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All-inorganic α-CsPbBrxI3-x perovskites featuring nano-sized crystallites show great potential for pure-red light-emitting diode (LED) applications. Currently, the CsPbBrxI3–x LEDs based on nano-sized α-CsPbBrxI3-x crystallites have been fabricated mainly via the classical colloidal route including a tedious procedure of nanocrystal synthesis, purification, ligand or anion exchange, film casting, etc. With the usually adopted conventional LED device structure, only high turn-on voltages (> 2.7) have been achieved for CsPbBrxI3-x LEDs. Moreover, this mix-halide system may suffer from severe spectra-shift under bias. In this report, CsPbBrxI3-x thin films featuring nano-sized crystallites are prepared by incorporating multiple ammonium ligands in a one-step spin-coating route. The multiple ammonium ligands constrain the growth of CsPbBrxI3-x nanograins. Such CsPbBrxI3-x thin films benefit from quantum confinement. The corresponding CsPbBrxI3-x LEDs, adopting a conventional LED structure of indium-doped tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/CsPbBrxI3-x/[6, 6]-phenyl C61 butyric acid methyl ester (PCBM)/bathocuproine (BCP)/Al, emit pure-red color at Commission Internationale de l’eclairage (CIE) coordinates of (0.709, 0.290), (0.711, 0.289), etc., which represent the highest color-purity for reported pure-red perovskite LEDs and meet the Rec. 2020 requirement at CIE (0.708, 0.292) very well. The CsPbBrxI3-x LED shows a low turn-on voltage of 1.6 V, maximum external quantum efficiency of 8.94%, high luminance of 2,859 cdm−2, and good color stability under bias.

The upscaling of perovskite solar cells to module scale and long-term stability have been recognized as the most important challenges for the commercialization of this emerging photovoltaic technology. In a perovskite solar module, each interface within the device contributes to the efficiency and stability of the module. Here, we employed a holistic interface stabilization strategy by modifying all the relevant layers and interfaces, namely the perovskite layer, charge transporting layers and device encapsulation, to improve the efficiency and stability of perovskite solar modules. The treatments were selected for their compatibility with low-temperature scalable processing and the module scribing steps. Our unencapsulated perovskite solar modules achieved a reverse-scan efficiency of 16.6% for a designated area of 22.4 cm2. The encapsulated perovskite solar modules, which show efficiencies similar to the unencapsulated one, retained approximately 86% of the initial performance after continuous operation for 2,000 h under AM1.5G light illumination, which translates into a T90 lifetime (the time over which the device efficiency reduces to 90% of its initial value) of 1,570 h and an estimated T80 lifetime (the time over which the device efficiency reduces to 80% of its initial value) of 2,680 h. The upscaling of layer treatments and processing that afford high efficiency and stability in small-area perovskite solar cells remains challenging. Liu et al. show how the efficiency and stability of perovskite modules can be improved using an integrated approach to interface and layer engineering.

A series of challenging issues such as field-driven spectral drift for the CsPbClxBr3–x system and mixed phases in quasi-two-dimensional structures still exist when devising blue-emitting perovskites. In this Letter, the CsPbBr3 quantum-dot (QD) system is proposed to overcome these challenges. However, to date, the CsPbBr3 QD films with tunable colors from green to blue still cannot be achieved using existing methods. Herein, a simple one-step spin-coating route incorporated with efficient ligand passivation is developed to realize this goal. The size restriction of CsPbBr3 QDs is enabled by a diammonium ligand, propane-1,3-diammonium bromide (PDAB). A mixed-ligand system of phenethylammonium bromide (PEAB) with PDAB is further explored to enhance their optical performance. The CsPbBr3 QDs experience a second growth process upon controlled air exposure, which is utilized to realize their size control and emission wavelength tunability. The CsPbBr3 QD-based devices exhibit no spectral drift in electroluminescence under voltage bias.

In recent years, the major factors that determine commercialization of perovskite photovoltaic technology have been shifting from solar cell performance to stability, reproducibility, device upscaling and the prevention of lead (Pb) leakage from the module over the device service life. Here we simulate a realistic scenario in which perovskite modules with different encapsulation methods are mechanically damaged by a hail impact (modified FM 44787 standard) and quantitatively measure the Pb leakage rates under a variety of weather conditions. We demonstrate that the encapsulation method based on an epoxy resin reduces the Pb leakage rate by a factor of 375 compared with the encapsulation method based on a glass cover with an ultraviolet-cured resin at the module edges. The greater Pb leakage reduction of the epoxy resin encapsulation is associated with its optimal self-healing characteristics under the operating conditions and with its increased mechanical strength. These findings strongly suggest that perovskite photovoltaic products can be deployed with minimal Pb leakage if appropriate encapsulation is employed. Lead leakage from damaged perovskite solar cells poses a challenge to the deployment of such technology. Here, Jiang, Qiu and co-workers quantify lead leakage caused by a simulated hail impact under a number of weather conditions and show that self-healing encapsulations can effectively reduce it.

To achieve industrially viable fabrication process for perovskite-based solar cells, every process step must be optimized for maximum throughput. We present a study of substituting laboratory-type UV-Ozone surface treatment with a high-throughput Corona treatment in a scalable perovskite solar cell fabrication process. It is observed that water contact angle measurements provide insufficient information to determine the necessary dose of Corona or UV-Ozone treatment, but the surface carbon signal measured by x-ray photoelectron spectroscopy accurately identifies when surface contamination has been completely removed. Furthermore, we observe highly accelerated de-contamination of ZnO surfaces by UV-Ozone treatment. The effect can be explained by photocatalytic O2− ion generation indicating that UV-Ozone treatment is also applicable in high-throughput processing.

To enable the use of high work-function metals as the cathode in organic light emitting diodes (OLEDs), different types of alkali-metal and alkaline-earth-metal based compounds are examined as the electron injection layer (EIL). All the studied compounds can improve the device performance for aluminum, gold, and silver cathodes. Potassium hydroxide (KOH) and potassium carbonate (K2CO3) are employed as EIL for the first time in OLEDs. The performance of the device with sodium hydroxide (NaOH)/silver cathode almost doubles the conventional barium/aluminum device’s efficiency. By examining the work-function of EIL-modified metals through Kelvin Probe, a general correlation between the device performance and cathode work-function is revealed. The image-charge effect is found to be a contributor to the reduction of the metal work-function.

Compared to the conventional cell-shaped pixel structure, linear bank structure for solution-processed organic light-emitting diode (OLED) displays has the advantage of simple process, and large aperture-ratio. However, the prolonged drying time of the inkjet-printed ink in the linear bank forms a convex surface profile in the longitude direction. To achieve uniform film surface of the inkjet-printed poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), a novel printing method by varying the droplet-spacing is introduced. It is found that a larger droplet density at the line ends than that at the line center can compensate the inward flow of the ink, enabling systematic control of the film thickness along the line direction. As a result, uniform PEDOT:PSS line pattern is achieved. OLED display panels with line pixels are fabricated with varied droplet-spacing demonstrating better brightness uniformity than those inkjet-printed with constant droplet-spacing.

Solvent treatment has been widely used to improve the device performance of both Organic Light Emitting Diodes (OLEDs) and Polymer Solar Cells (PSCs). One of the proposed mechanisms is the modification of the buried PEDOT:PSS layer underneath the organic active layer by the permeating solvent. By measuring the lateral electric conductivity of the PEDOT:PSS layer, the 3 orders of magnitude's enhancement on the conductivity after solvent treatment confirms that the solvent permeates through the top organic active layer and modifies the PEDOT:PSS layer. Using a “peel-off” method, the buried PEDOT:PSS layer is fully exposed and studied by UV–vis spectra, XPS spectra, and c-AFM images. The data suggest that the permeating solvent dissolves PSS, changes PEDOT:PSS′ core-shell structure into a linear/coiled structure, and moves PSS from the bulk to the surface. As a result, PEDOT becomes more continuous in the bulk. The continuous conducting PEDOT-rich domains create percolating pathways for the current which significantly improve electric conductivity.

Non-fullerene polymer solar cells have attracted extensive attention due to their potential for overcoming the performance bottleneck currently encountered in fullerene-based photovoltaics. Herein, we report non-fullerene polymer solar cells with a maximal power conversion efficiency of over 11% by introducing an n-type water/alcohol soluble conjugated polymer as a cathode interlayer. We found that the contact between the n-type interlayer and the donor provides an extra interface for charge dissociation and the matching of energy levels between the n-type interlayer and the acceptor allows efficient electron extraction from the bulk heterojunction, which eventually leads to much improved performance. This study proposes a significant design rule for designing new interfaces for high performance non-fullerene photovoltaics.

Inkjet-printing a solvent onto an insulating polymer layer is employed to in situ build microgrooves as bank structures in the application of a solution-processed OLED display. The inkjet-etching process not only eliminates photolithography's shadow mask and photo exposure, but is also capable of constructing bank structures on any functional layer. The orthogonal solubility between the CYtop polymer and the organic layer avoids any solvent erosion. A pixelated display is successfully fabricated by inkjet-printing a blue-emitting polymer onto inkjet-etched CYtop microgrooves with a pixel resolution of 140 lines per inch. Forming a bank structure in situ on any layer as needed offers more choices to design a new panel structure, device architecture, and deposition methods.

In order to achieve high device efficiency in blue-emitting polymer light-emitting diodes (PLED) and white-emitting PLED, low-conductive PEDOT:PSS Clevios ™ P CH 8000 (8000) has been used to replace widely adopted high-conductive PEDOT:PSS Clevios ™ P Al 4083 (4083). In a blue PLED with poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) as the emission layer, though the 8000 device has a higher efficiency and a lower turn-on voltage than the 4083 device, the 8000 device exhibits low peak luminance, sharp efficiency roll-off, and permanent degradation of the emission material. By analyzing the hole-only device and the X-ray photoelectron spectroscopy spectra, it's revealed that the insulating PSS layer on 8000 surface is responsible for the inefficient hole injection. A simple methanol treatment on the 8000 surface effectively removes the redundant PSS. Without the insulating PSS layer, the hole injection becomes efficient which extends the recombination zone. As the result, the methanol-treated 8000 device not only retains low turn-on voltage and the high device efficiency of the untreated 8000 device, but also achieves the peak luminance, mild efficiency roll-off, and device stability of the 4083 device.

Perovskite solar cells (PSCs) have attracted great attention in the past few years due to their rapid increase in efficiency and low-cost fabrication. However, instability against thermal stress and humidity is a big issue hindering their commercialization and practical applications. Here, by combining thermally stable formamidinium-cesium-based perovskite and a moisture-resistant carbon electrode, successful fabrication of stable PSCs is reported, which maintain on average 77% of the initial value after being aged for 192 h under conditions of 85 degrees C and 85% relative humidity (the "double 85" aging condition) without encapsulation. However, the mismatch of energy levels at the interface between the perovskite and the carbon electrode limits charge collection and leads to poor device performance. To address this issue, a thin-layer of poly(ethylene oxide) (PEO) is introduced to achieve improved interfacial energy level alignment, which is verified by ultraviolet photoemission spectroscopy measurements. Indeed as a result, power conversion efficiency increases from 12.2% to 14.9% after suitable energy level modification by intentionally introducing a thin layer of PEO at the perovskite/carbon interface.

Shrinking the device dimension has long been the pursuit of the semiconductor industry to increase the device density and operation speed. In the application of thin film transistors (TFTs), all-organic TFT arrays made by all-solution process are desired for low cost and flexible electronics. One of the greatest challenges is how to achieve ultrashort channel through a cost-effective method. In our study, ultrashort-channel devices are demonstrated by direct inkjet printing conducting polymer as source/drain and gate electrodes without any complicated substrate’s pre-patterning process. By modifying the substrate’s wettability, the conducting polymer’s contact line is pinned during drying process which makes the channel length well-controlled. An organic TFT array of 200 devices with 2 μm channel length is fabricated on flexible substrate through all-solution process. The simple and scalable process to fabricate high resolution organic transistor array offers a low cost approach in the development of flexible and wearable electronics.

In organic electronic devices, the interfacial dipole at organic/metal interfaces is critical in determining the carrier injection or extraction that limits the performance of the device. A novel technique to enable the direct measurement of underburied dipoles is developed and demonstrated. By tilting the shadow mask by a small angle, metal atoms diffuse into the opening slit to form an ultrathin metal layer during the evaporation process. As the ultrathin metal layer cannot screen out the dipole-induced surface work function change, the dipole strength and direction at the organic/metal interface can be revealed. It was found that the polarity of the organic material, the Fermi-level pinning and the interface morphology all play important roles in dipole formation. By comparing the energy level shifts at the organic/pre-deposited metal and organic/post-deposited metal interfaces, the dipole formed by molecular interactions could be distinguished from the dipole formed by Fermi-level pinning. A novel way to directly measure buried dipoles at metal–organic interfaces in organic electronic devices has been demonstrated. It is critical to match the energy levels at metal–organic interfaces in organic electronic devices such as organic solar cells and light-emitting diodes. To determine the most suitable material for the interfacial layer to achieve this, the work function of the cathode needs to be measured before and after depositing the interfacial layer, but this is difficult in conventional devices. Jian Wang and co-workers at South China University of Technology have solved this problem through forming an ultrathin metal layer by titling a shadow mask at a small angle during the evaporation process. Unlike thick metal layers, this ultrathin layer does not screen dipoles, allowing dipoles to be directly measured at interfaces. By thermally evaporated through a tilted shadow mask, metal vapor scatters and lands on the substrate to form an ultrathin layer. Absence of screening effect from bulk metal enables direct measurement of the buried dipole formed at the organic/metal interface. It is found that polar species of the organic, Fermi-level pinning and interface morphology can all influence the dipole orientation. The proposed method provides a quick and easy approach to assess interfacial materials and design electrode interfaces in organic electronic devices.

Superhydrophobicity is an important phenomenon in nature that inspires the design of numerous biomimetic functional materials. A superhydrophobic surface is expected to have a three-phase solid–liquid-vapor interface. To directly image the buried air phase in wetted superhydrophobic surfaces, we employed synchrotron radiation-based X-ray phase-contrast imaging to non-invasively probe the surfaces of natural lotus leaves and artificial carbon nanotube films in three dimensions. Reconstructed images of the three-phase distribution surrounding the superhydrophobic surfaces were presented with high resolution and contrast. Further extraction of the phases enabled direct analysis of the relationship between the surface morphology and its wetting property. We found that the protruding micro/nano-structures trapped a layer of air up to 14 μm thick, which played an important role in the observed superhydrophobicity. Direct evidence of the presence of a buffer layer air cushion deepens our understanding of hydrophobicity and opens new opportunities for designing novel bioinspired materials. Synchrotron X-ray imaging has given researchers the clearest view yet of how trapped air pockets can make surfaces extremely water repellent. Electron microscopy has helped researchers determine that rough, micro and nanoscale textures on natural surfaces such as lotus leaves cause water droplets to bead up. But below the droplets are hidden air cushions that elude direct observation. Now, Chunhai Fan and colleagues in Shanghai, China, have used non-invasive synchrotron radiation to clarify the role of the air phase in superhydrophobic lotus leaves and carbon nanotube films. High-colour and high-resolution images of interfacial electron density revealed that individual air pockets prevent water from infiltrating the surfaces grooves and can connect into large cushions that dramatically reduce the solid-water contact area. Any protruding structures on the superhydrophobic surfaces serve mainly as droplet anchor points. Synchrotron radiation-based X-ray phase-contrast imaging was employed to observe the solid–liquid-vapor interface on superhydrophobic surfaces. The reconstructed 3D images revealed that the water exists in the Cassie-to-Wenzel transit state. The micro/nano-structures trapped a large amount air on the supherhydrophobic surface to repel water.

An amino-functionalized copolymer with a conjugated backbone composed of fluorene, naphthalene diimide, and thiophene spacers (PFN-2TNDI) is introduced as an alternative electron transport layer (ETL) to replace the commonly used [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) in the p–i–n planar-heterojunction organometal trihalide perovskite solar cells. A combination of characterizations including photoluminescence (PL), time-resolved PL decay, Kelvin probe measurement, and impedance spectroscopy is used to study the interfacial effects induced by the new ETL. It is found that the amines on the polymer side chains not only can passivate the surface traps of perovskite to improve the electron extraction properties, they also can reduce the work function of the metal cathode by forming desired interfacial dipoles. With these dual functionalities, the resulted solar cells outperform those based on PCBM with power conversion efficiency (PCE) increased from 12.9% to 16.7% based on PFN-2TNDI. In addition to the performance enhancement, it is also found that a wide range of thicknesses of the new ETL can be applied to produce high PCE devices owing to the good electron transport property of the polymer, which offers a better processing window for potential fabrication of perovskite solar cells using large-area coating method.