Tuning the Optoelectronic Properties of Two-Dimensional Hybrid Perovskite Semiconductors with Alkyl Chain Spacers

Layered two-dimensional organo-metal halide perovskites are currently in the limelight, largely because their versatile chemical composition offers the promise of tunable photophysical properties. We report here on (time-dependent) density functional theory [(TD)­DFT] calculations of alkyl-ammonium lead iodide perovskites, where significant changes in the electronic structure and optical properties are predicted when using long versus short alkyl chain spacers. The mismatch between the structural organization in the inorganic and organic layers is epitomized for dodecyl chains that adopt a supramolecular packing similar to that of polyethylene, at the cost of distorting the inorganic frame and, in turn, opening the electronic band gap. These results rationalize recent experimental data and demonstrate that the optoelectronic properties of layered halide perovskite semiconductors can be modified through the use of electronically inert organic saturated chains

Combined Molecular Dynamics and Density Functional Theory Study of Azobenzene–Graphene Interfaces

The electronic properties of graphene can be tuned in a dynamic way from physical adsorption of molecular photoswitches. Here, we first investigate the formation of 4-(decyloxy)­azobenzene molecular monolayers on a single graphene layer through molecular dynamics (MD) simulations and assess the associated change in work function (WF) at the density functional theory (DFT) level. We show that the major contribution to the WF shift arises from electrostatic effects induced by the azobenzene electric dipole component normal to graphene and that the conformational distribution of the molecular switches in either their trans or cis forms can be convoluted into WF distributions for the hybrid systems. We next use this strategy to build a statistical ensemble for the work functions of graphene decorated with fluorinated azobenzene derivative designed to maximize the change in WF upon photoswitching. These findings pave the way to the possible use of photoswitchable graphene-based hybrid materials as optically controlled memories for light-assisted programming and high-sensitive photosensors

Influence of Surface Termination on the Energy Level Alignment at the CH₃NH₃PbI₃ Perovskite/C60 Interface

The impressive photovoltaic performance of hybrid iodide CH₃NH₃PbI₃ perovskite relies, among other factors, on the optimal alignment of the electronic energy levels of the semiconductor with respect to conventional hole transporting (HTM) and electron transporting (ETM) materials. Here, we first report on density functional theory electronic structure calculations of slab models of the (001) surface aiming to assess how the perovskite valence and conduction band edge (VBE and CBE) energies depend on the nature of the surface exposed to vacuum. We find that the surface termination plays a critical role in determining the energies of the frontier crystal orbitals, with PbI-terminated surface showing VBE and CBE energy ∼1 eV below the corresponding levels in the methylammonium-terminated surfaces. We next build perovskite/C60 interfaces based on two such surfaces and discuss the associated electronic structure in light of recent experimental data. The two interfaces are rather inert showing limited band bending/shifts with respect to the isolated components, in line with photoelectron spectroscopy data. They, however, yield very different electron extraction energies, possibly explaining the different behaviors reported in the literature

Coherent Electron Transmission across Nanographenes Tethered to Gold Electrodes: Influence of Linker Topology, Ribbon Width, and Length

The conductance of several well-defined and experimentally accessible graphene nanoribbons (GNRs) linked to gold electrodes by thiol groups to form single-molecule junctions is investigated within the nonequilibrium Green’s function formalism coupled to density functional theory. We focus on the change in conduction as a function of the width and length of the ribbons as well as the number and position of the linking groups. The calculations illustrate that the position of the linkers is a key parameter controlling the conductance through the GNRs investigated here, as can be anticipated from their Clar sextet representations. The increase in width yields higher conductance only if accompanied by an increasing number of linkers due to the opening of additional pathways. The decay of transmission with GNR length is close to exponential, with rather low attenuation factors (0.06–0.11 Å^–1) that depend on the ribbon topology

Energy Level Alignment and Charge Carrier Mobility in Noncovalently Functionalized Graphene

Density functional theory calculations have been performed to assess the electronic structure of graphene overlaid with a monolayer of electro-active conjugated molecules, being either electron donors or electron acceptors. Such a noncovalent functionalization results in a work function modification that scales with the amount of electron transfer from or to graphene, in line with the formation of an interfacial dipole. The charge transfer is accompanied by a pinning of the donor HOMO/acceptor LUMO around the Fermi level and a shift in the vacuum level. The use of the Boltzmann transport equation combined with the deformation potential theory shows that large charge carrier mobilities are maintained upon noncovalent functionalization of graphene, thereby suggesting that molecular doping is an attractive approach to design conductive graphene electrodes with tailored work function

Singlet Fission in Rubrene Derivatives: Impact of Molecular Packing

We examine the properties of six recently synthesized rubrene derivatives (with substitutions on the side phenyl rings) that show vastly different crystal structures. In order to understand how packing in the solid state affects the excited states and couplings relevant for singlet fission, the lowest excited singlet (S₁), triplet (T₁), multiexciton (TT), and charge-transfer (CT) states of the rubrene derivatives are compared to known singlet fission materials [tetracene, pentacene, 5,12-diphenyltetracene (DPT), and rubrene itself]. While a small difference of less than 0.2 eV is calculated for the S₁ and TT energies, a range of 0.50 to 1.2 eV in the CT energies and nearly 3 orders of magnitude in the electronic couplings are computed for the rubrene derivatives in their crystalline packings, which strongly affects the role of the CT state in facilitating SF. To rationalize experimental observations of singlet fission occurring in amorphous phases of rubrene, DPT, and tetracene, we use molecular dynamics (MD) simulations to assess the impact of molecular packing and orientations and to gain a better understanding of the parameters that control singlet fission in amorphous films compared to crystalline packings. The MD simulations point to a crystalline-like packing for thin films of tetracene; on the other hand, DPT, rubrene, and the rubrene derivatives all show various degrees of disorder with a number of sites that have larger electronic couplings than in the crystal, which can facilitate singlet fission in such thin films. Our analysis underlines the potential of these materials as promising candidates for singlet fission and helps understand how various structural motifs affect the critical parameters that control the ability of a system to undergo singlet fission

Which Oxide for Low-Emissivity Glasses? First-Principles Modeling of Silver Adhesion

Density functional theory (DFT) calculations were performed to assess the work of adhesion of silver layers deposited on metal oxide surfaces differing by their chemical nature (ZnO, TiO₂, SnO₂, and ZrO₂) and their crystallographic face. The calculated work of adhesion values range from ∼0 to 3 J m^–2 and are shown to originate from the interplay between ionic (associated with charge transfer at the interface) and covalent (as probed by atomic bond orders between silver and the metal oxide atoms) interactions. The results are discussed in the context of the design of silver/metal oxide interfaces for low-emissivity glasses

Maximizing Singlet Fission by Intermolecular Packing

A novel nonadiabatic molecular dynamics scheme is applied to study the singlet fission (SF) process in pentacene dimers as a function of longitudinal and lateral displacements of the molecular backbones. Detailed two-dimensional mappings of both instantaneous and long-term triplet yields are obtained, characterizing the advantageous and unfavorable stacking arrangements, which can be achieved by chemical substitutions to the bare pentacene molecule. We show that the SF rate can be increased by more than an order of magnitude through tuning the intermolecular packing, most notably when going from cofacial to the slipped stacked arrangements encountered in some pentacene derivatives. The simulations indicate that the SF process is driven by thermal electron–phonon fluctuations at ambient and high temperatures, expected in solar cell applications. Although charge-transfer states are key to construct continuous channels for SF, a large charge-transfer character of the photoexcited state is found to be not essential for efficient SF. The reported time domain study mimics directly numerous laser experiments and provides novel guidelines for designing efficient photovoltaic systems exploiting the SF process with optimum intermolecular packing

To Hop or Not to Hop? Understanding the Temperature Dependence of Spectral Diffusion in Organic Semiconductors

In disordered organic semiconductors, excited states and charges move by hopping in an inhomogeneously broadened density of states, thereby relaxing energetically (“spectral diffusion”). At low temperatures, transport can become kinetically frustrated and consequently dispersive. Experimentally, this is observed predominantly for triplet excitations and charges, and has not been reported for singlet excitations. We have addressed the origin of this phenomenon by simulating the temperature dependent spectral diffusion using a lattice Monte Carlo approach with either Miller–Abrahams or Förster type transfer rates. Our simulations are in agreement with recent fluorescence and phosphorescence experimental results. We show that frustrated and thus dispersive diffusion appears when the number of available hopping sites is limited. This is frequently the case for triplets that transfer by a short-range interaction, yet may also occur for singlets in restricted geometries or dilute systems. Frustration is lifted when more hopping sites become available, e.g., for triplets as a result of an increased conjugation in some amorphous polymer films

Electronic Polarization in Organic Crystals: A Comparative Study of Induced Dipoles and Intramolecular Charge Redistribution Schemes

Static dielectric tensors and charge carrier polarization energies of a wide set of organic molecules of interest for organic electronics application are calculated with two different approaches: intramolecular charge redistribution and induced dipoles (microlectrostatics). Our results show that, while charge redistribution is better suited for calculating the collective response to an external field, both methods reliably describe the effect of a localized charge carrier in the crystal. Advantages and limitations inherent to the different methods are discussed, also in relation to previous theoretical studies. The agreement with available experimental data confers to our results a predictive character where measurements are missing