Suppressed narrowband reflectance of nanopatterned silicon photovoltaic cells

Abstract

The increasing demand for efficient yet nonpolluting energy conversion technologies require the photovoltaic (PV) systems to have fine-tuned optical responses and suppressed thermalization. PV cells that are based on Silicon are commonly patterned via lithography and etching techniques to implement micro/nanoscale surface components to reduce their reflectance on a wide spectrum while enhancing their absorption of energies around and higher than its bandgap. In this way, the power output increases while increases in cell temperature (e.g., thermalization) is also expected. In this work, a nanopatterned Si PV cell is designed and optimized evaluating different surface nanostructures to suppress the reflectance only in the vicinity of Si bandgap energy, so the power output can be improved and the thermalization can be suppressed simultaneously. Two- and three-dimensional, periodic structures are simulated by finite-difference time-domain method and optimized via parameter sweep optimization technique. A figure of merit (FOM) is developed to compare the in-band and out-of-band front side reflectance. The results revealed that rectangular gratings provided higher FOM, thus better selectivity compared to triangular ones. Similarly, square prism nanostructures demonstrate better selectivity compared to pyramid structures. Rigorous correlation analyses revealed that the selectivity is more strongly correlated with the height than the width. It is demonstrated that with optimized square prism nanostructures, 20 % increase of the absorption of useful radiation is accompanied by a thermalization that is limited to 15 %. With pattern optimization, it is shown that the electrical power output can be improved without producing substantial increase in the cooling load of solar cells.