Excellent Optoelectrical Properties Research Articles

Stable Ultrathin Ag Electrodes by Tailoring the Surface of Plastic Substrates for Flexible Organic Light-Emitting Devices.

Flexible transparent metal electrodes (FTMEs) have significant application potentials in the fields of flexible optoelectronic devices due to their outstanding optical transmittance and electrical conductivity. However, obtaining excellent optoelectrical properties and mechanical flexibility of FTMEs is challenging because ultrathin metal layers usually follow an island growth mode. In this paper, flexible transparent ultrathin Ag electrodes with high mechanical stability and good optoelectrical properties were exploited by tailoring the surface properties of plastic substrates with ultraviolet-ozone (UVO) treatment for regulating the nucleation and growth kinetics of Ag films. The composite transparent electrodes of Ag (9 nm)/MoO3 (20 nm) fabricated on the UVO-treated polyethylene terephthalate (PET) substrates possess a low sheet resistance of ∼7.9 Ω/sq, a high optical transmittance of ∼87.2% at 550 nm, a long-period environmental stability of 30 days (∼65 °C, ∼80% humidity), and excellent mechanical flexibility of 100,000 bending cycles at a bending radius of 1.5 mm. These properties are derived from the surface treatment of PET substrates by UVO, which increases substrate surface energy and produces chemical nucleation sites of the phenolic hydroxyl groups. The phenolic hydroxyl groups generated on the PET surface not only provided efficient nucleation sites for subsequent Ag film growth but also formed C-O-Ag bonds between the substrate surface and the Ag layer, which act as "anchor chains" to fix firmly the Ag atoms on the substrate surface. As a universal applicability strategy, the composite electrodes on the UVO-treated polyethylene naphthalate (PEN) and norland optical adhesive 63 (NOA63) substrates also possess excellent optoelectrical properties and mechanical flexibility. Based on the ultrathin Ag composite electrodes, the flexible white organic light-emitting devices with PET, PEN, and NOA63 as substrates present the maximum current efficiencies of 53.0, 77.0, and 65.2 cd/A, respectively.

Highly Stable Silver Nanowires/Biomaterial Transparent Electrodes for Flexible Electronics.

Flexible transparent electrodes (FTEs) possess excellent optoelectrical properties, mechanical robustness, and environmental adaptability are important for the industrial scale development of flexible electronics. Silver nanowires (AgNWs) are widely used in FTEs owing to their excellent optoelectrical properties and mechanical flexibility. However, the high surface roughness and poor stability of AgNWs FTEs still limit their practical applications. Here, highly stable FTEs are demonstrated via combining AgNWs and biomaterial propolis which is eco-friendly and antioxidative. The AgNWs/propolis composite transparent electrodes exhibit excellent optoelectrical performance as well as a smooth surface (root-mean-square roughness ∼ 6.2 nm). Meanwhile, the composite electrodes possess high mechanical stability (10,000 bending cycles), thermal stability, and environmental adaptability (60 °C and 85 ± 3% humidity for 700 h). The versatile composite FTEs show great potential applications in organic light-emitting diodes and pressure sensors, which exhibit high performance, mechanical stability, and environmental adaptability. Our strategy of introducing biocompatible materials into metallic nanowires opens up new possibilities to achieve high-quality FTEs in a simple and eco-friendly way.

Continuous and controllable synthesis of CsxPbyBrz-based perovskite nanocrystals by a microfluidic system

All-inorganic perovskite CsPbBr3 shows great potential in optoelectronic device applications due to their excellent optoelectrical properties. However, its practical use is still dauntingly plagued by the poor stability. Recently, its derivatives Cs4PbBr6 and CsPb2Br5 featured with good stability and the versatility in crystal structure have emerged as promising alternatives. Nevertheless, synthesizing such a series of perovskites through a facile, scalable, and high-yield method is challenging, and their luminescence mechanisms are still elusive. Herein, we reported a facile and rapid method for continuous synthesis of CsxPbyBrz-based perovskites (including Cs4PbBr6, CsPbBr3/Cs4PbBr6, CsPbBr3, CsPbBr3/CsPb2Br5, CsPb2Br5) via a microfluidic system. The transformations of different dimensional perovskite nanocrystals were realized by simply adjusting the precursor flow rate. CsPbBr3/Cs4PbBr6 and CsPbBr3/CsPb2Br5 composite nanocrystals exhibited bright emissions in their solid forms, where CsPbBr3 nanocrystals uniformly embedded in Cs4PbBr6 or CsPb2Br5 matrix. Simultaneously, strong green fluorescence exclusively from CsPbBr3 nanocrystals was detected and related energy transfer in composite nanocrystals was further investigated. Finally, the highly stable CsPbBr3/Cs4PbBr6 nanocrystals were used for the construction of WLED with a wide color gamut of 131% NTSC. This work provides a novel path for large-scale and controlled synthesis of the ternary Cs-Pb-Br materials for specific light-emitting applications.

Effect of the thicknesses of the Al2SiO5 ion conductor on the opto-electrical properties for all-solid electrochromic devices

In this study, an all-solid-state electrochromic device (ECD) with the structure of ITO/WO3/Al2SiO5/NiOx/ITO was prepared, and the effect of the Al2SiO5 solid electrolyte thicknesses on the opto-electrical performance was investigated. The microstructure and surface morphology were characterized using XRD, SEM and AFM, and the surface morphology and degree of surface looseness demonstrate a significant influence on the opto-electrical properties of ECDs. The charge transfer dynamics at the solid-solid interface were characterized using EIS to obtain an ionic conductivity of 4.637 × 10-8 S/cm. CV, CA and UV–Visible spectra were employed to record the in situ electrochemical and optical properties. The results revealed that the highest optical modulation was 44.58%, the coloring and bleaching times were 14.8 s and 3.7 s, and the highest coloring efficiency was 98.17 cm2/C, which indicates that excellent opto-electrical properties were obtained. When the thickness increases, the degree of surface dense morphology transforms, and the loose morphology is more favorable for ion conductivity, which improves the opto-electrical properties. The results in this study provide insights into the understanding of Al3+-based all-solid-state ECDs, which promote the exploration of new types of Al3+ ionic conductors for all-solid-state ECDs.

N-Ga2O3/p-SnS heterojunction thin-films based transparent photovoltaic device

Due to the urgent requirement of transparent photovoltaics (TPVs) for building integrated applications, solar-blind photodetectors, and high-power devices, the immediate necessity is to develop solar cells with high transparency and good photovoltaic performance. The wide bandgap Ga2O3 has attracted significant attention for its excellent optoelectrical properties; however, the role of Ga2O3 in inorganic TPV is least explored yet. The wide bandgap Ga2O3 may give the device high optical transmittance in the UV to the NIR region by absorbing harmful UV radiations. Herein, we propose and study a novel, eco-friendly sputter-grown p-n heterojunction thin-films device by utilizing n-Ga2O3 and p-SnS for TPVs applications. The Ga2O3/SnS heterojunction device is tested for the transparent light harvester, and its photovoltaic performance is improved using variation in Ga2O3 layer thickness. All fabricated devices demonstrated optical transmittance of ~70 ± 2 % in the near-infrared (NIR) region and ~50 ± 2 % in the visible light region. By optimizing the layer thickness of Ga2O3 thin-film, the Ga2O3/SnS heterojunction quality was significantly improved and achieved a high photocurrent density of 1.15 mA/cm2 and the open-circuit voltage of 527 mV. Our fabricated device showed a power conversion efficiency of 2.9 % under UV light illumination. Furthermore, the proposed transparent photoelectric device exhibited an excellent photovoltaic effect, confirming the potential applications of the Ga2O3/SnS heterojunction for advanced optoelectronics.

All-Inorganic Perovskite Single Crystals for Optoelectronic Detection

Due to their many varieties of excellent optoelectric properties, perovskites have attracted large numbers of researchers in the past few years. For the hybrid perovskites, a long diffusion length, long carrier lifetime, and high μτ product are particularly noticeable. However, some disadvantages, including high toxicity and instability, restrict their further large-scale application. By contrast, all-inorganic perovskites not only have remarkable optoelectric properties but also feature high structure stability due to the lack of organic compositions. Benefiting from these, all-inorganic perovskites have been extensively explored and studied. Compared with the thin film type, all-inorganic perovskite single crystals (PSCs) with fewer grain boundaries and crystalline defects have better optoelectric properties. Nevertheless, it is important to note that only a few reports to date have presented a summary of all-inorganic PSCs. In this review, we firstly make a summary and propose a classification method according to the crystal structure. Then, based on the structure classification, we introduce several representative materials and focus on their corresponding growth methods. Finally, applications for detectors of all-inorganic PSCs are listed and summarized. At the end of the review, based on the current research situation and trends, some perspectives and advice are proposed.

Soft X-ray characterization of halide perovskite film by scanning transmission X-ray microscopy

Organic–inorganic metal halide perovskites (MHPs) have recently been receiving a lot of attention due to their newfound application in optoelectronic devices, including perovskite solar cells (PSCs) which have reached power conversion efficiencies as high as 25.5%. However, the fundamental mechanisms in PSCs, including the correlation of degradation with the excellent optoelectrical properties of the perovskite absorbers, are poorly understood. In this paper, we have explored synchrotron-based soft X-ray characterization as an effective technique for the compositional analysis of MHP thin films. Most synchrotron-based studies used for investigating MHPs so far are based on hard X-rays (5–10 keV) which include various absorption edges (Pb L-edge, I L-edge, Br K-edge, etc.) but are not suited for the analysis of the organic component in these materials. In order to be sensitive to a maximum number of elements, we have employed soft X-ray-based scanning transmission X-ray microscopy (STXM) as a spectro-microscopy technique for the characterization of MHPs. We examined its sensitivity to iodine and organic components, aging, or oxidation by-products in MHPs to make sure that our suggested method is suitable for studying MHPs. Furthermore, methylammonium triiodide with different deposition ratios of PbI2 and CH3NH3I (MAI), and different thicknesses, were characterized for chemical inhomogeneity at the nanoscale by STXM. Through these measurements, we demonstrate that STXM is very sensitive to chemical composition and homogeneity in MHPs. Thus, we highlight the utility of STXM for an in-depth analysis of physical and chemical phenomena in PSCs.

Study of CZTSSe-Based Solar Cells with Different ETMs by SCAPS

Third-generation thin-film solar cells based on CZTSSe are highly promising because of their excellent optoelectrical properties, earth-abundant, and non-toxicity of their constituent elements. In this work, the performance of CZTSSe-based solar cells with TiO2, CdS, and ZnSe as electron transporting materials (ETMs) was numerically investigated using the Solar Cell Capacitance Simulator (SCAPS). The effect of the active layer’s thickness and electron affinity, different buffer layers, and the contour plot of the operating temperature versus thickness of the CdS buffer layer were studied. The results show that the optimum power conversion efficiency for CdS, TiO2, and ZnSe, as the ETMs, is 23.16%, 23.13%, and 22.42%, respectively.

Fluorescent carbon dots for sensing metal ions and small molecules

As one kind of new carbon nanomaterials, carbon dots (CDs) have excellent optoelectric properties and indicate widespread applications in sensing, display, energy storage and catalysis. Although the precise structure of CDs needs to be further analyzed and investigated, it is generally accepted that there are large amounts of electron-rich groups and substituents on the surface of CDs, such as amino, hydroxyl, carboxyl, thiol, and heterocycles, depending on the precursors used and preparation route. Therefore, the inherent advantages of electron-rich CDs make them very suitable for sensing specific substances including metal ions and electron-deficient small molecules. The sensing performance of CDs can be further improved through physical or chemical surface modification. The recent progress in CDs application for sensing metal ions and small functional molecules in recent years is specifically summarized and the sensing mechanism behind that was elaborated. Finally, the research prospective and performance improvement for CDs are addressed.

Low Threshold Microlasers Based on Organic-Conjugated Polymers.

Conjugated polymers have emerged as ideal organic laser materials for the excellent optoelectrical properties and facile processability. During a typical lasing process, resonator configurations with specific geometry are essential to provide optical feedback and then amplified light. Herein, we summarized the geometry and working mechanism of several typical resonator configurations formed with conjugated polymers. Meanwhile, recent advances in fabrication techniques and lasing performance are also discussed to provide new ideas for the design and optimization of microcavity geometries. Followed by the advances of practical applications in fields of laser sensing, bioimaging, and laser illumination/display, we make a summary of the existing bottlenecks and future perspectives of electrically driven organic lasers toward laser display and illumination.

Nanoarray heterojunction and its efficient solar cells without negative impact of photogenerated electric field

Efficient, stable and low-cost solar cells are being desired for the photovoltaic conversion of solar energy into electricity for sustainable energy production. Nanorod/nanowire arrays of narrow-bandgap semiconductors are the promising light-harvesters for photovoltaics because of their excellent optoelectrical properties. Here, the array of preferentially oriented antimony trisulfide (Sb2S3) single-crystalline nanorods is grown on polycrystalline titania (TiO2) film by a tiny-seed-assisted solution-processing strategy, offering an Sb2S3/TiO2 nanoarray heterojunction system on a large scale. It is demonstrated that the Sb2S3 nanorod growth follows a tiny-seed-governed orientation-competing-epitaxial nucleation/growth mechanism. Using a conjugated polymer hole transporting layer on the heterojunction, we achieve a power conversion efficiency of 5.70% in the stable hybrid solar cell with a preferred p-type/intrinsic/n-type architecture featuring effectively straightforward charge transport channels and no negative impact of photogenerated electric field on device performance. An architecture-dependent charge distribution model is proposed to understand the unique photovoltaic behavior.

Tailoring of a Phenothiazine Core for Electrical Conductivity and Thermal Stability: Hole-Selective Layers in Perovskite Solar Cells.

Hole-selective layers are an indispensable component for the fabrication of effective perovskite solar cells. We designed and developed two phenothiazine-based hole transport materials: PTADAnCBZ with an electron-donating sulfur atom and PTODAnCBZ with an electron-withdrawing sulfone group in the core. PTODAnCBZ in contrast to PTADAnCBZ possesses a unique molecular orbital distribution and lower dihedral angles, which endowed it with excellent optoelectrical properties, improved charge transportation, and thermal stability. The solar cells fabricated with PTODAnCBZ yielded a higher photovoltaic (PV) performance as compared to PTADAnCBZ and were on par in terms of performance with those fabricated with Spiro-OMeTAD. Notably, the phenothiazine-based PV devices showed improved stability under multi-stress conditions including moisture, moisture and light, and moisture and heat. Phenothiazine-based molecules showed unparalleled thermal stability as compared to the doped Spiro-OMeTAD. Our findings pinpoint the advantages of cost-effective phenothiazine with dioxide as hole-selective layers and suggest its application in a variety of optoelectrical devices such as PVs and organic LED.

Selective Pattern Growth of Atomically Thin MoSe2 Films via a Surface-Mediated Liquid-Phase Promoter

Two-dimensional transition metal dichalcogenides (TMDs) offer numerous advantages over silicon-based application in terms of atomically thin geometry, excellent opto-electrical properties, layer-number dependence, band gap variability, and lack of dangling bonds. The production of high-quality and large-scale TMD films is required with consideration of practical technology. However, the performance of scalable devices is affected by problems such as contamination and patterning arising from device processing; this is followed by an etching step, which normally damages the TMD film. Herein, we report the direct growth of MoSe2 films on selective pattern areas via a surface-mediated liquid-phase promoter using a solution-based approach. Our growth process utilizes the promoter on the selective pattern area by enhancing wettability, resulting in a highly uniform MoSe2 film. Moreover, our approach can produce other TMD films such as WSe2 films as well as control various pattern shapes, sizes, and large-scale areas, thus improving their applicability in various devices in the future. Our patterned MoSe2 field-effect transistor device exhibits a p-type dominant conduction behavior with a high on/off current ratio of ∼106. Thus, our study provides general guidance for direct selective pattern growth via a solution-based approach and the future design of integrated devices for a large-scale application.

Design rules for high-efficiency both-sides-contacted silicon solar cells with balanced charge carrier transport and recombination losses

The photovoltaic industry is dominated by crystalline silicon solar cells. Although interdigitated back-contact cells have yielded the highest efficiency, both-sides-contacted cells are the preferred choice in industrial production due to their lower complexity. Here we show that omitting the layers at the front side that provide lateral charge carrier transport is the key to excellent optoelectrical properties for both-sides-contacted cells. This results in a conversion efficiency of 26.0%. In contrast to standard industrial cells with a front side p–n junction, this cell exhibits the p–n junction at the back surface in the form of a full-area polycrystalline silicon-based passivating contact. A detailed power-loss analysis reveals that this cell balances electron and hole transport losses as well as transport and recombination losses in general. A systematic simulation study led to some fundamental design rules for future >26% efficiency silicon solar cells and demonstrates the potential and the superiority of these back-junction solar cells. Front- and back-junction silicon photovoltaics dominate the market thanks to a lower manufacturing complexity compared with that of other device designs yet advances in efficiency remain elusive. Richter et al. now present an optimized design for the front and back junctions that leads to a 26.0%-efficient cell.

On Tailoring Co-Precipitation Synthesis to Maximize Production Yield of Nanocrystalline Wurtzite ZnS.

Pyroelectric materials can harvest energy from naturally occurring ambient temperature changes, as well as from artificial temperature changes, notably from industrial activity. Wurtzite- based materials have the advantage of being cheap, non-toxic, and offering excellent opto-electrical properties. Due to their non-centrosymmetric nature, all wurtzite crystals have both piezoelectric and pyroelectric properties. Nanocrystalline wurtzite ZnS, being a room temperature stable material, by contrast to its bulk counterpart, is interesting due to its still not well-explored potential in piezoelectric and pyroelectric energy harvesting. An easy synthesis method—a co-precipitation technique—was selected and successfully tailored for nanocrystalline wurtzite ZnS production. ZnS nanopowder with nanoparticles of 3 to 5 nm in size was synthesized in ethyl glycol under medium temperature conditions using ZnCl2 and thiourea as the sources of Zn and S, respectively. The purified and dried ZnS nanopowder was characterized by conventional methods (XRD, SEM, TEM, TG and FTIR). Finally, a constructed in-house pilot plant that is able to produce substantial amounts of wurtzite ZnS nanopowder in an environmentally friendly and cost-effective way is introduced and described.

Investigation of structural, optical, electrical and mechanical properties of transparent conducting ‘Ag’ electrodes

The high flexibility and excellent optoelectrical properties of thin metal films hold great potential for their use as transparent conducting electrodes in large-area flexible optoelectronic devices. Thermally evaporated transparent conducting silver (Ag) thin films (48–75 nm) are characterized using X-ray diffraction, scanning electron microscopy, scanning probe microscopy, UV–Vis–NIR spectroscopy, Hall effect, Nanoindentation and Kelvin probe techniques. Significant textured growth along (111) crystal plane direction is observed. The film with least thickness exhibits interesting wrinkled surface morphology. Surface roughness is observed to increase with film thickness. High transmittance and considerable transparency are observed in UV and NIR regions, respectively. Surface work function (4.7–4.9 eV) is determined from Kelvin probe measurements. Nanoindentation studies indicate reduced hardness with increasing film thickness. These experiments suggest that Ag film of thickness 63 nm shows better optical and electrical properties and may be a potential transparent conducting supplement for specific applications like transparent electrodes and interconnects.

High-Performance Flexible Transparent Conductive Films Enabled by a Commonly Used Antireflection Layer.

Recently, silver nanowire-based transparent conductive films (AgNW-based TCFs) with excellent comprehensive performance have aroused wide and great interest. However, it is always difficult to simultaneously improve the performances of TCFs in all aspects. In this work, by introducing silica nanoparticles (SiO2-NPs) with a smaller particle size, several properties of AgNW-based TCFs were optimized successfully. The transmittance and conductivity were improved simultaneously, and smaller particle size was proven to be more suitable to achieve TCFs with excellent optoelectrical properties. Typically, an AgNW/SiO2-based TCF with a sheet resistance of 250 Ω/sq and transmittance of 93.6% (including the poly (ethylene terephthalate) substrate, abbreviated as PET) could be obtained by using SiO2-NPs with a size of ∼21 nm, and this transmittance is even higher than that of the bare PET (91.8%) substrate. We demonstrated that the layer formed through self-assembly of SiO2-NPs can cut down the light scattering on the AgNW surface through total reflection, thus leading to a low haze of AgNW/SiO2-based TCFs. Very interestingly, the SiO2-NPs conducted away most of the heat generated during laser ablation, protecting the AgNWs from excessive melt and PET from empyrosis, and thus ensuring the TCFs with high transmittance and patterning accuracy. Besides, AgNW/SiO2-based TCFs have smaller surface roughness, better flexibility, and adhesive force. To the best of our knowledge, the comprehensive performance of the AgNW/SiO2-based TCFs reaches the highest level among recently reported novel TCFs.

Electrofluorochromic Device Based on a Redox-Active Europium(III) Complex

Electrofluorochromism owing to redox reactions on the center europium (Eu) ion in ionic liquids is examined for the helicate complexes (abbreviated as EuL) with a hexadentate pyridine derivative. Typical electrofluorochromism requires extra electroactive units complementing intra- or intermolecular energy transfer to quench fluorophores. Herein, an unprecedentedly simplified electrofluorochromic system overcoming such issues is demonstrated by utilizing reversible electrochemistry of EuL between Eu3+ and Eu2+, which accompanies large emission transition. A three-electrode electrochemical switching device is facilely prepared with an ionic liquid [BMIM][PF6] and EuL mixture. Benefiting from the stable helical coordinated structure of the ligand in [BMIM][PF6], highly enhanced red fluorescence of EuL with small quantity (≤1 wt %) is utilized. Rapid response and large contrast of luminescence are achieved: the emission is drastically quenched at the reduced state (Eu2+) and it is successfully restored by subsequent oxidation (Eu3+). The reversible fluctuation of excitation and emission spectra of an electrofluorochromic device is achieved in the potential window within ±2 V. The device affords excellent optoelectric properties in terms of well-controlled luminescence switching depending on the applied potentials and its durability. This work paves an efficient and smart way toward Eu luminescence control in optoelectronic devices.

Effects of doping graphene on the performance of graphene–silicon hybrid photoconductive detectors

The photoconductive detector based on a graphene–silicon heterostructure retains excellent optoelectrical properties, in which the graphene plays an indispensable role, acting as the carrier transporting channel. Herein, we systematically investigate by simulation and experiment how doping graphene will affect the performance of graphene–silicon hybrid photoconductors. Compared with lightly p-doped graphene devices, the responsivity can be made nine times better through increasing the p-type doping level. In addition, the net photocurrent can also be enhanced by about four times through increasing the n-type doping level of graphene. We attribute this improvement to the barrier height change adjusted by doping graphene, which can optimize the lifetime and transport of photocarriers. Such a graphene–doping method, that manipulates the junction region, could offer useful guidance for achieving high-performance graphene photodetectors.

High-performance p-channel transistors with transparent Zn doped-CuI

‘Ideal’ transparent p-type semiconductors are required for the integration of high-performance thin-film transistors (TFTs) and circuits. Although CuI has recently attracted attention owing to its excellent opto-electrical properties, solution processability, and low-temperature synthesis, the uncontrolled copper vacancy generation and subsequent excessive hole doping hinder its use as a semiconductor material in TFT devices. In this study, we propose a doping approach through soft chemical solution process and transparent p-type Zn-doped CuI semiconductor for high-performance TFTs and circuits. The optimised TFTs annealed at 80 °C exhibit a high hole mobility of over 5 cm2 V−1 s−1 and high on/off current ratio of ~107 with good operational stability and reproducibility. The CuI:Zn semiconductors show intrinsic advantages for next-generation TFT applications and wider applications in optoelectronics and energy conversion/storage devices. This study paves the way for the realisation of transparent, flexible, and large-area integrated circuits combined with n-type metal-oxide semiconductor.