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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

2015

Month Degree Awarded

September

First Advisor

Paul M. Lahti

Second Advisor

D. Venkataraman

Subject Categories

Materials Chemistry | Organic Chemistry | Polymer Chemistry

Abstract

Organic photovoltaic devices use an active layer of organic materials that absorbs light and creates free charges to generate electricity. Such organic photovoltaics have many desirable properties as the final devices can be very lightweight, thin, flexible, have low manufacturing costs, and be semitransparent. These properties make them particularly advantageous for myriad of applications: e.g., use on non-planar surfaces, as tinted coatings on windows, any use for charging low power devices. One of the biggest problems preventing organic photovoltaics from being commercialized is controlling the packing of the organic materials within the active layer from the molecular scale through the mesoscale. The state-of-the-art method for preparing active layer offers essentially no control over packing through multiple length scales. In this dissertation, I present a method for controlling the active layer morphology from the molecular scale through the mesoscale using the assembly of organic nanoparticles.

The miniemulsion method was used to prepare nanosized particles of each of two active layer components, an electron donor and an electron acceptor. During the nanoparticle synthesis, the internal molecular packing within the particles can be tuned by changing the “oil” (organic soluble) miniemulsion phase. The nanosized domains of each component were directly controlled by tuning the nanoparticle size during synthesis. This provides a large amount of control of the molecular scale packing and the nanoscale packing. Spray-coated assemblies of these nanoparticles, upon removing excess surfactant, were shown to transport charges with mobilities similar to those in films of pristine polymer made by standard drop-cast methods. Donor nanoparticles were mixed with acceptor nanoparticles and shown to form a random binary assembly of nanoparticles with charge conduction pathways. Organic photovoltaic devices were successfully prepared with a mixture of donor nanoparticles and acceptor nanoparticles, and gave performance comparable to analogous devices made by conventional film coating, bulk heterojunction methods. The internal morphology of the active layer films was shown to be easily tuned by changing the nanoparticle diameter or number ratio of donor nanoparticles to acceptor nanoparticles.

This method offers the ability to tune the molecular scale, nanoscale, or mesoscale, systematically, packing scales individually or collectively. Therefore, this method can be used to determine systematically what is the optimum active layer morphology for theoretically any donor plus acceptor combination of materials. Also, this method offers a way to control morphology and probe the properties of binary blends of organic materials, in a more general sense, e.g., for making other electronic devices or for charge mobility studies of solid films.

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