Carrier Blocking Layers Research Articles

Growth of monolithic full-color GaN-based LED with intermediate carrier blocking layers

Specially designed intermediate carrier blocking layers (ICBLs) in multi-active regions of III-nitride LEDs were shown to be effective in controlling the carrier injection distribution across the active regions. In principle, the majority of carriers, both holes and electrons, can be guided into targeted quantum wells and recombine to generate light of specific wavelengths at controlled current-densities. Accordingly we proposed and demonstrated a novel monolithic InGaN-based LED to achieve three primary colors of light from one device at selected current densities. This LED structure, which has three different sets of quantum wells separated with ICBLs for three primary red-green-blue (RGB) colors, was grown by metal-organic chemical vapor deposition (MOCVD). Results show that this LED can emit light ranging from 460 to 650 nm to cover the entire visible spectrum. The emission wavelength starts at 650 nm and then decreases to 460 nm or lower as the injection current increases. In addition to three primary colors, many other colors can be obtained by color mixing techniques. To the best of our knowledge, this is the first demonstration of monolithic full-color LED grown by a simple growth technique without using re-growth process.

Formation of all-oxide solar cells in atmospheric condition based on Cu2O thin-films grown through SILAR technique

We report formation of Cu2O thin-films through successive ionic layer adsorption and reaction (SILAR) process. We have fully characterized the thin-films grown in atmospheric condition with a non-vacuum process. Post-deposition annealing in oxygen environment has led to formation of CuO thin-films. We have formed pn-junctions with a layer of the p-type copper oxides and another layer of n-type SnO2 nanoparticles. Such pn-junctions, along with other oxide semiconductors as carrier blocking layers acted as all-oxide thin-film solar cells which were formed in an atmospheric condition. We report the effect of NiO as a hole-transporting and electron-blocking layer and ZnO as a buffer layer between p- and n-layers to forbid chemical changes occurring at the interface. From scanning tunneling spectroscopy (STS) measurements we could locate band-edges of the materials with respect to their Fermi energy. The NiO/Cu2O/ZnO/SnO2 heterojunction devices having a staircase-like energy level led to facile transport of electrons and holes to the opposite electrodes and thereby acting as all-oxide thin-film solar cells yielding an energy conversion efficiency in excess to 1%.

Suppression of dark current through barrier engineer for solution-processed colloidal quantum-dots infrared photodetectors

In an attempt to suppress the dark current, the barrier layer engineer for solution-processed PbSe colloidal quantum-dot (CQD) photodetectors has been investigated in the present study. It was found that the dark current can be significantly suppressed by implementing two types of carrier blocking layers, namely, hole blocking layer and electron blocking layer, sandwiched in between two active PbSe CQD layers. Meanwhile no adverse impact has been observed for the photo current. Our study suggests that this improvement resides on the transport pathway created via carrier recombination at intermediate layer, which provides wide implications for the suppression of dark current for infrared photodetectors.

Color stability, wave function overlap and leakage currents in InGaN-based LED structures: the role of the substrate orientation

We present a theoretical study of the performance of InGaN-based light-emitting diodes in terms of built-in fields, wave function overlap, emission wavelength shift with current density and leakage currents. We analyze polar (c-plane), semipolar ((11-22), (10-1-3), (20-21) and (20-2-1)) and non-polar systems. Compared with the c-plane, the semipolar planes show substantial improvements. An exceptionally high wave function overlap is expected for the (10-1-3) orientation, making it an attractive alternative to non-polar devices with low InN content. The (11-22) and (20-21) systems give rise to a natural carrier blocking layer, leading to significantly reduced leakage currents.

Minority carrier barrier heterojunctions for improved thermoelectric efficiency

We propose and demonstrate the beneficial use of minority carrier blocking layers to lessen bipolar conduction at elevated temperatures and increase the Seebeck coefficient in a thermoelectric material. Bipolar conduction, caused by thermally activated intrinsic carriers in semiconductors, causes a detrimental “roll-over” in the Seebeck coefficient and increase in thermal conductivity, measured as a function of temperature. We propose that these negative effects can be lessened or delayed in a small band gap host matrix by incorporating wider band gap semiconductor layers to impede the thermal diffusion of minority carriers over majority carriers. To prove this concept, a composite material grown by molecular beam epitaxy of p-type InAs layers and electron blocking p-type AlSb layers were grown and measured. We show improved thermoelectric properties over just p-type InAs in measurements from 110K to 600K.

Balancing high gain and bandwidth in multilayer organic photodetectors with tailored carrier blocking layers

We present detailed studies of the high photocurrent gain behavior in multilayer organic photodiodes containing tailored carrier blocking layers we reported earlier in a Letter [W. T. Hammond and J. Xue, Appl. Phys. Lett. 97, 073302 (2010)], in which a high photocurrent gain of up to 500 was attributed to the accumulation of photogenerated holes at the anode/organic active layer interface and the subsequent drastic increase in secondary electron injection from the anode. Here, we show that both the hole-blocking layer structure and layer thickness strongly influence the magnitude of the photocurrent gain. Temporal studies revealed that the frequency response of such devices is limited by three different processes with lifetimes of 10 μs, 202 μs, and 2.72 ms for the removal of confined holes, which limit the 3 dB bandwidth of these devices to 1.4 kHz. Furthermore, the composition in the mixed organic donor-acceptor photoactive layer affects both gain and bandwidth, which is attributed to the varying charge transport characteristics, and the optimal gain-bandwidth product is achieved with approximately 30% donor content. Finally, these devices show a high dynamic range of more than seven orders of magnitude, although the photocurrent shows a sublinear dependence on the incident optical power.

Multilayer rapid-drying blade coating for organic solar cells by low boiling point solvents

A bulk heterojunction organic solar cell with poly(3-hexylthiophene) (P3HT) as the donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor is deposited using blade coating on a hot plate at 80 °C with hot air of 70 °C applied from above. In contrast to the 30 min of conventional dichlorobenzene solvent annealing, the rapid-drying blade coating forms a dry film in 1 s. The fabrication throughput is substantially enhanced. The blade-coated film has a smoother surface roughness of 3.5 nm compared with 10.5 nm for solvent annealing; however, the desired phase separation in the 50 nm scale forms despite the rapid drying. A single layer solar cell exhibits power conversion efficiency of 4.1% with blade coating in chlorobenzene, which is the same as solvent annealing device. A multilayer device with carrier blocking layers fabricated entirely of the less toxic toluene also exhibits efficiency of 4.1%.

Improvement in the Photo-Bias Stability of Zinc Tin Oxide Thin-Film Transistors by Introducing a Thermal Oxidized ${\rm TiO}_{2}$ Film as a Hole Carrier Blocking Layer

This paper examined the morphological, structural, and electrical properties of thermal titanium oxide (TiOx) films as a function of the physical thickness. All the thermal TiOx films were assigned to a TiO2 chemical state irrespective of the film thickness. The thinner TiO2 films (≤ 5 nm) showed an amorphous phase, whereas the thicker TiO2 film (≥ 7 nm) had a nanocrystalline structure. This intriguing thickness-dependent crystallization behavior can be explained by the dimensional effect. The mobility of the resulting zinc tin oxide (ZTO) thin-film transistors (TFTs) with a gate-stack of silicon nitride (SiNx) and TiO2/SiNx was monotonously reduced with increasing TiO2 film thickness, which can be attributed to the enhanced Columbic scattering effect of TiO2 films. On the other hand, the negative bias illumination stress instability of the ZTO TFTs can be suppressed significantly to -2.4 V by the adoption of a 5-nm-thick TiO2 film compared with that (-14.4 V) of the ZTO device without a TiO2 film, which is discussed based on the valence band-off structure and the amorphous nature of thermal TiO2 films.

Annealing induced superconductivity in perovskite-related iron-based mixed anion compounds Sr2VFeAsO3−δ

Polycrystalline samples of Sr2VFeAsO3−δ with perovskite-type carrier blocking layers were prepared by a solid state reaction using an alumina tube in a sealed silica tube. Sr2VFeAsO3−δ samples which were prepared as nominally δ = 0 were not synthesized as a single phase, suggesting that finite oxygen deficiency is inevitable for the compound. Although a sample, which exhibits relatively larger lattice volume, does not exhibit bulk superconductivity at temperatures ≥5 K, finite superconducting transition temperatures appear in the sample which exhibits smaller lattice volumes. These results support making the electronic and magnetic phase diagram of Sr2VFeAsO3−δ.

Organic and Inorganic Blocking Layers for Solution‐Processed Colloidal PbSe Nanocrystal Infrared Photodetectors

AbstractA PbSe solution‐processed nanocrystal‐based infrared photodetector incorporating carrier blocking layers is demonstrated, and significant reduction of dark current is achieved. Detectivity values close to 1012 Jones are achieved by using poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butyl))diphenylamine)] (TFB) and ZnO nanocrystals (NC) as the electron blocker and hole blocker, respectively. An improvement in lifetime is also observed in the devices with the ZnO NCs hole blocking layer.

The ultralow driven current ultraviolet-blue light-emitting diode based on n-ZnO nanowires/i-polymer/p-GaN heterojunction

Under the ultralow driven current of 25 μA an ultraviolet (UV)-blue electroluminescence (EL) with a weak defect-related emission could be obtained for the n type (n-) ZnO nanowires (NWs)/insulating (i-) polymer/p-type (p-) GaN light-emitting diode (LED). The i-MgO layer was also explored as a carrier blocking layer for the comparison. For the i-polymer inserted LED the EL emission peak was located at 400 nm, by analyzing the spectra it is believed that the emission includes several compound originations from both ZnO and GaN. The flexible carrier blocking layer, such as i-polymer, could effectively confine the radiative recombination zone.

Analysis of heterostructures for electroluminescent refrigeration and light emitting without heat generation

We perform a self-consistent calculation to investigate the feasibility of electroluminescent refrigeration and light emitting without heat generation in AlGaAs/GaAs heterostructures, taking into account the effects of various recombination processes. The effect of radiation extraction on the cooling capacity and efficiency is also considered. Carrier blocking layers are used to almost eliminate current leakage and improve the injection efficiency to nearly 100%. An analysis is presented of the cooling power density, the cooling efficiency, and the radiative power density as functions of the applied voltage. We also explore the dependences of the cooling related quantities on the thickness and the doping of the active region. A GaAs active layer of thickness 5 μm at 300 K can give a limiting cooling power density of 97 W/cm2. We show that a net cooling power (>several W/cm2) and a high-power light emitting (>100 W/cm2) without heating are feasible. They require an overall efficiency of more than 90%, which is easily achieved if the photon recycling efficiency is high.

Improved White Organic Light-Emitting Diodes with Modified Dual-Emission-Layer Designs

In this work, the improved efficiency of highly pure white-light emission of white organic light-emitting diodes (WOLEDs), consisting of 2-(t-butyl)-9,10-bis(2'-naphthyl) anthracene (TBADN) and 5,6,11,12-tetraphenylnaphthacene (rubrene) has been achieved. Systematic design structures with a progressive improvement in efficiency and color purity have been successfully investigated. A dual emission layer (EML) of TBADN structures with modified rubrene doping, and the insertion of carrier-blocking layers such as tris(8-hydroxyquinoline) aluminum (Alq3), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) have been successfully designed and integrated to improve the device performance. The present cost-effective WOLED design, without the addition of blue dopants, demonstrated a superior high current efficiency of 6.09 cd/A with excellent Commission Internationale De L'Eclairage (CIE) coordinates of (0.32,0.32).

Nanostructured solid-state hybrid photovoltaic cells fabricated by electrostatic layer-by-layer deposition

We report on the fabrication of hybrid organic/inorganic photovoltaic cells utilizing layer-by-layer deposition of water-soluble polyions and nanocrystals. A bulk heterojunction structure was created consisting of alternating layers of the p-conductive polythiophene derivative poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] and n-conductive TiO2 nanoparticles. We fabricated working devices with the heterostructure sandwiched between suitable charge carrier blocking layers and conducting oxide and metal electrodes, respectively. We analyzed the influence of the thickness and nanostructure of the active layer on the cell performance and characterized the devices in terms of static and transient current response with respect to illumination and voltage conditions. We observed reproducible and stable photovoltaic behavior with photovoltages of up to 0.9 V.

High-efficiency blue multilayer polymer light-emitting diode fabricated by a general liquid buffer method

A bright, highly efficient blue polymer light-emitting diode is fabricated by spin-coating with multilayer structure. By introducing a liquid buffer layer, the dissolution problem is overcome successfully. This general method can be applied to fabricate arbitrary polymer multilayer structure, in particular the addition of the carrier blocking layers to balance the electron and hole currents and improve the device efficiency. By adding an electron-blocking layer, the external quantum efficiency for blue emission can be increased to 5.5% (9.1 cd/A) which is even higher than the theoretical limit of 5%. The luminance reaches 26,530 cd/m 2and the Commission Internationale de L’Eclairage (CIE) coordinates is (0.15, 0.23). Further more the lifetime is increased by three times relative to the single layer device.

High drivability μc-Si TFT device with a compound channel layer structure

In this paper, low-temperature deposited microcrystalline silicon (μc-Si) was applied to make a high-performance thin film transistor (TFT). The a-Si film is used as a carrier-blocking layer inserted into μc-Si TFT for suppressing unwanted OFF-state current. This compound TFT can achieve higher driving current (1.47×10 6 A) than the conventional a-Si TFT. Other parameters such as threshold voltage ( V th), sub-threshold swing (SS) and field effective carrier mobility value were 1.34 V, 1.32 V/dec and 1.05 cm 2/V s, respectively. It is noted that all the process for compound TFT are compactable with current a-Si industry.

Effects of carrier barrier on voltage controllable color tunable OLEDs

The effects of the carrier blocking layer on the emission color of the color tunable organic light emitting devices (OLEDs) have been investigated. Color tuning is controlled by the applied voltage. Both the experimental and theoretical results show that inserting a hole blocking layer between two adjacent emission layers will make the color tunable region move toward the wide bandgap emission layer of blue color in our case. By replacing the hole blocking layer with an electron blocking layer, the color tunable region will shift toward the small bandgap emission layer of red. Besides shifting the tunable region toward the pure spectral color, the introduction of the carrier blocking layer can extend the color tunable range.

Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters

We investigate quenching processes which contribute to the roll-off in quantum efficiency of phosphorescent organic light-emitting diodes (OLED's) at high brightness: triplet-triplet annihilation, energy transfer to charged molecules (polarons), and dissociation of excitons into free charge carriers. The investigated OLED's comprise a host-guest system as emission layer within a state-of-the-art OLED structure---i.e., a five-layer device including doped transport and thin charge carrier and exciton blocking layers. In a red phosphorescent device, $N,{N}^{\ensuremath{'}}$-di(naphthalen-2-yl)- $N,{N}^{\ensuremath{'}}$-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium $[\mathrm{Ir}(\mathrm{piq}{)}_{3}]$ as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,${4}^{\ensuremath{'}}$,${4}^{\ensuremath{''}}$-tris ($N$-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium $[\mathrm{Ir}(\mathrm{ppy}{)}_{3}]$. The triplet-triplet annihilation is characterized by the rate constant ${k}_{TT}$ which is determined by time-resolved photoluminescence experiments. To investigate triplet-polaron quenching, unipolar devices were prepared. A certain exciton density, created by continuous-wave illumination, is analyzed as a function of current density flowing through the device. This delivers the corresponding rate constant ${k}_{P}$. Field-induced quenching is not observed under typical OLED operation conditions. The experimental data are implemented in an analytical model taking in account both triplet-triplet annihilation and triplet-polaron quenching. It shows that both processes strongly influence the OLED performance. Compared to the red $\mathrm{Ir}(\mathrm{piq}{)}_{3}$ OLED, the green $\mathrm{Ir}(\mathrm{ppy}{)}_{3}$ device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime $\ensuremath{\tau}$ and a thinner exciton formation zone $w$.

Efficient blue lasers based on gain structure optimizing of vertical-external-cavity surface-emitting laser with second harmonic generation

We report on the demonstration of highly efficient blue lasers based on intracavity frequency doubling vertical-external-cavity surface-emitting lasers (VECSELs). By optimizing the number of InGaAs quantum wells and employing Al0.3Ga0.7As carrier blocking layers in resonant periodic gain structures, we observed the pump-power-limited output power of 4.5W at 920nm for an InGaAs∕GaAs quantum well VECSEL. With a frequency doubling LiB3O5 crystal inside the cavity, 1.9W continuous-wave 460nm blue output was demonstrated. Power conversion efficiencies (=output power∕pump input power) of 22.5% and 9.5% are realized for λ∼920nm and λ∼460nm, respectively.

Enhancing luminescence efficiency of InAs quantum dots at 1.5μm using a carrier blocking layer

The authors report an effective way to enhance the optical efficiency of InAs quantum dots (QDs) on GaAs emitting at the wavelength of 1.5μm. It is found that the loss of holes from QDs to their proximity via the high indium composition InGaAs overgrown layer, which is necessary for achieving long wavelength emission, is the origin of photoluminescence intensity degradation at high temperature. Inserting a 4nm thick Al0.45Ga0.55As layer, acting as a carrier blocking layer, into the GaAs capping matrix can improve the room temperature photoluminescence peak intensity by five and two times for the ground and first excited states, respectively.