Scheme 1. Several prominent discotic liquid crystalline compounds comprising a rigid aromatic central core
2. THESIS OVERVIEW
This thesis addresses the tailor‐made synthesis and characterization of different self‐organizing perylene derivatives as electronically active n‐type semiconducting dyes for application in organic photovoltaic devices (OPV). The primary question addressed the design of molecular architectures in order to obtain novel self‐assembling structures leading to new classes of n‐type semiconductors. The central focus of self‐assembly was set on two distinct supramolecular self‐
organization phenomena: 1) liquid crystalline order and 2) hydrogen‐bond or solvent directed self‐assembly. In this thesis the majority of work was devoted to the liquid crystalline ordering of discotic shaped perylene molecules (DLCs). Additionally, H‐bond/solvent directed self‐assembly for the creation of nanofibrous networks and their implementation into photovoltaic devices was also examined (Fig. 1).
Figure 1. Dependency between molecular structure, morphology and device performance. These three main aspects are addressed in this thesis by synthesizing self‐organizing perylene derivatives capable of either liquid crystalline ordering which allows for high intracolumnar charge carrier mobility or solvent or hydrogen‐bond assisted self‐assembly for the formation of nanostructured networks with large interface area and well defined charge carrier pathways.
Both self‐organization principles are promising approaches to improve device efficiency.
On the one hand, columnar liquid crystalline ordering allows for a high intracolumnar charge carrier mobility in these quasi one‐dimensional “nanowires”. On the other hand, a nanostructured fibrous network provides a large interface area between electron acceptor and a suitable donor material, together with well defined charge transport pathways in the bulk. Thus my endeavor addresses fundamental issues in photovoltaic devices, like morphology of the electron accepting component together with optical and electronic properties. The molecular architecture was varied to get new classes of n‐type semiconductors with extended absorption in the visible range and to obtain different kinds of ordering or packing such as columnar hexagonal or two‐
dimensional lamellar structures.
The dissertation contains six manuscripts that focus on different aspects of novel discotic perylene dyes. The chapters can basically be divided into two topics. The first part of the thesis (chapters 3‐6) concerns the synthesis and characterization of liquid crystalline perylene dyes with a focus on the extension of absorption in the visible region as well as tailoring the mesophase temperature‐width and packing behaviour. In this context, the major challenge was to effectively suppress the inherent tendency of the perylene moieties for crystallization. An overview of the synthetic strategies starting from perylene tetracarboxylic acid dianhydride (PTCDA) for the preparation of various classes of perylene dyes is presented in Scheme 1. A substitution at the bay‐positions of the perylene core was avoided in all cases, in order to maintain the planarity and strong π‐π interactions, which favour intermolecular order and charge carrier transport.
Scheme 1. Overview of synthetic strategies starting from perylene tetracarboxylic acid dianhydride (PTCDA) for the synthesis of symmetrically or unsymmetrically N‐substituted perylene bisimides (PBIs) and new
In this work a comprehensive structure‐property relationship to understand fundamental molecular design requirements to induce thermotropic liquid crystalline phases in PBIs was accomplished (Chapter 3). The introduction of branched (“swallow‐tail”) oligooxyethylene N‐substituents was crucial for obtaining liquid crystallinity and intracolumnar long‐range order.
Further, novel classes of n‐type semiconductors with an extended π‐conjugation system and an asymmetric substitution pattern, like perylene imide benzimidazoles (PIBI, chapter 4) and perylene diester benzimidazoles (PDBIs, Chapter 5) were synthesized. Chapter 4 demonstrates the introduction of a fused benzimidazole moiety on the perylene core, resulting in asymmetric perylene imide benzimidazoles (PIBIs). These dyes exhibit a significantly extended absorption to longer wavelengths in the visible spectrum and show for the first time self‐organization ability into columnar hexagonal liquid crystalline phases for this special type of material. Chapter 5 takes advantage of this result and expands the concept towards novel asymmetric perylene diester benzimidazoles (PDBIs). Likewise, these dyes exhibit a broad absorption in the visible wavelength regime and additionally exhibit room temperature mesophases. Thermotropic behavior of all novel discogens was investigated thoroughly by differential scanning calorimetry (DSC), polarization optical microscopy (POM) and X‐ray diffraction experiments (XRD). By these methods, liquid crystalline packing such as columnar hexagonal mesophases (Colh) for all three types of compounds, but as well compounds with a columnar hexagonal plastic phase (Colhp) or crystalline lamellar phases (CrL) were confirmed. Such systems are highly interesting for applications requiring high order in the molecular arrangement. Figure 2a illustrates typical X‐ray diffractograms for these types of phases.
Figure 2. (a) X‐ray diffractograms for the columnar hexagonal mesophase of a PBI, columnar hexagonal plastic phase of a PIBI and crystalline lamellar phase of a PDBI derivative. (b) UV/Vis absorption spectra of representative PTE, PBI, PDBI and PIBI derivatives. The absorption in the visible wavelength regime could be extended up to 680 nm.
The optical and electronic properties were studied via UV/Vis spectroscopy and cyclic voltammetry measurements (CV). Figure 2b gives an overview of absorption spectra of representative candidates of the different classes of perylene dyes. It can clearly be seen that the
absorption in the visible regime can be red‐shifted immensely from PTEs to PIBIs and PDBIs due to an extended π‐conjugation system. This is also the molecular origin for a decrease of the band‐
gap in PDBIs and PIBIs, which is mainly caused by a raise of the HOMO energy levels. Generally, HOMO values lie in between ‐5.5 and ‐6.1 eV and LUMO energy‐values between ‐3.5 eV and
‐3.8 eV. Finally in chapter 6, a detailed study of donor/acceptor blends of discotic perylene bisimide (based on the results of chapter 3,) and Cu‐phthalocyanine (CuPc) binary systems was investigated concerning blend morphology and for the purpose of elucidating their potential for light‐harvesting and charge separation.
The second part of this thesis (chapter 7‐8) spans the gap towards the second self‐organization principle and concentrates on the self‐assembly of PBIs into fibrillar nanowires. The morphology of the nanostructures was investigated by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Chapter 7 introduces a novel concept to generate donor‐acceptor interpenetrating nanostructures with inherent morphological stability by an organogel/polymer approach. Therefore the hydrogen bond directed self‐assembly of a PBI organogelator in presence of an amorphous hole conducting polymer matrix of poly(vinyldimethoxytetraphenylbenzidine) (pvDMTPD) was utilized to successfully generate the active layer for photovoltaic devices (Figure 7a). The presence of percolation paths for photogenerated electrons and holes in such organogel/polymer composite material was demonstrated directly in solar cells, which delivered appreciable photocurrents and photovoltages. These excellent results stimulated for an additional effort in chapter 8, which is directed towards the synthesis of novel self‐organizing PBIs with an increased chromophore content to further improve this concept. In this context, hydrogen‐bond directed and solvent directed self‐assembly of newly designed PBIs were studied (Figure 8).
Subsequently a brief summary of the main results is presented for each publication (Chapters 3‐
8). For a complete description of a particular topic and experimental details, the reader is referred to the respective chapter.
SWALLOW‐TAIL SUBSTITUTED LIQUID CRYSTALLINE PERYLENE BISIMIDES – SYNTHESIS &