While silicon is the most commonly used solar energy converter due to its high energy conversion efficiency, it is expensive, inflexible and “not green” (silicon is produced from silicon dioxide in a chemical process that produces carbon dioxide). In contrast, organic photovoltaic cells made of synthetic polymers are greener and vastly less expensive, yet are also less efficient in power conversion, as Barnes points out.
“The basic technology for organic photovoltaics has been in place for more than 20 years; the thrust is to make solar devices with power conversion efficiencies comparable to silicon at a tenth of the cost,” says Barnes, Professor of Chemistry who specializes in molecular level interactions of light and matter.
There are two materials problems that need to be solved simultaneously to achieve the goals of low cost and high efficiency for polymer-based solar devices. First is the problem of intimate (molecular-level) contact between polymers that separate positive and negative charge carriers. Second is the problem of enforcing long-range order so that charges can be efficiently shuttled to respective electrodes (charge reservoirs) and electrical work can ultimately be done.
Venkataraman and Barnes compare the chemistry problem in front of them to “integrating spaghetti with meatballs.” Polymers form wire-like (or spaghetti-like) structures, in contrast with the spherical (or meatball-like) structure of the molecule C60, a pure-form carbon known for its robust structure. Thus, the polymers and C60, prefer to form ‘spaghetti-only’ and ‘meatball-only’ islands, which confounds both problems of charge separation and charge transport. The team is looking at how different molecular architectures influence the ability to enhance structural integration and analyzing how different morphologies affect photovoltaic efficiency. Finding a polymeric semiconductor that will simultaneously charge an electron to jump from its ground state and do external work and charge the vacancy left by the “excited electron” is key to solving the problem. To accomplish this feat the team is “tweaking” the interface between the charged vacancy, or “hole,” and the electron, which cannot be more than 10 nanometers. The polymeric features must accomplish each task perfectly in order for the photophysical process to succeed.
“We have to do some tricks at the molecular level to get the electron to come out,” says Venkataraman.
Venkataraman demonstrates with a black light how they have synthesized polymers that can harness energy and release it (or ‘waste’ it) in the form of fluorescent light. Once Venkataraman and his students have synthesized a polymer and examined its assembly, Barnes analyzes how the polymer interacts with light. This final stage is crucial as it determines how successful the polymer is and why.
“Mike’s [Barnes] part of this is one of the most important parts because without him we wouldn’t know how the molecules are behaving in this sort of state,” says Venkataraman.
And the implications are infinite. Venkataraman explains that photovoltaic cells are commonly affordable only with a government subsidy, yet a polymeric solution would make solar energy available at a fraction of the cost. It would also release solar energy from its current bulky box—quite literally. The synthesized nanoparticles that the group is researching are easily printed or painted onto the surfaces of a wide variety of structures, so the dense solar panels would no longer be necessary. A smart phone, for example, could be painted with the polymer. In 50 years, Venkataraman explains, we may be walking around with our own power-generating devices. He explains that the technology would greatly benefit third-world communities; an affordable means of powering homes would change daily life for millions who still live “off the grid.”
The ability to charge an electron and the hole simultaneously would also open up a new chemical world of possibilities. Venkataraman explains that the push for the research stems from the global need for renewable energy, yet it goes beyond one accomplishment. The desired polymer enables a kind of chemistry that we can only begin to imagine; it would altogether transform energy transport.
Venkataraman and Barnes, along with co-authors and frequent collaborators Associate Physics Professor Anthony Dinsmore and Bobby Sumpter of Oak Ridge National Laboratory, describe related findings in the Journal of Physical Chemistry Letters (2011).
This research is one of a number of projects underway at the UMass Amherst PHaSE Center (Polymer-Based Materials for Harvesting Solar Energy), an Energy Frontier Research Center funded in large part by the Department of Energy. The Center carries out fundamental photovoltaic research and aims to design optimal, less expensive photovoltaic devices.
The PHaSE Center structure is ultimately a polymer assembly line. The first research group is devoted to synthesis and design of light-harnessing polymers and is coordinated by polymer scientist Todd Emrick and chemist Paul Lahti. The second research group, led by Venkataraman (whose work encompasses all three areas) and polymer scientist Thomas Russell, focuses on controlled assemblies and morphologies within the polymers. The third research group, led by Barnes and chemical engineer Dimitrios Maroudas, is dedicated to photophysical characterization to understand how the polymers are absorbing light and integrate this information into efficient solar device functioning.
Venkataraman and Barnes explain that their work is one piece of the larger puzzle. Solar energy breakthroughs will require the hands of many researchers working together in an interdisciplinary way to push new developments through to application.
“A hundred years from now, I hope that solar cells are no longer a novelty but are part of everyday use,” says Venkataraman.
Amanda Drane '12
The desired polymer enables a kind of chemistry that we can only begin to imagine; it would altogether transform energy transport.