Cocrystal engineering:towards high-performance near-infrared organic phototransistors based on donor-acceptor charge transfer cocrystals

Near-infrared organic phototransistors have wide application prospects in many fields.The active materials with the high mobility and near-infrared response are critical to building high-performance near-infrared organic phototransistors,which are scarce at present.Herein,a new charge transfer cocrystal using 5,7-dihydroindolo[2,3-b]carbazole(5,7-ICZ)as the donor and 2,2’-(benzo[1,2-b:4,5-b’]dithiophene-4,8-diylidene)dimalononitrile(DTTCNQ)as the acceptor is properly designed and prepared in a stoichiometric ratio(D:A=1:1),which not only displays a high electron mobility of 0.15 cm^(2)V^(-1)s^(-1) and very low dark current,but also can serve as the active layer materials in the region of near-infrared detection due to the narrowed band gap and good charge transport properties.A high photosensitivity of 1.8×10^(4),the ultrahigh photoresponsivity of 2,923 A W-1and the high detectivity of 4.26×10^(11)Jones of the organic near-infrared phototransistors are obtained.

High-performance floating-gate organic phototransistors based on n-type core-expanded naphthalene diimides

In the field of organic phototransistor, achieving both broad-spectral and high photosensitivity has always been a big challenge. The innovation of device structure has previously proven to be a possible solution to this problem. Here in this study, a novel organic phototransistor based on a high mobility n-type small molecule as the conducting layer and an isolated bulk heterojunction light-absorbing layer as the floating gate has been demonstrated in this study. With the special designed device structure, the phototransistor shows extremely high sensitivity to broad spectral and weak light irradiation, and the photoresponsivity and photocurrent/dark-current ratio of the device can reach up to 4840 mA/W and 1.8×10~5 respectively.For conclusion, this study suggests a potential way to obtain high-performance phototransistors at room temperature, which will further promote the commercial application of organic phototransistors.

Interface terminal group regulated organic phototransistors with tunable persistent and switchable photoconductivity

With both light detection and intrinsic amplification functions,organic phototransistors have demonstrated promising applications,including photodetection and photomemory.To achieve excellent photoresponse and superior photogain,a common and effective strategy is to modulate the trapping effect with the purpose of reducing recombination or prolonging the lifetime of the photogenerated charge carriers.However,introducing trapping sites delicately is challenging and might sacrifice the response rate together with a typical persistent photoconductivity.Here,we demonstrate a facile strategy for achieving high photo-responsive organic phototransistors with both persistent and switchable photoconductivity features via interface terminal group regulation.By varying the terminate groups of self-assembled monolayer(SAMs)from the strong electron withdrawing group-F,neutral−CH_(3) to electron donating−NH_(2) on the dielectric surface,we realize both minority carrier trapping and majority carrier trapping in the organic phototransistor based on the C8-BTBT active layer.The electron withdrawing effect of F significantly enhances the minority carrier trapping process and yields a high photoresponsivity with a long-lasting persistent photoconductivity.In contrast,the electron donating group−NH_(2) with a distinct majority carrier trapping ability causes switchable photoconductivity so that the photocurrent can rise pronouncedly and fully decay along with light on/off.Attractively,both cases can deliver high performance with photoresponsivities higher than 104 A W^(−1) together with a photosensitivity in the level of 107 and a detectivity of approximately 10^(15)–10^(16) Jones.Such a tunable,excellent photoresponse property enables the convenient exploration of organic phototransistors to satisfy different application requirements.

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.

Organic single-crystal phototransistor with unique wavelength-detection characteristics

Organic phototransistors based on high-quality 2,8-dichloro-5,11-dihexyl-indolo[3,2-b]carbazo(CHICZ)single crystals show the highest photoresponsivity of 3×10^3 A W^-1, photosensitivity of 2×10^4 and the detectivity can achieve 8.4×10^14 Jones. We also discovered good linear dependence of log(photosensitivity) versus the wavelength when the devices were illuminated with a series of sameintensity but different-wavelength lights. The organic phototransistors based on CHICZ single crystal have potential applications in wavelength-detection.

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.