In this report, a simple and general chemical route for fabricating MO semiconducting films at a relatively low temperature without any fuel additives or special annealing steps was demonstrated. The precursor, which consisted of perchlorate, nitrate, and water, is easily converted into In2O3 at an annealing temperature of 250 °C due to oxygen radical assisted decomposition and generation of a large amount of heat. It is found that perchlorate salt can decompose and form an oxide film with high quality at a lower temperature when assisted by nitrate salt. The optimized In2O3–TFT fabricated via this precursor exhibits a saturation mobility of 14.5 cm2 V−1 s−1. Furthermore, this approach has been expanded to the fabrication of ZnO films and attained improved performance, indicating its universality.

An array of inkjet-printed metal-oxide thin-film transistors (TFTs) is demonstrated for the first time with the assistance of surface-energy patterns prepared by printing pure solvent to etch the ultrathin hydrophobic layer. The surface-energy patterns not only restrained the spreading of inks but also provided a facile way to regulate the morphology of metal oxide films without optimizing ink formulation. The fully printed InGaO TFT devices in the array exhibited excellent electron transport characteristics with a maximum mobility of 11.7 cm2 V–1 s–1, negligible hysteresis, good uniformity, and good stability under bias stress. The new route lights a general way toward fully inkjet-printed metal-oxide TFT arrays.Keywords: Cytop; inkjet printing; metal oxide; surface-energy patterns; thin-film transistors;

•A novel planar p-type heteroacene (ICzDBT) was designed and synthesized.•The field-effect transistor properties show very low threshold voltage of −0.8 V.•The ICzDBT displays highly thermal and electric stability.A novel planar heteroacene, 8,17-dihexadecyl-8,17-dihydrobenzo[4′,5′]thieno[2′,3′:5,6] indolo[3,2-b]benzo[4,5]thieno[2,3-h]carbazole (ICzDBT), was designed and synthesized. The thermal, optical, electrochemical and field-effect transistor properties were investigated systematically. The highest occupied molecular orbital energy levels (EHOMO) measured by cyclic voltammetry and photoelectron yield spectra were −5.15 and −5.29 eV, respectively, which are close to the work function (WF) of Au (∼5.2 eV). This suitable energy level alignment between the EHOMO of ICzDBT and WF of Au is favorable for hole carrier injection from the Au electrode to the ICzDBT layer. The ICzDBT showed a hole mobility as high as 0.17 cm2 V−1 s−1 and a current on/off ratio of 1.2 × 106, with a very low threshold voltage of −0.8 V. Moreover, the device displayed excellent stability with little roll-off of hole mobility in air. Hence, this kind of molecule is a promising candidate for p-type organic field-effect transistors (OFETs) with high mobility and air stability.Download high-res image (240KB)Download full-size image

In this study, ambipolar field-effect transistors (FETs) based on organic–inorganic bilayer structures were investigated. InOx and pentacene were selected as n-type and p-type semiconductors, respectively. To improve the hole mobility, poly(perfluorobutenylvinylether) (Cytop) layer was introduced to modify the surface of InOx. The ambipolar FETs showed hole and electron mobilities of 1.1 and 0.1 cm2 V−1 s−1, respectively and excellent electrical stability under gate bias stress. Furthermore we found that ambipolar FETs could be integrated into functional complementary metal oxide semiconductor (CMOS)-like inverters and exhibited high peak gain (>50). This work provides a general method for realizing ambipolar FETs based on organic–inorganic hybrid structure.

A “green precursor” and a “green patterning technique” were used to fabricate low temperature processed indium oxide (InOx) semiconductors. For the InOx precursor, chloride ligand-based indium(III) was dissolved in deionized (DI) water without any additives to form a gel-like precursor. The as-spin-coated precursor films could be facilely patterned using the “green patterning technique”, which requires only ultraviolet (UV) irradiation and DI water. A systematic study was carried out to investigate the chemical reaction of the chloride-based precursor films as well as the semiconductor properties. It was found that UV irradiation and water treatment not only helped to transform In–Cl into In–OH, but also helped to remove the Cl-related impurities. It led to the activation of InOx films at temperatures as low as 180 °C. The mobility of InOx TFTs based on an anodized aluminium oxide (AlOx:Nd) insulator with patterning was improved by more than 1 order compared to that without patterning at an annealing temperature of 280 °C. In addition, flexible InOx TFTs on polyimide (PI) substrates were demonstrated. They showed only a little degradation in the subthreshold region of the transfer curve even at a bending curvature (R) of 5 mm.

Short-channel electronic devices several micrometers in length are difficult to implement by direct inkjet printing due to the limitation of position accuracy of the common inkjet printer system and the spread of functional ink on substrates. In this report, metal oxide thin-film transistors (TFTs) with channel lengths of 3.5 ± 0.7 μm were successfully fabricated with a common inkjet printer without any photolithography steps. Hydrophobic CYTOP coffee stripes, made by inkjet-printing and plasma-treating processes, were utilized to define the channel area of TFTs with channel lengths as short as ∼3.5 μm by dewetting the inks of the source/drain (S/D) precursors. Furthermore, by introduction of an ultrathin layer of PVA to modify the S/D surfaces, the spreading of precursor ink of the InOx semiconductor layer was well-controlled. The inkjet-printed short-channel TFTs exhibited a maximum mobility of 4.9 cm2 V–1 s–1 and an on/off ratio of larger than 109. This approach of fabricating short-channel TFTs by inkjet printing will promote the large-area fabrication of short-channel TFTs in a cost-effective manner.

Flexible solution-processed polymer thin-film transistors (PTFTs) with a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer as the active layer and electrochemically oxidized alumina (AlOx:Nd) as the gate insulator are fabricated on polyethylene naphthalate (PEN) substrates. The AlOx:Nd insulator exhibits excellent insulating properties with low leakage current, a high dielectric constant and a high breakdown field. To improve the interface coupling between the polymer active layer and the AlOx:Nd insulator, the AlOx:Nd insulator is treated with octadecyl-phosphonic acid (ODPA), forming self-assembled monolayers (SAMs) on the surface, and great improvement in TFT performance with the highest mobility of 2.88 cm2 V−1 s−1 is attained. The performance improvement is attributed to the smoother surface and lower surface energy of the ODPA-treated AlOx:Nd compared to those of bare AlOx:Nd. In addition, the flexible PTFT exhibits only small shifts in the transfer curves at bending curvatures (R) at 30 mm, but the device shows larger threshold voltage and higher off current (Ioff) after bent at R = 5–20 mm, which may be attributed to the damage in the insulator-semiconductor interface.

Solution-processed indium-zinc-oxide (IZO) thin-film transistors (TFTs) based on anodized aluminum oxide gate insulator modified with a zirconium oxide (ZrOx) interlayer were fabricated. By introduction of the ZrOx interlayer, the IZO-TFTs exhibited improved performance with a higher mobility of 7.8 cm2 V−1 s−1, a lower Vth of 4.6 V and a lower SS of 0.21 V dec−1 compared to those without the ZrOx interlayer. Comprehensive studies showed that the Al element easily diffused into the IZO film and formed AlOx clusters which acted as defects to deteriorate TFT performance; and after modification with a ZrOx interlayer, the diffusion of Al was suppressed and the Zr diffusing effect almost could be ignored. These results suggested that the introduction of an interlayer with less diffusing effect as well as an effect of blocking the elements from the gate insulator diffusing into the channel layer could be an effective way to improve the electrical performance for solution-processed oxide TFTs.

Flexible organic field-effect transistors (OFETs) with electrochemically oxidized gate insulators (AlOx:Nd) covered by a thin layer of hydroxyl-free poly(perfluorobutenylvinylether) known as Cytop were fabricated on a polyethylene naphthalate (PEN) substrate. The AlOx:Nd/Cytop bilayer insulator exhibited excellent insulating properties with low leakage current, high dielectric constant, high breakdown field, and low surface roughness. The pentacene film on AlOx:Nd without Cytop consisted of small grains, while the one on AlOx:Nd with Cytop exhibited a dendritic structure with a larger average grain size of ∼350 nm. The pentacene OFET with Cytop exhibited higher mobility (0.75 cm2 V−1 s−1) and better electrical stability under gate-bias-stress (in air condition) compared to that without Cytop. In addition, the flexible OFET was able to maintain a relatively stable performance under a certain degree of bending.

•IZO–TFT was fabricated with a two-step-annealing method.•This TFT had much better uniformity and stability than that with one annealing step.•Both the shallow and deep trap states were greatly reduced by this method.In this paper, a thin-film transistor (TFT) with indium zinc oxide (IZO) channel layer was fabricated using a two-step-annealing method in which the IZO film experienced annealing steps before the etch-stopper-layer formation and after the whole device completion. The device showed better uniformity and better stability under positive bias stress, negative bias illumination stress, and temperature stress, compared to those with only one post annealing step. The calculated falling rate of the Fermi lever of the IZO channel for the two-step annealing device was as high as 0.593 eV/V, compared to 0.213 eV/V for the only-post-annealing-step one. And the corresponding density of subgap state was 4.4 × 1015 and 1.6 × 1016 eV−1 cm−3 for the device with two annealing steps and with only one post annealing step, respectively.

•Ambipolar field-effect transistors (FETs) based on solution-processed organic-inorganic bilayer structures are investigated.•A self-assembled monolayer (SAM) of octadecyl-phosphonic acid is introduced to modify the organic-inorganic interface.•The complementary metal oxide semiconductor (CMOS)-like inverters based on ambipolar FETs are fabricated.Ambipolar field-effect transistors (FETs) based on solution-processed organic-inorganic bilayer structures were investigated. An amorphous indium oxide (InOx) film, as the n-type semiconducting layer, was prepared with an environmentally friendly method and annealed at a low temperature; and a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer, FBT-Th4(1,4), was spin-coated on the InOx film as the p-type semiconducting layer. To improve the p-type mobility, a self-assembled monolayer (SAM) of octadecyl-phosphonic acid was introduced to modify the surface of InOx. The ambipolar FETs showed high and well-balanced hole and electron mobilities of 1.1 and 1.5 cm2 V−1 s−1, respectively. Furthermore we found that ambipolar FETs could be integrated into functional complementary metal oxide semiconductor (CMOS)-like inverters.Download high-res image (233KB)Download full-size image

Flexible solution-processed polymer thin-film transistors (PTFTs) with a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer as the active layer and electrochemically oxidized alumina (AlOx:Nd) as the gate insulator are fabricated on polyethylene naphthalate (PEN) substrates. The AlOx:Nd insulator exhibits excellent insulating properties with low leakage current, a high dielectric constant and a high breakdown field. To improve the interface coupling between the polymer active layer and the AlOx:Nd insulator, the AlOx:Nd insulator is treated with octadecyl-phosphonic acid (ODPA), forming self-assembled monolayers (SAMs) on the surface, and great improvement in TFT performance with the highest mobility of 2.88 cm2 V−1 s−1 is attained. The performance improvement is attributed to the smoother surface and lower surface energy of the ODPA-treated AlOx:Nd compared to those of bare AlOx:Nd. In addition, the flexible PTFT exhibits only small shifts in the transfer curves at bending curvatures (R) at 30 mm, but the device shows larger threshold voltage and higher off current (Ioff) after bent at R = 5–20 mm, which may be attributed to the damage in the insulator-semiconductor interface.

In this report, a simple and general chemical route for fabricating MO semiconducting films at a relatively low temperature without any fuel additives or special annealing steps was demonstrated. The precursor, which consisted of perchlorate, nitrate, and water, is easily converted into In2O3 at an annealing temperature of 250 °C due to oxygen radical assisted decomposition and generation of a large amount of heat. It is found that perchlorate salt can decompose and form an oxide film with high quality at a lower temperature when assisted by nitrate salt. The optimized In2O3–TFT fabricated via this precursor exhibits a saturation mobility of 14.5 cm2 V−1 s−1. Furthermore, this approach has been expanded to the fabrication of ZnO films and attained improved performance, indicating its universality.

A “green precursor” and a “green patterning technique” were used to fabricate low temperature processed indium oxide (InOx) semiconductors. For the InOx precursor, chloride ligand-based indium(III) was dissolved in deionized (DI) water without any additives to form a gel-like precursor. The as-spin-coated precursor films could be facilely patterned using the “green patterning technique”, which requires only ultraviolet (UV) irradiation and DI water. A systematic study was carried out to investigate the chemical reaction of the chloride-based precursor films as well as the semiconductor properties. It was found that UV irradiation and water treatment not only helped to transform In–Cl into In–OH, but also helped to remove the Cl-related impurities. It led to the activation of InOx films at temperatures as low as 180 °C. The mobility of InOx TFTs based on an anodized aluminium oxide (AlOx:Nd) insulator with patterning was improved by more than 1 order compared to that without patterning at an annealing temperature of 280 °C. In addition, flexible InOx TFTs on polyimide (PI) substrates were demonstrated. They showed only a little degradation in the subthreshold region of the transfer curve even at a bending curvature (R) of 5 mm.