Controlling the Morphology of Polymer and Fullerene Blends in Organic Photovoltaics Through Sequential Processing and Self-Assembly

Organic photovoltaics are a potential source for cheap renewable energy. However oneof the main limitations to the field thus far has been scalability. Power conversionefficiencies of photovoltaic films made on the laboratory scale of a couple of mm2can be as high as 10%. However when the device area is increased to even tens of mm2power conversion efficiency plummets. This work presented in this dissertation focuseson understanding and circumventing the issues limiting the expansion of photovoltaicprocessing to larger device areas.One method of maintaining photovoltaic efficiency over a large range of device areasis to use self-assembling materials to control the active layer morphology. Thesematerials should give the preferred morphology regardless of substrate size. I first studyphotovoltaic devices utilizing self-assembling fullerenes designed to form nanometerscale wires within the film active layer. I show that fullerene that are able to form these nano-wires give a higher device range electron mobility through measuring the space charge limited current through a photovoltaic device. However the photovoltaic efficiencies of devices using these fullerenes remains low. I use time resolved microwave conductivity to measure the local nm-scale mobility of these fullerenes to show that there exists two ranges of mobilities in organic photovoltaic films. The nm-scale moiibility, governed by electronic overlap of neighboring molecules, and the device range mobility, governed by film morphology. I show that device performance is maximized when both mobility scales are taken into account.Self-assembly is not the only method to achieve scalable organic photovoltaic devices.Next, I show that the fabrication method of sequential processing can give identicaldevice performance between films fabricated on 7.2 mm2 and 34 mm2 substrates.This is because films produced by sequential processing allows the polymer layer toform prior to fullerene deposition, giving higher film quality. I show this scalability isnot seen in films that are fabricated through blendcasting, where the donor and acceptormaterials are blending together in solution and deposited together onto the substrate.Sequential processing proves to be a powerful fabrication technique in making scalableorganic photovoltaic films. Therefore I develop a method of selecting fullerenedeposition solvents that are compatible with any donor polymer. I show that polymerswelling is a key step in sequential processing film formation. I provide a procedureon tuning the c interaction parameter between the fullerene deposition solvent and thepolymer layer. This is done by mixing a good polymer solvent with a poor polymersolvent.This ensures the fullerene deposition solvent swells, but does not dissolve thepolymer film. By selecting the correct polymer solvent/non-solvent pair and ratio filmsfabricated by sequential processing can reach device performances matching those fabricated by traditional blendcasting