Molecular Engineering of Phthalocyanine Derivatives as Materials for Organic Photovoltaics

Abstract

Phthalocyanine (Pc) derivatives are π-conjugated molecules that have been explored as active layer components in organic photovoltaics (OPVs). The structure can be modified through incorporation of metals and installation of substituents which allows modulation of Pc-based materials toward desired solubility, photophysical properties and condensed phase organization. Research efforts in this thesis can be classified as (i) modulation of Pc absorption (Q-band) toward long wavelength to provide materials for near-infrared (NIR) absorbing OPVs, and (ii) development of solution processable Pc derivatives that self-organize in thin films to ease charge transport. Chapter 1 provides a brief review of OPV technology on its working mechanism, efficiency limitations and requirements for organic semiconductor materials. Characteristic properties of Pc materials, shortcoming for OPV applications and structural modulation strategies are discussed. Literature examples of Pc derivatives as (1) NIR absorbing materials; (2) self-organizing materials; and (3) OPV additives have been summarized. One major limitation of Pc materials is their relative narrow absorption band that lies in the UV-Vis region. Since more than 50% of solar radiation is incident in the NIR region, there has been increasing interest of tuning Pc absorption toward the NIR, which demands reasonable manipulation of the optical bandgap. In chapter 2 and chapter 3, we synthesized two series of alkylthio substituted MPc derivatives and successfully tuned Pc absorption in solution from ~680 nm to 850 nm. We also investigated thin film polymorphism and achieved Pc thin film photo response beyond 1000 nm. Pc materials have been used as dopants in P3HT/fullerene OPV devices for efficiency enhancement. In Chapter 3, we investigated the non-peripheral substituted Pcs as dopant in P3HT and utilized time-resolved microwave conductivity (TRMC) to probe free charge generation. For this series of Pcs, the free charge yield at the interface shows a clear trend of TiOPc > PbPc > H₂Pc, which can be correlated to (1) different metal species, (2) thermodynamic driving force of charge generation and (3) extent of Pc ring distortion. The charge transport behavior of Pc materials is highly dependent on long range organization of molecules, which is predominantly regulated by Pc π-π interactions. In chapter 4, we developed a new side-strapped Pc with 2-fold symmetry, which permits close π-π interaction along the substituent-absent axis and bears substituents on "side-straps" to ensure solubility. The substituent influence on Pc packing has been studied by UV-Vis spectroscopy in combination with single crystal XRD. We also investigated hole mobility of the Pc materials with conductive mode AFM (c-AFM) measurements and found that the side-strapped Pcs exhibit hole mobility up to 0.97 cm²V⁻¹s⁻¹, which is among the highest values recorded for solution processed Pc materials. Chapter 5 is a continuation of the work with side-strapped Pc. We explored the possibility of tuning side-strapped Pc energy levels to enable more diverse applications in organic electronics. We found alkoxy and alkylthio substitution on the non-peripheral positions effectively shifted frontier orbital levels and established a viable strategy to modulate optical and electronic properties of side-strapped Pcs. Chapter 6 summarizes the major discoveries in Chapter 2-5 and provides some future directions for current research may follow.