Molecular Organic Semiconductors Research Articles

High-throughput phase field simulation and machine learning for predicting the breakdown performance of all-organic composites

All-organic dielectric polymers are materials of choice for modern power electronics and high-density energy storage, and their performance can be significantly improved by doping trace amounts of organic molecular semiconductors with strong electron-affinity energy to suppress charge conduction losses. Insight into the breakdown mechanism of polymers/organic molecular semiconductor composites is essential for the design of high-performance dielectric polymers. This study investigates the impact of the doping concentration of organic molecular semiconductors, dielectric constants, and trap depths on the breakdown performance of dielectric polymers under high temperature and electric fields. A modified phase-field model, incorporating deep traps and carriers’ coulomb capture radius, has been developed to facilitate high-throughput simulations of electrical breakdown in polymer/organic molecular semiconductor composites. This work accurately predicted the breakdown strength of all-organic composites using high-throughput phase-field simulation data as input for machine learning, which provides crucial theoretical support for designing all-organic composite dielectric polymers for energy storage capacitors under extreme conditions.

Impact of Gate Voltage on Mobility of Charge Carriers in Conductive Channel of Organic Molecular Semiconductors: Transport Landscape Assessed by Alternating and Direct Current Mode Techniques

Impact of Gate Voltage on Mobility of Charge Carriers in Conductive Channel of Organic Molecular Semiconductors: Transport Landscape Assessed by Alternating and Direct Current Mode Techniques

Rapid Estimation of the Intermolecular Electronic Couplings and Charge-Carrier Mobilities of Crystalline Molecular Organic Semiconductors through a Machine Learning Pipeline.

Organic semiconductors (OSC) offer tremendous potential across a wide range of (opto)electronic applications. OSC development, however, is often limited by trial-and-error design, with computational modeling approaches deployed to evaluate and screen candidates through a suite of molecular and materials descriptors that generally require hours to days of computational time to accumulate. Such bottlenecks slow the pace and limit the exploration of the vast chemical space comprising OSC. When considering charge-carrier transport in OSC, a key parameter of interest is the intermolecular electronic coupling. Here, we introduce a machine learning (ML) model to predict intermolecular electronic couplings in organic crystalline materials from their three-dimensional (3D) molecular geometries. The ML predictions take only a few seconds of computing time compared to hours by density functional theory (DFT) methods. To demonstrate the utility of the ML predictions, we deploy the ML model in conjunction with mathematical formulations to rapidly screen the charge-carrier mobility anisotropy for more than 60,000 molecular crystal structures and compare the ML predictions to DFT benchmarks.

Exciton Transport in the Nonfullerene Acceptor O-IDTBR from Nonadiabatic Molecular Dynamics.

Theory, computation, and experiment have given strong evidence that charge carriers in organic molecular crystals form partially delocalized quantum objects that diffuse very efficiently via a mechanism termed transient delocalization. It is currently unclear how prevalent this mechanism is for exciton transport. Here we carry out simulation of singlet Frenkel excitons (FE) in a molecular organic semiconductor that belongs to the class of nonfullerene acceptors, O-IDTBR, using the recently introduced FE surface hopping nonadiabatic molecular dynamics method. We find that FE are, on average, localized on a single molecule in the crystal due to sizable reorganization energy and moderate excitonic couplings. Yet, our simulations suggest that the diffusion mechanism is more complex than simple local hopping; in addition to hopping, we observe frequent transient delocalization events where the exciton wave function expands over 10 or more molecules for a short period of time in response to thermal excitations within the excitonic band, followed by de-excitation and contraction onto a single molecule. The transient delocalization events lead to an increase in the diffusion constant by a factor of 3-4, depending on the crystallographic direction as compared to the situation where only local hopping events are considered. Intriguingly, O-IDTBR appears to be a moderately anisotropic 3D "conductor" for excitons but a highly anisotropic 2D conductor for electrons. Taken together with previous simulation results, two trends seem to emerge for molecular organic crystals: excitons tend to be more localized and slower than charge carriers due to higher internal reorganization energy, while exciton transport tends to be more isotropic than charge transport due to the weaker distance dependence of excitonic versus electronic coupling.

Excitons at the interface of 2D TMDs and molecular semiconductors.

Van der Waals heterostructures (vdWHs) of vertically stacked two-dimensional (2D) atomic crystals have been used to elicit intriguing phenomena stemming from strong electronic correlations, magnetic textures, and interlayer excitons spawned at the heterointerface. However, vdWHs comprised of heterointerfaces between these 2D atomic crystal lattices and molecular assemblies are emerging as equally intriguing platforms supporting properties to be harnessed for photovoltaic energy conversion, photodetection, spin-selective charge injection, and quantum emission. In this perspective, we summarize recent research examining exciton dynamics in heterostructures between semiconducting 2D transition metal dichalcogenides and molecular organic semiconductors. We discuss methods for assembly of these heterostructures, the nature of interlayer or charge-transfer excitons at transition-metal dichalcogenide (TMD)-molecule interfaces, explicit exciton transfer between organics and TMDs, and other interfacial phenomena driven by the merger of these two material classes. We also suggest key new research directions extending the remit of these 2D atomic-molecular lattice heterointerfaces into the domains of condensed matter physics, quantum sensing, and energy conversion.

Constructing a dual gradient structure of energy level gradient and concentration gradient to significantly improve the high-temperature energy storage performance of all organic composite dielectrics†

The severe electrical conduction loss at elevated temperature is the initial factor for the degradation of energy storage performance of polymer dielectric films, thus it urgently needs to develop highly heat-resistant polymer capacitive films for meeting the advancing electrical and electronic applications. For reducing conduction loss and improving the high-temperature energy storage performance of dielectrics, the current approaches are focusing on preventing carriers mobility at elevated temperature and high electric field. In this study, the molecular semiconductor ITIC (ITIC is an organic molecular semiconductor 2,2′-[[6,6,12,12-Tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]bis [methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] abbreviation) with high electron affinity was filled into the blends of polyether imide (PEI) and polyethersulfone (PESU). The energy level gradient was constructed due to the different energy bandgap structures among ITIC, PESU and PEI. In addition, the concentration structure gradient was constructed due to the concentration of ITIC which varies along the thickness direction. The results demonstrate that the dual gradients of energy level and concentration can effectively inhibit carrier migration and lower conduction loss, thus significantly improving the electric breakdown strength and energy storage performance at high temperature. The energy storage densities (Ue) of 5.14 J/cm3 and 3.6 J/cm3 at 150 °C and 200 °C, respectively were achieved in PEI/PESU blends with 9 layers of ITIC gradient and 0.25 % ITIC in the outer layer. This study puts forward a novel structural design combining the energy levels gradient with concentration gradient to optimise the high-temperature energy storage properties of all-organic dielectric films.

Size Isn't Everything: Geometric Tuning in Polycyclic Aromatic Hydrocarbons and Its Implications for Carbon Nanodots.

Recent developments in light-emitting carbon nanodots and molecular organic semiconductors have seen renewed interest in the properties of polycyclic aromatic hydrocarbons (PAHs) as a family. The networks of delocalized π electrons in sp2-hybridized carbon grant PAHs light-emissive properties right across the visible spectrum. However, the mechanistic understanding of their emission energy has been limited due to the ground state-focused methods of determination. This computational chemistry work, therefore, seeks to validate existing rules and elucidate new features and characteristics of PAHs that influence their emissions. Predictions based on (time-dependent) density functional theory account for the full 3-dimensional electronic structure of ground and excited states and reveal that twisting and near-degeneracies strongly influence emission spectra and may therefore be used to tune the color of PAHs and, hence, carbon nanodots. We particularly note that the influence of twisting goes beyond torsional destabilization of the ground-state and geometric relaxation of the excited state, with a third contribution associated with the electric transition dipole. Symmetries and peri-condensation may also have an effect, but this could not be statistically confirmed. In pursuing this goal, we demonstrate that with minimal changes to molecular size, the entire visible spectrum may be spanned by geometric modification alone; we have also provided a first estimate of emission energy for 35 molecules currently lacking published emission spectra as well as clear guidelines for when more sophisticated computational techniques are required to predict the properties of PAHs accurately.

Solution processable benzo[b]thiophene derivatives as small molecular organic semiconductors for organic field effect transistors

In this study, four new organic semiconductors based on benzo[b]thiophene derivatives were synthesized and characterized as the active layer of organic field-effect transistors (OFETs) to investigate the effect of different isomers and end-group moieties on the electrical performance. The top-contact/bottom-gate OFETs were fabricated via solution-shearing methods and exhibited p-channel activity. In particular, the thin film with a single BT moiety attached via the 6-position showed the highest mobility up to 0.055 cm2/Vs and current on/off ratio of 2.5 × 107. The microstructure and surface morphology of the thin films were analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM) to investigate the factors contributing to the difference in electrical performance of the devices.

Towards high-throughput exciton diffusion rate prediction in molecular organic semiconductors

Accurate property prediction is paramount to high-throughput screening of organic photovoltaics. Here, the accuracy of predicting exciton diffusion computationally is examined, and several simplifications towards high-throughput screening are explored.

Self-assembled π-conjugated chromophores: preparation of one- and two-dimensional nanostructures and their use in photocatalysis.

Photocatalytic systems have attracted research interest as a clean approach to generate energy from abundant sunlight. In this context, developing efficient and robust photocatalytic structures is crucial. Recently, self-assembled organic chromophores have entered the stage as alternatives to both molecular systems and (in)organic semiconductors. Nanostructures made of self-assembled π-conjugated dyes offer, on the one hand, molecular customizability to tune their optoelectronic properties and activities and on the other hand, provide benefits from heterogeneous catalysis that include ease of separation, recyclability and improved photophysical properties. In this contribution, we present recent achievements in constructing supramolecular photocatalytic systems made of chromophores for applications in water splitting, H2O2 evolution, CO2 reduction, or environmental remediation. We discuss strategies that can be used to prepare ordered photocatalytic systems with an emphasis on the effect of packing between the dyes and the resulting photocatalytic activity. We further showcase supramolecular strategies that allow interfacing the organic nanostructures with co-catalysts, molecules, polymers, and (in)organic materials. The principles discussed here are the foundation for the utilization of these self-assembled materials in photocatalysis.

Significantly improved high-temperature capacitive performance in polypropylene based on molecular semiconductor grafting

The high-temperature dielectric properties and energy storage performance of capacitive materials are of great significance for the sustainable development of new energy-related fields. However, the most widely used commercial capacitor dielectric biaxially oriented polypropylene (BOPP) films fail to satisfy the requirements of continuous operation above 105 °C at high electric fields. Here we demonstrate a molecular semiconductor-grafted polypropylene (PP) composite that possesses substantially enhanced dielectric and capacitive performance up to 120 °C by virtue of the modulated carrier transport behavior. The organic molecular semiconductor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is chemically grafted onto PP chains (PCBM-g-PP) to achieve strong interaction and decent compatibility between the PCBM and the polymer matrix. Based on computational simulation and experimental verification, it is confirmed that the grafted molecular semiconductor introduces deep traps to inhibit the migration of high-energy charge carriers excited by heat. In the meantime, the grafting also helps to intensify the regulation effect by exerting positive influences on the microstructure of the polymer. The PCBM-g-PP/PP composite films possess reduced leakage current and dielectric loss, as well as suppressed electric field distortion and elevated breakdown strength. At 120 °C, the energy storage density of the composite with an efficiency above 90% reaches 1.59 J/cm3, which is 683.62% that of the original PP film. The reported molecular semiconductor-grafting strategy is expected to promote the capacitive performance of polypropylene under hash-temperature conditions, facilitating the development of lightweight and compact-size dielectric capacitors.

Transiently delocalized states enhance hole mobility in organic molecular semiconductors.

Evidence shows that charge carriers in organic semiconductors self-localize because of dynamic disorder. Nevertheless, some organic semiconductors feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility organic semiconductors: dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (μ) with temperature (T) following μ ∝ T -n, the exponent n differs substantially. Modelling reveals that the differences between the two chemically similar semiconductors can be traced to the delocalization of the different states that are thermally accessible by charge carriers, which in turn depends on their specific electronic band structure. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states.

Solution-Processable Benzo[b]thieno[2,3-d]thiophene Derivatives as Organic Semiconductors for Organic Thin-Film Transistors

Two new benzo[b]thieno[2,3-d]thiophene (BTT) derivatives, 2-(benzo[b]thiophen-5-yl)benzo[b]thieno[2,3-d]thiophene (compound 2), and 2-(benzo[b]thieno[2,3-d]thiophene-2yl)dibenzo[b,d]thiophene (compound 3) have been synthesized and utilized as solution-processable small molecular organic semiconductors for organic field-effect transistors (OFETs). The physicochemical characteristics of the recently created substances were analyzed using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and UV-vis spectroscopy. Subsequently, the above-mentioned substances were employed as semiconductor layers in bottom-gate/top-contact OFETs through solution shearing methods for device fabrication, and their electrical performances were meticulously evaluated. The outcoming OFET device displayed p-channel behavior, demonstrating hole mobility of up to 0.005cm2/Vs and a current on/off ratio higher than 106.

Potential and anion effects on the adsorption of 3′,4′-bis(hexylthio)-2,2′:5′,2′'-terthiophene on Au(1 1 1) electrode characterized by in situ STM

The adsorption of organic molecules on gold electrodes serves as a model to understand the organic/inorganic electrified interface, which is relevant to the study of molecular electronics and organic thin film semiconductors. Our previous study on terthiophene (TT) adsorption on an Au(111) electrode shows that immersing Au(111) crystals in a TT ethanol dosing solution installs an ordered TT adlayer on the sample. The current study addresses the adsorption of 3′,4′-bis(hexylthio)-2,2′:5′,2′'-terthiophene (DTDST), a molecule with a TT backbone attached with two thiolhexyl chains, on an ordered Au(111) electrode. High-quality STM images were obtained to reveal the internal and 2D spatial structures of DTDST admolecules. The potential greatly influenced the organization of DTDST on the ordered Au(111) electrode. Although the pristine DTDST adlayer was disordered, it transformed into ordered Au(111) - (3√3 × 9) and (5√3 × 26) structures after applying a potential more negative than 0 V (vs. Ag/AgCl) in 0.1 M H2SO4 and HClO4, respectively. Shifting the potential more positive than 0.25 V resulted in coadsorption of bisulfate anions and restructuring of the DTDST adlayer. High-quality molecular resolution STM images were collected to reveal the azimuthal orientation of the DTDST admolecule on the Au(111) electrode. The thiolhexyl chains of DTDST admolecules could arrange in such a way that allowed intermolecular van der Waals interactions. Oxidation of adsorbed DTDST molecules to yield oligomers was also revealed by in situ STM.

High‐Efficiency Cd‐Free CZTSSe Solar Cells with Organic Semiconductor PCBM as a Buffer Layer

Replacing the traditional CdS buffer layer with more environmentally friendly materials is an urgent issue for environment‐friendly CZTSSe solar cells. Herein, a nontoxic, flexible, and conductive organic molecular semiconductor PCBM to replace CdS is used and a Cd‐free CZTSSe solar cell is obtained. By introducing mild spin‐coated ZnO window layers, the damage of PCBM in conventional sputtering ZnO process is effectively avoided, and a higher efficiency than previously reported organic semiconductor buffer layer systems and comparable to CdS buffer layer devices are achieved. The flexible PCBM covers the surface of the CZTSSe absorber layer densely and forms a flatter film surface, which is beneficial for the following ZnO deposition. The CZTSSe/PCBM interface exhibits a typical “spike‐like” energy band alignment with a conduction band offset value of only 0.1 eV. Compared with CZTSSe/CdS interface, the CZTSSe/PCBM heterojunction possesses larger interfacial recombination resistance and a longer carrier lifetime. The ideal coating ability, suitable energy band alignment, as well as the better interface carriers transfer behavior suggest that the organic molecular semiconductors can be used instead of the CdS buffer layer to pave the way for truly environment‐friendly kesterite photovoltaic devices.

Indoloindole‐Based Hole Transporting Material for Efficient and Stable Perovskite Solar Cells Exceeding 24% Power Conversion Efficiency

AbstractThe new concept of hole transporting materials (HTMs) has inspired researchers to develop high‐performing and stable perovskite solar cells (PSCs). In particular, small molecular organic semiconductors have been extensively studied for HTM due to their high reproducibility and easy synthesis. In this work, a novel linear‐type series of indoloindole (IDID)‐based hole transporting materials comprising a fluorinated IDID core(IDIDF) and multiple thiophene rings is developed. The structure‐property relationship in the IDIDF derivatives is investigated systematically by changing the alkyl position and length of the backbone. The intrinsic properties of the material are significantly different depending on the alkyl position of inner thiophene ring. The optimized material exhibits improved solubility, favorable molecular packing patterns, and superior hole mobility. The champion PSCs using the optimum molecule, IDIDF2, yield a power conversion efficiency of 23.16% in non‐doped and 24.24% in doped conditions, which represent one of the highest performances in n‐i‐p planar device configuration. For the first time, the IDIDF2‐based PSCs achieve outstanding thermal and moisture stabilities under thermal aging (85 °C) and relative humidity of 85%, respectively, for 1500 h.

Probing transport energies and defect states in organic semiconductors using energy resolved electrochemical impedance spectroscopy

AbstractDetermining the relative energies of transport states in organic semiconductors is critical to understanding the properties of electronic devices and in designing device stacks. Futhermore, defect states are also highly important and can greatly impact material properties and device performance. Recently, energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) is developed to probe both the ionization energy (IE) and electron affinity (EA) as well as sub‐bandgap defect states in organic semiconductors. Herein, ER‐EIS is compared to cyclic voltammetry (CV) and photoemission spectroscopies for extracting IE and EA values, and to photothermal deflection spectroscopy (PDS) for probing defect states in both polymer and molecular organic semiconductors. The results show that ER‐EIS determined IE and EA are in better agreement with photoemission spectroscopy measurements as compared to CV for both polymer and molecular materials. Furthermore, the defect states detected by ER‐EIS agree with sub‐bandgap features detected by PDS. Surprisingly, ER‐EIS measurements of regiorandom and regioregular poly(3‐hexylthiophene) (P3HT) show clear defect bands that occur at significantly different energies. In regioregular P3HT the defect band is near the edge of the occupied states while it is near the edge of the unoccupied states in regiorandom P3HT.

Modelling the effect of dipole ordering on charge-carrier mobility in organic semiconductors

The hopping mobility in organic semiconductors is strongly influenced by the correlated on-site noise — i.e. the dipole–dipole interaction between neighbouring molecules. In this paper, we use Kinetic Monte Carlo (KMC) to study the effect of dipole moment and structural order on the hole mobility of organic molecular semiconductors. While the effect on the charge mobility of the dipole–dipole interactions can be approximately reproduced by a global Gaussian disorder model, our present results show that there are non-trivial local effects that are not incorporated into such an approach. The dipole–dipole interactions give rise to large energy barriers that restrict charged particles to smaller and smaller regions as the dipole magnitude increases. While this effect can only reduce the mobility in any given system, we note that if the dipoles self-organize, potentially driven by the dipole moment itself or by an increase in packing density, the negative effect on the mobility can be reduced.

Subgap Absorption in Organic Semiconductors.

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

Effect of charge transport on electrical degradation in polypropylene/organic molecular semiconductor composites for HVDC cable insulation

Polypropylene is considered to be the material of choice for environmentally friendly high voltage direct current cable insulation. The high power transmission of electrical energy exposes insulating materials to high temperatures and electric fields, resulting in the degradation of material properties. This paper reports that organic molecular semiconductors with strong electron affinity can effectively modulate electrical properties of polypropylene. The charge injection and transport process are analyzed by considering a combination of relations describing various conduction models in dielectrics, including the Richardson–Schottky (RS) emission and the hopping conduction. Based on the performed experiments, a modified Wiesmann–Zeller (WZ) model is proposed to simulate the electrical treeing process of polypropylene. The electrical treeing results are well verified with the simulation results, which offer a valuable tool for further analysis of the effect of intrinsic barrier height, hopping distance, and activation energy on the electrical degradation in the material. This work provides an insightful analysis of multiple charge transport mechanisms affecting the electrical degradation of the polymer, which is crucially essential for the rational design of high-performance insulation materials.