Properties Of Organic Semiconductors Research Articles

High Mobility n‐Type Imide‐Based Semiconductor with Unusual Single‐Crystal Packing Structure in Solution‐Processed Thin Film

AbstractSolid‐state molecular arrangement has been recognized as the most important role in the charge transport properties of organic semiconductors. Although highly ordered molecular stacking is achieved in single crystals, maintaining single‐crystal molecular packing in solution‐processed thin films remains a significant challenge. Herein, a new type of n‐type organic semiconductors based on an asymmetric fluoranthene imide unit is reported, whose intermolecular packing and aggregation behavior in the thin film state can be effectively controlled by regulating the cyano substitution sites and alkyl chain types in the imide group. F10 with cyano groups at 4,9‐sites and branched 2‐ethylhexyl chain encouragingly shows a highly ordered single‐crystal‐like molecular packing in solution‐processed thin film after thermal annealing, and thus the resulting organic field‐effect transistors exhibit impressive charge transport performance, with the electron mobility as high as 0.116 cm2 V−1 s−1. This work opens a new avenue for developing high‐performance solution‐processed n‐type semiconductors.

Molecular hypergraph neural networks.

Graph neural networks (GNNs) have demonstrated promising performance across various chemistry-related tasks. However, conventional graphs only model the pairwise connectivity in molecules, failing to adequately represent higher order connections, such as multi-center bonds and conjugated structures. To tackle this challenge, we introduce molecular hypergraphs and propose Molecular Hypergraph Neural Networks (MHNNs) to predict the optoelectronic properties of organic semiconductors, where hyperedges represent conjugated structures. A general algorithm is designed for irregular high-order connections, which can efficiently operate on molecular hypergraphs with hyperedges of various orders. The results show that MHNN outperforms all baseline models on most tasks of organic photovoltaic, OCELOT chromophore v1, and PCQM4Mv2 datasets. Notably, MHNN achieves this without any 3D geometric information, surpassing the baseline model that utilizes atom positions. Moreover, MHNN achieves better performance than pretrained GNNs under limited training data, underscoring its excellent data efficiency. This work provides a new strategy for more general molecular representations and property prediction tasks related to high-order connections.

Achieving Zero-Temperature Coefficient Point Behavior by Defect Passivation for Temperature-Immune Organic Field-Effect Transistors.

Organic field-effect transistors (OFETs) have broad prospects in biomedical, sensor, and aerospace applications. However, obtaining temperature-immune OFETs is difficult because the electrical properties of organic semiconductors (OSCs) are temperature-sensitive. The zero-temperature coefficient (ZTC) point behavior can be used to achieve a temperature-immune output current; however, it is difficult to achieve in organic devices with thermal activation characteristics, according to the existing ZTC point theory. Here, the Fermi pinning in OSCs is eliminated using the defect passivation strategy, making the Fermi level closer to the tail state at low temperatures; thus threshold voltage (VT) is negatively correlated with temperature. ZTC point behaviors in OFETs are achieved by compensation between VT and mobility at different temperatures to improve its temperature immunity. A temperature-immune output current can be realized in a variable-temperature bias voltage test over 50000s by biasing the device at the ZTC point. This study provides an effective solution for temperature-immune OFETs and inspiration for their practical application.

Enhanced performance in doped micro-nano porous organic thin-film transistors

Molecular doping, as an effective technique for controlling the electrical property of organic semiconductors (OSCs) by introducing additional charges, has been proven to adjust important device parameters in organic thin-film transistors (OTFTs). Doping highly crystalline OSCs without disrupting structural order is a crucial challenge, as it significantly affects the charge carrier mobility. Here, we demonstrate a molecular doping method without disrupting the molecular ordering to improve the charge carrier mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) based OTFTs via a simple thermal spin-coating method. The key is to introduce micro-nano pores into C8-BTBT thin-film for channel doping, which is achieved by mixing with the unsubstituted BTBT as it can be easily removed from the thin-film through an ordinary annealing process. Micro-nano pores allow the dopant molecules (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, F4-TCNQ) to access the conductive channel of OTFT, which is beneficial for charge injection. Indeed, we further discover that F4-TCNQ doped porous C8-BTBT thin-films exhibit better charge mobility than those of neat and F4-TCNQ doped C8-BTBT films in OTFTs. This work proposes an effective way to expose OSC conjugated core to the dopant, which not only improves the charge transfer reaction between organic/dopant semiconductor through cofacial stacking, but also reduces the trap density and contact resistance.

Carrier Transport Switching of Ferroelectric BTBT Derivative.

Alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-NHCOC14H29 (1), which has ferroelectric N-H···O= hydrogen-bonding network of alkylamide group and two-dimensional (2D) electric structure of BTBT π-cores, was prepared to design the external electric field-responsive organic semiconductors. The short-chain derivative of BTBT-NHCOC3H7 (1') revealed the coexistence of a 2D electronic band structure based on the herringbone BTBT arrangement and the one-dimensional (1D) hydrogen-bonding chain. 1 formed a smectic E (SmE) liquid crystal phase above 412 K and showed ferroelectric hysteresis in the electric field-polarization (P-E) curves at 403-433 K. The remanent polarization (Pr) and coercive electric field (Ec) of 1 at 408 K, 0.1 Hz were 24.0 μC cm-2 and 5.54 V μm-1, respectively. By thermal annealing of thin-film 1 at 443 K, the molecular assembly structure of 1 changed from a monolayer to a bilayer structure with high crystallinity, resulting in conducting layers of BTBT parallel to the substrate surface. The organic field-effect transistor (OFET) device with thermally annealed thin-film 1 showed p-type semiconducting behavior with the hole mobility of 1.0 × 10-3 cm2 V-1 s-1. Furthermore, device 1 showed switching behavior of semiconducting properties by electric field poling and thermal annealing cycle. The electric field response of ferroelectrics modulated the molecular orientation and conduction properties of organic semiconductors, resulting in external electric field control of carrier transport properties.

Linear optical properties of organic microcavity polaritons with non-Markovian quantum state diffusion

Abstract Hybridisation of the cavity modes and the excitons to polariton states together with the coupling to the vibrational modes determine the linear optical properties of organic semiconductors in microcavities. In this article we compute the refractive index for such system using the Holstein–Tavis–Cummings model and determine then the linear optical properties using the transfer matrix method. We first extract the parameters for the exciton in our model from fitting to experimentally measured absorption of a 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl) fluorene (TDAF) molecular thin film. Then we compute the reflectivity of such a thin film in a metal clad microcavity system by including the dispersive microcavity mode to the model. We compute susceptibility of the model systems evolving just a single state vector by using the non-Markovian quantum state diffusion. The computed location and height of the lower and upper polaritons agree with the experiment within the estimated errorbars for small angles ( ≤ 30 ° ) $(\le 30{}^{\circ})$ . For larger angles the location of the polariton resonances are within the estimated error.

Pressure effects on both fluorescent emission and charge transport properties of organic semiconductors: a computational study.

External pressure can regulate the photophysical property and charge transport performance of organic semiconductors, however, the underlying mechanism at the microscopic level is still elusive. Using thermal vibrational correlation function coupled quantum mechanics/molecular mechanics and full quantum charge transfer rate theory, we systematically explore the influence of pressure on fluorescence emission and charge transport behaviours of representative cyclooctatetrathiophene (COTh). It is found that, upon pressurization, the intramolecular configurations of COTh became more twisted, leading to the blue-shifted emission. The fluorescence quantum efficiency (FQE) of COTh crystals decreases monotonically in a wide pressure range of 0-4.38 GPa, because the increase of intermolecular electronic energy transfer rate constant (keet) is larger than the decrease of internal conversion rate constant (kic), and the variation of keet is dominant. The decrease in kic is attributed to the decreasing reorganization energy, reflecting the suppression of the low-frequency flipping vibrations of four thiophene rings and the high-frequency stretching vibrations of central cyclooctatetraene, while the keet increase is due to the simultaneous increase in exciton coupling and spectra overlap. Moreover, we predicted that the hole mobility of COTh increases monotonically by nearly an order of magnitude from 0.39 to 3.00 cm2 V-1 s-1 upon compression, because of the increase in transfer integral and the decrease of charged reorganization energy. Furthermore, its hole mobility exhibits obvious anisotropy. Our work systematically builds the external pressure, molecular packing, luminescence and transport properties relationships of organic semiconductors and provides theoretical guidance for the rational design of pressure responsive organic semiconductors with excellent photoelectric performance.

Additive-assisted strategy for high-efficiency organic solar cells

Herein, we summarize the recent advances of the additive-assisted strategy with solvent/solid additives and binary additives and their working mechanism in OSCs.

Adaptable nanoparticles as multifunction semi-transparent conductive films and drug delivery system

Adaptable nanoparticles of reduced graphene oxide-curcumin as semi-transparent conductive films and graphene oxide-curcumin as drug delivery system (DDS) were evaluated. The reduced graphene oxide--curcumin nanoparticles showed outstanding organic semiconductor properties as semi-transparent conductive films (semi-TCFs). The optimum conditions for gaining maximum σdc/σop values of rGO-curcumin semi-TCFs were at pH = 11 in the absence of crosslinker, which may be a distinguished option in recent organic electronics. Moreover, the graphene-oxide-curcumin nanoparticles were used to increase water solubility of curcumin. Such nanoparticles could be used to deliver curcumin to delicate positions in human body. Graphene oxide (GO) and reduced graphene oxide (rGO) were synthesized from raw graphite, and characterized using attenuate total reflectance Fourier transform infrared (ATR-FTIR), X-ray pattern (XRD), thermogravimetry (TGA) for chemical structure, crystal structure and thermal stability, respectively. The successful exfoliation process of nanoparticles was also monitored by scanning electron microscope (SEM) technique. Pre-adjusted pH method was used at different pH values (i.e. pH = 4.0–7.4). Power law (ln/ln) constant values like diffusional exponent (n) and proportionality constant (k) demonstrated in-vitro Fickian diffusion mechanism. Successful drug delivery system (DDS) revealed maximum curcumin bioavailability of 97 % compared to 22 % curcumin alone. Conclusively, GO and rGO connected with curcumin could achieve adaptable and potential applications in medical and engineering sectors.

Recent Progress in Flexible and Wearable All Organic Photoplethysmography Sensors for SpO2 Monitoring.

Flexible and wearable biosensors are the next-generation healthcare devices that can efficiently monitor human health conditions in day-to-day life. Moreover, the rapid growth and technological advancements in wearable optoelectronics have promoted the development of flexible organic photoplethysmography (PPG) biosensor systems that can be implanted directly onto the human body without any additional interface for efficient bio-signal monitoring. As an example, the pulse oximeter utilizes PPG signals to monitor the oxygen saturation (SpO2 ) in the blood volume using two distinct wavelengths with organic light emitting diode (OLED) as light source and an organic photodiode (OPD) as light sensor. Utilizing the flexible and soft properties of organic semiconductors, pulse oximeter can be both flexible and conformal when fabricated on thin polymeric substrates. It can also provide highly efficient human-machine interface systems that can allow for long-time biological integration and flawless measurement of signal data. In this work, a clear and systematic overview of the latest progress and updates in flexible and wearable all-organic pulse oximetry sensors for SpO2 monitoring, including design and geometry, processing techniques and materials, encapsulation and various factors affecting the device performance, and limitations are provided. Finally, some of the research challenges and future opportunities in the field are mentioned.

Improved Light Utilization Efficiency for an ITO‐Free Semitransparent Organic Solar Cell Using a Multilayer Silver Back Electrode as Infrared Mirror

Semitransparent organic solar cells (STOSCs) exhibit promising application as power‐generating windows in buildings and agricultural greenhouses. Due to unique optical properties of organic semiconductors, they can efficiently absorb near‐infrared light while maintaining a high degree semitransparency in the visible range. Since power conversion efficiency (PCE) and average visible transmission (AVT) frequently stand in a trade‐off relationship, a major challenge in improving the overall performance of STOSCs is maximizing the product of both, called light utilization efficiency (AVT × PCE = LUE). Herein, using multiple layers of aluminium‐doped ZnO (AZO) and silver as an infrared reflecting back electrode, in order to increase current generation while maintaining high visible transparency, is proposed. Using optical modeling, the optimal layer thickness of the AZO layer sandwiched between two Ag layers is determined, leading to an increased photocurrent generation of up to 10%. Simultaneously, experimental findings show that the fill factor decreases with an increasing AZO layer thickness. By adjusting the thickness of the photoactive layer, the blend concentration, and improving the top electrode material the thus‐far highest reported LUE for indium tin oxide‐free STOSCs is attained, reaching 4.0% with a PCE of 8.7% and an AVT of 46.3%.

Different Energetics at Donor:Acceptor Interfaces in Bilayer and Bulk‐Heterojunction Polymer:Non‐Fullerene Organic Solar Cells

To understand the limitations placed on the open‐circuit voltage of bulk heterojunction (BHJ) organic solar cells, the energy levels of neat donor and acceptor samples are often characterized and applied to study BHJ blends. However, energy levels derived from neat samples may not necessarily reflect those at the donor:acceptor interface in blends. The properties of organic semiconductors are sensitive to microstructural changes, with non‐fullerene acceptors (NFAs) in particular known to exhibit different thin‐film polymorphs. To investigate the influence of differences in molecular packing in neat and blend films, temperature‐dependent current–voltage characteristics are measured for bilayer (BL) and BHJ devices. Herein, the fullerene acceptor PC71BM is compared—whose energy levels are expected to be less sensitive to molecular packing—with the NFA ITIC, paired with the same donor polymer PTB7‐Th. It is found that the interfacial energy levels differ for BL and BHJ devices for the PTB7‐Th:ITIC system but remain the same for the PTB7‐Th:PC71BM system. Furthermore, X‐ray scattering measurements identify that ITIC exhibits a different packing mode in neat films and in BHJ blends. Such microstructure‐dependent differences between neat and blend samples need to be considered when studying energy losses in NFA BHJ solar cells.

Advances in Organic Thermoelectric Devices for Multiple Applications

AbstractOrganic thermoelectric (OTE) devices composed of π‐conjugated molecules are the basic building blocks for self‐powered integrated organic electronics. In addition to molecular design and doping strategies, the highly tunable energy conversion process in OTE devices has drawn significant research interest. Specifically, the diverse physical properties of organic semiconductors, novel device geometry design, and advanced fabrication techniques combined enable the OTE device to be a powerful multiscale platform from single‐molecular scale to thin films for modulating the TE performance, studying the fundamental charge transport mechanism, exploring novel energy conversion phenomenon, and realizing various functionalities. Here, the authors comprehensively review the recent experimental and theoretical advances in related topics of OTE devices, including multifunctional, external physical fields, and temperature modulated, as well as quantum OTE devices. The remaining issues and perspectives toward future OTE device research are also discussed at the end.

Extremely Electron‐Withdrawing Lewis‐Paired CN Groups for Organic p‐Dopants

AbstractP‐type chemical doping (p‐doping) is a key technique to modulate the optical, electrical, and electronic properties of organic semiconductors. However, typical functional groups in organic p‐dopants have insufficient electron‐withdrawing strength, and the inevitable diffusion of dopants in host matrices degrades doping stabilities. Herein, we utilize extremely electron‐withdrawing Lewis‐paired CN groups as a new class of building blocks for designing unprecedentedly strong organic p‐dopants with excellent doping stability. Various Lewis acids are paired with CN‐functionalized conjugated molecules in the solution state, which strengthens the electron‐withdrawing properties of CN groups almost twofold. The large dopants afford outstanding doping stability against continuous heating and long‐term atmospheric exposure, which is promising for practical applications in devices. Given the broad applicability of this simple combinatorial approach, it may impact many fields of (opto)electronics.

Extremely Electron-Withdrawing Lewis-Paired CN Groups for Organic p-Dopants.

P-type chemical doping (p-doping) is a key technique to modulate the optical, electrical, and electronic properties of organic semiconductors. However, typical functional groups in organic p-dopants have insufficient electron-withdrawing strength, and the inevitable diffusion of dopants in host matrices degrades doping stabilities. Herein, we utilize extremely electron-withdrawing Lewis-paired CN groups as a new class of building blocks for designing unprecedentedly strong organic p-dopants with excellent doping stability. Various Lewis acids are paired with CN-functionalized conjugated molecules in the solution state, which strengthens the electron-withdrawing properties of CN groups almost twofold. The large dopants afford outstanding doping stability against continuous heating and long-term atmospheric exposure, which is promising for practical applications in devices. Given the broad applicability of this simple combinatorial approach, it may impact many fields of (opto)electronics.

Virtual mining of polymer monomers for photodetectors application and regression-aided reorganization energy prediction

Predicting and understanding the charge transport properties of organic semiconductors is a crucial target for constructing efficient electronic devices such as photodetectors. To this end, reorganization energy (Re) is a key parameter for molecular design, which provides quantitative strength of electron-photon coupling process (charge transport parameter). Although modern density functional theory-based approaches can accurately simulate intramolecular RE trends, such computations are time-consuming and costly. In this study, we present machine-learning tools for the accurate and fast prediction of intramolecular Re. These tools are effectively used for screening of new compounds with low internal Re. Moreover, virtual mining of polymer monomers is performed for photodetector applications. Machine learning models are developed with molecular descriptors (features). Regression analysis is used to show how two or more variables are related. The dependent variable yields the best outcomes and helps to predict Re. This research shows that chemical designs can accurately predict the REs of organic semiconductor materials. Out of five different regressions models, gradient booster regression gives the best prediction capability (R2 = 0.761). Moreover, chemical similarity analysis (CSA) is used to search for a new group of molecules with high performance. Library enumeration is used for structure determination and for studying the properties of these small molecules. The fitness score of newly designed quinoxaline is more than 0.9, which shows best outcome. The new strategy holds immense potential for the virtual mining of polymer monomers for photodetectors applications and regression-aided reorganization energy predictions.

P‐Type Doping of PM6:Y6‐Based Photoactive Layer with Formic Acid

The introduction of molecular doping process is necessary to enhance the optic and electronic properties of organic semiconductors for facilitating charge transport. In particular, since the doping process has a positive influence on the charge transfer interaction between host semiconductor and dopant, improved mobility has been efficiently achieved. Despite its advantages, doping technologies in organic solar cells (OSCs) are emphasized to the development of n‐type dopants used for balancing the electron and hole. In addition, since the bulk‐heterojunction microstructure in OSCs has randomly blended phases of the donor and acceptor, it is important to optimize charge extraction without loss by controlling the morphology. Herein, OSCs by p‐type doping with formic acid (FA) into a photoactive layer is reported. The champion device yields a significantly improved power conversion efficiency from 14.3% to 15.3% with a high fill factor of 71.7%. It is found that the p‐doped photoactive layer exhibits enhanced conductivity, improved carrier mobilities, suppressed charge recombination, and lowered leakage current. The FA simultaneously acts as a film morphology controller of the photoactive layer with enhanced phase separation to transport the charge efficiently. Thus, the doping process with FA can maintain the device performance in stability tests (95.6% remaining).

General Spatial Confinement Recrystallization Method for Rapid Preparation of Thickness-Controllable and Uniform Organic Semiconductor Single Crystals.

Organic semiconductor single crystals (OSSCs) are ideal materials for studying the intrinsic properties of organic semiconductors (OSCs) and constructing high-performance organic field-effect transistors (OFETs). However, there is no general method to rapidly prepare thickness-controllable and uniform single crystals for various OSCs. Here, inspired by the recrystallization (a spontaneous morphological instability phenomenon) of polycrystalline films, a spatial confinement recrystallization (SCR) method is developed to rapidly (even at several second timescales) grow thickness-controllable and uniform OSSCs in a well-controlled way by applying longitudinal pressure to tailor the growth direction of grains in OSCs polycrystalline films. The relationship between growth parameters including the growth time, temperature, longitudinal pressure, and thickness is comprehensively investigated. Remarkably, this method is applicable for various OSCs including insoluble and soluble small molecules and polymers, and can realize the high-quality crystal array growth. The corresponding 50 dinaphtho[2,3-b:2″,3″-f]thieno[3,2-b]thiophene (DNTT) single crystals coplanar OFETs prepared by the same batch have the mobility of 4.1 ± 0.4 cm2 V-1 s-1 , showing excellent uniformity. The overall performance of the method is superior to the reported methods in term of growth rate, generality, thickness controllability, and uniformity, indicating its broad application prospects in organic electronic and optoelectronic devices.

Resonance Effect in Brunel Harmonic Generation in Thin Film Organic Semiconductors

AbstractNonlinear optical properties of organic semiconductors (OSCs) have been extensively investigated in the perturbative regime, while strong light induced high‐order processes in solid‐state OSCs are less studied. Here, below‐threshold harmonic generation is examined, both experimentally and theoretically, in two solid‐state thin film OSCs, that is, tetraphenylporphyrin and zinc tetraphenylporphyrin. Results show that the π–π* excitations of the porphyrin ring system generate the harmonic emission. The contribution of the Brunel harmonic to the 5th harmonic emission is uncovered, where the resonant 5‐photon transition (S0 → S2 transition) is found to lead to an early onset of non‐perturbative behavior. A similar resonance effect is expected in Brunel harmonic generation in other organic materials.

Are Coarse-Grained Structures as Good as Atomistic Ones for Calculating the Electronic Properties of Organic Semiconductors?

The quality of amorphous molecular morphologies obtained with a recently introduced coarse-grained model, representing molecules in terms of connected anisotropic beads (Phys. Chem. Chem. Phys. 2019, 21, 26195), is benchmarked against reference atomistic data. Typical small-molecule organic semiconductors in their pristine and doped forms are chosen as a challenging and technologically relevant case study for our comparison, which includes both structural features and the resulting electronic properties, such as charge carrier energy levels, energetic disorder, and intermolecular charge transfer couplings. Our analysis shows that our accurate coarse-grained model leads to molecular glasses that are very similar to native atomistic samples, with the discrepancy being further reduced upon back-mapping. The electronic properties computed for back-mapped morphologies are almost indistinguishable from the atomistic reference, especially for multibranched poly(hetero)cyclic hydrocarbons usually employed as organic semiconductors. This study provides a proof of principle for highly accurate large-scale simulations of complex molecular systems at a reduced computational cost.