Lateral Near-infrared Organic Photodetector Based On Electron/hole Dual Trap System

Near-infrared(NIR)photodetectors have great potential for industrial and scientific applications,such as quality inspection,bioimaging,health monitoring,and medical applications.Organic NIR photodetectors(OPDs),with advantages of low-cost,lightweight,flexibility,wearability,tunability of wavelength range and working at room temperature,have a big development potential.However,in the NIR range,a narrow bandgap with a low energy induces overlap of the adjacent energy levels,resulting in low photoelectric conversion efficiency(small photocurrent)and high intrinsic carrier concentration(large dark noise),limiting the performance of NIR OPDs such as detectivity.In particular,the parameters of photodetectors are interrelated in general.For photomultiplier photodetectors,the external quantum efficiency is much greater than 1,and the response speed is often very slow.To solve the above scientific problems,this paper took the bulk heterojunction as the active layer,combined with nanoparticles to form a unique electron/hole double trapping structure,in the lateral photodetector and phototransistor,explored the method of gain enhancement,dark current reduction and response acceleration,and developed the high comprehensive performance photomultiplier near-infrared organic photodetector.The mechanism study showed that the dual electron/hole storage effectively enhance the internal electric field,regulate the multi-carrier capture,and provide the carrier compound channel,which was the key to achieve the unity of optoelectric current,dark current and response time.The main research contents of this paper are as follows:1.Simultaneously improve the gain and response time of near-infrared lateral organic photodetectors(L-OPDs)by the charge trapping effect of silver nanoparticles(Ag-NPs)and the bulk heterojunction(BHJ)structure.Firstly,we explored the influence of PC₆₁BM on the performance of responsivity(R)and detectivity(D*)in a new near-infrared organic photodetector.It was found that the dark current of L-OPDs increased as the PC₆₁BM content increased,while the photocurrent first increased and then decreased with the increase of PC₆₁BM content.Therefore,with the increase of PC₆₁BM content,D*increases because of the decrease of the dark current,while R increases first and then decreases as the PC₆₁BM content increased.Secondly,we explored the effect of Ag-NPs on the detection performance of L-OPDs and verified that the hole trapping effect of Ag-NPs could effectively reduce the dark current,improve D*and the response rate.Finally,in L-OPDs containing Ag-NPs,the gain(>1)and response speed of the lateral photodetectors are promoted simultaneously and effectively by increasing the trap ratio.The gain increases from 12.7 to 19.8 and the fall time decreases from 313.4 ms to 172.9 ms as the PC₆₁BM ratio(D/A)increase from 5:1 to 1:1.The lateral photodetector structure with long electrode distance has been testified to play the key role for simultaneous promotion compared with vertical photodiodes,allowing the charges well trapped in PC₆₁BM-rich phase at a high PC₆₁BM ratio and accumulation of multiple built-in electric fields.The long channel distance and silver nanoparticles also effectively restrain the increment of dark current with PC₆₁BM loading,resulting in a high detectivity of1.1×10¹² Jones under 0.031 m W cm^-2@820 nm.It is of great theoretical and practical value for the high-performance photodetectors with simultaneous high photomultiplication and quick response.2.Ultrahigh performance near-infrared organic phototransistors(OPTs)were obtained by means of a spatially-separated,electron/hole,dual traps formed by PC₆₁BM and Ag-NPs.In phototransistors,the photovoltaic-induced current is proportional to the turn-on voltage shift and the total number of trapped charges.However,it is challenging to obtain a high turn-on voltage shift simply by using minority carrier trap sites because high-concentration carrier trap sites introduce strong current traps and carrier recombination.To obtain a high turn-on voltage shift,spatially-separated,hole/electron,dual traps were introduced into a phototransistor.The current quenching caused by the charge trap sites was effectively compensated by the increase in the drain source current caused by the threshold voltage shift,and obtained a large photocurrent.When the thickness of Ag-NPs is 1 nm and PDPPBTT:PC₆₁BM is 5:1,a high I_light/I_dark(1×10⁶)alongside a turn-on voltage shift of 28 V were obtained,resulting in a high photoresponsivity(5.26×10³ A W^-1)and an ultra-high detectivity(D*_shot=8.21×10¹⁶ Jones,D*_1/f=1.88×10¹⁵ Jones,V_g=0 V,0.031 m W cm^-2@820 nm).In addition,in order to improve the response rate of OPTs,the effects of classic solvent additive 1,8-diiodooctane(DIO)on the morphology and detection performance of OPTs active layer were studied.It was found that DIO causes BHJ in OPTs to form an interpenetrating discontinuous phase,effectively improving the response rate(t_r=27.1 ms and t_f=113.1 ms for control device,t_r=15.7 ms and t_f=69.6 ms for DIO device).3.The electron/hole dual traps system was extended to the double electron traps system,and the more moderate charge capture Zn O nanoparticles(Zn O-NPs)was selected to achieve a good balance and synchronous promotion among the multiple parameters,Zn O-NPs has a weak trapping effect compared with Ag-NPs,which can effectively improve the I_light/I_darkwithout damaging the response speed.The effect of Zn O-NPs on the detection performance of near infrared organic phototransistors was investigated.Zn O-NPs,whose charge trapping effect is weaker than that of metal nanoparticles,are added between the silicon wafer and PMMA.By adjusting the thickness of Zn O-NPs and PMMA,the charge trapping effect was different in light and without light.In dark,Zn O-NPs did not capture electrons and has a low dark current,while under light,Zn O-NPs effectively captured electrons in the active layer,which leads to the right-shift of the turn-on voltage,obtaining a high photocurrent and a large I_light/I_dark,thus,high OPTs detection performance was obtained.When the thickness of Zn O-NPs and PMMA was 16 nm and 30 nm,respectively,OPTs had extremely high R,Gain(G),D*_shot and D*_1/f,which were 3.23×10³A·W^-1?4.92×10³,2.78×10¹⁶Jones and 8.98×10¹⁴Jones(V_g=2 V,0.031 m W cm^-2),respectively.Due to the relatively weak electron capture effect,ZnO-NPs had no effect on the response rate of NIR OPTs under above conditions.