Improved materials stability and morphology in organic photovoltaic cells

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Plastic solar cells use organic or polymer semiconductor materials instead of more traditional materials, such as silicon or cadmium telluride. Efficiencies of plastic solar cells, also known as organic photovoltaics (OPV), have increased drastically over the last decade, from a few percent to over 12%, making their performance competitive with more established solar cell technologies. Plastic solar cells have the potential to significantly reduce the price of solar-generated electricity, but they currently suffer from relatively short lifetimes and low-cost production has yet to be fully demonstrated. The short lifetimes (compared to the industry standard of 20-25 years) are due to the use of unstable, reactive materials, and low-cost production would require both inexpensive semiconductor materials and fabrication techniques. This proposal aims to address these issues by 1) designing a scientific model of OPV electrical characteristics to improve our understanding of the operation and limitations of OPV. This model will guide our materials and device structure choices in the other two initiatives, 2) developing new molecules for solar cells that exhibit high efficiencies and long-term stability, and 3) developing new low-cost manufacturing methods using laminated films of plastic semiconductors. To guide our choices of materials and structures, it is important that we understand, from a first-principles physics point of view, the operation and fundamental limitations of OPV technology. These issues have been well-understood for traditional solar cells for many years, but the operation of OPV cells is significantly more complicated, and new models are required. Understanding these limitations will allow us to focus our efforts on properties of the solar cells where the most improvement can be realized. Every high-efficiency OPV cell uses a fullerene-derived molecule as the “electron acceptor”. Although a few non-fullerenes were used in early devices, the success of fullerenes due to their excellent electron transport, and high electron affinity, has made their use wide-spread. Unfortunately, there are several reasons why fullerenes are not suitable for a successful commercial OPV technology. First, fullerenes are very unstable in the presence of oxygen and light. The resulting photo-oxidation drastically diminishes their electrical properties, and degrades solar cell efficiency. Second, fullerenes require a lot of energy to produce, and a sustainable solar cell technology should require as little energy as possible, to minimize the energy payback time. Finally, fullerenes absorb light very poorly, so we must rely on the electron donor material to harvest the solar photons. By developing non-fullerene acceptors, we aim to produce stable, more sustainable, and more efficient plastic solar cells. The lowest cost methods for producing OPV cells are solution-based, where an “ink” containing the plastic semiconductor is used to coat the cell. Unfortunately, this limits the complexity (and therefore efficiency) of cells that can be produced, as trying to coat a second ink on top of an already-coated film will typically dissolve the first. We propose to develop new fabrication techniques, where complex, efficient, multilayer cells are produced by laminating semiconductor films, rather than ink-coating. In this procedure, the ink is used to coat a temporary surface called a stamp. The ink dries, and the film is transferred, or laminated, onto a second surface that contains previously deposited films. This dry-transfer process avoids the problems of dissolving earlier layers, and complex multilayer films can be formed. This technology will enable low-cost production of efficient multilayer OPV cells.