MIT researchers use heat to improve efficiency of silicon-based PV cells

A nanophotonic solar thermophotovoltaic device composed of an array of multi-walled carbon nanotubes as the absorber, a one-dimensional silicon/silicon dioxide photonic crystal as the emitter, and a 0.55 eV photovoltaic cell.

Researchers at the Massachusetts Institute of Technology (MIT) have developed a new approach to harvesting solar energy that could improve efficiency by using sunlight to heat a high-temperature material whose infrared radiation would then be collected by a conventional PV cell. This technique could also make it easier to store the energy for later use, the researchers say. The technology would improve harvesting of solar energy by taking advantage of wavelengths of light that are not ordinarily used by a conventional silicon-based solar cell.
To address the limitation of conventional silicon-based cells, the MIT researchers inserted a two-layer absorber-emitter device – made of novel materials including carbon nanotubes and photonic crystals – between the sunlight and the PV cell. This intermediate material collects energy from a broad spectrum of sunlight, heating up in the process. When it heats up, it emits light of a particular wavelength, which in this case is tuned to match the bandgap of the PV cell mounted nearby.
In theory, such solar thermophotovoltaic (STPV) systems could provide a way to circumvent the theoretical limit on the energy-conversion efficiency of semiconductor-based PV devices. That theoretical limit – the so-called the Shockley-Queisser limit –imposes a cap of 33.7% on the conversion efficiency of conventional silicon-based cells. However, MIT associate professor of mechanical engineering Evelyn Wang says that with TPV systems, »the efficiency would be significantly higher – it could ideally be over 80%.«
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