Department of Chemistry

Research

Our research considers fundamental questions of optical energy conversion relating to plasmonic and inorganic nanoscale materials. Our experiments are principally designed to identify and optimize unique nanoscale phenomena useful for solar energy conversion, as well as related opportunities at the intersection of nanophotonics and chemistry for broad application beyond the scope of solar energy.

The current world record solar cell operates at 44.4% power conversion efficiency. Thermodynamic analyses indicate that much higher efficiency is theoretically possible. Indeed, technical challenges, rather than laws of nature, limit current solar power convertors from achieving the maximum thermodynamic efficiency of 95%.

  • We explore how nanofabricated optoelectronic and plasmonic materials can provide systematic control of the thermodynamic parameters governing optical power conversion for optimization that can shape, confine, and interconvert the energy and entropy of a radiation field.
  • We employ optical and electrical characterization techniques with high spatial and energy resolution to probe optical excitation and relaxation mechanisms in nanostructured metals and semiconductors.

Shockley and Queisser pointed out that the maximum efficiency for a single junction photovoltaic solar cell is 33%. Their calculation is based on several assumptions about the microscopic process of absorption and re-emission of radiation by the solar cell. We explore how optical nanomaterials may modify these underlying assumptions to provide opportunities for achieving even higher conversion efficiencies.

 

Anisotropic Optical Emitters

Semiconductor nanorods have the unique ability to rectify light into a solar cell by changing the angles of light entering or leaving the device. We aim to define the maximum benefits provided by this effect.

 

Nanoparticle Heterostructures

We are interested in using more complex doped or heterogeneous, highly fluorescent nanoparticles to promote luminescent up-conversion – that is, to absorb two low energy photons and emit one higher energy photon.

 

Fundamentally plasmon oscillations are an opto-mechanical phenomenon. Incident optical fields induce the coherent motion of electrons that produce large fluctuations of charge density at ‘hot spots’ in resonant structures. We are interested to know if this microscopic electronic motion can provide useful electrical or chemical work directly – without the use of semiconductors or other non-plasmonic structural elements. Our ultimate vision is an all-metal solar cell.

 

Reaching High Temperatures Through Plasmonic Metal Films

What is the highest temperature a material can reach when exposed to sunlight? We are determining if the remarkable thermal and optical energy concentration provided by plasmonic resonances can enable new hybrid semi-thermal power cycles, whereby photo-excited ‘hot electrons’ and resonant photothermal heating provide a dual excitation mechanism for electron transfer phenomenon.

 

Plasmonic Tunneling

Can sunlight induce a tunneling current and therefore provide a new way to achieve solar energy conversion? A fascinating phenomenon in quantum mechanics is tunneling. Even when electrons do not have sufficient energy, they can still “tunnel” through a potential barrier and reappear on the other side. We are exploring how plasmonic nanostructures can promote electron tunneling when excited with light, due to the strong electric field enhancement.

 

The Inverse Faraday Effect

Due to the inverse Faraday effect (IFE), circularly polarized light can induce static magnetic fields and drift currents that are enhanced by plasmonic resonances. We are studying the fundamentals of this phenomenon and exploring opportunities for optical-to-electrical energy conversion.