Inside every leaf and blade of grass, chlorophyll molecules power tiny chemical factories that transform the energy in sunlight into sugars trees and plants need to grow. Hoping to replicate that factory, NC State professors Jon Lindsey and Gregory Parsons are working on a solar cell technology that uses organic materials. The inorganic materials currently used in photovoltaic cells are expensive to process, Parsons says, and making them often produces harmful greenhouse gases as well. “Nature can collect energy from the sun cheaply,” he says. “Humans ought to be able to do it with cheap organic materials, too.”
When chlorophyll absorbs sunlight, a chain-reaction begins in which small packets of energy are passed from one molecule to the next. The chlorophyll molecules funnel the energy into reaction centers within cells, where it is used to turn water and carbon dioxide into sugar and oxygen. Lindsey, the Glaxo Distinguished University Professor of Chemistry, is creating molecules that mimic chlorophyll. The symmetrical, snowflake-like molecules, called porphyrins, have a metal atom at the core surrounded by four nitrogen atomssimilar to chlorophyll. Lindsey has experimented with different metals, such as zinc and magnesium, and also is working to chain the porphyrins together in different arrays. “Simply having chlorophyll in a plant is not enough,” Parsons says. “The molecules need to be arranged in a certain matrix for the energy to be transferred efficiently from one to the next.”
Getting that matrix in the right order to interact with two electrodes and complete an electrical circuit is Parsons’ task. A professor of chemical and biomolecular engineering, he assembles the porphyrins on an electrode through evaporative techniques. Based on test results, Lindsey then modifies the molecular structure to optimize light absorption and energy transfer.
Parsons is trying to develop electrodes that matchup well with the porphyrins to transfer electrical charges. “We can’t go uphill with the reaction. We need an energetically favorable transfer,” he says. Indium tin oxide (ITO) has so far provided the best results as one electrode because it provides a critical combination of conductivity and transparencythe latter allows sunlight to pass through to the porphyrins. At the other end of the circuit, he is working with buckyballs, carbon molecules that act as conductors. Although initial attempts at an organic solar cell have proven less efficient than existing photovoltaic devicesthey are even farther away from matching the chlorophyll-run factories inside plantsParsons says the concept will develop over time. “Mother Nature has had millions of years to perfect the system,” he says. “Humans are just getting started.”