Fabrication of a planar water gated organic field effect transistor using a hydrophilic polythiophene for improved digital inverter performance

A planar water gated OFET (WG-OFET) structure is fabricated by patterning gate, source and drain electrodes on the same plane at the same time. Transistor output characteristics of this novel structure employing commercial regioregular poly(3-hexylthiophene) (rr-P3HT) as polymer semiconductor and deionized (DI) water as gate dielectric show successful field effect transistor operation with an on–off current ratio of 43 A/A and transconductance of 2.5 μA/V. These output characteristics are improved using P3HT functionalized with poly(ethylene glycol) (PEG) (P3HT-co-P3PEGT), which is more hydrophilic, leading to on–off ratio of 130 A/A and transconductance of 3.9 μA/V. Utilization of 100 mM NaCl solution instead of DI water significantly increases the on–off ratio to 141 A/A and transconductance to 7.17 μA/V for commercial P3HT and to 217 A/A and to 11.9 μA/V for P3HT-co-P3PEGT. Finally, transistors with improved transconductances are used to build digital inverters with improved characteristics. Gain of the inverters employing P3HT and P3HT-co-P3PEGT are measured as 2.9 V/V and 10.3 V/V, respectively, with 100 mM NaCl solution.     

Solution-processed semiconducting aluminum-zinc-tin-oxide thin films and their thin-film transistor applications

We prepared aluminum-zinc-tin-oxide (AZTO) thin films by the solution spin-coating method and investigated their physical and electrical properties according to different incorporated amounts of Al. AZTO films annealed at 400 °C were amorphous. Though SnO₂ crystallites were detected in films annealed at temperatures higher than 500 °C, the number of crystallites decreased as the Al content increased. Thin films had a smooth and uniform surface morphology with an optical transmittance value higher than 92% in the visible range. Electrical conductivity and its temperature dependence varied markedly according to the amount of Al incorporated in the film. We therefore systematically investigated activation energies for carrier transport for each film composition. Thin-film transistors (TFTs) were fabricated using solution-processed AZTO as an active channel layer. The effects of the amount of Al incorporated in the thin film on TFT characteristics were also evaluated. The best device performance was observed for a TFT with a 5 mol%-Al-incorporated AZTO channel. Field effect mobility, subthreshold swing, and on/off ratio were approximately 0.24 cm² V⁻¹ s⁻¹, 0.69 V/dec, and 1.03×10⁶, respectively.     

Advances in Cost-Efficient Thin-Film Photovoltaics Based on Cu(In,Ga)Se2

In this article, we discuss the leading thin-film photovoltaic (PV) technology based on the Cu(In,Ga)Se₂ (CIGS) compound semiconductor. This contribution includes a general comparison with the conventional Si-wafer-based PV technology and discusses the basics of the CIGS technology as well as advances in world-record-level conversion efficiency, production, applications, stability, and future developments with respect to a flexible product. Once in large-scale mass production, the CIGS technology has the highest potential of all PV technologies for cost-efficient clean energy generation.     

Detailed analysis and contact properties of low-voltage organic thin-film transistors based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and its didecyl and diphenyl derivatives

Low-voltage organic thin-film transistors (TFTs) based on four different small-molecule semiconductors (pentacene, DNTT (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), C₁₀-DNTT and DPh-DNTT) were fabricated, and a detailed comparison of the semiconductor thin-film morphology, of the current-voltage characteristics of transistors with channel lengths ranging from 100 to 1 μm, and of the contact resistances is provided. The three thienoacene derivatives DNTT, C₁₀-DNTT and DPh-DNTT all have significantly larger charge-carrier mobilities and smaller contact resistances than pentacene. In terms of the intrinsic channel mobility (determined using the transmission line method), C₁₀-DNTT and DPh-DNTT perform quite similarly and notably better than DNTT, suggesting that the decyl substituents in C₁₀-DNTT and the phenyl substituents in DPh-DNTT provide a similar level of enhancement of the charge-transport characteristics over DNTT. However, the DPh-DNTT TFTs have a substantially smaller contact resistance than both the DNTT and the C₁₀-DNTT TFTs, resulting in notably larger effective mobilities, especially in transistors with very small channel lengths. For DPh-DNTT TFTs with a channel length of 1 μm, an effective mobility of 0.68 cm²/V was determined, together with an on/off ratio of 10⁸ and a subthreshold swing of 100 mV/decade.     

A course on thin‐film transistor circuit design for modern displays

A new subject‐specific course on thin‐film transistor (TFT) circuit design is introduced, covering related knowledge of display technologies, TFT device physics, processing, characterization, modeling and circuit design. A design project is required for students to deepen the understanding even more and get hands‐on design experience. This course can be an intense 1‐week course to offer a full training of design engineers in an organized way to meet the ever‐increasing needs in display industry for TFT circuit design specialists. It can also be organized in one semester for electrical engineering Master's and Ph.D. students.     

Solution-processed ytterbium oxide dielectrics for low-voltage thin-film transistors and inverters

High permittivity (high k) metal-oxide thin films fabricated via solution processes have recently received much attention for the construction of low-operating voltage and high-performance thin-film transistors (TFTs). In this report, amorphous ytterbium oxide (Yb₂O₃) thin films were fabricated by spin coating and their applications in TFTs were explored. The physical properties of the solution-processed Yb₂O₃ thin films processed at different annealing temperatures were systematically investigated using various characterization techniques. To explore the feasibility of the Yb₂O₃ thin films as gate dielectrics for oxide TFTs, In₂O₃ TFTs based on Yb₂O₃ dielectrics were integrated. All the devices could be operated at 3 V, which is critical for the applications in portable, battery-driven, and low-power electronic devices. The optimized In₂O₃/Yb₂O₃ TFT exhibits high electrical performances, including field-effect mobility of 4.98 cm²/V s, on/off current ratio of ~ 10⁶, turn-on voltage around 0 V, and subthreshold swing of 70 mV/decade, respectively. To demonstrate the potential of In₂O₃/Yb₂O₃ TFT toward more complex logic application, the unipolar inverter was further constructed.     

Accounting for variability in the design of circuits with organic thin-film transistors

We experimentally verify that the methodology to account for local parameter variations and transistor mismatch known in Si CMOS technologies can be transposed to organic thin-film transistor technologies, and we present a design case that makes use of design for variability. Transistor parameter variation decreases with the square root of the transistor footprint. As a consequence, Monte Carlo simulations which take this effect into account can be executed to better predict the final circuit yield. The design case in this work is an 8-bit, organic RFID transponder chip. The yield prediction by simulations corresponds to the finally observed circuit yield.     

Carboxylic acid mediated self-assembly of small molecules for organic thin film transistors

A p-type small molecule bearing dicarboxylic acid functional group (–COOH) is synthesized and evaluated for field-effect transistor properties. We discover and report for the first time, that the –COOH groups assist in the passivation of surface traps on the dielectric layer and simultaneously facilitate the self-assembly of the molecules via inter-molecular hydrogen bonding resulting in crystalline active channels. A 9-fold decrease in the threshold voltage was observed for the transistors made using the –COOH functionalized molecule, QT-DA, compared to its ester analogue, QT-ES, providing an evidence of surface passivation. This resulted in an increase in the hole mobility of QT-DA by up to 2 orders of magnitude. It was shown that QT-DA adopts a vertical alignment with respect to the substrate due to preferential interaction between the –COOH groups and the SiO₂ surfaces.     

原子层沉积Al2O3 保护层氢终端金刚石MISFETs的电学特性

研究了Zr-Si-N氢终端金刚石（H-diamond）绝缘栅场效应晶体管（MISFET）在有无Al₂O₃保护层情况下的电学特性。分别采用原子层沉积法（ALD）和射频溅射法（RF）制备了Al₂O₃保护层和Zr-Si-N栅介质层。MISFETs的转移特性曲线表明,其栅阈值电压在有无Al₂O₃保护的情况下从-2.5 V变化到3 V,表明器件从常关型转换为常开型。输出和转移特性曲线揭示了氧化铝的存在保护了氢终端,使其免受磁控溅射过程的损伤。