Investigating Light Induced Charge Transfer in Interfacial Molecular Assemblies

Abstract

To improve the overall efficiency of an organic photovoltaic (OPV) device, the charge transfer at the organic/transparent conductive oxide (TCO) interface is one of the key parameters to investigate. Modifying the electrode surface with self-assembled monolayers (SAMs) of the organic active materials can serve as a model to study the factors that affect this efficiency. This dissertation covers projects that study the interface of the TCO and organic active layer of OPV devices. The electron transfer process across this interface is one of the factors that limit the efficiency of these devices. Common materials for these devices that are also used in these projects are Indium tin oxide (ITO) for the TCO, phthalocyanines (Pc’s) for the organic donor material, and pereylene diimide (PDI) for the organic acceptor material. The projects are covered in Chapters 2 through 5. The first project covered in Chapter 2 introduces the design of zinc phthalocyanines (ZnPc’s) to bind to the ITO surface with a rigid orientation. The charge transfer processes across the ITO/ZnPc interface are studied and compared to previous reports with flexible linkers. The next project covered in Chapter 3 also builds on the previous reports of charge transfer processes across the ITO/ZnPc interface with flexible linkers. Here, an acceptor PDI is attached onto the ZnPc already anchored to the ITO surface, creating a monolayer OPV. Photoelectrochemical studies are carried out to further study the charge transfer processes. Chapters 4 and 5 study donor-acceptor dyads for Pc’s and PDI’s covalently bonded to each other using transient absorption spectroscopy to monitor the processes that occur after a photon of light hits the organic layer. In Chapter 4, the charge separated state lifetime is measured with and without a PDI directly attached to a ruthenium phthalocanine (RuPc) to show the effect that spatially separating out the electron-hole pair on the surface of ITO has on the electron transfer process across the ITO/Pc interface. Chapter 5 uses a series of dyads in which the functional groups in the bay-position of the PDI are varied to change the frontier orbital levels, ultimately changing the energy gap between the highest occupied molecular orbital (HOMO) of the Pc and the lowest unoccupied molecular orbital (LUMO) of the PDI. Transient absorption spectroscopy is used to identify how these energy gap changes affect the lifetimes of the charge separated states.