Alok P. Vasudev

Semiconductor nanowire optical antenna solar absorbers.

Photovoltaic (PV) cells can serve as a virtually unlimited clean source of energy by converting sunlight into electrical power. Their importance is reflected in the tireless efforts that have been devoted to improving the electrical and structural properties of PV materials. More recently, photon management (PM) has emerged as a powerful additional means to boost energy conversion efficiencies. Here, we demonstrate an entirely new PM strategy that capitalizes on strong broad band optical antenna effects in one-dimensional semiconductor nanostructures to dramatically enhance absorption of sunlight. We show that the absorption of sunlight in Si nanowires (Si NWs) can be significantly enhanced over the bulk. The NWs optical properties also naturally give rise to an improved angular response. We propose that by patterning the silicon layer in a thin film PV cell into an array of NWs, one can boost the absorption for solar radiation by 25% while utilizing less than half of the semiconductor material (250% increase in the light absorption per unit volume of material). These results significantly advance our understanding of the way sunlight is absorbed by one-dimensional semiconductor nanostructures and provide a clear, intuitive guidance for the design of efficient NW solar cells. The presented approach is universal to any semiconductor and a wide range of nanostructures; as such, it provides a new PV platform technology.

Electrically Controlled Nonlinear Generation of Light with Plasmonics

A plasmonic structure is used to electrically produce frequency-doubled light. Plasmonics provides a route to develop ultracompact optical devices on a chip by using extreme light concentration and the ability to perform simultaneous electrical and optical functions. These properties also make plasmonics an ideal candidate for dynamically controlling nonlinear optical interactions at the nanoscale. We demonstrate electrically tunable harmonic generation of light from a plasmonic nanocavity filled with a nonlinear medium. The metals that define the cavity also serve as electrodes that can generate high direct current electric fields across the nonlinear material. A fundamental wave at 1.56 micrometers was frequency doubled and modulated in intensity by applying a moderate external voltage to the electrodes, yielding a voltage-dependent nonlinear generation with a normalized magnitude of ~7% per volt.

Nanophotonic light trapping with patterned transparent conductive oxides

Transparent conductive oxides (TCOs) play a crucial role in solar cells by efficiently transmitting sunlight and extracting photo-generated charge. Here, we show how nanophotonics concepts can be used to transform TCO films into effective photon management layers for solar cells. This is accomplished by patterning the TCO layer present on virtually every thin-film solar cell into an array of subwavelength beams that support optical (Mie) resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. We first demonstrate these concepts with a model system consisting of a patterned TCO layer on a thin silicon (Si) film and outline a design methodology for high-performance, TCO-based light trapping coatings. We then show that the short circuit current density from a 300 nm thick amorphous silicon (a-Si) cell with an optimized TCO anti-reflection coating can be enhanced from 19.9 mA/cm2 to 21.1 mA/cm2, out of a possible 26.0 mA/cm2, by using an optimized nanobeam array. The key differences and advantages over plasmonic light trapping layers will be discussed.

Light Trapping for Solar Fuel Generation with Mie Resonances

The implementation of solar fuel generation as a clean, terawatt-scale energy source is critically dependent on the development of high-performance, inexpensive photocatalysts. Many candidate materials, including for example α-Fe2O3 (hematite), suffer from very poor charge transport with minority carrier diffusion lengths that are significantly shorter (nanometer scale) than the absorption depth of light (micrometer scale near the band edge). As a result, most of the photoexcited carriers recombine rather than participate in water-splitting reactions. For this reason, there is a tremendous opportunity for photon management. Plasmon-resonant nanostructures have been employed to effectively enhance light absorption in the near-surface region of photocatalysts, but this approach suffers from intrinsic optical losses in the metal. Here, we circumvent this issue by driving optical resonances in the active photocatalyst material itself. We illustrate that judiciously nanopatterned photocatalysts support optical Mie and guided resonances capable of substantially enhancing the photocarrier generation rate within 10-20 nm from the water/photocatalyst interface.