Synthesis And Application Of Thiothiadiazole-based Narrow Band Gap Conjugated Polymers For Photodetectors

Organic photodetectors（OPDs）are favored by researchers because of their abundant material sources,light weight,flexible manufacturing and other advantages.In recent years,how to improve the sensitivity of OPDs and broaden the spectrum response range has become a research hot spot.With the development of research,it has been found that modifying the molecular structure of conjugated polymers is one of the effective ways to regulate the energy level,band gap,charge transfer and other properties of polymers,so as to obtain a wide spectral response and high sensitivity of OPDs.In addition,the performance of OPDs can also be improved by modifying the interface layer to obtain low dark current.In this thesis,a series of narrow band gap donor polymers were synthesized,and mixed with fullerene acceptor and non-fullerene acceptor respectively to prepare bulk heterojunction organic photodetectors,and the properties of the detectors were characterized.（1）In chapter three,using benzodithiophene（BDT）as donor unit and benzodithiophene as acceptor unit,D-A conjugated polymer was constructed,and different number of F atoms were introduced into the side chain of BDT unit,and finally three narrow band gap conjugated polymers,PT-TZEH,PTF-TZEH and PTDF-TZEH,were synthesized.The optical,electrical and aggregation properties of the copolymers were systematically characterized.The ultraviolet-visible absorption spectra showed that the copolymers PT-TZEH,PTF-TZEH and PTDF-TZEH have strong absorption in the range of 350 nm-1070 nm.With the introduction of different F atoms in the BDT side group,the highest occupied molecular orbital（HOMO）energy level of the copolymers gradually decreases from-5.26 e V to-5.40e V.（2）In chapter four,BHJ-OPDs based on narrow band gap copolymers（PT-TZEH,PTF-TZEH,PTDF-TZEH）and fullerene acceptor PC₇₁BM were prepared.With the introduction of one F atom and two F atoms respectively on the side chain of the donor polymers,The Open-circuit voltage(V_OC)of BHJ-OPDs devices increases from 0.549V to0.579V and 0.595V,respectively.The research showed that the introduction of F atom on the polymers side chain is one of the effective ways to increase V_OCof BHJ-OPDs devices.On this basis,the effects of single-cathode interface（Zn O）and double-cathode interface（Zn O+PFN）on the photovoltaic performance of the devices were compared.It is found that the Dark Current（J_d）of the dual-cathode interface device is reduced by one order of magnitude compared with the single-cathode interface device.The bimolecular recombination and trap-assisted recombination of the device are also effectively inhibited.More importantly,the device with dual cathode interface achieve high D^*.When the thickness of the active layer is about 350 nm,the performance of the BHJ-OPDs is studied.The thicker active layer effectively reduces the trap in the film,thus reducing the leakage current of the device,and thus significantly reducing the J_dof the BHJ-OPDs.Under the bias voltage of-0.5V,compared with the device with the thickness of the active layer of 80 nm,The D^*of thick film devices is improved by 1-2 orders of magnitude.It is suggested that increasing the thickness of active layer can be an effective method to improve the sensitivity of BHJ-OPDs.（3）In chapter five,BHJ-OPDs based on narrow band gap copolymers（PT-TZEH,PTF-TZEH,PTDF-TZEH）and non-fullerene acceptor Y6 were prepared.Through a series of tests,we found that with the introduction of F atoms on the polymers side chain can effectively improve the exciton dissociation rate of BHJ-OPDs.At the same time,bimolecular recombination and trap assisted recombination are inhibited.Then,the D^*of the devices under different bias were measured and calculated.At 0 V bias and 800 nm peak value,the D^*values of the devices based on PTF-TZEH:Y6 and PTDF-TZEH:Y6 are up to 1.55×10¹³Jones and 1.52×10¹³Jones,respectively.It showed that high D^*BHJ-OPDs can be achieved by introducing F atom into the side chain of copolymers.In the non-fullerene system,the effect of thicker active layer（about 350 nm）on the devices performance is explored.At a bias voltage of-0.5V,J_ddecreases by about 3 orders of magnitude and D^*increases significantly compared with the device with an active layer thickness of about 80 nm.