| Due to its application in the weak light detection,flame detection,environmental monitoring,optical communication,remote control device and image sensor,etc.,the organic photodetector receives more and more attention from the scientific research institutions and the industry.At present,the organic photodetector device structure is dominated by vertical-photovoltaic type(electrode distance ? 100 nm).Although its response speed is quick,there exists the problem of large dark current which lowers the sensitivity of the photodetector.High-performance organic photodetector entails high gain(or EQE),excellent response rate and high photo-current(photochemical factor).At the same time,fast response speed and low dark current(electricity factors)are important indexes of measuring device sensitivity.In recent years,with its unique and brilliant properties of quantum size effect,surface effect,macroscopic quantum tunnel effect and volume effect,etc.,nanoparticles present excellent qualities such as electrical effect,surface enhanced fluorescence,energy transfer,scattering effect,etc.in the photoelectric device.Therefore,it is a simple and effective method of improving the performance of photoelectric device,attracting an increasingly number of attention from scientists.For the problem of large dark current in organic photodetector,we designed experiment according to the silver nanoparticles(Ag-NPs)electrical effect and optical effect in order to achieve the goal of enhancing organic photodetector performance.As for electrical effect of Ag-NPs,first of all,we used the single carrier transmission characteristics of field-effect transistor,making a systematic and quantitative analysis of the electrical effect of Ag-NPs,confirming the charge trapping mechanism of Ag-NPs.At the same time,we introduced weak light to the Ag-NPs electrical effect,successfully utilizing light to activate electric effect storage of Ag-NPs floating gate device,and profoundly studying charge storage properties shown by the Ag-NPs electrical effect.Based on the mechanism of Ag-NPs electrical effect,a new lateral-photovoltaic type organic photodetector structure was designed,providing a feasible approach to reduce the dark current in the organic photodetector by Ag-NPs capturing charges and by the released holes reuniting with residual photo-generated electrons in theactive layer.Meanwhile,we used localized surface plasmon resonance(LSPR)of Ag-NPs to enhance light absorption,and the photo-generated electric field induced by accumulation of photo-generated electrons was conducive to the separation of carriers,thus realizing the increase in organic photodetector responsivity.To enhance the photo-current density of organic photodetector,we also designed phototransistors,where carriers were more concentrated in the course of transmission,to further improve the performance of photodetector.The work of this paper was mainly divided into the following three parts:The first part of the work aimed to solve problems existing in the study of the electrical effects of metal nanoparticles from this two aspects: the quantitative analysis of electrical effects and the electrical properties of charge storage.Firstly,by making use of the single carrier transmission characteristics of OFETs,we evaporated Ag-NPs and regulated its morphology to effectively control the electrical effect,therefore drawing a systematic conclusion that the nanoparticles size was dependent on charge trapping.Secondly,by introducing the weak light to electrical effect of Ag-NPs,utilizing light to activate electric effect storage of Ag-NPs floating gate device was perfectly achieved.There was no storage behavior under dark condition in these devices.However,after exerting a weak light(0.015mW/cm2),the memory window of 12.5-35 V could be effectively achieved.Results showed that the Ag-NPs surface could store the hole,and the photo-generated electrons accumulated in the active layer or interface of the active layer and insulating layer could provide a photo-generated electric field,playing a positive role in charge trapping process and stable storage of trapped charges in the floating gate.In the second part,based on the mechanism of the electrical effect and the optical effect of Ag-NPs,a new lateral-photovoltaic type organic photodetector structure was designed.On the one hand,in the high-performance PffBT4T-2OD:PC61BM devices,the trapping effect of the Ag-NPs and the unition of the released hole with photo-generated electrons in the active layer could reduce the dark current(79.6%),and could improve the photocurrent(32.2%)under the action of the photo-generated electric field and LSPR.The additional photocurrent ensured a high stability in responsivity(R)and gain(G)of the Ag-NPs device at most light intensities.At the meantime,the lifetime of photo-generated carriers of the Ag-NPs device(?= 6.1 ms)was significantly shorter than that of the control device(? = 70 ms),which greatlyimproved the response speed of photoswitch(tr = 22 ms,tf = 13 ms).On the other hand,with the use of lateral-photovoltaic type organic photodetector structure,the PTB7:PC61BM material system also showed outstanding responsivity,gain and specific detectivity.Moreover,the performance of the photodetector was associated with the intensity of the applied voltage.The stability performance of the photoswitch was impacted by the mobility of materials,photo-generated carrier's lifetime,and PC61 BM and Ag-NPs doping.Through effective adjustment,low mobility PTB7:PC61BM system could also present relatively high photoswitch stablility.Adding Ag-NPs,the lifetime of photo-generated carriers of the PTB7:PC61BM device decreased from 4.958 s to 0.368 s.In the third part,considering the problem of low concentration of carriers in organic light diode detector,a phototransistor based on FBT-Th4(1,4):PC61BM system of high mobility was designed,where carriers were more concentrated in the course of transmission,further improving the responsivity and specific detectivity of organic photodetector.The organic phototransistor of FBT-Th4(1,4):PC61BM materials exhibited excellent optical detection performance under weak light(0.0031 mW/cm2): when Vg was 0 V,responsivity of FBT-Th4(1,4)and FBT-Th4(1,4):PC61BM(D/A ratio 5:1)device were 5565(A/W)and 7265(A/W),respectively,with a high specific detectivity(2.1 × 1015(Jones)and 7.7 × 1014(Jones)).The value of gate voltage affected the ratio of the gate-controlled photoswitch,using different on-state gate voltage could effectively regulate the gate-controlled photoswitch: when Vg was0 V,the on-off ratio of FBT-Th4(1,4):PC61BM(D/A ratio 5:1)devices in the dark and weak light(0.0031 mW/cm2)were Ion/Ioff = 3 × 103 and Ion/Ioff = 1 × 105,respectively.This illustrated that more excellent performance was obtained under light.With distinguishable on-state and off-state current state of organic phototransistor in dark and light,four logic gate-controlled photoswitches were achieved,which greatly met the application requirement of the photoelectric device on the gate-controlled switch under different conditions. |
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