Professor S Hall
The average efficiency of a solar PV module is between 6 and 15%. Hence, 85-94% of the solar radiation is rejected as waste heat at about 333k. An alternative approach to photovoltaics is the rectenna concept, which is a combination of a receiving antenna and a rectifier. The sunlight is collected in the antenna as an alternating current and rectified to direct current (DC). The rectenna concept has been successfully demonstrated for microwave power transmission with high efficiency of ~84%.
The capture and conversion of solar energy >200THz and therefore efficiency of rectennas in the infrared and visible regime, is currently limited by the lack of a diode nanostructure that can work at THz frequencies. High-frequency response of semiconductor diodes, like p-n junctions or Schottky, is limited by charge storage and parasitic capacitance respectively, making them inoperable for rectification at frequencies beyond 5 THz.
The practical rectifying mechanism proposed is based on a tunneling effect of electrons having fast response times of the order of femto-seconds. The device is based on a Metal Insulator Insulator Metal (MIIM) structure, where electrons tunnel from one metal electrode to the other under forward bias, with the aid of a 'stepping stone' offered by a notch between the two dielectric (insulator layers) resonant tunnelling. In the reverse bias direction, electrons have to tunnel directly through the combined thickness of the two dielectrics making the 'off current' much smaller. Another possibility is to use a ferroelectric dielectric between two unbiased metal electrodes (MFM), with which the spontaneous polarization inside the ferroelectric dielectric would force the electron to tunnel in one direction but not the other. The dielectric layers are a few nms thick only and are realised by atomic layer deposition (ALD). The project is aimed at solar energy harvesting but is also of interest in the general area of THz electronics, at lower THz frequencies.