FB-ECDA: Fragment-based Electronic Coupling Decomposition Analysis for Organic Amorphous Semiconductors

We present a fragment-based decomposition analysis tool (FB-ECDA) for the electronic coupling of charge transfer processes. This tool provides insight on the sophisticated relationship between molecular packing, electronic coupling, and the molecular transport network present in organic amorphous semiconductors. On the basis of atomic orbitals, FB-ECDA decomposes the total electronic coupling into individual electronic coupling terms arising from each molecular building blocks in a straightforward manner. The usefulness of this approach is demonstrated by revealing the structure-packing-property relationships for two series of molecules differing by the number of arm substituents and acene core length. Overall, we provide insight on the design of organic semiconductors exhibiting efficient charge transport network through achieving a subtle balance between molecular packing and electronic structure. We expect FB-ECDA to be a valuable tool for understanding sophisticated charge transfer processes in amorphous systems and guiding the rational design of organic semiconductors.

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

Kun-Han Lin

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 (ÃÅ) 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 (Î») remains unchanged, while the electronic coupling (V) and energetic disorder (ÃÂ) decrease simultaneously. Since V decreases much faster than ÃÂ does, ÃÅ gradually decreases with increasing alkyl chain length. Finally, we reveal the underlying mechanism of the multiarm effect on ÃÅ. By increasing the number of carbazole arms substituted on the TPA core, all transport parameters are decreased, which overall leads to an increase in ÃÅ. 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 ÃÂ» 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 ÃÂ» of a dopant-free HTM (TPA1PM, ÃÂ» = 213 meV) by nearly 50% (TPD3PM, ÃÂ» = 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.

EPFL2020

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

Photophysics and Photochemistry from First Principles

Matteo Guglielmi

Quantum chemical methods are important tools for the predictions of electronic structure and energetics of molecules providing the interpretation of spectroscopic experiments and reaction mechanisms. In this thesis, density functional theory (DFT) and wavefunction-based methods are used in order to evaluate their ability in predicting experimental data with a particular attention to photoprocesses occurring in both gas and condensed phase systems. The cases studied in this work are inspired by problems both from biology and material science. In the first study, the photoexcitation properties of the molecule 7-hydroxyquinoline·(NH3)3, a prototypical molecular wire, are investigated. A concerted mechanism according to which all protons are transferred simultaneously in a fast process (∼ 100 fs) that amounts to the net transport of one proton from the oxygen to the nitrogen of 7-hydroxyquinoline is observed. In addition, the proton transfer pathway involves all three ammonia molecules and not only two as previously proposed. The presence of potential energy surface crossings with a dark-state is responsible for the low experimentally observed quantum-yield. To better understand the complex photophysics of the amino acid tryptophan, which is widely used as a probe of protein structure, we performed DFT and TDDFT calculations of gas phase tryptophan solvated with a controlled number of water molecules. The addition of two water molecules sufficiently lengthens the excited state lifetime enabling vibrationally resolved spectra. Quantum chemical calculations at the RI-CC2/aug-cc-pVDZ level, together with TDDFT/pw based first-principles MD simulations of the excited state dynamics, clearly demonstrate how interactions with water destabilize the photodissociative states and increase the excited state lifetime. Due to the availability of high-resolution experimental data, this system is ideally suited as benchmark model for the evaluation of the performance of different quantum chemical methods. An extensive low-energy isomer search was carried out for [TrpH·(H2O)n=0,1,2]+ using a hierarchy of theoretical methods ranging from classical force fields to a variety of DFT methods (BLYP, B3LYP, M05, M05-2X, M06, M06-2X and M06-HF with different basis sets) up to the CBS-C level. For the low-energy structures, the harmonic vibrational frequencies are calculated and compared with high resolution experimental IR spectra. It turns out that in all three cases, CBS-C is able to predict the lowest energy isomers that are in agreement with the experimentally observed vibrational spectra. In most cases, M06 provides energetics in close agreement with the CBS-C results. On the other hand, M05-2X is the only functional that yields highly-reliable predictions of the vibrational spectra. A natural extension of the tryptophan model to condensed phase was performed by an investigation of the [Trp-Lys]+ motif studied in the protein Human Serum Albumin via QM/MM simulations. Like in the case of gas phase [TrpH]+, solvation effects lead to an increase of the σN-H orbital energies. As a result, the first photoaccessible state of sW K+ is primarily a photostable π orbital, while in dW K+, photoexcitation leads to a dissociative pathway. The last application of this work concerns the optical properties of a novel dye for the sensitization of titanium dioxyde nanoparticles in Grätzel solar cells. The presence of a new absorption band in the visible region opens the way for a rational design of new dyes with improved properties.