This award on solar energy research is co-funded by the Divisions of Chemistry, Materials Research, and Mathematical Sciences of the Directorate for Mathematical and Physical Sciences. A collaboration of chemistry, physics, and applied mathematics groups at the University of North Carolina will explore the limitations of bulk-heterojunction polymer solar cells, which have been identified as economical and easy to manufacture. An interdisciplinary team will design, fabricate, and optimize photonic crystal solar cells that specifically address the disparate length scales in polymer photovoltaic materials, thereby confronting the major challenge in solar cell technology: efficiency. The aim is to achieve simultaneously an efficient absorption of photons with effective carrier extraction, but unfortunately the two processes have opposing requirements. Efficient absorption of light calls for thicker modules whereas carrier transport always benefits from thinner ones, and this dichotomy is at the heart of an efficiency/cost conundrum that has kept solar energy expensive relative to fossil fuels. The UNC approach, based on the advanced optical control possible with photonic crystals, enables efficient light capture by a photonic arrangement of nanostructures while significantly reducing the effective path carriers need to travel to reach contacts. Integrating an applied mathematics component into the team will enable rigorous and simultaneous optimization of both photonic crystal designs and carrier extraction pathways. Moreover, the optimized nanostructures will be scalable to large areal dimensions via a technique called PRINT (Pattern Replication In Nonwetting Templates), a recent breakthrough in roll-to-roll nanomolding pioneered in Chapel Hill.
NONTECHNICAL SUMMARY:
Polymer solar cells have demonstrable advantages such as ease and economy of fabrication, but to compete with the lower cost of fossil fuels they must attain higher efficiencies. A collaborative group of researchers at the University of North Carolina at Chapel Hill will use an interdisciplinary approach to optimize a novel type of photonic-crystal solar cell. Inspired by nature's own design, a design manifested in the beautiful iridescent colors of minerals, gems, insects, and butterflies photonic crystals enhance the absorption of light and are expected to increase solar cell efficiency. The unorthodox alliance of three disciplines at UNC (applied mathematics, chemistry and physics) will leverage rigorous mathematical modeling to optimize the design and fabrication of photonic crystal solar cells. Moreover, this atypical conjunction of a mathematician, a chemist and a physicist working together on the challenging problem of polymer solar cell efficiency will provide a unique educational environment for undergraduate and graduate students, one wherein the power of a diversity of backgrounds is emphasized in approaches to complex, interdisciplinary problems.
This SOLAR project dealt from its inception with two conflicting elements: electronic and optical properties of organic polymer solar cells. Both usually limit the performance of a solar cell. Solar cells absorb light within their active layer, creating electrical charges that migrate to opposite contacts and ultimately do useful (electrical) work in an external circuit. The distance the charges travel before reaching the contacts is limited by the probability of those charges finding each other (positve and negatives) and anihilating one and other in a so-called recombination process. So, in general, to capture light we need materials be as thick as necessary, but to minimize recombination we need the materials to be as thin as possible. The compromise is usually the highest efficiency cell one can achieve in a conventional planar, multilayer device. Although this limiting dichotomy is prevalent in all solar cells, in this SOLAR project we focused on attacking it for polymeric photovoltaics, which for a while appeared to be limited by poor charge transport. We introduced a photonic structure, i.e., a 3D surface structure that goes well beyond a simple flat cell. We did this to manipulate the light interaction with the photovoltaic polymer material in an attempt to overcome this general limitation of polymer based solar cells. We spent considerable effort on multiple fronts: characterizing charge transport and optical properties of many polymer materials, developing processes to create the 3D photonic surface structures, measuring the embossed planar photonic solar cells, and modelling in detail the process and impact of our structural mofications. We found that generally, our initial intuition was true and sound: A 3D photonic surface structure exhibits the best characteristics among all standard flat solar cell structures. We found however, that the degree of solar cell improvement could be quite limited or large dependeing of the exact extent of the trade -ff between optical enhancements and electrical transport characteristics. In the particular set of polymer blends we set ot use as a work-horse for our studies, we found the potential for improvement was not larger than a relative 10% improvement, i.e., if a polymer cell exhibited in the best flat-cell case a 4% overall photoconversion efficiency, a photonic surface cell would show an overall inprovement of only 4.4%. The effect in that case was clearly small. Too small to warrant the extra effort in nanofabricating the photonic crystal surface on the cell. We have thus fdelineated some overall design rules that allow researchers to predict the maximum improvement possible, and to judge whether the improvement is worth the extra nanofabrication steps. It is important to mention that our observed overall figures of improvement are taken from the entire universe of possible solar cell devices. In the technical literature it is common to find research work that claims improvements via photonic nanostructuring, but in most cases they are comparing observations with a substandard flat device. The guiding principles we have developed, are materials agnostic and are based on large scale comparisons, because for the real world what matters is the best and lowest cost device. In summary, the SOLAR project yielded experimental and modelling tools that allowed us to identify candidate material systems that could surpase silicon solar cell performance at a lower cost via 3D photonic surface structuring. We developed new nanostructuring methods and imparted experience to a new generation of young researchers capable of bringing novel cell designs to the marketplace.
