Hole Transport Bilayer Structure for Quasi‐2D Perovskite Based Blue Light‐Emitting Diodes with High Brightness and Good Spectral Stability

Substantial achievements have been made in green and red perovskite light emitting diodes (PeLEDs) recently. However, blue PeLEDs still lag behind with much lower performances. One of the main reasons is the mass undesirable nonradiative recombination at interfaces and within the perovskite films. In this work, an efficient hole transport bi‐layer structure composed of PSSNa and NiO_x is demonstrated to simultaneously inhibit the nonradiative decays between NiO_x and perovskite films by reducing NiO_x surface defects and improving quasi‐2D perovskite thin film quality by minimizing its pin‐holes and reducing the film roughness. The results show that the dipole feature of PSSNa improves the hole transportation and thus PeLED performances. Moreover, by introducing KBr into the perovskite, its film quality improves and trap states reduce. Eventually, the blue PeLEDs is achieved with a very low turn‐on voltage of 3.31 V accompanied with an external quantum efficiency of 1.45% and a remarkable luminance of 4359 cd m^‐2. With further optimization of the perovskite precursor concentration, the highest luminance reaches 5737 cd m^‐2, which represents the brightest blue PeLEDs reported to date as far as it is known. Furthermore, the devices also show better spectral stability and operation lifetime as compared to other blue PeLEDs.     

Synergistic Effect of Dual Ligands on Stable Blue Quasi‐2D Perovskite Light‐Emitting Diodes

Since the emergence of inorganic–organic hybrid perovskites a few years ago, there have been many promising achievements in the field of green and red perovskite light‐emitting diodes (PeLEDs). Nevertheless, the performance of blue‐light PeLEDs faces challenges. In this work, the unique synergy obtained by introducing two different ligands to successfully form quasi‐2D perovskite films, which can exhibit stable blue‐light emission, is utilized. The fabricated PeLEDs have a maximum external quantum efficiency of 2.62% and a half lifetime (T₅₀) of 8.8 min. Meanwhile, the electroluminescence spectrum with its peak located at 485 nm, demonstrates improved stability by applying different voltage bias. The finding in this work offers a new way to achieve steady blue PeLEDs with high performance.     

Efficient Ruddlesden–Popper Perovskite Light‐Emitting Diodes with Randomly Oriented Nanocrystals

Ruddlesden–Popper phase (RP‐phase) perovskites that consist of 2D perovskite slabs interleaved with bulky organic ammonium (OA) are favorable for light‐emitting diodes (LEDs). The critical limitation of LED applications is that the insulating OA arranged in a preferred orientation limits charge transport. Therefore, the ideal solution is to achieve a randomly connected structure that can improve charge transport without hampering the confinement of the electron–hole pair. Here, a structurally modulated RP‐phase metal halide perovskite (MHP), (PEA)₂(CH₃NH₃)_m?1Pb_mBr_3m+1 is introduced to make the randomly oriented RP‐phase unit and ensure good connection between them by applying modified nanocrystal pinning, which leads to an increase in the efficiency of perovskite LEDs (PeLEDs). The randomly connected RP‐phase MHP forces contact between inorganic layers and thereby yields efficient charge transport and radiative recombination. Combined with an optimal dimensionality, (PEA)₂(CH₃NH₃)₂Pb₃Br₁₀, the structurally modulated RP‐phase MHP exhibits increased photoluminescence quantum efficiency, from 0.35% to 30.3%, and their PeLEDs show a 2,018 times higher current efficiency (20.18 cd A^?1) than in the 2D PeLED (0.01 cd A^?1) and 673 times than in the 3D PeLED (0.03 cd A^?1) using the same film formation process. This approach provides insight on how to solve the limitation of RP‐phase MHP for efficient PeLEDs.     

Process Pathway Controlled Evolution of Phase and Van‐der‐Waals Epitaxy in In/In2O3 on Graphene Heterostructures

Many applications of 2D materials require deposition of non‐2D metals and metal‐oxides onto the 2D materials. Little is however known about the mechanisms of such non‐2D/2D interfacing, particularly at the atomic scale. Here, atomically resolved scanning transmission electron microscopy (STEM) is used to follow the entire physical vapor deposition (PVD) cycle of application‐relevant non‐2D In/In₂O₃ nanostructures on graphene. First, a “quasi‐in‐situ” approach with indium being in situ evaporated onto graphene in oxygen‐/water‐free ultra‐high‐vacuum (UHV) is employed, followed by STEM imaging without vacuum break and then repeated controlled ambient air exposures and reloading into STEM. This allows stepwise monitoring of the oxidation of specific In particles toward In₂O₃ on graphene. This is then compared with conventional, scalable ex situ In PVD onto graphene in high vacuum (HV) with significant residual oxygen/water traces. The data shows that the process pathway difference of oxygen/water feeding between UHV/ambient and HV fabrication drastically impacts not only non‐2D In/In₂O₃ phase evolution but also In₂O₃/graphene out‐of‐plane texture and in‐plane rotational van‐der‐Waals epitaxy. Since non‐2D/2D heterostructures' properties are intimately linked to their structure and since influences like oxygen/water traces are often hard to control in scalable fabrication, this is a key finding for non‐2D/2D integration process design.     

Suppressing Ion Migration Enables Stable Perovskite Light‐Emitting Diodes with All‐Inorganic Strategy

Stability issue is one of the major concerns that limit emergent perovskite light‐emitting diodes (PeLEDs) techniques. Generally, ion migration is considered as the most important origin of PeLEDs degradation. In this work, an all‐inorganic device architecture, LiF/perovskite/LiF/ZnS/ZnSe, is proposed to address this imperative problem. The inorganic (Cs_1?xRb_x)_1?yK_yPbBr₃ perovskite is optimized with achieving a photoluminescence quantum yield of 67%. Depth profile analysis of X‐ray photoelectron spectroscopy indicates that the LiF/perovskite/LiF structure and the ZnS/ZnSe cascade electron transport layers significantly suppress the electric‐field‐induced ion migrations of the perovskite layers, and impede the diffusion of metallic atoms from cathode into perovskites. The as‐prepared PeLEDs display excellent shelf stability (maintaining 90% of the initial external quantum efficiency [EQE] after 264 h) and operational stability (half‐lifetime of about 255 h at an initial luminance of 120 cd m^?2). The devices also exhibit a maximum brightness of 15 6155 cd m^?2 and an EQE of 11.05%.     

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

In this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiN_x, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO₂/SiN_x passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO₂/SiN_x passivation layers, without inducing appreciable damage or defects on the surface of the n⁺ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.     

CuOx/a‐Si:H heterojunction thin‐film solar cell with an n‐type µc‐Si:H depletion‐assisting layer

We showed that thin n‐type CuO_x films can be deposited by radio‐frequency magnetron reactive sputtering and demonstrated the fabrication of n‐CuO_x/intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) heterojunction solar cells (HSCs) for the first time. A highly n‐doped hydrogenated microcrystalline Si (n‐µc‐Si:H) layer was introduced as a depletion‐assisting layer to further improve the performance of n‐CuO_x/i‐a‐Si:H HSCs. An analysis of the external quantum efficiency and energy‐band diagram showed that the thin depletion‐assisting layer helped establish sufficient depletion and increased the built‐in potential in the n‐CuO_x layer. The fabricated HSC exhibited a high open‐circuit voltage of 0.715 V and an efficiency of 4.79%. Copyright © 2015 John Wiley & Sons, Ltd.     

High‐Performance,Air‐Stable,Top‐Gate,p‐Channel WSe2 Field‐Effect Transistor with Fluoropolymer Buffer Layer

High‐performance, air‐stable, p‐channel WSe₂ top‐gate field‐effect transistors (FETs) using a bilayer gate dielectric composed of high‐ and low‐k dielectrics are reported. Using only a high‐k Al₂O₃ as the top‐gate dielectric generally degrades the electrical properties of p‐channel WSe₂, therefore, a thin fluoropolymer (Cytop) as a buffer layer to protect the 2D channel from high‐k oxide forming is deposited. As a result, a top‐gate‐patterned 2D WSe₂ FET is realized. The top‐gate p‐channel WSe₂ FET demonstrates a high hole mobility of 100 cm²_­ V^?1 s^?1 and a I_ON/I_OFF ratio > 10⁷ at low gate voltages (V_GS ca. ?4 V) and a drain voltage (V_DS) of ?1 V on a glass substrate. Furthermore, the top‐gate FET shows a very good stability in ambient air with a relative humidity of 45% for 7 days after device fabrication. Our approach of creating a high‐k oxide/low‐k organic bilayer dielectric is advantageous over single‐layer high‐k dielectrics for top‐gate p‐channel WSe₂ FETs, which will lead the way toward future electronic nanodevices and their integration.     

Quantitative Correlation of Perovskite Film Morphology to Light Emitting Diodes Efficiency Parameters

This paper reports quantitative correlation of CH₃NH₃PbBr₃ (MAPbBr₃) thin film morphology to light emitting diode efficiency parameters. Sequential (spin coating) deposition is used for highly reproducible and dense film morphology of MAPbBr₃ thin‐film. In this fabrication process using an orthogonal solvent approach, control of morphology, coverage, thickness, and optical properties in these compact thin‐films is demonstrated. Optical studies show direct correlation between morphology to dynamics of photoluminescence (PL) and absolute PL yield. Perovskite light emitting diodes (PeLEDs) are fabricated from these films to find the best ratio of PbBr₂ versus MABr for optimal performance. This study demonstrates PeLEDs with high brightness, ≈1050 cd m^?2 at 4.7 V (luminance efficiency ≈0.1 cd A^?1), for optimal thin‐film process with state‐of‐the‐art device performance. This quantitative analysis suggests that these state‐of‐the‐art PeLEDs suffer from poor charge carrier balance (≈2%) and out‐coupling efficiency (≈6%). Interestingly, charge carrier balance and PL yield together can explain the change in PeLED efficiency modulation with film morphology. Studies on single carrier devices show that these PeLEDs are electron current dominated and charge carrier balance increases with operating bias voltage.     

Reduced Efficiency Roll‐Off and Improved Stability of Mixed 2D/3D Perovskite Light Emitting Diodes by Balancing Charge Injection

Perovskite light emitting diodes (PeLEDs) have reached external quantum efficiencies (EQEs) over 21%. Their EQE, however, drops at increasing current densities (J) and their lifetime is still limited to just a few hours. The mechanisms leading to EQE roll‐off and device instability require thorough investigation. Here, improvement in EQE, EQE roll‐off, and lifetime of PeLEDs is demonstrated by tuning the balance of electron/hole transport into a mixed 2D/3D perovskite emissive layer. The mixed 2D/3D perovskite layer induces exciton confinement and beneficially influences the electron/hole distribution inside the perovskite layer. By tuning the electron injection to match the hole injection in such active layer, a nearly flat EQE for J = 0.1–200 mA cm^?2, a reduced EQE roll‐off until J = 250 mA cm^?2, and a half‐lifetime of ≈47 h at J = 10 mA cm^?2 is reached. A model is also proposed to explain these improvements that account for the spatial electron/hole distributions.     

High‐Performance CO2 Capture through Polymer‐Based Ultrathin Membranes

Thin film composite (TFC) membranes have attracted great research interest for a wide range of separation processes owing to their potential to achieve excellent permeance. However, it still remains challenging to fully exploit the superiority of thin selective layers when mitigating the pore intrusion phenomenon. Herein, a facile and generic interface‐decoration‐layer strategy collaborating with molecular‐scale organic–inorganic hybridization in the selective layer to obtain a high‐performance ultrathin film composite (UTFC) membrane for CO₂ capture is reported. The interface‐decoration layer of copper hydroxide nanofibers (CHNs) enables the formation of an ultrathin selective layer (≈100 nm), achieving a 2.5‐fold increase in gas permeance. The organic part in the molecular‐scale hybrid material contributes to facilitating CO₂‐selective adsorption while the inorganic part assists in maintaining robust membrane structure, thus remarkably improving the selectivity toward CO₂. As a result, the as‐prepared membrane shows a high CO₂ permeance of 2860 GPU, superior to state‐of‐the‐art polymer membranes, with a CO₂/N₂ selectivity of 28.2. The synergistic strategy proposed here can be extended to a wide range of polymers, holding great potential to produce high‐efficiency ultrathin membranes for molecular separation.     

Highly Efficient p‐i‐n and Tandem Organic Light‐Emitting Devices Using an Air‐Stable and Low‐Temperature‐Evaporable Metal Azide as an n‐Dopant

Cesium azide (CsN₃) is employed as a novel n‐dopant because of its air stability and low deposition temperature. CsN₃ is easily co‐deposited with the electron transporting materials in an organic molecular beam deposition chamber so that it works well as an n‐dopant in the electron transport layer because its evaporation temperature is similar to that of common organic materials. The driving voltage of the p‐i‐n device with the CsN₃‐doped n‐type layer and a MoO₃‐doped p‐type layer is greatly reduced, and this device exhibits a very high power efficiency (57 lm W^?1). Additionally, an n‐doping mechanism study reveals that CsN₃ was decomposed into Cs and N₂ during the evaporation. The charge injection mechanism was investigated using transient electroluminescence and capacitance–voltage measurements. A very highly efficient tandem organic light‐emitting diodes (OLED; 84 cd A^?1) is also created using an n–p junction that is composed of the CsN₃‐doped n‐type organic layer/MoO₃ p‐type inorganic layer as the interconnecting unit. This work demonstrates that an air‐stable and low‐temperature‐evaporable inorganic n‐dopant can very effectively enhance the device performance in p‐i‐n and tandem OLEDs, as well as simplify the material handling for the vacuum deposition process.     

Compositional and Dimensional Control of 2D and Quasi‐2D Lead Halide Perovskites in Water

Blue light emitting two dimensional (2D) and quasi‐2D layered halide perovskites (LHPs) are gaining attention in solid‐state lighting applications but their fragile stability in humid condition is one of the most pressing issues for their practical applications. Though water is much greener and cost effective, organic solvents must be used during synthesis as well as the device fabrication process for these LHPs due to their water‐sensitivity/instability and consequently, water‐stable blue‐light emitting 2D and quasi‐2D LHPs have not been documented yet. Here, water‐mediated facile and cost‐effective syntheses, characterizations, and optical properties of 16 organic–inorganic hybrid compounds are reported including 2D (A′)₂PbX₄ (A′ = butylammonium, X = Cl/Br/I) (8 compounds), 3D perovskites (4), and quasi‐2D (A′)_pA_x?1B_xX_3x+1 LHPs (A = methylammonium) (4) in water. Here, both composition and dimension of LHPs are tuned in water, which has never been explored yet. Furthermore, the dual emissive nature is observed in quasi‐2D perovskites, where the intensity of two photoluminescence (PL) peaks are governed by 2D and 3D inorganic layers. The Pb(OH)₂‐coated 2D and quasi‐2D perovskites are highly stable in water even after several months. In addition, single particle imaging is performed to correlate structural–optical property of these LHPs.     

Improved Performance and Stability of All‐Inorganic Perovskite Light‐Emitting Diodes by Antisolvent Vapor Treatment

All‐inorganic perovskite light‐emitting diodes (LEDs) reveal efficient luminescence with high color purity, but their modest brightness and poor stability are still critical drawbacks. Here, the luminescent efficiency and the stability of perovskite LEDs (PeLEDs) are boosted by antisolvent vapor treatment of CsPbBr₃ embedded in a dielectric polymer matrix of polyethylene oxide (PEO). A unique method is developed to obtain high quality CsPbBr₃ emitting layers with low defects by controlling their grain sizes. CsPbBr₃ in PEO matrix is post‐treated with antisolvent of chloroform (CF), leading to microcrystals with a size of ≈5 µm along the in‐plane direction with active emitting composite of 90%. A device based on CF post‐treatment (CsPbBr₃‐PEO‐CF) film displays a brightness of up to 51890 cd m^?2 with an external quantum efficiency of 4.76%. CsPbBr₃‐PEO‐CF PeLED still maintains 82% of its initial efficiency after 80 h continuous operation in ambient air, which indicates relatively good device stability. This work highlights that film quality is not only key to promoting fluorescence in CsPbBr₃, but also to achieving higher performance PeLEDs.     

Thermally Stable Two‐Dimensional Hexagonal Mesoporous Nanocrystalline Anatase,Meso‐nc‐TiO2: Bulk and Crack‐Free Thin Film Morphologies

Herein a novel synthetic route is described for the production of thermally stable, structurally well‐defined two‐dimensional (2D) hexagonal mesoporous nanocrystalline anatase (meso‐nc‐TiO₂), with a large pore diameter, narrow pore‐size distribution, high surface area, and robust inorganic walls comprised of nanocrystalline anatase. The synthetic approach involves the evaporation‐induced co‐assembly of a non‐ionic amphiphilic triblock‐copolymer template and titanium tetraethoxide, but with a pivotal change in the main solvent of the system, where the commonly used ethanol is replaced with 1‐butanol. This seemingly minor modification in solvent type from ethanol to 1‐butanol turns out to be the key synthetic strategy for achieving a robust, structurally well‐ordered meso‐nc‐TiO₂ material in the form of either thick or thin films. The beneficial “solvent” effect originates from the higher hydrophobicity of 1‐butanol than ethanol, enhancing microphase separation and templating, lower critical micelle concentration of the template in 1‐butanol, and the ability to increase the relative concentration of the inorganic precursor to template in the co‐assembly synthesis. Moreover, thin films with dimensions of several centimeters that are devoid of cracks down to the length scale of the mesostructure itself, having high porosity, well‐defined mesostructural features, and semi‐crystalline pore walls were straightforwardly and reproducibly obtained as a result of the physicochemical property advantages of 1‐butanol over ethanol within our synthesis scheme.     

Low‐Dimensional Lead‐Free Inorganic Perovskites for Resistive Switching with Ultralow Bias

3D organic–inorganic and all‐inorganic lead halide perovskites have been intensively pursued for resistive switching memories in recent years. Unfortunately, instability and lead toxicity are two foremost challenges for their large‐scale commercial applications. Dimensional reduction and composition engineering are effective means to overcome these challenges. Herein, low‐dimensional inorganic lead‐free Cs₃Bi₂I₉ and CsBi₃I₁₀ perovskite‐like films are exploited for resistive switching memory applications. Both devices demonstrate stable switching with ultrahigh on/off ratios (≈10⁶), ultralow operation voltages (as low as 0.12 V), and self‐compliance characteristics. 0D Cs₃Bi₂I₉‐based device shows better retention time and larger reset voltage than the 2D CsBi₃I₁₀‐based device. Multilevel resistive switching behavior is also observed by modulating the current compliance, contributing to the device tunability. The resistive switching mechanism is hinged on the formation and rupture of conductive filaments of halide vacancies in the perovskite films, which is correlated with the formation of AgI_x layers at the electrode/perovskite interface. This study enriches the library of switching materials with all‐inorganic lead‐free halide perovskites and offers new insights on tuning the operation of solution‐processed memory devices.     

Donor–Acceptor Poly(3‐hexylthiophene)‐block‐Pendent Poly(isoindigo) with Dual Roles of Charge Transporting and Storage Layer for High‐Performance Transistor‐Type Memory Applications

Field‐effect transistor memories usually require one additional charge storage layer between the gate contact and organic semiconductor channel. To avoid such complication, new donor–acceptor rod–coil diblock copolymers (P3HT₄₄‐b‐Piso_n) of poly(3‐hexylthiophene) (P3HT)‐block‐poly(pendent isoindigo) (Piso) are designed, which exhibit high performance transistor memory characteristics without additional charge storage layer. The P3HT and Piso blocks are acted as the charge transporting and storage elements, respectively. The prepared P3HT₄₄‐b‐Piso_n can be self‐assembled into fibrillar‐like nanostructures after the thermal annealing process, confirmed by atomic force microscopy and grazing‐incidence X‐ray diffraction. The lowest‐unoccupied molecular orbital levels of the studied polymers are significantly lowered as the block length of Piso increases, leading to a stronger electron affinity as well as charge storage capability. The field‐effect transistors (FETs) fabricated from P3HT₄₄‐b‐Piso_n possess p‐type mobilities up to 4.56 × 10^?2 cm² V^?1 s^?1, similar to that of the regioregular P3HT. More interestingly, the FET memory devices fabricated from P3HT₄₄‐b‐Piso_n exhibit a memory window ranging from 26 to 79 V by manipulating the block length of Piso, and showed stable long‐term data endurance. The results suggest that the FET characteristics and data storage capability can be effectively tuned simultaneously through donor/acceptor ratio and thin film morphology in the block copolymer system.     

Fine Control of Perovskite Crystallization and Reducing Luminescence Quenching Using Self‐Doped Polyaniline Hole Injection Layer for Efficient Perovskite Light‐Emitting Diodes

Organic–inorganic hybrid perovskites (OHPs) are promising emitters for light‐emitting diodes (LEDs) due to the high color purity, low cost, and simple synthesis. However, the electroluminescent efficiency of polycrystalline OHP LEDs (PeLEDs) is often limited by poor surface morphology, small exciton binding energy, and long exciton diffusion length of large‐grain OHP films caused by uncontrolled crystallization. Here, crystallization of methylammonium lead bromide (MAPbBr₃) is finely controlled by using a polar solvent‐soluble self‐doped conducting polymer, poly(styrenesulfonate)‐grafted polyaniline (PSS‐g‐PANI), as a hole injection layer (HIL) to induce granular structure, which makes charge carriers spatially confined more effectively than columnar structure induced by the conventional poly(3,4‐ethylenedioythiphene):polystyrenesulfonate (PEDOT:PSS). Moreover, lower acidity of PSS‐g‐PANI than PEDOT:PSS reduces indium tin oxide (ITO) etching, which releases metallic In species that cause exciton quenching. Finally, doubled device efficiency of 14.3 cd A^‐1 is achieved for PSS‐g‐PANI‐based polycrystalline MAPbBr₃ PeLEDs compared to that for PEDOT:PSS‐based PeLEDs (7.07 cd A^‐1). Furthermore, PSS‐g‐PANI demonstrates high efficiency of 37.6 cd A^‐1 in formamidinium lead bromide nanoparticle LEDs. The results provide an avenue to both control the crystallization kinetics and reduce the migration of In released from ITO by forming OIP films favorable for more radiative luminescence using the polar solvent‐soluble and low‐acidity polymeric HIL.     

“Layer‐Filter Threshold” Technique for Near‐Infrared Laser Ablation in Organic Semiconductor Device Processing

Although conventional laser ablation (CLA) method has widely been used in patterning of organic semiconductor thin films, its quality control still remains unsatisfied due to the ambiguous photochemical and photothermal processes. Based on industrial available near‐infrared laser source, herein, a novel “layer‐filter threshold” (LFT) technique is proposed, which involves the decomposition of targeted “layer‐filter” and subsequent explosive evaporation process to purge away the upper layers instead of layer‐by‐layer ablation. For photovoltaic device with structure of metal/blend/PEDOT:PSS/ITO/glass, the PEDOT:PSS layer as the “layer‐filter” is first demonstrated to be effective, and then the merged P1–P2 line and metal electrode layer are readily patterned through the “self‐aligned” effect and regulation of ablation direction, respectively. The correlation between laser fluence and explosive ablation efficacy is also investigated. Finally, photovoltaic modules based on classical P3HT:PC₆₁BM and low‐bandgap PBDT‐TFQ:PC₇₁BM systems are separately fabricated following the LFT technique. It is found that over 90% of geometric fill factor is achieved while device performances maintain in a limited change with increased number of series cells. In comparison to conventional laser ablation methods, the LFT technique does not require sophisticated instruments but reaches comparable processing accuracy, which shows promising potential in the fabrication and commercialization of organic semiconductor thin‐film devices.     

Efficient Conjugated‐Polymer Optoelectronic Devices Fabricated by Thin‐Film Transfer‐Printing Technique

The fabrication of functional multilayered conjugated‐polymer structures with well‐defined organic‐organic interfaces for optoelectronic‐device applications is constrained by the common solubility of many polymers in most organic solvents. Here, we report a simple, low‐cost, large‐area transfer‐printing technique for the deposition and patterning of conjugated‐polymer thin films. This method utilises a planar poly(dimethylsiloxane) (PDMS) stamp, along with a water‐soluble sacrificial layer, to pick up an organic thin film (～20 nm to 1 µm) from a substrate and subsequently deliver this film to a target substrate. We demonstrate the versatility of this transfer‐printing technique and its applicability to optoelectronic devices by fabricating bilayer structures of poly(9,9‐di‐n‐octylfluorene‐alt‐(1,4‐phenylene‐((4‐sec‐butylphenyl)imino)‐1,4‐phenylene))/poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (TFB/F8BT) and poly(3‐hexylthiophene)/methanofullerene([6,6]‐phenyl C₆₁ butyric acid methyl ester) (P3HT/PCBM), and incorporating them into light‐emitting diodes (LEDs) and photovoltaic (PV) cells, respectively. For both types of device, bilayer devices fabricated with this transfer‐printing technique show equal, if not superior, performance to either blend devices or bilayer devices fabricated by other techniques. This indicates well‐controlled organic‐organic interfaces achieved by the transfer‐printing technique. Furthermore, this transfer‐printing technique allows us to study the nature of the excited states and the transport of charge carriers across well‐defined organic interfaces, which are of great importance to organic electronics.