Optoelectric Devices Research Articles

Atomically thin heterostructure with gap-mode plasmon for overcoming trade-off between photoresponsivity and response time

Two-dimensional (2D) materials have recently provided a new perspective on optoelectronics because of their unique layered structure and excellent physical properties. However, their potential use as optoelectric devices has been limited by the trade-off between photoresponsivity and response time. Here, based on a vertically stacked atomically thin p-n junction, we propose a gap-mode plasmon structure that simultaneously enables enhanced responsivity and rapid photodetection. The atomically thin 2D materials act as a spacer for enhancing the gap-mode plasmons, and their short transit length in the vertical direction allows fast photocarrier transport. We demonstrate a high responsivity of up to 8.67 A/W with a high operation speed that exceeds 35 MHz under a 30 nW laser power. Spectral photocurrent, absorption, and a numerical simulation are used to verify the effectiveness of the gap-mode plasmons in the device. We believe that the design strategy proposed in this study can pave the way for a platform to overcome the trade-off between responsivity and response time.

Wetting-Induced Fabrication of Graphene Hybrid with Conducting Polymers for High-Performance Flexible Transparent Electrodes.

Graphene has attracted extensive attention for the supply of electrically conductive, optically transparent, and mechanical robust electrodes for flexible optoelectrical devices, as an alternative to commercial indium tin oxide, due to its superior mechanical, electrical, and optical properties. However, conventional chemical vapor deposition is impeded by harsh conditions and complicated processes, and it is still a challenge to fabricate high-performance graphene transparent electrode in a facile and scalable solution-processable route. Herein, a wetting-induced scalable solution-processable approach to fabricate graphene hybrid with conductive ionogel and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), i.e., graphene/ionogel@PEDOT:PSS (G/Ionogel@PEDOT:PSS), for high-performance flexible transparent electrode (FTE) is reported, achieving a low sheet resistance of 30 Ω sq-1 and a high transmittance of 88% at 550 nm. The as-fabricated trinary hybrid FTE as both transparent electrode and electrochromic layer is applied to a compact indium tin oxide (ITO)-free three-layered flexible electrochromic device, showing fast switching response, good electrochromic contrast, and reliable stability. Our work enables a scalable solution-processable approach for the generation of graphene-based FTE and functional devices.

Large and controlled Goos–Hänchen shift in monolayer graphene covered multilayer photonic crystals grating

Enhancing and engineering the Goos–Hänchen (GH) shift of two dimensional layered materials provides a powerful way to realize the novel optoelectric devices. Here we report an enhanced and controlled GH shift in the hybrid structure consisting of monolayer graphene and multilayer photonic crystals grating structure by using the rigorous coupled-wave analysis and stationary phase method, whose value can reach up to more than two thousand times of incident wavelength. Such an enhanced GH shift can be ascribed to the excitation of the guide mode resonances in the multilayer photonic crystal grating structure. In addition, the various geometrical parameters of the hybrid structure allow for the control of the GH shift in monolayer graphene based multilayer photonic crystal grating structure. Our work highlights the possibility of improving the performance of the GH shift of monolayer graphene by well designing the structures, which is essential for the optoelectronic sensors and detectors.

Distinctive Interfacial Charge Behavior and Versatile Photoresponse Performance in Isotropic/Anisotropic WS2/ReS2 Heterojunctions.

Van der Waals (vdWs) heterostructures based on in-plane isotropic/anisotropic 2D-layered semiconducting materials have recently received wide attention because of their unique interlayer coupling properties and hold a bright future as building blocks for advanced photodetectors. However, a fundamental understanding of charge behavior inside this kind of heterostructure in the photoexcited state remains elusive. In this work, we carry out a systematic investigation into the photoinduced interfacial charge behavior in type-II WS2/ReS2 vertical heterostructures via polarization-dependent pump-probe microscopy. Benefiting from the distinctive (ultrafast and anisotropic) charge-transfer mechanisms, the photodetector based on the WS2/ReS2 heterojunction displays more superior optoelectronic properties compared to its constituents with diverse functionalities including moderate photoresponsivity, polarization sensitivity, and fast photoresponse speed. Additionally, this device can function as a self-driven photodetector without the external bias. These results of our work tangibly corroborate the intriguing interlayer interaction in in-plane isotropic/anisotropic heterostructures and are expected to shed light on designing balanced-performance multifunctional optoelectrical devices.

Current methods for GaN synthesis and the limitations

The application of GaN has greatly enhanced the performance of optoelectrical and microelectronic devices. However, the industrial application still faces huge challenges due to the difficulty during the synthesis process. This paper introduces several current methods for GaN synthesis and talks about their advantages and limitations. Four methods are introduced: high pressure nitrogen solution growth process, ammonothermal method, sodium flux process and hydride vapor phase epitaxy. As a result, they are only suitable for specific use like scientific research and can’t be used for massive production. This paper reviews main GaN synthesis methods and points out the limitations they have to overcome for commercial applications.

980 nm-excited multiphoton photocarrier separation process of Yb3+ ions under internal electric field and its upconverting modification on Eu3+ ions

In this work, we report a new optical properties and modification methods for the rare-earth (RE) ion doped upconversion (UC) materials. Here, The Yb3+- Eu3+ co-doped BiOCl layered micrometer crystals were synthesized via the traditional solid state reaction. The experiment results show that due to efficient photocarrier space separation role of IEF, a band-to-band multiphoton excitation process of Yb3+ ions and the subsequent photopolarization role have been triggered inside BiOCl crystals via a commercial 980 nm laser. Then, it combines the photopolarization electric field (PPF) of the laser beam to generate a multiplexed tunable photopolarization on the Eu3+ ions. Consequently, the uponverting Stark splitting and emission intensity ratio of electrical dipole transitions (EDTs) of Eu3+ ions could be tailored simultaneously, either by pumping power or Yb3+ ion doping level, but showing an opposite trend. This research provides new insights into the interaction between the light and materials as well as the UC tailored method of lanthanide ions, which may inspire the design of opto-electric multifunctional devices.

An A-π-D-π-A-Type Organic Semiconductor Based Optoelectrical Device With Photo Response and Optical Memory Behaviors

A novel photoactive semiconductor (named as IDTOT-4F) with an A-π-D-π-A-type configuration is synthesized. It contains an electron-donating fused ring (D) as the core flanked with two π-spacers and is end-capped with two electron-withdrawing units (A). The intramolecular charge transfer effect endows IDTOT-4F with strong and broad light absorption and a relatively narrow band gap (1.46 eV). Thin-film optoelectrical devices based on IDTOT-4F exhibit both n-type and p-type switching behaviors. Besides, the p-channel device shows significantly photoresponsive performance with the maximum P (photo/dark current ratio), R (photoresponsivity), and D* (detectivity) values of around 60, 0.07 A W−1, and 2.5 × 1010 Jones, respectively. Further, IDTOT-4F based optoelectrical devices exhibit good optical memory characteristics with a time constant τ1 of 4.6 h, indicating its applicability to nonvolatile optical memory devices. The results provide new insights into the photoresponsive behavior of fused-ring semiconductors and pave the way for the design of nonvolatile optical memory devices.

Insights into the physical properties and anisotropic nature of ErPdBi with an appearance of low minimum thermal conductivity

We theoretically study the structural, elastic and optical properties of ErPdBi together with its anisotropic behaviors using density functional theory. It is observed that ErPdBi satisfies the Born stability criteria nicely and possesses high quality of machinability. The anisotropic behavior of ErPdBi is reported with the help of theoretical anisotropy indices incorporating 3D graphical presentation, which suggests that ErPdBi is highly anisotropic in nature. It is noticed that the minimum thermal conductivity is very low for ErPdBi compared to the several species. This low value of minimum thermal conductivity introduces the potentiality of ErPdBi in high-temperature applications such as thermal barrier coatings. In addition, deep optical insights of ErPdBi reveal that our material can be used in different optoelectronic and electronic device applications ranging from organic light-emitting diodes, solar panel efficiency, waveguides etc. to integration of integrated circuits. Therefore, we believe that our results will provide a new insight into high-temperature applications and will benefit for the development of promising optoelectric devices as well.

Multistage opto-neuromorphic process system based on commercial optoelectric devices

This paper proposes a structure for opto-neuromorphic processes by using multistage connections of photo-transmitters. The basic unit consists of a light-emitting diode and a photoresistor for a non-inverting photo signal processor. By constructing a cascade structure of these basic units, multistage photo transmission of input light signal is examined in pulse signals with various periods and duty cycles as the input to the first opto-neuromorphic stage. This approach verifies that optical signals can be transmitted through our proposed structure and that by controlling the amount of the transmitted light between the basic units, the gain between stages can be controlled. It is demonstrated that this structure can be used for opto-neuromorphic processes by transmitting light signals as if transmitting signals between neurons with different weights and activation patterns. The proposed structure can be modified with an inverter circuit to build a photo-inverter and by using a combination of photo-transmitter and photo-inverters, it can serve as the basic structure to build more complicated opto-neuromorphic process systems.

Direct Observation of the Thickness-Dependent Dielectric Response of MoS2 and WSe2

We report a direct observation of the thickness-dependent dielectric response of MoS2 and WSe2 via mobile charge carrier measurements based on second-harmonic electrostatic force microscopy (EFM). The measured second harmonic (2ω) signals in the EFM are related to the dielectric polarization of MoS2 and WSe2 due to their charge carrier concentration and mobility. We find that the 2ω signals increase with the thickness in thin layers and saturate to bulk values for MoS2 and WSe2 flakes. These thickness-dependent dielectric behaviors are consistent with previous first-principles calculations for layered transition metal dichalcogenide (TMDC) materials. Our work provides useful information about enhancing the performance of the optoelectrical devices based on layered two-dimensional materials.

Evolution of electrical and magnetotransport properties with lattice strain in La0.7Sr0.3MnO3 film**Project supported by the National Natural Science Foundation of China (Grant No. 10974105), the Double-Hundred Talent Plan, Shandong Province, China (Grant No. WST2018006), the Recruitment Program of High-end Foreign Experts, China (Grant Nos. GDW20163500110 and GDW20173500154), and the Top-notch Innovative Talent Program of Qingdao City, China (Grant No. 13-CX-8).

In this paper, we investigate the effects of lattice strain on the electrical and magnetotransport properties of La0.7Sr0.3MnO3 (LSMO) films by changing film thickness and substrate. For electrical properties, a resistivity upturn emerges in LSMO films, i.e., LSMO/STO and LSMO/LSAT with small lattice strain at a low temperature, which originates from the weak localization effect. Increasing film thickness weakens the weak localization effect, resulting in the disappearance of resistivity upturn. While in LSMO films with a large lattice strain (i.e., LSMO/LAO), an unexpected semiconductor behavior is observed due to the linear defects. For magnetotransport properties, an anomalous in-plane magnetoresistance peak (pMR) occurs at low temperatures in LSMO films with small lattice strain, which is caused by two-dimensional electron gas (2DEG). Increasing film thickness suppresses the 2DEG, which weakens the pMR. Besides, it is found that the film orientation has no influence on the formation of 2DEG. While in LSMO/LAO films, the 2DEG cannot form due to the existence of linear defects. This work can provide an efficient way to regulate the film transport properties.

Layer-Selective Synthesis of MoS2 and WS2 Structures under Ambient Conditions for Customized Electronics

Transition metal dichalcogenides (TMDs) have attracted significant interest as one of the key materials in future electronics such as logic devices, optoelectrical devices, and wearable electronics. However, a complicated synthesis method and multistep processes for device fabrication pose major hurdles for their practical applications. Here, we introduce a direct and rapid method for layer-selective synthesis of MoS2 and WS2 structures in wafer-scale using a pulsed laser annealing system (λ = 1.06 μm, pulse duration ∼100 ps) in ambient conditions. The precursor layer of each TMD, which has at least 3 orders of magnitude higher absorption coefficient than those of neighboring layers, rigorously absorbed the incoming energy of the laser pulse and rapidly pyrolyzed in a few nanoseconds, enabling the generation of a MoS2 or WS2 layer without damaging the adjacent layers of SiO2 or polymer substrate. Through experimental and theoretical studies, we establish the underlying principles of selective synthesis and optimize the laser annealing conditions, such as laser wavelength, output power, and scribing speed, under ambient condition. As a result, individual homostructures of patterned MoS2 and WS2 layers were directly synthesized on a 4 in. wafer. Moreover, a consecutive synthesis of the second layer on top of the first synthesized layer realized a vertically stacked WS2/MoS2 heterojunction structure, which can be treated as a cornerstone of electronic devices. As a proof of concept, we demonstrated the behavior of a MoS2-based field-effect transistor, a skin-attachable motion sensor, and a MoS2/WS2-based heterojunction diode in this study. The ultrafast and selective synthesis of the TMDs suggests an approach to the large-area/mass production of functional heterostructure-based electronics.

Potassium Doping to Enhance Green Photoemission of Light‐Emitting Diodes Based on CsPbBr3 Perovskite Nanocrystals

AbstractMetal halide perovskite nanocrystals (PeNCs) are emerging as one of the most promising materials for optoelectrical devices such as light‐emitting diode (LED) owing to their tunable band gaps, high color purity, and tolerance to defects. Nevertheless, the materials have not yet demonstrated their full potential in practical applications due to the poor stability of PeNCs and their unsatisfactory performance in devices. In this research, LED devices based on CsPbBr3 NCs are successfully demonstrated as an active light‐emitting layer with enhanced efficiency via potassium doping. The study of the effect of potassium dopant on physicochemical properties of inorganic CsPbBr3 PeNCs shows that potassium cations (K+) remain in the crystal structure of perovskite compound as interstitial defects, which results in better coverage of surface ligands on the PeNCs and improved energy band alignment with adjacent electron injection layer (2,2′,2′‐(1,3,5‐benzinetriyl)‐tris(1‐phenyl‐1‐H‐benzimidazole)) and hole injection layer (poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate) in an LED. As a consequence, the green LED with the K‐doped CsPbBr3 PeNCs shows luminance up to 5759 cd m‐2 and external quantum efficiency (EQE) of 5.6%, which is superior to the pristine device without K‐doping (luminance: 4579 cd m‐2, EQE: 4.8%).

Optoelectric Property and Flexibility of Tin-Doped Indium Oxide (ITO) Thin Film.

Transparent conducting electrodes (TCEs) are key materials for electronic devices such as flat panel displays (e.g., a liquid crystal display and a light emitting diode display), photovoltaic cells, and transparent transistors. Tin-doped indium oxide (ITO) is known to be highly conductive/transparent, but rigid. In this study, very thin (<35 nm) ITO films with amorphous phases were prepared on flexible substrates and their optoelectric properties investigated. A 10 nm-thick ITO film was also fabricated. Because of their low thickness, their transmittances were above 80% at ˜550 nm wavelength. Their sheet resistances were below 0.7 kΩ/sq and decreased with increasing film thickness. An interesting observation was that their sheet resistances were nearly unchanged even at a bending radius of ˜2 mm. These optoelectric properties and flexibility demonstrate that the ITO films fabricated in this study are suitable transparent conducting oxides for the electrodes of flexible optoelectric devices.

Solution Process Fabrication of Silver Nanowire Composite Transparent Conductive Films with Tunable Work Function

Solution processed silver nanowire transparent conductive films (AgNW TCFs) have attracted substantial attention for various devices, and optoelectrical properties, surface roughness and work function are major issues affecting commercial applications. In this work, work function tunable AgNW-metal oxide composite TCFs (AgNW composite TCFs) were fabricated by filling AgNW networks with ZnO and Au/MoO3 thin films respectively. The introduction of nanoparticle constructed ZnO thin film as an electron transport layer decreases the work function from 3.82 eV of pristine AgNW TCFs to 2.76 eV due to the charges transport between AgNWs and ZnO nanoparticles, while the Au/MoO3 thin film as a hole injection layer increases the work function to 4.46 eV. The work function tunable AgNW composite TCFs show excellent optoelectronic properties with a sheet resistance of 17 Ω/sq and a transmittance of about 90%. Conductive metal oxides and metal nanoparticles filling the holes of AgNW networks improve the rough surface, uneven electronic field and chemical durability especially for better operating time and stability of organic optoelectrical devices, and provide a simple strategy for regulating the performance of AgNW TCFs.

Improved photovoltaic performance of graphene-based solar cells on textured silicon substrate

Abstract Graphene has attracted much interest as an active layer in heterojunction solar cells due to its outstanding properties such as flexibility, transparency, mechanical strength and elevated carrier mobility. In this research, a new technique was presented in order to enhance the efficiency of graphene–based heterojunction solar cells by employing a textured silicon (Si) substrate. Here, two sets of devices were fabricated based on flat and pyramidal structure of Si and the photovoltaic properties of graphene/Si heterojunction solar cells were compared. Selective chemical dissolution of Si wafers was carried out in order to produce pyramidal skeleton. Reduced graphene oxide (rGO) was then transferred on pyramidal Si through electrophoretic deposition (EPD) technique. The evidence of graphene layers on Si substrates was studied using Raman spectroscopy, X–ray diffractometry (XRD) and atomic force microscopy (AFM) analysis. The morphology of samples indicated an enhancement in rGO/Si interface area when the pyramidal structure is applied. Moreover, the enhanced surface area of this sample which is due to elevated roughness of pyramidal structure and wrinkles of graphene layers promotes its antireflective behavior which was proven using reflectance spectroscopy. The average reflectance of the graphene layer on the textured Si was ∼14% in the wavelength range of 400–800 nm which is lower than that of rGO on flat Si. The improved optical properties of graphene on pyramidal silicon can broaden its potential applications in optoelectrical devices such as high-efficiency solar cells. In order to study the photovoltaic properties of rGO/Si samples, a passive layer was formed on Si substrate and a square frame of Ag was coated on it which was acted as a top contact. The current–voltage characteristics showed that the efficiency of rGO/Si heterojunction solar cells was improved when textured silicon was applied.

WO3–ZnO and CuO–ZnO nanocomposites as highly efficient photoanodes under visible light illumination

We prepared ZnO nanocomposites with WO3 or CuO nanostructures to improve the photocatalytic performance of ZnO nanostructures. Characterization of the nanocomposites using scanning electron microscopy, x-ray diffraction, UV–vis spectrometry and photoluminescence revealed the morphologies and wide light absorption range of the materials. The highest current densities of WO3/ZnO and CuO/ZnO nanocomposites were 1.28 mA cm−2 and 2.49 mA cm−2 at 1.23 V (versus a reversible hydrogen electrode) under AM 1.5 100 mW cm−2, which are ~1.2- and 3.5-fold greater than those of bare ZnO nanostructures, respectively. The easy fabrication process suggests that nanocomposites with narrow bandgap materials, such as WO3 and CuO, will improve the performance of electrochemical and optoelectrical devices such as dye-sensitized solar cells and biosensors.

Degradation problem in silver nanowire transparent electrodes caused by ultraviolet exposure

Silver nanowire (AgNW) transparent electrode inevitably encounters ultraviolet (UV) irradiation from the environment, leading to stability and durability problems when in operation. Since UVA is the most abundant UV band and highly penetrating to AgNW related optoelectrical devices, it is crucial to understand the UVA damage caused to AgNWs. In this study, transparent electrodes composed of pristine AgNWs and glass substrates were manufactured with optimized processing parameters, and then used as model samples for aging tests. UVA exposure was conducted at elevated temperatures including 45 °C, 60 °C and 75 °C at 12 ± 5.5% relative humidity (RH) conditions. Comparative aging tests using conditions of damp heat (85 °C/85% RH) and 105 °C without UV (dark conditions) were also conducted. The relationship between optoelectrical property degradation, morphological changes and photo-corrosion was discussed. Under UVA exposure, the sheet resistance of electrodes increased gradually in an induction period before an abrupt change occurred. A nominal sheet resistance value of 200 Ω/sq was considered as a predestined failure of electrical property. It took 16, 24 and 60 h for UVA exposure at 75 °C, 60 °C and 45 °C, respectively, and 288 h by damp heat aging to degrade to the same status of predestined failure. Aging results of dark conditions indicated no degradation effect on AgNWs for 126 d aging. Moisture caused a different mechanism in damaging the capping agents on AgNWs. Nanocubes of silver chloride and sodium chloride were prone to precipitate at higher aging temperature such as 75 °C with UVA exposure. Sulfidation accounted for deterioration of optical transmittance, and occurred significantly at 45 °C with UVA irradiation and under damp heat conditions. The synergistic aging effect of UVA irradiance at elevated temperature on AgNW degradation has been unambiguously demonstrated. The results of this study provide guidelines for the design of optoelectronic devices when utilizing AgNW transparent electrodes.

Temperature-Dependent Dynamic Carrier Process of FAPbI3 Nanocrystals’ Film

The temperature-dependent optoelectronic properties of FAPbI3 nanocrystals’ (NCs) film are studied by steady photoluminescence (PL), transient PL, and photocurrent measurements. Steady PL shows that FAPbI3 NCs experience three phases of α, β, and γ, as temperature decreases from 295 to 77 K. Analyzed with the two-carrier model of free carrier and trapped carrier, the dynamic PL shows that the lifetime and the percentage of free carrier in α phase are enhanced obviously with temperature relative to β and γ phase, while those of the trapped carrier are insensitive to the detailed FAPbI3 phase. Meanwhile, the excitation intensity-dependent photocurrent follows a similar behavior, suggesting that the photocurrent is determined by the free carrier migration controlled by the trapping state distribution and phonon scattering. This result helps to further understand the generation, diffusion, and transport of carriers in FAPbI3 NCs film and to design and optimize the optoelectrical devices of high performance.

Effects of composition and temperature on the exciton emission behaviors of Mo(SxSe1−x)2 monolayer: experiment and theory

Exploring the excitonic behavior of two-dimensional (2D) alloys is of great significance, which not only could promote the understanding of fundamental photophysics in optoelectric devices, but could also guide the functional improvement of future applications. Here, we demonstrate the controllable synthesis of monolayer Mo(SxSe1−x)2 nanosheets using a one-step chemical vapor deposition method and systematical investigation on the exciton emission characteristics based on the temperature-dependent photoluminescence spectroscopy (PL) experiments. As a result, the tunable bandgap of Mo(SxSe1−x)2 alloys between 1.52 and 1.85 eV can be achieved, which is consistent with the theoretical results calculated by the ab initio density function theory. Besides, both the exciton and trion behaviors in Mo(SxSe1−x)2 are observed from the PL spectra at T = 80 K. More intriguingly, the differences between the emission energy of exciton and trion (ΔE), known as the dissociation energy of the trion, are positively correlated to the concentrations of the sulfur (S) elements, which is also proved by the theoretical calculation. Combining the experimental and theoretical results, the phenomena can be explained by the reduced dielectric screening effect and the increasing Fermi energy (EF) along with the increasing of sulfur in Mo(SxSe1−x)2 nanosheets, jointly leading to the increase of ΔE. Furthermore, the evolutions of ΔE in Mo(SxSe1−x)2 alloys as a function of temperature have been also discovered, which lay the foundation for the potential uses of 2D alloys in optoelectronic devices.