FV-TR-SMS · Time Resolved Single Molecule Spectroscopy Studies of Photoinduced Charge Separation and Charge Transfer in Model Photovoltaic Solar Energy Devices

Final Activity Report Summary - FV-TR-SMS (Time Resolved Single Molecule Spectroscopy Studies of Photoinduced Charge Separation and Charge Transfer in Model Photovoltaic Solar Energy Devices)

Many of the most promising strategies for solar energy conversion involve charge separation of an exciton in a photovoltaic device comprised of a of nanostructured composite of various types. For photovoltaic devices of this type there are many unresolved issues regarding how the charge separation and charge transfer processes depend upon the chemical structure, morphology and charging of the layers and interfaces. Unfortunately, the extreme heterogeneity of nanostructured materials makes it difficult to obtain an adequate understanding of these devices by standard 'bulk' methods, such as ensemble time-resolved fluorescence measurements and device-type measurements, e.g. current versus voltage (I-V) curves. Furthermore, interfaces in photovoltaic devices are typically 'imbedded' and thus inaccessible to surface imaging tools such as scanning tunnelling microscopy (STM), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), which operate without destroying the device. This research used novel single-molecule, i.e. particle, modulation spectroscopy to investigate charge separation and charge transfer reactions in model photovoltaic solar energy devices. The project took a new and distinct direction by combining single molecule spectroscopy (SMS) with induced external electric field and light intensity modulation to create a new spectroscopic technique, namely the 'fluorescence voltage-time resolved-single molecule spectroscopy' (FV-TR-SMS). The key goal of the project focused on the elucidation of photo-induced charge injection, charge separation and charge transfer in organic nanomaterials on a molecular level. We investigated hole injection from a layer of a carbazole derivative, which was a strong organic hole-donor, into isolated, single-polymer chains of the conjugated polymer poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH- PPV). Hole injection was studied by using a fluorescence-voltage single molecule spectroscopy approach that was developed by the Barbara group. Through modulation of both fluorescence excitation light and device bias it was determined that hole injection from carbazole into single chain MEH-PPV was a purely light-driven process, leading to the efficient storage of charge on MEH-PPV. This effect might underlie critical, poorly understood organic electronic device phenomena such as the build-up of functional deeply trapped charge layers. This concept of light-induced single molecule charge storage was then adapted to demonstrate a novel image capture device.

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