Electrical Transport Properties Of Two-dimensional Organic Semiconductors

Since the first organic field effect transistor(OFET)was developed in 1980s,it has attracted strong attention from industry and academia.OFETs are widely used in electronics and display industries due to their unique advantages,such as portability,low cost and flexibility.Charge carrier transport in organic semiconductors occurs two dimensionally within several molecular layers near the dielectric interface.While the mobility of bulk organic semiconductors has been greatly proved in recent years,it is still a chanllenge to directly probe the structure-dependent properties at the two-deminsional(2D)limit.The characteristics of ultrathin organic thin film with a few molecular layers are often dominated by disorders and traps and cannot reflect intrinsic properties of organic semiconductors.Furthermore,semiconductor heterojunctions are fundamental buliding blocks of modern optoelectronic and electronic devices,such as light-emitting diodes,laser diodes.High quality heterojunctions can be obtained using traditional molecular beam epitaxy(MBE)technique,but the high requirement of lattice match at the interface results in the limination of material selection to form heterojunction.With the development of graphene and other 2D layered materials which have weak interlayer van der Waals(vdW)interactions,vdW heterojunctions were developed at the proper time.They can be formed by stacking different materials and bonded by vdW interactions byond the limitations of traditional MBE technique.To date,various high quality lateral and vertical heterojunctions have been prepared using chemical vapor deposition(CVD)and mechanical transfer,but nearly all the vdW heterojunctions are composed of 2D atomic crystals.If the building blocks are expanded to organic materials,it will provide significantly more design space for heterojunctions.In this thesis,we explore the electrical transport properties of organic semiconductor at 2D limit,including the structure-dependent charge carrier transport in pentacene,the properties of lateral and vertical heterojunctions combined by 2D layered materials pentacene and C8-BTBT.We make a systematic and comprehensive study on 2D organic semiconductors.The main contents are as follows:(i)Highly ordered single-crystalline 2D pentacene crystals can be grown on BN substrate via vdW epitaxy,and the thickness can be precisely controlled from monolayer to tetralayer.Through atomic force microscope(AFM)characterization,we demonstrate that the molecular packing near the dielectric interface is very different.The average thickness of the first three layers(WL,1L and 2L)is 0.5nm,1.14nm and 1.58nm,respectively.The subsequent layers have the same thickness as 2L which is consistent with the thin film phase of pentacene crystals.WL adopt the face-on configuration,and 1L molecular packing is more tilted than 2L.The lattice constants for 1L and 2L are a=6.23±0.07A,b=7.77±0.08A and a=6.03±0.05A,b=7.76±0.05A,respectively,obtained by high-resolution AFM(HRAFM).The lattice constant of 1L expands by 0.2A(3.3%)along a axis,which can be well explained by DFT calculations.The tilt of 1L pentacene occurs along b axis and then reorient the molecule shorter axis more parallel to a axis,resulting in a shorter distance and more repulsion between neighbouring molecules,so the lattice will expand along a axis to release the repulsion.(ii)Molecular packing structure-dependent electrical transport properties of pentacene crystals near the interface of BN are studied using the ideal system in(i).WL is not conductive due to the absence of ?-? stacking.At room temperature,1L pentacene crystals exhibit textbook features of high quality OFETs:high field-effect mobility(1?2cm2/Vs),nearly zero thresthod voltage,small subthreshold swing,ohmic contact and little hysteresis.At low temperature,charge carriers adopt 2D hopping transport and the localization length?1nm can be obtained by fitting the transfer curves at different temperature.For 2L,a surprising band-like transport can be observed.At room temperature,the mobility(?3cm2/Vs)is slightly higher than 1L devices,but it will be improved as the temperature is lowered.The mobility can reach up to?5cm2/Vs at low temperature and is up to 50 times of 1L device at similar temperature.DFT calculations were carried out to explain the modulation of charge transport properties near the dielectric interface.In 2L,the molecule orbital overlaps along a and b axis forming a fully extended density of states which is likely the origin for the band-like transport.Furthermore,the molecular packing in 1L is more tilted,which modifies the spatial distribution of the bonding states,so the molecule orbital can only span for five molecules.Charge carriers in 1L can only move?lnm before it is localized at the WL-1L interface,the calculated distance is consistent with the localization length obtained from experiment.Besides the main reason,another reason is the different interlayer coupling.1L pentacene has strong interlayer coupling with WL,holes in 1L can easily hop to WL and then be further localized.However,the coupling between 2L and 1L can be negligible,so 1L pentacene has no effect on 2L charge transport.(iii)High quality lateral and vertical vdW heterojunctions composed of pentacene and Cs-BTBT are prepared via a two-step physical vapor deposition(PVT)process.Both of them show clean and sharp interface and exhibit a strong built-in electric field near the interface,which are similar to their inorganic counterparts.A strong epitaxial relationship between pentacene and C8-BTBT can be observed though they are bonded by weak vdW interaction.The vertical heterojunction(VHJ)devices show pronounced negative differential conductance(NDC),while the lateral heterojunction(LHJ)devices present excellent rectifying effect.We extend the concept of vdW heterojunctions to organic materials,providing possibility for realizing more complicated functionalities.