Interface Modifications for Applications in Organic and Hybrid Photovoltaics

Thesis (Ph.D.)--University of Washington, 2014Considerable research has been conducted in the area of organic photovoltaics due to several intrinsic advantages, including their high throughput solution processability, light weight, and their applicability on flexible substrates. Product development has been limited, however, due to the low mobilities and short exciton diffusion lengths of organic materials relative to inorganic materials used for photovoltaics. In this dissertation, we look at interfacial phenomena in attempt to control the charge transport dynamics in different parts of photovoltaic systems. The first chapter provides an overview of the field of organic photovoltaics, including their benefits, operating procedures, and a brief history of materials and device development. Chapter 2 examines some donor-acceptor small molecules as the electron donors in all organic bulk heterojunction solar cells with soluble fullerene derivatives as the electron acceptors. The donor-acceptor small molecules are unique because their energy levels agree well with the theoretical optimal HOMO and LUMO energy levels required for high efficiency organic photovoltaics. Even with energy level matching, however, we found that we were only able to obtain modest device efficiencies due to the formation of large domains that are greater than the exciton diffusion length and result in large interfacial areas. In chapter 3 we examine some of the optical, physical, and charge transport properties of a series of fully conjugated brush copolymers that are comprised of a carbazole-diketopyrrolorpyrrole donor-acceptor backbone copolymerized with different lengths of poly(3-hexylthiophene) pendant chains. It was found that there was a sufficient break in conjugation between the two copolymers such that the absorbance characteristics of both could be realized independently. In addition, the physical and charge transport properties could be tuned to primarily show influence from either the ambipolar low band gap backbone or the p-type pendant chains. Chapter 4 examines the synthesis of poly(3-methylthiophene) via surface initiated Kumada catalyst transfer polymerization from indium tin oxide where it was found that the thickness of the polymer layers could be controlled by controlling the monomer concentration in solution. These films proved to be robust interface layers that exhibit work functions that are tunable by electrochemical doping; when in the doped state, they show fast electron dynamics, while in the neutral state, they may be applicable as electron blocking layers for organic photovoltaics. In chapter 5, a new method for the in-situ functionalization of CdSe quantum dots with π-conjugated ligands during synthesis is presented. This technique is useful for controlling the composition of the surface of colloidal CdSe quantum dots when traditional ligand exchange processes prove difficult. This synthetic technique is then used in chapter 6 to functionalize CdSe nanocrystals with poly(3-hexylthiophene) in an attempt to promote good interfacial charge transport properties for use in hybrid photovoltaics. The photophysics of a series of these hybrid CdSe/polymer materials were investigated by steady state and time-resolved spectroscopies, and ultimately it was found that there is a strong propensity for fluorescence resonance energy transfer between the two materials owing to their intimate contact, good resonance, and large spectral overlap. Finally, a brief future work section is presented that is required to wrap up the study of the photophysical processes for these hybrid materials