Recently, it was demonstrated that electrochemical doping fronts in organic semiconductors ex- hibit a new fundamental instability growing from multidimensional perturbations [Phys. Rev. Lett. 107, 016103 (2011)]. In the instability development, linear growth of tiny perturbations goes over into a nonlinear stage of strongly distorted doping fronts. Here we develop the nonlinear theory of the doping front instability and predict the key parameters of a distorted doping front, such as its velocity, in close agreement with the experimental data. We show that the instability makes the electrochemical doping process considerably faster. We obtain the self-similar properties of the front shape corresponding to the maximal propagation velocity, which allows for a wide range of controlling the doping process in the experiments. The theory developed provides the guide for optimizing the performance of organic optoelectronic devices.
The interfacial structure plays a critical role in modern optoelectronics. Currently multilayer electrodes are used to optimize the injection and lifetime properties. The choice of interlayer is not universal, with different effects for the same material with different capping metals. Using a novel in-situ characterization method with X-ray photoelectron spectroscopy, the organic/inorganic interface in OLEDs was examined for two common cathode metals with a LiF interlayer. The impact of the interfacial layer on the performance of devices can be attributed to the bulk lattice matching of the in…
In this work we address the microscopic effects related to the implementation of a Bathophenanthroline (BPhen) exciton blocking layer (EBL) sandwiched between Ag cathode and molecular Diindenoperylene (DIP)/C60 bilayer of a photovoltaic cell. Complementary studies of current density, external quantum efficiency, and photoluminescence quenching for EBL thicknesses up to 50 nm indicate that Ag atoms are able to penetrate through the whole 35 nm thick C60 film into the crystalline DIP layer underneath, thereby enhancing exciton quenching if no blocking layer is applied. In contrast, an optimal tr…
Thermally stable polymers have attracted a lot of interest due to their potential use as the active component in electronic, optical and optoelectronic applications, such as light-emitting diodes, light emitting electrochemical cells, photodiodes, photovoltaic cells, field effect transistors, optocouplers and optically pumped lasers in solution and solid state.We report results of investigations into the use of thermal treatment of poly(p-phenylene vinylene) (PPV) films grown on a variety of substrates (quartz and glass). Film thickness, morphology and structural properties were investigated b…
Organic-inorganic hybrids may offer material properties not available from their inorganic components. However, they are typically less stable and disordered. Long-term stability study of the hybrid materials, over the anticipated lifespan of a real-world electronic device, is practically nonexistent. Disordering, prevalent in most nanostructure assemblies, is a prominent adversary to quantum coherence. A family of perfectly ordered II-VI based hybrid nanostructures has been shown to possess a number of unusual properties and potential applications. Here, using a prototype structure ZnTe(en)0.…
Structural stability, electronic structure and optical properties of CH3NH3BaI3 hybrid perovskite is examined both from theory as well as experiment. Solution-processed thin films of CH3NH3BaI3 exhibited a high band gap of approximately 3.87 eV, which is in excellent agreement with the theoretical estimate of 4 eV. Also, the XRD patterns of the thin films match well with the l-peaks of the simulated pattern obtained from the relaxed unit cell of CH3NH3BaI3, crystallizing in the I4/mcm space group, with lattice parameters, a = 9.30 A, c = 13.94 A. Atom projected density of state and band struct…
The hybrid organic inorganic perovskites (HOIPs) have attracted much attention for their potential applications as novel optoelectronic devices. Remarkably, the Rashba band splitting, together with specific spin orientations in k space (i.e., spin texture), has been found to be relevant for the optoelectronic performances. In this work, by using first principles calculations and symmetry analyses, we study the electric polarization, magnetism, and spin texture properties of the antiferromagnetic (AFM) HOIP ferroelectric TMCM_MnCl3 (TMCM = (CH3)3NCH2Cl, trimethylchloromethyl ammonium). This rec…
Covalent organic frameworks (COFs) offer a high degree of chemical and structural flexibility. There is a large family of COFs built from 2D sheets that are stacked to form extended crystals. While it has been common to represent the stacking as eclipsed with one repeating layer ("AA"), there is growing evidence that a more diverse range of stacking sequences is accessible. Herein, we report a computational study of layer stacking in two prototypical COFs, Tp-Azo and DAAQ-TFP, which have shown high performance as Li-ion battery electrodes. We find a striking preference for slipped structures w…
Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are introducing new directions in the 2D materials landscape. The coexistence of ferroelectricity and spin-orbit interactions play a key role in their optoelectronic properties. We perform a detailed study on a recently synthesized ferroelectric 2D-HOIP, (AMP)PbI$_4$ (AMP = 4-aminomethyl-piperidinium). The calculated polarization and Rashba parameter are in excellent agreement with experimental values. We report a striking new effect, i.e., an extraordinarily large Rashba anisotropy that is tunable by ferroelectric polarization:…
An electrode contact-related mechanism for the operational instability of organic electronic devices is proposed and confirmed via observation of a water-induced change in charge-injection barrier eights at the electrode/organic-semiconductor interfaces. Water molecules in air penetrate into the organic crystal via diffusion, and an external electric field orients the electric dipole of the water molecules at the electrode surfaces, leading to dipolar switching of the charge-injection barrier height. As a result of the switching, current-voltage curves of two-terminal Au-rubrene-Au devices cha…
By applying Monte Carlo simulations we found that the extraction of bound polaron pairs (PP) at the electrodes is an important loss factor limiting the efficiency of organic optoelectronic and photovoltaic devices. Based upon this finding, we developed a unified analytic model consisting of exact Onsager theory, describing the dissociation of PP in organic donor-acceptor heterojunctions, the Sokel-Hughes model for the extraction of free polarons at the electrodes, as well as of PP diffusion leading to the aforementioned loss mechanism, which was not considered previously. Our approach allows t…
Organometallic complexes have potential applications as the optically active components of organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). Development of more effective complexes may be aided by understanding their excited state properties. Here we discuss two key theoretical approaches to investigate these complexes: first principles atomistic models and effective Hamiltonian models. We review applications of these methods, such as, determining the nature of the emitting state, predicting the fraction of injected charges that form triplet excitations, and explaining the…
Atomistic modeling of energetic disorder in organic semiconductors (OSCs) and its effects on the optoelectronic properties of OSCs requires a large number of excited-state electronic-structure calculations, a computationally daunting task for many OSC applications. In this work, we advocate the use of deep learning to address this challenge and demonstrate that state-of-the-art deep neural networks (DNNs) are capable of predicting the electronic properties of OSCs at an accuracy comparable with the quantum chemistry methods used for generating training data. We extensively investigate the perf…
Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic semiconductors are of current interest for their room-temperature polariton formation. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polariton physics. Here we demonstrate polariton population from the triplet manifold via triplet-triplet annihilatio…
Hybrid organic-inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron-phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report the first high-precision measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high energy resolution and fully deuterated s…
Water permeation pathways in electronic devices should be eliminated for the suppression of operational instabilities. We investigated possible pathways in field-effect transistors based on a laminated single crystal (SC) of an organic semiconductor, rubrene. Water-induced instabilities were found to be more obvious with a thicker rubrene SC. Furthermore, under our simulation conditions, molecular dynamics calculations of water diffusion on a rubrene SC showed that no water molecules penetrated the SC. These findings indicate that a space at the SC/substrate interface is a dominant pathway. Th…
Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our …
Hybrid organic-inorganic halide perovskites have emerged as a disruptive new class of materials, exhibiting optimum properties for a broad range of optoelectronic applications, most notably for photovoltaics. The first report of highly efficient organic-inorganic perovskite solar cells in 2012 marked a new era for photovoltaics research, reporting a power conversion efficiency of over 10%. Only five years after this discovery, perovskite photovoltaic devices have reached a certified efficiency of 22.7%, making them the first solution processable technology to surpass thin film and multi-crysta…
Chalcogenide perovskites have received considerable interest in photovoltaic research community owing to their stability (thermal and aqueous), non-toxicity and lead free composition. However, to date a theoretical study mainly focusing on the excitonic and polaronic properties are not explored rigorously, due to its huge computational demand. Herein, we capture the excitonic and polaronic effects in a series of chalcogenide perovskites ABS$_3$ where A=Ba, Ca, Sr, and B=Hf, Sn by employing state-of-the art hybrid density functional theory and many body perturbative approaches viz. GW and BSE. …
Metal-organic frameworks (MOFs) symbolize the particular class of hybrid crystalline, nano-porous materials made of either discrete metal ions or clusters with organic linkers. Past studies on MOFs-based materials largely focused on porosity, chemical and structural diversity, gas sorption, sensing, drug delivery, catalysis, and separation applications. However, initial efforts either neglected or have not gained much attention to refine the electrical conductivity of MOFs materials. MOFs reported earlier with poor electrical conductivity impeded to employ in electronics, optoelectronics, and …
We investigate the charge and lattice states in a quasi-one-dimensional organic ferroelectric material, TTF-QCl$_{4}$, under pressures of up to 35 kbar by nuclear quadrupole resonance experiments. The results reveal a global pressure-temperature phase diagram, which spans the electronic and ionic regimes of ferroelectric transitions, which have so far been studied separately, in a single material. The revealed phase diagram clearly shows that the charge-transfer instability and the lattice symmetry breaking, which coincide in the electronic ferroelectric regime at low pressures, bifurcate at a…
Our research shows that commercially available graphene on quartz modified with rhenium oxide meets the requirements for its use as a conductive and transparent anode in optoelectronic devices. The cluster growth of rhenium oxide enables an increase in the work function of graphene by 1.3 eV up to 5.2 eV, which guarantees an appropriate adjustment to the energy levels of the organic semiconductors used in OLED devices.
Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. The crystalline layers are atomically s…
Hybrid systems consisting of organic molecules at inorganic semiconductor surfaces are gaining increasing importance as thin film devices for optoelectronics. The efficiency of such devices strongly depends on the collective behavior of the adsorbed molecules. In the present paper we propose a novel, coarse-grained model addressing the condensed phases of a representative hybrid system, that is, para-sexiphenyl (6P) at zinc-oxide (ZnO). Within our model, intermolecular interactions are repre- sented via a Gay-Berne potential (describing steric and van-der-Waals interactions) combined with the …
The broad use of organic semiconductors for optoelectronic applications relies on quantitative understanding and control of their spectroscopic properties. Of paramount importance are the transport gap - the difference between ionization potential and electron affinity - and the exciton binding energy - inferred from the difference between the transport and optical absorption gaps. Transport gaps are commonly established via photoemission and inverse photoemission spectroscopy (PES/IPES). However, PES and IPES are surface-sensitive, average over a dynamic lattice, and are subject to extrinsic …
Excitons are widely found in semiconductors. In contrast to inorganic-based semiconductors, where the presence of excitons may need special morphonology, like low-dimensional layer structure, the excitons prevailingly exist in molecular organic semiconductors in a three-dimension solid state. The excitons can undergo a charging process to yield another kind of quasiparticles originating from the coupling of excitons and free charges. Herein, we investigated the interaction between excitons and free charges in a representative arylamine-derivative organic semiconductor(NPB), by the time-of-flig…
Exciton diffusion in organic films is crucial phenomenon in optoelectronic devices such as organic light-emitting diodes and organic photovoltaics. However, diffusion of exciplexes has not been actively studied despite their ever-growing importance in such devices due to the lack of apparent charge-transfer absorption, resulting in the absence of energy transfer (ET) to exciplex-forming pairs. Here, the ET from exciplexes to exciplex-forming pairs is reported by analysis of transient photo-luminescent profiles. Recent reports and our own observation of the sub-bandgap charge-transfer absorptio…
The semiconducting p-n junction is a simple device structure with great relevance for electronic and optoelectronic applications. The successful integration of low-dimensional materials in electronic circuits has opened the way forward for producing gate-tunable p-n junctions. In that context, here we present an organic (Cu-phthalocyanine)-2D layered material (MoS2) hybrid p-n junction with both gate-tunable diode characteristics and photovoltaic effect. Our proof-of-principle devices show multifunctional properties with diode rectifying factors of up to 10^4, while under light exposure they e…
Despite the highly porous nature with significantly large surface area, metal organic frameworks (MOFs) can be hardly used in electronic, and optoelectronic devices due to their extremely poor electrical conductivity. Therefore, the study of MOF thin films that require electron transport or conductivity in combination with the everlasting porosity is highly desirable. In the present work, thin films of Co3(NDC)3DMF4 MOFs with improved electronic conductivity are synthesized using layer-by-layer and doctor blade coating techniques followed by iodine doping. The as-prepared and doped films are c…
Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown remarkable progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over thr…
The electronic wavefunctions of an atom or molecule are affected by its interactions with its environment. These interactions dictate electronic and optical processes at interfaces, and is especially relevant in the case of thin film optoelectronic devices such as organic solar cells. In these devices, charge transport and interfaces between multiple layers occur along the thickness or vertical direction, and thus such electronic interactions are crucial in determining the device properties. Here, we introduce a new in-situ spectroscopic ellipsometry data analysis method called DART with the a…
