Promise For Optoelectronic Devices Research Articles

Stretchable OLEDs based on a hidden active area for high fill factor and resolution compensation

Stretchable organic light-emitting diodes (OLEDs) have emerged as promising optoelectronic devices with exceptional degree of freedom in form factors. However, stretching OLEDs often results in a reduction in the geometrical fill factor (FF), that is the ratio of an active area to the total area, thereby limiting their potential for a broad range of applications. To overcome these challenges, we propose a three-dimensional (3D) architecture adopting a hidden active area that serves a dual role as both an emitting area and an interconnector. For this purpose, an ultrathin OLED is first attached to a 3D rigid island array structure through quadaxial stretching for precise, deformation-free alignment. A portion of the ultrathin OLED is concealed by letting it ‘fold in’ between the adjacent islands in the initial, non-stretched condition and gradually surfaces to the top upon stretching. This design enables the proposed stretchable OLEDs to exhibit a relatively high FF not only in the initial state but also after substantial deformation corresponding to a 30% biaxial system strain. Moreover, passive-matrix OLED displays that utilize this architecture are shown to be configurable for compensation of post-stretch resolution loss, demonstrating the efficacy of the proposed approach in realizing the full potential of stretchable OLEDs.

Low-Temperature Cross-Linked Hole Transport Layer for High-Performance Blue Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs), a kind of promising optoelectronic device, demonstrate potential superiority in next-generation display technology. Thermal cross-linked hole transport materials (HTMs) have been employed in solution-processed QLEDs due to their excellent thermal stability and solvent resistance, whereas the unbalanced charge injection and high cross-linking temperature of cross-linked HTMs can inhibit the efficiency of QLEDs and limit their application. Herein, a low-temperature cross-linked HTM of 4,4'-bis(3-(((4-vinylbenzyl)oxy)methyl)-9H-carbazol-9-yl)-1,1'-biphenyl (DV-CBP) with a flexible styrene side chain is introduced, which reduces the cross-linking temperature to 150 °C and enhances the hole mobility up to 1.01 × 10-3 cm2 V-1 s-1. More importantly, the maximum external quantum efficiency of 21.35% is successfully obtained on the basis of the DV-CBP as a cross-linked hole transport layer (HTL) for blue QLEDs. The low-temperature cross-linked high-mobility HTL using flexible side chains could be an excellent alternative for future HTL development.

Effective Charge Collection of Electron Transport Layers for High-Performance Quantum Dot Infrared Solar Cells.

Infrared (IR) solar cells, capable of converting low-energy IR photons to electron-hole pairs, are promising optoelectronic devices by broadening the utilization range of the solar spectrum to the short-wavelength IR region. The emerging PbS colloidal quantum dot (QD) IR solar cells attract much attention due to their tunable band gaps in the IR region, potential multiple exciton generation, and facile solution processing. In PbS QD solar cells, ZnO is commonly utilized as an electron transport layer (ETL) to establish a depleted heterostructure with a QD photoactive layer. However, band gap shrinkage of large PbS QDs makes it necessary to tailor the behaviors of the ZnO ETL for efficient carrier extraction in the devices. Herein, the characteristics of ZnO ETL are efficiently and flexibly tailored to match the QD layer by handily adjusting the postannealing process of ZnO ETL. With a suitable temperature, the well-matched energy level alignment and suppressed trap states are simultaneously achieved in the ZnO ETL, effectively reducing the nonradiative recombination and accelerating the electron injection from the QD layer to ETL. As a consequence, a high-performance PbS QD photovoltaic device with power conversion efficiencies (PCEs) of 10.09% and 1.37% is obtained under AM 1.5 and 1100 nm filtered solar illumination, demonstrating a simple and effective approach for achieving high-performance IR photoelectric devices.

Surface Microstructure Engineering in MAPbBr3 Microsheets for Performance-Enhanced Photodetectors.

Metal halide-perovskite-based photodetectors have recently emerged as a class of promising optoelectronic devices in various fields. Meanwhile, nano/microstructuring perovskite-based photodetectors are a facile integration with complementary metal-oxide semiconductors for miniaturized imaging systems. However, there are still challenges to be overcome in reducing the losses caused by light reflection on the surface of microstructural perovskites. In this work, surface microstructure engineering is employed in MAPbBr3 microsheets for reducing light reflection and improving light absorption, resulting in high-performance perovskite photodetectors. MAPbBr3 microsheets, which possess different surface morphologies of flat, upright hemisphere arrays and inverted hemisphere arrays (IHAs), are fabricated by a simple microstructure template-assisted space confinement process. The light absorption capacity of IHA MAPbBr3 is significantly higher than that of the other two structures. Hence, IHA photodetectors with excellent figures of merit, including low dark current, decent responsivity, and fast speed, are achieved. Furthermore, the noise of the IHA photodetectors is only ∼10-13 A/, which results in the superior sensitivity for weak light detection with a specific detectivity up to 1011 Jones. Our results demonstrate that surface engineering is a simple, low-cost, yet effective approach to improve the performance of nano-/micro-optoelectronic devices.

Exciton-polaron interactions in metal halide perovskite nanocrystals revealed via two-dimensional electronic spectroscopy.

Metal halide perovskite nanocrystals have been under intense investigation for their promise in optoelectronic devices due to their remarkable physics, such as liquid/solid duality. This liquid/solid duality may give rise to their defect tolerance and other such useful properties. This duality means that the electronic states are fluctuating in time, on a distribution of timescales from femtoseconds to picoseconds. Hence, these lattice induced energy fluctuations that are connected to polaron formation are also connected to exciton formation and dynamics. We observe these correlations and dynamics in metal halide perovskite nanocrystals of CsPbI3 and CsPbBr3 using two-dimensional electronic (2DE) spectroscopy, with its unique ability to resolve dynamics in heterogeneously broadened systems. The 2DE spectra immediately reveal a previously unobserved excitonic splitting in these 15nm NCs that may have a coarse excitonic structure. 2D lineshape dynamics reveal a glassy response on the 300fs timescale due to polaron formation. The lighter Br system shows larger amplitude and faster timescale fluctuations that give rise to dynamic line broadening. The 2DE signals enable 1D transient absorption analysis of exciton cooling dynamics. Exciton cooling within this doublet is shown to take place on a slower timescale than within the excitonic continuum. The energy dissipation rates are the same for the I and Br systems for incoherent exciton cooling but are very different for the coherent dynamics that give rise to line broadening. Exciton cooling is shown to take place on the same timescale as polaron formation, revealing both as coupled many-body excitation.

An all ambient, room temperature-processed solar cell from a bare silicon wafer.

Solar cells are a promising optoelectronic device for the simultaneous solution of energy resource and environmental problems. However, their high cost and slow, laborious production process so far severely hinder a sufficient widespread of clean, renewable photovoltaic energy as a major alternative electricity generator. This undesirable situation is mainly attributed to the fact that photovoltaic devices have been manufactured through a series of vacuum and high-temperature processes. Here we realize a poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/Si heterojunction solar cell fabricated only in ambient and room temperature conditions from a plain Si wafer, with an over 10% energy conversion efficiency. Our production scheme is based on our finding that PEDOT:PSS photovoltaic layers actively operate even on highly doped Si substrates, which substantially mitigates the condition requirements for electrode implementation. Our approach may pave the way for facile, low-cost, high-throughput solar cell fabrication, useful in various fields even including developing countries and educational sites.

Study of the perovskites CaZrO3-xSx (x=0, 1, 2 and 3) for photovoltaic applications

In this work, we present the calculations for the structural, electronic and optical properties of simple perovskites CaZrO3-xSx (x = 0, 1, 2 and 3) using density functional theory (DFT) with GGA-PBE as implemented in ABINIT package. The crystal structure of CaZrO3-xSx (x = 0,1,2 and 3) compound changes with the change of x value. The calculated electronic properties revealed the indirect semiconducting behavior of CaZrO3-xSx (x = 0, 1, 2 and 3) perovskites. However, the indirect band gap value Eg (R-Γ) drops from 3.36 to 0.48 eV when the parameter “x" increases from 0 to 3. In any case, the optical parameters such as dielectric constant ε(ω), reflectivity R(ω), refractive index n(ω), absorption coefficient α(ω), extinction coefficient k(ω), energy-loss function L(ω), and optical conductivity σ(ω), are also investigated. As results, it can be deduced that CaZrO2S and CaZrOS2 materials have the promising optoelectronic devices in photovoltaic applications.

Color-tunable encapsulated perylene-labeled silica fluorescent hybrid nanoparticles

Novel fluorescent nanoparticles were prepared via the encapsulation of perylene diimide into silica nanoparticles. The fluorescent nanoparticles were characterized by various analytical techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier-transform infrared spectra (FT-IR), and fluorescent optical microscopy (FOM). The diameters of the perylene-labeled silica nanoparticles were detected in the range of 40–180 nm. The emission properties of the prepared perylene-labeled silica nanoparticles were explored. The perylene-labeled silica nanoparticles displayed good photo and thermal properties. Depending on the type of the perylene salt, the composite nanoparticles showed tunable colors ranging between yellow, orange, and red. The diluted solutions of perylenes in aqueous media were highly fluorescent, displaying different colors between green and yellow, with a maximum emission at 510 nm and 650 nm, respectively. Those fluorescent perylene-labeled silica nanoparticles pave the road for the preparation of promising fluorescent substrates, including photonics, smart textiles, electronic displays, and optoelectronic devices.

Promoting Light Extraction Efficiency of Ultraviolet Light Emitting Diodes by Nanostructure Optimization

Ultraviolet (UV) light-emitting diodes (LEDs), as one of the more promising optoelectronic devices, are intrinsically limited by poor light extraction efficiencies (LEEs). To unlock the full potential of UV-LEDs, we propose a simple and effective strategy to promote the LEEs of UV-LEDs by screening and tailoring suitable optical structures/designs through rigorous numerical simulations. The photonic crystals (PCs) and/or nano-patterned sapphire substrates (NPSSs) equipped with the nano-pillar, nano-cone, nano-oval, and their derivates, are particularly investigated. The simulated results show that individual PC with an average transmittance of 28% is more efficient than that of individual NPSS (24.8%). By coupling PC and NPSS structures, a higher LEE with an average transmittance approaching 29% is obtained, much higher than that of the flat one (23.6%). The involved mechanisms are clarified and confirm that the promotion of optical performance of the nanostructured devices should be attributed to the widened response angles (from 0 to 60°), rather than the enhanced transmittances in the small angles within 30°.

The transverse magnetic electromagnetic bound mode in a black phosphorene inserted multilayer dielectric structure

The electromagnetic wave of the bound mode along the black phosphorene (BP) interface and the field distribution in a BP inserted planar multilayer dielectric structure have been investigated. The dispersion results indicate that the transverse magnetic surface plasmon wave is a bound mode and anisotropic with different electric field polarization directions. The effective refractive index increases as a function of the incident optical energy. It can also be tuned by the chemical potential and the thickness of the middle dielectric layer. The variable coupling strength between the light and matter based on the BP intraband optical conductivity makes the influence of the thickness of the middle dielectric layer significant or negligible in the lower and higher frequency regimes, respectively. The presented results can be applied to design BP-based promising optoelectronic devices.

Enhanced terahertz detection of multigate graphene nanostructures

Abstract Terahertz (THz) waves have revealed a great potential for use in various fields and for a wide range of challenging applications. High-performance detectors are, however, vital for exploitation of THz technology. Graphene plasmonic THz detectors have proven to be promising optoelectronic devices, but improving their performance is still necessary. In this work, an asymmetric-dual-grating-gate graphene-terahertz-field-effect-transistor with a graphite back-gate was fabricated and characterized under illumination of 0.3 THz radiation in the temperature range from 4.5 K up to the room temperature. The device was fabricated as a sub-THz detector using a heterostructure of h-BN/Graphene/h-BN/Graphite to make a transistor with a double asymmetric-grating-top-gate and a continuous graphite back-gate. By biasing the metallic top-gates and the graphite back-gate, abrupt n+n (or p+p) or np (or pn) junctions with different potential barriers are formed along the graphene layer leading to enhancement of the THz rectified signal by about an order of magnitude. The plasmonic rectification for graphene containing np junctions is interpreted as due to the plasmonic electron-hole ratchet mechanism, whereas, for graphene with n+n junctions, rectification is attributed to the differential plasmonic drag effect. This work shows a new way of responsivity enhancement and paves the way towards new record performances of graphene THz nano-photodetectors.

Tunable Schottky and ohmic contacts in the Ti2NF2/α-Te van der Waals heterostructure.

Two dimensional α-Te holds great promise in optoelectronic devices because of its high mobility and excellent environmental stability. In this study, the electronic structures and interfacial contact characteristics of the Ti2NF2/α-Te van der Waals heterostructure are investigated by means of first-principles calculations. It is found that p-type Schottky contacts with a Schottky barrier (SB) of 0.21 eV are formed at the Ti2NF2/α-Te interface. By applying external electric fields or controlling the interlayer coupling between the Ti2NF2 and α-Te monolayers, the SB height can be effectively tuned, and all the n-type Schottky, p-type Schottky, n-type ohmic and p-type ohmic contacts can be achieved. Such an extremely high tunability is further found to be closely associated with the charge transfer at the interface, as well as the interface dipole and the potential step. Our results provide an avenue for the design of future α-Te-based electronic devices with high performance.

One-step synthesized PbSe nanocrystal inks decorated 2D MoS2 heterostructure for high stability photodetectors with photoresponse extending to near-infrared region

Two-dimensional layered transition metal dichalcogenides (TMDs) have been widely employed as functional materials in promising electronics and optoelectronic devices due to their unique physical and outstanding electronic properties.

Enhancement of Photoresponse on Narrow-Bandgap Mott Insulator α-RuCl3 via Intercalation.

Charge doping to Mott insulators is critical to realize high-temperature superconductivity, quantum spin liquid state, and Majorana fermion, which would contribute to quantum computation. Mott insulators also have a great potential for optoelectronic applications; however, they showed insufficient photoresponse in previous reports. To enhance the photoresponse of Mott insulators, charge doping is a promising strategy since it leads to effective modification of electronic structure near the Fermi level. Intercalation, which is the ion insertion into the van der Waals gap of layered materials, is an effective charge-doping method without defect generation. Herein, we showed significant enhancement of optoelectronic properties of a layered Mott insulator, α-RuCl3, through electron doping by organic cation intercalation. The electron-doping results in substantial electronic structure change, leading to the bandgap shrinkage from 1.2 eV to 0.7 eV. Due to localized excessive electrons in RuCl3, distinct density of states is generated in the valence band, leading to the optical absorption change rather than metallic transition even in substantial doping concentration. The stable near-infrared photodetector using electronic modulated RuCl3 showed 50 times higher photoresponsivity and 3 times faster response time compared to those of pristine RuCl3, which contributes to overcoming the disadvantage of a Mott insulator as a promising optoelectronic device and expanding the material libraries.

Point and complex defects in monolayer PdSe2 : Evolution of electronic structure and emergence of magnetism

Layered transition-metal dichalcogenides (TMDs) constitute an emerging class of materials that provide researchers a platform to realize fundamental studies and to design promising optoelectronic devices. While defects are an almost unavoidable part of TMDs, they bring additional interesting properties absent in defect-free layers. Moreover, the controlled introduction of defects in TMDs makes it possible to tailor the electromagnetic properties of the materials. Here we report defect-induced properties of single-layer $\mathrm{Pd}{\mathrm{Se}}_{2}$ and demonstrate the emergence of magnetism at the nanoscale. Our first-principle calculations indicate that Se vacancies create in-gap defect states, which can be responsible for trapping of carriers. The complex square ${V}_{\mathrm{Pd}+4\mathrm{Se}}$ vacancy induces spin-polarized states with a total local magnetic moment of $2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per defect, making it possible to introduce magnetization into $\mathrm{Pd}{\mathrm{Se}}_{2}$ and therefore expand the family of two-dimensional (2D) magnets. The defect formation energies are much lower compared to many other TMD materials that can explain the presence of a large number of Se defects after mechanical exfoliation of $\mathrm{Pd}{\mathrm{Se}}_{2}$ layers, while the central location of the Pd atoms preserves them from exfoliation-induced defect formation. The negatively charged vacancies are prone to form and in many cases demonstrate spin-polarized states. The small diffusion barrier of ${\mathrm{V}}_{\mathrm{Se}}$ in 2D $\mathrm{Pd}{\mathrm{Se}}_{2}$ leads to an easy room-temperature migration, while ${\mathrm{V}}_{\mathrm{Pd}}$ demonstrates a high diffusion barrier preventing its spontaneous migration. As a guide for experimentalists, we simulate and characterize scanning tunneling microscope images in valence and conduction states and estimate the electron-beam energy for a controllable production of various defect patterns. These intriguing findings make $\mathrm{Pd}{\mathrm{Se}}_{2}$ an ideal platform to study defect-induced phenomena.

Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons.

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier–carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10 000 cm2 V–1 s–1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump–probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices.

Indium tin oxid/germanium Schottky photodetectors modulated by ultra-thin dielectric intercalation

Germanium (Ge) photodetectorhas been considered as one of the promising optoelectronic devices for optoelectronic integration. So far, most of reported Ge photodetectors with bulk Ge show high dark currents and low responsivities. In this paper, ultra-thin dielectric interlayer-modulated indium tin oxid (ITO)/Ge Schottky photodetectors with high responsivities and low dark currents are investigated, in which the ultra-thin dielectric interlayers are deposited through atomic layer deposition. The characteristics of ITO/Al<sub>2</sub>O<sub>3</sub> (or MoO<sub>3</sub>)/Ge Schottky photodiodes fabricated on bulk Ge wafers with various doping concentrations and Ge epilayer on silicon substrates are comparatively studied. It is found that the 2-nm-thick Al<sub>2</sub>O<sub>3</sub> intercalation between ITO transparent electrode and Ge can effectively enhance the Schottky barrier heights of the photodetectors and trap holes at interface states, rendering their dark currents low and responsivities high. The effective Schottky barrier heights increase from 0.34 eV (ITO/i-Ge) to 0.55 eV (ITO/Al<sub>2</sub>O<sub>3</sub>/i-Ge), and from 0.24 eV (ITO/n-Ge) to 0.56 eV (ITO/Al<sub>2</sub>O<sub>3</sub>/n-Ge). While MoO<sub>3</sub> intercalation between ITO and Ge has no significant effect on the characteristics of all of the photodetectors due to its large electron affinity. The best performance is realized on the ITO/Al<sub>2</sub>O<sub>3</sub>/i-Ge photodetector with a low dark current of 5.91 mA/cm<sup>–2</sup> at –4 V, sharply dropping by two orders of magnitude, compared with that of the ITO/i-Ge photodetector without the Al<sub>2</sub>O<sub>3</sub> interlayer, and the responsivity is significantly improved to 4.11 A/W at 1310 nm. The ITO/Al<sub>2</sub>O<sub>3</sub>/epi-Ge photodetector fabricated on 500 nm Ge epilayer on a silicon substrate also shows the improved performance with a dark current density of 226.70 mA/cm<sup>2</sup> at –3 V and a responsivity of 0.38 A/W at 1310 nm, compared with ITO/epi-Ge photodetector. Finally, experiment studies of single-point infrared images at 1310 nm and 1550 nm are carried out with the ITO/Al<sub>2</sub>O<sub>3</sub>/i-Ge photodetector on a two-dimensional <i>XY</i> displacement platform, which contains 25 pixels and a total detection size of 1750 μm × 1750 μm. The clear and distinguishable images of the infrared spot position are obtained. Consequently, these results suggest that the dielectric interlayer- modulated Schottky photodetectors are competitive in low power consumption and high responsivity, and have great potential applications in the civil field of short wave infrared imaging.

Drift-diffusion simulation of photodetector with controlled relocation of carrier density peaks

A photodetector with controlled relocation of carrier density peaks is a promising optoelectronic device designed for the operation as a part of on-chip optical interconnections together with a high-speed laser-modulator. Previously, we estimated the back-edge performance of the sensor using a combined quantum-mechanical model that had not taken into account certain physical aspects of charge carrier transport in the device structure. In this paper, we propose a two-dimensional drift-diffusion model allowing for the comprehensive analysis of transients in the photodetector with controlled relocation within the semiclassical approach. To implement the model, we develop a technique of finite difference numerical simulation and applied software. The obtained simulation results enable us to clarify the operation mode and connection principle of the photodetector with controlled relocation and to improve its performance through the formation of special carrier-holding layers.

Interlayer Trions in 2D Transition-Metal Dichalcogenide Heterostructures

A dimensionally confined dielectric constant and reduced dielectric screening lead to two-dimensional transition-metal dichalcogenides (TMDs). The confined dielectric constant triggers a strong Coulomb interaction and high exciton binding energies between TMD heterostructures. Here we show a highly efficient interlayer charged exciton or trion formation and its generation sites are present in materials at room temperature. Two different transition metals (Mo, W) and chalcogenide (S, Se) elements were investigated in two different heterostructure combinations (MoS2—WS2 and MoSe2—WS2). Room temperature photoluminescence measurements demonstrate a highly efficient trion formation between and close to the heterostructure interfaces. This study highlights an effective band alignment, strong photoexciting Coulomb interaction, and formation of interlayer trions with different recombination energies. This investigation suggests the possibility of utilizing interlayer trions in promising optoelectronic devices in the future.

Toward See‐Through Optoelectronics: Transparent Light‐Emitting Diodes and Solar Cells

AbstractTransparent light‐emitting diodes (LEDs) and solar cells have attracted extensive attention as the most promising optoelectronic devices surpassing conventional opaque displays and photovoltaics. These transparent devices are particularly suitable for special applications including see‐through display and building‐integrated photovoltaics, for example, head‐mounted displays, navigation displays on car windshields, smart windows, roofing, skylights, and facades. This review systematically evaluates the pros and cons of representative transparent conductive electrodes (TCEs) including transparent conductive oxides, ultrathin metal films, metal nanowires/meshes, conductive flexible polymers, carbon nanotubes, and graphene. A full‐scale overview of recent studies on transparent organic LEDs, quantum dot LEDs, perovskite LEDs, dye‐sensitized solar cells, organic solar cells, and perovskite solar cells using the above‐mentioned TCEs as top electrodes, is also provided, summarizing information on state‐of‐the‐art device performances. Finally, the key challenges in current transparent optoelectronic devices and a few potential proposals for developing highly efficient transparent devices are addressed in the last part. It is believed that, with the rapid development of novel optoelectronic materials and fabrication technologies, there will be a bright prospect for transparent optoelectronic devices and their related applications in the near future.