Performance Of Optoelectronic Devices Research Articles

Impact of oxygen plasma power on the performance of Ga2O3 passivated GaN ultraviolet photodetectors

The role of oxygen plasma power on the extent of surface passivation of Gallium Nitride (GaN) epitaxial layers and the photo-response of Au/Ni/GaN Schottky detectors is investigated. The surface morphology is found to be preserved in oxide passivated GaN at intermediate plasma power which significantly deteriorates at higher power. The photoluminescence spectrum shows a fall in the intensity of band edge and defects related yellow luminescence at higher power indicating the generation of non-radiative recombination centres. The chemical and electronic structures of the passivated layer investigated by x-ray photoelectron spectroscopy revealed the formation of stoichiometric Ga2O3 up to intermediate plasma power and partial decomposition of Ga2O3 along with formation of gallium sub-oxides at higher power. This leads to an enhancement in the responsivity of the photodetector after passivation at an intermediate plasma power followed by a considerable reduction at higher power. From the electrical transport measurement across the Au/Ni/GaN Schottky diode, it is observed that the diode manifests more Schottky (Ohmic) character after oxygen plasma treatment at intermediate (higher) power. The present investigations provide a clear guideline on the selection of oxygen plasma power for surface passivation on GaN, which is highly beneficial in optimizing the performance of GaN-based optoelectronic and power devices.

Real-time detection of aging status of methylammonium lead iodide perovskite thin films by using terahertz time-domain spectroscopy

The inadequate stability of organic–inorganic hybrid perovskites remains a significant barrier to their widespread commercial application in optoelectronic devices. Aging phenomena profoundly affect the optoelectronic performance of perovskite-based devices. In addition to enhancing perovskite stability, the real-time detection of aging status, aimed at monitoring the aging progression, holds paramount importance for both fundamental research and the commercialization of organic–inorganic hybrid perovskites. In this study, the aging status of perovskite was real-time investigated by using terahertz time-domain spectroscopy. Our analysis consistently revealed a gradual decline in the intensity of the absorption peak at 0.968 THz with increasing perovskite aging. Furthermore, a systematic discussion was conducted on the variations in intensity and position of the terahertz absorption peaks as the perovskite aged. These findings facilitate the real-time assessment of perovskite aging, providing a promising method to expedite the commercialization of perovskite-based optoelectronic devices.Graphical

All-solution preparation of perovskite light-emitting diodes in ambient air through film-forming regulation of light-emitting layers based on dual-additive doping and oxygen plasma treatment

Organic-inorganic hybrid perovskites are promising for optoelectronics owing to their high color purity, tunable bandgaps, and low nonradiative recombination rates. However, challenges such as rough surfaces and poor film-forming properties constrain the optoelectronic performance of light-emitting devices. Herein, the PEDOT:PSS film treated by oxygen plasma serves as the hole transport layer, while the formation of perovskite films is optimized by dual-additive doping of PEO and TPBi in ambient air. The results show that the PEDOT:PSS films with PSSH-rich after oxygen plasma treatment provide more nucleation sites, favoring the formation of the perovskite film. Meanwhile, the dual-additive prompts to form a dense and uniform film as well. The intrinsic mechanisms involve PEO as a binder to fill grain gaps and improve water-oxygen stability in the perovskite film, while TPBi acts as a passivator to stimulate uniform nucleation and dense crystallization, improving the grain coverage of the light-emitting layer. Under atmospheric conditions, all-solution processed perovskite devices reach a maximum luminance of 810.4 cd m−2 and a maximum current efficiency of 0.437 cd A−1. This work highlights the significance of using oxygen plasma treatment and additives to improve the performance of perovskite films and achieve superior PeLEDs.

Synthesis of copper oxide (CuO) via coprecipitation method: Tailoring structural and optical properties of CuO nanoparticles for optoelectronic device applications

Oxide-based nanoparticles have been studied extensively due to their unique optical and passivation properties, which are significantly distinct from those of bulk materials. The present study aimed to synthesize copper oxide nanoparticles through the co-precipitation method. The adequate substitution of sodium hydroxide (NaOH) into CuO is analyzed by different characterizations, i.e., X-ray diffraction (XRD), Scanning Electron Microscope (SEM), UV–visible spectroscopy (UV–visible), Fourier transform infrared (FTIR), and EDX. The X-ray diffraction spectra indicate that the prepared copper oxide nanoparticles were crystalline, nano-sized, and highly pure. The SEM micrographs showed that nanoparticles were spherical at lower contents of NaOH, while at higher contents, agglomeration of nanoparticles occurs, and the grain size changes. The decreased optical bandgap of the nanoparticles was observed due to different concentrations of sodium hydroxide. The crystalline size and strain values were also determined through the Williamson-Hall method. The compressive micro-stain in CuO materials confirms the negative slope value of the line fit in the W–H Plot. From this study, we concluded that synthesized CuO NPs have the potential to combine CuO materials with antireflecting films for improved optical performance in optoelectronic devices, precisely current density properties.

CsPbI3 Perovskite Quantum Dot-Based WORM Memory Device with Intrinsic Ternary States.

The migration of mobile ionic halide vacancies is usually considered detrimental to the performance and stability of perovskite optoelectronic devices. Taking advantage of this intrinsic feature, we fabricated a CsPbI3 perovskite quantum dot (PQD)-based write-once-read-many-times (WORM) memory device with a simple sandwich structure that demonstrates intrinsic ternary states with a high ON/OFF ratio of 103:102:1 and a long retention time of 104 s. Through electrochemical impedance spectroscopy, we proved that the resistive switching is achieved by the migration of mobile iodine vacancies (VIs) under an electric field to form conductive filaments (CFs). Using in situ conductive atomic force microscopy, we further revealed that the multilevel property arises from the different activation energies for VIs to migrate at grain boundaries and grain interiors, resulting in two distinct pathways for CFs to grow. Our work highlights the potential of CsPbI3 PQD-based WORM devices, showcasing intrinsic multilevel properties achieved in a simple device structure by rationally controlling the drift of ionic defects.

Rational synthesis of core/shell heterostructured quantum dots for improved self-powered photodetection performance enabled by energy band engineering

Colloidal quantum dots (CQDs), especially core-shell QDs, have attracted dramatic interests as emerging materials in optoelectronic devices due to their size tuneable band-gap, high absorption coefficient, facile and low-cost synthesis routing, etc. However, the precise design of core/shell band engineering along with charge carrier transfer efficiency are still challenging, inevitably impacting the performance of optoelectronic devices. Herein, three different types of CdSe/CdS core-shell heterostructured QDs have been obtained via a proposed synthesis strategy of adjusting the core size but fixing the shell thickness. The performance of the core-shell QDs based self-powered photodetectors can be efficiently regulated by the optimization of synthesis strategy with the design of energy band alignment. Compared to the quasi type-II and the type-I QDs devices, the Ilight/Idark ratio of type-II heterostructure based photodetector is increased by ∼2 and ∼5 orders of magnitude, respectively. The champion device exhibits extraordinary optoelectronic performance in terms of ultrahigh light-to-dark current ratio of 105, photoresponsivity of 10.64 mA/W, at bias of 0 V, respectively. The charge carrier dynamics and transfer mechanisms of the QDs were further investigated, revealing the improved carrier separation and transportation efficiency induced by a strong build-in electric field in type-II CdSe/CdS core-shell heterostructure. This work provides a new strategy for the next-generation self-powered QDs optoelectronics enabled by the proposed synthesis approach along with the design of energy band alignment.

Symmetry-breaking-enhanced power conversion efficiency of 2D van der Waals heterostructures

Symmetry-breaking plays a crucial role in determining the property and functionality of materials. Here, we demonstrate that symmetry-breaking can dramatically enhance the power conversion efficiency (PCE) of a two-dimensional (2D) van der Waals (vdW) heterostructure solar cell by taking a γ-phosphorus-carbide (PC)-based vdW heterostructure as a model. Thanks to its four-atom-layer structure of γ-PC, both alternately arranging P and C atoms to form a Janus structure and sliding C atom layer to change space group are two effective methods to break the symmetry. We find that in comparison with a symmetric configuration, the PCE of γ-PC/MoS2N4 with symmetry-breaking could be increased by 257.2% and 270% via forming a Janus structure and the change in space group, respectively. Particularly, the PCE of symmetry-broken γ-PC/MoSi2N4 can be further increased to 21.35% under an appropriate tensile strain, which could be attributed to small conduction band offset between constituent monolayers and suitable donor bandgap. Our study showcases that tuning the symmetry of multi-atom-layer 2D materials is an effective strategy to realize enhancement of the performance for 2D materials-based optoelectronic devices.

A Dibenzo[g,p]chrysene-Based Organic Semiconductorwith Small Exciton Binding Energy via Molecular Aggregation.

Exciton binding energy (Eb) is understood as the energy required to dissociate an exciton in free-charge carriers, and is known to be an important parameter in determining the performance of organic opto-electronic devices. However, the development of a molecular design to achieve a small level of Eb in the solid state continues to lag behind. Here, to investigate the relationship between aggregation and Eb, star-shaped π-conjugated compounds DBC-RD and TPE-RD were developed using dibenzo[g,p]chrysene (DBC) and tetraphenylethylene (TPE). Theoretical calculations and physical measurements in solution showed no apparent differences between DBC-RD and TPE-RD, indicating that these molecules possess similar properties on a single-molecule level. By contrast, pristine films incorporating these molecules showed significantly different levels of electron affinity, ionization potential, and optical gap. Also, DBC-RD had a smaller Eb value of 0.24 eV compared with that of TPE-RD (0.42 eV). However, these molecules showed similar Eb values under dispersed conditions, which suggested that the decreased Eb of DBC-RD in pristine film is induced by molecular aggregation. By comparison with TPE-RD, DBC-RD showed superior performances in single-component organic solar cells and organic photocatalysts. These results indicate that a molecular design suitable for aggregation is important to decrease the Eb in films.

Controlled Crystal Growth of All-Inorganic CsPbI2Br via Sequential Vacuum Deposition for Efficient Perovskite Solar Cells.

Vacuum deposition of perovskites is a promising method for scale-up fabrication and uniform film growth. However, improvements in the photovoltaic performance of perovskites are limited by the fabrication of perovskite films, which are not optimized for high device efficiency in the vacuum evaporation process. Herein, we fabricate CsPbI2Br perovskite with high crystallinity and larger grain size by controlling the deposition sequence between PbI2 and CsBr. The nucleation barrier for perovskite formation is significantly lowered by first evaporating CsBr and then PbI2 (CsBr-PbI2), followed by the sequential evaporation of multiple layers. The results show that the reduced Gibbs free energy of CsBr-PbI2, compared with that of PbI2-CsBr, accelerates perovskite formation, resulting in larger grain size and reduced defect density. Furthermore, surface-modified homojunction perovskites are fabricated to efficiently extract charge carriers and enhance the efficiency of perovskite solar cells (PeSCs) by modulating the final PbI2 thickness before thermal annealing. Using these strategies, the best PeSC exhibits a power conversion efficiency of 13.41% for a small area (0.135 cm2), the highest value among sequential thermal deposition inorganic PeSCs, and 11.10% for a large area PeSC (1 cm2). This study presents an effective way to understand the crystal growth of thermally deposited perovskites and improve their performance in optoelectronic devices.

Enhanced ambient-stability and charge transport property of the CsPbBr3@Polyaniline

Although all-inorganic lead halide perovskites (CsPbX3 (X = Cl, Br, I)) have received widespread attention due to excellent optoelectronic properties, their low carrier mobility and poor stability have limited their application in photodetection. In this study, to alleviate these problems, CsPbBr3@Polyaniline composite is fabricated by passivating CsPbBr3 quantum dots (QDs) into a CSA-doped polyaniline conducting polymer. Notably, polyaniline is employed as a protective layer to block the structure damage of CsPbBr3 QDs against water, light and heat. The contact between polyaniline and CsPbBr3 QDs forms a type II heterojunction for charge transfer, which greatly reduces the recombination rate of charge carriers compared to pristine CsPbBr3 QDs. Benefiting from the high stability and enhanced charge mobility, CsPbBr3@Polyaniline-based photodetector achieves improved photocurrent signal and optical response time. The results demonstrate that the surface passivation could resolve the crucial stability problem and simultaneously facilitate the superior performance of perovskite optoelectronic devices.

Plasma-Etched Black GaAs Nanoarrays with Gradient Refractive Index Profile for Broadband, Omnidirectional, and Polarization-Independent Antireflection.

Black GaAs nanotip arrays (NTs) with 3300 nm lengths were fabricated via self-masked plasma etching. We show, both experimentally and numerically, that these NTs, with three gradient refractive index layers, effectively suppress Fresnel reflections at the air-GaAs interface over a broad range of wavelengths. These NTs exhibit exceptional UV-Vis light absorption (up to 99%) and maintain high NIR absorption (33-60%) compared to bare GaAs. Moreover, possessing a graded layer with a low refractive index (n = 1.01 to 1.12), they achieve angular and polarization-independent antireflection properties exceeding 80° at 632.8 nm, aligning with perfect antireflective coating theory predictions. This approach is anticipated to enhance the performance of optoelectronic devices across a wide range of applications.

Piezotronically tuned photosensitivity in ZnO varistor-type interface devices

The study of piezotronic and piezo-phototronic features in semiconducting piezoelectrics has gained considerable attention for optimizing electronic and optoelectronic device performance. In this study, we explore the photocurrent response within a ZnO varistor-type interface device and examine its UV photosensitivity enhancement through piezo-phototronic modulation of the potential barriers at grain boundaries. Doped ZnO bicrystals were synthesized using the epitaxial solid-state transformation method with polarization vectors perpendicular to the interface. The vectors can be directed both into and out of the interfaces depending on the crystallographic orientation of the crystals designated as (0001̅)|(0001̅) and (0001)|(0001) interfaces, respectively. These varistor-type boundaries demonstrate an exceptional photoelectric response to UV photons. Furthermore, our results indicate that the photoresponsivity of bicrystals with (0001)|0001 crystallographic orientation can be further enhanced under piezo-phototronic influence. Application of compressive stress of 100 MPa not only increased photocurrent formation by 1000 % but also boosted photosensitivity by 1400 %, attributed to the elevated double Schottky barrier (DSB) height at the interface resulting from mechanical loading on the bicrystal. Our findings highlight the significant role of piezoelectrically tuned potential barriers at grain boundaries in enhancing the separation and extraction of photogenerated excitons for photocurrent formation. This study introduces the engineered varistor-type interface as a promising candidate for future optoelectronic devices and contributes to our understanding of the piezo-phototronic concept in semiconductor-semiconductor interface devices.

Advances and challenges in molecular engineering of 2D/3D perovskite heterostructures.

Organic-inorganic hybrid perovskites have been intensively studied in past decades due to their outstanding performance in solar cells and other optoelectronic devices. Recently, the emergence of two-dimensional/three-dimensional (2D/3D) heterojunctions have enabled many solar cell devices with >25% power conversion efficiency, driven by advances in our understanding of the structural and photophysical properties of the heterojunctions and our ability to control these properties through organic cation configuration in 2D perovskites. In this feature article, we discuss a fundamental understanding of structural characteristics and the carrier dynamics in the 2D/3D heterojunctions and their impact factors. We further elaborate the design strategies for the molecular configuration of organic cations to achieve thorough management of these properties. Finally, recent advances in 2D/3D heterostructures in solar cells, light-emitting devices and photodetectors are highlighted, which translate fundamental understandings to device applications and also reveal the remaining challenges in ligand design for the next generation of stable devices. Future development prospects and related challenges are also provided, with wide perspectives and insightful thoughts.

Noise equivalent temperature difference study of type-II superlattice MWIR focal plane arrays for high operating temperature performance

Achieving high operating temperature (HOT) plays a crucial role in miniaturizing type-II superlattice (T2SL) mid-wavelength infrared (MWIR) focal plane arrays (FPAs). However, their full potential has yet to be realized due to a lack of complete understanding of their operation from the perspective of detection principles. Here, by investigating the photon transmission path and optoelectronic performance of the simulated devices, a detailed noise equivalent temperature difference (NETD) model of the T2SL MWIR FPAs was established. The NETD limitations in the optics-limited and detector-limited modes were revealed by studying the effects of the source, optical system, and FPA-related parameters. Although NETD exhibits sensitivity to dark currents, improvements in the quantum efficiency and well capacity can further boost its performance. When the defects and carrier lifetimes are well controlled to completely suppress the dark current, the NETD of an MWIR system with optimized integration times, which operates between 150 K and 200 K, is predicted to be below 10 mK when detecting room-temperature targets. The results provide new insights into the model and sources contributing to the NETD and demonstrate the possibility of high-temperature operation of T2SLs MWIR FPAs.

Nanoscale phase management of the 2D/3D heterostructure toward efficient perovskite solar cells

The stabilization of the formamidinium lead iodide (FAPbI3) structure is pivotal for the development of efficient photovoltaic devices. Employing two-dimensional (2D) layers to passivate the three-dimensional (3D) perovskite is essential for maintaining the α-phase of FAPbI3 and enhancing the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, the role of bulky ligands in the phase management of 2D perovskites, crucial for the stabilization of FAPbI3, has not yet been elucidated. In this study, we synthesized nanoscale 2D perovskite capping crusts with <n> = 1 and 2 Ruddlesden-Popper (RP) perovskite layers, respectively, which form a type-II 2D/3D heterostructure. This heterostructure stabilizes the α-phase of FAPbI3, and facilitates ultrafast carrier extraction from the 3D perovskite network to transport contact layer. We introduced tri-fluorinated ligands to mitigate defects caused by the halide vacancies and uncoordinated Pb2+ ions, thereby reducing nonradiative carrier recombination and extending carrier lifetime. The films produced were incorporated into PSCs that not only achieved a PCE of 25.38% but also maintained 95% of their initial efficiency after 2000 hours of continuous light exposure without encapsulation. These findings underscore the effectiveness of a phase-pure 2D/3D heterostructure-terminated film in inhibiting phase transitions passivating the iodide anion vacancy defects, facilitating the charge carrier extraction, and boosting the performance of optoelectronic devices.

EFFECT OF NIOBIUM DOPANT CONCENTRATION ON ZNO NANOSTRUCTURES ON SI POROUS SUBSTRATE

Zinc oxide (ZnO) nanostructures have gained popularity in sensor technology due to their unique properties and facile device integration. However, the intrinsic optical and electrical properties of ZnO are suboptimal for direct industrial applications, necessitating enhancements through doping. This study investigates the impact of niobium (Nb) doping concentrations on the physical, optical, and electrical characteristics of ZnO nanostructures. FESEM analyses indicate successful deposition of ZnO on silicon nanoparticles substrates, with nanostructure sizes peaking at 88 nm and 115 nm for 6% and 8% Nb dopant concentrations, respectively. XRD results suggest that the structural integrity of the nanostructures is maintained up to a 10% doping level, with an optimal concentration at 6%. UV-Vis spectroscopy shows a shift in the absorption peak into the visible range and a decrease in maximum reflectivity from 47.5990% at 2% doping to 40.9205% at 6% doping, indicating enhanced light absorption suitable for optoelectronic applications. Electrical measurements reveal that a 6% Nb dopant concentration yields the lowest resistivity and highest conductivity among the tested samples. These findings underscore the potential of Nb-doped ZnO nanostructures in improving the performance of optoelectronic devices through tailored doping strategies.

Photocurrent interference spectroscopy of solids and characterization of semiconductor solar-cell materials

Photocurrent measurements are widely used to study the intrinsic optoelectronic properties of semiconductors, such as photocarrier generation efficiency and carrier mobility, as well as evaluate the performance of optoelectronic devices, such as solar cells and photodetectors. Interferometric spectroscopy precisely measures optical properties and gathers optical spectra information on semiconductors. Consequently, photocurrent-based interferometric measurements, with high signal-to-noise ratios, high resolution, and broad frequency bandwidths, can probe the energy distributions of low-density defects and impurities and investigate charge transport in materials and devices. Here, we demonstrate that photocurrent interference spectroscopy reveals the intrinsic properties of solar-cell materials: bulk crystals of GaAs and halide perovskite, and thin films of halide perovskite and quantum dot. We show that homodyne interference spectroscopy of photocurrent can monitor low-density localized states in semiconductors and that it can be used in combination with other spectroscopy techniques, such as photoluminescence measurements, to provide a deep understanding of photocurrent generation processes. Furthermore, we show that heterodyne interference spectroscopy of photocurrent can be used to investigate the frequency dependence of material parameters, such as the dielectric constant, absorption coefficient, and reflectance. As an application, we used interference spectroscopy of photocurrent to show the impact of multiexcitons on the photoabsorption and photocarrier generation processes in quantum dot solar cells. Finally, we used it to reveal the distinctive spectral characteristics at the band edge of a halide perovskite, which is considered to be an exceptional solar-cell material with high energy conversion efficiency.

Charge Transfer Kinetics in Halide Perovskites: On the Constraints of Time-Resolved Spectroscopy Measurements.

Understanding photophysical processes in lead halide perovskites is an important aspect of optimizing the performance of optoelectronic devices. The determination of exact charge carrier extraction rate constants remains elusive, as there is a large and persistent discrepancy in the reported absolute values. In this review, we concentrate on experimental procedures adopted in the literature to obtain kinetic estimates of charge transfer processes and limitations imposed by the spectroscopy technique employed. Time-resolved techniques (e.g., transient absorption-reflection and time-resolved photoluminescence spectroscopy) are commonly employed to probe charge transfer at perovskite/transport layer interfaces. The variation in sample preparation and measurement conditions can produce a wide dispersion of the measured kinetic parameters. The selected time window and the kinetic fitting model employed introduce additional uncertainty. We discuss here evaluation strategies that rely on multiexponential fitting protocols (regular or stretched) and show how the dispersion in the reported values for carrier transfer rate constants can be resolved.

Nanocrystal Materials for Resistive Memory and Artificial Synapses: Progress and Prospects.

Resistive random-access memory (RRAM) is considered to be the most promising next-generation non-volatile memory because of its low cost, low energy consumption, and excellent data storage characteristics. However, the on/off (SET/RESET) voltages of RRAM are too random to replace the traditional memory. Nanocrystals (NCs) offer an appealing option for these applications since they combine excellent electronic/optical properties and structural stability and can address the requirements of low-cost, large-area, and solution-processed technologies. Therefore, the doping NCs in the function layer of RRAM are proposed to localize the electric field and guide conductance filaments (CFs) growth. The purpose of this article is to focus on a comprehensive and systematical survey of the NC materials, which are used to improve the performance of resistive memory (RM) and optoelectronic synaptic devices and review recent experimental advances in NC-based neuromorphic devices from artificial synapses to light-sensory synaptic platforms. Extensive information related to NCs for RRAM and artificial synapses and their associated patents were collected. This review aimed to highlight the unique electrical and optical features of metal and semiconductor NCs for designing future RRAM and artificial synapses. It was demonstrated that doping NCs in the function layer of RRAM could not only improve the homogeneity of SET/RESET voltage but also reduce the threshold voltage. At the same time, it could still increase the retention time and provide the probability of mimicking the bio-synapse. NC doping can significantly enhance the overall performance of RM devices, but there are still many problems to be solved. This review highlights the relevance of NCs for RM and artificial synapses and also provides a perspective on the opportunities, challenges, and potential future directions.

Evidence of two-dimensional lateral quantum confinement in self-formed core–shell InGaN nanowires on Si (111) emitting in the red

The proof of strong two-dimensional lateral quantum confinement in the In-rich core of red-light emitting self-formed core–shell InGaN nanowires is given. The nanowires are directly grown on Si (111) by plasma-assisted molecular beam epitaxy. After the initial InGaN nucleation, straight nanowires with quantum-size core radius determined by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray mappings develop. Detailed comparison of the photoluminescence from the core, the In contents of the core and shell, and the core radius with theoretical modeling reveals a parabolic confinement potential with large ground state quantum confinement energies of electrons and holes. Such strong lateral quantum confinement in a vertical quantum wire active region is ideal for the performance of optoelectronic devices, in particular of our reported red InGaN light emitting diode with high brightness and color stability.