Photonic crystal, or the “semiconductor of light”, has revolutionized photonics since its inception in 1986. Most of the applications involving photonic crystal rely on its ability to suppress electromagnetic density of states, leading to electromagnetic band-gaps. However, photonic crystals also have the unique capability of doing just the opposite i.e. to greatly increase the electromagnetic density of states. Our recent endeavor has been to harness the light-trapping capability of silicon photonic crystals through formation of densely-packed wave-interference assisted resonances over a broad range of frequencies. The central topic of this talk will be various light-trapping schemes we used in recent years to design high-efficiency, ultra-thin silicon solar cells. Although the room-temperature thermodynamic efficiency-limit of a single-junction silicon solar cell is close to 33%, the world-record conversion efficiency of the conventional cells has inched from 25% to 26.7% over the past 20 years. Moreover, the efficiencies of the commercial silicon cells are in the 15 - 20% range, way below in comparison to the research cells. In stark contrast to this scenario, our recent photonic crystal based cell-design has broken the longstanding ray-optics based Lambertian light-trapping limit (also known as the 4n 2 limit), paving the way for single-junction, flexible, thin-film silicon solar cells that are low-cost, amenable to mass-production, an order of magnitude thinner than the present- day world-record holding cell and most importantly, capable of yielding close to 30% efficiency. I will also briefly touch upon the future strategies that could boost the efficiency further beyond 30%. Last but not the least, the scope of this general idea of photonic crystal based light-trapping goes far beyond photovoltaics applications. In this context, I will share some conceptual insights into how similar light-trapping mechanism hints towards the realization of all-optical transistors and logic gates based on room-temperature Bose-Einstein condensation (BEC) of exciton-polaritons in a photonic crystal.
Sayak’s primary research area of interest is strong light-matter interaction in photonic crystals. He obtained his M.Tech from the Dept. of Electronics and Electrical Communication Engineering (specialization: RF & Microwave Engineering), Indian Institute of Technology Kharagpur in 2010. During his PhD from the Dept. of Electrical Engineering, Indian Institute Technology Delhi, he worked on Plasmonics. After completion of his PhD in 2016, he joined Prof. Sajeev John’s group at the Dept. of Physics, University of Toronto, as a postdoctoral fellow. Presently, he continues to work in the same group and carries on research on light-trapping and solar energy harvesting in silicon photonic crystals. He is also responsible for providing computational support to the experimental collaborators and is actively involved in designing of slanted-pore (SP) photonic crystals with SP 2 symmetry. He was the recipient of SPIE Best Student Paper Award in the 12 th International Conference on Fiber Optics and Photonics (held in IIT Kharagpur) in December, 2014. His paper (co-authored with Prof. John), “Designing High-Efficiency Thin Silicon Solar Cells Using Parabolic-Pore Photonic Crystals", was featured as Editor’s Suggestion in the April, 2018 issue of Physical Review Applied for illustrating a roadmap towards 28 ? 29% conversion efficiency (well beyond the current world-record efficiency of 26.7%) in silicon solar cells. Recently, his research on high-efficiency silicon photovoltaics, carried out jointly with his group members, has been the topic of the prestigious Robert Resnick Lecture Series: https://science.rpi.edu/physics/events/robert-resnick-lecture-series.