Study On The Application Of Thieno[3,4-b] Pyrazine-containing D-A Conjugated Polymers In Optoelectronic Device

As a class of electron-deficient nitrogen-containing heterocyclic compounds with largeπ-conjugated rigid planar structures,thieno[3,4-b]pyrazine（TP）unit is often used to synthesize donor-acceptor（D-A）conjugated polymers.The TP-based polymers have been widely used in organic photodetectors（OPDs）,organic light-emitting diodes（OLEDs）,organic solar cells（OSCs）and other optoelectronic devices.In this paper,six D-A conjugated polymers have been synthesized based on TP acceptor units Use different donor units to adjust their energy levels and photophysical properties.Polymer photodetectors and dual-function devices have been successfully fabricated and their preliminary applications have been explored.1.Three ambipolar D-A polymers,namely TP-FL,TP-CZ and TP-Si,are developed based on the same acceptor unit TP combining with varied benzene-based arenes donor units such as fluorene（FL）,carbazole（CZ）and dibenzo[b,d]silole（Si）derivatives.The photophysical,electrochemical and carrier transport properties of polymers were analyzed by a series of tests.The results show that these polymers exhibit UV-Visible dual-band absorbance,intramolecular charge transfer（ICT）effect and ambipolar transport characteristics.They are evaluated as the photosensitive layers in single-active-layer OPDs.The test result indicates that OPDs all present premium UV-visible dual-band photoresponse.The optimal TP-Si device presents a high external quantum efficiency（EQE）over 40%and a high on/off ratio of 2.17×10~5 with fast temporal response under 550 nm at-1.5 V,which are superior to the most of reported single-active-layer OPDs.The high performance could be ascribed to the well-balanced photogenerated carriers and the efficient charge separation and transport of TP-Si active-layer.In addition,the TP-Si OPD can be used as a color temperature（CT）sensor for white light illumination and applied in encrypted optical communication.2.The rubrene/TP-Si and rubrene/TP-Si:PC₇₁BM heterojunction films were fabricated and characterized.The OLED-OPD bi-functional devices were fabricated based on the heterojunction films.By testing the photoelectric performances,two devices respectively present the stable orange-red and orange light emissions when they are working in luminous mode.The rubrene/TP-Si device shows the promising electroluminescent（EL）performances.On the other hand,the rubrene/TP-Si:PC₇₁BM device exhibits the better performances when they are working in photodetection mode.Through analysis,although PC₇₁BM produced a quenching effect on the radiative excitons generated in TP-Si,the efficient charge separation at interface of the rubrene/TP-Si:PC₇₁BM and the energy level difference between donor and acceptor will facilitate the dissociation and transport of photogenerated carriers.In addition,we demonstrated the application of rubrene/TP-Si:PC₇₁BM bifunctional device in the bidirectional optical communication.3.Three D-A polymers,namely TP-BDT,TP-CPP,TP-DTP,are developed based on the same acceptor unit TP combining with varied thienyl-based arenes donor units,such as the dibromide derivatives of dibenzothiophene（BDT）,cyclopentanedithiophene（CPP）and dithieno[b,d]pyrroledibenzo（DTP）.The experimental results show that three polymers have strong ICT properties with a broad absorption spectrum covering the near infrared region.Furthermore,three donor polymers were doped with PC₇₁BM as the active layers to fabricate heterojunction OPDs with a device configuration of ITO/PEDOT:PSS/polymer:PC₇₁BM/Li F/Al.These OPDs show the stable photoresponse in the wide spectral range of 300～1000 nm.The optimal TP-CPP:PC₇₁BM device exhibits the better performances with a low dark current density of 1.29×10^-8 A/cm~2,a detectivity of 1.67×10¹³ Jones,and a linear dynamic range of 102 d B.It is found that the TP-CPP:PC₇₁BM device possesses lower trap-assisted recombination rate and a wider depletion region.It will suppress the dark current of the device and facilitate the dissociation of photogenerated excitons with increased carriers’concentration,which effectively improves the detectivity and responsivity of the device.