Molecular Engineering Strategies for Morphology Control in Organic Semiconductors for Optoelectronics

It is understood that the optoelectronic performance of organic electronic devices is determinant on the macroscopic solid state self-assembly of organic semiconductors (OSC), which is driven by the supramolecular Ï-Ï stacking interactions between the conjugated segments in its molecular structure. Molecular engineering approaches directly impacts the intrinsic self-assembling properties of the OSCs, but the flexibility of organic chemistry on modulating the backbone architecture of the Ï-conjugated system has led a vast library of OSCs with various functionalities, which fundamentally changes the electronic properties of the Ï-conjugated system. It is highlighted that introducing conjugation break spacers (flexible linkers) between the Ï-conjugated segments in the OSC, showed great promise in controlling supramolecular self-assembly without altering the semiconducting core of the OSC. Previous examples on controlling the morphology of the molecular OSC, DPP(TBFu)2 using horizontal and vertical dimers, and a horizontal flexibly linked polymer analogues, have shown great promise in effecting the self-assembly behaviour and stabilizing the morphology of the parent (non-flexibly linked) DPP(TBFu)2 molecule, when used as an additive. In this thesis, an extension of this work is presented whereby the flexible linking approach is used to design and synthesize two molecular compatibilizers that consists of the donor component, DPP(TBFu)2 that is linked with an aliphatic spacer to an acceptor component, based on a fullerene and perylenediimde small molecule OSC. In chapter 2, a comparison between the compatiblizer (CP) and an in-situ linker approach (ISL) was explored to elucidate the impact of stabilizing a multi-component bulk heterojunction (BHJ) morphology and its device performance for organic photovoltaics (OPVs). It was concluded that the CP approach shows the most promise in stabilizing the BHJ morphology for OPVs, which was then applied onto a highly crystalline BHJ system with a perylenediimde acceptor to demonstrate its versatility as described in chapter 3. Taking leverage from this demonstration, chapter 4 discusses how the CP approach is used to tune the phase-domain size of the BHJ that is processed from a homogeneous single-phase melt, to obtain a photoactive BHJ in an OPV device. This unique demonstration, ultimately opens up vast new possibilities for solvent free âgreenâ processing of OPVs. Lastly in Chapter 5, the approached used to address BHJ morphological stabilization is slightly different from that of previous chapters, where the (kinetic) stability of a binary donor-acceptor BHJ is addressed. In this chapter, a fully-conjugated block copolymer (BCP) consisting of donorâacceptor blocks is used to demonstrate its applicability for a single-component BHJ for OPVs. However, the main challenge for this approach is in the synthetic methodology, and to overcome this, a modular synthetic strategy using Heck coupling between two functionalized donor and accepting marcromonomers showed promise in obtaining a fully conjugated BCP for OPVs

On-Surface Assembly and Reactions of Organic Molecules in Ultra-High Vacuum

Claudius Morchutt

This thesis comprises the study of two types of 2D materials, those stabilized by non-covalent and those by covalent interactions. They are synthesized and studied on well-defined metallic surfaces in ultra-high vacuum (UHV). The self-assembly of terephthalic acid (TPA) on Ag(100) is explored. TPA stays intact upon deposition at room temperature (RT) and forms islands stabilized by hydrogen bonds. TPA islands influence the homoepitaxial growth of silver and reduce the sticking coefficient of Ag atoms. At RT Ag atoms intercalate TPA islands which are not disrupted. In contrast, TPA molecules in conjunction with Mg atoms on Ag(100) results in tip-induced altering of the surface. The electric field between tip and sample interacts with Mg atoms and TPA, which leads to a restructuring of the step-edges during scanning. Moreover, the self-assembly of the organic semiconductor 2,7-dicyano[1]benzothiene[3,2-b]benzothiophene (cBTBT) on Ag(111) at RT is presented. The pro-chiral molecules form compact islands with a chevron-like structure containing both enantiomers. Deposition of Fe atoms leads to an amorphous metal-organic coordination network (MOCN). Statistical analysis reveals that conformational entropy plays a critical part in its stabilization. Phase segregation into spatially homogeneously crystalline domains of molecules and metal atoms upon annealing suggests that the amorphous network is kinetically trapped at RT during the preparation process. The second part presents the on-surface synthesis of 2D polymers via different coupling schemes. First, the Ullmann coupling on Au(111) is explored. The molecule 1,3,5-tris(4-bromophenyl)benzene (TBPB) debrominates upon annealing and polyphenylene is formed. To investigate the influence of metal substrates on the dehalogenation, a single layer of hexagonal boron nitride as well as graphene grown on Ni(111) is introduced. On both surfaces TBPB forms self-assembly islands stabilized by halogen bonds, while on bare Ni(111) the substrate-molecule interaction dominates resulting in structures without long-range order. Upon annealing on both surfaces dehalogenation is induced and 2D nanostructures are formed. On bare Ni(111) the precursor molecules merely decompose. The experimental annealing temperatures are consistent with debromination barriers calculated by DFT. An example of the synthesis of tailor-made 2D structures by using particularly designed precursor molecules is also given. A terminal alkyne with a triazine core undergoes on-surface Glaser coupling and cyclotrimerization on Au(111) resulting in nitrogen doped 2D polymers. Finally, a comprehensive study of the on-surface decarboxylation reaction of 1,3,5-tris(4-carboxyphenyl)benzene (TCPB) on Cu(111) is presented. TCPB deprotonates upon deposition on Cu(111) at RT and self-assembles in compact islands. The self-assembly is stabilized by ionic hydrogen bonds between deprotonated and negatively charged oxygen atoms and hydrogen atoms of neighboring molecules. Annealing leads to decarboxylation and formation of 2D nanostructures. The reaction occurs in a clean fashion because CO2 leaves the surface. In addition, STS reveals that the lowest unoccupied molecular orbital (LUMO) of the 2D polymer is destabilized compared to the LUMO of the monomer although the pi-electron system is more extended in the polymer. The decarboxylation impacts the energy position of the LUMO to a greater extend which is corroborated by DFT calculations.

EPFL2016

Position-Dependent Extension of π-Conjugation in D-π-A Dye Sensitizers and the Impact on the Charge-Transfer Properties

Thomas Wesley Holcombe, Jinhyun Kim, Jacques-Edouard Moser, Mateusz Barnaba Wielopolski, Yuxiang Yu, Shaik Mohammed Zakeeruddin

A series of five organic donor-π-bridge-acceptor (D-π-A) sensitizers is investigated within the context of their photoinduced charge-transfer properties. Thereby, the focus is set on the impact of structural modifications of the molecular architecture on the π-systems of the dyes. In particular, two different modes of systematic extension of the sensitizers’ π-systems, namely, (i) within the electron donating site and (ii) within the π-bridge, are investigated by means of steady-state and time-resolved spectroscopic methods. The photophysical studies of the molecules in solution and as deposited on Al2O3 or TiO2 films reveal that different effects on the charge-transfer characteristics evolve dependent where – within the molecular structure – the modification of the π-system is performed. Hence, π-extension of the donor sites, for instance, leads to a strong red shift of the absorption features and a variation of light-harvesting properties. Modifying the π-bridges results in a spatial decoupling of the HOMO and LUMO orbitals, which goes along with changes of the electronic coupling to TiO2. Furthermore, solution studies show that the electronic structure of the dyes governs their singlet excited-state features. As shown, the results obtained from these studies then allow important predictions about the deactivation of the excited states of these molecules adsorbed on TiO2. Finally, quantum chemical methods – among others, time-dependent density functional theory calculations – provide conclusive insight into the relationship between the electronic structure of the dyes and its impact on the photoinduced charge-transfer characteristics.

American Chemical Society2013

On-Surface Assembly and Reactions of Organic Molecules in Ultra-High Vacuum

Claudius Morchutt

EPFL2016