Flexible Optoelectronic Devices Research Articles

Temperature and Flexural Endurances of Aluminum-Doped Zinc Oxide Thin Films on Flexible Polyethylene Terephthalate Substrates: Pathways to Enhanced Flexibility and Conductivity

This work investigates the endurance and performance of aluminum-doped zinc oxide (AZO) thin films fabricated on flexible polyethylene terephthalate (PET) substrates, providing new insights into their degradation linked to mechanical flexing and accelerated thermal cycling (ATC). The current study uniquely combines cyclic bending fatigue at 23 °C and 70 °C with ATC between 0 °C and 100 °C, simulating operational stresses in real-world environments, in contrast to previous research that has focused primarily on either isolated mechanical or thermal effects. The 425 nm thick films showed high transparency and conductivity, making them suitable materials for flexible electronics and optoelectronic devices. In this work, electrical resistivity, one of the most important performance parameters, was investigated after each mechanical or thermal cycle. The results indicate that mechanical cycling at high temperatures can drastically enhance the crack formation and electrical degradation, with an over 250% change in the electrical resistance (PCER) after 12,000 cycles at 70 °C and more than 300% after 500 thermal cycles. The highly deleterious effects of combined stressors on the structural integrity and electrical properties of AZO films are underlined by these observations. This study further suggests that the design of more robust AZO-based materials/coatings would contribute toward achieving better durability in flexible electronic applications. These findings also go hand in glove with the ninth goal of the United Nations’ Sustainable Development Goals, specifically Target 9.5: Enhance Research and Upgrade Industrial Technologies.

Ultra Stable X‐Ray Imaging Through a Mutually Reinforcing Strategy Between Perovskite Nanocrystal‐Polymethyltrifluoropropylsiloxane

AbstractThe instability of halide perovskite nanocrystals (PNCs) and related composite polymer films posed considerable challenges for application in flexible optoelectronic devices. Herein, perovskite nanocrystal‐polymethyltrifluoropropylsiloxane (PNCs‐PMFS) composites are developed that exhibit outstanding optical stability and irradiation resistance through a mutually reinforcing strategy. The photoluminescence (PL) intensity of PNCs‐PMFS remained stable after four heating cycles, whereas perovskite nanocrystal‐polydimethylsiloxane (PNCs‐PDMS) composites exhibited a 31% decrease in PL intensity. Moreover, PNCs‐PMFS demonstrated superior luminescence stability under UV and X‐ray irradiation due to strong ion‐dipole interactions between PNCs and trifluoromethyl (CF3) dipoles. Under γ‐ray irradiation (300 kGy), PNCs‐PMFS retained 73% (2.86 MPa) of the initial mechanical strength, while PMFS without PNCs retained only 51%. This enhancement is attributed to the effective reduction of free radical concentration in the system by PNCs, as confirmed by electron spin resonance (ESR) and curing curve. Density‐functional theory (DFT) calculations further indicated that PNCs adsorbed free radicals, thereby facilitating interfacial charge transfer and forming a stable resonance structure. These advancements enabled PNCs‐PMFS to serve as scintillation screens for X‐ray detection and imaging, achieving a spatial resolution of 19.0 lp mm−1 and a detection limit of 3.78 µGy s−1, offering novel insights for designing of X‐ray detectors in high‐energy radiation environments.

Two-Dimensional MoS2-Based Photodetectors

Nanomaterials can significantly improve the analytical performance of optical sensors for environmental pollutants. Two-dimensional (2D) molybdenum sulfide (MoS2) exhibits some unique physicochemical properties, such as strong light–matter interactions, bandgap tunability, and high carrier mobility, which are beneficial for constructing flexible optoelectronic devices. In this review, the principle and classification of 2D MoS2-based photodetectors (PDs) are introduced, followed by a discussion about the physicochemical properties of 2D MoS2, as well as the structure–property relationships of 2D MoS2-based photoactive materials for PDs to understand the modulation strategies for enhancing the photodetection performance. Furthermore, we discuss significant advances in the surface modification and functionalization of 2D MoS2 for developing high-performance PDs, particularly focusing on synthesis pathways, modification strategies, and underlying physiochemical mechanisms for enhanced photodetection capabilities. Finally, conclusions and research perspectives on resolving significant bottlenecks or remaining challenges are offered based on recent developments in 2D MoS2-based PDs.

Plasticizer Design Principle of "Like Dissolves Like": Semiconductor Fluid Plasticized Stretchable Fully π-Conjugated Polymers Films for Uniform Large-Area and Flexible Deep-Blue Polymer Light-Emitting Diodes.

Physical blending of fully π-conjugated polymers (FπCPs) is an effective strategy to achieve intrinsically stretchable films for the fabrication of flexible optoelectronic devices, but easily causes phase separation, nonuniform morphology and uncontrollable photo-electronic processing. This may cause low efficiency, unstable and nonuniform emission, and poor color purity, which are undesirable for deep-blue flexible polymer light-emitting diodes (FPLEDs). Herein, a "Like Dissolves Like" design principle to prepare semiconductor fluid plasticizers (SFPs) is established and intrinsically stretchable FπCPs films via external plasticization for high-performance deep-blue FPLEDs are developed. Three fundamental requirements are proposed, "similar conjugated skeleton, similar molecular polarity, and similar electronic structures," to prepare model-matched nonpolar M1 and polar M2 plasticizers for poly(9,9-dioctylfluorene) (PFO). Large-area plasticized PFO films exhibit an efficient, narrowband, and stable ultra-deep-blue electroluminescence (FWHM < 40nm, CIE: 0.12, 0.04), uniform morphology, and excellent intrinsic stretchability (fracture strain >20% and crack-onset strain >120%). Efficient and uniform deep-blue FPLEDs based on stretchable PFO films are fabricated with a high brightness of ≈3000cd cm-2. Finally, blended PFO films exhibit outstanding stretch-deformation cycling stability of their deep-blue electroluminescent behavior, confirming the effectiveness of the "Like Dissolves Like" principle to design matched SFPs for stretchable FπCP films in flexible electronics.

Strain engineered structural, mechanical and electronic properties of monolayer phosphorene: A DFT study

Role of strain in engineering structural, mechanical and electronic properties and their anisotropy in monolayer phosphorene has been investigated using density functional calculations. The in-plane Youngs's modulus, estimated as 102.45 N/m and 23.43 N/m along zigzag and armchair directions, respectively confirms substantial elastic anisotropy. Phosphorene is also verified as a super flexible material which can withstand a tensile strain up to 35 % (70 %) along zigzag (armchair) direction. Furthermore, band gap (Eg) engineering, band dispersion mechanism, multiple direct-to-indirect band gap transition, semiconductor-to-metal transition, anisotropy in carrier effective mass in phosphorene due to application of strain has been comprehensively investigated and explained. It is predicted that, Eg of phosphorene can be widely tuned in the range, 0–1.12 eV by applying compressive and tensile strains. The observed super flexible nature, wide tunable electronic properties and their anisotropy recommends phosphorene a preferred material for designing flexible optoelectronic devices with directional selectivity.

Piezoelectric dispenser printing and intense pulsed light sintering of AgNW/PEDOT:PSS hybrid transparent conductive films

A hybrid transparent conductive films (TCFs) combining silver nanowires (AgNWs) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was fabricated using a piezoelectric dispenser printing method. The innovation lies in optimizing the ink composition and employing intense pulsed light sintering to enhance the TCF's performance. The optimized AgNW/PEDOT:PSS mixture, with an 8:2 ratio, achieved a figure of merit (FOM) of 28.05 × 10-3Ω-1, corresponding to a sheet resistance of 9.93 Ω sq-1and a transmittance of 88.0%. This represents a significant improvement over the pre-sintering FOM of 24.09 × 10-3Ω-1. Additionally, the hybrid TCFs exhibited outstanding structural stability, maintaining functionality after 7000 mechanical bending cycles. The results enable applications in flexible optoelectronic devices, and highlight the potential of this method to produce high-performance, flexible, and durable transparent electrodes, advancing the development of next-generation optoelectronic devices.

Damage-free non-mechanical transfer strategy for highly transparent, stretchable embedded metallic micromesh electrodes

Stretchable, flexible, transparent electrodes garner significant research interest as indispensable components of flexible optoelectronic devices. However, frequent mechanical transfers during processing pose a considerable challenge in preparing electrodes of scalable size with superior performance and intact structure. Herein, we present a stretchable embedded metallic micromesh (SEMM) electrode with high optoelectronic and robust mechanical properties. The SEMM electrode is fabricated via a damage-free non-mechanical transfer strategy with the assistance of a bifunctional metal transition layer that serves as both a seed layer during electrodeposition and a sacrificial layer during stripping of the electrode. Consequently, the SEMM electrode features a scalable size and an intact structure. By optimizing the electrodeposition parameters, the SEMM achieves high optical transmittance (∼83 %) and low sheet resistance (0.22 Ω sq−1), with a figure of merit reaching 8600–53 times greater than that of commercial polyethylene terephthalate-indium tin oxide (PET-ITO). Furthermore, the SEMM exhibits excellent mechanical stability, enduring up to 60 % of tensile strain and maintaining almost constant normalized resistance after 20,000 bending cycles. Based on the SEMM, a transparent film heater yields rapid response time, low operating voltage, and fast defogging capability. This non-mechanical transfer strategy offers a compelling approach for enhancing the structural integrity and scalability of stretchable embedded transparent electrodes.

Flexible poly (ethylene-co-vinyl acetate)/copper oxide nanocomposites: A promising avenue for energy storage applications

The use of metal oxide nanoparticles in polymeric materials to improve optical properties, dielectric constant, mechanical strength, and electrical conductivity has generated significant interest in fabricating flexible optoelectronic and energy storage devices. Herein, copper oxide (CuO) nanoparticle-reinforced poly (ethylene-co-vinyl acetate) (EVA) nanocomposites were prepared using a solvent-free two roll mill mixing method. Fourier transform infrared (FTIR) analysis reveals the distinct absorption peaks of CuO in the EVA matrix. The addition of CuO nanoparticles improved the crystallinity of EVA, as confirmed by X-ray diffraction (XRD). The addition of CuO to EVA led to an increase in the refractive index and a decrease in bandgap energy, as well as a broadening and intensification of UV–visible absorption, indicating strong interactions between CuO nanoparticles and the EVA matrix. Field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) revealed a homogeneous dispersion of CuO nanoparticles throughout the EVA matrix. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) demonstrated that the incorporation of CuO nanoparticles into EVA significantly enhanced its thermal properties. The electrical characteristics studies showed that the AC conductivity and dielectric constant of EVA increased significantly with increasing temperatures and CuO nanoparticle loading levels. EVA containing 5 wt% CuO exhibited the highest conductivity and the lowest activation energy. CuO nanoparticle reinforcement significantly enhanced the tensile, tear, and impact strength of EVA while reducing elongation at break up to a particular concentration. The nanocomposites containing 5 wt% CuO exhibited the highest tensile, tear resistance, and impact strengths, outperforming virgin EVA by 85 %, 103.6 %, and 83.16 %, respectively. These findings suggest that EVA/CuO nanocomposites are promising candidates for flexible dielectric materials.

Overcoming the Tradeoff of Visible Transparency and Electrical Conductance via Dual Smoothing of Dielectric/Metal Interfaces in Cu-Thin-Layer-Based Transparent Electrodes.

The rapid advancement of flexible optoelectronic devices, such as light-emitting diodes, solar cells, and electrochromic devices, necessitates the development of high-performance flexible transparent electrodes (TEs). Dielectric/metal/dielectric (DMD)-type TEs are promising alternatives to conventional indium tin oxide (ITO) due to the high electrical conductance, excellent visible transparency, and sufficient mechanical flexibility. However, the tradeoff between electrical conductance and visible transparency poses a challenge to performance enhancement. This study introduces an Ar-ion-mediated interface modification method to address this tradeoff by dual smoothing of dielectric/metal interfaces in TiOx/Cu/ZnO TEs. Implementing this dual smoothing methodology significantly enhances both electrical conductance and visible light transmittance, achieving a Haacke figure of merit 200% higher than that of an unmodified otherwise identical structure. The highest figure of merit is 0.113 Ω-1, a record high for Cu-thin-layer-based DMD TEs, far surpassing ITO electrode values. Further, the enhanced optoelectronic performance remains highly durable under severe and simultaneous electrical, thermal, and mechanical stresses, showcasing the potential for significant advances in flexible optoelectronics.

Flexibility, Dispersion Property, and Giant Sunlight Absorption of Monolayer MX2 (M = Zr, Hf; X = S, Se) and Related Janus MXY (M = Zr, Hf; X = S, Y = Se)

IVB–VIA transition metal dichalcogenides (TMDs) MX2 (M = Zr, Hf; X = S, Se) and related Janus MXY (M = Zr, Hf; X = S, Y = Se) have garnered significant attention due to their unique optoelectronic properties. To further explore their potential in flexible photonics, their mechanical, band dispersion, and optical dielectric properties using orbital hybrid theory are investigated. The results reveal that the out‐of‐plane linear compression resistance of these materials is considerably lower than their in‐plane resistance, primarily because van der Waals interactions are much weaker compared to ionic bond interactions. This characteristic makes them less susceptible to external pressure and torsional forces, positioning them as promising candidates for flexible optoelectronic devices. Furthermore, the presence of multiple energy bands at the same momentum state suggests a tendency toward metal–insulator transitions. Notably, the materials exhibit a high photon flux, indicating robust photoelectric conversion capabilities, making them suitable for applications in photoelectric transducers. This study not only deepens the understanding of the optoelectronic and mechanical properties of IVB–VIA TMD materials, but also offers theoretical guidance for the development and application of flexible optoelectronic devices based on IVB‐VIA TMDs and related Janus materials.

Abnormal Response of Indirect Excitons to Non‐Uniform Elastic Strain in MoS2 Flakes

AbstractRegulating the electronic structures and carrier dynamics in 2D materials via elastic strain is a fascinating avenue for tailoring their optoelectronic properties at atomic scale. Here, the abnormal response of indirect excitons to non‐uniform elastic strain in MoS2 flakes is reported. The non‐uniform elastic strain in the MoS2 flakes is introduced by transferring them to pre‐prepared convex cross structures on a SiO2/Si substrate, where the topography and strain distribution are determined by atomic force microscopy and Raman spectroscopy, respectively. It is observed that the emission energy of the indirect excitons shows an unexpected blue‐shift followed by a red‐shift with increasing the local tensile strain, which is in sharp contrast to the linear red‐shift of the direct excitons. Density functional theory calculations reveal that the abnormal energy shift of the indirect excitons in the MoS2 flakes arises from the strain‐induced competition between two indirect bandgap transitions and the spatial exciton funnel effect in the non‐uniform strain field. This work provides new insights into the elastic strain effect on the indirect excitons in 2D semiconductors, which possesses potential applications in flexible optoelectronic devices.

Body-worn and self-powered flexible optoelectronic device for metronomic photodynamic therapy

Photodynamic therapy (PDT) as a clinical method relies on appropriate light delivery to activate photosensitizers, usually necessitates the utilization of cumbersome surgical instruments and high irradiation intensity, along with the requirement for hospitalization. To extend the applicability of PDT beyond hospital for better patient mobility, we design a wearable and self-powered metronomic PDT (mPDT) system for chronic wound infection treatment. A flexible alternative current electroluminescent (ACEL) device is constructed through sandwiching an emissive layer between conductive hydrogel electrodes. This ACEL device works as a therapeutic patch by loading photosensitizer (PS) in its bottom hydrogel electrode, thus aviods the intravenous administration to patients. Under the triboelectric nanogenerator generated AC pulse, the electroluminescence produced from emissive layer can be absorbed by the PS-loaded electrode to generate reactive oxygen species for mPDT. Benefited from its arbitrary tailorability, this device can be customized into on-demand shapes and sizes. Using diabetic infected wound as a model condition, this ACEL mPDT device effectively eliminates drug-resistant bacteria and accelerates wound healing. Thus, the body-worn optoelectronic device successfully avoids the utilization of extracorporeal physical light and power sources, providing a promising strategy for convenient, user-friendly, and prolonged treatment of superficial diseases.

Enhanced Carrier Lifetime and Mobility in Monolayer NbOI2.

The lifetime and mobility of hot carriers are critical parameters for assessing the performance of optoelectronic materials, as they directly impact the response speed and operational efficiency of devices. Combining first-principles calculations with nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigated the electronic properties and carrier dynamics of monolayer NbOI2. Our findings indicate that, at room temperature, this material demonstrates a carrier lifetime of up to ∼13 ns and an electron mobility reaching as high as ∼9 × 103 cm2 V-1 s-1. The low Young's modulus makes it susceptible to deformation under external stress, and we found that the carrier lifetime extends to ∼40 ns under a 4% tensile strain, along with a significant increase in hole mobility. This study elucidates the carrier dynamics in monolayer NbOI2, facilitating its potential application in future flexible optoelectronic devices.

Intrinsically stretchable fully π-conjugated polymer film via fluid conjugated molecular external-plasticizing for flexible light-emitting diodes

Fully π-conjugated polymers with rigid aromatic units are promising for flexible optoelectronic devices, but their inherent brittleness poses a challenge for achieving high-performance, intrinsically stretchable fully π-conjugated polymer. Here, we are establishing an external-plasticizing strategy using semiconductor fluid plasticizers (Z1 and Z2) to enhance the optoelectronic, morphological, and stretchable properties of fully π-conjugated polymer films for flexible light-emitting diodes. The synergistic effect of hierarchical structure and optoelectronic properties of Z1 in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films enable excellent stretchable deformability (~25%) and good conductivity. PLEDs based on F8BT/Z1 films show stable electroluminescence and efficiency under 15% stretch and 100 cycles at 10% strain, revealing outstanding stress tolerance. This strategy is also improving the stretchable properties of polymers like poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) (Super Yellow), demonstrating its general applicability. Therefore, this strategy can provide effective guidance for designing high-performance stretchable fully π-conjugated polymers films for flexible electronic devices.

Temperature endurable face-sealing polyisobutylene film with contactable liquid desiccant for the encapsulation of OLEDs

We propose a face-sealed encapsulation solution with simple process and low cost. The polyisobutylene encapsulation patch mixed with calcium oxide particles is attached to the surface of the OLEDs that is exposed to the hygroscopic agent in a face-sealing manner. We find that this encapsulation scheme exhibited excellent waterproof performance. The encapsulated OLED has a long shelf lifetime in water under high temperature conditions, which is manifested in a long underwater shelf lifetime of T95 (luminance drops to 95 % of the initial luminance) of over 336 h at 60°C. We believe that this encapsulating technology can be applied to other organic optoelectronic devices, and the face-sealed encapsulating patch is flexible. This strategy can be extended to roll-to-roll processes to expand the application of flexible organic optoelectronic devices underwater.

Flexible organic optoelectronic devices: Design, fabrication, and applications

Flexible organic optoelectronic devices: Design, fabrication, and applications

Improved performance of MgxZn1-xO flexible UV photodetectors by piezo-phototronic effect through alloying treatment

MgxZn1-xO films (x = 0.22, 0.39, and 0.52) with a single hexagonal wurtzite structure were grown on flexible polyethylene terephthalate (PET) substrates using radio frequency (RF) magnetron sputtering technique. The piezo-phototronic properties of flexible ultraviolet (UV) photodetectors with MgxZn1-xO films were investigated by examining the relationship between their photoelectric characteristics and strain, and the impact of Mg alloying on their piezo-phototronic effect. As the tensile strain of the detectors increased from ε = 0 to ε = 0.2, the responsivity of Mg0.22Zn0.78O, Mg0.39Zn0.61O, and Mg0.52Zn0.48O flexible UV photodetectors increased by ∼ 260 %, ∼308 %, and ∼ 441 %, respectively, showing an enhanced piezo-phototronic effect with increasing Mg content. It could be attributed to the fact that the alloying process increased the piezoelectric coefficient of the MgxZn1-xO materials, thereby increasing the piezopotential. This work provides a technical approach for the preparation and performance optimization of novel flexible optoelectronic functional devices.

Tuning the optical properties of PMMA polymer by using subphthalocyanine dye and metal oxide nanoparticles for flexible optoelectronic devices

Tuning the optical properties of PMMA polymer by using subphthalocyanine dye and metal oxide nanoparticles for flexible optoelectronic devices

High Electron Mobility in Si-Doped Two-Dimensional β-Ga2O3 Tuned Using Biaxial Strain.

Two-dimensional (2D) semiconductors have attracted much attention regarding their use in flexible electronic and optoelectronic devices, but the inherent poor electron mobility in conventional 2D materials severely restricts their applications. Using first-principles calculations in conjunction with Boltzmann transport theory, we systematically investigated the Si-doped 2D β-Ga2O3 structure mediated by biaxial strain, where the structural stabilities were determined by formation energy, phonon spectrum, and ab initio molecular dynamic simulation. Initially, the band gap values of Si-doped 2D β-Ga2O3 increased slightly, followed by a rapid decrease from 2.46 eV to 1.38 eV accompanied by strain modulations from -8% compressive to +8% tensile, which can be ascribed to the bigger energy elevation of the σ* anti-bonding in the conduction band minimum than that of the π bonding in the valence band maximum. Additionally, band structure calculations resolved a direct-to-indirect transition under the tensile strains. Furthermore, a significantly high electron mobility up to 4911.18 cm2 V-1 s-1 was discovered in Si-doped 2D β-Ga2O3 as the biaxial tensile strain approached 8%, which originated mainly from the decreased quantum confinement effect on the surface. The electrical conductivity was elevated with the increase in tensile strain and the enhancement of temperature from 300 K to 800 K. Our studies demonstrate the tunable electron mobilities and band structures of Si-doped 2D β-Ga2O3 using biaxial strain and shed light on its great potential in nanoscale electronics.

Patterned flexible Silver Nanowire/2D MXene transparent conducting electrode for organic light-emitting diodes

With the advent of numerous flexible optoelectronic applications, the importance of flexible and stable transparent conducting electrodes (TCEs) has been considerable. The conventional silver nanowire (AgNW) is a promising candidate as TCEs due to its decent opto-electrical properties and solution processibility. However, AgNW's high surface root mean square (RMS) roughness, relatively high sheet resistance, and low work function (∼4.5 eV) limit the usage for display applications. In this work, we design and study the AgNW and MXene (Ti3C2Tx) hybrid TCEs for flexible organic light-emitting diodes (OLEDs) to enhance the optoelectrical properties and the hole injection within the device. Experiment investigation is carried out via an application of AgNW/MXene (AgMX) hybrid TCEs as a bottom electrode for OLEDs devices in which the photolithography process and lift-off technique are employed for such fabrication. The hybrid TCEs indicate high optical transparency (86.8 % at 550 nm) and low sheet resistance (24.1 Ω/cm2) as well as improved surface morphology, enabling the efficient OLEDs operation. The optimized OLEDs exhibit a clear and stable red electroluminescence (EL) peak with an improved maximum external quantum efficiency (EQE) and luminance of 5.25 % and 3421.7 cd/m2, respectively. Furthermore, the hybrid TCEs are readily formed on poly(ethylene terephthalate) (PET) substrate, showing high durability under the bending test up to 100,000 cycles. The results demonstrate that adopting AgMX TCEs is a reasonable strategy for obtaining proper flexible optoelectronic devices.