Charge-Transfer Interactions in Organic Functional Materials

Abstract

Our goal in this review is three-fold. First, we provide an overview of a number of quantum-chemical methods that can abstract charge-transfer (CT) information on the excited-state species of organic conjugated materials, which can then be exploited for the understanding and design of organic photodiodes and solar cells at the molecular level. We stress that the Composite-Molecule (CM) model is useful for evaluating the electronic excited states and excitonic couplings of the organic molecules in the solid state. We start from a simple polyene dimer as an example to illustrate how interchain separation and chain size affect the intercahin interaction and the role of the charge transfer interaction in the excited state of the polyene dimers. With the basic knowledge from analysis of the polyene system, we then study more practical organic materials such as oligophenylenevinylenes (OPVn), oligothiophenes (OTn), and oligophenylenes (OPn). Finally, we apply this method to address the delocalization pathway (through-bond and/or through-space) in the lowest excited state for cyclophanes by combining the charge-transfer contributions calculated on the cyclophanes and the corresponding hypothetical molecules with tethers removed. This review represents a step forward in the understanding of the nature of the charge-transfer interactions in the excited state of organic functional materials.

Theoretical Design of Benzoselenadiazole Based Organic Donor Molecules for Solar Cell Applications

“…It is found from previous investigations that a minimum energy difference of 0.3 eV is required to split the exciton [28]. Finally, planar conjugated backbone structures are preferable as they form tightly-packed stacking arrangements which may enhance strong intra-and intermolecular electronic coupling and facilitate minimal reorganization energy which is a measure of the charge transport in these organic molecular systems [29][30][31][32]. [6,6]-Phenyl-C 61 -butyric acid methyl ester (PCBM) and its derivatives have been used as electron acceptors in many organic solar cells because of high carrier (electron) mobility [33].…”

Computational Molecular Electronic Spectroscopy with TD-DFT

“…Photoinduced charge-transfer excited states play a key role in several applications, notably in dye sensitized solar cells (DSSC) [159][160][161][162][163]. In DSSC, the absorption of light by a dye anchored on a semi-conducting surface, typically a metallic oxide, induces a CT on the dye which eventually leads to charge separation, the electron (or the hole) being injected into the semi-conductor.…”

Theoretical Investigation of the Intermolecular Charge‐Transfer Interactions of Poly‐p‐Phenylene Oligomers

“…(2), where the basis sets of individual molecules are used to construct the combined electronic states for the dimer by considering intermolecular interactions including CT interactions, in which the four diagonal blocks stand for Frenkel exciton ( F ), CT exciton ( C ), electron transfer ( t e ), and hole transfer ( t h ) subspaces, and the off‐diagonal blocks ( V and U ) represent the interaction among them. The detailed matrix elements of the CM Hamiltonian of the CTE(f) model have been described in our previous study …”

“…This model has been used extensively in the studies of π‐conjugated systems and proven to give reliable results for spectroscopic and other linear or nonlinear optical properties with respect to the experimental results and computationally highly extensive ab initio calculations . The Pariser–Parr–Pople model has been shown to well reproduce the excitation energies as well as excited‐state properties compared to the all‐valence INDO/S model which provides reliable data in comparison with the UV–vis and UPS spectra of many conjugated molecules . To obtain the contribution of the CT characters to the lowest two excitation energies, four truncated models were used to compare with the full CM Hamiltonian (CTE(f) model).…”