Synergetic Transparent Electrode Architecture for Efficient Non-Fullerene Flexible Organic Solar Cells with >12% Efficiency

Stretchable, Electrochemically-Stable Electrochromic Devices Based on Semi-Embedded Ag@Au Nanowire Network

Stretchable electrochromic (EC) devices that can adapt the irregular and dynamic human surfaces show promising applications in wearable display, adaptive camouflage, and visual sensation. However, challenges exist in lacking transparent conductive electrodes with both tensile and electrochemical stability to assemble the complex device structure and endure harsh electrochemical redox reactions. Herein, a wrinkled, semi-embedded Ag@Au nanowire (NW) networks are constructed on elastomer substrates to fabricate stretchable, electrochemically-stable conductive electrodes. The stretchable EC devices are then fabricated by sandwiching a viologen-based gel electrolyte between two conductive electrodes with the semi-embedded Ag@Au NW network. Because the inert Au layer inhibits the oxidation of Ag NWs, the EC device exhibits much more stable color changes between yellow and green than those with pure Ag NW networks. In addition, since the wrinkled semi-embedded structure is deformable and reversibly stretched without serious fractures, the EC devices still maintain excellent color-changing stability under 40% stretching/releasing cycles.

Critical Role of Non-Halogenated Solvent Additives in Eco-Friendly and Efficient All-Polymer Solar Cells

Organic solar cells (OSCs) demonstrating high power conversion efficiencies have been mostly fabricated using halogenated solvents, which are highly toxic and harmful to humans and the environment. Recently, non-halogenated solvents have emerged as a potential alternative. However, there has been limited success in attaining an optimal morphology when non-halogenated solvents (typically o-xylene (XY)) were used. To address this issue, we studied the dependence of the photovoltaic properties of all-polymer solar cells (APSCs) on various high-boiling-point non-halogenated additives. We synthesized PTB7-Th and PNDI2HD-T polymers that are soluble in XY and fabricated PTB7-Th:PNDI2HD-T-based APSCs using XY with five additives: 1,2,4-trimethylbenzene (TMB), indane (IN), tetralin (TN), diphenyl ether (DPE), and dibenzyl ether (DBE). The photovoltaic performance was determined in the following order: XY + IN < XY + TMB < XY + DBE ≤ XY only < XY + DPE < XY + TN. Interestingly, all APSCs processed with an XY solvent system had better photovoltaic properties than APSCs processed with chloroform solution containing 1,8-diiodooctane (CF + DIO). The key reasons for these differences were unraveled using transient photovoltage and two-dimensional grazing incidence X-ray diffraction experiments. The charge lifetimes of APSCs based on XY + TN and XY + DPE were the longest, and their long lifetime was strongly associated with the polymer blend film morphology; the polymer domain sizes were in the nanoscale range, and the blend film surfaces were smoother, as the PTB7-Th polymer domains assumed an untangled, evenly distributed, and internetworked morphology. Our results demonstrate that the use of an additive with an optimal boiling point facilitates the development of polymer blends with a favorable morphology and can contribute to the widespread use of eco-friendly APSCs.

Effects of Solvent Additive and Micro-Patterned Substrate on the Properties of Thin Films Based on P3HT:PC70BM Blends Deposited by MAPLE

Lately, there is a growing interest in organic photovoltaic (OPV) cells due to the organic materials' properties and compatibility with various types of substrates. However, their efficiencies are low relative to the silicon ones; therefore, other ways (i.e., electrode micron/nanostructuring, synthesis of new organic materials, use of additives) to improve their performances are still being sought. In this context, we studied the behavior of the common organic bulk heterojunction (P3HT:PC70BM) deposited by matrix-assisted pulsed laser evaporation (MAPLE) with/without 0.3% of 1,8-diiodooctane (DIO) additive on flat and micro-patterned ITO substrates. The obtained results showed that in the MAPLE process, a small quantity of additive can modify the morphology of the organic films and decrease their roughness. Besides the use of the additive, the micro-patterning of the electrode leads to a greater increase in the absorption of the studied photovoltaic structures. The inferred values of the filling factors for the measured cells in ambient conditions range from 19% for the photovoltaic structures with no additive and without substrate patterning to 27% for the counterpart structures with patterning and a small quantity of additive.

Enhancement of color and photovoltaic performance of semi-transparent organic solar cell via fine-tuned 1D photonic crystal

Semi-transparent organic solar cells’ (ST-OSCs) photovoltaic and high optical performance parameters are evaluated in innovative applications such as power-generating windows for buildings, automobiles, and aesthetic designs in architectural and industrial products. These parameters require the precision design of structures that optimize optical properties in the visible region and aim to achieve the required photon harvest in UV and IR. These designs can be realized by integrating wavelength-selective photonics-based systems into ST-OSC to increase localized absorption in wavelengths greater than 600 nm and NIR and provide modifiable optical properties. In this study, methodologically, we followed highly detailed light management engineering and transfer matrix method-based theoretical and experimental approaches. We discussed the optimal structures by evaluating color, color rendering index, correlated color temperature, and photovoltaic performances for ST-OSCs, including one-dimensional photonic crystal (1D-PC) designed at different resonance wavelengths (λ_B) and periods. Finally, by integrating fine-tuned (MgF₂/MoO₃)^N 1D-PC, we report the inherently dark purple-red color of the P3HT:PCBM bulk-heterojunction-based ST-OSC neutralizes with the optimal state was 0.3248 and 0.3733 by adjusting close to the Planckian locus. We also enhanced short current density from 5.77 mA/cm² to 6.12 mA/cm² and PCE were increased by 7.34% from 1.77% to 1.90% designed for the N = 4 period and λ_B = 700 nm.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Slip-Stacked J-Aggregate Materials for Organic Solar Cells and Photodetectors

Dye-dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near-infrared region and the latter an ultra-narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip-stacked dyes, distinguishing classical J-aggregates with predominant long-range Coulomb coupling from charge transfer (CT)-mediated or -coupled J-aggregates, whose red-shifts are primarily governed by short-range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength-selective OPDs, providing important insights into the rational design of thin-film optoelectronic materials.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Simultaneous Enhancement in Visible Transparency and Electrical Conductivity via the Physicochemical Alterations of Ultrathin-Silver-Film-Based Transparent Electrodes

A methodology for the simultaneous modulation of the chemical and physical states of an amorphous TiO_x layer surface and its impact on the subsequent deposition of a polycrystalline Ag layer are presented. The smoothened TiO_x layer surface comprising chemically altered, oxygen-deficient states serves as a nucleating platform for Ag deposition, facilitating a marked increase (∼75%) in the nucleation number density, which strongly enhances the wettability of ultrathin Ag layers. The physically smoothened TiO_x/Ag interface further reduces the optical and electrical losses. When the proposed methodology is applied to TiO_x/Ag/ZnO transparent conductive electrodes (TCEs), exceptional TCE properties are yielded owing to the simultaneous improvement in visible transparency and electrical conductivity; specifically, a record-high 0.22 Ω^-1 Haacke figure of merit is realized. TCEs are prepared on flexible substrates to verify their applicability as stand-alone flexible transparent heaters and as integrated heaters within electrochromic devices to enhance color-switching reactions.

Synergetic effects of a front ITO nanocylinder array and a back square Al array to enhance light absorption for organic solar cells

Efficient light management is critical to obtain high performance for organic solar cells (OSCs), which aims to solve the contradiction between limited carrier extraction and light absorption for the normally employed photoactive layers generally having both short exciton diffusion lengths and low extinction coefficients. In this study, we introduce a simple and efficient light management structure consisting of a front indium tin oxide nanocylinder (ITO-NC) array and a back square Al array. Thanks to the synergetic effects of antireflection and light scattering induced by the ITO-NC array, together with the secondary scattering and localized surface plasmon resonance because of the square Al array, remarkably enhanced light absorption in a broad spectral range can be achieved. Taking the most investigated photoactive layer of the P3HT:PC₆₁BM blend as an example, simulation results reveal that, compared with the planar control device of the ITO/PEDOT:PSS/P3HT:PC₆₁BM(80nm)/ZnO/Al, the short-circuit current density and power conversion efficiency can be enhanced by 36.58% and 38.38% after incorporating the light management structure with the optimal structural parameters. Furthermore, good omnidirectional light management can be achieved for the proposed device structure. Given the excellent performance and simple structure, we believe that this study would provide a meaningful exploration of developing light management structures applicable for thin film-based optoelectronic devices.

A wireless optoelectronic skin patch for light delivery and thermal monitoring

Wearable optoelectronic devices can interface with the skin for applications in continuous health monitoring and light-based therapy. Measurement of the thermal effect of light on skin is often critical to track physiological parameters and control light delivery. However, accurate measurement of light-induced thermal effects is challenging because conventional sensors cannot be placed on the skin without obstructing light delivery. Here, we report a wearable optoelectronic patch integrated with a transparent nanowire sensor that provides light delivery and thermal monitoring at the same location. We achieve fabrication of a transparent silver nanowire network with >92% optical transmission that provides thermoresistive sensing of skin temperature. By integrating the sensor in a wireless optoelectronic patch, we demonstrate closed-loop regulation of light delivery as well as thermal characterization of blood flow. This light delivery and thermal monitoring approach may open opportunities for wearable devices in light-based diagnostics and therapies.

Zirconium-Doped Zinc Oxide Nanoparticles as Cathode Interfacial Layers for Efficiently Rigid and Flexible Organic Solar Cells

Low-temperature zinc oxide nanoparticles (ZnO NPs) are widely applied as cathode interfacial layers (CILs) for rigid and flexible organic solar cells. However, the inferior optoelectronic properties of ZnO NPs constrain the improvement in the photovoltaic performance and enhance the thickness sensitivity. Herein, upon application of this ZnO:Zr NP as a CIL for inverted device construction, the maximum power conversion efficiency (PCE_max) is increased to 17.7%, with an enhancement of 12.0% compared to that of the pristine ZnO-based devices (15.8%). A series of optoelectronic characterizations have revealed that the Zr doping methodology would enhance the charge generation and extraction process and suppress trap-assisted recombination, which is beneficial for the synergistic improvement of the thickness tolerance and shelf stability. Encouragingly, ZnO:Zr NPs can be easily fabricated through a doctor-blade coating technique with remarkable performance (16.6%). More critically, this approach can be applied to the development of high-performance flexible solar cells, with a superb PCE of 16.0%.

Ultra-Flexible Organic Photovoltaics with Low-Temperature Deposited IZTO on a Cyclic Polymer Substrate Having Excellent Mechanical Properties

In this study, ultra-flexible organic photovoltaics (OPVs) with a new device structure are developed as a power source for ultra-flexible wearable devices with excellent biocompatibility. To fabricate ultra-flexible OPVs with excellent mechanical properties, we develop an ultra-flexible substrate with a bilayer structure based on polymers and transparent conducting oxides. An amorphous perfluorinated polymer (cyclic transparent optical polymer, CYTOP) is introduced as an ultra-flexible substrate by a solution process. An indium zinc tin oxide (IZTO) transparent electrode possessing an amorphous structure is fabricated via pulsed DC magnetron sputtering at room temperature using a target containing 80 atom % In₂O₃-10 atom % ZnO-10 atom % SnO₂. Ultra-flexible OPVs with a one-dimensional (1D) grating pattern are fabricated on the buffer layer and photoactive layer. These OPVs exhibit an increase of 12% in power conversion efficiency (PCE) (maximum PCE: 8.52%) compared to the reference, thereby minimizing reliance on the incident angle of light. In addition, even after 1000 compression/relaxation tests with a compression strain of 33%, the PCE of the ultra-flexible OPVs is maintained up to 94.8% of its initial value.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Ultra-Flexible Organic Solar Cell Based on Indium-Zinc-Tin Oxide Transparent Electrode for Power Source of Wearable Devices

We have developed a novel structure of ultra-flexible organic photovoltaics (UFOPVs) for application as a power source for wearable devices with excellent biocompatibility and flexibility. Parylene was applied as an ultra-flexible substrate through chemical vapor deposition. Indium-zinc-tin oxide (IZTO) thin film was used as a transparent electrode. The sputtering target composed of 70 at.% In₂O₃-15 at.% ZnO-15 at.% SnO₂ was used. It was fabricated at room temperature, using pulsed DC magnetron sputtering, with an amorphous structure. UFOPVs, in which a 1D grating pattern was introduced into the hole-transport and photoactive layers were fabricated, showed a 13.6% improvement (maximum power conversion efficiency (PCE): 8.35%) compared to the reference device, thereby minimizing reliance on the incident angle of the light. In addition, after 1000 compression/relaxation tests with a compression strain of 33%, the PCE of the UFOPVs maintained a maximum of 93.3% of their initial value.

Mechanical Robust Flexible Single-Component Organic Solar Cells

Owing to the advantages of being lightweight and compatible with surfaces with different deformations, flexible organic solar cells (OSCs) have broad scopes of applications, including wearable electronics and portable devices. Most flexible OSCs focus on the two-component bulk-heterojunction (BHJ) photo-active layers, but they usually suffer from degradation problems both in efficiency and mechanical durability derived from the limited phase stability under mechanical and thermal stress. Whereas, single-component organic solar cells (SCOSCs) based on the double-cable conjugated polymer are supposed to possess excellent mechanical robustness and long-term stability. Here, the first flexible SCOSCs based on a double-cable polymer are fabricated on a transparent silver nanowires (AgNWs) electrode on a plastic foil. Impressively, the obtained flexible SCOSCs exhibited a power conversion efficiency (PCE) of 7.21%. The flexible SCOSCs are further demonstrated to possess superior mechanical robustness (>95% retention after 1000 bending cycles) and storage stability (>97% retention after 430 h in nitrogen atmosphere) compared to several BHJ-type flexible OSCs. The pseudo-free-standing tensile test and morphology investigation are conducted to reveal the distinction in mechanical durability of the single-component polymer film and the BHJ-type films. Besides, ultraflexible SCOSCs are also fabricated, indicating the application prospect and superiority in flexible devices and wearable electronic products.

Digital printing of a novel electrode for stable flexible organic solar cells with a power conversion efficiency of 8.5

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) mixed with single-wall nanotubes (SWNTs) (10:1) and doped with (0.1 M) perchloric acid (HClO4) in a solution-processed film, working as an excellent thin transparent conducting film (TCF) in organic solar cells, was investigated. This new electrode structure can be an outstanding substitute for conventional indium tin oxide (ITO) for applications in flexible solar cells due to the potential of attaining high transparency with enhanced conductivity, good flexibility, and good durability via a low-cost process over a large area. In addition, solution-processed vanadium oxide (VOx) doped with a small amount of PEDOT-PSS(PH1000) can be applied as a hole transport layer (HTL) for achieving high efficiency and stability. From these viewpoints, we investigate the benefit of using printed SWNTs-PEDOT-PSS doped with HClO4 as a transparent conducting electrode in a flexible organic solar cell. Additionally, we applied a VOx-PEDOT-PSS thin film as a hole transporting layer and a blend of PTB7 (polythieno[3,4-b] thiophene/benzodithiophene): PC71BM (phenyl-C71-butyric acid methyl ester) as an active layer in devices. Zinc oxide (ZnO) nanoparticles were applied as an electron transport layer and Ag was used as the top electrode. The proposed solar cell structure showed an enhancement in short-circuit current, power conversion efficiency, and stability relative to a conventional cell based on ITO. This result suggests a great carrier injection throughout the interfacial layer, high conductivity and transparency, as well as firm adherence for the new electrode.

Optical Expediency of Back Electrode Materials for Organic Near-Infrared Photodiodes

Organic semiconductor devices, including organic photodetectors (OPDs) and organic photovoltaics (OPVs), have undergone vast improvements, thanks to the development of non-fullerene acceptors. The absorption range of such NFA-based systems is typically shifted toward the near-infrared (near-IR) region compared to early-generation fullerene-based systems, rendering organic semiconductor devices suitable for near-IR sensing applications. While most efforts are concentrated on the photoactive materials, less attention is paid to the impact of the back electrodes on the device performance. Therefore, this work focuses on the optical expediency of gold (Au), silver (Ag), aluminum (Al), and graphite as back electrode materials in organic optoelectronics. This work shows that the "one size fits all" methodology is not a valid approach for choosing the back electrode material. Instead, considering the active layer absorption, the active layer thickness, and the intended application is necessary. A traditional polymer/fullerene-based system, poly(3-hexylthiophene) with [6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PC₆₀BM), and a state-of-the-art narrow-band gap non-fullerene-based system, poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethy-lhexyl)3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-(2-6-diyl)] and 2,2'-((2Z,2'Z)-((5,5'-(4,4-bis(2-ethylhexyl)4H-cyclopenta[1,2-b:5,4-b']dithiophene-2,6-diyl)bis(4-((2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene)) bis(5,6-difluoro3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (PCE10:COTIC-4F), are investigated by combining optical transfer matrix modeling simulations with experimentally determined recombination and extraction losses. We find that the narrow-band gap system shows performance gains when employing Au as the back electrode. Furthermore, we show that these performance gains are dependent on active layer thickness, yielding the most significance for thin active layers (<100 nm). Such thin, ultra-narrow-band gap devices are the focus of near-IR sensing applications, highlighting the importance of methodically choosing the back electrode. Lastly, the impact of the back electrode on the OPV device performance is outlined.

Molecular Doping Inhibits Charge Trapping in Low-Temperature-Processed ZnO toward Flexible Organic Solar Cells

There has been a growing interest in the development of efficient flexible organic solar cells (OSCs) due to their unique capacity to provide energy sources for flexible electronics. To this end, it is required to design a compatible interlayer with low processing temperature and high electronic quality. In this work, we present that the electronic quality of the ZnO interlayer fabricated from a low-temperature (130 °C) sol-gel method can be significantly improved by doping an organic small molecule, TPT-S. The doped TPT-S, on the one hand, passivates uncoordinated Zn-related defects by forming N-Zn bonds. On the other hand, photoinduced charge transfer from TPT-S to ZnO is confirmed, which further fills up electron-deficient trap states. This renders ZnO improved electron transport capability and reduced charge recombination. By illuminating devices with square light pulses of varying intensities, we also reveal that an unfavorable charge trapping/detrapping process observed in low-temperature-processed devices is significantly inhibited after TPT-S doping. OSCs based on PBDB-T-2F:IT-4F with ZnO:TPT-S being the cathode interlayer yield efficiencies of 12.62 and 11.33% on rigid and flexible substrates, respectively. These observations convey the practicality of such hybrid ZnO in high-performance flexible devices.

Hybrid nanostructured plasmonic electrodes for flexible organic light-emitting diodes

Improved performance in flexible organic light-emitting diodes (OLEDs) is demonstrated by using a hybrid nanostructured plasmonic electrode consisting of silver nanowires (AgNWs) decorated with silver nanoparticles (AgNPs) and covered by exfoliated graphene sheets. Such all-solution processed electrodes show high optical transparency and electrical conductivity. When integrated in an OLED with super yellow polyphenylene vinylene as the emissive layer, the plasmon coupling of the NW-NP hybrid plasmonic system is found to significantly enhance the fluorescence, demonstrated by both simulations and photoluminescence measurements, leading to a current efficiency of 11.61 cd A^-1 and a maximum luminance of 20 008 cd m^-2 in OLEDs. Stress studies reveal a superior mechanical flexibility to the commercial indium-tin-oxide (ITO) counterparts, due to the incorporation of exfoliated graphene sheets. Our results show that these hybrid nanostructured plasmonic electrodes can be applied as an effective alternative to ITO for use in high-performance flexible OLEDs.

Syntheses of Silver Nanowires Ink and Printable Flexible Transparent Conductive Film: A Review

Nowadays, flexible transparent conductive film (FTCF) is one of the important components of many flexible electronic devices. Due to comprehensive performances on optoelectronics, FTCF based on silver nanowires (AgNWs) networks have received great attention and are expected to be a new generation of transparent conductive film materials. Due to its simple process, printed electronic technology is now an important technology for the rapid production of low-cost and high-quality flexible electronic devices. AgNWs-based FTCF fabricated by using printed electronic technology is considered to be the most promising process. Here, the preparation and performance of AgNW ink are introduced. The current printing technologies are described, including gravure printing, screen printing and inkjet printing. In addition, the latest methods to improve the conductivity, adhesion, and stability of AgNWs-based FTCF are introduced. Finally, the applications of AgNWs-based FTCF in solar cells, transparent film heaters, optoelectronic devices, touch panel, and sensors are introduced in detail. Therefore, combining various printing technologies with AgNWs ink may provide more opportunities for the development of flexible electronic devices in the future.

Solvent Vapor-Assisted Magnetic Manipulation of Molecular Orientation and Carrier Transport of Semiconducting Polymers

Effective control of the molecular orientation and the degree of ordering in organic semiconductors is important to achieve high-performance organic electronics. Herein, we have successfully achieved highly oriented films in centimeter scale for a naphthalenedicarboximide-based semiconducting polymer (P(NDI2OD-T2)) by solvent vapor annealing (SVA) of precast films under a high magnetic field (HMF). As revealed by the microstructural studies, the SVA-HMF films exhibit a remarkably higher degree of chain alignment and high morphological uniformity compared to the HMF-guided drop-cast films. Based on the structural evolution of the films with the SVA time, a mechanism is proposed to elucidate the alignment process, which emphasizes that the chain aggregates re-formed in the swollen films trigger magnetic alignment and determine the film order. Compared with the unaligned films, field-effect transistors of the magnetic aligned P(NDI2OD-T2) films have exhibited a 19-fold enhancement of electron mobility and an extraordinarily large mobility anisotropy of 125. Furthermore, a significantly reduced energetic barrier for activated transport is observed on the aligned devices from temperature-variable measurements. The improved performance achieved by the HMF-SVA process has indicated its potential for high-performance organic electronic applications.

Rational Interface Engineering for Efficient Flexible Perovskite Light-Emitting Diodes

Although perovskite light-emitting diodes (PeLEDs) are promising for next-generation displays and lighting, their efficiency is still considerably below that of conventional inorganic and organic counterparts. Significant efforts in various aspects of the electroluminescence process are required to achieve high-performance PeLEDs. Here, we present an improved flexible PeLED structure based on the rational interface engineering for energy-efficient photon generation and enhanced light outcoupling. The interface-stimulated crystallization and defect passivation of the perovskite emitter are synergistically realized by tuning the underlying interlayer, leading to the suppression of trap-mediated nonradiative recombination losses. Besides approaching highly emissive perovskite layers, the outcoupling of trapped light is also enhanced by combining the silver nanowires-based electrode with quasi-random nanopatterns on flexible plastic substrate. Upon the collective optimization of the device structure, a record external quantum efficiency of 24.5% is achieved for flexible PeLEDs based on green-emitting CsPbBr₃ perovskite.

Realizing Ultrahigh Mechanical Flexibility and >15% Efficiency of Flexible Organic Solar Cells via a "Welding" Flexible Transparent Electrode

The power conversion efficiencies (PCEs) of flexible organic solar cells (OSCs) still lag behind those of rigid devices and their mechanical stability is unable to meet the needs of flexible electronics at present due to the lack of a high-performance flexible transparent electrode (FTE). Here, a so-called "welding" concept is proposed to design an FTE with tight binding of the upper electrode and the underlying substrate. The upper electrode consisting of solution-processed Al-doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by utilizing the capillary force effect and secondary growth of AZO, leading to a reduction of the AgNWs junction site resistance. Meanwhile, the poly(ethylene terephthalate) is modified by embedding the AgNWs, which are then used to link with the AgNWs in the upper hybrid electrode, thus enhancing the adhesion of the electrode to the substrate. By this welding strategy, critical bottleneck issues relating to the FTEs in terms of optoelectronic and mechanical properties are comprehensively addressed. The single-junction flexible OSCs based on this welded FTE show a high performance, achieving a record high PCE of 15.21%. In addition, the PCEs of the flexible OSCs are less influenced by the device area and display robust bending durability even under extreme test conditions.

Recent Progress in Organic Photodetectors and their Applications

Organic photodetectors (OPDs) have attracted continuous attention due to their outstanding advantages, such as tunability of detecting wavelength, low-cost manufacturing, compatibility with lightweight and flexible devices, as well as ease of processing. Enormous efforts on performance improvement and application of OPDs have been devoted in the past decades. In this Review, recent advances in device architectures and operation mechanisms of phototransistor, photoconductor, and photodiode based OPDs are reviewed with a focus on the strategies aiming at performance improvement. The application of OPDs in spectrally selective detection, wearable devices, and integrated optoelectronics are also discussed. Furthermore, some future prospects on the research challenges and new opportunities of OPDs are covered.

Recent progress of light manipulation strategies in organic and perovskite solar cells

Organic and perovskite solar cells are suffering from the insufficient utilization of incident light and thus low light harvesting efficiency despite their rapid progress in the past decade. In this regard, light manipulation strategies have attracted numerous attention to solve this inherent limit. Herein, the recent advances in light manipulation techniques in this area are overviewed. The light manipulation mechanisms are illustrated to classify the structures. Various light manipulation structures, fabrication techniques, and corresponding results are given and discussed, addressing the suppression of surface reflection, nano/micro-structure-induced light scattering, and the plasmonic effects with periodic metallic patterns and metallic nanoparticles. A brief perspective on future research is also proposed for pursuing broadband light harvesting.

Enhanced Electron Transportation by Dye Doping in Very Low-Temperature (<130 °C)-Processed Sol-Gel ZnO toward Flexible Organic Solar Cells

A perylene bisimide functionalized with four 4-carboxyphenoxy substituents at bay area (PBI-COOH) was embedded in sol-gel-derived zinc oxide (ZnO) to fabricate organic-inorganic hybrid photoconductive cathode interlayers (ZnO:PBI-COOH) that can be annealed at a rather low temperature of 120-130 °C as desired for plastic substrates for flexible devices. For these interlayers, the structural defects including oxygen vacancy and residual hydroxy groups are reduced that leads to increased electron mobility, and a photoinduced electron transfer from the organic dopant into the conduction band of ZnO endows the hybrid thin film with relatively higher conductivity when compared to the undoped ZnO thin film. The low-temperature-processed hybrid thin films were applied on indium tin oxide electrodes to produce inverted organic solar cells (OSCs) with power conversion efficiencies of 11.68 and 13.48% when using J71:ITIC and PBDB-T-2Cl:IT4F as active layers, respectively. Finally, flexible OSCs are fabricated on poly(ethylene terephthalate) substrates that maintained stability with relatively high performance after 100 times bending.