Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir–Shäfer Deposition

The semiconductor–electrode interface impacts the function and the performance of (opto)electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution‐processed organic ultrathin films on electrodes typically form islands due to dewetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer–electrode interface impossible. Also, this has hampered the development of devices including ultrathin conjugated polymer layers. Here, Langmuir–Shäfer‐manufactured homogenous mono‐ and multilayers of semiconducting polymers on metal electrodes are reported and the energy level bending using photoelectron spectroscopy is tracked. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. These findings provide new insights of the energetics of the polymer–electrode interface and opens up for new high‐performing devices based on ultrathin semiconducting polymers.     

Systematic Control of Nucleation Density in Poly(3‐Hexylthiophene) Thin Films

While molecular ordering via crystallization is responsible for many of the impressive optoelectronic properties of thin‐film semiconducting polymer devices, crystalline morphology and its crucial influence on performance remains poorly controlled and is usually studied as a passive result of the conditions imposed by film deposition parameters. A method for systematic control over crystalline morphology in conjugated polymer thin films by very precise control of nucleation density and crystal growth conditions is presented. A precast poly(3‐hexylthiophene) film is first swollen into a solution‐like state in well‐defined vapor pressures of a good solvent, while the physical state of the polymer chains is monitored using in situ UV–vis spectroscopy and ellipsometry. Nucleation density is selected by a controlled deswelling of the film or by a self‐seeding approach using undissolved crystalline aggregates that remain in the swollen film. Nucleation densities ranging successively over many orders of magnitude are achieved, extending into the regime of spherulitic domains 10 to 100 μm in diameter, a length scale highly relevant for typical probes of macroscopic charge transport such as field‐effect transistors. This method is presented as a tool for future systematic study of the structure‐function relation in semicrystalline semiconducting polymers in a broad range of applications.     

Cover Picture: High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing (Adv. Funct. Mater. 15/2007)

A new method for direct patterning of organic optoelectronic/electronic devices using a reconfigurable and scalable printing method is reported by Vladimir Bulovic and co‐workers on p. 2722. The printing technique is applied to the fabrication of high‐resolution printed organic light emitting devices (OLEDs) and organic field effect transistors (OFETs). Remarkably, the final print‐deposited films are evaporated onto the substrate (rather than solvent printed), giving high‐quality, solvent‐free, molecularly flat structures that match the performance of comparable high‐performance unpatterned films. We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Improving Charge Transport via Intermediate‐Controlled Crystal Growth in 2D Perovskite Solar Cells

Reduced‐dimensional hybrid perovskite semiconductors have recently attracted significant attention due to their promising stability and optoelectronic properties. However, the issue of poor charge transport in 2D perovskites limits its application. Here, studies on intermediate‐controlled crystal growth are reported to improve charge carrier transport in 2D perovskite thin films. It is shown that the coordination strength of solvents with perovskite precursor affects the initial state of intermediate phase formation as well as the subsequent perovskite layer growth. Tuning the solvent composition with a mixture (5:5) of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) leads to the growth of highly orientated 2D perovskite films with much‐improved optoelectronic properties (faster transport by ≈50x, longer carrier lifetime by ≈4x, and lower defect density by ≈30x) than the film prepared with pure DMF. Consequently, perovskite solar cells based on DMF/DMSO (5:5) show >80% efficiency improvement than the devices based on pure DMF.     

Solution‐Processed Laminated Perovskite Layers for High‐Performance Solar Cells

Laminated multilayers of perovskite films with different optical and electronic characteristics will easily realize high‐performance optoelectronic devices because it is widely demonstrated that differential distribution of film properties in the vertical direction of devices plays particularly important roles in device performance. However, the existing laminated perovskite films are hardly prepared by a solution process because there is no solvent with sufficient selectivity of solubility for different perovskite materials. Here, it is demonstrated that aniline (AN) has a largely different solubility toward the perovskite MAPbI₃ and the MAPbI₃ blend with an additive of hydrochloride diethylammonium chloride. By using AN as the solvent in the perovskite precursor solution, two laminated perovskite layers with different crystal size and optical and electrical characteristics are achieved. Inverted perovskite solar cells with the laminated films as active layers achieve an averaged power conversion efficiency of 20.65% originating from the high V_OC 1.112 V and fill factor of 80.8%. The devices maintain 98% efficiency after 400 h under 65% RH. This work provides a very simple and feasible method for production of laminated perovskite films to achieve high‐performance perovskite solar cells.     

Copper (I) Selenocyanate (CuSeCN) as a Novel Hole‐Transport Layer for Transistors,Organic Solar Cells,and Light‐Emitting Diodes

The synthesis and characterization of copper (I) selenocyanate (CuSeCN) and its application as a solution‐processable hole‐transport layer (HTL) material in transistors, organic light‐emitting diodes, and solar cells are reported. Density‐functional theory calculations combined with X‐ray photoelectron spectroscopy are used to elucidate the electronic band structure, density of states, and microstructure of CuSeCN. Solution‐processed layers are found to be nanocrystalline and optically transparent (>94%), due to the large bandgap of ≥3.1 eV, with a valence band maximum located at ?5.1 eV. Hole‐transport analysis performed using field‐effect measurements confirms the p‐type character of CuSeCN yielding a hole mobility of 0.002 cm² V^?1 s^?1. When CuSeCN is incorporated as the HTL material in organic light‐emitting diodes and organic solar cells, the resulting devices exhibit comparable or improved performance to control devices based on commercially available poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate as the HTL. This is the first report on the semiconducting character of CuSeCN and it highlights the tremendous potential for further developments in the area of metal pseudohalides.     

Composition Waves in Solution-Processed Organic Films and Its Propagations from Kinetically Frozen Surface Mesophases

Organic thin films deposited from solution attract wide interest for next-generation (opto-)electronic and energy applications. During solvent evaporation, the phase evolution dynamics for different components at different locations are not synchronic within the incrementally concentrated liquid films, determining the final anisotropic morphology and performance. Herein, by examining tens of widely investigated optoelectronic organic films, the general existence of composition wave propagating along the surface-normal direction upon solidification is identified. The composition wave is initiated by a few nanometers thick surface mesophase kinetically forming at the foremost stage of phase transition, and afterward propagates toward the substrate during solvent evaporation. The composition waves exhibit well-defined wave properties, including spatial wavelength, period, amplitude, and propagation velocity. These wave properties are closely correlated with the evaporation rate and the diffusion rate of organic molecules, which determines the dynamically varied local composition gradient along the surface-normal direction. Such composition waves are commonly found for more than 80% of randomly examined solution-processed thin films for high-performance organic electronic devices including photovoltaic cells and field-effect transistors.     

Hybrid Silver Nanowire and Graphene‐Based Solution‐Processed Transparent Electrode for Organic Optoelectronics

The research on transparent conductive electrodes is in rapid ascent in order to respond to the requests of novel optoelectronic devices. The synergic coupling of silver nanowires (AgNWs) and high‐quality solution‐processable exfoliated graphene (EG) enables an efficient transparent conductor with low‐surface roughness of 4.6 nm, low sheet resistance of 13.7 Ω sq^?1 at high transmittance, and superior mechanical and chemical stabilities. The developed AgNWs–EG films are versatile for a wide variety of optoelectronics. As an example, when used as a bottom electrode in organic solar cell and polymer light‐emitting diode, the devices exhibit a power conversion efficiency of 6.6% and an external quantum efficiency of 4.4%, respectively, comparable to their commercial indium tin oxide counterparts.     

Fabrication and Characterization of Organic Single Crystal‐Based Light‐Emitting Devices with Improved Contact Between the Metallic Electrodes and Crystal

Organic single crystals have attracted great attention because of their advantages of high charge‐carrier mobility, high chemical purity, and potential for flexible optoelectronic devices. However, their intrinsic properties of sensitive to organic solvent and fragile result in a difficulty in the fabrication of the organic crystal‐based devices. In this work, a simple and non‐destructive technique of template stripping is employed to fabricate single‐crystal‐based organic light‐emitting devices (OLEDs). Efficient and uniform carrier injection induced by an improved contact between crystals and both top and bottom electrodes is realized, so that a homogeneous and bright electroluminescence (EL) are obtained. Highly polarized EL and even white emission is also observed. Moreover, the crystal‐based OLEDs exhibit good flexibility, and keep stable EL under a small bending radius and after repeated bending. It is expectable that this technique would support broad applications of the organic single crystals in the crystal‐based optoelectronic devices.     

Bulk Heterojunction Organic Photovoltaics Based on Carboxylated Polythiophenes and PCBM on Glass and Plastic Substrates

The optoelectronic and photophysical properties of four regioregular poly[3‐(carboxyalkyl)thiophene‐2,5‐diyl] (P3CAT) with different carboxyalkyl chain lengths (propyl to hexyl) are reported. Each P3CAT is combined with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester to form the photoactive bulk heterojunction layer for organic photovoltaic devices. The extent of hydrogen bonding and polymer crystallinity in the films has been determined through infrared spectroscopy and X‐ray diffraction. The mechanical properties of films are analyzed with nanoindentation, and the measurements suggest that P3CATs are suitable for use in flexible devices. Power conversion efficiencies of up to 2.6% and 1.6% are obtained for devices fabricated in air, and supported on glass and flexible poly(ethylene terephthalate) substrates, respectively.     

Polarization‐Sensitive Single‐Wall Carbon Nanotubes All‐in‐One Photodetecting and Emitting Device Working at 1.55 µm

Functional and easy‐to‐integrate nanodevices operating in the telecom wavelength ranges are highly desirable. Indeed, the pursuit for faster, cheaper, and smaller transceivers for datacom applications is fueling the interest in alternative materials to develop the next generation of photonic devices. In this context, single wall carbon nanotubes (SWNTs) have demonstrated outstanding electrical and optical properties that make them an ideal material for the realization of ultracompact optoelectronic devices. Still, the mixture in chirality of as‐synthesized SWNTs and the necessity of precise positioning of SWNT‐based devices hinder the development of practical devices. Here, the realization of operational devices obtained using liquid solution‐based techniques is reported, which allow high‐purity sorting and localized deposition of aligned semiconducting SWNTs (s‐SWNTs). More specifically, devices are demonstrated by combining a polymer assisted extraction method, which enables a very effective selection of s‐SWNTs with a diameter of about 1–1.2 nm, with dielectrophoresis, which localizes the deposition onto silicon wafers in aligned arrays in‐between prepatterned electrodes. Thus, long semiconducting nanotubes directly contact the electrodes and, when asymmetric contacts (i.e., source and drain made of different metals) are used, each device can operate both as photoemitter and as photodetector in the telecom band around 1.55 µm in air at room temperature.     

Additive‐Free Bulk‐Heterojuction Solar Cells with Enhanced Power Conversion Efficiency,Comprising a Newly Designed Selenophene‐Thienopyrrolodione Copolymer

Bulk‐heterojunction solar cells are reported with an enhanced power conversion efficiency (PCE) based on a newly designed semiconducting selenophene‐thienopyrrolodione (TPD) copolymer blended with [6,6]‐phenyl C₇₁ butyric acid methyl‐ester. The solar cells are fabricated using simple solution processing (implying low‐cost fabrication). The relatively deep highest occupied molecular orbital (HOMO) level leads to a correspondingly high open‐circuit voltage of 0.88 V. The PCE approaches 5.8% when Clevious P VP AI4083 is used as the hole‐transport interlayer, with an optimized active layer thickness of approximately 95 nm, and a donor‐acceptor blend ratio of 1:1. A fill factor (FF) of 0.62 is achieved. The use of additives does not seem to be beneficial in this blended system, due to the achievement of proper phase separation in the as‐cast films. Also, the BHJ devices with a 3% ratio of a 1‐chloronaphthalene (CN) additive exhibit much more severe oxidative degradation from the decreased FF with a high series resistance than BHJ devices without additive. The selenophene‐TPD based BHJ solar cell is a promising candidate for high‐performance single cells with a low‐cost additive‐free fabrication and a long‐term stable operation.     

Reduced Graphene Oxide Micromesh Electrodes for Large Area,Flexible, Organic Photovoltaic Devices

A laser‐based patterning technique—compatible with flexible, temperature‐sensitive substrates—for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes is presented. The mesh patterning can be accurately controlled in order to significantly enhance the electrode transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers. In particular, rGO films with an initial transparency of ≈20% are patterned, resulting in rGOMMs films with a ≈59% transmittance and a sheet resistance of ≈565 Ω sq^?1, that is significantly lower than the resistance of ≈780 Ω sq^?1, exhibited by the pristine rGO films at the same transparency. As a proof‐of‐concept application, rGOMMs are used as the transparent electrodes in flexible organic photovoltaic (OPV) devices, achieving power conversion efficiency of 3.05%, the highest ever reported for flexible OPV devices incorporating solution‐processed graphene‐based electrodes. The controllable and highly reproducible laser‐induced patterning of rGO hold enormous promise for both rigid and flexible large‐scale organic electronic devices, eliminating the lag between graphene‐based and indium–tin oxide electrodes, while providing conductivity and transparency tunability for next generation flexible electronics.     

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

Here, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 10²² m^?3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.     

Solution‐Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells

The detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA)₂(OAc)]⁺ ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni³⁺ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.     

A General Method: Designing a Hypocrystalline Hydroxide Intermediate to Achieve Ultrasmall and Well‐Dispersed Ternary Metal Oxide for Efficient Photovoltaic Devices

Solution‐process fine metal‐oxide nanoparticles are promising carrier transport layer candidates for unlocking the full potential of solution process in solar cells, due to their low cost, good stability, and favorable electrical/optical properties. However, exotic organic ligands adopted for achieving small size and monodispersion can mostly cause poor conductivity, which thus impedes their electrical application. In this work, a concept of constructing a hypocrystalline intermediate is proposed to develop a general method for synthesizing various ternary metal oxide (TMO) nanoparticles with a sub‐ten‐nanometer size and good dispersibility without exotic ligands. Particularly, a guideline is summarized based on the understandings about the impact of metal ion intercalation as well as water and anion coordination on the hypocrystalline intermediate. A general method based on the proposed concept is developed to successfully synthesize various sub‐ten‐nanometer TMO nanoparticles with excellent ability for forming high‐quality (smooth and well‐coverage) films. As an application example, the high‐quality films are used as hole transport layers for achieving high‐performance (stability and efficiency) organic/perovskite solar cells. Consequently, this work will contribute to the development of TMO for large‐scale and high‐performance optoelectronic devices and the concept of tailoring intermediate can leverage the fundamental understandings of synthesis strategies for other metal oxides.     

Fast,Self‐Driven,Air‐Stable,and Broadband Photodetector Based on Vertically Aligned PtSe2/GaAs Heterojunction

Group‐10 layered transitional metal dichalcogenides including PtS₂, PtSe₂, and PtTe₂ are excellent potential candidates for optoelectronic devices due to their unique properties such as high carrier mobility, tunable bandgap, stability, and flexibility. Large‐area platinum diselenide (PtSe₂) with semiconducting characteristics is far scarcely investigated. Here, the development of a high‐performance photodetector based on vertically aligned PtSe₂‐GaAs heterojunction which exhibits a broadband sensitivity from deep ultraviolet to near‐infrared light, with peak sensitivity from 650 to 810 nm, is reported. The I_light/I_dark ratio and responsivity of photodetector are 3 × 10⁴ and 262 mA W^?1 measured at 808 nm under zero bias voltage. The response speed of τ_r/τ_f is 5.5/6.5 µs, which represents the best result achieved for Group‐10 TMDs based optoelectronic device thus far. According to first‐principle density functional theory, the broad photoresponse ranging from visible to near‐infrared region is associated with the semiconducting characteristics of PtSe₂ which has interstitial Se atoms within the PtSe₂ layers. It is also revealed that the PtSe₂/GaAs photodetector does not exhibit performance degradation after six weeks in air. The generality of the above good results suggests that the vertically aligned PtSe₂ is an ideal material for high‐performance optoelectronic systems in the future.     

Self‐Assembly of a Donor‐Acceptor Dyad Across Multiple Length Scales: Functional Architectures for Organic Electronics

Molecular dyads based on polycyclic electron donor (D) and electron acceptor (A) units represent suitable building blocks for forming highly ordered, solution‐processable, nanosegregated D‐A domains for potential use in (opto)electronic applications. A new dyad, based on alkyl substituted hexa‐peri‐hexabenzocoronene (HBC) and perylene monoimide (PMI) separated by an ethinylene linker, is shown to have a high tendency to self‐assemble into ordered supramolecular arrangements at multiple length scales: macroscopic extruded filaments display long‐range crystalline order, nanofiber networks are produced by simple spin‐coating, and monolayers with a lamellar packing are formed by physisorption at the solution‐HOPG interface. Moreover, highly uniform mesoscopic ribbons bearing atomically flat facets and steps with single‐molecule heights self‐assemble upon solvent‐vapor annealing. Electrical measurements of HBC‐PMI films and mesoscopic ribbons in a transistor configuration exhibit ambipolar transport with well balanced p‐ and n‐type mobilities. Owing to the increased level of order at the supramolecular level, devices based on ribbons show mobility increases of more than one order of magnitude.     

A General Solvent Selection Strategy for Solution Processed Quantum Dots Targeting High Performance Light‐Emitting Diode

An all‐solution‐processed quantum dots (QDs) light emitting diode (QLED) consists of different layers deposited from various orthogonal solvents. Here, the authors develop a general solvent selection strategy to obtain orthogonal solubility properties as well as high film quality. It is found that a “poor” QDs film morphology with striation defects often occurs when the QDs film is deposited from “bad” solvent. A physical model is presented to rationalize the observed striation defects, and then a general solvent selection strategy is proposed to prevent both surface striation defects and the dissolving of the underlying layers by carefully choosing the “good” solvent for QDs. A compact QDs film can be fabricated without altering the original morphology of underlying functional layers in a QLED device, leading to significant device performance improvement. An external quantum efficiency of 15.45% is achieved in a green QLED with uniform emitting region. This solvent selection strategy provides a general way to deposit high quality films for most of the solution‐processed multilayer optoelectronic devices.     

Ortho‐ vs Bay‐Functionalization: A Comparative Study on Tetracyano‐Terrylenediimides

In this paper n‐type semiconductors synthesized via selective fourfold cyanation of the ortho‐ and bay‐positions (2,5,10,13‐ and 1,6,9,14‐positions respectively) of teyrrylenediimides are reported. A detailed study about the impact of the diverse functionalization topologies on the optoelectronic properties, self‐organization from solution, solid‐state packing, and charge carrier transport in field‐effect transistors is presented. The ortho‐substitution preserves the planarity of the core and favors high order in solution processed films. However, the strong intermolecular interactions lead to a microstructure with large aggregates and pronounced grain boundaries which lower the charge carrier transport in transistors. In contrast, the well‐soluble bay‐functionalized terrylenediimide forms only disordered films which surprisingly result in n‐type average mobilities of 0.17 cm²/Vs after drop‐casting with similar values in air. Processing by solvent vapor diffusion enhances the transport to 0.65 cm²/Vs by slight improvement of the order and surface arrangement of the molecules. This mobility is comparable to highest n‐type conductivities measured for solution processed PDI derivatives demonstrating the high potential of TDI‐based semiconductors.