Study Of The Heterostructure Of Ultrathin Organic And 2D Materials And Devices

Photoelectric conversion is widely used in modern society.By selecting semiconductor materials with appropriate band gap,optoelectronic devices with different wavelength can be constructed.In recent years,two-dimensional materials have been widely used in optoelectronic devices due to their rich variety and excellent electrical and optical properties.However,due to the ultra-thin properties of two-dimensional materials,their absolute light absorption is limited,which affects their applications in the field of optoelectronics.By constructing two-dimensional organic-inorganic heterojunction,the problem of insufficient light absorption of two-dimensional inorganic materials can be effectively solved,and the complementary properties between materials can be realized,which provides the basis for building high-performance optoelectronic devices.In this paper,the photoelectric detection performance of graphene and Mo S₂ was improved by organic/inorganic heterojunction.Graphene is widely used in photodetectors because of its high mobility,wide spectrum light absorption and ultrafast photo generated carrier dynamics.A large number of photosensitive materials have been used to form heterojunction with graphene to improve the light absorption and detection sensitivity of the device.In these devices,photogating effect is widely used.Its working principle is based on the built-in electric field formed between graphene and photosensitive materials:after illumination,excitons in photosensitive materials are separated under the built-in electric field,and photogenerated electrons(holes)are transferred to graphene to form photocurrent.At the same time,photogenerated carriers are captured by defects,and the lifetime of the photogenerated carriers is prolonged,which leads to high gain and high responsivity in graphene based photodetectors.However,it is inevitable to sacrifice the response speed by prolonging the carrier lifetime.As a result,ultra-high speed performance can not be obtained at the same time.In this paper,through the heterojunction pentacene/PTCDA,the efficiency of separating and collecting photo-generating carriers are improved,leading to the high performance organic/graphene photodetectors.High quality single crystal PTCDA was prepared by van der Waals epitaxy on mechanically exfoliated graphene.By controlling the growth temperature,the controllable thickness from 1 nm to 20 nm was achieved.The flatness of PTCDA was found of 280 pm by atomic force microscopy(AFM).On this basis,type II organic heterojunction pentacene/PTCDA was prepared.Pentacene can be controlled by the upper layer number of PTCDA by repeated growth.The built-in electric field of pentacene/PTCDA was studied by scanning Kelvin probe microscope(SKPM)and fluorescence microscope.It is found that there is an effective built-in electric field between pentacene and PTCDA,which can realize the efficient exciton separation.At the same time,pentacene/PTCDA/graphene hybrid photodetectors were prepared.Their responsivity,response speed and spectral response were studied.It is found that the built-in electric field of heterojunction improves the quantum efficiency,the responsivity of the detector is obtained,and the response speed is also improved.Compared with pentacene/graphene and PTCDA/graphene devices,the quantum efficiency of pentacene/PTCDA/graphene heterojunction devices is increased by 800 times.Due to the mechanism of independent defect trapping carriers,the responsivity of organic heterojunction/graphene hybrid photodetectors is improved by one order of magnitude to 10⁵ A/W,and the response speed is increased by two orders of magnitude to 28?s.Similarly,by constructing type II heterojunction C₈-BTBT/PTCDA/graphene heterojunction,the response can be improved without loss of response speed.It is proved that the band and device structure design are universal.However,the band gap of graphene is zero.It is difficult to suppress the dark current.By selecting a two-dimensional semiconductor Mo S₂ with band gap,the dark current can be suppressed.In this paper,I-type heterojunction Me-PTCDI/Mo S₂ device is constructed to realize the simultaneous transfer of electrons and holes from photogenerated electrons to Mo S₂.It can enhance the light absorption and avoid the defect trapping process in the grating effect.On this basis,the device response is improved and the sacrificial response speed is avoided.At the same time,the existence of organic/inorganic heterojunction also greatly improves the optical properties of Mo S₂.The exciton peak of Mo S₂ in heterojunction reaches 6 me V at low temperature.It is very important to improve the performance of two-dimensional photodetectors by using organic/inorganic heterojunction with different band combinations,which is of great significance for the further construction of high-performance two-dimensional optoelectronic devices.High performance photodetectors need high responsivity,fast speed response and low dark current.Through organic/inorganic heterojunction,the above goals can be achieved on the basis of energy band design.This is of great significance for the further use of two-dimensional materials to construct high-performance optoelectronic devices.