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 π-π 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 π-conjugated system has led a vast library of OSCs with various functionalities, which fundamentally changes the electronic properties of the π-conjugated system. It is highlighted that introducing conjugation break spacers (flexible linkers) between the π-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 “green” 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 donor–acceptor 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