Ultrafast spectroscopy linking molecular and device properties of organic solar cells

Plastic- and biomaterial-based organic photovoltaic devices (OPVs) are an emerging field of research and technology targeted at complimenting existing energy sources. Despite the recent progress, the power-conversion efficiency of OPVs is still insufficient for commercialisation. A key bottleneck to OPV efficiency is the fact that photogenerated charges have to overcome multiple molecular-scale energetic barriers on their way to the electrodes. These barriers are induced by the electrostatic interactions between charge carriers and by the disorder in organic materials. To tackle the problem, researchers have been developing new materials with the optimised energy-level structure of molecular orbitals. However, recently it became evident that the molecular-level charge dynamics are poorly correlated with the overall OPVs power-conversion efficiency. This is because cell efficiency is a macroscopic quantity - an amalgamation of numerous microscopic effects and processes. Therefore, a clear discrepancy exists between molecular and device-scale understandings of OPVs. The existing paradigm of OPVs photophysics lends insufficient support for material and device optimisation. The proposed research is targeted at bridging the gap between molecular- and device-level approaches to OPVs characterisation. We will develop a novel experimental approach combining instrumentation of ultrafast infrared spectroscopy, coherent control, and photocurrent device-characterization techniques. In a typical experiment, the OPV device will first be irradiated with a light pulse mimicking sunlight. Subsequently, charge dynamic will be manipulated with low-energy infrared and/or terahertz photons. The resulting charge dynamics will be simultaneously detected by optical and electrical techniques. This new hybrid type of spectroscopy will resolve in time the free-energy landscape for photon-to-current conversion in the OPVs. Furthermore, the optical response will be separated into interface- and bulk-specific components by utilizing the surface-enhanced sun-frequency-generation spectroscopy. All data will be analyzed together to yield a unified multiscale picture of the cell performance, thereby, providing a comprehensive feedback for the material and device improvement.