FAUNA

1.3.1.1 Phthalocyanines

Since their discovery in 1934, phthalocyanines (Pcs) have been used as dyes in inks, and colouring for plastics and clothes (Figure 1.7a-c).[88] More recently interest in this class of molecule has been in their use in organic electronics including organic field- effect transistors (OFET), sensing elements, organic light emitting diodes (OLED) and OPVs, which demonstrates the versatility of Pcs.[26, 89-91]

Pcs are 18 π-electron heteroaromatics, derived from porphyrins, with a large π- system. These macrocycles, also defined as the phthalocyanato anion (C32H16N82-), can act as a metal chelating ligand. The central ligand cavity can accommodate as many as 70 different metal ions and metal oxides (MPc), as well as hydrogen in the simple metal-free type (H2Pc).[92] Pcs prove to be chemically and thermally stable and can be vacuum deposited. In addition to the variety of metal centres, the molecules can be tuned in their solvent solubility as well as electronic and crystalline properties by substitution of the hydrogen groups at the outer ring. Halogenation, for example, shifts the HOMO and LUMO further away from Evac due to the introduction of electron withdrawing groups making it a good electron acceptor.[93] _{Substitution by ionic sodium sulfonic acid groups} (-SO2Na) drastically increases the solubility of otherwise poorly soluble Pcs in water.[94]

Blue coloured CuPc and its derivative, 3,4’,4’’,4’’’-copper(II) phthalocyanine- tetrasulfonic acid tetrasodium salt (TSCuPc), show good absorption in the range 550-700 nm. With a HOMO at -5.1 eV and the LUMO at -3.5 eV they exhibit suitable electron donor properties in combination with fullerenes for OPV devices.[26]

Planar Pcs such as CuPc have the ability to undergo co-facial intermolecular stacking based on π−π system overlap of adjacent molecules. A typical molecular crystal structure adopted by CuPc and other planar phthalocyanines is the so-called herringbone

structure shown in Figure 1.8. In this structure the individual molecular stacks are arranged with a well-defined angle to each other. The crystal arrangement and morphology depends greatly on thermal treatment, type of substrate, underlying layer and the nature of any substituents, with bulky substituents leading to larger inter-stack separation.[95, 96] Material properties such as charge mobility and exciton diffusion rely greatly on larger crystalline domains of higher order. In the case of charge mobility Pc thin films exhibit an anisotropic mobility with enhanced charge transport along the π-π

stacking axis, which influences device behaviour.

Figure 1.8 a) Crystal structure of CuPc in its α-phase. The Cu central atoms are highlighted by the black

markers. b) CuPc crystal alignment on a weakly interacting flat substrate surface.

1.3.1.2 Subphthalocyanines

Subphthalocyanines (SubPcs), a different class of small molecule semiconductor derived from Pcs, were first synthesised in 1972 by Meller and Ossko in an attempt to synthesise boron phthalocyanines.[97] _{Boron subphthalocyanine consists of only three N-} fused diiminoisoindole rings arranged around the central B atom with a substituent, usually a halogen, bound directly to B at the axially accessible top site. The molecule adapts a non-planar, cone-shaped structure with a 14 π-electron system.[98] Compared to Pcs, it was only possible to synthesise boron based subphthalocyanines. SubPcs have two target sites which can be substituted including the organic ligand ring and the open axial site directly bound to the B centre. SubPcs absorb visible light in the range of 500-650

3.4 Å 3.8 Å 24.0 Å b b a) b)

Chapter 1: Introduction nm. Unsubstituted boron subphthalocyanine chloride (SubPc – see Figure 1.7d) serves as an electron donor with a LUMO of -3.6 eV and a HOMO of -5.6 eV.[99] If substituted on the ring with appropriate electron withdrawing groups such as halogens, SubPc derivatives can also act as an electron acceptor.[100] The molecular arrangement in a thin film was found to be mainly amorphous due to the non-planar molecular structure and the sterically hindering axial substituent also leading to lower charge mobility compared to planar Pcs.[54, 101] This also leads to pair formation through weak van der Waals interactions rather than efficient π−π stacking.[98]

1.3.1.3 Polythiophenes

Conjugated polymers can undergo an efficient photoinduced charge transfer from the polymer to a fullerene which was discovered in 1992 by Sariciftci et al.[39] Since then the field of semiconducting organic polymers has progressed dramatically with polythiophenes (PTs) and poly(phenylene vinylene) (PPV) being just two well-studied polymer groups out of many with versatile applications not just in OPVs, but also OLEDs, OFETs and other organic electronic applications.[102, 103] Usually, such a polymer chain consists of a huge number of repeat units which create long π-conjugated sequences. The sequences are divided by chain twists and folds which interrupt the conjugated system. The π-conjugated system is formed by sp2-hybridised carbon atoms and is stretched along the polymer chain. The linear combination of the pz–orbital wavefunctions adds up to an entire band-like π-MO with a broad energy level distribution.[60] To avoid confusion with inorganic bandgap materials it will still be referred to as HOMO and LUMO.

To improve processability, polymers such as PTs and PPV were functionalised with alkyl and alkoxy side chains to make them more soluble. PTs consist of four carbon atoms and one sulphur atom per repeat unit. A well studied example of PT is P3HT (Figure 1.7e), which is functionalised with a hexyl side chain. Due to its low lying LUMO of about -3.0 eV to -3.3 eV and HOMO of -5.0 eV to -5.2 eV and with a broad absorption range from about 400-650 nm, it proved to be a suitable electron donor material.[104, 105] _{Hole transport takes place through the conjugated polymerbackbone but}

also between chains. For highly regioregular P3HT the hole mobility was greatly improved to 5 x 10-2-10-1 cm2V-1s-1.[106] P3HT is very soluble in non-polar or weakly polar solvents such as toluene, chlorobenzene and dichlorobenzene and is mainly deposited as a blend with PCBM to form BHJ active layers, where it undergoes phase separation to form larger polymer domains during film formation. A different derivative is water-soluble sodium poly[2-(3-thienyl)ethoxy-4-butylsulfonate] (PTEBS – see Figure 1.7f) with methoxyalkyl sulfonate side chain group.[107]