Impact of Molecular Orientation and Packing Density on Electronic Polarization in the Bulk and at Surfaces of Organic Semiconductors

Electronic Couplings for Triplet-Triplet Annihilation Upconversion in Crystal Rubrene

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process able to repackage two low-frequency photons into light of higher energy. This transformation is typically orchestrated by the electronic degrees of freedom within organic compounds possessing suitable singlet and triplet energies and electronic couplings. In this work, we propose a computational protocol for the assessment of electronic couplings crucial to TTA-UC in molecular materials and apply it to the study of crystal rubrene. Our methodology integrates sophisticated yet computationally affordable approaches to quantify couplings in singlet and triplet energy transfer, the binding of triplet pairs, and the fusion to the singlet exciton. Of particular significance is the role played by charge-transfer states along the b-axis of rubrene crystal, acting as both partial quenchers of singlet energy transfer and mediators of triplet fusion. Our calculations identify the π-stacking direction as holding notable triplet energy transfer couplings, consistent with the experimentally observed anisotropic exciton diffusion. Finally, we have characterized the impact of thermally induced structural distortions, revealing their key role in the viability of triplet fusion and singlet fission. We posit that our approaches are transferable to a broad spectrum of organic molecular materials, offering a feasible means to quantify electronic couplings.

The impact of molecular orientations on the energy levels of A-D-A acceptors: implications for the charge separation driving force of organic solar cells

The energetic landscape of charge carriers, namely the ionization potential (IP) and electron affinity (EA), can play a crucial role in the charge separation and migration processes for organic solar cells (OSCs). However, the impact of molecular orientations on the energy levels remains elusive, especially in acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) with intrinsic anisotropy. Using the self-consistent quantum mechanics/embedded charge (sc-QM/EC) approach, we have investigated the energy level shifts from the edge-on or face-on surfaces to the bulk phase for three typical NFA crystals, IDIC-4F, INIC-4F, and Y6. The results point out that the surface-to-bulk changes in IP are limited within 0.2 eV for both the orientations due to the mutual counteraction between the electrostatic and induction effects. In sharp contrast, the EA values are substantially decreased from the bulk to the surfaces; especially, for the face-on orientation, the reduction reaches 0.5-0.8 eV. This indicates that the face-on orientation can provide a significant driving force for electrons moving from the surface or the interface to the bulk phase and thus improve the charge separation efficiency. Our work indicates that enhancing the face-on orientation is an effective method to increase the charge separation driving force for the OSCs based on A-D-A NFAs.

The ionization energy of α-sexithiophene and p-sexiphenyl in 2D and 3D thin films grown on silicon oxide surfaces

The ionization energy (IE) of molecular thin films is sensitive to the molecular environments surrounding ionized molecules. In this study, we investigate the evolution of the IE on dimension-controlled 2D and 3D thin films of rod-like molecules, namely α-sexithiophene (α-6T) and p-sexiphenyl (p-6P), grown on SiO2 surfaces via photoelectron yield spectroscopy. In 2D thin films, the IE of α-6T and p-6P showed nearly constant values of 4.93 eV and 5.50 eV, respectively. In 3D thin films, however, with increasing coverage, the IE of the α-6T thin films gradually decreased to 4.84 eV at 15 ML (ML: monolayer) and the IE of the p-6P thin films gradually increased to 5.65 eV at 7 ML. These experimental findings underline the less significant impact of the domain size on the IE in 2D thin films and the significant impact of the neighboring layers on the IE in 3D thin films.

Toward Quantifying the Relation between Exciton Binding Energies and Molecular Packing

Reducing the exciton binding energy E_b of organic photoactive materials is critical to minimize the energy loss and improve the photovoltaic efficiency of organic solar cells. However, the relation between the E_b and molecular packing is not well understood. Herein, the E_b in the crystals of a series of A-D-A type nonfullerene acceptors with different lengths of alkyl side chains has been examined by self-consistent quantum mechanics/embedded charge calculations. The variation of molecular packing induced by the different alkyl chains can have an important impact on the polarization effect of charge carriers and thereby the E_b. More interestingly, the E_b values are found to be linearly increased with the ratio of the void fraction vs the packing coefficient of molecular backbones in the solid crystals. Owing to the smallest ratio, a remarkable low E_b of several tens of meV is achieved for the acceptor with an optimal length of alkyl chains.

Effect of the π-bridge on the light absorption and emission in push-pull coumarins and on their supramolecular organization

A family of eight π-extended push-pull coumarins with cross-conjugated (amide) and directly conjugated (p-phenylene, alkyne, alkene) bridges were synthesized through a convergent strategy. Using an experimentally calibrated computational protocol, their UV-Visible light absorption and emission spectra in solution were investigated. Remarkably, amide-, alkyne- and alkene-bridges undergo comparable vertical excitations. The different nature of these bridges manifests during excited-state relaxation and fluorescence. We predict that these molecules can serve as building blocks for p-type semiconductors with low reorganization energies, below 0.2 eV. Since solid-state self-assembly is crucial for this application, we examined the effect of the π-bridge over the supramolecular organization in this family of compounds to determine if stacking prevails in these π-extended coumarin derivatives. Amide and alkyne spacers allow coplanar conformations which crystallize readily; p-phenylene hinders planarity yet allows facile crystallization; alkene-bridged molecules eluded all crystallization attempts. All the crystals obtained feature dense face-to-face π-stacking with 3.5-3.7 Å interlayer distances, expected to facilitate charge transfer processes in the solid state.

Evolution of the Ionization Energy in Two- and Three-Dimensional Thin Films of Pentacene Grown on Silicon Oxide Surfaces

The ionization energy (IE) of pentacene in two- and three-dimensional (2D and 3D) thin films and its evolution with coverage were studied via photoelectron yield spectroscopy in ambient conditions. In the 2D thin films, the IE of pentacene was found to be nearly constant at 4.91 eV, irrespective of its island size, for an average island size exceeding 1.6 × 10⁴ nm². In the 3D thin films, however, a reduction in IE by 0.04 eV was clearly observed upon stacking an additional molecular layer on top of the monolayer film, and the IE decreased to 4.73 eV at 20 monolayers. These experimental findings demonstrate the IE evolution in the buried layers of the 3D thin films and the significant impact of the neighboring molecular layers on the IE in layered systems with molecular aggregation.

Solid state structure and properties of phenyl diketopyrrolopyrrole derivatives

Crystal structures of the title compounds show diverse packing by interactions of auxochromes giving materials with varied optoelectronic properties.

Barrier-Free Charge Separation Enabled by Electronic Polarization in High-Efficiency Non-fullerene Organic Solar Cells

The separation of charge-transfer states into free charges at the donor/acceptor (D/A) interfaces plays a central role in organic solar cells (OSCs). Because of strong Coulomb attraction, the separation mechanisms are elusive, particularly for the high-efficiency non-fullerene (NF) OSCs with low exciton-dissociation driving forces. Here, we demonstrate that the Coulomb barriers can be substantially overcome by electronic polarization for OSCs based on a series of A-D-A acceptors (ITIC, IT-4F, and Y6). In contrast to fullerene-based D/A heterojunctions, the polarization energies for both donor holes and acceptor electrons are remarkably increased from the interfaces to pure regions in the NF heterojunctions because of strong stabilization on electrons but destabilization on holes by electrostatic interactions in the A-D-A acceptors. In particular, upon incorporation of fluorine substituents and electron-poor cores into ITIC, the increased polarization energies can completely compensate for the Coulomb attraction in the IT-4F- and Y6-based heterojunctions, leading to barrierless charge separation.

Tailoring the growth and electronic structures of organic molecular thin films

In the rapidly developing electronics industry, it has become increasingly necessary to explore materials that are cheap, flexible and versatile which have led to significant research efforts towards organic molecular thin films. Organic molecules are unique compared to their inorganic atomic counterparts as their properties can be tuned drastically through chemical functionalization, offering versatility, though their extended shape and weak intermolecular interactions bring significant challenges to the control of both the growth and the electronic structures of molecular thin films. In this paper, we will review the self-assembly process and how to establish long-range ordered organic molecular thin films. We will also discuss how the electronic structures of thin films are impacted by the molecule's local electrostatic environment and its interaction with the substrate, within the context of controlling interfacial energy level alignment between organic semiconductors and electrodes in electronic devices.

Concentration dependent energy levels shifts in donor-acceptor mixtures due to intermolecular electrostatic interaction

Recent progress in the improvement of organic solar cells lead to a power conversion efficiency to over 16%. One of the key factors for this improvement is a more favorable energy level alignment between donor and acceptor materials, which demonstrates that the properties of interfaces between donor and acceptor regions are of paramount importance. Recent investigations showed a significant dependence of the energy levels of organic semiconductors upon admixture of different materials, but its origin is presently not well understood. Here, we use multiscale simulation protocols to investigate the molecular origin of the mixing induced energy level shifts and show that electrostatic properties, in particular higher-order multipole moments and polarizability determine the strength of the effect. The findings of this study may guide future material-design efforts in order to improve device performance by systematic modification of molecular properties.

Intra- and intermolecular interaction of anthracene moieties in 7,8-disilabicyclo[3.3.0]octadienyl-bridged bisanthracenes

V-shaped 9-anthrylarylsilane dimers prepared through ruthenium-catalyzed C–H cleavage showed excimer emission and bathochromic shifts by promoted contact of π-orbitals.

Disentangling "Bright" and "Dark" Interactions in Ordered Assemblies of Organic Semiconductors

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10-15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how "dark" (ground-state) interactions in organic semiconductors can be distinguished from "bright" (excited-state) exciton coupling typically assessed by spectral measurements alone.

Tuning the ambipolar charge transport properties of tricyanovinyl-substituted carbazole-based materials

The ambipolar charge transport properties of a series of push–pull carbazole-based semiconductors are here evaluated. The ambipolar characteristics depend on the supramolecular organization. Experimental results were confirmed and justified through the X-ray analysis of single crystals and by theoretical calculations.

Planarity and multiple components promote organic photovoltaic efficiency by improving electronic transport

Establishing how the conformation of organic photovoltaic (OPV) polymers affects their electronic and transport properties is critical in order to determine design rules for new OPV materials and in particular to understand the performance enhancements recently reported for ternary blends. We report coupled classical and ab initio molecular dynamics simulations showing that polymer linkage twisting significantly reduces optical absorption efficiency, as well as hole transport rates in donor polymers. We predict that blends with components favoring planar geometries contribute to the enhancement of the overall efficiency of ternary OPVs. Furthermore, our electronic structure calculations for the PTB7-PID2-PC71BM system show that hole transfer rates are enhanced in ternary blends with respect to their binary counterpart. Finally, our results point at thermal disorder in the blend as a key reason responsible for device voltage losses and at the need to carry out electronic structure calculations at finite temperature to reliably compare with experiments.

Polarization Energies at Organic-Organic Interfaces: Impact on the Charge Separation Barrier at Donor-Acceptor Interfaces in Organic Solar Cells

We probe the energetic landscape at a model pentacene/fullerene (C60) interface to investigate the interactions between positive and negative charges, which are critical to the processes of charge separation and recombination in organic solar cells. Using a polarizable force field, we find that polarization energy, i.e., the stabilization a charge feels due to its environment, is larger at the interface than in the bulk for both a positive and a negative charge. The combination of the charge being more stabilized at the interface and the Coulomb attraction between the charges results in a barrier to charge separation at the pentacene/C60 interface that can be in excess of 0.7 eV for static configurations of the donor and acceptor locations. However, the impact of molecular motions, i.e., the dynamics, at the interface at room temperature results in a distribution of polarization energies and in charge separation barriers that can be significantly reduced. The dynamic nature of the interface is thus critical, with the polarization energy distributions indicating that sites along the interface shift in time between favorable and unfavorable configurations for charge separation.