Sooji Nam

High-resolution patterns of quantum dots formed by electrohydrodynamic jet printing for light-emitting diodes.

Here we demonstrate materials and operating conditions that allow for high-resolution printing of layers of quantum dots (QDs) with precise control over thickness and submicron lateral resolution and capabilities for use as active layers of QD light-emitting diodes (LEDs). The shapes and thicknesses of the QD patterns exhibit systematic dependence on the dimensions of the printing nozzle and the ink composition in ways that allow nearly arbitrary, systematic control when exploited in a fully automated printing tool. Homogeneous arrays of patterns of QDs serve as the basis for corresponding arrays of QD LEDs that exhibit excellent performance. Sequential printing of different types of QDs in a multilayer stack or in an interdigitated geometry provides strategies for continuous tuning of the effective, overall emission wavelengths of the resulting QD LEDs. This strategy is useful to efficient, additive use of QDs for wide ranging types of electronic and optoelectronic devices.

Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics

We investigated the effects of direct solvent exposure on the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT)/PCBM blend films employed as active layers in, respectively, organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The crystallinity, morphology, and OFET characteristics of the PCBM thin films were significantly influenced by direct exposure to solvent, especially to select alcohols. Control over the nanoscale morphology of the PCBM film, achieved via direct solvent exposure, yielded highly efficient poly(3-hexylthiophene) (P3HT)/PCBM OPVs with a short-circuit current density of 10.2 mA cm−2, an open-circuit voltage of 0.64 V, and a power conversion efficiency of 3.25% under AM 1.5 illumination with a light intensity of 100 mW cm−2. These results indicated that optimal phase separation in the P3HT/PCBM films could be obtained simply by exposing the active layer films for a few seconds to solvent.

Effect of the hydrophobicity and thickness of polymer gate dielectrics on the hysteresis behavior of pentacene-based field-effect transistors

We demonstrate the origin and mechanism of the hysteresis behavior that is frequently observed during the operation of organic field-effect transistors (OFETs) based on polymer gate dielectrics. Although polar functionalities, such as hydroxyl groups, present in the polymer gate dielectrics are known to induce hysteresis, there have only been a few detailed investigations examining how the presence of such end functionalities both at the polymer surface—forming an interface with the semiconductor layer—and in the bulk influences the hysteresis. In this study, we control the hydrophobicity of the polymer by varying the number of hydroxyl groups, and use an ultrathin polymer/SiO2 bilayer and a thick single polymer as the gate dielectric structure so that the hysteresis behavior is divided into contributions from hydroxyl groups present at the polymer surface and in the bulk, respectively. Electrical characterizations of the OFETs, performed both in vacuum (≈10−3 Torr) and in ambient air (relative humidity o...

Hysteresis-free pentacene field-effect transistors and inverters containing poly(4-vinyl phenol-co-methyl methacrylate) gate dielectrics

The influence of hydroxyl groups on the hysteresis of pentacene field-effect transistors (FETs) and metal-insulator-semiconductor diodes containing poly(4-vinyl phenol) and poly(4-vinyl phenol-co-methyl methacrylate) (PVP-PMMA) gate dielectrics was investigated. The electrical characteristics and Fourier transform infrared spectroscopy measurements show that hysteresis is intimately related to the presence of free OH groups in the polymer gate dielectrics. The methyl methacryl moieties in PVP-PMMA minimize residual water in the polymer and form hydrogen bonds with the hydroxyl groups, thus reducing the number of free OH species. Therefore, pentacene FETs and inverters using PVP-PMMA gate dielectrics exhibit high, hysteresis-free performances.

High Efficiency and Optical Anisotropy in Double-Heterojunction Nanorod Light-Emitting Diodes

Recent advances in colloidal quantum dot light-emitting diodes (QD-LEDs) have led to efficiencies and brightness that rival the best organic LEDs. Nearly ideal internal quantum efficiency being achieved leaves light outcoupling as the only remaining means to improve external quantum efficiency (EQE) but that might require radically different device design and reoptimization. However, the current state-of-the-art QD-LEDs are based on spherical core/shell QDs, and the effects of shape and optical anisotropy remain essentially unexplored. Here, we demonstrate solution-processed, red-emitting double-heterojunction nanorod (DHNR)-LEDs with efficient hole transport exhibiting low threshold voltage and high brightness (76,000 cd m(-2)) and efficiencies (EQE = 12%, current efficiency = 27.5 cd A(-1), and power efficiency = 34.6 lm W(-1)). EQE exceeding the expected upper limit of ∼ 8% (based on ∼ 20% light outcoupling and solution photoluminescence quantum yield of ∼ 40%) suggests shape anisotropy and directional band offsets designed into DHNRs play an important role in enhancing light outcoupling.

Double-heterojunction nanorods.

As semiconductor heterostructures play critical roles in todays electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will enable the use of solution-processable colloidal quantum dots in future devices. Such heterojunctions incorporated into colloidal nanorods may be especially promising, since the inherent shape anisotropy can provide additional benefits of directionality and accessibility in band structure engineering and assembly. Here we develop double-heterojunction nanorods where two distinct semiconductor materials with type II staggered band offset are both in contact with one smaller band gap material. The double heterojunction can provide independent control over the electron and hole injection/extraction processes while maintaining high photoluminescence yields. Light-emitting diodes utilizing double-heterojunction nanorods as the electroluminescent layer are demonstrated with low threshold voltage, narrow bandwidth and high efficiencies.

Reduced Water Vapor Transmission Rate of Graphene Gas Barrier Films for Flexible Organic Field-Effect Transistors

Preventing reactive gas species such as oxygen or water is important to ensure the stability and durability of organic electronics. Although inorganic materials have been predominantly employed as the protective layers, their poor mechanical property has hindered the practical application to flexible electronics. The densely packed hexagonal lattice of carbon atoms in graphene does not allow the transmission of small gas molecules. In addition, its outstanding mechanical flexibility and optical transmittance are expected to be useful to overcome the current mechanical limit of the inorganic materials. In this paper, we reported the measurement of the water vapor transmission rate (WVTR) through the 6-layer 10 × 10 cm(2) large-area graphene films synthesized by chemical vapor deposition (CVD). The WVTR was measured to be as low as 10(-4) g/m(2)·day initially, and stabilized at ∼0.48 g/m(2)·day, which corresponds to 7 times reduction in WVTR compared to bare polymer substrates. We also showed that the graphene-passivated organic field-effect transistors (OFETs) exhibited excellent environmental stability as well as a prolonged lifetime even after 500 bending cycles with strain of 2.3%. We expect that our results would be a good reference showing the graphenes potential as gas barriers for organic electronics.

Double-heterojunction nanorod light-responsive LEDs for display applications

Multifunctional displays As we head toward the “Internet of things” in which everything is integrated and connected, we need to develop the multifunctional technology that will make this happen. Oh et al. developed a quantum dot-based device that can harvest and generate light and process information. Their design is based on a double-heterojunction nanorod structure that, when appropriately biased, can function as a light-emitting diode or a photodetector. Such a dual-function device should contribute to the development of intelligent displays for networks of autonomous sensors. Science, this issue p. 616 A dual-function device capable of light harvesting and emission is demonstrated for intelligent displays. Dual-functioning displays, which can simultaneously transmit and receive information and energy through visible light, would enable enhanced user interfaces and device-to-device interactivity. We demonstrate that double heterojunctions designed into colloidal semiconductor nanorods allow both efficient photocurrent generation through a photovoltaic response and electroluminescence within a single device. These dual-functioning, all-solution-processed double-heterojunction nanorod light-responsive light-emitting diodes open feasible routes to a variety of advanced applications, from touchless interactive screens to energy harvesting and scavenging displays and massively parallel display-to-display data communication.

Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular "Solders": Approaching Single Crystal Mobility.

Crystalline silicon-based complementary metal-oxide-semiconductor transistors have become a dominant platform for todays electronics. For such devices, expensive and complicated vacuum processes are used in the preparation of active layers. This increases cost and restricts the scope of applications. Here, we demonstrate high-performance solution-processed CdSe nanocrystal (NC) field-effect transistors (FETs) that exhibit very high carrier mobilities (over 400 cm(2)/(V s)). This is comparable to the carrier mobilities of crystalline silicon-based transistors. Furthermore, our NC FETs exhibit high operational stability and MHz switching speeds. These NC FETs are prepared by spin coating colloidal solutions of CdSe NCs capped with molecular solders [Cd2Se3](2-) onto various oxide gate dielectrics followed by thermal annealing. We show that the nature of gate dielectrics plays an important role in soldered CdSe NC FETs. The capacitance of dielectrics and the NC electronic structure near gate dielectric affect the distribution of localized traps and trap filling, determining carrier mobility and operational stability of the NC FETs. We expand the application of the NC soldering process to core-shell NCs consisting of a III-V InAs core and a CdSe shell with composition-matched [Cd2Se3](2-) molecular solders. Soldering CdSe shells forms nanoheterostructured material that combines high electron mobility and near-IR photoresponse.

Hysteresis-free organic field-effect transistors and inverters using photocrosslinkable poly(vinyl cinnamate) as a gate dielectric

We have fabricated organic field-effect transistors (OFETs) and inverters using photocrosslinkable poly(vinyl cinnamate) (PVCN) as a gate dielectric. The photocrosslinked PVCN dielectric film has superior insulating properties and does not require thermal curing. The high water resistance of the dielectric, which arises because PVCN is hydroxyl group-free, means that the devices were found to be hysteresis-free in all operations. The OFETs with the PVCN dielectric were found to exhibit a carrier mobility of 0.51cm2∕Vs, an on/off ratio of 106, and a subthreshold swing of 0.913V/decade. An organic inverter consisting of two OFETs exhibited a high inverter gain of 17.9.