UCLA

Organic photovoltaics are a potential source for cheap renewable energy. However one

of the main limitations to the field thus far has been scalability. Power conversion

efficiencies of photovoltaic films made on the laboratory scale of a couple of mm2

can be as high as 10%. However when the device area is increased to even tens of mm2

power conversion efficiency plummets. This work presented in this dissertation focuses

on understanding and circumventing the issues limiting the expansion of photovoltaic

processing to larger device areas.

One method of maintaining photovoltaic efficiency over a large range of device areas

is to use self-assembling materials to control the active layer morphology. These

materials should give the preferred morphology regardless of substrate size. I first study

photovoltaic 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 identical

device performance between films fabricated on 7.2 mm2 and 34 mm2 substrates.

This is because films produced by sequential processing allows the polymer layer to

form prior to fullerene deposition, giving higher film quality. I show this scalability is

not seen in films that are fabricated through blendcasting, where the donor and acceptor

materials are blending together in solution and deposited together onto the substrate.

Sequential processing proves to be a powerful fabrication technique in making scalable

organic photovoltaic films. Therefore I develop a method of selecting fullerene

deposition solvents that are compatible with any donor polymer. I show that polymer

swelling is a key step in sequential processing film formation. I provide a procedure

on tuning the c interaction parameter between the fullerene deposition solvent and the

polymer layer. This is done by mixing a good polymer solvent with a poor polymer

solvent.This ensures the fullerene deposition solvent swells, but does not dissolve the

polymer film. By selecting the correct polymer solvent/non-solvent pair and ratio films

fabricated by sequential processing can reach device performances matching those fabricated by traditional blendcasting.