Theoretical Study of the Charge Transfer Exciton Binding Energy in Semiconductor Materials for Polymer:Fullerene-Based Bulk Heterojunction Solar Cells

Recent efforts and progress in polymer solar cell research have boosted the photovoltaic efficiency of the technology. This efficiency depends not only on the device architecture but also on the material properties. Thus, insight into the design of novel semiconductor materials is vital for the advancement of the field. This paper looks from a theoretical viewpoint into two of the factors for the design of semiconductor materials with applications to bulk heterojunction solar cells: the charge transfer exciton binding energy and the nanoscale arrangement of donor and acceptor molecules in blend systems. Being aware that the exciton dissociation of local excitons in charge transfer states initiates the charge generation process, the excited state properties of four oligomers (one donor-type: PEO–PPV; and three donor–acceptor-types: PTFB, PTB7, and PTB7–Th) and two fullerene derivatives ([60]­PCBM and [70]­PCBM), previously reported in the literature as having high electrical conductance, are studied. With such a study, the donor molecules, either of donor-type or donor–acceptor type, are screened as candidates for [60]­PCBM- and/or [70]­PCBM-based bulk heterojunctions. The charge transfer energy and charge transfer exciton binding energy of suitable donor:acceptor bulk heterojunctions, some of them not yet fabricated, are studied. Further, the charge transfer exciton binding energies of [60]­PCBM- and [70]­PCBM-based blends are compared. A combination of molecular dynamics simulations with calculations based on Kohn–Sham density functional theory (KS-DFT) and its time-dependent extension (KS-TDDFT) is used. An important feature of this work is that it incorporates the effect of the environment of the quantum chemical system in KS-DFT or KS-TDDFT calculations through a polarizable discrete reaction field (DRF). Our predictions in terms of the influence of the nanoscale arrangement of donor and acceptor molecules on the performance of organic solar cells indicate that bulk heterojunction morphologies for donor–acceptor-type oligomers lead to their lowest excited states having charge transfer character. Further, we find that in terms of favorable charge transfer exciton binding energy, the PTB7–Th:[70]­PCBM blends outperform the other blends

Electronic couplings for singlet fission:Orbital choice and extrapolation to the complete basis set limit

For the search for promising singlet fission candidates, the calculation of the effective electronic coupling, which is required to estimate the singlet fission rate between the initially excited state (S 0S 1) and the multiexcitonic state ( 1TT, two triplets on neighboring molecules, coupled into a singlet), should be sufficiently reliable and fast enough to explore the configuration space. We propose here to modify the calculation of the effective electronic coupling using a nonorthogonal configuration interaction approach by: (a) using only one set of orbitals, optimized for the triplet state of the molecules, to describe all molecular electronic states, and (b) only taking the leading configurations into consideration. Furthermore, we also studied the basis set convergence of the electronic coupling, and we found, by comparison to the complete basis set limit obtained using the cc-pVnZ series of basis sets, that both the aug-cc-pVDZ and 6–311++G** basis sets are a good compromise between accuracy and computational feasibility. The proposed approach enables future work on larger clusters of molecules than dimers

Quantifying the conceptual problems associated with the isotropic NICS through analyses of its underlying density

The isotropic Nucleus Independent Chemical Shift (NICSiso) is widely considered to be a suitable descriptor for aromaticity based on the correlations it exhibits with other aromaticity descriptors. To gain more insight into the origin of these correlations, we establish causal relations between the NICSiso and the underlying current density patterns by resolving the NICSiso into its underlying density. Our results indicate that the origin of the behavior of the NICSiso can be radically different from what is generally assumed. Not only does this bring into question the robustness of applying the NICSiso beyond the realms of where good correlations with other measures of aromaticity have been established, it also points to an inherent weakness in all interpretations of NICSiso values that are not based on additional data.</p

Can the Dielectric Constant of Fullerene Derivatives Be Enhanced by Side-Chain Manipulation? A Predictive First-Principles Computational Study

The low efficiency of organic photovoltaic (OPV) devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn Sham method to calculate the electronic contribution to the dielectric constant for fullerene C-60 derivatives, a ubiquitous class of molecules in the field of OPVs. The results show good agreement with experimental data when available and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: in all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant, and consequently, their electronic dielectric constant is upper bound by the one of C-60

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

Q-Force:Quantum Mechanically Augmented Molecular Force Fields

The quality of molecular dynamics simulations strongly depends on the accuracy of the underlying force fields (FFs) that determine all intra- and intermolecular interactions of the system. Commonly, transferable FF parameters are determined based on a representative set of small molecules. However, such an approach sacrifices accuracy in favor of generality. In this work, an open-source and automated toolkit named Q-Force is presented, which augments these transferable FFs with molecule-specific bonded parameters and atomic charges that are derived from quantum mechanical (QM) calculations. The molecular fragmentation procedure allows treatment of large molecules (&gt;200 atoms) with a low computational cost. The generated Q-Force FFs can be used at the same computational cost as transferable FFs, but with improved accuracy: We demonstrate this for the vibrational properties on a set of small molecules and for the potential energy surface on a complex molecule (186 atoms) with photovoltaic applications. Overall, the accuracy, user-friendliness, and minimal computational overhead of the Q-Force protocol make it widely applicable for atomistic molecular dynamics simulations.</p

Bulk Heterojunction Morphologies with Atomistic Resolution from Coarse-Grain Solvent Evaporation Simulations

Control over the morphology of the active layer of bulk heterojunction (BHJ) organic solar cells is paramount to achieve high efficiency devices. However, no method currently available can predict morphologies for a novel donor:acceptor blend. An approach which allows to reach relevant length scales, retain chemical specificity, mimic experimental fabrication conditions, and which is suited for high-throughput schemes has been proven challenging to find. Here, we propose a method to generate atom-resolved morphologies of BHJs which conforms to these requirements. Coarse-grain (CG) molecular dynamics simulations are employed to simulate the large-scale morphological organization during solution-processing. The use of CG models which retain chemical specificity translates into a direct path to the rational design of donor and acceptor compounds which differ only slightly in chemical nature. Finally, the direct retrieval of fully atomistic detail is possible through backmapping, opening the way for improved quantum mechanical calculations addressing the charge separation mechanism. The method is illustrated for the poly(3-hexyl-thiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) mixture, and found to predict morphologies in agreement with experimental data. The effect of drying rate, P3HT molecular weight and thermal annealing are investigated extensively, resulting in trends mimicking experimental findings. The proposed methodology can help reduce the parameter space which has to be explored before obtaining optimal morphologies not only for BHJ solar cells but for any other solution-processed soft matter device.</p

BxGe120/+ Clusters with x=1-4:Germanium Tubes Stabilized by Three and Four Boron Dopants

Some boron-doped germanium clusters BxGe12q with x = 1, 2, 3, and 4 and q = 0, 1 were designed as stabilized double ring tubes. While the B2Ge12 constitutes the smallest deltahedral germanium cluster, both B3Ge12+ and B4Ge12 clusters present us, for the first time, with an endohedral tubular motif in which either the B-3 or the B-4 cycle is encapsulated inside a Ge-12 hexagonal prism tube. Both B-3 and B-4 units thus satisfy a geometry requirement to create an endohedral structure within the Ge-12 double ring. Keeping their high symmetry, both B-3 and B-4 units generate delocalized bonds upon interaction with the Ge-12 tubular framework and thereby induce an aromatic character for the resulting B3Ge12+ and B4Ge12, respectively. Their aromaticity was probed by the magnetic responses of electron densities. Such a tubular aromaticity appears to greatly contribute to the high thermodynamic stability of the binary hexagonal germanium tubes

Resolving Donor-Acceptor Interfaces and Charge Carrier Energy Levels of Organic Semiconductors with Polar Side Chains

Organic semiconductors consisting of molecules bearing polar side chains have been proposed as potential candidates to overcome the limitations of organic photovoltaics owing to their enhanced dielectric constant. However, introducing such polar molecules in photovoltaic devices has not yet resulted in higher efficiencies. A microscopic understanding of the impact of polar side chains on electronic and structural properties of organic semiconductors is paramount to rationalize their effect. Here, the impact of such side chains on bulk heterojunction overall morphology, molecular configurations at donor-acceptor (DA) interfaces, and charge carrier energy levels is investigated. The multiscale modeling approach used allows to resolve DA interfaces with atomistic resolution while taking into account the large-scale self-organization process which takes place during the processing of an organic thin film. The polar fullerene-based blends are compared to the well-studied reference system, poly(3-hexyl-thiophene) (P3HT):phenyl-C-61-butyric acid methyl ester (PCBM). Introduction of polar side chains on a similar molecular scaffold does not affect molecular orientations at the DA interfaces; such orientations are, however, found to be affected by processing conditions and polymer molecular weight. Polar side chains, instead, are found to impact considerably the charge carrier energy levels of the organic blend, causing electrostatic-induced broadening of these levels