Low Cost Universal High‐k Dielectric for Solution Processing and Thermal Evaporation Organic Transistors

Abstract

SAM is needed to reduce the leakage current through the gate and dielectric. Some of the SAMs would require relatively long preparation time [12][13][14] and usually the functional head groups of SAMs is dielectric specifi c. It is challenging to develop a universal SAMs suitable for different dielectric insulators. More importantly, the SAM also modifi es the surface energy of the dielectric and affects the crystallinity of the organic semiconductors. It is necessary to employ a suitable SAM to achieve high crystallility and carrier mobility in the OFETs. [ 14 ] To date, it is still a great challenge in the research fi eld to develop a universal high-k material which does not require SAM, and can be processed at moderate temperature, ambient air environment, and adjustable surface energy for both thermal evaporation and solution processing of organic semiconductors.To address these challenges, we recently reported a high-k (ε r = 11) barium strontium titanate (BST) thin fi lm treated by UV-Ozone and deposited by multi-layer spin coating deposition. [ 15 ] The high-k fi lms are spin coated in ambient air and heated to a temperature of around 85 °C during the UV-Ozone process. The UV emits at 184.9 nm and 253.7 nm under ambient air conditions converting oxygen into reactive O species, which then diffuse into the amorphous BST thin fi lm and reacts with the oxygen vacancies to form lattice oxygen. The reactive oxygen also removes the organic impurities in the fi lm, which results in a high quality amorphous BST thin fi lm with better stoichiometry. [ 16 ] The compatibility with ambient air environment, low processing temperature, and no SAM requirement suggest the BST dielectric has extremely high potentials to be applied in developing low cost OFETs on plastic fl exible substrates and have advantages over other polymers and inorganic materials. In the current study, we investigate the surface energy modifi cation effect by varying the storage environment and time of the BST thin fi lm in order to achieve a universal high-k material for OFET fabrications without using SAM. Based on the contact angle measurements, we calculated the surface energy and the wetting envelope of the BST high-k dielectric stored under different fi ve different conditions. The fresh BST with hydrophilic surface is suitable for spin coating or drop casting of organic semiconductor deposition where surface wetting is needed for the alignment of organic molecules. On the other hand, the BST thin fi lms stored under different conditions show different surface energy, which will directly affect the crystallinity of the organic semiconductors. On the current BST high-k dielectric without SAM and further optimization, dinaphtho[2,3-b:2′,3′-f ] thieno[3,2-b] thiophene (DNTT) and 6,13-Bis (triisopropylsilylethynyl) pentacene (TIPS-pentacene) organic transistors with average mobility of 1.51 cm 2 V −1 s −1 and 0.11 cm 2 V −1 s −1 are formed by thermal evaporation and droplet-pinned crystallization (DPC) method (one of the solution processing methods...

Modifying the thermal conductivity of small molecule organic semiconductor thin films with metal nanoparticles

“…2(c) , we can see that there are three diffraction peaks, (001), (002) and (003). The (001) peak at around 2θ = 5.57° shows the highest intensity, thus suggesting that the d-spacing (between lattice planes parallel to the substrate) in the z-axis direction is 1.59 nm, which is similar to previously reported values 28 30 . The values of the 2θ diffraction peaks are the same for DNTT films deposited at different temperatures, which indicates that the d-spacings in the z-axis direction are identical among these films.…”

Organic Field‐Effect Transistor for Energy‐Related Applications: Low‐Power‐Consumption Devices, Near‐Infrared Phototransistors, and Organic Thermoelectric Devices

“…[67] The dielectric was fabricated by exposing the thermally evaporated Al to oxygen plasma to form an ≈2 nm thick oxide layer on top, followed by the formation of [96] www.advancedsciencenews.com a phosphonic acid monolayer through self-assembly. [72][73][74][75][76][77][78][79][80][81][82][83] More importantly, the solution processing of sol-gel dielectrics, such as spin coating, bar coating, spray coating, and inkjet printing, always generates an extremely smooth dielectric surface with a root-mean-square (RMS) roughness of ≈0.3 nm. As a result, both the thermal evaporated organic p-type and n-type FETs turned on at a −3 V gate bias.…”

“…Later, the role of SAMs was intensively studied by varying the alky chain length, the tail group, and the effect of deposition methods. [76,79] The composition of 184.9 and 253.7 nm UV light converted oxygen into a reactive O spices and then diffused into a BST thin film and reacted with its oxygen vacancies to form lattice oxygen at the same time to remove organic impurities. [28] Other than SAMs, chemical vapor-deposited parylene-C can also be used to enhance the electrical strength of oxidized ultrathin AlO x , and an off-to-on switching within a 1.5 V gate bias is achieved at an optimized parylene-C thickness.…”

“…To realize all solution‐processed organic devices, metal oxide dielectrics made with sol–gel methods have drawn great research attention due to their high‐ k values, optical transparency, and chemical stability . More importantly, the solution processing of sol–gel dielectrics, such as spin coating, bar coating, spray coating, and inkjet printing, always generates an extremely smooth dielectric surface with a root‐mean‐square (RMS) roughness of ≈0.3 nm .…”

Single‐Crystal‐Like Organic Thin‐Film Transistors Fabricated from Dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (DNTT) Precursor–Polystyrene Blends

“…We assume that the density of the materials (1.473 g cm −3 for DNTT and 0.96–1.04 g cm −3 for PS) is not directly related to the position of DNTT layer observed and instead the surface energy of the substrate can influence the position of the DNTT layer. The surface energies of the OTS‐ and UV/O 3 ‐treated substrates determined by the Owens–Wendt–Rabel–Kaelble method have been reported to be 23.5 and 72 mN m −1 , respectively, and the surface energy of DNTT (43.87 mN m −1 ) falls between the two values . This suggests that the lower surface energy of the OTS‐treated substrate than that of DNTT could induce favorable nucleation of the DNTT crystals on the substrate, inducing DNTT layer formation under the PS layer.…”