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45293-G10
Revealing Microscopic Nature of Light-Induced Defects in Hydrogenated Amorphous Silicon Using Ultrafast Difference 2D IR Spectroscopy
We have used funds from the ACS Petroleum Research Fund to initiate an entirely new area of ultrafast infrared spectroscopy research targeted at organic solar cells. We originally proposed to study defect formation in hydrogenated amorphous silicon. However, we found the study of electron and excitation transport in organic solar cells to be a much more promising direction. In particular, funds from the award have been used to equip a materials processing laboratory to prepare thin films of organic photovoltaic (PV) materials.
We have demonstrated our ability to study excitation and electron transport to and from interfaces in polymeric photovoltaic materials that are used in organic solar cells. The materials are composed of electron accepting functionalized fullerenes (C60) that when blended with a conjugated polymer form 50 nm nanospheres surrounded by layers of the polymer. Using our methods, we excite the polymer layers with an ultrashort visible light pulse and induce electron transfer to the fullerene nanospheres. We exploit the sensitivity of the vibrational frequencies of materials to the proximity of interfaces in order to examine excitation and electron transport to and from the fullerene nanospheres. In particular, we find that the vibrational frequencies of the functionalized fullerenes shift to higher frequency when the molecules are close to the polymer layers. This sensitivity allows us to directly observe the movement of electrons as they diffuse from the interfaces into the centers of the nanospheres. In conjunction with SEM imaging, our methods allow us to determine the intrinsic mobility of electrons which provides insight into the fundamental dynamics that determine the efficiency of organic solar cells.
The sensitivity of vibrational frequencies to the presence of excited electronic states also enables us to measure the diffusion of excitons in the polymer layers that form when they absorb visible light. The rate of electron transfer into the fullerene nanospheres is limited by the diffusion of excitons to the interfaces. By measuring the electron transfer dynamics using the vibrations of the fullerene molecules, we are able to directly resolve the dynamics of exciton diffusion. This approach allows us to examine the exciton diffusion length in conjugated polymers in situ – in the morphologies that exist in organic solar cells – a capability that is particularly important because the exciton diffusion length depends strongly on the polymer morphology.
