Thiophene-based molecular and polymeric semiconductors for organic field effect transistors and organic thin film transistors

Organic electronics has been a popular field for the last two decades, due to its potential to commercialize cheap-price and large-area flexible electronics. The devices based on organic compounds heavily rely on organic semiconductors (OSs). Primary challenge for materials chemist is the new OSs construction that has ameliorated attainment in organic thin film transistors (OTFTs) and organic field effect transistors (OFETs). The construction of air-stable (stable in air) n-channel OSs (electron-conducting materials) is particularly needed with capability comparable to that of p-channel materials (hole-conducting materials). In the last 10 years, there have been significant advancements in thiophene-based OSs. Thiophene-mediated molecules have a prominent role in the advancement of OSs. The main significance in thiophene-based molecules is their cheap-price (in comparison to silicon), processability at low temperature, structural flexibility, ability to be applied on flexible substrates, and high charge transport characteristics. In this paper, we review the progress in the performance of thiophene-based OSs that has been reported in the last 18 years, with a major emphasis on the last 10 years. This approach provides a crisp introduction to organic devices and catalogs progress toward the fabrication of thiophene containing p, n and ambipolar channel OSs, and discusses their characteristics. Finally, review discusses current challenges and future research directions for thiophene based OSs. This review would be beneficial for further developments in the technological performance. Moreover, this review will serve to accelerate knowledge and lays the foundation for improved applications. Hopefully, this struggle pushes the reader’s mind to consider new perspectives, think differently and forge new connections.     

Nanometer-scale organic thin film transistors from self-assembled monolayers

A survey of the most interesting results on nanometer-scale organic thin film transistors (nano-OTFT) is presented. Additionally, we discuss our recent results on the properties of end-group functionalized organic self-assembled monolayers and on their use in the fabrication of nanometer-scale field-effect transistors. Nanometer-scale organic transistors (channel length 30 nm) were fabricated, with a self-assembled monolayer as gate insulator. The carrier transport in these transistors, as a function of the channel length, was investigated, and a transition from a dispersive to a ballistic transport at a channel length of 200 nm was observed. On a molecular scale, alkyl monolayers functionalized at their omega-ends by aromatic moieties were prepared. A high anisotropic conductivity in molecular insulator/semiconductor heterostructures of monolayer thickness was observed. These molecular architectures provide a basis for the building blocks of molecular transistors.     

Synthesis and characterization of polystyrene brushes for organic thin film transistors

We synthesized and characterized polystyrene brushes on a silicon wafer using surface-initiated atom transfer radical polymerization. The thickness of the polymer brush was controlled by adjusting the reaction time. We investigated monomer conversion as well as the molecular weight and density of the polymer brushes. When the monomer conversion reached 100%, the number-average molecular weight and film thickness reached 135,000 and 113 nm, respectively. The estimated densities of the synthesized polystyrene brushes were in the range 0.34-0.54 chains/nm2, high enough to be categorized in the "concentrated brush" regime. The synthesized polymer brush was used as an insulating layer in an organic thin-film transistor. Organic thin-film transistors were fabricated using pentacene as an active p-type organic semiconductor and a polystyrene brush on a SiO2 layer as a gate dielectric. The pentacene based organic thin-film transistor with the polystyrene brush exhibited a field-effect mobility microFET of 0.099 cm2/(V x s).     

Emerging inorganic compound thin film photovoltaic materials: Progress,challenges and strategies

The efficient conversion of solar energy to electricity for human utilization heavily relies on the development of solar cells. Nowadays, a variety of high-performance solar cells are constantly emerging. Thin-film solar cells made from inorganic materials have constituted one of the major categories of solar cells showing potential in the fast growing photovoltaic (PV) market. In order to provide an overall grasp of and insight into the future direction of inorganic thin-film solar cell development, we review key emerging and representative inorganic photovoltaic materials including chalcopyrite Cu(In,Ga)Se₂ (CIGSe), kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe), CdTe, Sb₂Se₃ and inorganic perovskite CsPb(I_1−xBr_x)₃ in this paper. Absorber materials, evolution of device development, and current challenges and key strategies for performance enhancement are detailed.     

Anthracene derivatives substituted with phenyl or pentafluorophenyl for organic thin film transistors

Four anthracene derivatives (TIPSAntP, TIPSAntFP, TIPSAntPA and TIPSAntFPA) containing phenyl or pentafluorophenyl have been synthesized by using Suzuki coupling or Sonogashira cross-coupling reactions. DSC measurement showed that TIPSAntP and TIPSAntFP were crystalline and thermally stable to 300 degrees C under nitrogen atmosphere, comparing to TIPSAntPA and TIPSAntFPA with acetylene. From cyclic voltammetry measurement, it can be observed that all compounds were electrochemically stable in the voltage range of -2.0-2.0 V. The obtained data revealed that fluorinated phenyl groups could decrease HOMO as well as LUMO levels of the compounds. UV-vis absorption and fluorescent emission spectra of them showed red shift in solid film state in comparison to their solution state, revealing that the molecular packing formed in solid state. Further proofs for the study of the packing order of crystals by X-ray analysis showed that phenyl groups provided TIPSAntP and TIPSAntPA planar conjugated unit along the anthracene core and favorable face-to-face interactions between their neighboring molecules. Fluorinated phenyl groups were twisted to the anthracene core and formed interleaved layer packing in TIPSAntFP. Preliminary device results were achieved from TIPSAntP and TIPSAntPA with p-type mobility of 10(-5) and 10(-6) cm2N x s, respectively, and n-type mobility for TIPSAntFP as 10(-7) cm2/V x s.     

Studies on thin film materials on acrylics for optical applications

Deposition of durable thin film coatings by vacuum evaporation on acrylic substrates for optical applications is a challenging job. Films crack upon deposition due to internal stresses and leads to performance degradation. In this investigation, we report the preparation and characterization of single and multi-layer films of TiO₂, CeO₂, Substance2 (E Merck, Germany), Al₂O₃, SiO₂ and MgF₂ by electron beam evaporation on both glass and PMMA substrates. Optical micrographs taken on single layer films deposited on PMMA substrates did not reveal any cracks. Cracks in films were observed on PMMA substrates when the substrate temperature exceeded 80°C. Antireflection coatings of 3 and 4 layers have been deposited and characterized. Antireflection coatings made on PMMA substrate using Substance2 (H2) and SiO₂ combination showed very fine cracks when observed under microscope. Optical performance of the coatings has been explained with the help of optical micrographs.     

A low-temperature-cross-linked poly(4-vinylphenol) gate-dielectric for organic thin film transistors

We present experimental results using an alternative cross-linker for poly(4-vinylphenol) (PVP) that allows lowering of the thermal budget of the cross-linking reaction to 130 °C which compares very favorable to the 180 °C required by the commonly used cross-linking agent poly(melamine-co-formaldehyde). Furthermore, we characterize the dielectric properties of a 200 nm thick layer and realize high quality organic thin film transistors using this low-temperature PVP as a gate-dielectric.     

Surface energy and adhesion energy of solution-based patterning in organic thin film transistors

This study elucidates the patterning of pentacene by adjusting its surface energy. The surface energy was modified by self-assembled monolayer treatment and exposure to ultra-violet (UV) light through a quartz-glass mask. Then, following pentacene deposition, dipping in water was used to remove pentacene from the UV-exposed area. The adhesion energy and the intrusion energy were analyzed to determine the mechanism of this patterning process. The variation of the intrusion energy with the surface energy was found to be the main issue in pentacene patterning. The characteristics of pentacene-organic thin film transistors were also measured to confirm the proposed method.     

Nitrogen-added Al rare-earth alloys for thin film transistors

The effects of nitrogen addition to aluminum-rare-earth alloys were investigated. Yttrium and gadolinium were employed as respective alloy components. The electrical properties and hillock densities of alloy films were investigated, and their nanostructures were studied by cross-sectional transmission electron microscopy. Nitrogen effectively decreases the grain size, and causes the columnar structure, that is generally present in aluminum-based alloys, to disappear. Nitric aluminum-rare-earth alloys have a strong resistance to hillock formation, and formed no micro-voids even after annealing at 450°C. An N₂ flow rate of 1.3–10% in the sputtering gas gives a low hillock density of 2.0E+1–7.7E−1 pcs/mm² after annealing at 350°C in both nitric Al-rare-earth alloys. In the case of patterned nitric aluminum-yttrium alloys, an N₂ flow rate of less than 1.3% causes large side-hillocks after annealing at over 350°C. As an optimum value, an N₂ flow rate of 2.5% results in a low hillock density of 1.9E+1 pcs/mm² and a low resistivity of 8.6 μΩ cm after annealing at 350°C.     

Molecular layer deposition of ZrO2-based organic-inorganic nanohybrid thin films for organic thin film transistors

Molecular layer deposition (MLD) technique can be used for preparation of various organic-inorganic nanohybrid superlattices at a gas-phase. The MLD method is a self-controlled layer-by-layer growth process under vacuum conditions, and is perfectly compatible with the atomic layer deposition (ALD) method. In this paper, we fabricated a new type organic-inorganic nanohybrid thin film using MLD method combined with ALD. A self-assembled organic layer (SAOL) was formed at 170 °C using MLD with repeated sequential adsorptions of CC terminated alkylsilane and zirconium hydroxyl with ozone activation. A ZrO₂ inorganic nanolayer was deposited at the same temperature using ALD with alternating surface-saturating reactions of Zr(OC(CH₃)₃)₄ and H₂O. The prepared SAOL-ZrO₂ organic-inorganic nanohybrid films exhibited good mechanical stability, excellent insulating properties, and relatively high dielectric constant k (~ 16). They were then used as a 23 nm-thick dielectric for low voltage pentacene-based thin film transistors, which showed a maximum field effect mobility of 0.63 cm²/V s, operating at − 1 V with an on/off current ratio of ~ 10³.     

Co-sputtered oxide thin film encapsulated organic electronic devices with prolonged lifetime

Effective top-side thin film encapsulation for organic light-emitting devices (OLEDs) was achieved by deposition of a multi-layer water diffusion barrier stack to protect the device against moisture permeation. The barrier stack was formed by alternative depositions of co-oxide and fluorocarbon (CF_x) films. The co-oxide layer was fabricated by magnetron co-sputtering of silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃). While the CF_x layer was formed by plasma enhanced chemical vapor deposition. The water vapor transmission rate of the optimized diffusion barrier stack can be down to 10^− 6 g/m²/day. The OLEDs encapsulated with the multilayer stack have been shown to have operation lifetime of over 18,000 h which is nearly the same as devices with conventional glass-cover encapsulation.     

Innovative aspects in thin film technologies for nanostructured materials in gas sensor devices

This work reports on the use of two innovative techniques in the field of gas sensors for preparing nano-structured materials: sol-gel and supersonic cluster beam deposition. By means of sol-gel, nano-structured In₂O₃ thin films have been prepared and deposited under different deposition parameters on silicon wafer. In this way the results have shown a good compatibility of the method with silicon technology, then potentially suitable to be used in the fabrication of integrated devices. The second technique has been applied for the preparation of nano-structured TiO₂ thin films showing its capability to be used in the fabrication of gas sensor devices, mainly when a good control of the grain dimension is required.     

无机薄膜电致发光器件中绝缘层材料的研究

薄膜电致发光（TFEL）技术将戍为平板显示技术的潮流和主体。简要介绍了平板显示技术的发展，同时对无机薄膜电致发光器件中绝缘层材料的选择进行了探讨。     

Using chemical-mechanical polishing for planarizing a high-kappa nanocomposite polyimide insulator for organic thin film transistors application

To solve the large leakage current of the heavily blended nanocomposite (Polyimide and nano-TiO2 particles) gate dielectric film exhibiting a high-kappa, the chemical-mechanical polishing (CMP) was adopted to flatten the surface of the PI-TiO2 nanocomposite film. An extremely high dielectric constant (is congruent to 13) of the nanocomposite with CMP treatment is obtained and its leakage current is comparable to that of the neat polyimide in our studies. An OTFT based on the nanocomposite gate dielectric exhibiting high capacitance and a smooth surface after CMP treatment shows very promising performance. Compared with the OTFT based on the neat polyimide gate dielectric, the threshold voltage is improved from -22 to -5 (V), the sub-threshold voltage is decreased from 3.44 to 0.50 (V/dec), the current on/off ratio is increased from 1.6 x 10(6) to 3.53 x 10(8), and the mobility is increased from 0.416 to 0.624 (cm2V(-1)s(-1)). Moreover, it is worth noting that the hysteresis effect of OTFT based the nanocomposite can be significantly reduced due to the few charge trapped in the interface when the nanocomposite dielectric surface was further polished by CMP treatment.     

Development of a hybrid inverter through integration of organic and inorganic thin film transistors

In this paper, complementary thin-film transistor (TFT) inverter is fabricated with organic-inorganic hybrid channels. By adopting p-channel pentacene and n-channel ZnO, we have fabricated a device of hybrid complementary TFT inverter by using same electrode in organic-inorganic hybrid channels. To be accomplished Ohmic-contact in organic-inorganic hybrid channels, we adapted to n-channel staggered TFT and p-channel coplanar TFT. In results, a hybrid inverter built through integration of organic and inorganic TFT shows that the typical inverter response to stage switching is clearly observed between 0 and 40 V, for both input directions, displaying a high voltage gain — (dV_OUT/dV_IN) > 16.     

A hybrid mask-mould lithography scheme and its application in nanoscale organic thin film transistors

Nanoimprint lithography (NIL) has stimulated great interest in both academic research and industrial development due to its high resolution, high throughput and low cost advantages. Though NIL has been demonstrated to be very successful in replicating nanoscale features, it also has its limitations as a general lithography technique. Its fundamental moulding characteristics (i.e.?physically displacing polymer materials) frequently lead to pattern defects when replicating arbitrary patterns, especially patterns with broad size distribution. To solve this problem, we have developed a combined nanoimprint and photolithography technique that uses a hybrid mould to achieve good pattern definitions. In this work, we applied this technique to fabricate finger-shaped nanoelectrodes, and demonstrated nanoscale pentacene organic thin film transistors (OTFTs). Methods of the hybrid mask-mould (HMM) fabrication and results on the device electrical characteristics are provided. With combined advantages of both photolithography and NIL, and the applicability to general nanoscale device and system fabrication, this method can become a valuable choice for low cost mass production of micro-?and nanoscale structures, devices and systems.     

Shelf-life time test of p- and n-channel organic thin film transistors using copper phthalocyanines

P- and n-type channel thin film transistors (OTFTs) were fabricated by using hexadecahydrogen copper phthalocyanine (H₁₆CuPc) and hexadecafluoro copper phthalocyanine (F₁₆CuPc) molecules, respectively. Top-contact and bottom-contact source-drain configurations were used for both semiconductors. Furthermore, the temperature and film thickness dependences on the mobility values were measured in the saturation regime of source-drain current. Unipolar mobilities in such single-layer OTFTs were correlated to thin film morphology by X-ray diffraction analysis and atomic force microscopy measurements. Shelf-life time tests of p-type and n-type OTFTs are detailed as OTFT configuration and substrate temperature dependence over a time period of 100 days.