Non-innocent side-chains with dipole moments in organic solar cells improve charge separation

Providing sustainable energy is one of the biggest challenges nowadays. An attractive answer is the use of organic solar cells to capture solar energy. Recently a promising route to increase their efficiency has been suggested: developing new organic materials with a high dielectric constant. This solution focuses on lowering the coulomb attraction between electrons and holes, thereby increasing the yield of free charges. In here, we demonstrate from a theoretical point of view that incorporation of dipole moments in organic materials indeed lowers the coulomb attraction. A combination of molecular dynamics simulations for modelling the blend and ab initio quantum chemical calculations to study specific regions was performed. This approach gives predictive insight in the suitability of new materials for application in organic solar cells. In addition to all requirements that make conjugated polymers suitable for application in organic solar cells, this study demonstrates the importance of large dipole moments in polymer side-chains

On the relation between local and charge-transfer exciton binding energies in organic photovoltaic materials

In organic photovoltaic devices two types of excitons can be generated for which different binding energies can be defined: the binding energy of the local exciton generated immediately after light absorption on the polymer and the binding energy of the charge-transfer exciton generated through the electron transfer from polymer to PCBM. Lowering these two binding energies is expected to improve the efficiency of the devices. Using (time-dependent) density functional theory, we studied whether a relation exists between the two different binding energies. For a series of related co-monomers, we found that the local exciton binding energy on a monomer is not directly related to that of the charge-transfer exciton on a monomer-PCBM complex because the variation in exciton binding energy depends mainly on the variation in electron affinity, which does not affect in a direct way the charge-transfer exciton binding energy. Furthermore, for the studied co-monomers and their corresponding trimers, we provide detailed information on the amount of charge transfer upon excitation and on the charge transfer excitation length. This detailed study of the excitation process reveals that the thiophene unit that links the donor and acceptor fragments of the co-monomer actively participates in the charge transfer process

The behaviour of charge distributions in dielectric media

Screened Coulomb interaction in dielectrics is often used as an argument for a lower exciton binding energy and easier exciton dissociation in a high dielectric material. In this paper, we show that at length scales of excitons (10-20 angstrom), the screened Coulomb law is invalid and a microscopic (quantum chemical) description is necessary to describe the medium effect on exciton dissociation. The exciton dissociation energy decreases with increasing dielectric constant, albeit deviating from the inversely proportional relationship. The electron-hole interaction energy, approximated with a point charge model, is apparently not affected by the dielectric constant of the environment. (C) 2014 Elsevier B.V. All rights reserved

Stabilizing cations in the backbones of conjugated polymers

We synthesized a cross-conjugated polymer containing ketones in the backbone and converted it to a linearly conjugated, cationic polyarylmethine via a process we call "spinless doping" to create a new class of materials, conjugated polyions. This process involves activating the ketones with a Lewis acid and converting them to trivalent cations via the nucleophilic addition of electron-rich aryl moieties. Spinless doping lowers the optical band gap from 3.26 to 1.55 eV while leaving the intrinsic semiconductor properties of the polymer intact. Electrochemical reduction (traditional doping) further decreases the predicted gap to 1.18 eV and introduces radicals to form positive polarons; here, n-doping produces a p-doped polymer in its metallic state. Treatment with a nucleophile (NaOMe) converts the cationic polymer to a neutral, non-conjugated state, allowing the band gap to be tuned chemically, postpolymerization. The synthesis of these materials is carried out entirely without the use of Sn or Pd and relies on scalable Friedel-Crafts chemistry

Explorative computational study of the singlet fission process

Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the 1TT state can be identified using multi-reference techniques. The geometrical and vibrational properties of the 1TT state are such that they can be approximated with those of the 5TT state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the 1TT and S1 states are close in energy with a large non-adiabatic coupling matrix element at the S1 geometry, and the energy of the S0 state is well below that of the 1TT state at the 1TT geometry. The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The 1TT states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and 1TT[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and 1TT[2] excited states.

Synthesis of telechelic and three-arm polytetrahydrofuran-block-amylose

Telechelic amine terminated polytetrahydrofuran (PTHF) is prepared via cationic ring opening polymerization (CROP) of THF, initiated by trifluoromethanesulphonic anhydride (triflic anhydride). Hexamethylene tetramine (HMTA) is used as a terminating agent. The resulting HMTA terminated PTHF is hydrolyzed to result in an amine terminated PTHF. Reductive amination is carried out by reacting the PTHF with maltoheptaose resulting in maltoheptaose-b-PTHF-b-maltoheptaose. The product is prepared as a primer for the enzymatic polymerization to synthesize amylose-b-PTHF-b-amylose. In addition, a three-arm PTHF is prepared via CROP of THF. The initiator is synthesized in situ by the reaction of triflic anhydride and triethanol amine. The resulting amine terminated three-arm PTHF is reacted with maltoheptaose to synthesize a three-arm PTHF-b-maltoheptaose which can be used for the enzymatic synthesis of three-arm PTHF-b-amylose. Characterization of the products is difficult due to the amphiphilic behavior of both telechelic amylose-b-PTHF-b-amylose and three-arm PTHF-b-amylose. Therefore, the analysis of the products is mainly based on attenuated total reflectance Fourier transform infrared spectroscopy. Telechelic and three-arm polytetrahydrofuran-block-amylose can be synthesized by coupling telechelic and three-arm polytetrahydrofuran with maltoheptaose, followed by enzymatic polymerization to elongate the saccharide blocks. The covalently attached amylose in these block copolymers can form inclusion complexes with suitable guest molecules which leads to a facile approach in modifying block copolymers to result in highly ordered supramolecules

Influence of push-pull group substitution patterns on excited state properties of donor-acceptor co-monomers and their trimers

Organic electronics form a very promising new generation of cheap, lightweight and flexible devices. Of special interest is the ability to engineer photo-physical properties of organic molecules by chemical modification. In this regard, the purpose of this research is to understand the influence of push-pull group substitution patterns on excited state properties of several donor-acceptor co-monomers an their trimers. Part of this work focuses on organic photovoltaic applications to demonstrate the practical use of the structure-property relations. In this context, the strong exciton binding energy determined by the electron-hole interaction is an important property. (Time-dependent) Density Functional Theory calculations showed a significant difference between linear- and cross-conjugated push-pull group pathways for the electron-hole interaction and the vertical exciton binding energy, which can be understood from simple Huckel theory. A linear relation between the dipole moment change upon excitation and the vertical exciton binding energy hints to a possible correlation, although this relation is less pronounced for the trimers. The overlap density between the frontier molecular orbitals alone already reveals valuable information about the relative size of the electron-hole interaction and the vertical exciton binding energy. Application of our findings in the context of organic photovoltaics results in significant support for cross-conjugated mesomeric push-pull group pathways in order to spatially separate the HOMO and LUMO. (C) 2014 Elsevier B.V. All rights reserved

Promising Strategy To Improve Charge Separation in Organic Photovoltaics: Installing Permanent Dipoles in PCBM Analogues

A multidisciplinary approach involving organic synthesis and theoretical chemistry was applied to investigate a promising strategy to improve charge separation in organic photovoltaics: installing permanent dipoles in fullerene derivatives. First, a PCBM analogue with a permanent dipole in the side chain (PCBDN) and its reference analogue without a permanent dipole (PCBBz) were successfully synthesized and characterized. Second, a multiscale modeling approach was applied to investigate if a PCBDN environment around a central donor-acceptor complex indeed facilitates charge separation. Alignment of the embedding dipoles in response to charges present on the central donor-acceptor complex enhances charge separation. The good correspondence between experimentally and theoretically determined electronic and optical properties of PCBDN, PCBBz, and PCBM indicates that the theoretical analysis of the embedding effects of these molecules gives a reliable expectation for their influence on the charge separation process at a microscopic scale in a real device. This work suggests the following strategies to improve charge separation in organic photovoltaics: installing permanent dipoles in PCBM analogues and tuning the concentration of these molecules in an organic donor/acceptor blend