Beyond Silicon: Solar power gets atom-thin

November 21, 2015

A physicist from sunny Barcelona who traps and bends light and a Yale engineering professor who makes atom-thin plastic solar cells have teamed up to deliver the sun’s energy towards new ends. Flexible, lightweight and nearly two-dimensional, the carbon-based materials they are developing could lead to products such as solar house paint and self-charging cars.

More than 60 years after Bell Laboratories introduced the first solar cell, Silicon continues to be the element most people think of when they think of solar power.  Indeed, as the price of solar panels falls, Silicon-photovoltaic (SPV) panels continue to proliferate on rooftops; solar power provides an increasing fraction of U.S. energy demand. Innovations in solar cell design, and the use of exotic transition metals, have produced an eight-fold increase in solar cell power conversion efficiency.  And yet, for all those advances, the U.S. Energy Information Agency reports that solar power accounted for only 0.4% of national energy production in 2014.

SPV has multiple inherent problems: ultra-high purity Silicon needed for manufacture must be mined transported and milled.   A 2012 study found long-term silica dust exposure associated with substantially increased mortality. SPV manufacture requires energy-intensive high temperatures and vacuum conditions. Panels are bulky, inflexible and lose efficiency when not trained properly on the sun.

Carbon — more specifically, graphene, a two-dimensional lattice of carbon molecules first isolated in the lab in 2003 – presents an alternative substrate to Silicon that could vastly expand solar’s contribution to the grid.  Graphene acts as a metal and as a semiconductor, making it a “semi-metal.” In addition to having a strength-to-weight ratio 207 times greater than steel, graphene can be manufactured at room temperature and normal atmospheric pressure.  It is cheap and abundant, and its honeycomb structure – mimicked by other two-dimensional materials discovered in the past three years — allows fine-tuning for desirable optoelectronic qualities.

Andre Taylor of the Yale School of Applied Science and Engineering is the maestro tuning those materials.  Working with Nilay Hazari of the Yale chemistry department, who synthesizes various organic and organo-metallic molecules for study, Taylor’s lab characterizes their properties and develops techniques for incorporating them in energy conversion and storage. Taylor and Hazari – recipients of a $100,000 seed grant from the Yale Climate and Energy Institute – published research findings in Nano Letters on a new method to precision coat tiny Carbon nanotubes with metal atoms, controlling their polarity, and making them more effective electrodes in nanoscale solar design. The “n-type” (negatively charged) Carbon nanotubes they developed are 450 times more efficient than previously existent technologies.  But even that dramatic advance, says Taylor, is just “one step further towards our goal of improving the efficiency of hybrid solar cells.”  Cells thin enough to work in clothing or flexible enough to coat buildings and vehicles are still a long way off.

While Taylor and Hazari develop more efficient materials and methods to convert photons into electricity, they needed to address the other side of the challenge, gathering more photons and retaining them long enough within an atom-thick structure to make the conversion.

They recruited Marina Mariano-Juste, recent recipient of a PhD in photonics, the study of light, to help. While at the International Center of Photonics in Spain, Mariano-Juste developed a 15% interest in a patent that describes how a fiber array atop a traditional solar cell captures incident light and reflects it more directly onto the active layer. The light-scattering fibers effectively replace the need for rotating a panel to keep it normal to the sun.

Supported by a Yale Climate and Energy Institute fellowship to work with Taylor for two years, Mariano-Juste now investigates how techniques similar to her patent can be applied in the microscopic world of Taylor and Hazari. “Whispering Gallery Modes,” an acoustic phenomena that describes how sonic waves travel along an appropriately curved arch, and total internal refraction, the means by which electromagnetic signals travel through optical cables, are two inspirations that guide her research.

On October 26 and October 28, Mariano-Juste and Taylor will each talk about their Yale Climate and Energy Institute-supported research.  The sequential talks are part of a series over the next five months that highlights the research results of YCEI postdoctoral fellows and their mentors and colleagues.  The talks are open to the public at Yale’s Klein Geology Laboratory on 210 Whitney Avenue in New Haven, CT.  Further details on the speakers series are on the Yale Climate and Energy Institute website at www.climate.yale.edu