Solution OFETs

OFETs with wet-deposited films of 48 as the active layer in bottom contact geometry were fabricated on highly doped n-type silicon wafers with the organic semiconductor layer (ca. 50 nm) deposited from chloroform solution [70], OFETs made from 48 exhibited negative amplification, which is typical of... 

OFETs with spin coated film of H2TPP from a chloroform solution as active semiconductor layers, Al as gate electrode, and Cd as source-drain electrodes were fabricated by Checcoli and co-workers in 2003 [100]. This device exhibited zero-bias hole transfer mobility of 0.007 cm2 V-1 s-1 with a threshold of —7.5 V and field-dependent mobility, as high as 0.012 cm2 V-1 s-1. 

Miskiewicz. P. Mas-Torrent. M. Jung J. Kotarba S. Glowacki. I. Gomar-Nadal. E. Amabilino. D. B. Veciana. J. Krause. B. Carbone. D. Rovira. C. Ulanski. J. (2006). Efficient High Area OFETs by Solution Based Processing of a pi-Electron Rich Donor. Chemistry of Materials, vol. 18, no. 20, 4724-9. 

Presently there exists a strong research interest in the understanding, development, and optimisation of organic field effect transistors (OFETs) [1, 2]. Two classes of semiconducting organic materials are considered, namely molecular materials which are processed into thin films by vacuum sublimation [1, 2], and polymers which are deposited onto substrates in the form of solutions, for instance by spin coating [3]. In this chapter we report on OFETs based on thin polycrystalline films of the molecular material pentacene (Pc) as the semiconducting material. 

In this chapter we investigate and discuss the thermal, optical, electrical properties of the oligothiophene derivatives by means of differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and UV-Vis spectroscopy. The thin films of these compounds produced by solution cast and vacuum deposition methods are characterised by AFM measurements in contact and non-contact mode, and by X-ray diffraction. Finally, an ultra-thin OFET is built, and the transistor characteristics are determined. 

The oligothiophene films were prepared by solution cast methods and by vacuum deposition. Depending on the cast procedure, different layer thicknesses could be achieved. Preparation from solution either by drop casting or spincoating resulted in films ranging from 50 nm to 200 nm in thickness. Vacuum sublimation yielded very thin layers in the range of 10 nm to 15 nm, which nevertheless allowed to build-up and operate an OFET structure of molecular thickness as demonstrated in this chapter. 

Using DHPT-SC as the semiconductor in OFETs, a field effect mobility of 0.012 cmWs and a current on/off ratio of >10 can be realised, which is among the highest OFET mobilities fabricated achieved with solution-processed oligothiophenes. 

CPB Materials. Various types of silane crosslinkers were employed to fabricate crosslinked polymer blend (CPB) dielectrics in this study (Fig. 4). The reactivity of each crosslinker was tested via in situ NMR kinetic studies. The CPB dielectrics were fabricated on various substrates using mixture of polymer and crosslinker solution via spin-coating and gravure-printing. Organic semiconductors and source/ drain electrodes were vacuum-deposited to complete the OFET device. Dielectric and OFET properties were measured under vacuum and ambient as described previously. 

Single-crystal OFETs provide a unique tool for the express analysis of transport characteristics of new molecular materials with defect densities much smaller than those in TFTs. Therefore, even though large-scale applications will ultimately rely on thin films, research on single-crystal OFETs can play an important role in the material selection for applied devices. The case of rubrene perfectly illustrates this point. The unprecedented quality of OFETs based on vapor-grown rubrene crystals has stimulated work on the deposition of rubrene thin-films from solution Stingelin-Stutzmann et al. have recently demonstrated solution-processed rubrene TFTs with high mobility (up to 0.7 cmWs at room temperature) [111]. 

To illustrate the current capabilities of OFET technology in this application area. Figure 2.3.2 shows an optical micrograph of an A5 active matrix display demonstrator on a flexible polyethyleneterephtalate (PET) substrate made by Plastic Logic. The display was fabricated by laminating the OFET backplane with an E Ink Imaging Film. The display has a resolution of 100 pixels per inch (ppi) and displays four levels of gray. It contains 480,000 solution-processed OFETs (600 x 800 rows and columns). No substrate encapsulation is needed. The display exhibits excellent... 

OFET Contact Resistance Values Solution-Deposited Polymers and Oligomers... 

FIGURE 3.1.1 Evolution of OFET performance with time for variousp-channel (pentacene, rubrene, other small molecules, and polymers) and -chaimel organic semiconductors, (v) Vacuum deposition (s) solution deposition (sc) single crystal. A range of mobilities for hydrogenated amorphous silicon (a-Si H) is shown for reference. Please refer to Table 3.1.1 for the respective molecule indices. 

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