External-force-driven solution epitaxy of large-area 2D organic single crystals for high-performance field-effect transistors

Growth of two-dimensional(2D)organic single crystals(2DOSCs)on water surface has attracted increasing attention,because it can serve as a molecularly flat and defect-free substrate.However,large-area growth of 2DOSCs with controllable crystal orientation on water surface remains a key challenge.Herein,we develop a simple method,i.e.external-force-driven solution epitaxy(EFDSE),for the large-area growth of 2DOSCs at air/water interface.Using 2,7-didecylbenzothienobenzothiophene(C1o-BTBT)as an example,high-quality 2D C10-BTBT crystals on centimeter scale are generated by directionally controlling the spreading of organic solution on water surface with external force.Benefiting from the controllable crystal orientation with optimal charge transport,the corresponding 2DOSC-based organic field-effect transistors(OFETs)exhibit a high carrier mobility of 13.5 cm^2·V^-1·s^-1(effective mobility=5.4 cm^2·V^-1·s^-1 according to a reliability factor of 40%),which represents the best result achieved for water-surface-assembled 2DOSC based OFETs.Furthermore,by transterring the C1o-BTBT 2DOSCs to flexible substrates,devices with excellent bending stability are achieved.It is anticipated that our method will provide new insight into the controllable growth of large-area 2DOSCs for high-performance organic devices.

Epitaxial etching of organic single crystals

Organic single crystals(OSCs)have received increasing interest in the last decades for their potential applications in flexible electronics.Although there are various subtractive manufacturing methods of organic electronics,the subtractive manufacturing of OSCs is still a challenge,since OSCs are assembled via weak van-der-Waals interactions which are vulnerable and cannot afford damages and suffer the degradation of performances after the process.Here,we develop an epitaxial etching strategy which clips the OSCs and keeps high-quality crystalline nature of the resulting materials.As a result,high-quality organic micro-ribbon arrays are fabricated which maintains 89%charge mobility in average compared with original OSCs,showing great potential of this subtractive manufacturing method in future organic electronics.

Alignment and patterning of organic single crystals for field-effect transistors

Organic field-effect transistors are of great importance to electronic devices.With the emergence of various preparation techniques for organic semiconductor materials,the device performance has been improved remarkably.Among all of the organic materials,single crystals are potentially promising for high performances due to high purity and well-ordered molecular arrangement.Based on organic single crystals,alignment and patterning techniques are essential for practical industrial application of electronic devices.In this review,recently developed methods for crystal alignment and patterning are described.

Liquid-liquid interface‐assisted growth of two‐dimensional organic semiconductor single crystals:Mechanisms,properties,and applications

Two‐dimensional organic semiconductor single crystals(2D OSSCs)represent the promising candidates for the construction of high‐performance electronic and optoelectronic devices due to their ultrathin thicknesses,free of grain boundaries,and long‐range ordered molecular structures.In recent years,substantial efforts have been devoted to the fabrication of the large‐sized and layer‐controlled 2D OSSCs at the liquid‐liquid interface.This unique interface could act as the molecular flat and defect‐free substrate for regulating the nucleation and growth processes and enabling the formation of large‐sized ultrathin 2D OSSCs.Therefore,this review focuses on the liquid-liquid interface‐assisted growth methods for the controllable preparation of 2D OSSCs,with a particular emphasis on the advantages and limitations of the corresponding methods.Furthermore,the typical methods employed to control the crystal sizes,morphologies,structures,and orientations of 2D OSSCs at the liquid-liquid interface are discussed in detail.Then,the recent progresses on the 2D OSSCs‐based optoelectronic devices,such as organic field‐effect transistors,ambipolar transistors,and phototransistors are highlighted.Finally,the key challenges and further outlook are proposed in order to promote the future development of the 2D OSSCs in the field of the next‐generation organic optoelectronic devices.

Vapor-Induced Coating Method for Well-Aligned and Uniform Organic Semiconductor Single Crystals

Uniform and well-aligned organic semiconductor single crystals(OSSCs)are critical for high-performance electronic and optoelectronic device applications due to their long-range order and low defect density.However,it is still challenging to fabricate uniform and well-aligned OSSCs by an efficient and facile method.Here,we report a vapor-induced coating method to prepare uniform organic semiconductor stripe single crystals with well-aligned orientation.The coating velocity and solution concentration are important to control the stripe crystals’morphology,which influence the triple-phase contact line dewetting behavior and then change the mass transport of the meniscus.Insufficient solute supply causes the generation of dendritic crystals.Uniform stripe single crystals of high quality and pure orientation are prepared in the condition of a sufficient and suitable solute supply.Moreover,the electronic and optoelectronic properties are evaluated.Notably,the polarization-sensitive photodetectors based on the uniform stripe crystals exhibit high polarization sensitivity and its dichroic ratio of photocurrent is 1.98.This method is efficient for the preparation of various high-quality and uniform organic semiconductor stripe single crystals,opening an opportunity for high-performance organic functional devices.

Long-range ordering of composites for organic electronics:TIPS-pentacene single crystals with incorporated nano-fibers

Multi-component active materials are widely used for organic electronic devices, with every component contributing complementary and synergistic optoelectronic functions. Mixing these components generally leads to lowered crystallinity and weakened charge transport. Therefore, preparing the active materials without substantially disrupting the crystalline lattice is highly desired. Here, we show that crystallization of TIPS-pentacene from solutions in the presence of fluorescent nanofibers of a perylene bisimide derivative （PBI） leads to formation of composites with nanoflber guest incorporated in the crystal host. In spite of the binary composite structure, the TIPS-pentacene maintains the single- crystalline nature. As a result, the incorporation of the PB1 guest introduces additional fluorescence function but does not significantly reduce the charge transport property of the TIPS-pentacene host, exhibiting field-effect mobility as high as 3.34 cm^2 V^-1 s^-1 even though 26.4% of the channel area is taken over by the guest. As such, this work provides a facile approach toward high-performance multifunctional organic electronic materials.

Few-layered organic single-crystalline heterojunctions for high-performance phototransistors

Photogating and electrical gating are key physical mechanisms in organic phototransistors(OPTs).However,most OPTs are based on thick and polycrystalline films,which leads to substantially low efficiency of both photogating and electrical gating and thus reduced photoresponse.Herein,high-performance OPTs based on few-layered organic single-crystalline heterojunctions are proposed and the obstacle of thick and polycrystalline films for photodetection is overcome.Because of the molecular scale thickness of the type I organic single-crystalline heterojunctions in OPTs,both photogating and electrical gating are highly efficient.By synergy of efficient photogating and electrical gating,key figures of merit of OPTs reach the highest among those based on planar heterojunctions so far as we know.The production of few-layered organic single-crystalline heterojunctions will provide a new type of advanced materials for various applications.

Organic single crystal phototransistors:Recent approaches and achievements

Organic phototransistors(OPTs),compared to traditional inorganic counterparts,have attracted a great deal of interest because of their inherent flexibility,light-weight,easy and low-cost fabrication,and are considered as potential candidates for next-generation wearable electronics.Currently,significant advances have been made in OPTs with the development of new organic semiconductors and optimization of device fabrication protocols.Among various types of OPTs,small molecule organic single crystal phototransistors(OSCPTs)standout because of their exciting features,such as long exciton diffusion length and high charge carrier mobility relative to organic thinfilm phototransistors.In this review,a brief introduction to device architectures,working mechanisms and figure of merits for OPTs is presented.We then overview recent approaches employed and achievements made for the development of OSCPTs.Finally,we spotlight potential future directions to tackle the existing challenges in this field and accelerate the advancement of OSCPTs towards practical applications.

Controlled 2D growth of organic semiconductor crystals by suppressing “coffee-ring” effect

Owing to enhanced charge transport efficiency arising from the ultrathin nature,two-dimensional(2D)organic semiconductor single crystals(OSSCs)are emerging as a fascinating platform for high-performance organic field-effect transistors(OFETs).However,ucoffee-ring"effect induced by an evaporation-induced convective flow near the contact line hinders the large-area growth of 2D OSSCs through a solution process.Here,we develop a new strategy of suppressing the"coffee-ring"effect by using an organic semiconductor:polymer blend solution.With the high-viscosity polymer in the organic solution,the evaporation-induced flow is remarkably weakened,ensuring the uniform molecule spreading for the 2D growth of the OSSCs.As an example,wafer-scale growth of crystalline film consisting of few-layered 2,7-didecylbenzothienobenzothiophene(C10-BTBT)crystals was successfully accomplished via blade coating.OFETs based on the crystalline film exhibited a maximum hole mobility up to 12.6 cm^2·V^-1·s^-1,along with an average hole mobility as high as 8.2 cm^2·V^-1·s^-1.Our work provides a promising strategy for the large-area growth of 2D OSSCs toward high-performance organic electronics.

Spin injection and transport in single-crystalline organic spin valves based on TIPS-pentacene

Single crystals of organic semiconductors with perfect crystal structure and minimal density of defects can exhibit high mobility and low spin scattering compared with their amorphous or polycrystalline counterparts.Therefore,these materials are promising candidates as the spin transport media to obtain long spin relaxation times and spin diffusion lengths in spintronic devices.However,the investigation of spin injection and transport properties in organic single crystals is hindered by the inability to construct devices such as single-crystalline organic spin valves(OSVs).Herein,thin and large organic single crystals of 6,13-bis(triisopropylsilylethynyl)pentacene(TIPS-pentacene)were grown on a liquid substrate and transferred to a target substrate carrying ferromagnetic electrodes to construct single-crystalline OSVs.The magnetoresistance(MR)responses of the single crystals were investigated to study their spin injection and transport properties.MR value as high as 17%was probed with an intermediate layer thickness of 269 nm.More importantly,spin transport was still observed in a single crystal of a thickness up to 457 nm,which was much larger than that of polycrystalline thin film.Our research provides a general methodology for constructing single-crystalline OSVs and paves the way to probe the intrinsic spin transport properties of organic semiconductors based on single crystals.

Highly efficient modulation of the electronic properties of organic semiconductors by surface doping with 2D molecular crystals

Doping is a critically important strategy to modulate the properties of organic semiconductors(OSCs) to improve their optoelectrical performances. Conventional bulk doping involves the incorporation of foreign molecular species(i.e., dopants) into the lattice of the host OSCs, and thus disrupts the packing of the host OSCs and induces structural defects, which tends to reduce the mobility and(or) the on/off ratio in organic field-effect transistors(OFETs). In this article, we report a highly efficient and highly controllable surface doping strategy utilizing 2D molecular crystals(2DMCs) as dopants to boost the mobility and to modulate the threshold voltage of OFETs. The amount of dopants, i.e., the thickness of the 2DMCs, is controlled at monolayer precision, enabling fine tuning of the electrical properties of the OSCs at unprecedented accuracy. As a result, a prominent increase of the average mobility from 1.31 to 4.71 cm2 V-1 s-1 and a substantial reduction of the threshold voltage from -18.5 to -1.8 V are observed. Meanwhile, high on/off ratios of up to 108 are retained.

Vertical‐organic‐nanocrystal‐arrays for crossbar memristors with tuning switching dynamics toward neuromorphic computing

Memristors proposed by Leon Chua provide a new type of memory device for novel neuromorphic computing applications.However,the approaching of distinct multi‐intermediate states for tunable switching dynamics,the con-trolling of conducting filaments(CFs)toward high device repeatability and reproducibility,and the ability for large‐scale preparation devices,remain full of challenges.Here,we show that vertical‐organic‐nanocrystal‐arrays(VONAs)could make a way toward the challenges.The perfect one‐dimensional structure of the VONAs could confine the CFs accurately with fine‐tune resistance states in a broad range of 103 ratios.The availability of large‐area VONAs makes the fabrication of large‐area crossbar memristor arrays facilely,and the analog switching characteristic of the memristors is to effectively imitate different kinds of synaptic plasticity,indicating their great potential in future applications.