Organic Semiconductors Research Articles

Synthesis, Structural, and Optical Properties of 2-(2-methyl 8-hydroxyquinoline) Magnesium Nanorods for Optical Display Systems.

Organic semiconductors find widespread applications in the realm of organic light-emitting diodes (OLEDs) as well as organic photovoltaic cells. In the domain of OLED devices, it is plausible for nano-based Mg metal complexes to play a role as electron and hole-transport layers. In the present investigation, we synthesized 2(2-methyl 8-hydroxyquinoline) magnesium [Mg(mq)2] nanorods through the employment of the precipitation method, using 2-methyl 8- hydroxyquinoline and magnesium acetate. We employed various techniques to characterize the Mg(mq)2 nanorods, including powder XRD, FTIR spectroscopy, SEM, EDX, UV-Vis, and PL spectroscopy studies. The structural aspects of Mg(mq)2 were ascertained through P-XRD analysis. The elemental composition of Mg(mq)2 and its surface texture were established via EDX and HR-SEM analyses. FTIR spectroscopy confirmed the existence of functional groups within the sample. UV-Vis spectroscopy was utilized to evaluate the optical absorbance, bandgap, and Urbach energy of Mg(mq)2. The luminescence properties of the Mg(mq)2 nanorods were determined from the photoluminescence study. The characterization results were compared with the Zn(mq)2 nano samples. The experimental results presented herein serve to demonstrate the practicality of employing Mg(mq)2 nanorods in OLED devices.

Enhancing the Maturation of Human Pluripotent Stem Cell-Derived Cardiomyocytes with an n-Type Organic Semiconductor Coating.

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are a promising cell source for cardiac regenerative medicine and in vitro modeling. However, hPSC-CMs exhibit immature structural and functional properties compared with adult cardiomyocytes. Various electrical, mechanical, and biochemical cues have been applied to enhance hPSC-CM maturation but with limited success. In this work, we investigated the potential application of the semiconducting polymer poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) as a light-sensitive material to stimulate hPSC-CMs optically. Our results indicated that P(NDI2OD-T2)-mediated photostimulation caused cell damage at irradiances applied long-term above 36 μW/mm2 and did not regulate cardiac monolayer beating (after maturation) at higher intensities applied in a transient fashion. However, we discovered that the cells grown on P(NDI2OD-T2)-coated substrates showed significantly enhanced expression of cardiomyocyte maturation markers in the absence of a light exposure stimulus. A combination of techniques, such as atomic force microscopy, scanning electron microscopy, and quartz crystal microbalance with dissipation monitoring, which we applied to investigate the interface of the cell with the n-type coating, revealed that P(NDI2OD-T2) impacted the nanostructure, adsorption, and viscoelasticity of the Matrigel coating used as a cell adhesion promoter matrix. This modified cellular microenvironment promoted the expression of cardiomyocyte maturation markers related to contraction, calcium handling, metabolism, and conduction. Overall, our findings demonstrate that conjugated polymers such as P(NDI2OD-T2) can be used as passive coatings to direct stem cell fate through interfacial engineering of cell growth substrates.

Inhibiting Photo-Oxidation and Enhancing Visible-Light-Driven Photocatalytic Water Oxidation over Covalent Organic Frameworks Through the Coordination of Cobalt with Bipyridine.

Photocatalytic water splitting using covalent organic frameworks (COFs) is a promising approach for harnessing solar energy. However, challenges such as slow kinetic dynamics in the photocatalytic oxygen evolution reaction (OER) and COFs' self-oxidation hinder its progress. In this study, an enamine-based COF coordinated is introduced with cobalt dichloride, CoCl2 (CoCl2-TpBPy). The coordination of cobalt ions with bipyridines in CoCl2-TpBPy enhances charge-carrier separation and migration, leading to effective photocatalytic OER. Under visible light irradiation, CoCl2-TpBPy achieves a notable OER rate of up to 1 mmol·g-1·h-1, surpassing the reported organic semiconductor analogs. Additionally, CoCl2-TpBPy shows minimal nitrogen evolution compared to TpBPy and ethanol-treated TpBPy (E-TpBPy), indicating cobalt plays a pivotal role in improving charge utilization and minimizing photo-oxidation. In situ X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analyses revealed that Co(IV) species are key to the high OER efficiency. This work highlights Co(IV) species in the efficient OER and inhibiting photo-oxidation of CoCl2-TpBPy.

Vinyl-Group-Anchored Covalent Organic Framework for Promoting the Photocatalytic Generation of Hydrogen Peroxide.

The artificial photosynthesis of H2O2 from water and oxygen using semiconductor photocatalysts is attracting increasing levels of attention owing to its green, environmentally friendly, and energy-saving characteristics. Although covalent organic frameworks (COFs) are promising materials for promoting photocatalytic H2O2 production owing to their structural and functional diversity, they typically suffer from low charge-generation and -transfer efficiencies as well as rapid charge recombination, which restricts their use as catalysts for photocatalytic H2O2 production. Herein, we report a strategy for anchoring vinyl moieties to a COF skeleton to facilitate charge separation and migration, thereby promoting photocatalytic H2O2 generation. This vinyl-group-bearing COF photocatalyst exhibits a H2O2-production rate of 84.5 μmol h-1 (per 10 mg), which is ten-times higher than that of the analog devoid of vinyl functionality and superior to most reported COF photocatalysts. Both experimental and theoretical studies provide deep insight into the origin of the improved photocatalytic performance. These findings are expected to facilitate the rational design and modification of organic semiconductors for use in photocatalytic applications.

Vinyl‐Group‐Anchored Covalent Organic Framework for Promoting the Photocatalytic Generation of Hydrogen Peroxide

AbstractThe artificial photosynthesis of H2O2 from water and oxygen using semiconductor photocatalysts is attracting increasing levels of attention owing to its green, environmentally friendly, and energy‐saving characteristics. Although covalent organic frameworks (COFs) are promising materials for promoting photocatalytic H2O2 production owing to their structural and functional diversity, they typically suffer from low charge‐generation and ‐transfer efficiencies as well as rapid charge recombination, which restricts their use as catalysts for photocatalytic H2O2 production. Herein, we report a strategy for anchoring vinyl moieties to a COF skeleton to facilitate charge separation and migration, thereby promoting photocatalytic H2O2 generation. This vinyl‐group‐bearing COF photocatalyst exhibits a H2O2‐production rate of 84.5 μmol h−1 (per 10 mg), which is ten‐times higher than that of the analog devoid of vinyl functionality and superior to most reported COF photocatalysts. Both experimental and theoretical studies provide deep insight into the origin of the improved photocatalytic performance. These findings are expected to facilitate the rational design and modification of organic semiconductors for use in photocatalytic applications.

Radical electron-induced cellulose-semiconductors.

Bio-semiconductors are expected to be similar to organic semiconductors; however, they have not been utilized in application yet. In this study, we show the origin of electron appearance, N- and S-type negative resistances, rectification, and switching effects of semiconductors with energy storage capacities of up to 418.5mJ/m2 using granulated amorphous kenaf cellulose particles (AKCPs). The radical electrons in AKCP at 295K appear in cellulose via the glycosidic bond C1-O1·-C4. Hall effect measurements indicate an n-type semiconductor with a carrier concentration of 9.89 × 1015/cm3, which corresponds to a mobility of 10.66 cm2/Vs and an electric resistivity of 9.80 × 102 Ωcm at 298K. The conduction mechanism in the kenaf tissue was modelled from AC impedance curves. The light and flexible cellulose-semiconductors may open up new avenues in soft electronics such as switching effect devices and bio-sensors, primarily because they are composed of renewable natural compounds.

The Importance of Semiconductor Miscibility for the Formation of Light‐Harvesting Polymer:Fullerene Nanoparticle Dispersions

Light‐harvesting layers in organic bulk‐heterojunction solar cells are commonly fabricated from aromatic solvents which are often toxic or hazardous. To fully omit harmful solvents, (surfactant‐free) nanoparticle dispersions of organic semiconductor blends are investigated, but only a very limited choice of materials form blend nanoparticles. This work is a systematic study of the nanoparticle formation by nanoprecipitation of electrostatically stabilized polymer: fullerene blends. It turns out that miscibility of both components is crucial for the incorporation of the fullerenes into the nanoparticles. This finding demystifies why certain polymer:fullerene combinations seemingly randomly form stable nanoparticle dispersion while others do not. The need of miscibility constitutes an important design criterion for processing future semiconductor blends along the nanoparticle route. Some mitigation to the unwanted release of larger amounts of fullerenes can be achieved by using ternary semiconductor blends.

Low Bandgap Organic Semiconductors for Photovoltaic Applications

Photovoltaic is one of the best low-cost alternative energy sources. In this paper, organic semiconductors were explored as light harvesters due to their extraordinary properties, but initially, the scientific community focused on synthesizing the large bandgap organic semiconductors, which were unsuitable for photovoltaic applications. Wide bandgap organic semiconductors can capture light only from the UV–Vis region of the solar spectrum, thus showing solar cells’ power conversion efficiency (PCE) of less than 5%. By reducing the bandgap, organic semiconductors can show good compatibility with the air mass (AM) of 1.5 solar spectrums. So, the attention came toward synthesizing low bandgap semiconductors, which led to the enhancement of PCE from 5% to 17%. This review summarizes the overall development of the last 15 years in the field of organic photovoltaic semiconductors. The significant parameters in organic semiconductors are their bandgap, the position of the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) that can easily be tuned chemically by molecular designing. One of the issues in organic photovoltaic solar devices is the need for absorbers with a narrow bandgap to maximize power conversion efficiencies to a great extent. Moreover, several factors, such as conjugation length, bond length alteration, intermolecular interaction, donor–acceptor charge transfer, planarity, and physical states, are responsible for tuning the bandgap. This review also classifies low bandgap semiconductors based on the core skeleton with their bandgaps, structures, and power conversion efficiencies.

Enhancing Electrical Conductivity and Power Factor in Poly‐Glycol‐Bithienylthienothiophene with Oligoethylene Glycol Side Chains Through Tris (pentafluorophenyl) Borane Doping

AbstractDoping of organic semiconductors has served as an effective method to achieve high electrical conductivity and large thermoelectric power factor. This is of importance to the development of flexible/wearable electronics and green energy‐harvesting technologies. The doping impact of the Lewis acid tris (pentafluorophenyl) borane (BCF) on the thermoelectric performance of poly(2‐(4,4′‐bis(2‐methoxyethoxy)‐5′‐methyl‐[2,2′‐bithiophen]‐5‐yl)‐5‐methylthieno[3,2‐b]thiophene (pgBTTT), a thiophene‐based polymer featuring oligoethylene glycol side chains is investigated. Tetrafluorotetracyanoquinodimethane (F4TCNQ), a well‐established dopant, is utilized as a comparison; however, its inability to co‐dissolve with pgBTTT in less polar solvents hinders the attainment of higher doping levels. Consequently, a comparative study is performed on the thermoelectric behavior of pgBTTT doped with BCF and F4TCNQ at a very low doping level. Subsequent investigation is carried out with BCF at higher doping levels. Remarkably, at 50 wt% BCF doping level, the highest power factor of 223 ± 4 µW m−1 K2 is achieved with an electrical conductivity of 2180 ± 360 S cm−1 and a Seebeck coefficient of 32 ± 1.3 µV K−1. This findings not only contribute valuable insights to the dopant interactions with oxygenated side chain polymers but also open up new avenues for high conductivity thermoelectric polymers in flexible electronic applications.

A minimalist approach to 3D photoemission orbital tomography: algorithms and data requirements

Abstract Photoemission orbital tomography provides direct access from laboratory measurements to the real-space molecular orbitals of well-ordered organic semiconductor layers. Specifically, the application of phase retrieval algorithms to photon-energy- and angle-resolved photoemission data enables the direct reconstruction of full 3D molecular orbitals without the need for simulations using density functional theory or the like. A major limitation for the direct approach has been the need for densely-sampled, well-calibrated 3D photoemission patterns. Here, we present an iterative projection algorithm that completely eliminates this challenge: for the benchmark case of the pentacene frontier orbitals, we demonstrate the reconstruction of the full orbital based on a dataset containing only four simulated photoemission momentum measurements. We discuss the algorithm performance, sampling requirements with respect to the photon energy, optimal measurement strategies, and the accuracy of orbital images that can be achieved.

Dirac Cones and Room Temperature Polariton Lasing Evidenced in an Organic Honeycomb Lattice.

Artificial 1D and 2D lattices have emerged as a powerful platform for the emulation of lattice Hamiltonians, the fundamental study of collective many-body effects, and phenomena arising from non-trivial topology. Exciton-polaritons, bosonic part-light and part-matter quasiparticles, combine pronounced nonlinearities with the possibility of on-chip implementation. In this context, organic semiconductors embedded in microcavities have proven to be versatile candidates to study nonlinear many-body physics and bosonic condensation, and in contrast to most inorganic systems, they allow the use at ambient conditions since they host ultra-stable Frenkel excitons. A well-controlled, high-quality optical lattice is implemented that accommodates light-matter quasiparticles. The realized polariton graphene presents with excellent cavity quality factors, showing distinct signatures of Dirac cone and flatband dispersions as well as polariton lasing at room temperature. This is realized by filling coupled dielectric microcavities with the fluorescent protein mCherry. The emergence of a coherent polariton condensate at ambient conditions are demonstrated, taking advantage of coupling conditions as precise and controllable as in state-of-the-art inorganic semiconductor-based systems, without the limitations of e.g. lattice matching in epitaxial growth. This progress allows straightforward extension to more complex systems, such as the study of topological phenomena in 2D lattices including topological lasers and non-Hermitian optics.

Spectroscopic and electronic investigations on tin(II) 2,3-naphthalocyanine/p-Si heterojunction for optoelectronic applications

The spectroscopic and electronic properties of small-molecular organic semiconductors in thin films are a significant aspect that controls the performance of their optical/optoelectronic devices. A heterojunction of thermal vacuum deposited film of tin(II)2,3-naphthalocyanine (SnNc) dye onto a p-type Si wafer (SnNc/p-Si) was fabricated. The morphology of SnNc thin film onto Si wafer was explored using the Field Emission Scanning Electron Microscope (FE-SEM). The specular reflectance and the fluorescent properties of SnNc thin film onto Si wafer were investigated. The PL emission spectra of SnNc/p-Si emission bands are related to the B and Q absorption bands for SnNc. The spectral behavior of the refractive index and the specific polarizability for both SnNc/Glass and SnNc/p-Si were extracted. Under varying temperatures (298–378 K), the electronic properties of the SnNc/p-Si heterojunction were analyzed in dark conditions. The diode parameters of SnNc/p-Si heterojunctions, which exhibited temperature-dependent behavior, were extracted from the current–voltage (I-V) procedure. According to the Cheung-Cheung and Norde procedure, the essential electrical diode parameters of the SnNc/p-Si device, such as the ideality factor, the series resistance, and barrier height, were estimated. The photoconductivity parameters, such as the photocurrent, the sensitivity, and the responsivity of the SnNc/p-Si devices, were estimated. These findings can be helpful and utilized in optoelectronic and organic electronic applications.

Evolution of the pentacene exciton band width in pentacene-tetracene blends.

Pentacene is one of the most investigated organic semiconductors. It is well known that the motion of excitons in pentacene and other organic semiconductors is determined by inter-molecular exciton coupling based on charge-transfer processes. In the present study, we demonstrate the impact of the admixture of tetracene, which has a larger band gap and interrupts the pentacene-pentacene interaction, on the exciton behavior in pentacene. Using a combination of optical absorption and electron energy-loss spectroscopy, we show that both the Davydov splitting and the exciton band width in pentacene strongly decrease with increasing tetracene concentration, while the decrease of the exciton band width is substantially larger.

Conformational Analysis of Conjugated Organic Materials: What Are My Heteroatoms Really Doing?

Organic semiconductor small molecules and polymers often incorporate heteroatoms into their chemical structures to affect the electronic properties of the material. A particular design philosophy has been to use these heteroatoms to influence torsional potentials, since the overlap of adjacent π-orbitals is most efficient in planar systems and is critical for charge delocalization in these systems. Since these design rules became popular, the messages from the earlier works have become lost in a sea of reports of "conformational locks", where the non-covalent interactions have relatively small contributions to planarizing torsional potentials. Greater influences can be found in the stabilization by extended conjugation, consideration of steric repulsion, and the interactions involving solubilizing chains and neighboring molecules or polymer chains in condensed phases.

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.

Room temperature phosphorescence from natural, organic emitters and their application in industrially compostable programmable luminescent tags.

Organic semiconductors provide the potential of biodegradable technologies, but prototypes do only rarely exist. We present transparent, ultrathin programmable luminescent tags for minimalistic yet efficient information storage that are fully made from biodegradable or at least industrially compostable, ready-to-use materials (bioPLTs). As natural emitters, the quinoline alkaloids show sufficient room temperature phosphorescence when being embedded in a polymer matrix with cinchonine exhibiting superior performance. Polylactic acid provides a solution for both the matrix material and the flexible substrate. Room temperature phosphorescence can be locally controlled by the oxygen concentration in the film by using Exceval as additional oxygen blocking layers. These bioPLTs exhibit all function-defining characteristics also found in their regular non-environmentally degradable analogs and, additionally, provide a simplified, high-contrast readout under continuous-wave illumination as a consequence of the unique luminescence properties of the natural emitter cinchonine. Limitations for flexible devices arise from limited thermal stability of the polylactic acid foil used as substrate allowing only for one writing cycle and preventing an annealing step during fabrication. Few-cycle reprogramming is possible when using the architecture of the bioPLTs on regular quartz substrates. This work realizes the versatile platform of PLTs with less harmful materials offering a more sustainable use in future. This article is protected by copyright. All rights reserved.

Interplay of intermolecular and intramolecular vibrations mediates ultrafast singlet fission

Abstract Singlet fission (SF) is an exciton multiplication process that splits a singlet exciton in organic semiconductors into two triplet excitons and thus, can overcome the Shockley-Queisser limit to improve the solar energy conversion efficiency in photovoltaics. In this paper, we construct a unified model for the ultrafast primary step of the SF process. To achieve that we investigate the dynamics of vibrational modes and their interactions to the relevant electronic excited states in prototypical SF materials, pentacene (exothermic SF) and tetracene (endothermic SF) single crystals. Additionally, the functional role of charge transfer (CT) state is also examined. Using the refined parameters obtained from the reported experimental results, we deduce that the intermolecular vibrations mediate the SF in pentacene with the assistance from strong vibronic couplings to intramolecular modes, which drives the SF process to occur within 100 fs. In this timescale, the CT state has a limiting role towards the SF process in pentacene. However, the CT state plays an important role in a relatively slower SF process of tetracene. Our results disentangle the role of underlying vibrational coherences and clarify the importance of the CT state in tetracene crystal. Hence, with our unified model, we can study the coherent dynamics of SF process, which can principally be extended to the other SF materials as well.

Influence of gate dielectric property on the capacitance dispersion in organic-metal-insulator semiconductor capacitors

Abstract The capacitance dispersion with frequency in an organic metal insulator semiconductor (OMISCAP) capacitor is a vital phenomenon for predicting high-frequency applications. In this paper, the influence of the dielectric constant on the capacitance dispersion of OMISCAP is investigated by varying the dielectric constant of poly-4-vinyl-phenol (PVP) using crosslinker concentration. The cut-off frequency of capacitance dispersion fC at which the capacitance drops by 10% of the plateau is introduced to measure the capacitance dispersion and the range of operating frequency. The variation of fC from 34 KHz to 50 kHz upon changing the dielectric constant of the insulator from 3.8 to 4.8 appears to be due to the combined effect of the capacitance variation in step with dielectric constant and the resistance variation due to field-induced charge accumulation in the semiconductor. The experimental results are validated using TCAD simulation and parameter extraction.

High Open-Circuit Voltage Organic Solar Cells with 19.2% Efficiency Enabled by Synergistic Side-Chain Engineering.

Restricted by the energy-gap law, state-of-the-art organic solar cells (OSCs) exhibit relatively low open-circuit voltage (VOC) because of large nonradiative energy losses (ΔEnonrad). Moreover, the trade-off between VOC and external quantum efficiency (EQE) of OSCs is more distinctive; the power conversion efficiencies (PCEs) of OSCs are still <15% with VOCs of >1.0V. Herein, the electronic properties and aggregation behaviors of non-fullerene acceptors (NFAs) are carefully considered and then a new NFA (Z19) is delicately designed by simultaneously introducing alkoxy and phenyl-substituted alkyl chains to the conjugated backbone. Z19 exhibits a hypochromatic-shifted absorption spectrum, high-lying lowest unoccupied molecular orbital energy level and ordered 2D packing mode. The D18:Z19-based blend film exhibits favorable phase separation with face-on dominated molecular orientation, facilitating charge transport properties. Consequently, D18:Z19 binary devices afford an exciting PCE of 19.2% with a high VOC of 1.002V, surpassing Y6-2O-based devices. The former is the highest PCE reported to date for OSCs with VOCs of >1.0V. Moreover, theΔEnonrad of Z19- (0.200eV) and Y6-2O-based (0.155eV) devices are lower than that of Y6-based (0.239eV) devices. Indications are that the design of such NFA, considering the energy-gap law, could promote a new breakthrough in OSCs.

Graphitic Carbon Nitride as a Metal‐Free Heterogeneous Semiconductor Photocatalyst for Fluoroalkylation Reactions

AbstractThe increasing use of fluorine‐containing bioactive molecules necessitates efficient strategies for fluorinated group installation. Despite the impressive development of photoinduced radical fluoroalkylation as a powerful tool for introducing fluorine, the persistent issues, including the recyclability and reaction specificity of homogeneous photocatalysts, still leave great room for further advancement in a sustainable and general fashion. Herein, we report a conceptually different approach toward multiple types of fluoroalkylations by using recoverable and versatile graphitic carbon nitride (g‐CN) nanosheets as a heterogeneous photocatalyst. This photocatalytic system enables diverse intermolecular fluoroalkylations of alkenes with fluoroalkanesulfinates and intramolecular fluoroalkyl migrations of alkenyl triflates. Detailed characterizations and mechanism studies substantiate the stability of this organic semiconductor and the crucial role of photogenerated electron‐hole pairs.