How do Plastic Solar Cells Work?

1. University of Washington Ginger Research Group How do Plastic Solar Cells Work? The various forms of organic photoactive layers: Absorption: Photo-active materials capture and transfer energy from light particles to charge particles. Charge Transfer: Energized charges become useful for generation of electricity. Charge Collection: Free charges must overcome many obstacles to ‘find’ electrodes, before they can produce electricity. Stacked (bi-layer heterojunction) The photo-active material is sandwiched between two metal electrodes. Blended (blended heterojunction) transparent metal electrode Inside the photo-active layer Finger (interdigitated heterojunction) photo-active layer metal electrode How Can We Make Them More Efficient? (% Efficiency) = (% Absorption) x(% Charge Transfer) x(% Charge Collection) 1 2 3 Collect more of the free charges Increase charge transfer efficiency Better light absorption The finger structure ( ) allows for , and , but has not yet been realized due to the challenge in creating such a highly ordered structure at the nanoscale. The blended structure ( ) increases the surface area for efficient charge transfer, allowing for & , but will negatively affect . More light is absorbed as it penetrates into thicker mediums, but this can affect . Current Mode 20 nA Topography The atomic force microscope (AFM):The AFM is sensitive to forces between charges, atoms, and molecules. The microscopic tip of this cantilever allows us to “see” the surface of materials with +5 nm 0 nA Ginger Lab, UW -5 nm 2 µm Waste-not want-not: Using more of the solar energy spectrum. We are currently investigating techniques for guided phase separation (structuring of solid materials that are deposited in solution). The AFM images above shows surface height as well as electrical current conduction, which can be measured simultaneously. The AFM and various spectroscopic techniques (those in which we observe the interactions between light and matter and how they vary with the energy of photons) allow us to investigate the chemistry which is responsible for the ordering of materials. resolution in the nanometers. The AFM has lead us to find that the interactions between materials (charge transfer) in organic solar cells, and specifically the way in which they are structured at the nanoscale, are of utmost importance in efficiently generating electricity from sun light. Quantum dots By confining the size of these semiconductor crystals to nanometers, we can choose what color of light they absorb (or emit). What is Nanotechnology? Spectroscopy is the study of interactions between light and matter. Photoinduced absorption spectroscopy can show us which materials (such as quantum dots) yield in blended structures: Nanotechnology is science of phenomena which occur at length scales which are 100’s of nanometers or smaller. Nanoprisms Similarly, nano-scale chemistry also allows for selective scattering of light, which can be used to capture more light for other absorbing materials. Physics and chemistry haven’t changed, but the tools which are now available to us have. Using nanotechnology, we can solve big problems using little things. 1 nanometer = 1 billionth of 1 meter, which is about 70,000 times smaller than the width of human hair. Disclaimer: Elvis’s hair is would not be considered as nanotechnology. We can also use this technique to check for when incorporating our nanoprisms. 100 nm 100 nm This poster includes work supported by the National Science Foundation (DMR-0120967, DMR-044942, the Graduate Research Fellowship Program, and the Integrative Graduate Education and Research Traineeship), the Air Force Office of Scientific Research, the Department of Energy, the Camille Dreyfus Teacher-Scholar Awards Program, the Research Corporation, and the Alfred P. Sloan Foundation. The Ginger Research Group (of the Department of Chemistry, University of Washington) also thanks the Center for Nanotechnology at the University of Washington for sponsoring us at Shoreline Solarfest 2009.