A Review of Low‐Temperature Solution‐Processed Metal Oxide Thin‐Film Transistors for Flexible Electronics

Nonfullerene (NF) organic solar cells (OSCs) have been attracting significant attention in the past several years. It is still challenging to achieve high-performance flexible NF OSCs. NF acceptors are chemically reactive and tend to react with the low-temperature-processed low-work-function (low-WF) interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the "S" shape in the current-density characteristics of the cells. In this work, the chemical interaction between the NF active layer and the polymer interfacial layer of PEIE is deactivated by increasing its protonation. The PEIE processed from aqueous solution shows more protonated N+ than that processed from isopropyl alcohol solution, observed from X-ray photoelectron spectroscopy. NF solar cells (active layer: PCE-10:IEICO-4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room-temperature processing and effective WF reduction enable the demonstration of high-performance (12.5%) flexible NF OSCs.

12.5% Flexible Nonfullerene Solar Cells by Passivating the Chemical Interaction Between the Active Layer and Polymer Interfacial Layer

Wide bandgap oxide semiconductors constitute a unique class of materials that combine properties of electrical conductivity and optical transparency. They are being widely used as key materials in optoelectronic device applications, including flat-panel displays, solar cells, OLED, and emerging flexible and transparent electronics. In this article, an up-to-date review on both the fundamental understanding of materials physics of oxide semiconductors, and recent research progress on design of new materials and high-performing thin film transistor (TFT) devices in the context of fundamental understanding is presented. In particular, an in depth overview is first provided on current understanding of the electronic structures, defect and doping chemistry, optical and transport properties of oxide semiconductors, which provide essential guiding principles for new material design and device optimization. With these principles, recent advances in design of p-type oxide semiconductors, new approaches for achieving cost-effective transparent (flexible) electrodes, and the creation of high mobility 2D electron gas (2DEG) at oxide surfaces and interfaces with a wealth of fascinating physical properties of great potential for novel device design are then reviewed. Finally, recent progress and perspective of oxide TFT based on new oxide semiconductors, 2DEG, and low-temperature solution processed oxide semiconductor for flexible electronics will be reviewed.

Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices

Solution combustion synthesis (SCS) has been widely used to produce simple and complex oxides with a desired morphology (size and shape). SCS is valuable due to low cost, simplicity and energy efficient synthesis. To guarantee the best molecular-level mixing of reactants in an aqueous or solvent-based solution some parameters need to be controlled, such as fuel type, metal cations precursors, stoichiometry ratio (φ), pH effect, atmosphere and initiation type. These determine the final properties of the oxide materials, providing the potential to reach different morphologies, which are essential for their final applications. This Review article focuses on the crucial parameters in SCS and how these affect the overall materials properties from nanostructures to thin films. To finalize, special attention is given to the application of SCS to form metal oxide thin films at low temperature and their application in thin film transistors (TFTs).

Solution Combustion Synthesis: Towards a Sustainable Approach for Metal Oxides.

Mechanically robust, highly compressible, and conductive thermoplastic polyurethane/carbon black foams were successfully fabricated via 3D printing. Thixotropic inks were formulated by dispersing n...

Reprocessable 3D-Printed Conductive Elastomeric Composite Foams for Strain and Gas Sensing

The high-performance ZnO thin-film transistors (TFTs) were fabricated on indium tin oxide glass with high-capacitance atomic layer deposition (ALD)-processed ZrO2 as the gate dielectric. The 5-nm ultrathin ZrO2 film showed a very high areal capacitance of 820 nF/cm2 at 20 Hz, a relatively high breakdown field of 14 MV/cm, and low surface root-mean-square (rms) roughness of 0.22 nm, making it possible for ZnO/ZrO2 TFT to not only be operated by an ultralow operating voltage of 1 V but also present a near theoretical limit subthreshold swing of 69 mV/dec. Furthermore, the ZnO TFT with a 5-nm ZrO2 gate dielectric exhibited excellent performance, such as a high Ion/Ioff of 107, large field effect mobility of 36.8 cm2/Vs, low-density of trapping states ( ${N}_{{\text {trap}}}$ ) of $1.6\times 10^{{11}}$ eV−1cm−2, and negligible hysteresis. In addition, the electron transport mode was built to explain the high mobility of nanocrystalline ZnO TFT. As a result, the ultralow operating voltage TFTs exhibited great potential for low-powered electronics applications.

High-Performance 1-V ZnO Thin-Film Transistors With Ultrathin, ALD-Processed ZrO 2 Gate Dielectric

Solution-processed thin film transistors (TFTs) in the next generation of large-area flexible electrics not only need oxide semiconductors with high performance but also require them with low-temperature processability. To meet these requirements, we developed a low-temperature method, that is, combining solution combustion synthesis and deep-ultraviolet irradiation to prepare p-type Li-doped NiOx (Li:NiOx) thin films at 150 °C. The incorporation of Li into the NiOx matrix significantly improves p-type conductivity of NiOx because the Fermi level (EF) shifts toward the valence band maximum. To confirm the possible application of Li:NiOx as the channel layer in TFTs, Li:NiOx/ZrO2/ITO was fabricated on each of rigid glass and flexible PET substrates. This is the first report of application of Li:NiOx semiconductor in TFT devices. The field-effect mobility, Ion/Ioff and subthreshold swing for rigid and flexible Li5%:NiOx TFTs are 1.69 and 1.41 cm2 V−1 s−1, 8 × 106 and 105, 0.21 and 0.54 V dec−1, respectively. Furthermore, the counterclockwise hysteresis of transfer curves is negligible (0.1 V). Thus, the high-performance and cost-effective electronics with low fabrication temperature will definitely contribute to future large-scale flexible and wearable electronics.

Low-temperature combustion synthesis and UV treatment processed p-type Li:NiOx active semiconductors for high-performance electronics

Flexible information displays hold great promise for future optoelectronic applications. The performance of white quantum dot light-emitting diodes (QLEDs) has been dramatically improved in the past few years. However, most of the reported white QLEDs are fabricated on glass substrates; less progress has been made in white QLEDs based on flexible substrates. Herein, we report the fabrication of an efficient flexible white QLED with mixed red, green and blue QDs as emitters by an all-solution process. The resulting flexible white QLED shows pure white light emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.34, 0.33) by controlling the Forster energy transfer process between the differently colored RGB QDs, and it also exhibits remarkable current efficiency of 10.5 cd A−1 and brightness of up to 3554 cd m−2.

Highly efficient, all-solution-processed, flexible white quantum dot light-emitting diodes

We developed a novel method to fabricate Zr-doped ZnO (ZrZnO) thin films via low-temperature atomic layer deposition technique. ZrZnO films were deposited by diethylzinc (DEZ)/tetrakiszirconium (TDMAZr)/H2O cycles instead of the traditional DEZ/H2O/TDMAZr/H2O cycles and applied in thin-film transistors (TFTs). It is found that ZrZnO-TFTs with a Zn-Zr-O:ZnO atomic ratio of 1:49, i.e., ZrZnO (1:49) exhibit excellent properties, such as a minimum subthreshold swing value of 0.37 V/dec, a maximum ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ value of $2.4\times 10^{{7}}$ , a largermobility of 12.38 cm2/ $\text {V}\cdot \text {s}$ , and a smaller threshold voltage shift ( $\Delta {V}_{\text {th}}$ ) of 0.61 V under temperature stress from 25 °C to 105 °C, which are superior compared with TFTs with ZnO channel doped by ZrO2 layer. The stability of ZnO TFTs was improved greatly by Zr-Zn-O doping. Moreover, the field-effect mobility of ZrZnO-TFTs was rarely influenced. Temperature-stress test was carried out to build up the correlation model in terms of the defect structures, subgap states, and stability. These results could provide a newaccess to understand the device instability of ZrZnO TFTs.

Enhanced Stability in Zr-Doped ZnO TFTs With Minor Influence on Mobility by Atomic Layer Deposition

High-performance nitrogen-doped ZnO (ZnON)-based thin-film transistors (TFTs) were fabricated by atomic layer deposition (ALD) with a rapid purging time of only 5 s at a temperature as low as 150 °C. It is the first time to report ALD ZnON TFT using NH3 as N source. It is found that ZnON TFT with a N: Zn atomic ratio of 1:19 (ZnON 1:19) exhibited excellent properties, such as lower subthreshold swing of 0.47 V/decade and smaller $\Delta {V}_{\textsf {th}}$ of 0.71 V under the temperature stress from 25 to 105 °C. The ${I}_{ \mathrm{\scriptscriptstyle ON}}$ and ${I}_{ \mathrm{\scriptscriptstyle OFF}} $ decreased as N-doping concentration increased, and ZnON 1:19 TFT presented a high ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio of 1.75 $\times \,\,10^{\textsf {7}}$ . The density of state was calculated by temperature stress. Combining with the X-ray photoelectron spectroscopy analysis, we built a model to explain the reaction mechanism that the moderate amount of N doping could significantly suppress the creation of oxygen defects.

Xingwei Ding

Papers

High-Performance 1-V ZnO Thin-Film Transistors With Ultrathin, ALD-Processed ZrO 2 Gate Dielectric

Low-temperature combustion synthesis and UV treatment processed p-type Li:NiOx active semiconductors for high-performance electronics

Highly efficient, all-solution-processed, flexible white quantum dot light-emitting diodes

Enhanced Stability in Zr-Doped ZnO TFTs With Minor Influence on Mobility by Atomic Layer Deposition

Nitrogen-Doped ZnO Film Fabricated Via Rapid Low-Temperature Atomic Layer Deposition for High-Performance ZnON Transistors