Zheng Cui

Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices

A method for direct inkjet printing of silver nanowire (Ag NW) to form transparent conductive network as the top electrode for inverted semi-transparent organic photovoltaic devices (OPV) was developed. The highest power conversion efficiency of the poly(3-hexylthiophene):phenyl-C61–butyric acid methyl ester (P3HT:PC61BM) based OPV was achieved to be 2.71% when the top electrode was formed by 7 times of printing. In general, devices with printed Ag NW top electrode had similar open-circuit voltage (VOC, around 0.60u2009V) but lower fill factor (FF, 0.33–0.54) than that of device with thermally deposited Ag opaque electrode (reference device). Both FF and short-circuit current density (JSC), however, were found to be increasing with the increase of printing times (3, 5, and 7), which could be partially attributed to the improved conductivity of Ag NW network electrodes. The solvent effect on device performances was studied carefully by comparing the current density-voltage (J-V) curves of different devices. Th...

Homoleptic tris-cyclometalated iridium( iii ) complexes with phenylimidazole ligands for highly efficient sky-blue OLEDs

As an extension of our previous study, three sky-blue homoleptic iridium(III) complexes 1–3 with fluorine-free phenylimidazole ligands were synthesized and their photophysical, electrochemical and thermal properties were studied. All the complexes showed high photoluminescence quantum yields ranging from 0.50 to 0.53. The introduction of a bulky isopropyl group at the 2,6-position of N-phenyl of the phenylimidazole ligands increases the quantum yield and the decomposition temperature of the iridium(III) complexes. As the conjugated system of the ligand becomes larger, a very small bathochromic-shift of 1 nm was observed in complex 3 (475 nm) compared with complexes 1 (474 nm) and 2 (474 nm). All the OLED devices showed high current efficiencies of over 20 cd A−1 at the luminance of 1000 cd m−2. Devices incorporated with complexes 1–3 all showed longer lifetime in comparison with that of a FIrpic-based device.

Effect of surface wettability properties on the electrical properties of printed carbon nanotube thin-film transistors on SiO2/Si substrates.

The precise placement and efficient deposition of semiconducting single-walled carbon nanotubes (sc-SWCNTs) on substrates are challenges for achieving printed high-performance SWCNT thin-film transistors (TFTs) with independent gates. It was found that the wettability of the substrate played a key role in the electrical properties of TFTs for sc-SWCNTs sorted by poly[(9,9-dioctylfluorene-2,7-diyl)-co-(1,4-benzo-2,1,3-thiadiazole)] (PFO-BT). In the present work we report a simple and scalable method which can rapidly and selectively deposit a high concentration of sc-SWCNTs in TFT channels by aerosol-jet-printing. The method is based on oxygen plasma treatment of substrates, which tunes the surface wettability. TFTs printed on the treated substrates demonstrated a low operation voltage, small hysteresis, high mobility up to 32.3 cm(2) V(-1) s(-1), and high on/off ratio up to 10(6) after only two printings. Their mobilities were 10 and 30 times higher than those of TFTs fabricated on untreated and low-wettability substrates. The uniformity of printed TFTs was also greatly improved. Inverters were constructed by printed top-gate TFTs, and a maximum voltage gain of 17 at Vdd = 5 V was achieved. The mechanism of such improvements is that the PFO-BT-functionalized sc-SWCNTs are preferably immobilized on the oxygen plasma treated substrates due to the strong hydrogen bonds between sc-SWCNTs and hydroxyl groups on the substrates.

Printed thin-film transistors with functionalized single-walled carbon nanotube inks

A simple and scalable method to print high performance thin-film transistors (TFTs) with high yield by an aerosol jet printer has been developed, using water-based and chemically functionalized single-walled carbon nanotube (SCNT) inks as the semiconducting layer. The absorption spectra, Raman spectra and photoluminescence excitation (PLE) spectra of the SCNT ink indicated that metallic species and small diameter semiconducting species in CoMoCat 76 were effectively eliminated after reaction with 2,2-azobisisobutyronitrile (AIBN) in dimethyformamide (DMF). The functionalized SCNTs were dispersed in 1% SDS aqueous solution, and the as-prepared SCNTs inks were directly used to print TFTs using an aerosol jet printing system without any additional purification. Printed TFTs with high yield were obtained by tuning the concentrations of SCNTs and surfactants in the inks. The printed side-gate and bottom-gate TFTs exhibited the effective mobility of ∼1 cm2 V−1 s−1 and an on/off ratio over 103. In addition, these printed TFTs were hysteresis-free and able to operate at low-voltage when using printable high-capacitance ion gel as the dielectric layer. The new method has opened the route to fabricate all-printed SCNT TFTs on flexible substrate with high performance.

Fabrication and electrical properties of all-printed carbon nanotube thin film transistors on flexible substrates

In this manuscript, we developed a high-performance, printable and water-based semiconducting SCNT ink, and fabricated all-printed chemically functionalized CoMoCat 76 SCNT thin film transistors (TFTs) on flexible substrates via a suite of printing technologies. The metallic species in the pristine CoMoCat 76 SCNTs were effectively eliminated by organic radicals, and the functionalized SCNTs were characterized by UV-Vis-NIR spectroscopy and Raman spectra. The high quality, printable and water-based functionalized SCNT inks were obtained by tuning the ink ingredients, such as the concentrations of surfactants and additives. The printing methods we investigated include inkjet printing, aerosol jet printing and a hybrid with nanoimprinting. Large area source and drain electrode patterns were first fabricated on flexible substrates by a hybrid printing method, and then the optimal SCNT ink was printed on the channel of the TFT devices by ink-jet printing. Subsequently, the silver side-gate electrode and ion gel dielectric layer were deposited by aerosol jet printing. The all-printed flexible TFTs exhibited an effective mobility up to 1.5 cm2 V−1 s−1 and an on/off ratio up to 4 × 103. This work opens up a way to fabricate scalable and all-printed flexible electronics, and it is of benefit to generalize the practical applications of flexible electronics in the future.

Performance improvement of organic light emitting diode with aluminum oxide buffer layer for anode modification

A thin Al2O3 insulating buffer layer deposited on indium tin oxide (ITO) anode by atomic layer deposition has been investigated for organic light-emitting diodes (OLEDs). With an optimal thickness of 1.4u2009nm and low density of structural defects of the Al2O3 film, the OLEDs current efficiency and power efficiency were simultaneously improved by 12.5% and 23.4%, respectively. The improvements in both current and power efficiency mean lower energy loss during holes injection process and better balanced charge injection. To understand the mechanism behind the enhanced performance of OLED by the buffer layer, a series of Al2O3 films of different thicknesses were deposited on ITO anode and characterized. The roughness, sheet resistance, and surface potential (EF′) of the Al2O3 modified ITO were characterized. Also, the properties of Al2O3 films were investigated at the device level. It is believed that the block of holes injection by the Al2O3 buffer layer makes more balanced carrier density in the emitting lay...

Novel ternary bipolar host material with carbazole, triazole and phosphine oxide moieties for high efficiency sky-blue OLEDs

A novel bipolar host material 9-(3-(5-(4-(diphenylphosphoryl)phenyl)-4-phenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole (CTPO) with carbazole, triazole and phosphine oxide moieties was designed and synthesized. CTPO was found to exhibit a high glass transition temperature (Tg = 127 °C), suitable HOMO and LUMO levels (5.65 and 2.42 eV), a high triplet energy (3.06 eV) and excellent bipolar properties. A device with 15 wt% doping concentration showed a low turn-on voltage of 2.5 V and maximum current and power efficiencies of 41.6 cd A−1 and 43.0 lm W−1, respectively. A high efficiency of 40.1 cd A−1 was achieved at the brightness of 100 cd m−2. Even at a high luminance of 1000 cd m−2, the efficiency remained as high as 35.2 cd A−1.

Flexible CMOS-Like Circuits Based on Printed P-Type and N-Type Carbon Nanotube Thin-Film Transistors

P-type and n-type top-gate carbon nanotube thin-film transistors (TFTs) can be selectively and simultaneously fabricated on the same polyethylene terephthalate (PET) substrate by tuning the types of polymer-sorted semiconducting single-walled carbon nanotube (sc-SWCNT) inks, along with low temperature growth of HfO2 thin films as shared dielectric layers. Both the p-type and n-type TFTs show good electrical properties with on/off ratio of ≈105 , mobility of ≈15 cm2 V-1 s-1 , and small hysteresis. Complementary metal oxide semiconductor (CMOS)-like logic gates and circuits based on as-prepared p-type and n-type TFTs have been achieved. Flexible CMOS-like inverters exhibit large noise margin of 84% at low voltage (1/2 Vdd = 1.5 V) and maximum voltage gain of 30 at Vdd of 1.5 V and low power consumption of 0.1 μW. Both of the noise margin and voltage gain are one of the best values reported for flexible CMOS-like inverters at Vdd less than 2 V. The printed CMOS-like inverters work well at 10 kHz with 2% voltage loss and delay time of ≈15 μs. A 3-stage ring oscillator has also been demonstrated on PET substrates and the oscillation frequency of 3.3 kHz at Vdd of 1 V is achieved.

Performance improvement for printed indium gallium zinc oxide thin-film transistors with a preheating process

High performance indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) were fabricated by printing and spin-coating IGZO inks as a semiconductor layer at low temperature annealing. A preheating strategy was developed, which significantly enhanced the performance of IGZO TFTs while the post-annealing temperature was kept constant at 300 °C. It was found that when the temperature of preheating on a hotplate increased from 40 °C to 275 °C, the field effect mobility improved from 0.31 cm2 V−1 s−1 to 4.93 cm2 V−1 s−1 for printed IGZO TFTs and from 1.44 cm2 V−1 s−1 to 7.9 cm2 V−1 s−1 for spin-coated IGZO TFTs. The surface roughness of the IGZO films significantly decreased by increasing the preheating temperature from 40 °C to 95 °C. In addition, the analysis of IGZO film composition revealed that an additional nitrate bidentate configuration appeared in the films with preheating at 275 °C, though the substitution of a N atom for O sub-lattice (N)O was found in the film regardless of the preheating temperature. It was suggested that the performance enhancement was primarily attributed to the improvement in film texture brought about by the preheating strategy. Furthermore, the mobility enhancement at high preheating temperature was also related to the appearance of a bidentate configuration (M–O2–N).

Highly Air-Stable Electron-Transport Material for Ink-Jet-Printed OLEDs

A novel cross-linkable electron-transport material has been designed and synthesized for use in the fabrication of solution-processed OLEDs. The material exhibits a low LUMO level of -3.51u2005eV, a high electron mobility of 1.5×10-5 u2005cm2 u2009V-1 u2009s-1 , and excellent stability. An average 9.3u2009% shrinkage in film thickness was observed for the film after thermal curing. A maximum external quantum efficiency (EQE) of 15.6u2009% (35.0u2005cdu2009A-1 ) was achieved for blue-phosphorescent OLEDs by spin-coating and 13.8u2009% (31.0u2005cdu2009A-1 ) for an ink-jet-printed device, both of which are better than the EQE of a control device prepared by vacuum-deposition (see figure).