Ultraclean Transfer And Surface Modification Of 2D Materials For High-Performance Optoelectronic Devices

Atomically thin two-dimensional(2D)layered materials,such as graphene,have provided a broad platform for the development of electronic and optoelectronic applications due to their unique 2D structure,and electrical and optical properties.Chemical vapor deposition(CVD)on metal substrates such as Cu,Ni,Pt and Au has been one of the most promising methods to grow large-area and high-quality 2D layered materials.However,2D materials need to be transferred to another substrate for a particular application,such as SiO2/Si,glass and poly(ethylene terephthalate)(PET).It has been reported that the commonly used macromolecular support layer,such as PMMA,often leaves large particulate residues on the surface of the 2D material,leading to non-ohmic contact or high leakage currents in device applications.While the small molecule support layer has strict requirements for the solution environment,support strength and gentle operation,which is not suitable for large-area transfer and the transfer of 2D materials growing on the noble metal substrates.In addition,mismatch of work function,poor interface compatibility with neighbor layer and difficulty in patterning also limit the application of 2D materials in the optoelectronic devices.For example,as the transparent cathode of optoelectronic devices,graphene has high work function and electron inj ection barriers to electronic inj ection layer(EIL).Moreover,EIL is difficult to be evenly deposited on the surface of graphene with hydrophobic nature,resulting in poor interface between EIL and graphene cathode and then poorer device performance.In addition,the traditional patterning method gernerally leave large particle residues on the graphene.The above problems greatly reduce the device efficiency,and increase the difficulty in device processing.In this dissertation,we developed the new transfer and modification strategy to achieve ultraclean transfer of the 2D materials and to improve its work function match and interface compatibility with neighbor layer.Furthermore,we explored the applications of these 2D materials in field effect transistor(FET),organic photovoltaic(OPV)cell,organic light-emitting diode(OLED)and active matrix OLED(AMOLED).The main results are as follows:We proposed a PMMA/rosin double support layer for ultraclean and damage-free transfer of CVD-grown WSe2,WS2 and large-area graphene films on different metal substrates to the target substrates(SiO2/Si and PET).The bottom rosin layer ensures clean transfer,while the top PMMA layer improves the strength and screens the rosin from the transfer conditions.The transferred WSe2,WS2 and graphene films have high smoothness and optical and electrical properties.Their average surface roughness is only 0.26 nm,1.98 nm and 1.5 nm,respectively.As a result of the clean surface,the transferred WSe2 does not require annealing,enabling high-performance FETs with a carrier mobility and ON/OFF ratio up to 79 cm2 V-1 s-1 and 6.8 × 106,which is～10 times higher than those made by PMMA-transferred WSe2.The average surface resistance of the large area(15×15 cm2)graphene film transferred on the flexible substrate was less than 400 ohm sq-1,and the light transmittance was as high as～97.1%.The photoelectric conversion efficiency of OPV cell using PMMA/rosin-transferred monolayer graphene as anode is up to 6.4%,which is 1.7 times higher than that of the devices made by PMMA-transferrd graphene.This universal method for the clean transfer of 2D materials provides the possibilities for their application in high-performance electronic and optoelectronic devices.We developed polyethylenimine(PEIE)to modify the graphene.The adsorption of PEIE can reduce the vacuum level of graphene,and then reduce the work function of the graphene electrode.And PEIE has better hydrophilicity,which can improve the surface wettability of graphene.Therefore,the coating of PEIE causes the work function of graphene to reduce from 4.3 eV to 3.8 eV and water contact angle to decrease from 88° to 33°.The good wettability and low work function enable the uniform deposition of the electron inj ection layer,and then improved electron inj ection efficiency.Furthermore,we designed and fabricated a flexible inverted OLED device by using PEIE modified graphene as transparent cathode.The maximum current efficiency,power efficiency,and external quantum efficiency can achieve up to 66 cd A-1,57 lm W-1,and 17%,respectively,which is the best result of the reported graphene inverted OLED.In addition,the device has excellent flexibility,and the luminance of the OLED could remain 85%of the original brightness after repeatedly bending 10,000 times.Moreover,we developed a laser ablation method to directly pattern graphene film.And a rectangular graphene matrix pattern with well arranged and sharp edges were obtained.Furthermore,the one-to-one transfer of the graphene electrodes matrix to the carbon nanotube thin film transistor(CNT-TFT)array is realized.The electron inj ection capability of the graphene matrix is improved,and the passivation between the patterned graphene pixel electrodes is achieved by using PEIE modification,which effectively avoild the leakage current of the electron inj ection layer.Driven by a flexible CNT-TFT array,a 16×16 pixel flexible AMOLED device based on graphene electrodes was fabricated,promoting the application of graphene in flexible display devices.