Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Joseph J. Berry

Second Advisor

Charles Rogers

Third Advisor

David S. Ginley

Fourth Advisor

Garry Rumbles

Fifth Advisor

Ivan Smalyukh

Abstract

Organic photovoltaics (OPV) offers a potential for solar-electric power generation to be affordable. Crucial to OPV device performance is the incorporation of interlayers, ultra-thin films deposited between the photoactive material and the electrical contacts. These interlayers have various, targeted functionalities: optical window, encapsulation, or electronic bridge. The last category is known as "transport layers", and is the focus of this thesis.

In this thesis, we explore and investigate the physics that leads to improvements in OPV device performance when a transport layer is employed. We focus on the use of non-stoichiometric nickel oxide (NiOx) as a hole transport layer (HTL) in poly(3-hexylthiophene):phenyl-C61-butryric acid methyl ester (P3HT:PCBM) solar cells. NiOx deposited by physical vapor deposition is chosen for this study because of its successful use as HTL, the ease to engineer its electronic properties by varying deposition parameters, and it leading to improved device lifetime.

Our initial studies indicate that the well-known \high work-function" rule is not adequate to explain the trends observed in the devices. The series of investigations discussed in this thesis reveal that band-offsets at the NiOx / P3HT:PCBM interface need to be taken into account in order to explain the observed trends. Non-optimal band-offsets lead to either sigmoidal current-voltage characteristics or reduced photocurrent.

The optimal energy level alignment depends on the energy levels of the photo-active material, which are measurable. This means that an HTL material must be optimized for different photoactive material. A simple and practical set of rules are proposed to achieve this optimal energy level alignment for a given photoactive material. The rules not only include the pervasively-used \high work-function" rule, but also the impacts of band-offsets investigated in this work.

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