Search

Semiconductor nanocrystals represent a promising class of solution-processable optical-gain media that can be manipulated via inexpensive, easily scalable colloidal techniques. Due to their extremely small sizes (typically

In this study, we investigate phosphomolybdic acid (PMA), which allows solution processing of quantum dot light-emitting diodes. With its low cost, easy solution processes, and excellent physical and optical properties, PMA is a potential candidate as the hole injection layer (HIL) in optoelectronic devices. We evaluate the physical and electrical properties of PMA using various solvents. The surface morphology of the PMA film was improved using a solvent with appropriate boiling points, surface tension, and viscosity to form a smooth, pinhole-free film. The energy level was regulated according to the solvent, and PMA with the appropriate electronic structure provided balanced charge carrier transport in quantum dot electroluminescent (QD-EL) devices with enhanced efficiency. A device using PMA dissolved in cyclohexanone was demonstrated to exhibit improved efficiency compared to a device using PEDOT:PSS, which is a conventional solution HIL. However, the stability of PMA was slightly poorer than PEDOT:PSS; there needs to be further investigation.

Colloidal quantum-dot-based light-emitting diodes (QD-LEDs) have gained tremendous attention as great candidates to potentially replace current emissive display technologies. The luminescence efficiency of a QD LED has increased rapidly in the past decade; this was triggered by the use of metal oxides in the charge transport layers, particularly zinc oxide (ZnO) for the electron transport layer (ETL). However, the ZnO ETL often results in undesirable device performance such as efficiency roll-off and poor device stability because of excessive electron injection into the QD emissive layer. Here, we explore solution-processable tin dioxide (SnO2) nanoparticles (NPs) as alternatives to ZnO NPs for the ETL in QD-LEDs. We evaluated the thin-film quality and electrical performance of SnO2 NPs and then applied them to the ETL for constructing QD-LEDs. As a result of the smooth surface morphology, moderate electron-transport ability, and lower carrier concentration compared to ZnO NPs, the QD-LED with SnO2 NP-ETL exhibited improved performance in terms of lower turn-on and operating voltages, maximum luminance, improved efficiency roll-off, and improved power efficiency over the reference device with the ZnO NP-ETL. This shows promising potential for SnO2 NPs in optoelectronic applications.

Self-assembled monolayers (SAMs) of organic molecules are frequently employed to improve the electrical performance of organic field-effect transistors (OFETs). However, the relationship between SAM properties and OFET performance has not been fully explored, leading to an incomplete understanding of the system. This study investigates the effect of the SAM alkyl chain length on the crystalline phase of pentacene films and OFET performance. Two types of SAMs—with alkyl chain lengths of 10 (decyltrichlorosilane, DTS) and 22 (docosyltrichlorosilane, DCTS)—were examined, and variations in the performance of pentacene-based OFETs with the nature of the SAM treatment were observed. Despite the similar surface morphologies of the pentacene films, field-effect mobility in the DCTS-treated OFET was twice that in the DTS-treated OFET. To find the reason underlying the dependence of the OFET’s electrical performance on the SAM alkyl chain length, X-ray diffraction measurements were conducted, followed by a phase analysis of the pentacene films. Bulk and thin-film phases were observed to coexist in the pentacene film grown on DTS, indicating several structural defects in the film; this can help explain the dependence of the OFET electrical performance on the SAM alkyl chain length, mediated by the different crystalline phases of pentacene.

Due to their anisotropic structure, quantum rods (QRs) feature unique properties that differ from quantum dots, such as suppression of non-radiative Auger recombination and linearly polarized light emission. Despite many potential advantages, the progress of QR-based light-emitting diodes (QR-LEDs) is left behind due to the difficulty in aligning QRs. In this study, polarized electroluminescence emission is reported in high-performance QR-LEDs by employing the Langmuir-Blodgett (LB) technique. The adoption of the LB technique successfully produces a highly dense and smooth QR film with a high degree of alignment. As a result, the aligned QR films exhibit polarized photoluminescence emission with a degree of linear polarization of 2.1. Advantageous features of the LB technique, such as nondestructiveness, precise thickness control, and the nonnecessity of an additional matrix material, allow to fabricate QR-LEDs with the same procedure as the standard spin coating-based scheme. The device is fabricated via the LB technique, which shows excellent device performance, such as the low turn-on voltage of 1.8 V, peak luminance of 56 287 cd m-2 , and peak external quantum efficiency (EQE) of 10.33%. Furthermore, these devices clearly exhibit an indication of polarized electroluminescence emission, which opens new opportunities for QRs in display technologies.

The emerging technology of colloidal quantum dot electronics provides an opportunity for combining the advantages of well-understood inorganic semiconductors with the chemical processability of molecular systems. So far, most research on quantum dot electronic devices has focused on materials based on Pb- and Cd chalcogenides. In addition to environmental concerns associated with the presence of toxic metals, these quantum dots are not well suited for applications in CMOS circuits due to difficulties in integrating complementary n- and p-channel transistors in a common quantum dot active layer. Here, we demonstrate that by using heavy-metal-free CuInSe2 quantum dots, we can address the problem of toxicity and simultaneously achieve straightforward integration of complimentary devices to prepare functional CMOS circuits. Specifically, utilizing the same spin-coated layer of CuInSe2 quantum dots, we realize both p- and n-channel transistors and demonstrate well-behaved integrated logic circuits with low switching voltages compatible with standard CMOS electronics., Designing efficient toxic-element-free technologies in solution-processable CMOS electronics remains a challenge. Here, the authors demonstrate integrated logic CMOS circuits based on heavy-metal-free colloidal CuInSe2 quantum dots with low switching voltages and with degradation-free performance on month-long time scales.

Electrical annealing (EA) is one of the post treatments to enhance the electrical performances of organic devices. To date, the improvements using EA have only been reported for the solution-processed devices because its mechanism has been known as the alignments of ionic impurities or polymer chains. In this paper, we applied EA to thermally evaporated organic diodes which not have ionic impurities or polymer chains. After EA, the turn-on voltage of the diode was reduced, and the forward-bias current of the diode was increased without changing the reverse-bias current, resulting in an improvement of the cutoff frequency of the rectifier. In addition, we proposed a new mechanism to explain why the EA can be applied to the thermally evaporated organic devices. Based on time-of-flight secondary ion mass spectrometry and impedance spectra, we suggest that this improvement is due to the creation of a MoO3:pentacene mixed layer, leading to ease of charge injection. We believe that our finding will be helpful to understand change at the organic/metal interfaces and useful to apply a wide range of organic devices such as organic photovoltaics, organic light-emitting diodes, and organic thin-film transistors.

Interface engineering for the improved injection properties of all-solution-processed n-type organic field-effect transistors (OFETs) arising from the use of an inkjet-printed ZnO electron injection layer were demonstrated. The characteristics of ZnO in terms of electron injection and transport were investigated, and then we employed ZnO as the electron injection layer via inkjet-printing during the fabrication of all-solution-processed, n-type OFETs. With the inkjet-printed ZnO electron injection layer, the devices exhibited approximately five-fold increased mobility (0.0058 cm2/V s to 0.030 cm2/V s), more than two-fold increased charge concentration (2.76 × 1011 cm−2 to 6.86 × 1011 cm−2), and two orders of magnitude reduced device resistance (120 MΩ cm to 3 MΩ cm). Moreover, n-type polymer form smoother film with ZnO implying denser packing of polymer, which results in higher mobility.

A recent study has shown that the use of a semiconducting surfactant in hybrid bulk heterojunction solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxyl)-1,4-phenylene vinylene] and zinc oxide significantly increases their performance. To further study the mechanism underlying the improved performance resulting from the addition of a semiconducting surfactant, we compared the temperature and light intensity dependence of the current voltage characteristics and impedance spectra of devices that used no surfactant, an insulating oleic acid (OA) surfactant, or a semiconducting 2-(2-ethylhexyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinoline-6,7-dicarboxylic acid (BQ) surfactant. The temperature and light intensity dependence data enabled us to extract the activation energy for different devices, and we measured the junction resistance through impedance spectroscopy. The device that used BQ had significantly lower activation energy (9–12 meV) than that without surfactant or oleic acid (17–21 meV), thus indicating improved charge transport and consequently an increased short-circuit current. Furthermore, the impedance characteristics showed that the injection resistance, when the BQ surfactant was used, was lower than that when no surfactant or oleic acid was used. These experiments provide further evidence that the use of semiconducting surfactants is essential to improve the performance of hybrid bulk heterojunction solar cells.

In this work, we analyze characteristics of Ohmic, Schottky forward and reverse contact through a low-frequency noise (LFN) measurement, combining two types of metals (Pd and Au) as the source and drain (S/D) contacts that enable

We have investigated the possibility of fabricating quantum dot light-emitting diodes (QLEDs) using inkjet printing technology, which is the most attractive method for the full-color patterning of QLED displays. By controlling the quantum dot (QD) ink formulation and inkjet printing condition, we successfully patterned QLED pixels in the 60-in ultrahigh definition TV format, which has a resolution of 73 pixels per inch. The inkjet-printed QLEDs exhibited a maximum luminance of 2500 cd/m2. Although the performance of inkjet-printed QLEDs is low compared with that of QLEDs fabricated using the spin-coating process, our results clearly indicate that the inkjet printing technology is suitable for patterning QD emissive layers to realize high-resolution, full-color QLED displays.

High-efficiency organic photovoltaics (OPV) with a PTB7: PC70BM photoabsorption layer were fabricated by adopting Li- and Cd-doped ZnO electron extraction layers. For devices with both Li- and Cd-doped ZnO, the performances were significantly improved, including the short-circuit current, open-circuit voltage, fill factor (FF), and power conversion efficiency. Furthermore, film characterization with techniques including X-ray diffraction, high-resolution transmission electron microscopy, and resistivity analysis was performed. The results suggest that the improved electrical properties of Li- and Cd-doped ZnO electron extraction layers with a doping ratio of approximately 6 at% are highly effective in achieving a power conversion efficiency of up to 8%.

Large hysteresis behaviors on molybdenum disulfide (MoS2) field-effect transistors (FETs) without passivation have been typically observed, which can be one of the obstacles to understanding the intrinsic properties of MoS2 layers. Reported herein is the fact that the negligible hysteresis gap of MoS2 can be achieved through thermal annealing without any further passivation layer. The hysteresis gap of the MoS2 FETs with thermal annealing was reduced to 0.08 V, and the device showed good field-effect mobility (23.3 cm2/V s) and a high on-to-off ratio (∼107). The hysteresis-free MoS2 FETs were systematically investigated by analyzing both the contact property and the bias stability. Furthermore, low contact resistance (6.21 Ω cm) and excellent bias stability with a 1.29 × 108 sec relaxation time were obtained.

We investigated the contact resistance of the OTFTs based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) as a representative organic semiconductors with deep HOMO level. We confirmed a dramatic decrease in the contact resistance with the MoOx injection layer by transmission line method and y-function method.

We report the improved oscillation frequency of the organic complementary ring oscillators by balancing mobilities of p- and n-type organic field-effect transistors (OFETs) with a functional polymer interfacial layer. By employing poly (methyl methacrylate) (PMMA) as the interfacial layer, the balanced performance of the p- and n-type OFETs was achieved, which resulted in the improved performance of the complementary inverters. With the PMMA interfacial layer, the noise margin was increased from 35 % to 80 % of 1/2 supply voltage, and the maximum gain was 1.5 times higher than the one of the complementary inverter without the interfacial layer. The oscillation frequency of the organic complementary ring oscillators was remarkably improved from 106.7 Hz to 1.4 kHz. The polymer interfacial layer to balance the performance of p- and n-type OFETs is found to be a simple and efficient way to facilitate the operation of the organic complementary ring oscillators.

Switching on quantum dot lasers The optical properties of quantum dots can be tailored with alterations to their size and composition. Developing quantum dots as low-threshold laser sources requires overcoming problems associated with carrier recombination and stability. Kozlov et al. report success in this direction by demonstrating that a mix of processing and postsynthesis charging can overcome these problems. Synthesis of compositionally graded core-shell quantum dots followed by a carrier charging step provides stable, low-threshold lasing. Science , this issue p. 672

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS

A vapor-phase-processed fluorinated silazane self-assembled monolayer (SAM), 1,3-bis(trifluoropropyl)-1,1,3,3-tetramethyldisilazane (FPDS), was introduced as a surface modifier for pentacene-based organic thin-film transistors (OTFTs). A remarkable improvement in the field effect mobility from 0.25 cm2/Vs (without SAM-treatment) to 0.42 cm2/Vs (with FPDS-treatment) was observed, which was attributed to the better pentacene growth on a hydrophobic surface. A significant reduction in the contact resistance was also observed by FPDS treatment due to the improved bulk conductivity and diminished charge trapping at the gate dielectric surface by the SAM treatment. In addition, FPDS treatment efficiently improved the bias stability of the OTFTs; the drain-to-source current degradation by the bias stress was greatly reduced from 80% to 50% by FPDS treatment, and the characteristic time for charge trapping of the FPDS treated OTFTs was approximately one order of magnitude larger than that of the OTFTs without SAM treatment.

New thermally curable organic/inorganic hybrid polymers were designed and synthesized as insulators for organic thin film transistors (OTFTs). Cyclotetrasiloxane (CTS) was reacted with allyl alcohols through a hydrosilylation reaction in the presence of a catalytic amount of Pt(0) to give the alcohol-functionalized cyclotetrasiloxane (CTS-OH). The synthesized CTS-OH was then thermally cured with hexamethoxymethylmelamine (HMMM) at 80 °C in the presence of a catalytic amount of p-toluenesulfonic acid to form a hard and smooth thin film composed of a highly cross-linked network polymers (CTS-MMs). Devices with indium-tin-oxide/CTS-MM/Au configuration were fabricated to investigate electrical properties of the polymers such as capacitance, dielectric constant, and leakage current. The CTS-MM showed lower leakage current level than the well-known curable insulator consisting of poly(vinylphenol) (PVP) and a melamine derivative. Pentacene-based OTFTs were fabricated using the synthesized insulators as the gate dielectric layers, and their performances were compared to those of the device fabricated using PVP. The OTFTs fabricated using CTS-MM showed higher field-effect mobility than that of the PVP. The hole mobility of the pentacene based-OTFTs fabricated using CTS-MM as gate dielectric was 0.36 cm2/V s and the on/off current ratio was >107.

New low-temperature curable organic/inorganic hybrid polymers were designed and synthesized as gate dielectrics for organic thin-film transistors (OTFTs). Allyl alcohols were introduced to polyhedral oligomeric silsesquioxane (POSS) via hydrosilyation to produce an alcohol-functionalized POSS derivative (POSS-OH). POSS-OH was then reacted with hexamethoxymethylmelamine at carrying molar ratios at 80 °C in the presence of a catalytic amount of p-toluenesulfonic acid to give highly cross-linked network polymers (POSS-MM). The prepared thin films were smooth and hard after the thermal cross-linking reaction and had very low leakage currents (

A polyhedral oligomeric silsesquioxane (POSS)-based insulating material with photocurable propyl-cinnamate groups (POSS-CYNNAM) was designed and synthesized through simple single step reaction for use as a gate dielectric in organic thin-film transistors (OTFT). POSS-CYNNAM was soluble in common organic solvents and formed a smooth thin film after spin-casting. A thin film of POSS-CYNNAM was cross-linked and completely solidified under UV irradiation without the use of additives such as photoacid generators or photoradical initiators. ITO/insulator/Au devices were fabricated and characterized to measure the dielectric properties of POSS-CYNNAM thin films, such as leakage current and capacitance. A pentacene-based OTFT using the synthesized insulator as the gate dielectric layer was fabricated on the transparent indium tin oxide (ITO) electrode, and its performance was compared to OTFTs using thermally cross-linked poly(vinyl phenol) (PVP) as the insulator. The fabricated POSS-CYNNAM OTFT showed a comparable performance to devices based on the PVP insulator with 0.1 cm 2 /Vs of the field effect mobility and 4.2 × 10 5 of an on/off ratio.

We demonstrated modulation of charge carrier densities in all-solution-processed organic field-effect transistors (OFETs) by modifying the injection properties with self-assembled monolayers (SAMs). The all-solution-processed OFETs based on an n-type polymer with inkjet-printed Ag electrodes were fabricated as a test platform, and the injection properties were modified by the SAMs. Two types of SAMs with different dipole direction, thiophenol (TP) and pentafluorobenzene thiol (PFBT) were employed, modifying the work function of the inkjet-printed Ag (4.9 eV) to 4.66 eV and 5.24 eV with TP and PFBT treatments, respectively. The charge carrier densities were controlled by the SAM treatment in both dominant and non-dominant carrier-channel regimes. This work demonstrates that control of the charge carrier densities can be efficiently achieved by modifying the injection property with SAM treatment; thus, this approach can achieve polarity conversion of the OFETs.

In this study, we investigated the effect of the donor/acceptor mixing ratio and the substrate temperature (T(SUB)) during the co-deposition process on the performance of bulk heterojunction organic photovoltaic cells. We found that the ratio of dispersed donor islands (less than 10 nm), which hinders charge carrier transport, increased as the donor concentration (C(D)) increased in the film processed at room temperature. By contrast, the donor cluster (larger than 10 nm), providing percolation paths for the carriers, was enlarged in the film containing a high C(D) fabricated at high T(SUB) (70 degrees C). This enhanced phase separation in the mixed layer led to an improved fill factor and a decreased activation energy of the short-circuit current (J(SC)). Therefore, we demonstrated a 23% improvement in the device performance by employing an elevated T(SUB) and optimized mixing ratio in comparison with the device fabricated at room temperature.

The effect of a nanoscale boron subphthalocyanine chloride (SubPc) interfacial layer on the performance of inverted polymer solar cells based on poly (3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) was studied. When a 1 nm SubPc layer was introduced between the active layer (P3HT:PC(71)BM) and MoO(x) in the device with ITO/ZnO/P3HT:PC(71)BM/SubPc/MoO(x)/Al configuration, the power conversion efficiency (PCE) was increased from 3.42 (without SubPc) to 3.59%. This improvement is mainly attributed to the enhanced open-circuit voltage from 0.62 to 0.64 V. When the Flory-Huggins interaction parameters were estimated from the solubility parameters through the contact angle measurement, it revealed that the interaction between SubPc and PC(71)BM is more attractive than that between SubPc and P3HT at the interface of P3HT:PC(71)BM/SubPc, through which charges are well transported from the active layer to the anode. This is supported by a decrease of the contact resistance from 5.49 (SubPc 0 nm) to 0.94 MΩ cm (SubPc 1 nm). The photoelectron spectra provide another evidence for the enhanced PCE, exhibiting that the 1 nm thick SubPc layer extracts more photoelectrons from the active layer than other thicknesses.

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS2 ) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS2 FETs by using self-assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS2 FETs in an enhancement mode with preservation of electrical parameters such as field-effect mobility, subthreshold swing, and current on-off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature-dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS2 FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 VDD . More impressively, quantum dot light-emitting diodes, driven by enhancement mode MoS2 FETs, stably demonstrate 120 cd m-2 at the gate-to-source voltage of 5 V, exhibiting promising opportunities for future display application.

The main issue of the organic rectifier, the key element in radio frequency identification tags, is to improve forward-bias current density of an organic diode in the rectifier, which increases the frequency response of the rectifier. One approach to achieve high current density is inserting a hole injection layer (HIL) between the anode and the active layer to enhance the charge injection efficiency. Here we study the effect of HILs in pentacene rectifying diodes. Three different hole injection layers are applied to the pentacene diode: molybdenum trioxide (MoO3), 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN), and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). A rectifier consists of the diode with a capacitor. The results show that current density of diodes with HILs is increased by more than three orders of magnitude compared with the diode without a HIL. The diode with MoO3 and that with HAT-CN shows similar forward bias current density, while that of the diode with PEDOT:PSS is slightly lower than those. Finally, the output voltage of the rectifier with a HIL is 4.6 V at 100 MHz when input voltage of 10 V is applied.

n-type organic thin film transistors (OTFTs) with high operational stability was demonstrated. The operational stability of N,N`-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13)-based OTFTs was improved by engineering organic semiconductor-gate insulator interface with hydrophobic polymers. Under high gate bias stress (equivalent to the electric field of 3MV/cm) for two hours, OTFTs with hydrophobic polymers showed small threshold voltage shift around 2 V. Moreover, the electron mobility was improved from 0.038 to 0.14–0.15 cm 2 /Vs with hydrophobic polymers.