Energy Pooling as a Novel Thermodynamic Mechanism for Organic Photovoltaics.
DU physicist turns to strange quantum realm to boost energy yields in plastic solar panels
Sean Shaheen, a physicist at the University of Denver, says the world’s energy future can be found at the quantum mechanical level of reality – the subatomic world of very tiny particles and their strange behavior. Armed with a grant from Research Corporation for Science Advancement, America’s second-oldest foundation, Shaheen is attempting to prove his statement in a project that, if successful, would greatly enhance our ability to produce electricity directly from sunlight in organic photovoltaic panels – basically, cheap plastic solar collectors. “Sean’s work is a long shot,” says RCSA President and CEO James M. Gentile. “That’s why we’re supporting it. We can’t have real progress without taking some big risks in fundamental science.” First, some background on how solar energy works:
When sunlight strikes a solar panel, photons—the basic units of sunlight—are absorbed into the panel. To say they are “absorbed” in the photoelectric process means that certain electrons in the atoms that compose the solar panel are actually struck by the photons. When this happens, an electron absorbs the photon’s energy and becomes excited. Some atoms have many electrons, all of which exist in “shells” (the old word was “orbits”) around the atom’s nucleus, or center. Most solar energy panels today work because electrons in the outer shells of certain atoms aren’t held very tightly by their nuclei, and when photons strike these outer electrons, they instantly acquire enough extra energy to jump ship. To oversimplify a bit: if enough electrons abandon their nuclei and wander off, they can create electric current – free electricity from sunlight. This process can occur again and again, indefinitely. The most efficient solar panels today require rare minerals such as indium and tellurium. And those that function well with less expensive materials are often expensive to make. Costs are driven up because of the energy input required for manufacturing and the complicated procedures required to fabricate the devices, Shaheen notes. That‘s why his research is aimed at greatly reducing, or even eliminating, the need for expensive minerals and complicated manufacturing steps, all while improving the electrical properties of cheap plastic panels. He hopes to do so through a phenomenon called “energy pooling.” Basically, this pooling effect occurs when a number of excited electrons are collected in a single spot on a molecule. Shaheen predicts that pooling will increase a solar panel’s voltage (potential for energy), while reducing the proportional energy loss (as heat) inherent in the process of electrical current production. He hopes to do this by carefully adding “plasmonic nanostructures” to the plastic. More background:
A nanostructure is very tiny. One nanometer equals one-billionth of a meter, roughly the difference between the earth and a child’s marble. True nanostructures are usually defined as being around 100 nanometers or smaller. A “plasmonic nanostructure” is really just a tiny trap for photons. So tiny, in fact, that it is smaller than the wavelength of the photon. (In the strange world of quantum physics a photon is both a wave and a particle – a wavicle.) When photons get jammed into this tiny physical space, nearby electrons in the nanostructure are driven into a tizzy of excess energy and they congregate into an excited state called “plasma.” Think of plasma as a swarm of angry bees. A plasmon, or single unit of this plasma, is a mode of oscillation of the electrons that are confined in a nanoparticle. The exotic (and useful) behavior in this process is that the resonance of the electrons in the nanoparticle can be strongly coupled to electrons in the surrounding plastic molecules, forcing them to collectively "trap" or absorb many more photons coming from the sun. In fact, photons can be trapped hundreds or thousands of times faster by molecules in the vicinity of a nanoparticle due to plasmonic resonance. This is akin to one bee (the plasmon) exciting a whole slew of nearby bees to buzz collectively… The plasmonic energy generated in this way is not captured. The idea is for this energy to resonate with surrounding molecules to induce the molecules’ electrons to absorb more solar photons much more rapidly. Shaheen’s plasmonic studies are done in collaboration with colleagues at the University of Colorado at Boulder and the National Renewable Energy Laboratory. Organic – that is, plastic—solar cells have "amorphous" molecules, meaning that there is a lot of randomness in the positions and orientations of the molecules. This plastic molecular structure is much different from the more elegant “semiconductor” material found in many other electronic devices. Semiconductors generally have crystal-lattice atomic structures that seem perfect, or nearly so, for the creation and movement of stray elections. In other words, generating electricity from this inexpensive plastic material, as Shaheen hopes to do, is not easy. And never mind that, as Shaheen said recently, “I should point out that this work is purely theoretical and that no one has demonstrated this in any experiment.” In fact, nobody in science is doing a very good job of what he wants to do in this instance, namely controlling coherent (that is fixed and predictable) processes in quantum mechanics for solar energy production. And that’s precisely why RCSA is funding his project. RCSA President Gentile, noted that RCSA founder Frederic Cottrell once wrote: “Bet on the youngsters. They are long shots, but some of them pay off.” Over the past 100 years, Gentile pointed out, that philosophy has led RCSA to support fundamental science leading directly to space travel, lasers, MRI machines for peering into the living human body, so-called “wonder” drugs such as corticosteroids, nuclear medicine and atomic science, among many other modern achievements. Shaheen is funded through RCSA’s Scialog® program, which Gentile said is specifically designed to support “high-risk/potentially high-reward research on challenges of great global significance.” Developing clean, renewable sources of energy is one such urgent challenge, he added. Scialog is formed from the words “science” and “dialog.” The program requires Scialog fellows such as Shaheen to attend annual meetings where they are encouraged to discuss their most innovative and “far-out” ideas in a supportive environment, Gentile said. “Scialog is an experiment in accelerating the pace of breakthrough scientific discoveries,” Gentile said. “Creative researchers like Sean Shaheen represent the world’s best hope for a brighter tomorrow. RCSA is proud to support them.”