Carrier Injection Efficiency Research Articles

Fe2O3 hierarchical tubular structure decorated with cobalt phosphide (CoP) nanoparticles for efficient photoelectrochemical water splitting

• Fabrication of large surface area and high crystallinity CoP/SnO 2 :Fe 2 O 3 HTS for PEC application. • The role of SnO 2 and CoP on the oxidation reaction kinetics in the photoanode system was proved. • The highest photocurrent density of 3.54 mA cm −2 at 1.23 V RHE among the Fe 2 O 3 -based photoanodes. Severe charge recombination and poor water oxidation kinetics limit the performance of hematite-based (Fe 2 O 3 ) photoanodes far from their theoretical levels and restrict their applicability for photoelectrochemical (PEC) water splitting devices. In this study, a hierarchical tubular structure (HTS) CoP/SnO 2 :Fe 2 O 3 with a large surface area and well-defined crystalline structures was fabricated via a facile template-engaged redox etching method. The SnO 2 nano-layer served as a dopant source to improve the bulk charge carrier transport efficiency along the Fe 2 O 3 HTS growth axis; therefore, the achieved photocurrent could be up to 2.0 mA cm −2 at 1.23 V RHE (V vs. reversible hydrogen electrode). The CoP nanoparticles (NPs) were introduced as a high-speed hole collector from the light-absorber layer as well as a catalyst to enhance charge carrier injection efficiency on the photoelectrode surface. As a result, CoP/SnO 2 :Fe 2 O 3 shows the highest photocurrent density of 3.50 mA cm −2 (at 1.23 V RHE ) with a negative shift in the onset potential of 180 mV.

Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells.

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

Accurate Extraction of Schottky Barrier Height and Universality of Fermi Level De‐Pinning of van der Waals Contacts

AbstractDue to Fermi level pinning (FLP), metal‐semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal—an essential requirement for several applications, including light‐emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky–Mott limit of de‐pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this study a simple technique is proposed to accurately estimate the SBH at the vdW contact interfaces by circumventing several ambiguities associated with SBH extraction. Using this technique on several vdW contacts, including metallic 2H‐TaSe2, semi‐metallic graphene, and degenerately doped semiconducting SnSe2, it is demonstrated that vdW contacts exhibit a universal de‐pinned nature. Superior ambipolar carrier injection properties of vdW contacts are demonstrated (with Au contact as a reference) in two applications, namely, a) pulsed electroluminescence from monolayer WS2 using few‐layer graphene (FLG) contact, and b) efficient carrier injection to WS2 and WSe2 channels in both n‐type and p‐type field effect transistor modes using 2H‐TaSe2 contact.

Photoelectrochemical investigation of charge injection efficiency for quantum dot light-emitting diode

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

Combined CdS/In2S3 heterostructures with cocatalyst for boosting carriers separation and photoelectrochemical water splitting

CdS with a nature of serious photocorrosion and poor photogenerated electron-hole pairs separation defects would restrict its further development. Herein, we reported that In2S3 and the cocatalyst NiOOH were modified CdS by successive ionic layer adsorption and reaction (SILAR) and photoassisted electrodeposition method. In this ternary system, the type-II heterojunction constructed by CdS and highly dispersed In2S3 (silar 15 cycles) to greatly improve the efficiency of photogenerated carrier separation, while NiOOH could avoid direct contact between CdS/In2S3 and electrolyte to protect it from photocorrosion. Concretely, the CdS/15In2S3 showed the photogenerated carrier injection efficiency (ηinj) and surface carrier separation efficiency (ηsep) achieved 18.4% and 24.6% at 1.23 V vs. RHE, respectively. The CdS/15In2S3/NiOOH (C/15I/N) demonstrated the photostability decreased only by 30.7% under 1 h illumination. This work reveals a feasible measure for sustainable and effective photoelectrochemical (PEC) water splitting via fabricating heterojunction with highly dispersed materials on CdS-based surface.

Nanowire-based telecom-band light-emitting diodes with efficient light extraction

Semiconductor nanowires grown by the gold-free, self-catalyzed, vapor–liquid–solid approach have great potential for realizing monolithically integrable light sources in the telecom band towards integrated optoelectronic circuits. However, light-emitting diodes based on such nanowires operating in the telecom band range remain challenging to achieve, due to difficulties in growing nanowires with suitable materials to cover the telecom band range, their geometrical shape to enhance light extraction efficiency, and their junction structure to suppress current leakage. Here we report telecom-band light-emitting diodes operating at room temperature using InP/InAs axial-heterostructure nanowires grown by self-catalyzed vapor–liquid–solid mode with efficient light extraction. Conically tapered nanowire structures with a single axial junction structure are advantageous for increasing light extraction and carrier injection efficiencies. We realize the structure by making use of a distinctive growth phenomenon in self-catalyzed vapor–liquid–solid growth; i.e. continuously tunable size of catalyst particles during growth.

Enhanced performance of N-polar AlGaN-based deep-ultraviolet light-emitting diodes.

We numerically investigated the performance of N-polar AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) with different Al contents in quantum wells (QWs) and barriers. We found that N-polar structures could improve the maximum internal quantum efficiency (IQE) and suppress the efficiency droop, especially for deep-UV LEDs. Compared to metal-polar LEDs, N-polar ones retained higher IQE values even when the acceptor concentrations in the p-layers were one order of magnitude lower. The enhanced performance originated from the higher injection efficiencies of N-polar structures in terms of efficient carrier injection into QWs and suppressed electron overflow at high current densities.

Robust and Efficient Electrical Injection of Spin-Polarized Carriers from a Zero-Spin-Gap Half-Metal V2NiAl into n-Si at Room Temperature

In recent times, an engrossing and newly discovered class of materials with unique electronic band properties, namely, zero-spin-gap half-metal, has gained acute attraction worldwide because of its...

Quantum Transport in Two-Dimensional WS2 with High-Efficiency Carrier Injection through Indium Alloy Contacts.

Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress. Quantum transport studies are scant as low-quality contacts are intractable at cryogenic temperatures. Here, temperature-dependent transfer length measurements are performed on chemical vapor deposition grown single-layer and bilayer WS2 devices with indium alloy contacts. The devices exhibit low contact resistances and Schottky barrier heights (∼10 kΩ μm at 3 K and 1.7 meV). Efficient carrier injection enables high carrier mobilities (∼190 cm2 V-1 s-1) and observation of resonant tunnelling. Density functional theory calculations provide insights into quantum transport and properties of the WS2-indium interface. Our results reveal significant advances toward high-performance WS2 devices using indium alloy contacts.

Remarkably improved photoelectric performance of AlGaN-based deep ultraviolet luminescence by using dual-triangle quantum barriers

This work investigated the impact of using the dual-triangle quantum barriers (DTQBs) in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs). The DUV-LED with DTQBs notably exhibits higher internal quantum efficiency and light output power as compared to the conventional DUV-LED. The advantage of utilizing DTQBs can be ascribed to the significant improvements of carrier injection efficiency, which modulates the distribution of holes and electrons and consequently increases the radiative recombination rate. More importantly, by looking into the band structure characteristics in the active region of DUV-LEDs, it is found that the hole–electron’s wave-function overlap has been enhanced due to the Band-filling effect by the DTQBs design, even though the electrostatic field and Quantum-confinement-stark-effect increase. Therefore, the proper design of the energy-band-structure engineering of QBs can enhance photoelectric performance of DUV-LEDs.

Enhancement of the optoelectronic characteristics of deep ultraviolet nanowire laser diodes by induction of bulk polarization charge with graded AlN composition in AlxGa1-xN waveguide

AlGaN based Nanowire laser diodes (NW-LDs) grown on sapphire substrates have strong polarization induced electric field. Such electric field has the ability to degrade the optoelectronic characteristics of deep ultraviolet (DUV) NW-LD. In this work, a graded AlN composition AlxGa1-xN waveguide (WG) layer is used for the enhancement of DUV NW-LD performance. Grading of WG induces bulk polarization charges which compensates the effect of polarization induced electric field. According to the calculated optoelectronic characteristics of NW-LD, it is found that grading of n-type WG (n-WG) increases the optical confinement factor (OCF) by 82%. Fortunately, the proposed graded n-WG structure suppresses the leakage of optical field from active region and enhances carrier injection efficiency. Furthermore, if both n-WG and p-WG graded layers are used, the improvement is not obvious because of the current leakage from graded p-WG. Thus, graded n-WG based NW-LD gives highest 33.5% OCF with the lowest 7 mA and 4.59 V threshold current and voltage respectively.

Nanocomposite Electron-Transport Layer Incorporated Highly Efficient OLED

The realization of highly efficient devices depends on efficient charge carrier injection, employment of materials, and proper fabrication methodologies. However, it is difficult to manufacture hig...

Preparation of Sb2O3/Sb2S3/FeOOH composite photoanodes for enhanced photoelectrochemical water oxidation

Abstract A novel Sb2O3/Sb2S3/FeOOH photoanode was fabricated via a simple solution impregnation method along with chemical bath deposition and post-sulfidation. The X-ray diffractometry, Raman measurement, and X-ray photoelectron spectroscopy show that the Sb2O3/Sb2S3/FeOOH thin films are successfully prepared. SEM−EDS analyses reveal that the surface of Sb2O3/Sb2S3 thin films becomes rough after the immersion in the FeCl3 solution. The optimized impregnation time is found to be 8 h. The FeOOH co-catalyst loaded Sb2O3/Sb2S3 electrode exhibits an enhanced photocurrent density of 0.45 mA/cm2 at 1.23 V versus RHE under simulated 1 sun, which is approximately 1.41 times compared to the photocurrent density of the unloaded one. Through the further tests of UV−Vis spectroscopy, the electrochemical impedance spectra, and the PEC measurements, the enhancement can result from the increased light-harvesting ability, the decreased interface transmission impedance, and the remarkably enhanced carrier injection efficiency.

Efficient Mid-Infrared Germanium Variable Optical Attenuator Fabricated by Spin-on-Glass Doping

We demonstrated an efficient carrier-injection germanium (Ge) variable optical attenuator (VOA) operating at 1.95 μm on a Ge-on-insulator (GeOI) platform for mid-infrared (MIR) integrated photonics. Numerical analysis illustrated that the Ge VOA can realize a significantly higher modulation efficiency than a Si VOA owing to the larger free-carrier absorption in Ge at MIR wavelengths. We proved that by introducing phosphorus-doped spin-on-glass (P-SOG) doping technology into the formation of a Ge lateral PIN junction, we can improve the doping concentration in the n+ region as compared with P ion implantation, resulting in improved modulation efficiencies of 380–460 dB/A for Ge VOAs. The P-SOG-doped Ge VOA exhibited a comparable modulation efficiency to a Si VOA operating at 1.55 μm despite the shorter carrier lifetime of 0.4–0.8 ns determined by numerical fitting. Thus, the Ge VOA formed by SOG doping has promising applications in communication and sensing at MIR wavelengths.

Influence of the excitation density and temperature on the optical properties of type I InAs/GaAsSb quantum dots

In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs1−xSbx (x = 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing excitation density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs- capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the excitation power density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.

Exploring the Impact of Counterion Chemistry on Carrier Doping of Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (s-SWCNT) are viewed as a promising candidate for optical, electronic, and energy conversion applications. Often the performance of carbon nanotubes in these applications is dependent on being able to control the interplay between their optical and electronic properties. One strategy toward achieving this control is tuning the position of the Fermi level through chemical doping to inject charge carriers into the electronic density of states.We have previously demonstrated that p-type doping using the one-electron oxidant, triethyloxonium hexachloroantimonate (OA) results in very efficient injection of free carriers into near-monochiral s-SWCNT networks. Here we build on our previous work by employing a series of charge-transfer dopants based on boron clusters appended with various organic moieties that control the redox properties of the dopant. These dopants are unique, since their chemical structure means that the electron that is extracted from the nanotube is localized on the boron cluster and spatially separated from hole density injected into the carbon nanotube. We explore the charge-transfer doping process using these boron-cluster dopants, the resulting charge-carrier transport properties of the doped s-SWCNT networks, and make a comparison to networks doped using either OA or 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We will present spectroscopic and bulk conductivity measurements of charge-transport in high-mobility s-SWCNT networks and will discuss the impact that the chemistry and structure of the dopant counterion has on charge transport and the implications for electronic devices comprised of these tailored s-SWCNT networks.

Highly Rectifying Heterojunctions Formed by Annealed ZnO Nanorods on GaN Substrates.

We study the effect of thermal annealing on the electrical properties of the nanoscale p-n heterojunctions based on single n-type ZnO nanorods on p-type GaN substrates. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and on the substrates locally patterned by focused ion beam lithography. Electrical properties of single nanorod heterojunctions are measured with a nanoprobe in the vacuum chamber of a scanning electron microscope. The focused ion beam lithography provides a uniform nucleation of ZnO, which results in a uniform growth of ZnO nanorods. The specific configuration of the interface between the ZnO nanorods and GaN substrate created by the focused ion beam suppresses the surface leakage current and improves the current-voltage characteristics. Further improvement of the electrical characteristics is achieved by annealing of the structures in nitrogen, which limits the defect-mediated leakage current and increases the carrier injection efficiency.

Vacancy defect modulation in hot-casted NiO film for efficient inverted planar perovskite solar cells

Abstract Nickel oxide (NiOx) has exhibited great potential as an inorganic hole transport layer (HTL) in perovskite solar cells (PSCs) due to its wide optical bandgap and superior stability. In this study, we have modulated the Ni2+ vacancies in NiOx film by controlling deposition temperature in a hot-casting process, resulting the change of coordination structure and charge state of NiOx. Moreover, the change of the HOMO level of NiOx makes it more compatible with perovskite to decrease energy losses and enhance hole carrier injection efficiency. Besides, the defect modulation in the electronic structure of NiOx is beneficial for increasing the electrical conductivity and mobility, which are considered to achieve the balance of charge carrier transport and avoid charge accumulation at the interface between perovskite and HTL effectively. Both experimental analyses and theoretical calculations reveal the increase of nickel vacancy defects change the electronic structure of NiOx by increasing the ratio of Ni3+/Ni2+ and improving the p-type characteristics. Accordingly, an optimal deposition temperature at 120 °C enabled a 36.24% improvement in the power conversion efficiency compared to that deposited at room temperature (25 °C). Therefore, this work provides a facile method to manipulate the electronic structure of NiOx to improve the charge carrier transport and photovoltaic performance of related PSCs.

Analysis of dominant non-radiative recombination mechanisms in InGaN green LEDs grown on silicon substrates

Relationship between the external quantum efficiency (EQE) curves and the dominant non-radiative recombination mechanisms of InGaN green LEDs grown on silicon substrates were investigated. Through the analysis of the ABC+f(n) model, the significant drop in EQE at low current levels is due to an increasingly defect-related Shockley-Read-Hall (SRH) recombination. Under extremely low current densities, the defect traps can even become the dominant channel for the leakage current through the tunneling process, thereby reducing the efficiency of carrier injection into the active region. These observations were further supported by the carrier lifetime measurement. However, this fails to explain the droop in EQE at high current densities, especially when SRH recombination has been saturated. Our results show that carrier leakage has becomes dominant at high current density when Auger recombination has been less impossible. Reduced carrier leakage may lead to increased carrier injection efficiency, which in turn alleviates EQE droop.

Core–Shell Tunnel Junction Nanowire White-Light-Emitting Diode

We have demonstrated a new class of phosphor-free white LEDs with the use of tunnel junction structure in nonpolar core-shell InGaN nanowires. It is confirmed that the tunnel junction based nanowire LEDs can eliminate the use of the resistive p-GaN:Mg contact layer, leading to significantly enhanced hole injection and dramatically reduced voltage loss. The nonpolar core-shell nanowire heterostructure showed the enhanced carrier injection efficiency through the widened shell n-GaN contact area. The TEM analysis verified that the core-shell Al tunnel junction layers were uniformly grown on nonpolar surfaces of the GaN wurtzite crystal nanowire structure. We have also showed the monolithic integration of multiple-color emission on a single chip by using the multiple-stacked tunnel junction core-shell nanowire heterostructure. Compared to the conventional film based quantum well LEDs, the demonstrated nonpolar core-shell tunnel junction nanowire LEDs will be a very promising candidate for future solid-state lighting applications as well as phosphor-free white LEDs.