Photolithography-assisted Precise Patterning Of Organic Single-crystal Microwire Arrays For Device Applications

In recent years, small-molecule organic semiconductor micro/nanocrystals have attracted much attention. Compared to their inorganic counterpart, they have unique properties such as facile fabrication process, excellent electronic/optical properties and suitable for flexible devices, which are promising for new-generation low cost, high-performance and wearable devices. In device applications like integrated devices or logic circuits, organic micro/nanocrystals are required to be grown in the desired locations, which can reduce material waste, save costs, and reduce crosstalk among devices. However, organic micro/nanocrystals fabricated by conventional methods usually dispersed in solution or roughly grew along the direction on the substrate, which were not precisely grown and aligned in the desired locations. Several methods have been developed to pattern organic micro/nanocrystals in the desired positions, but they always suffered from the problems like complicated process, low space resolution or low crystal quality. Therefore, it is much desired to precisely align and pattern high-quality organic micro/nanocrystals by facile and high-efficiency methods.In this thesis, we developed the photolithography-assisted method to fabricate precise alignment and patterning of organic micro/nanocrystal arrays and explored their application in devices. In addition, we developed an efficient metal-assisted photolithography method to achieve high-integrated organic field-effect transistors(OFETs) on photoresist-unwettable insulators.1. Photolithography-assisted dip-coating method to precisely align and pattern TIPS-PEN single-crystal microwire arrays for organic field-effect transistorsA facile photolithography-assisted dip-coating method was utilized to realize large-area precise patterning of 6,13-bis(triisopropyl-silylethynyl)pentacene(TIPS-PEN) microwires(MWs). Firstly, periodic photoresist pattern was fabricated on SiO2/Si substrate, then the substrate was dipped into TIPS-PEN/CH2Cl2 solution and pulled up in a certain speed. After dip-coating process, TIPS-PEN MWs were grown along the edge of photoresist strips. Morphology and crystal quality were related to pull speed, and smooth and uniform single-crystal TIPS-PEN MW arrays were obtained at optimized pull speed. Furthermore, different periodicity and complicated pattern of TIPS-PEN MW arrays were achieved through designing different photoresist patterns. This method was also applicable to other small-molecule organic materials. OFETs based on single-crystal TIPS-PEN MW arrays showed high performance with low threshold voltage and high mobility. The highest mobility was 3.5 cm2 V-1 s-1. Moreover, OFETs displayed excellent operating cycle stability. Photolithography-assisted dip-coating method could realize large-area uniform precise alignment and patterning of organic single-crystal arrays, verifying the great potential for integrated device applications in the future.2. Photolithography-assisted method to precisely align and pattern organic single-crystal p-n junctions for device applicationsOrganic p-n junction micro/nanocrystals have been widely used in ambipolar OFETs. However, conventional methods could only obtain individual p-n junction or roughly control p-n junction arrays’ directional growth, and could not realize p-n junction arrays to grow and align in the desired locations, which was not suitable for large-scale device applications. Dif-TES-ADT(p-type) and BPE-PTCDI(n-type) were employed to fabricate precise alignment and patterning of organic p-n junction MW arrays through photolithography-assisted method. In this method, mixed solution of pand n-type material was dropped on the substrate and the solvent was slowly evaporated under saturated solvent vapor. Then dif-TES-ADT MWs were grown along the edge of photoresist strips, and BPE-PTCDI MWs were grown laterally stacked to dif-TES-ADT MWs, forming precise alignment and patterning of p-n junction MW arrays. P-n junction MWs had single-crystal nature, and ambipolar OFETs based on them showed high performance with highest electron mobility of 0.43 cm2 V-1 s-1 and highest hole mobility of 0.32 cm2 V-1 s-1. Moreover, p-n junction MW arrays were fabricated on polyethylene naphthalate(PEN) substrate, and flexible ambipolar OFETs were successfully obtained. In addition, high-performance inverters were fabricated based on p-n junction MW arrays. Photolithography-assisted method could realize large-area uniform precise patterning of p-n junction MW arrays, which was promising for future devices like organic light-emitting transistors(OLETs) and complementary-like organic circuits.3. Fabrication of high-integrated organic field-effect transistors on photoresist-unwettable insulators with remarkable stabilityEnvironmental stability is one of the most important parameters for high-performance OFETs. Hydrophobic insulators are suitable for air-stable OFETs which usually possess much better air stability than some conventional inorganic and hydrophilic organic insulators. However, high-integrated devices can not be directly fabricated by photolithography method on hydrophobic insulators due to their photoresist-unwettable properties. In this work, a facile and efficient metal-assisted photolithography method is developed to achieve large-scale fabrication of high-integrated organic electronic devices on photoresist-unwettable insulators. By utilizing copper(Cu) as sacrificial layer, photolithography can be performed on these insulators with nearly the same resolution and uniformity as conventional photolithography. This method shows excellent flexiblity and is capable of fabricating high-integrated devices on a variety of hydrophobic insulators including hydrophobic amorphous fluoropolymer(CYTOP), poly(dimethylsiloxane)(PDMS), and octadecyltrichlorosilane(OTS)-modified SiO2. OFETs based on 6,13-dichloropentacene(DCP) microwires(MWs) with CYTOP as the insulator layer were fabricated, which exhibited excellent device performance. Much improved device stability with very low mobility degradation was observed after 24 days. While devices fabricated on hydrophilic insulators of poly-(vinyl phenol)(PVP) and bare SiO2 experienced dramatic performances decay within 24 days. In addition, by using metal-assisted photolithography method, flexible OFETs arrays could be fabricated on polyethylene naphthalate(PEN) substrate, which showed excellent mechanical bending flexibility and stability. This work unveils the great potential of metal-assisted photolithography method for air-stable high-integration organic electronic devices.