Investigation Of Organic Phototransistors Based On Vertical Field-effect Transistor

With the tremendous development of conjugated polymers,organic phototransistors(OPTs),combining organic field-effect transistors(OFETs)with photodetectors,have gained considerable interest in organic electronics.Due to its higher optical gain and signal/noise ratio compared to photodiodes,OPTs show promising potential applications in image sensors,optical communications,space exploration and biomedical sensors.However,the present OPTs are based on conventional OFETs with long channel length(tens of micrometer)and it has been proved that the long channel length rises the bimolecular recombination and scattering induced by structural defects and grain boundaries during carrier transfer process,resulting in a lower collection efficiency,which dramatically decreases the photoresponse performance.Therefore,it is crucial to design a new device architecture that enables downscaling of channel length to achieve high photoelectric properties OPTs.In this work,high performance flexible organic phototransistors based on vertical field-effect transistor are proposed for the first time,then the impact of the morphology of active layers on the photoelectric properties in OPTs are investigated systematically by the addition of second solvent to optimize the device photoelectric performance,and finally we further investigate the potential application of the vertical organic phototransistors in the next-generation flexible electronics.The main research contents are as follows:1.Using PDVT-8:PCBM polymer bulk heterojunction(BHJ)as channel layers and active layers materials,the organic phototransistors based on vertical field-effect transistor are invented for the first time,which enables reducing the channel length to nanometer level(150 nm).The results reveal that the vertical structure design has significant enhanced the OPTs performance,including photosensitivity,photoresponsivity,detectivity and photoresponse speed.Under 0.1 m W/cm² light illumination,the OPTs exhibit a high photoresponsivity of 328 A/W,high photosensitivity(I_light/I_dark)of 1×10⁵ and detectivity of 9.1×10¹⁴ Jones,which is much better than those traditional OPTs with the same PDVT-8:PCBM active layers.2.The impact of the morphology of active layers on the photoelectric properties in OPTs is investigated systematically,in which the active layers morphology is tuned by the mixture solvent.The results reveal that the crystalline area and crystal size of PDVT-8 are increased with the addition of high boiling point solvent chlorobenzene(CB),while the maximum surface area of crystals are obtained at 30%CB addition,leading optimal photoelectric properties.The results also reveal that the performance of OPTs is dominant by the surface area of crystals,which facilitate the exciton dissociation and carriers transfer.The optimized OPTs show a high photoresponsivity of 750 A/W,ultrahigh photosensitivity(I_light/I_dark)of 1×10⁶,detectivity of more than4.5×10¹⁵ Jones and real-time photoresponse of 0.72 s,which is even much better than those lateral organic phototransistors ever reported.3.To meet the growing development for the next-generation organic electronics with light weights,excellent flexibility and portability,the vertical structure OPTs was fabricated on the flexible PI substrate.The performances of the device under various bending radius and 1000 bending times are characterized.And the results show that the vertical structure can effectively reduce the influence of internal cracks and dislocations in the channel induced by mechanical bending on carriers'transport,leading excellent mechanical stability.