Luminescent High‐Mobility 2D Organic Semiconductor Single Crystals

Channel-restricted meniscus self-assembly for uniformly aligned growth of single-crystal arrays of organic semiconductors

Polarization, a fundamental property of light, encodes unique information critical for technological innovations such as bionic navigation, anti‐interference imaging, and secure communication. Conventional polarization‐sensitive materials suffer from inherent limitations, including low linear dichroism, complex fabrication processes, and bulky device integration. Organic semiconductors single crystals (OSSCs) have emerged as a transformative alternative, leveraging intrinsic structural anisotropy from long‐range ordered molecular packing, tunable optical and charge‐transport synergy, and low‐cost solution processability. This perspective systematically elaborates on the core merits of OSSCs for polarization perception, focusing on their intrinsic molecular packing anisotropy, polarization‐dependent optical absorption, and charge transport features. We then highlight recent breakthroughs in OSSC‐based polarization‐sensitive devices, encompassing high‐performance polarization photodetectors, biomimetic polarization vision systems, advanced polarization imaging, and encrypted communication. Furthermore, future directions for advancing OSSCs, including materials engineering, scalable patterned growth of OSSCs, and multifunctional integration, are subsequently outlined. This perspective aims to inspire the great potential of OSSCs for driving transformative progress in next‐generation polarization‐enabled technologies and unlock new possibilities in flexible wearables, autonomous robotics, and energy‐efficient intelligent perception systems.

Previous studies have demonstrated the use of crystalline organic semiconductors to detect and localize the passage of charged particles and energetic radiation. [1-6] In this context, polycrystalline bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) was printed onto polyethylene naphthalate (PEN) substrates patterned with parylene-C dielectric, PEDOT electrodes and gold pads to form fully organic flexible x-ray detectors. The electrodes were patterned using orthogonal photolithography and oxygen reactive ion etching to define a width/length (W/L) = 100 µm/10 µm. An organic voltage divider built using these materials was hot bar bonded to a printed circuit board (PCB) via a flexible conducting tape to form a complete sensor system. The devices were irradiated with a variety of localized and large area sources and the output was extracted from the node between the two resistors and then connected to an operational amplifier via a second PCB. Dark currents for each resistor were in the 100 pA - 1 nA range. The device demonstrated has the potential to be applied in microdosimetry to allow for detection using a cross-section that matches organic tissue forming a solid state tissue equivalent detector (SSTED) [7].

The use of organic materials as radiation detectors has grown, due to the easy processability in liquid phase at room temperature and the possibility to cover large areas by means of low cost deposition techniques. Direct charged-particle detectors based on solution-grown Organic Semiconducting Single Crystals (OSSCs) are shown to be capable to detect charged particles in pulse mode, with very good peak discrimination. The direct charged-particle detection in OSSCs has been assessed both in the planar and in the vertical axes, and a digital pulse processing algorithm has been used to perform pulse height spectroscopy and to study the charge collection efficiency as a function of the applied bias voltage. Taking advantage of the charge spectroscopy and the good peak discrimination of pulse height spectra, an Hecht-like behavior of OSSCs radiation detectors is demonstrated. It has been possible to estimate the mobility-lifetime value in organic materials, a fundamental parameter for the characterization of radiation detectors, whose results are equal to μτcoplanar = (5 .5 ± 0.6 ) × 10−6 cm2/V and μτsandwich = (1 .9 ± 0.2 ) × 10−6 cm2/V, values comparable to those of polycrystalline inorganic detectors. Moreover, alpha particles Time-of-Flight experiments have been carried out to estimate the drift mobility value. The results reported here indicate how charged-particle detectors based on OSSCs possess a great potential as low-cost, large area, solid-state direct detectors operating at room temperature. More interestingly, the good detection efficiency and peak discrimination observed for charged-particle detection in organic materials (hydrogen-rich molecules) are encouraging for their further exploitation in the detection of thermal and high-energy neutrons.

In this paper, we report an efficient alkali metal doping system for organic single crystals. Our system employs an enhanced diffusion method for the introduction of alkali metal into organic single crystals by controlling the sample temperature to induce secondary thermal activation. Using this system, we achieved intercalation of potassium into picene single crystals with closed packed crystal structures. Using optical microscopy and Raman spectroscopy, we confirmed that the resulting samples were uniformly doped and became K2picene single crystal, while only parts of the crystal are doped and transformed into K2picene without secondary thermal activation. Moreover, using a customized electrical measurement system, the insulator-to-semiconductor transition of picene single crystals upon doping was confirmed by in situ electrical conductivity and ex situ temperature-dependent resistivity measurements. X-ray diffraction studies showed that potassium atoms were intercalated between molecular layers of picene, and doped samples did not show any KH- nor KOH-related peaks, indicating that picene molecules are retained without structural decomposition. During recent decades, tremendous efforts have been exerted to develop high-performance organic semiconductors and superconductors, whereas as little attention has been devoted to doped organic crystals. Our method will enable efficient alkali metal doping of organic crystals and will be a resource for future systematic studies on the electrical property changes of these organic crystals upon doping.

The standard technique to separately and simultaneously determine the carrier concentration per unit volume (N, cm-3 ) and the mobility (μ) of doped inorganic single crystals is to measure the Hall effect. However, this technique has not been reported for bulk-doped organic single crystals. Here, the Hall effect in bulk-doped single-crystal organic semiconductors is measured. A key feature of this work is the ultraslow co-deposition technique, which reaches as low as 10-9 nm s-1 and enables us to dope homoepitaxial organic single crystals with acceptors at extremely low concentrations of 1 ppm. Both the hole concentration per unit volume (N, cm-3 ) and the Hall mobility (μH ) of bulk-doped rubrene single crystals, which have a band-like nature, are systematically observed. It is found that these rubrene single crystals have (i) a high ionization rate and (ii) scattering effects because of lattice disturbances, which are peculiar to this organic single crystal.

Organic semiconducting single crystals are perfect for both fundamental and application-oriented research due to the advantages of free grain boundaries, few defects, and minimal traps and impurities, as well as their low-temperature processability, high flexibility, and low cost. Carrier mobilities of greater than 10 cm2 V-1 s-1 in some organic single crystals indicate a promising application in electronic devices. The progress made, including the molecular structures and fabrication technologies of organic single crystals, is introduced and organic single-crystal electronic devices, including field-effect transistors, phototransistors, p-n heterojunctions, and circuits, are summarized. Organic two-dimensional single crystals, cocrystals, and large single crystals, together with some potential applications, are introduced. A state-of-the-art overview of organic single-crystal electronics, with their challenges and prospects, is also provided.

All-Solution-Processed 5 V FeFETs with Organic Semiconductor Single Crystals

Organic semiconductors (OSCs) have attracted growing attention for optoelectronic applications such as field-effect transistors (FETs), and coherent (or band-like) carrier transport properties in OSC single crystals (SCs) have been of interest as they can lead to high carrier mobilities. Recently, such p-type OSC SCs compatible with a printing technology have been used to achieve high-speed FETs; therefore, developments of n-type counterparts may be promising for realizing high-speed complementary organic circuits. Herein, coherent electron transport properties in a printed SC of a state-of-the-art, air-stable n-type OSC, PhC2 -BQQDI, by means of variable-temperature gated Hall effect measurements and X-ray single-crystal diffraction analyses in conjunction with band structure calculations, are reported. Furthermore, the SC FET is tested for high-speed operations, which obtains a cutoff frequency of 4.3MHz at an operation voltage of 20V in air. Thus, PhC2 -BQQDI is shown as a new candidate for practical applications of SC-based, organic complementary devices.

We report a method to fabricate thin films of large-domain organic semiconductor single crystals dispersed over the whole surface of centimeter-scale substrates for field-effect transistors. Growing less than 500-nm thick film-like organic crystals of sub-millimeter sizes densely in a furnace independently of substrates by physical vapor transport, the collection of the single crystals is mechanically attached to the surface of gate dielectric layers. The organic transistors made of large-domain benzo-annulated pentathienoacene crystals exhibited pronounced transistor performances with mobility values of ∼ 0.2-2 cm2/Vs, which is as high as devices of one-piece crystals. The result demonstrates that the above technique provides a method to apply high performance of organic single crystal transistors to real circuitry devices on large-area substrates.

Developing high-performance polarization-sensitive ultraviolet photodetectors is crucial for their application in military remote sensing, detection, bio-inspired navigation, and machine vision. However, the significant absorption in the visible light range severely limits the application of polarization-sensitive ultraviolet photodetectors, such as high-quality anti-interference imaging. Here, based on a wide-bandgap organic semiconductor single crystal (trans-1,2-bis(5-phenyldithieno[2,3-b:3',2'-d]thiophen-2-yl)ethene, BPTTE), high-performance polarization-sensitive solar-blind ultraviolet photodetectors with a dichroic ratio close to 4.26 are demonstrated. The strong anisotropy of 2D grown BPTTE single crystals in molecular vibration and optical absorption is characterized by various techniques. Under voltage modulation, stable and efficient detection of polarized light is demonstrated, attributed to the intrinsic anisotropy of transition dipole moment in the bc crystal plane, rather than other factors. Finally, high-contrast polarimetric imaging and anti-interference imaging are successfully demonstrated based on BPTTE single crystal photodetectors, highlighting the potential of organic semiconductors for polarization-sensitive solar-blind ultraviolet photodetectors.

The electronic properties of organic single crystals have been intensely studied for well over 40 years. Until recently, organic single-crystal field-effect transistors have generated results that are comparable to and sometimes better in performance than hydrogenated amorphous silicon. Organic thin-film transistors are being actively pursued for a broad area of electronic applications, but their charge-carrier mobilities are limited by structural imperfections (i.e., grain boundaries) and impurities. Organic single crystals, on the other hand, have been limited to charge-transport studies mainly because the fabrication of single-crystal transistors poses a technological challenge. Novel methods for fabricating single-crystal devices include the flip-crystal technique, elastomeric stamp platforms, and freestanding devices, where the source–drain electrodes, dielectric, and gate are all fabricated onto the crystal surface. For the most part, a relatively thick and rigid single crystal is employed (5–500 lm thick). Because the fragility makes them difficult to handle, their use has been restricted to simple and basic devices and wide-ranging applications in sensors or plastic transistors for flexible electronics have not yet been possible. Thus, there is a strong need for the development of mechanically flexible, nondestructive, single-crystal devices with prospective applications in organic electronics while maintaining the intrinsic properties and characteristics of organic single crystals. We demonstrate field-effect transistors fabricated from thin and conformable organic single crystals. We report on proofof-concept “flexible” organic single-crystal field-effect transistors with performance exceeding those of previously reported organic thin-film flexible devices. Rubrene single-crystal devices constructed on low-cost flexible substrates (Fig. 1b) yielded mobilities as high as 4.6 cm V s and on/off ratios of approximately 10.

Electronic band structure serves as the foundation for understanding the physics of semiconductors. The electronic band structure of inorganic semiconductors has been well understood based on ideal single crystal samples, thus actually laid the foundation for the prospering development of inorganic semiconductor devices. As for organic semiconductors, which are currently being pervasive in our daily life, however, the ‘physics’ of organic semiconductors is still far from complete understanding compared to their inorganic counter-parts which could hinder the rapid development of organic electronics. Only few organic single crystals (e.g., rubrene, pentacene) have been well investigated to probe the electronic structure and their relationship with their electrical properties, sufficient experimental evidence is still lacking for intrinsic properties of organic single crystal, which mainly hindered by the limited crystal size and low conductivity. Two-dimensional molecular crystals (2DMCs) of organic semiconductors are intriguing materials because their unique advantages, such as long-range molecular packing, low defect density and lack of grain boundaries, make 2DMCs an ideal platform for exploring the structure-property relationship, revealing the intrinsic properties and probable carrier transport mechanism, fabricating high-performance optoelectronic devices. Especially, unique optoelectronic properties exhibited in 2DMCs are not found in their bulk counterparts. With breakthrough in crystal engineering for producing large-area (e.g., millimeter or centi-meter even wafer-sized) 2DMCs and material engineering for designing novel organic semiconductors (e.g., C10-DNTT, C6-DPA), all provide a great opportunity to explore the physical origin behind the novel optoelectronic properties of kinds of organic semiconductors and their correlation with optoelectronic device performance, which further guiding material design and facilitating flourishing of organic electronics. The aim of this thesis is to investigate the electronic structure of 2DMCs and correlate them with their optoelectronic properties. The 2DMCs were mainly produced by space-confined strategy and layer-defining strategy, the produced 2DMCs could be transferred to any substrates for further characterization. One of selected organic materials is 2,6-Bis(4-hexylphenyl)anthracene (C6-DPA), which belongs to anthracene derivates family, typically known for their high luminescence efficiency and carrier mobility. All characterized 2DMCs of C6-DPA show a high quality. Firstly, we fabricated integrated organic effect field transistors e.g., organic phototransistors, organic memory phototransistors based on 2DMCs of C6-DPA to elucidate the high performance and potential applications. To clarify the physical origin of opto-electronic properties, some advanced surface science experimental techniques were used to determine the electronic structure of 2DMCs of C6-DPA. Resonant photoemission spectroscopy reveals a room temperature band dispersion of C6-DPA, which is well explained by calculated band structure. Angle-resolved photoemission spectroscopy results confirm the room temperature dispersion and exhibit anisotropic band dispersion in plane. The anisotropic charge carrier mobility is 2 in plane, where the highest mobility obtained along the molecular direction with obvious band dispersion, suggesting the electronic property and electrical property quite match well. We then investigated the in-fluence of degree of crystallinity on electronic structure by ultraviolet photoemission spectroscopy, all results indicate that high crystallinity help to overcome Coulomb interaction and facilities charge to be delocalized on whole 2D crystal, while in verse in less degree of thin film. We then selected perylene as the second material to explore the exciton band structure by electron energy-loss spectroscopy. The observed negative band dispersion is rationalized by effective inter-dimer coupling with an additional charge transfer contribution. This result could provide guidance for understanding the in-plane charge transport properties in 2D crystal of perylene.

Polypyrrole hexagonal microplates (PHMs) (50-100 microm long, 10-20 microm wide, and 0.8-1.2 microm thick) with a quasicrystalline structure and high electrical conductivity (up to 400 S/cm) are simply fabricated using single crystals of 4-sulfobenzoic acid monopotassium salt (KSBA) in aqueous medium. Moreover, the fabrication process described here differs strikingly from traditional methods, such as template-free, soft template, and hard template methods. Synthetic time-resolved polypyrrole (PPy) morphology dynamics reveals that the fabrication process of PHMs composed of PPy nanostructures combines a shape-copying process for forming a PPy preform that imitates the shape of a KSBA single crystal and the self-assembly process of PPys on the preform. The PHMs exhibit the improved pi-stacking and bipolaron structure. The strong pi-stacks among PPy rings of bipolaron structures lead to a high quasicrystalline structural order and the metallic conduction. Other single organic crystals that can act as dopants could also be grown using this approach, which will also enable the fabrication of complex micro/nanostructures on organic single crystals.

Single-Crystal Organic Semiconductors