Enhanced energy harvesting from shadow-effect: mechanism and a new device geometry

Energy harvesting from shadow-effect is the generation of electrical power from a Schottky junction when a part of it is kept in shadow and the remaining under illumination. It has been recently invented in Au/n-Si junctions, where modulation of work function of the Au top electrode under contrasting illumination has been invoked to explain the effect. In this paper, a different physical mechanism for energy harvesting from shadow-effect in a Schottky junction is proposed that does not assume change in work function of the top electrode under illumination. The device, termed shadow-effect energy generator (SEG), is modelled as two parallel Schottky junction solar cells, one at the shadowed and the other at the illuminated part, connected with each other in a closed loop circuit through the Si substrate and the top electrode. To test the proposed mechanism, ITO/n-Si junction based SEGs have been fabricated. The values of open circuit voltage in the SEGs have been found to be matching with the difference of photovoltages of the two cells corrected for the potential drop across the Si substrate, that validates the proposed mechanism. To further corroborate the mechanism, the conventional SEG geometry has been modified by applying a continuous ohmic coating at the back of the Si substrates that bypasses the resistance of the Si substrate for current flow and results in higher open circuit voltage and short circuit current. Moreover, ITO/n-Si based SEGs have been found to produce higher output power density compared to that reported in Au/n-Si devices in both the conventional and the new geometry. Although the closed loop present in the equivalent circuit of the SEG devices lead to wastage of harvested energy, the ITO/n-Si SEG devices can nevertheless be used as self-powered sensor for light, object and movement detection as well as for producing electricity from contrasting illumination.