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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.

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.

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 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.

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.

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.

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.

Pentacene diodes with 2,3,4,5,6-pentafluorobenzenethiol-coated Au anodes show high current densities of 100 A cm−2 at 3 V with rectification ratios of 107. Using such diodes in rectifier circuits allows an output voltage of 3.8 V to be achieved from a 10 V sinusoidal input at 1 GHz.

We studied the effect of surface polarity of gate dielectric buffer layer on operational stability in organic thin film transistors (OTFTs). By employing four types of polymers with different polarity, we found out the operational stability of OTFTs can be improved with low-polarity polymer gate dielectric buffer layer. With low-polarity polymers, the devices showed good operational stability with longer relaxation time than 106 sec. Also, threshold voltage of the devices shifted less than 2 V under high gate bias stress of 3 MV/cm for two hours. Good stability may be resulted from low surface trap density as well as good grain interconnection of pentacene on low-polarity polymer.

We demonstrate that nanocrystalline Al-doped zinc oxide (n-AZO) thin film used as an electron-extraction layer can significantly enhance the performance of inverted polymer solar cells based on the bulk heterojunction of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM). A synergistic study with both simulation and experiment on n-AZO was carried out to offer a rational guidance for the efficiency improvement. As a result, An n-AZO film with an average grain size of 13 to 22 nm was prepared by a sol-gel spin-coating method, and a minimum resistivity of 2.1 x 10(-3) Omega.cm was obtained for an Al-doping concentration of 5.83 at.%. When an n-AZO film with a 5.83 at.% Al concentration was inserted between the ITO electrode and the active layer (PCDTBT:PC70BM), the power conversion efficiency increased from 3.7 to 5.6%. (C) 2015 Optical Society of America

We demonstrated highly stable multilayer molybdenum disulfide (MoS2) field-effect transistors (FETs) with negligible hysteresis gap (ΔV(HYS) ∼ 0.15 V) via a multiple annealing scheme, followed by systematic investigation for long-term air stability with time (∼50 days) of MoS2 FETs with (or without) CYTOP encapsulation. The extracted lifetime of the device with CYTOP passivation in air was dramatically improved from 7 to 377 days, and even for the short-term bias stability, the experimental threshold voltage shift, outstandingly well-matched with the stretched exponential function, indicates that the device without passivation has approximately 25% larger the barrier distribution (ΔE(B) = k(B)T(o)) than that of a device with passivation. This work suggests that CYTOP encapsulation can be an efficient method to isolate external gas (O2 and H2O) effects on the electrical performance of FETs, especially with low-dimensional active materials like MoS2.

This study examined the relationship between the mobility and bias stability of pentacene-based organic field-effect transistors (OFETs) regarding a self-assembled monolayer (SAM) treatment. For this systematic study, four types of silazane-based SAMs with different alkyl chain lengths in the range of 1–8 were used. Silazane-based SAMs have an advantage of processability due to the mild reaction conditions. The mobility was increased from 0.29 without SAM to 0.46, 0.61, 0.65, and 0.84 cm2/V s after the SAM-treatment with an alkyl chain length of 1, 3, 4, and 8, respectively. On the other hand, inverse proportional relationship was observed between the bias stability and SAM alkyl chain length. Under high gate bias stress (equivalent to electric field of 3 MV/cm) for 2 h, the threshold voltage shift of the OFET was decreased from 12.19 V without SAM to 5.69 V with a short SAM-treatment (alkyl chain length of 1) and 7.14 V with a long SAM-treatment (alkyl chain length of 8). This is the significant finding that there was a tradeoff relationship between the mobility and bias stability of OFETs concerning the SAM alkyl chain length. To overcome this tradeoff, a method for surface engineering using two-step SAM-treatment was introduced. By treating long SAM and short SAM in sequence, both the high mobility and good bias stability were achieved. With two-step SAM-treatment, the OFET showed high mobility as a long SAM-treated OFET and good bias stability as a short SAM-treated OFET.