Crystalline Organic Semiconductors Research Articles

Evolution of surface and interface in soluble acene/polymer blends and its critical effects on gas diffusion dynamics in gas sensors

Organic field-effect transistor (OFET)-based gas sensors are highly valued for their sensitivity, cost-effectiveness, and operation at room temperature, making them ideal candidates for gas detection applications. For gas sensor performance, gas molecules must not only adsorb onto the organic semiconductor surface but also diffuse into the organic semiconductor-insulator interface that the conductive channel is formed. Therefore, the thickness of the active OSC layer is crucial in determining the efficiency of gas diffusion and the overall sensing response. This study analyzed the impact of semiconductor thin film thickness on gas sensing properties by adjusting the thickness of crystalline organic semiconductors. Using 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), known for its excellent response toward NO2, surface and interface characteristics were modulated through a blend approach. With all other conditions constant, the semiconductor film thickness significantly influenced NO2 detection properties, with the highest sensitivity observed in films around 10 nm thick. A quantitative analysis using Langmuir isotherms demonstrated that reducing the OSC layer thickness from 20 nm to 10 nm could enhance the response speed by a factor of five. Furthermore, the limit of detection (LOD) for TIPS-pentacene films at this reduced thickness was found to be in the low parts per billion (ppb) range.

Discovery of Crystallizable Organic Semiconductors with Machine Learning.

Crystalline organic semiconductors are known to have improved charge carrier mobility and exciton diffusion length in comparison to their amorphous counterparts. Certain organic molecular thin films can be transitioned from initially prepared amorphous layers to large-scale crystalline films via abrupt thermal annealing. Ideally, these films crystallize as platelets with long-range-ordered domains on the scale of tens to hundreds of microns. However, other organic molecular thin films may instead crystallize as spherulites or resist crystallization entirely. Organic molecules that have the capability of transforming into a platelet morphology feature both high melting point (Tm) and crystallization driving force (ΔGc). In this work, we employed machine learning (ML) to identify candidate organic materials with the potential to crystallize into platelets by estimating the aforementioned thermal properties. Six organic molecules identified by the ML algorithm were experimentally evaluated; three crystallized as platelets, one crystallized as a spherulite, and two resisted thin film crystallization. These results demonstrate a successful application of ML in the scope of predicting thermal properties of organic molecules and reinforce the principles of Tm and ΔGc as metrics that aid in predicting the crystallization behavior of organic thin films.

Anion Bulk Doping of Organic Single‐Crystalline Thin Films for Performance Enhancement of Organic Field‐Effect Transistors

AbstractChemical doping is a powerful way to enhance the electrical performance of organic electronics. To avoid perturbing the ordered molecular packing of organic semiconducting hosts, molecular dopants are deposited on the surface of highly crystalline organic semiconductor thin films. However, such surface doping protocols not only limit charge‐transfer efficiency but also cause dopant diffusion problems, which significantly reduce charge carrier mobility and device stability. Here, an innovative anion bulk doping strategy is reported that allows effective doping of organic single‐crystalline films (OSCFs) without disrupting molecular ordering to improve the performance of organic field‐effect transistors (OFETs). This method is mediated by anion dopants and can be pictured as an effective charge transfer of dopants with organic semiconductors in liquid phase. The direct introduction of dopant anions overcomes limitations of partial charge transfer while avoiding interference from dopant aggregation with crystallization. Using this method, the average carrier mobility of the OSCFs is boosted by ≈2.5 times. Significantly, low‐voltage OFETs developed from anion‐doped OSCFs exhibit a near‐ideal subthreshold swing of 59.2 mV dec−1 and unparalleled mobility as high as 19.8 cm2 V−1 s−1 together with excellent stability. The concept of anion doping opens new avenues for improving the electrical performance of organic electronics.

Sulfone‐functionalized stable molecular single crystals for photocatalytic hydrogen evolution

AbstractHighly crystalline organic semiconductors are ideal materials for photocatalytic hydrogen evolution in water splitting. However, the instability and complex synthesis processes of most reported organic molecule‐based photocatalysts restrict their applications. In this study, we introduce benzo [1,2‐b:4,5‐bʹ] bis [1] benzothiophene‐3,9‐dicarboxylic acid, 5,5,11,11‐tetraoxide (FSOCA), a highly crystalline, stable molecular crystal that is easy to synthesize and serves as an efficient photocatalyst for the hydrogen evolution reaction. FSOCA exhibits high efficiency in sacrificial hydrogen evolution reaction (760 µmol h−1, 76 mmol g−1 h−1 at 330 mW cm−2; 570 µmol h−1, 57 mmol g−1 h−1 at 250 mW cm−2), and FSOCA remains stable during photocatalysis for up to 400 h. Experiments and theoretical studies confirmed the presence of hydrogen bonds between the sulfone group and the sacrificial agent (ascorbic acid). This interaction significantly improved the oxidation reaction kinetics and boosted the photocatalytic performance. This study presents a scalable and convenient approach to synthesize highly crystalline, active, and stable organic photocatalysts with potential applications in large‐scale photocatalysis.

High-speed blade-coating using liquid crystalline organic semiconductor Ph-BTBT-10

We have investigated the fabrication of crystalline thin films of a liquid crystalline organic semiconductor (2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene: Ph-BTBT-10) by high-speed blade-coating at 140 mm s−1. Uniform crystalline Ph-BTBT-10 films were fabricated at temperatures over 50 °C, which is the temperature of the liquid crystal phase, without inhomogeneous recrystallization, despite the high-speed blade-coating. Transistors fabricated using these films showed a high average carrier mobility of 4.8 cm2 Vs−1 for 26 devices, with a coefficient of variation of only 7.3%. We concluded that liquid crystalline organic semiconductors are useful materials for high-speed meniscus-guided-coating for practical use.

High-Performance Organic Field-Effect Transistors of Liquid Crystalline Organic Semiconductor by Laser Mapping Annealing.

Organic semiconductors (OSCs), especially small molecule semiconductors, have received increasing attention due to their good designability and variability. Phase transitions and interfacial properties have a decisive influence on device performance. Here, 2-Dodecyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-12) devices are treated with low-power laser annealing, which is able to avoid the influence of the dewetting effect on the hole mobility of organic semiconductor materials. Ultraviolet ozone treatment and self-assembled monolayer treatment can improve the performance and stability of the device. Moreover, after low-temperature thermal annealing, the hole mobility of the device can even reach as high as 4.80 cm2 V-1 s-1, and we tested the optical response of the device to the ultraviolet wavelength and found that its maximum optical responsivity was 8.2 AW-1.

Large‐Area Processable Ultrathin Organic Transistors with High Mobility and Mechanical Stabilities

AbstractFor applications involving the implantation of organic thin‐film transistors (OTFTs) in flexible environments, maintaining uniform device performance and ultrathin profiles with excellent mechanical stability is crucial for dynamic deformable conditions. This study presents high‐performance, mechanically flexible ultrathin OTFTs with a thickness under 5 µm, designed to be scalable for utilization in large areas. The OTFTs utilized an organosilicon polymer dielectric layer synthesized via initiated chemical vapor deposition combined with the crystalline organic semiconductor 2‐Decyl‐7‐phenyl[1]benzothieno[3,2‐b][1]benzothiophene (Ph‐BTBT‐10). Compared with conventional organic transistors, the OTFTs exhibited exceptional features, such as a charge carrier mobility of up to 7 cm2 V−1 s−1 and a near‐zero threshold voltage. Their performance is showcased across 900 devices on a 10 cm × 10 cm substrate to ensure uniformity and scalability. Additionally, the OTFTs demonstrate remarkable resilience to mechanical deformation, with a 97.4% mobility retention rate under a 20% tensile compression. After 1000 cycles of repeated deformation stress, they maintain a robust 94.6% performance level compared with the initial state. In summary, the ultrathin flexible OTFTs exhibit exceptional resilience to mechanical deformation, extended air exposure, uniformity in large arrays, and high mobility. This makes them promising for diverse applications, such as in vivo organ monitoring and skin displays.

Effect of fullerene doping on electronic and photovoltaic properties of the cubic bicontinuous phase

Fullerene doping allows for tuning the conductivity and photovoltaic properties of cubic bithiophene-based liquid crystalline organic semiconductors.

The anisotropic nature of singlet fission in single crystalline organic semiconductors

The escalating global energy predicament implores for a revolutionary resolution—one that converts sunlight into electricity—holding the key to supreme conversion efficiency. This comprehensive review embarks on the exploration of the principle of generating multiple excitons per absorbed photon, a captivating concept that possesses the potential to redefine the fundamental confines of conversion efficiency, albeit its application remains limited in photovoltaic devices. At the nucleus of this phenomenon are two principal processes: multiple exciton generation (MEG) within quantum-confined environments, and singlet fission (SF) inside molecular crystals. The process of SF, characterized by the cleavage of a single photogenerated singlet exciton into two triplet excitons, holds promise to potentially amplify photon-to-electron conversion efficiency twofold, thereby laying the groundwork to challenge the detailed balance limit of solar cell efficiency. Our discourse primarily dissects the complex nature of SF in crystalline organic semiconductors, laying special emphasis on the anisotropic behavior of SF and the diffusion of the subsequent triplet excitons in single-crystalline polyacene organic semiconductors. We initiate this journey of discovery by elucidating the principles of MEG and SF, tracing their historical genesis, and scrutinizing the anisotropy of SF and the impact of quantum decoherence within the purview of functional mode electron transfer theory. We present an overview of prominent techniques deployed in investigating anisotropic SF in organic semiconductors, including femtosecond transient absorption microscopy and imaging as well as stimulated Raman scattering microscopies, and highlight recent breakthroughs linked with the anisotropic dimensions of Davydov splitting, Herzberg–Teller effects, SF, and triplet transport operations in single-crystalline polyacenes. Through this comprehensive analysis, our objective is to interweave the fundamental principles of anisotropic SF and triplet transport with the current frontiers of scientific discovery, providing inspiration and facilitating future ventures to harness the anisotropic attributes of organic semiconductor crystals in the design of pioneering photovoltaic and photonic devices.

Solution-Processable liquid crystalline organic semiconductors based on 6-(Thiophen-2-yl)benzo[b]thiophene for organic thin-film transistors

Liquid crystalline organic semiconductors have garnered significant attention in the realm of organic thin-film transistors (OTFTs) due to their inherent controllability of phase and electrical properties with temperature variations. Typically, liquid crystalline semiconductors possess distinct structures comprising rigid anisotropic cores and flexible chains. Here, we sought to identify liquid crystalline semiconductor materials by appending a thiophene ring and octyl chain to the anisotropic 6-(thiophen-2-yl)benzo[b]thiophene (TBT) backbone. Among the investigated TBT derivatives, the compound featuring additional thiophene ring and octyl chains on both sides exhibited pronounced liquid crystalline characteristics, specifically the SmE phase. The optical, thermal, and electrical characteristics as well as thin film properties of the TBT derivatives were investigated. The TBT compound was employed through solution-shearing as an active layer for top-contact/bottom-gate OTFTs. The liquid-crystalline TBT (2,6-bis(5-octylthiophen-2-yl)benzo[b]thiophene) thin film annealed at low temperature (30 °C) showed hole mobility as high as 0.025 cm2/Vs and current on/off ratio of > 106. This results in a significant enhancement by an order of magnitude compared to the sample annealed at 70 °C in the SmE phase, and nearly a two-fold improvement compared to the performance observed in the sample annealed at 130 °C in the transition phase. The utilization of the TBT derivative as a p-channel material represents a significant step toward the development of novel liquid crystalline semiconductors.

Benchmark Data Set of Crystalline Organic Semiconductors.

This work reports a Benchmark Data set of Crystalline Organic Semiconductors to test calculations of the structural and electronic properties of these materials in the solid state. The data set contains 67 crystals consisting of mostly rigid molecules with a single dominant conformer, covering the majority of known structural types. The experimental crystal structure is available for the entire data set, whereas zero-temperature unit cell volume can be reliably estimated for a subset of 28 crystals. Using this subset, we benchmark r2SCAN-D3 and PBE-D3 density functionals. Then, for the entire data set, we benchmark approximate density functional theory (DFT) methods, including GFN1-xTB and DFTB3(3ob-3-1), with various dispersion corrections against r2SCAN-D3. Our results show that r2SCAN-D3 geometries are accurate within a few percent, which is comparable to the statistical uncertainty of experimental data at a fixed temperature, but the unit cell volume is systematically underestimated by 2% on average. The several times faster PBE-D3 provides an unbiased estimate of the volume for all systems except for molecules with highly polar bonds, for which the volume is substantially overestimated in correlation with the underestimation of atomic charges. Considered approximate DFT methods are orders of magnitude faster and provide qualitatively correct but overcompressed crystal structures unless the dispersion corrections are fitted by unit cell volume.

Leveling up Organic Semiconductors with Crystal Twisting.

The performance of crystalline organic semiconductors depends on the solid-state structure, especially the orientation of the conjugated components with respect to device platforms. Often, crystals can be engineered by modifying chromophore substituents through synthesis. Meanwhile, dissymetry is necessary for high-tech applications like chiral sensing, optical telecommunications, and data storage. The synthesis of dissymmetric molecules is a labor-intensive exercise that might be undermined because common processing methods offer little control over orientation. Crystal twisting has emerged as a generalizable method for processing organic semiconductors and offers unique advantages, such as patterning of physical and chemical properties and chirality that arises from mesoscale twisting. The precession of crystal orientations can enrich performance because achiral molecules in achiral space groups suddenly become candidates for the aforementioned technologies that require dissymetry.

Doping of molecular semiconductors through proton-coupled electron transfer.

The chemical doping of molecular semiconductors is based on electron-transfer reactions between the semiconductor and dopant molecules; here, the redox potential of the dopant is key to control the Fermi level of the semiconductor1,2. The tunability and reproducibility of chemical doping are limited by the availability of dopant materials and the effects of impurities such as water. Here we focused on proton-coupled electron-transfer (PCET) reactions, which are widely used in biochemical processes3,4; their redox potentials depend on an easily handled parameter, that is, proton activity. We immersed p-type organic semiconductor thin films in aqueous solutions with PCET-based redox pairs and hydrophobic molecular ions. Synergistic reactions of PCET and ion intercalation resulted in efficient chemical doping of crystalline organic semiconductor thin films under ambient conditions. In accordance with the Nernst equation, the Fermi levels of the semiconductors were controlled reproducibly with a high degree of precision-a thermal energy of about 25 millielectronvolts at room temperature and over a few hundred millielectronvolts around the band edge. A reference-electrode-free, resistive pH sensor based on this method is also proposed. A connection between semiconductor doping and proton activity, a widely used parameter in chemical and biochemical processes, may help create a platform for ambient semiconductor processes and biomolecular electronics.

Band Structure Engineering in Highly Crystalline Organic Semiconductors

Band Structure Engineering in Highly Crystalline Organic Semiconductors

High-performance organic field effect transistor of liquid crystalline organic semiconductor using silver electrodes with bottom-gate top-contact configuration

We have investigated organic field effect transistors (OFETs) fabricated with liquid crystalline organic semiconductor, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), and silver (Ag) source-drain electrodes. Ph-BTBT-10 OFETs using Ag source-drain electrodes in bottom-gate top-contact configuration exhibit high mobility of 8.2 cm2 Vs−1, which is similar to those of gold (Au) electrodes. Contact resistance of Ph-BTBT-10 OFETs using Ag electrodes achieved 0.49 kΩcm by transfer length method, which is lower than those of Au electrodes, 2.7 kΩcm. Ph-BTBT-10 OFET using Ag source-drain electrodes in bottom-gate top-contact configuration is a promising candidate for high-performance and low-cost OFETs.

Thin-Film Organic Heteroepitaxy.

Incorporating crystalline organic semiconductors into electronic devices requires understanding of heteroepitaxy given the ubiquity of heterojunctions in these devices. However, while rules for commensurate epitaxy of covalent or ionic inorganic material systems are known to be dictated by lattice matching constraints, rules for heteroepitaxy of molecular systems are still being written. Here, it is found that lattice matching alone is insufficient to achieve heteroepitaxy in molecular systems, owing to weak intermolecular forces that describe molecular crystals. It is found that, in addition, the lattice matched plane also must be the lowest energy surface of the adcrystal to achieve one-to-one commensurate molecular heteroepitaxy over a large area. Ultraviolet photoelectron spectroscopy demonstrates the lattice matched interface to be of higher electronic quality than a disordered interface of the same materials.

Polymer-Gated Transistors with Only One Solution-Processed, Single Crystalline Organic Microwire for Light and Oxygen Detection.

Organic semiconductors employed in single crystalline form have several advantages over polycrystalline films, such as higher charge carrier mobility and better environmental stability. Herein, we report the fabrication and characterization of a solution-processed microsized single-crystalline organic wire of n-type N,N'-dipentyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C5). The crystal was applied as an active layer in polymer-gated organic field-effect transistors (OFETs) and organic complementary inverter circuits. The single crystaiiline nature of PTCDI-C5 wires were characterized using two-dimensional grazing incidence wide-angle X-ray diffraction (2D-GIXD) and polarized optical microscopy. OFETs with the PTCDI-C5 crystals exhibited high n-type performance and air stability under ambient conditions. To investigate the electrical properties of the single-crystalline PTCDI-C5 wire more precisely, OFETs with only one PTCDI-C5 microwire in the channel were fabricated, and clear n-type characteristics with satisfactory saturation behavior were observed. The device with only one crystal wire exhibited characteristics with significantly lower variation compared to the multicrystal devices, which shows that the density of crystal wires is a critical factor in precisely investigating device performance. The devices exhibited a reversible threshold voltage shift under vacuum and oxygen conditions, without changing the charge carrier mobility. Light-sensitive characteristics were also observed. Additionally, this solution-processed, highly crystalline organic semiconductor can be used in high-performance organic electronic circuits as well as in gas or light sensors.

Step‐Edge‐Induced Patterning and Orientation Control of Crystalline Organic Semiconductor Films

AbstractOrientation control over micropatterned crystalline organic semiconductor film is crucial to achieve high device performance with small variation in organic electronics. Here, using Au patterned SiO2 substrates, a method is reported to realize high‐resolution patterning and orientation control of crystalline organic thin films simultaneously. The vacuum deposited N,N‐dioctyl‐3,4,9,10‐perylene tetracarboxylic diimide (PTCDI‐C8) molecules can selectively diffuse to and nucleate on prepatterned Au stripes, resulting in patterned crystalline films with a preferential orientation distribution. Scanning tunneling microscope and high‐resolution atomic force microscope images reveal that the PTCDI‐C8 molecules first assemble on Au with perylene diimide cores in a laying down configuration, providing nucleation sites at the step edges for directional growth along the a‐axis of the molecule crystal. Subsequent deposition of molecules leads to orientated crystalline films growth due to the π packing of the perylene diimide cores, resulting in orientation control on micropatterned crystalline films. The finding provides a new strategy to grow oriented crystalline films for high device performance uniformity.

Efficiently predicting directional carrier mobilities in organic materials with the Boltzmann transport equation.

Describing charge carrier anisotropy in crystalline organic semiconductors with ab initio methods is challenging because of the weak intermolecular interactions that lead to both localized and delocalized charge transport mechanisms. Small polaron hopping models (localized) are generally used to describe materials with small charge carrier mobilities, while periodic band models (delocalized) are used to describe materials with high charge carrier mobilities. Here, we prove the advantage of applying the constant relaxation time approximation of the Boltzmann transport equation (BTE) to efficiently predict the anisotropic hole mobilities of several unsubstituted (anthracene, tetracene, pentacene, and hexacene) and substituted (2,6-diphenylanthracene, rubrene, and TIPS-pentacene) high-mobility n-acene single crystals. Several density functionals are used to optimize the crystals, and the composite density functional PBEsol0-3c/sol-def2-mSVP predicts the most experimentally similar geometries, adequate indirect bandgaps, and the theoretically consistent n-acene charge transport mobility trend. Similarities between BTE and Marcus mobilities are presented for each crystal. BTE and Marcus charge carrier mobilities computed at the same geometry result in similar mobility trends, differing mostly in materials with more substitutions or structurally complex substituents. By using a reduced number of calculations, BTE is able to predict anisotropic carrier mobilities efficiently and effectively for a range of high-mobility organic semiconductors.

Crystalline organic thin films for crystalline OLEDs (II): weak epitaxy growth of phenanthroimidazole derivatives.

The ordered molecular arrangement of crystalline organic semiconductors facilitates high carrier mobility and light emission in organic light-emitting diode (OLED) devices. It has been demonstrated that the weak epitaxy growth (WEG) process is a valuable crystallization route for fabricating crystalline thin-film OLEDs (C-OLEDs). Recently, C-OLEDs based on crystalline thin films of phenanthroimidazole derivatives have exhibited excellent luminescent properties such as high photon output at low driving voltage and high power efficiency. Achieving effective control of organic crystalline thin film growth is crucial for the development of new C-OLEDs. Herein, we report the studies on morphology structure and growth behavior of the phenanthroimidazole derivative WEG thin films. The oriented growth of WEG crystalline thin films is determined by channeling and lattice matching between the inducing layer and active layer. Large-size and continuous WEG crystalline thin films can be obtained by controlling the growth conditions.