Study On The Photoresponse Enhancement Of Organic Phototransistors By Minority Carrier Trap States And Application In Weak Light Detection

The presence of minority carrier trap states in organic semiconductors(OSCs)is one of the important reasons for the high photoresponse of organic phototransistors(OPTs).However,as the source of minority carrier trap states in OSCs is not determined,the enhancement mechanism of photoresponse is not clear,which poses a challenge for rational design and optimization of devices.To this end,this paper focuses on the origin of the minority carrier trap state in OSC and the mechanism of the trap state enhancing the phototransistor light response,and as a guide to design a new photoelectric transistor device structure,to achieve its application in weak light detection,specific research is as follows.1.Study on the origin of minority carrier trap state in Dif-TESADT and the mechanism of trap state enhancing the photoresponse of organic phototransistorIn this work,we elucidate the main origins of the photoresponse within 2,8-difluoro5,11-bis(triethylsilylethynyl)anthradithiophene(Dif-TES-ADT).Due to the existence of minority carrier trap in Dif-TES-ADT,extra gate voltage is formed under illumination condition,leading to enhanced photoresponse of OPTs with light-induced instability behavior.After annealing,density of minority carrier trap states(Ntrap)decreases from the initial 6.3 × 1017 cm-3 to 2.5 × 1017 cm-3,photocurrent decreases from 10-7 A to 10-11 A by about 4 orders of magnitude,and the photon accumulation rate decreases by about 99.7%.This suggests that the residual solvent is identified as a key factor in the formation of minority carrier traps within the OSCs thin film.Further,density functional theory(DFT)calculations confirm that water and oxygen molecules in the residual solvent are the main origin of the formation of the minority carrier traps(-2.66 eV).In particular,the electron density distribution on the Dif-TES-ADT LUMO level is mostly concentrated on the oxygen molecules and OH radicals(-2.74 eV)and thus generate the deeper traps.By solvent purification,the OSCs Ntrap decreased by 34%,OPTs photocurrent decreased to 10-9 cm-3.Through a series of controlled experiments and combined with photoluminescence spectroscopy,we first identify those hydrated impurities(water and oxygen)in the residual organic solvent buried in the OSCs primarily contribute to the formation of minority carrier traps within the OSCs bandgap,giving rise to photo-induced electrical instability of OPTs.In addition,a molecular additive strategy is developed to improve the OPTs photostability of the OSCs film by releasing trapped electrons from the levels of minority carrier traps(-4.19 eV).Our work not only elucidates the significant role of water and oxygen in the residual organic solvents in forming minority carrier traps but also provides guidelines for improving OPTs stability for practical applications.2.C8-BTBT Organic phototransistor with PbS CQDs and polymer co-blended optical floating gate and its weak light detection performanceIn this work,we propose an photofloating gate organic phototransistor(OPTs)based on 2,7-dioctyl[1]benzothiophene[3,2-b][1]Benzothiophene(C8-BTBT)crystalline film and a photosensitive layer blended with lead sulfide colloidal quantum dots(PbS CQDs)and polyvinyl cinnamate(PVCn),which significantly improves the weak light detection capability of C8-BTBT OPTs.By blending of PVCn and PbS CQDs,a large number of minority carrier trap states were introduced in the gate insulating layer.The photo-induced electrons are trapped by trap states in the gate dielectric layer to generate a strong gate voltage and improve the photocurrent of OPTs.The device design strategy,which avoids the introduction of carrier trap states within the carrier transport channel,improves the OPTs photoresponse while ensuring the operating stability.We characterize the surface potential of the co-blended photosensitive layer before and after illumination by Kelvin scanning probe(SKPM),and shows that a gate voltage of about-0.35 V is generated during illumination,which experimentally verifies the photorespons enhancement mechanism of the OPTs.Compared with the OPTs with pure PbS film,the blending photosensitive layer has higher defect capturing cross section,which improves the detection limit from 36 nW cm-2 to 1 n W cm-2 while the photosensitivity increases by 100 times and the imaging dynamic range increases from 69.63 dB to 109.47 dB.Further,we improve the operating stability of the device by optimizing the passivation layer on the gate dielectric and successfully achieve the detection of dim light of 0.57 nW cm-2.This work provides a feasible idea for improving the weak light detection ability of OPTs and a new possibility for the development and research of bionic vision.