Comment on “Energy harvesting from shadow-effect” by Q. Zhang, Q. Liang, D. K. Nandakumar, S. K. Ravi, H. Qu, L. Suresh, X. Zhang, Y. Zhang, L. Yang, A. T. S. Wee and S. C. Tan, Energy Environ. Sci., 2020, 13, 2404

Abstract

In their recent paper published in Energy and Environmental Science (DOI: 10.1039/D0EE00825G), Zhang et al. demonstrated that if a part of the Au/n-Si Schottky junction is kept in shadow and the remaining under illumination, a potential difference is generated between the illuminated and the shadowed part. The authors explained the effect by invoking modulation of the work function of the Au electrode under illumination contrast. In this comment, we provide an alternative physical mechanism for the observed effect by considering the Schottky junction under illumination contrast to consist of 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, we have fabricated ITO/n-Si and Au/n-Si Schottky junction devices. The values of open circuit voltage in the devices under illumination contrast are found to be matching with the difference of photovoltages of the two cells corrected for the potential drop across the Si substrate, which validates the proposed mechanism. To further corroborate the mechanism, the resistance of the Si substrate is short-circuited using ohmic contacts, which results in higher open circuit voltage and short circuit current from the device under illumination contrast. Although energy can be harvested from illumination contrast in a Schottky junction following the methods of Zhang et al., the presence of a closed loop in the equivalent circuit of the devices would lead to wastage of harvested energy.