Recent advances in flexible photodetectors based on 1D nanostructures

Semiconductor nanowires have demonstrated excellent electronic and optoelectronic properties.When integrated into photodetectors,excellent device performance can be easily attained.Apart from the exceptional performance,these nanowires can also enable robust and mechanically flexible photodetectors for various advanced utilizations that the rigid counterparts cannot perform.These unique applications include personal healthcare,next-generation robotics and many others.In this review,we would first discuss the nanowire fabrication techniques as well as the assembly methods of constructing largescale nanowire arrays.Then,the recent development of flexible photodetectors based on these different nanowire material systems is evaluated in detail.At the same time,we also introduce some recent advancement that allows individual photodetectors to integrate into a more complex system for advanced deployment.Finally,a short conclusion and outlook of challenges faced in the future of the community is presented.

High-performance photodetectors based on two-dimensional perovskite crystals with alternating interlayer cations

Organic-inorganic halide perovskite,as a low-cost,solution-processable material with remarkable optoelectronic properties,is ideal candidate to fabricate high-performance photodetectors and is expected to significantly reduce device costs.Compared to the common Dion-Jacobson and Ruddlesden-Popper two-dimensional(2D)layered hybrid perovskite compounds,the perovskites with alternating cations in the interlayer(ACI)phase show higher crystal symmetry and narrower optical bandgaps,which exhibit great potential for excellent photodetection performance.Herein,we report a high-performance photodetector based on the 2D bilayered hybrid lead halide perovskite single crystal with the ACI phase(GAMA_(2)Pb_(2)I_(7);GA=C(NH_(2))_(3)and MA=CH_(3)NH_(3)).The single-crystal photodetector exhibits high photoresponsivity of 1.56,2.54,and 2.60 A/W for incident light wavelengths of 405,532,and 635 nm under 9.82 nW,respectively,together with the correspondingly high detectivity values of 1.86×10^(12),3.04×10^(12),and 3.11×10^(12)Jones under the same operating conditions.Meanwhile,a high-resolution imaging sensor is built based on the GAMA_(2)Pb_(2)I_(7)single-crystal photodetector,confirming the high stability and photosensitivity of the imaging system.These results show that the 2D hybrid lead halide perovskites with alternating interlayer cations are promising for high-performance visible light photodetectors and imaging systems.

Wafer-scale synthesis of monolayer WS2 for high-performance flexible photodetectors by enhanced chemical vapor deposition

Two-dimensional （2D） nanomaterials have recently attracted considerable attention due to their promising applications in next-generation electronics and optoelectronics. In particular, the large-scale synthesis of high-quality 2D materials is an essential requirement for their practical applications. Herein, we demonstrate the wafer-scale synthesis of highly crystalline and homogeneous monolayer WS2 by an enhanced chemical vapor deposition （CVD） approach, in which precise control of the precursor vapor pressure can be effectively achieved in a multi-temperature zone horizontal furnace. In contrast to conventional synthesis methods, the obtained monolayer WS2 has excellent uniformity both in terms of crystallinity and morphology across the entire substrate wafer grown （e.g., 2 inches in diameter）, as corroborated by the detailed characterization. When incorporated in typical rigid photodetectors, the monolayer WS2 leads to a respectable photodetection performance, with a responsivity of 0.52 mA/W, a detectivity of 4.9 × 10^9 Jones, and a fast response speed （〈 560μs）. Moreover, once fabricated as flexible photodetectors on polyimide, the monolayer WS2 leads to a responsivity of up to 5 mA/W. Importantly, the photocurrent maintains 89% of its initial value even after 3,000 bending cycles. These results highlight the versatility of the present technique, which allows its applications in larger substrates, as well as the excellent mechanical flexibility and robustness of the CVD-grown, homogenous WS2 monolayers, which can promote the development of advanced flexible optoelectronic devices.

Crystalline InGaZnO quaternary nanowires with superlattice structure for high-performance thin-film transistors

Amorphous indium-gallium-zinc oxide (a-IGZO) materials have been widely explored for various thin-film transistor (TFT) applications;however, their device performance is still restricted by the intrinsic material issues especially due to their non-crystalline nature. In this study, highly crystalline superlattice-structured IGZO nanowires (NWs) with different Ga concentration are successfully fabricated by enhanced ambient-pressure chemical vapor deposition (CVD). The unique superlattice structure together with the optimal Ga concentration (i.e., 31 at.%) are found to effectively modulate the carrier concentration as well as efficiently suppress the oxygen vacancy formation for the superior NW device performance. In specific, the In1.8Ga1.8Zn2.4O7 NW field-effect transistor exhibit impressive device characteristics with the average electron mobility of ~ 110 cm^2-V^-1·s^-1 and on/off current ratio of ~ 10^6. Importantly, these NWs can also be integrated into NW parallel arrays for the construction of high-performance TFT devices, in which their performance is comparable to many state-of-the-art IGZO TFTs. All these results can evidently indicate the promising potential of these crystalline superlattice-structured IGZO NWs for the practical utilization in next-generation metal-oxide TFT device technologies.

Solution-processed lead-free double perovskite microplatelets with enhanced photoresponse and thermal stability

Due to the better stability and environmentfriendly nature,lead-free halide double perovskites are widely explored as promising materials for next-generation photovoltaics and optoelectronics;however,to date,their photoelectric device performance is still not satisfactory.Herein,we report a facile solution-process method to synthesize the recently most popular lead-free halide double perovskite,MA_(2)Ag Bi Br_(6),and its all-inorganic counterpart,Cs_(2)Ag Bi Br_(6).The obtained MA_(2)Ag Bi Br_(6)and Cs_(2)Ag Bi Br_(6)films exhibit the microplatelet morphology with excellent crystallinity,distinctly contrasting the ones fabricated by the conventional spin-coating method.Once fabricated into simple photodetectors,the Cs_(2)Ag Bi Br_(6)microplatelet devices yield a respectable responsivity of 245 m A W^(-1) that is two orders of magnitude larger than that of the spin-coated films.More importantly,the response speed of the Cs_(2)Ag Bi Br_(6)microplatelets device is as fast as 145μs,which is higher than most of the values reported in the community of halide double perovskites.When subjected to the thermal stability testing,the Cs_(2)Ag Bi Br_(6)microplatelet device can maintain its initial performance after heating to 160℃ and cooling down to room temperature in the ambient environment.All these results suggest that the facile solution-process method is capable of fabricating high-quality lead-free double perovskites,enabling their advanced device applications.

Direct drop-casting synthesis of all-inorganic lead and lead-free halide perovskite microcrystals for high-performance photodetectors

Due to the exciting photoelectric properties,better stability,and environmental-friendly nature,all-inorganic halide perovskites(AIHPs),especially the lead-free perovskites,have attracted worldwide attention.However,the film quality of AIHPs fabricated by typical spin-coating and subsequent high-temperature annealing is still not satisfactory,restricting their further development.Herein,we demonstrate a simple low-temperature solution-processed drop-casting method to achieve highly-crystalline cubic CsPbBr_(3)and lead-free layer-structured Cs_(3)Sb_(2)I_(9)microcrystals(MCs).This drop-casting technique not only consumes the less amount of precursor solution but also eliminates the high-temperature annealing as compared with those of spin coating.When these MCs are configured into photodetectors,they exhibit superior device performance,which is in distinct contrast to the one of spin-coated counterparts.Specifically,the responsivity of CsPbBr_(3)MCs is found to be as large as 8,990 mA/W,being 13 times larger than the spin-coated films and even better than many state-of-the-art solution-processed AIHPs devices.This device performance enhancement is attributed to the better film quality and phase purity obtained by the drop-casting method.All these results can evidently fill the“technology gap”for further enhancing the material quality of solution-processed AIHPs and breaking down the barriers that hinder the development of AIHPs based optoelectronic devices.

The origin of gate bias stress instability and hysteresis in monolayer WS2 transistors

Due to the ultra-thin nature and moderate carrier mobility,semiconducting two-dimensional(2D)materials have attracted extensive attention for next-generation electronics.However,the gate bias stress instability and hysteresis are always observed in these 2D materials-based transistors that significantly degrade their reliability for practical applications.Herein,the origin of gate bias stress instability and hysteresis for chemical vapor deposited monolayer WS2 transistors are investigated carefully.The transistor performance is found to be strongly affected by the gate bias stress time,sweeping rate and range,and temperature.Based on the systematical study and complementary analysis,charge trapping is determined to be the major contribution for these observed phenomena.Importantly,due to these charge trapping effects,the channel current is observed to decrease with time;hence,a rate equation,considering the charge trapping and time decay effect of current,is proposed and developed to model the phenomena with excellent consistency with experimental data.All these results do not only indicate the validity of the charge trapping model,but also confirm the hysteresis being indeed caused by charge trapping.Evidently,this simple model provides a sufficient explanation for the charge trapping induced gate bias stress instability and hysteresis in monolayer WS2 transistors,which can be also applicable to other kinds of transistors.