Enhanced Efficiency and Stability of Sky Blue Perovskite Light-Emitting Diodes via Introducing Lead Acetate.

All-inorganic metal halide perovskite is promising for highly efficient and thermally stable perovskite light-emitting diodes (PeLEDs). However, there is still great room for improvement in the film quality, including low coverage and high trap density, which play a vital role in achieving high-efficiency PeLEDs. In this work, lead acetate (Pb(Ac)2) was introduced into the perovskite precursor solution as an additive. Experimental results show that perovskite films deposited from a one-step anti-solvent free solution process with increased surface coverage and reduced trap density were obtained, leading to enhanced photoluminescence (PL) intensity. More than that, the valence band maximum (VBM) of perovskite films was reduced, bringing about a better energy level matching the work function of the hole-injection layer (HIL) poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS), which is facilitated for the hole injection, leading to a decrease in the turn-on voltage (Vth) of PeLEDs from 3.4 V for the control device to 2.6 V. Finally, the external quantum efficiency (EQE) of the sky blue PeLEDs (at 484 nm) increased from 0.09% to 0.66%. The principles of Pb(Ac)2 were thoroughly investigated by using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). This work provides a simple and effective strategy for improving the morphology of perovskite and therefore the performance of PeLEDs.

Dual-Color Lasers in Interlayer-Free Solution Processed Polymeric Bilayer Devices.

Multiwavelength organic lasers have attracted considerable interest in recent years due to the cost efficiency, wide luminescence coverage, and simple processability of organics. In this work, by simply spin coating immiscible polymeric gain media in sequence, dual-wavelength (blue-green or blue-red) amplified spontaneous emission (ASE) was achieved in bilayer devices. The blue emission, water/alcohol-soluble conjugated polyelectrolyte, poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), was used as the bottom layer. The commercially available nonpolar solvent soluble polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and its blend with poly(3-hexylthiophene) (P3HT) were used as the top active layers offering green and red emission, respectively. This novel compact configuration, without interlayers between the two active layers, offers potential for developing various applications. The carefully selected top and bottom layer polymers not only meet the conditions of immiscibility and different emission wavelength range but also have a common absorption band in UV, which allows simultaneous blue-green or blue-red dual-color ASE behaviors observed in the bilayer devices under the same 390 nm laser excitation. By introducing two-dimension (2D) square distributed feedback (DFB) gratings with different periods (300 nm for blue, 330 nm for green, and 390 nm for red) as cavities, single mode blue-green (Eth = 245 µJ cm-2) and blue-red (Eth = 189 µJ cm-2) lasers were achieved by focusing the excitation laser spot on different 2D DFB gratings area. Furthermore, we found it possible to gain sufficient light confinement for red emission along its diagonal direction (Λ âˆ¼424 nm), whereas the 2D DFB gratings offer feedback for blue emission from the 300 nm period along the rectangle direction. Therefore, both blue and red lasers were eventually achieved in the same PFN-Br/F8BT:P3HT bilayer device on the single 2D DFB gratings with a period of 300 nm in this work.

Enhancing the Performance of Perovskite Solar Cells by Introducing 4-(Trifluoromethyl)-1H-imidazole Passivation Agents.

Defects in perovskite films are one of the main factors that affect the efficiency and stability of halide perovskite solar cells (PSCs). Uncoordinated ions (such as Pb2+, I-) act as trap states, causing the undesirable non-radiative recombination of photogenerated carriers. The formation of Lewis acid-base adducts in perovskite directly involves the crystallization process, which can effectively passivate defects. In this work, 4-(trifluoromethyl)-1H-imidazole (THI) was introduced into the perovskite precursor solution as a passivation agent. THI is a typical amphoteric compound that exhibits a strong Lewis base property due to its lone pair electrons. It coordinates with Lewis acid Pb2+, leading to the reduction in defect density and increase in crystallinity of perovskite films. Finally, the power conversion efficiency (PCE) of PSC increased from 16.49% to 18.97% due to the simultaneous enhancement of open-circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF). After 30 days of storage, the PCE of the 0.16 THI PSC was maintained at 61.9% of its initial value, which was 44.3% for the control device. The working mechanism of THI was investigated. This work provides an attractive alternative method to passivate the defects in perovskite.

Highly Efficient Perovskite Solar Cell Based on PVK Hole Transport Layer.

A π-conjugated small molecule N, N'-bis(naphthalen-1-yl)-N, N'-bis(phenyl)benzidine (NPB) was introduced into poly(9-vinylcarbazole) (PVK) as a hole transport layer (HTL) in inverted perovskite solar cells (PSCs). The NPB doping induces a better perovskite crystal growth, resulting in perovskite with a larger grain size and less defect density. Thus, the VOC, JSC, and FF of the PSC were all enhanced. Experimental results show that it can be ascribed to the reduction of surface roughness and improved hydrophilicity of the HTL. The effect of NPB on the aggregation of PVK was also discussed. This work demonstrates the great potential of PVK as the HTL of PSCs and provides an attractive alternative for HTL to realize high-efficiency PSCs.

Internal Interactions between Mixed Bulky Organic Cations on Passivating Defects in Perovskite Solar Cells.

In perovskite solar cells (PSCs), bulky organic cation halide salt additions play a significant role in suppressing nonradiative recombination by passivating intrinsic defects in perovskites. Herein, a passivation treatment is developed by applying mixed bulky cations [guanidinium cation (GA+) and phenylethylammonium cations (PEA+)] as the additive for perovskite thin films. The internal interactions between the two bulky cations could result in lower carrier trap-state densities, a sharper Urbach tail, and better carrier transport in perovskite films in comparison with a control film. As a result, in comparison to the control device, which has a power conversion efficiency (PCE) of 18.92%, the mixed-cation-based device exhibits a dramatic enhancement of PCE of 20.64%. Importantly, after 720 h of storage in an ambient atmosphere with a relative humidity (RH) of 60-80% at room temperature, the mixed-cation-based device retains 62.7% of its original performance, whereas the control devices decay to less than 40% of their original performance.

Defect Passivation Using Trichloromelamine for Highly Efficient and Stable Perovskite Solar Cells.

Nonradiative recombination losses caused by defects in the perovskite layer seriously affects the efficiency and stability of perovskite solar cells (PSCs). Hence, defect passivation is an effective way to improve the performance of PSCs. In this work, trichloromelamine (TCM) was used as a defects passivator by adding it into the perovskite precursor solution. The experimental results show that the power conversion efficiency (PCE) of PSC increased from 18.87 to 20.15% after the addition of TCM. What's more, the environmental stability of PSCs was also improved. The working mechanism of TCM was thoroughly investigated, which can be ascribed to the interaction between the -NH- group and uncoordinated lead ions in the perovskite. This work provides a promising strategy for achieving highly efficient and stable PSCs.

High-quality WS2 film as a hole transport layer in high-efficiency non-fullerene organic solar cells.

Liquid-exfoliated 2D transition metal disulfides (TMDs) are potential substitutes for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layers (HTLs) in Organic Solar Cells (OSCs). Herein, high-yield and high-quality WS2 flake layers are prepared by comprehensively controlling the initial concentration, sonication processing time and centrifugal speed. The WS2 layers deposited on in situ transparent indium tin oxide (ITO) without plasma treatment show higher uniformity and conductivity than that formed on ITO after plasma treatment. With a significant increase in the short-circuit current density (JSC), the power conversion efficiency (PCE) of PM6:Y6-based non-fullerene OSCs using optimized WS2 as the HTL is higher than that using PEDOT:PSS as the HTL(15.75% vs. 15.31%). Combining the morphology characteristics with carrier recombination characteristics, the higher quality of the ITO/WS2 composite substrate leads to better charge transport and a lower bimolecular recombination rate in OSCs, thereby improving the device performance.

Porphyrin Acceptors with Two Perylene Diimide Dimers for Organic Solar Cells.

Three small-molecule acceptors (Por-PDI, TEHPor-PDI, and BBOPor-PDI) with different side chains were synthesized by using a porphyrin core as the electron-donating unit and connecting electron-withdrawing perylene diimide dimers via acetylene bridges. The bulk heterojunction organic solar cells based on the three acceptors and a polymer donor provided power conversion efficiencies (PCEs) of 3.68-5.21 % when the active layers were fabricated with pyridine additives. Though the synthesis of Por-PDI is easier with fewer reaction steps and higher yields, the devices based on Por-PDI showed the best performance with a PCE of 5.21 %. The more ordered intermolecular packing due to the reduced steric hindrance at the porphyrin core of Por-PDI could contribute to the more balanced hole/electron mobilities, higher maximum charge generation rate, and less bimolecular recombination in Por-PDI devices, which are beneficial for the higher PCE.

Roll-to-roll printed high voltage supercapattery in lead-contaminated aqueous electrolyte.

This work investigated the potential application of roll-to-roll printed PEDOT:PSS on an ITO/PET substrate using Pb2+ containing 0.1 M NaCl aqueous solution for a supercapattery. The PEDOT:PSS/ITO/PET electrode achieved 2.2 µAh cm-2 (46.5 mAh g-1) in 0.1 M NaCl and 10 µAh cm-2 (216.8 mAh g-1) in 2 mM Pb2+/0.1 M NaCl at a current density of 0.2 mA cm-2 (4.34 A g-1). The electrode also shows good cyclic performance that retains 63% of its initial capacitance after 1000 charge-discharge cycles. A device operating at a high voltage of 1.8 V was built using PEDOT:PSS/ITO/PET in aqueous electrolyte. The energy density of the symmetric PEDOT:PSS/ITO/PET device is 6.2 Wh kg-1 in 0.1 M NaCl and is improved to 11 Wh kg-1 in 3 mM Pb2+/0.1 M NaCl.

Electrocatalytic reduction of oxygen at platinum nanoparticles dispersed on electrochemically reduced graphene oxide/PEDOT:PSS composites.

Composites of commercially available graphene oxide (GO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with solvent additive ethylene glycol (EG) were investigated as an alternative support for Pt nanoparticles towards the electrocatalytic reduction of oxygen. The surface characteristics of the materials were examined using atomic force microscopy (AFM), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS). Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) at rotating disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) were used to characterise the electrocatalytic activities of the composites materials. The structural and electrochemical studies reveal that the addition of EG favours the homogeneous distribution of Pt particles with reduced particle size and improves the electrocatalytic properties. A 30% and 16% increase in electrochemically active surface area (ECSA), a 1.2 and 1.1 fold increase in specific area activity (SA), and a 1.5 and 1.2 fold increase in mass activity (MA) were observed for 30% and 40% Pt loading on PEDOT:PSS after the addition of EG. A composite of rGO and PEDOT:PSS(EG) was investigated for different (w/w) ratios of PEDOT:PSS and rGO. The 1 : 2 w/w ratio showed an enhanced catalytic activity with high limiting current, more positive onset potential, higher SA and MA with lower H2O2 yield compared to PEDOT:PSS(EG) and rGO and previously reported values for PEDOT:PSS.

Mechanism study on highly efficient polymer light-emitting diodes utilizing double-layered alkali halide electron injection layer.

Enhancing the injection of electron is an effective strategy to improve the performance of polymer light-emitting diodes (PLEDs). In this work, we reported a 286% improvement in current efficiency (CE) of PLEDs by using double-layered alkali halide electron injection layer (EIL) NaCl/LiF instead of LiF. A significant enhancement of electron injection was observed after inserting the NaCl layer. To understand the mechanism of such improvement, the devices with KBr/LiF and CsF/LiF as EILs were also investigated. Experimental results show that metal cation migrated under the effect of built-in electric field (Vbi), which plays the main role on the improvement of electron injection in PLEDs.

Morphology control of organic halide perovskites by adding BiFeO3 nanostructures for efficient solar cell.

The morphology of perovskite light-absorption layer plays an important role in the performance of perovskite solar cells (PSCs). In this work, BiFeO3 (BFO) nanostructures were used as additive for CH3NH3PbI3 (MAPbI3) via anti-solvent method. The addition of BFO nanostructures greatly enhanced the crystallinity, grain size and film uniformity of MAPbI3. As a result, the charge carrier mobility and electron diffusion length increased, leading to the increase of the short circuit current (JSC) of PSCs. This work provides a very simple but effective approach to improve the morphology of perovskite layer for efficient PSCs.

Nanoparticles: Oral Delivery for Protein and Peptide Drugs.

Protein and peptide drugs have many advantages, such as high bioactivity and specificity, strong solubility, and low toxicity. Therefore, the strategies for improving the bioavailability of protein peptides are reviewed, including chemical modification of nanocarriers, absorption enhancers, and mucous adhesion systems. The status, advantages, and disadvantages of various strategies are systematically analyzed. The systematic and personalized design of various factors affecting the release and absorption of drugs based on nanoparticles is pointed out. It is expected to design a protein peptide oral delivery system that can be applied in the clinic.

Improving the Performances of Perovskite Solar Cells via Modification of Electron Transport Layer.

The commonly used electron transport material (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) for perovskite solar cells (PSC) with inverted planar structures suffers from properties such as poor film-forming. In this manuscript, we demonstrate a simple method to improve the film-forming properties of PCBM by doping PCBM with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the electron transport layer (ETL), which effectively enhances the performance of CH3NH3PbI3 based solar cells. With 5 wt % F8BT in PCBM, the short circuit current (JSC) and fill factor (FF) of PSC both significantly increased from 17.21 ± 0.15 mA·cm-2 and 71.1 ± 0.07% to 19.28 ± 0.22 mA·cm-2 and 74.7 ± 0.21%, respectively, which led to a power conversion efficiency (PCE) improvement from 12.6 ± 0.24% to 15 ± 0.26%. The morphology investigation suggested that doping with F8BT facilitated the formation of a smooth and uniform ETL, which was favorable for the separation of electron-hole pairs, and therefore, an improved performance of PSC.

Concurrent Optical Gain Optimization and Electrical Tuning in Novel Oligomer:Polymer Blends with Yellow-Green Laser Emission.

Electrically pumped organic lasing requires the integration of electrodes contact into the laser cavity in an organic light-emitting diode (OLED) or organic field effect transistor configuration to enable charge injection. Efficient and balanced carrier injection requires in turn alignment of the energy levels of the organic active layers with the Fermi levels of the cathode and anode. This can be achieved through chemical substitution with specific aromatic functional groups, although paying the price for a substantial (and often detrimental) change in the emission and light amplifying properties of the organic gain medium. Here, using host-guest energy transfer mixtures with hosts bearing a systematic and gradual shift in molecular orbitals is proposed, which reduces the amplified spontaneous emission (ASE) threshold of the organic gain medium significantly while leaving the peak emission unaffected. By virtue of the low guest doping required for complete host-to-guest energy transfer, the injection levels in the blends are attributed to the host whereas the gain properties solely depend on the guest. It is demonstrated that the ASE peak and thresholds of blends with different hosts do not differ while the current efficiency of OLEDs devices is deeply influenced by molecular orbital tuning of the hosts.

Enhancing the Performance of Inverted Perovskite Solar Cells via Grain Boundary Passivation with Carbon Quantum Dots.

Nonradiative recombination, the main energy loss channel for open circuit voltage ( Voc), is one of the crucial problems for achieving high power conversion efficiency (PCE) in inverted perovskite solar cells (PSCs). Usually, grain boundary passivation is considered as an effective way to reduce nonradiative recombination because the defects (uncoordinated ions) on grain boundaries are passivated. We added the hydroxyl and carbonyl functional groups containing carbon quantum dots (CQDs) into a perovskite precursor solution to passivate the uncoordinated lead ions on grain boundaries. Higher photoluminescence intensity and longer carrier lifetime were demonstrated in the perovskite film with the CQD additive. This confirmed that the addition of CQDs can reduce nonradiative recombination by grain boundary passivation. Additionally, the introduction of CQDs could increase the thickness of the perovskite film. Consequently, we achieved a champion device with a PCE of 18.24%. The device with CQDs retained 73.4% of its initial PCE after being aged for 48 h under 80% humidity in the dark at room temperature. Our findings reveal the mechanisms of how CQDs passivate the grain boundaries of perovskite, which can improve the efficiency and stability of PSCs.

Photophysical and Fluorescence Anisotropic Behavior of Polyfluorene ß-Conformation Films.

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (ß-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene ß-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) ß-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, ß-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to ß-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF ß-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.

Highly Promoting the Performances of Polymer Light-Emitting Diodes via Control of the Residue of a Polar Solvent on an Emissive Layer.

The polar solvent dimethylformamide (DMF) was used to treat the emissive layer (EML) of polymer light-emitting diodes (PLEDs). The formation of a dipole layer at the EML/cathode interface after DMF treatment was proven, which led to a reduction of the electron-injection barrier. The dipole layer was formed mainly because of the intrinsic polarity of DMF. By control of the residue of DMF on the EML, a maximum enhancement of the peak luminous efficiency from 5.33 ± 0.57 to 12.05 ± 1.2 cd/A was achieved. This study suggests that solvent treatment is a simple and efficient approach to realizing highly efficient PLEDs with a high-work-function metal cathode.

Highly pH-responsive sensor based on amplified spontaneous emission coupled to colorimetry.

We demonstrated a simple, directly-readable approach for high resolution pH sensing. The method was based on sharp changes in Amplified Spontaneous Emission (ASE) of a Stilbene 420 (ST) laser dye triggered by the pH-dependent absorption of Bromocresol Green (BG). The ASE threshold of BG:ST solution mixtures exhibited a strong dependence on BG absorption, which was drastically changed by the variations of the pH of BG solution. As a result, ASE on-off or off-on was observed with different pH levels achieved by ammonia doping. By changing the concentration of the BG solution and the BG:ST blend ratio, this approach allowed to detect pH changes with a sensitivity down to 0.05 in the 10-11 pH range.

Morphology Analysis and Optimization: Crucial Factor Determining the Performance of Perovskite Solar Cells.

This review presents an overall discussion on the morphology analysis and optimization for perovskite (PVSK) solar cells. Surface morphology and energy alignment have been proven to play a dominant role in determining the device performance. The effect of the key parameters such as solution condition and preparation atmosphere on the crystallization of PVSK, the characterization of surface morphology and interface distribution in the perovskite layer is discussed in detail. Furthermore, the analysis of interface energy level alignment by using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy is presented to reveals the correlation between morphology and charge generation and collection within the perovskite layer, and its influence on the device performance. The techniques including architecture modification, solvent annealing, etc. were reviewed as an efficient approach to improve the morphology of PVSK. It is expected that further progress will be achieved with more efforts devoted to the insight of the mechanism of surface engineering in the field of PVSK solar cells.