoid this restriction. The efficiency increase comes from quantum dots harvesting energy that would otherwise be lost as heat in conventional semiconductors. The amount of heat loss is reduced and the resulting energy is funneled into creating more electrical current.

By harnessing the power of a process called multiple exciton generation (MEG), the researchers were able to show that on average, each blue photon absorbed can generate up to 30 percent more current than conventional technology allows. MEG works by efficiently splitting and using a greater portion of the energy in the higher-energy photons.

The researchers for both of these new developments will present their findings at the AVS 59th International Symposium and Exhibition, held Oct. 28 — Nov. 2, in Tampa, Fla.