Conductive PEDOT-Dominant Surface of Transparent Electrode Patch via Selective Phase Transfer for Efficient Flexible Photoelectronic Devices.

In this study, a conductive patch for a flexible organic optoelectronic device is proposed and implemented using a poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) polymer electrode based on a transfer process to achieve its high conductivity with an efficient conductive pathway. This PEDOT-dominant surface is induced by phase inversion during the transfer process owing to the solvent affinity of the PSS phase. The PEDOT:PSS patch formed by the transfer process minimizes the power loss in a flexible optoelectronic device due to the improved charge collection and suppressed leakage current responses. In addition, the bending stability of the flexible photoelectronic device is also enhanced by maintaining performance for 1000 bending cycles. Therefore, in the fabrication of a transparent flexible conductive PEDOT:PSS patch, the transfer process of a conducting polymer constitutes an effective strategy that can improve conductivity and embellished morphology.

Surface defect mitigation via alkyl-ligand-controlled purification for stable and high-luminescence perovskite quantum dots.

Perovskite quantum dots (PQDs) have received considerable attention as fluorescent materials due to their excellent optical properties. However, because PQDs contain ionic bonds, they have the disadvantage of being vulnerable to environmental conditions, so improving their stability is essential. Indeed, recent research has focused on improving both the stability and luminescence of PQDs by mixing them with methyl acetate (MeOAc) to suppress surface defects via purification. MeOAc reacts with the surface ligands of PQDs, resulting in ligand-controlled purification. However, while the ligands are limited for the PQD synthesis, the effect of ligand alkyl-chain length has not been reported. Therefore, we report herein a strategy for obtaining stable PQDs with tunable performances by using amine ligands of various chain lengths. The amine ligand is selected because it is very effective in interacting with the halide vacancies present on the surface of the perovskite crystal structure. The results indicate that MeOAc becomes less effective as the chain length of the ligand is increased, and more effective as the chain length is decreased. Consequently, PQDs treated with MeOAc and a short-chain ligand afford a quantum yield (QY) of 79.2% and are highly stable when exposed to thermal and ambient conditions. Therefore, we suggest a facile approach to suppressing the degradation of PQDs during the fabrication process.

A Fluorination Strategy and Low-Acidity Anchoring Group in Self-Assembled Molecules for Efficient and Stable Inverted Perovskite Solar Cells.

Herein, we synthesized two donor-acceptor (D-A) type small organic molecules with self-assembly properties, namely MPA-BT-BA and MPA-2FBT-BA, both containing a low acidity anchoring group, benzoic acid. After systematically investigation, it is found that, with the fluorination, the MPA-2FBT-BA demonstrates a lower highest occupied molecular orbital (HOMO) energy level, higher hole mobility, higher hydrophobicity and stronger interaction with the perovskite layer than that of MPA-BT-BA. As a result, the device based-on MPA-2FBT-BA displays a better crystallization and morphology of perovskite layer with larger grain size and less non-radiative recombination. Consequently, the device using MPA-2FBT-BA as hole transport material achieved the power conversion efficiency (PCE) of 20.32 % and remarkable stability. After being kept in an N2 glove box for 116 days, the unsealed PSCs' device retained 93 % of its initial PCE. Even exposed to air with a relative humidity range of 30±5 % for 43 days, its PCE remained above 91 % of its initial condition. This study highlights the vital importance of the fluorination strategy combined with a low acidity anchoring group in SAMs, offering a pathway to achieve efficient and stable PSCs.

In Situ Interfacial-Assembly Perovskite Quantum Dot via Marangoni and Capillary Convection Manipulation for Robust Luminescence.

In solid substrates, colloidal solutions produce irregular deposits on the surface by Marangoni flow and capillary flow during evaporation. Reportedly, perovskite quantum dots (PQDs) as a colloidal solution have irregular surfaces based on a similar principle as the coffee ring effect in QD systems when droplets evaporate from the substrate. Given that this issue is due to the direction of Marangoni and capillary flows, the substrate is tilted to change the direction of the flows. The appropriate angle is determined by controlling the angle of the substrate so that the two flows circulate similarly; this method is called "assembly-coating". Herein, we compare the PL intensity before and after the thermal evaporation of the thin films prepared by conventional and assembly-coating. Moreover, by characterizing the diode device (hole-only space charge limited current) for each coating process, the charge carrier characteristics are investigated in detail. Therefore, we suggest a facile strategy to obtain a uniform surface and thermal evaporative stability using colloidal solutions. This strategy is effective in designing surface uniformity and light-emitting layers for colloidal solution deposition and assembly.

One-Pot Synthesis of All-Inorganic CsPbClBr2 Blue Perovskite Quantum Dots via Stoichiometric Precursor.

The synthesis of perovskite-based blue light-emitting particles is valuable for several applications as the excellent optical properties and performances of the constituting materials associated with multi-exciton generation can be exploited. However, the preparation of perovskite precursors requires high temperatures, resulting in a complex manufacturing process. This paper proposes a one-pot method to synthesize CsPbClBr2 blue light-emitting quantum dots (QDs). In the case of nonstoichiometric precursor synthesis, the CsPbClBr2 QDs coexisted with additional products. The solvent for synthesizing mixed perovskite nanoparticles (containing chloride) was selected by mixing dimethylformamide (DMF) and/or dimethyl sulfoxide (DMSO) in different ratios. When only DMF was used with the stoichiometric CsBr and PbX2 (X = Cl, Br) ratio, the quantum yield was 70.55%, and superior optical properties were achieved. Moreover, no discoloration was observed for 400 h, and a high photoluminescence intensity was maintained. When deionized water was added to form a double layer with hexane, the luminescence was maintained for 15 days. In other words, the perovskite did not easily decompose even when in contact with water, which suppressed the release of Pb2+, which are heavy metal atoms in the structure. Overall, the proposed one-pot method for all-inorganic-based perovskite QDs provides a platform for synthesizing superior blue light-emitting materials.

Versatile Pendant Polymer for Selective Charge Carrier Transport via Controlling the Supramolecular Self-Assembly.

Invited for this month's cover is the group of Jin Kuen Park and Dong Hwan Wang at two different universities in South Korea. The image shows how the supramolecular interaction between pendant polymers can play a role in controlling the electronic properties in perovskite-based electronics such as solar cells and photodetectors. The Full Paper itself is available at 10.1002/cssc.202101785.

Versatile Pendant Polymer for Selective Charge Carrier Transport via Controlling the Supramolecular Self-Assembly.

Polyvinyl carbazole (P0)-based pendant polymers were synthesized by modifying carbazole motifs with pyrene derivatives (P1 and P4) to manipulate the bandgap and frontier orbital energy levels. To establish the electronic properties of pendant polymers according to structural differences, the polymers were utilized as additional hole transport layers in planar-type perovskite solar cells and organic photovoltaic cells. When P4 with thiophene-pyrene pendant was used as hole transport layer, all device parameters, except open-circuit voltage, were significantly improved in comparison with P0 and P1 (conjugated with t-butyl pyrene derivatives). Since P4 had more electrically conductive thiophene units than benzene units with fewer alkyl groups, the supramolecular assembly of P4 was found to be more favorable in electronic devices. Furthermore, devices with P4 demonstrated lower dark current than others, which could potentially be useful for charge carrier transport and sensitive photo detecting devices.

A Facile and Effective Ozone Exposure Method for Wettability and Energy-Level Tuning of Hole-Transporting Layers in Lead-Free Tin Perovskite Solar Cells.

Lead-free perovskite solar cells (PSCs) have attracted interest among scientists searching for eco-friendly energy harvesting devices. Herein, the effects of ozone exposure on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in lead-free tin halide PSCs as a facile and low-cost process for improving device performance are analyzed. Two types of tin-based PSCs and one typical lead-based PSC were fabricated. The ozone exposure on PEDOT:PSS increases the short-circuit current density (JSC) and the fill factor (FF) of PSCs in all cases with perovskite grain enlargement and hole-mobility enhancement of the devices, respectively. For open-circuit voltage (VOC), the outcome depends on the band gap and the energy levels of the perovskite films. While ozone exposure treatment is favorable for PEA0.15FA0.85SnI3-based tin PSCs, VOC decreases with ozone exposure in the case of Ge:EDA0.01FA0.98SnI3-based tin PSCs because of a misalignment of the energy levels. Regardless, the efficiency of PEA0.15FA0.85SnI3-based tin PSCs increases from 8.7 to 10.1% when measured inside a glovebox upon ozone exposure of PEDOT:PSS. The efficiency of Ge:EDA0.01FA0.98SnI3-based tin PSCs increases from 6.8 to 8.1%, and the devices retain an efficiency of 5.0% even after 50 days in air.

High-Valent Iodoplumbate-Rich Perovskite Precursor Solution via Solar Illumination for Reproducible Power Conversion Efficiency.

The power conversion efficiency (PCE) of solution-processed organic-inorganic hybrid perovskite solar cells has been drastically improved. Despite this considerable progress, systematic research on precursor solution chemistry and its effects on photovoltaic parameters has been limited thus far. Herein, we report on the tracking of changes in chemical species in a precursor solution under solar illumination and investigate the correlation between the equilibrium change and the corresponding perovskite film formation. The illuminated perovskite precursors display a higher density of high-valent iodoplumbate, where the resulting perovskite film exhibits reduced defect density with uniform film formation. Conclusively, the perovskite solar cells prepared by the photoaged precursor solution demonstrate not only improved average PCE but also enhanced reproducibility with a narrow PCE distribution. This discovery shows robust control of perovskite precursor solutions from a simple treatment and suggests that the resulting uniform film may be applicable to various halide perovskite-based devices.

Fully Inorganic CsSnI3-Based Solar Cells with >6% Efficiency and Enhanced Stability Enabled by Mixed Electron Transport Layer.

Fully inorganic black orthorhombic (B-γ) CsSnI3 has become a promising candidate for perovskite solar cell (PSC) thanks to its low toxicity and decently high theoretical power conversion efficiency (PCE). However, so far, the reported PCE of the B-γ CsSnI3 PSC is still not comparable with its lead-based or organotin-based counterparts. Herein, a mixed electron transport layer (ETL) composed of ZnO nanoparticles (NPs) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is incorporated into inverted B-γ CsSnI3 PSCs. The mixed ETL exhibits the merits of both ZnO and PCBM. The highest PCE of 6.08% was recorded for the PSC with mixed ZnO-PCBM ETL, which is 34.2% higher than that of the device with plain PCBM ETL (PCE of 4.53%) and 28.8% superior to that of plain ZnO ETL-based device (PCE of 4.72%). Meanwhile, the mixed ZnO-PCBM ETL-based PSC retained 71% of its initial PCE under inert conditions at room temperature after 60 days of storage and maintained 67% PCE after 20 days of storage under ambient air at 30% relative humidity and room temperature.

Light-Emitting Transistors with High Color Purity Using Perovskite Quantum Dot Emitters.

The class of organic-inorganic lead halides with perovskite crystal structures has recently emerged as promising materials for a variety of practical optoelectronic applications. In particular, hybrid halide perovskite quantum dots possess excellent intrinsic optoelectronic properties such as high color purity (full width at half-maximum of 24.59 nm) and photoluminescence quantum yields (92.7%). In this work, we demonstrate the use of perovskite quantum dot materials as an emissive layer of hybrid light-emitting transistors. To investigate the working mechanism of perovskite quantum dots in light-emitting transistors, we investigated the electrical and optical characteristics under both p-channel and n-channel operation. Using these materials, we have achieved perovskite quantum dot light-emitting transistors with high electron mobilities of up to 12.06 cm2·V-1 s-1, high brightness of up to 1.41 × 104 cd m-2, and enhanced external quantum efficiencies of up to 1.79% operating at a source-drain potential of 40 V.

Selective Soxhlets extraction to enhance solubility of newly-synthesized poly(indoloindole-selenophene vinylene selenophene) donor for photovoltaic applications.

An electron-rich fused indoloindole-based poly(indoloindole-selenophene vinylene selenophene) was synthesized and characterized. Soxhlet can be obtained by continuously purifying the product with a specific solvent and obtaining a pure polymer with a high concentration. Molecular weight is affected by the vapor pressure of marginal solvent, and the polymer was fractionated using tetrahydrofuran, chloroform, and chlorobenzene. Solubility is closely related to the morphology of bulk heterojunction and device parameters. In the solution process of fabricating the organic solar cell, securement of solubility has a great effect on the performance of the device, because morphology and orientation of a photo-active layer which significantly affect charge transport in the device. Since tetrahydrofuran (THF) Soxhlet solvents have high vapor pressure and appropriate solubility parameters, THF induced the best solubility of P-IDI-SVS materials for organic solvents. And through additive optimization, the performance of the device based on P-IDI-SVS from THF-Soxhlet extraction was enhanced. This is expected to be a meaningful study because the effect on solubility of Soxhlet solvent suggests factors to be considered in the solution process in organic solar cell research. In addition, surface modified bulk heterojunction was observed using atomic force microscopy, photoluminescence, time-correlated single photon counting and Raman spectroscopy analysis.

Acidity Suppression of Hole Transport Layer via Solution Reaction of Neutral PEDOT:PSS for Stable Perovskite Photovoltaics.

Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) is typically used for hole transport layers (HTLs), as it exhibits attractive mechanical, electrical properties, and easy processability. However, the intrinsically acidic property can degrade the crystallinity of perovskites, limiting the stability and efficiency of perovskite solar cells (PSCs). In this study, inverted CH3NH3PbI3 photovoltaic cells were fabricated with acidity suppressed HTL. We adjusted PEDOT:PSS via a solution reaction of acidic and neutral PEDOT:PSS. And we compared the various pH-controlled HTLs for PSCs devices. The smoothness of the pH-controlled PEDOT:PSS layer was similar to that of acidic PEDOT:PSS-based devices. These layers induced favorable crystallinity of perovskite compared with acidic PEDOT:PSS layers. Furthermore, the enhanced stability of pH optimized PEDOT:PSS-based devices, including the prevention of degradation by a strong acid, allowed the device to retain its power conversion efficiency (PCE) value by maintaining 80% of PCE for approximately 150 h. As a result, the pH-controlled HTL layer fabricated through the solution reaction maintained the surface morphology of the perovskite layer and contributed to the stable operation of PSCs.

Tuning the energy level of TAPC: crystal structure and photophysical and electrochemical properties of 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methoxyphenyl)aniline].

The energy level of a hole-transporting material (HTM) in organic electronics, such as organic light-emitting diodes (OLEDs) and perovskite solar cells (PSCs), is important for device efficiency. In this regard, we prepared 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methoxyphenyl)aniline] (TAPC-OMe), C46H46N2O4, to tune the energy level of 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methylphenyl)aniline] (TAPC), which is a well-known HTM commonly used in OLED applications. A systematic characterization of TAPC-OMe, including 1H and 13C NMR, elemental analysis, UV-Vis absorption, fluorescence emission, density functional theory (DFT) calculations and single-crystal X-ray diffraction, was performed. TAPC-OMe crystallized in the triclinic space group P-1, with two molecules in the asymmetric unit. The dihedral angles between the central amine triangular planes and those of the phenyl groups varied from 26.56 (9) to 60.34 (8)° due to the steric hindrance of the central cyclohexyl ring. This arrangement might be induced by weak hydrogen bonds and C-H...π(Ph) interactions in the extended structure. The emission maxima of TAPC-OMe showed a significant bathochomic shift compared to that of TAPC. A strong dependency of the oxidation potentials on the nature of the electron-donating ability of substituents was confirmed by comparing oxidation potentials with known Hammett parameters (σ).

Long-Term Stable Transferred Organic Photoactive Layer-Based Photodiode with Controlled Wetting through Interface Stabilization.

The stamping transfer process, which provides a precise patterning of the target material without the limitation of an underlying layer, has attracted significant attention for large-scale roll-to-roll fabrication. Despite the need to minimize the peeling energy, expressed as the sum of adhesion energies, for a simple transfer process, many studies have not considered this effect. In this study, we introduced a wetting coefficient related with adhesions between polymers for the transfer design of organic photosensitive materials. We observed a difference in adhesion between polymer blends depending on the surface energy of the mold. We designed high-surface-energy polyurethane acrylate to enable a residue-free transfer process. The transfer process significantly contributed to the device stability through changes in dark currents, photocurrents, responsivity, and detectivity over time, compared to spin coating. In particular, the detectivity was maintained over 95% after 360 h, and no burn-in loss of internal resistance was observed in the device with a transferred active layer. X-ray photoelectron spectroscopy showed that a large interfacial change between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5- b']dithiophene-2,6-diyl- alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophene-4,6-diyl):[6,6] phenyl C71 butyric acid methyl ester obtained through spin coating occurred owing to solution penetration, whereas the transfer process provided a constant interface owing to morphology stabilization. Therefore, the transfer process with optimized adhesion properties can improve the device operation durability without burn-in loss, enabling a cost-effective fabrication of organic optoelectronic devices.

Covalent organic nanosheets for effective charge transport layers in planar-type perovskite solar cells.

Herein, solvent-treated bandgap-tunable covalent organic nanosheets (CONs) were prepared via the Stille cross-coupling reaction. These materials are considered useful as interlayers in photovoltaic devices upon the alignment of energy levels between other components. Among various types of solar cells, according to the organic-interlayer study, inverted planar perovskite solar cells (PSCs) are mostly demanded to effectively transport and collect charge carriers due to their high performance. At first, the C-V analysis proved the energy levels of the frontier orbitals for CON-10 and CON-16 nanosheets; this verified the suitability of these nanosheets as hole transport layers (HTLs) with the PEDOT:PSS upon casting both films from DMSO. It became evident, however, that the hole transport property of the PEDOT:PSS on the CON-16 layer was unfavorable with the increasing UPS-proven hole injection barrier. In addition, both CONs induced a rough surface morphology; however, CON-10 showed a relatively smooth surface as compared to CON-16 based on the Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) profiles; furthermore, their surface properties influenced both the PEDOT:PSS layers and the perovskite layers. Especially, the XRD profiles presented an enhanced crystallinity of the perovskite layers with CON-10. All these aspects indicate that CON-10 is a more effective HTL material, and several versions of perovskite solar cells (PSCs) have been fabricated with/without CON-10 and CON-16 together with the PEDOT:PSS to determine the more-HTL-suitable CON. As a result, the power conversion efficiencies (PCEs) of the optimized devices with CON-10 exhibited a value of 10.2%, which represented a 1% increase over those of the reference devices without the CONs and was 4% higher than that of the CON-16 devices. Moreover, the devices with CON-10 were further optimized with TiOx using Al electrodes, leading to a PCE increase of these devices that became slightly higher than the PCEs of the device with CON-10 and without TiOx. This tendency was supported by photoluminescence (PL) spectroscopy, photocurrent density (Jph), and space-charge-limited current (SCLC) mobility results.

The Investigation of the Seebeck Effect of the Poly(3,4-Ethylenedioxythiophene)-Tosylate with the Various Concentrations of an Oxidant.

Poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) can be synthesized through an in situ polymerization and doping process with iron(III) p-toluenesulfonate hexahydrate as an oxidant. Both the Seebeck coefficient and the electrical conductivity were modified by varying the concentration of the oxidant. We investigated the effects of varying the concentration of the oxidant on the particle sizes and doping (oxidation) levels of PEDOT-Tos for thermoelectric applications. We demonstrated that an increase in the oxidant enabled an expansion of the particle sizes and the doping levels of the PEDOT-Tos. The modification of the doping levels by the concentration of the oxidant can provide another approach for having an optimal power factor for thermoelectric applications. De-doping of PEDOTs by reduction agents has been generally investigated for changing its oxidation levels. In this study, we investigated the effect of the concentration of the oxidant of PEDOT-Tos on the oxidation levels, the electrical conductivities and the Seebeck coefficients. As loading the oxidant of PEDOT-Tos, the Seebeck coefficient was compromised, while the electrical conductivity increased.

Facile NiOx Sol-Gel Synthesis Depending on Chain Length of Various Solvents without Catalyst for Efficient Hole Charge Transfer in Perovskite Solar Cells.

Nickel oxide (NiOx)⁻based perovskite solar cells (PSCs) have recently gained considerable interest, and exhibit above 20% photovoltaic efficiency. However, the reported syntheses of NiOx sol-gel used toxic chemicals for the catalysts during synthesis, which resulted in a high-temperature annealing requirement to remove the organic catalysts (ligands). Herein, we report a facile "NiOx sol-gel depending on the chain length of various solvents" method that eschews toxic catalysts, to confirm the effect of different types of organic solvents on NiOx synthesis. The optimized conditions of the method resulted in better morphology and an increase in the crystallinity of the perovskite layer. Furthermore, the use of the optimized organic solvent improved the absorbance of the photoactive layer in the PSC device. To compare the electrical properties, a PSC was prepared with a p-i-n structure, and the optimized divalent alcohol-based NiOx as the hole transport layer. This improved the charge transport compared with that for the typical 1,2-ethanediol (ethylene glycol) used in earlier studies. Finally, the optimized solvent-based NiOx enhanced device performance by increasing the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), compared with those of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)⁻based devices.

Rapid Formation of a Disordered Layer on Monoclinic BiVO4 : Co-Catalyst-Free Photoelectrochemical Solar Water Splitting.

A surface disordered layer is a plausible approach to improve the photoelectrochemical performance of TiO2 . However, the formation of a crystalline disordered layer in BiVO4 and its effectiveness towards photoelectrochemical water splitting has remained a big challenge. Here, we report a rapid solution process (within 5 s) that is able to form a disordered layer of a few nanometers thick on the surface of BiVO4 nanoparticles using a specific solution with a controllable reducing power. The disordered layer on BiVO4 alleviates charge recombination at the electrode-electrolyte interface and reduces the onset potential greatly, which in turn results in a photocurrent density of approximately 2.3 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE). This value is 2.1 times higher than that of bare BiVO4 . The enhanced photoactivity is attributed to the increased charge separation and transfer efficiencies, which resolve the intrinsic drawbacks of bare BiVO4 such as the short hole diffusion length of around 100 nm and poor surface oxygen evolution reactivity.