Rational Design of Hole Transport Materials: Tools and Structure-Packing-Property Relationships

The growing demand for low-cost, large-scale and flexible electronics has led to a boost in research on organic semiconductors (OSC). Understanding structure-packing-property relationships (SPPR) and the role of molecular building blocks in charge transport enables the development of effective rational design strategies, which helps in accelerating the search of promising of OSCs. Therefore, the original work presented in this thesis addresses these two aspects, with a focus on organic hole transport materials (HTM) utilized in perovskite solar cells. In the first part of this thesis, we place emphasis on revealing the SPPR of triphenylamine (TPA)-based HTMs utilizing multi-scale simulations involving molecular dynamics, density-functional theory and kinetic Monte-Carlo techniques. In the first example, we investigated the role of heteroatoms, presence of hexyl chains and substitution positions on charge transport for truxene derivatives. In particular, the impact of the presence of hexyl chain on hole mobility (ÎŒ) was identified, where hexyl-substituted truxene compounds possess hole mobilities an order of magnitude lower than those of their bare-core counterparts. The lower mobility originates primarily from the steric effect of hexyl chains, which prevents the truxene dimer from adopting a face-to-face arrangement. Due to the crucial role of the hexyl chain, we further investigated the effect of alkyl chain length on charge transport. As the alkyl chain is elongated, the reorganization energy (λ) remains unchanged, while the electronic coupling (V) and energetic disorder (σ) decrease simultaneously. Since V decreases much faster than σ does, ÎŒ gradually decreases with increasing alkyl chain length. Finally, we reveal the underlying mechanism of the multiarm effect on ÎŒ. By increasing the number of carbazole arms substituted on the TPA core, all transport parameters are decreased, which overall leads to an increase in ÎŒ. The second part focuses on the development of a fragment-based decomposition scheme for reorganization energy (FB-REDA) and electronic coupling (FB-ECDA). The total λ of a molecule is partitioned into fragment reorganization energies utilizing local fragment vibrational modes. We demonstrate the usefulness of FB-REDA by successfully reduce the λ of a dopant-free HTM (TPA1PM, λ = 213 meV) by nearly 50% (TPD3PM, λ = 108 meV) using rational design strategies based on the FB-REDA results. Similarly, the total V between two molecules in a dimer was decomposed into fragment-fragment electronic coupling terms using FB-ECDA. Combined with percolation and dimer composition analysis, we successfully established the relationship between molecular packing and electronic coupling for two series of molecules.