Novel Film‐Casting Method for High‐Performance Flexible Polymer Electrodes

A new film‐casting method for polymer electrodes is reported, in which thickness‐controlled drop‐casting (TCDC), using polyaniline doped with camphorsulfonic acid (PANI:CSA) is used. By combining the advantages of conventional spin‐casting and drop‐casting methods, and by rigorously controlling the film formation parameters, flexible polymer electrodes with high conductivity and excellent transmittance can be produced. The PANI:CSA electrodes cast by the TCDC method exhibited constant thickness‐independent conductivities of ～600 S cm^?1 down to a film thickness of 0.2 μm, and a high optical transmittance of about 85% at 550 nm. Furthermore, the new casting method significantly reduced the sheet resistance (～90 Ω/square) of the PANI:CSA electrodes compared with the conventional spin‐cast films, enhancing the performance of the devices deposited on plastic substrates. The flexible polymer light‐emitting diode produced a brightness of 6000 cd m^?2, and the flexible polymer solar cell exhibited a power conversion efficiency of 2%, both of which were much higher than those of the devices fabricated by the conventional spin‐casting method.     

Nanoscale Morphology of Conjugated Polymer/Fullerene‐Based Bulk‐ Heterojunction Solar Cells

The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk‐heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid‐state blends of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)]‐1,4‐phenylenevinylene (MDMO‐PPV) and the soluble fullerene C₆₀ derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl [6,6]C₆₁ (PCBM), spin‐cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar‐cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin‐film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase‐separated blends cast from toluene. For toluene‐cast films the scale of phase separation depends strongly on the ratio of MDMO‐PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO‐PPV, in blend films cast from both toluene and chlorobenzene.     

Solvent‐Induced Morphology in Polymer‐Based Systems for Organic Photovoltaics

Studies on the influence of four different solvents on the morphology and photovoltaic performance of bulk‐heterojunction films made of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) via spin‐coating for photovoltaic applications are reported. Solvent‐dependent PCBM cluster formation and P3HT crystallization during thermal annealing are investigated with optical microscopy and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and are found to be insufficient to explain the differences in device performance. A combination of atomic force microscopy (AFM), X‐ray reflectivity (XRR), and grazing‐incidence small‐angle X‐ray scattering (GISAXS) investigations results in detailed knowledge of the inner film morphology of P3HT:PCBM films. Vertical and lateral phase separation occurs during spin‐coating and annealing, depending on the solvent used. The findings are summarized in schematics and compared with the IV characteristics. The main influence on the photovoltaic performance arises from the vertical material composition and the existence of lateral phase separation fitting to the exciton diffusion length. Absorption and photoluminescence measurements complement the structural analysis.     

Effect of Mesoscale Crystalline Structure on the Field‐Effect Mobility of Regioregular Poly(3‐hexyl thiophene) in Thin‐Film Transistors

Regioregular poly(3‐hexyl thiophene) (RR P3HT) is drop‐cast to fabricate field‐effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well‐dispersed nanofibrils to well‐developed spherulites by changing solution processing conditions. The morphological correlation with the charge‐carrier mobility in RR P3HT thin‐film transistor (TFT) devices is investigated. The TFT devices show charge‐carrier mobilities in the range of 10^–4 ～ 10^–2 cm² V^–1 s^–1 depending on the solvent used, although grazing‐incidence X‐ray diffraction (GIXD) reveals that all films develop the same π–π‐stacking orientation, where the <100>‐axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the <100>‐axis orientation distribution of the π–π‐stacking plane with respect to the film normal play important roles on the charge‐carrier mobility of RR P3HT for TFT applications.     

Self‐Assembled Conjugated Thiophene‐Based Rotaxane Architectures: Structural,Computational, and Spectroscopic Insights into Molecular Aggregation

A comparative study of the self‐assembly at a variety of surfaces of a dithiophene rotaxane 1 ?β‐CD and its corresponding dumbbell, 1, by means of atomic force microscopy (AFM) imaging and scanning tunneling microscopy (STM) imaging on the micrometer and nanometer scale, respectively. The dumbbell is found to have a greater propensity to form ordered supramolecular assemblies, as a result of π–π interactions between dithiophenes belonging to adjacent molecules, which are hindered in the rotaxane. The fine molecular structure determined by STM was compared to that obtained by molecular modelling. The optical properties of both rotaxane and dumbbell in the solid state were investigated by steady‐state and time‐resolved photoluminescence (PL) experiments on spin‐cast films and diluted solutions. The comparison between the optical features of the threaded and unthreaded systems reveals an effective role of encapsulation in reducing aggregation and exciton migration for the rotaxanes with respect to the dumbbells, thus leading to higher PL quantum efficiency and preserved single‐molecule photophysics for longer times after excitation in the threaded oligomers.     

The Influence of the Solvent Evaporation Rate on the Phase Separation and Electrical Performances of Soluble Acene‐Polymer Blend Semiconductors

The phase‐separation characteristics of spin‐cast difluorinated‐triethylsilylethynyl anthradithiophene (F‐TESADT)/poly(methyl methacrylate) (PMMA) blends are investigated with the aim of fabricating transistors with a high field‐effect mobility and stability. It is found that the presence of PMMA in the F‐TESADT/PMMA blends prevents dewetting of F‐TESADT from the substrate and provides a platform for F‐TESADT molecules to segregate and crystallize at the air–film interface. By controlling the solvent evaporation rate of the spin‐cast blend solution, it is possible to regulate the phase separation of the two components, which in turn determines the structural development of the F‐TESADT crystals on PMMA. At a low solvent evaporation rate, a bilayer structure consisting of highly ordered F‐TESAT crystals on the top and low‐trap PMMA dielectric on the bottom can be fabricated by a one‐step spin‐casting process. The use of F‐TESADT/PMMA blend films in bottom gate transistors produces much higher field‐effect mobilities and greater stability than homo F‐TESADT films because the phase‐separated interface provides an efficient pathway for charge transport.     

Solution Processable Fluorenyl Hexa‐peri‐hexabenzocoronenes in Organic Field‐Effect Transistors and Solar Cells

The organization of organic semiconductor molecules in the active layer of organic electronic devices has important consequences to overall device performance. This is due to the fact that molecular organization directly affects charge carrier mobility of the material. Organic field‐effect transistor (OFET) performance is driven by high charge carrier mobility while bulk heterojunction (BHJ) solar cells require balanced hole and electron transport. By investigating the properties and device performance of three structural variations of the fluorenyl hexa‐peri‐hexabenzocoronene (FHBC) material, the importance of molecular organization to device performance was highlighted. It is clear from ¹H NMR and 2D wide‐angle X‐ray scattering (2D WAXS) experiments that the sterically demanding 9,9‐dioctylfluorene groups are preventing π–π intermolecular contact in the hexakis‐substituted FHBC 4 . For bis‐substituted FHBC compounds 5 and 6 , π–π intermolecular contact was observed in solution and hexagonal columnar ordering was observed in solid state. Furthermore, in atomic force microscopy (AFM) experiments, nanoscale phase separation was observed in thin films of FHBC and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) blends. The differences in molecular and bulk structural features were found to correlate with OFET and BHJ solar cell performance. Poor OFET and BHJ solar cells devices were obtained for FHBC compound 4 while compounds 5 and 6 gave excellent devices. In particular, the field‐effect mobility of FHBC 6 , deposited by spin‐casting, reached 2.8 × 10^?3 cm² V^?1 s and a power conversion efficiency of 1.5% was recorded for the BHJ solar cell containing FHBC 6 and PC₆₁BM.     

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.     

Enhanced Performance Consistency in Nanoparticle/TIPS Pentacene‐Based Organic Thin Film Transistors

In this study, inorganic silica nanoparticles are used to manipulate the morphology of 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS pentacene) thin films and the performance of solution‐processed organic thin‐film transistors (OTFTs). This approach is taken to control crystal anisotropy, which is the origin of poor consistency in TIPS pentacene based OTFT devices. Thin film active layers are produced by drop‐casting mixtures of SiO₂ nanoparticles and TIPS pentacene. The resultant drop‐cast films yield improved morphological uniformity at ～10% SiO₂ loading, which also leads to a 3‐fold increase in average mobility and nearly 4 times reduction in the ratio of measured mobility standard deviation (μ_Stdev) to average mobility (μ_Avg). Grazing‐incidence X‐ray diffraction, scanning and transmission electron microscopy as well as polarized optical microscopy are used to investigate the nanoparticle‐mediated TIPS pentacene crystallization. The experimental results suggest that the SiO₂ nanoparticles mostly aggregate at TIPS pentacene grain boundaries, and 10% nanoparticle concentration effectively reduces the undesirable crystal misorientation without considerably compromising TIPS pentacene crystallinity.     

Exciton Dynamics and Effects of Structural Order in Morphology‐Controlled J‐Aggregate Assemblies

Narrow‐band photoluminescence (PL) together with high quantum efficiency from organic molecules is essential for high‐color‐purity emitters. Supramolecular assemblies like J‐aggregates are promising materials due to their narrow PL signal with full‐width at half maximum <20 nm. However, their microcrystalline nature and coherent exciton migration results in strong nonradiative exciton recombination at the grain boundaries that diminish the photoluminescence quantum yield (PLQY), and possibilities for improving the crystallinity by tuning the growth mechanism are limited. Here, two distinct routes to grow different J‐aggregate morphologies like platelets and lamellar crystals with improved crystallinity by surface‐guided molecular assembly are demonstrated, thereby suppressing nonradiative decay and improving PLQY. Both platelets and lamellar crystals show similar absorbance at room temperature. However, temperature‐dependent PL studies show sevenfold (twofold) higher PLQY for lamellar films compared to platelets at 6 K (300 K). Using time‐resolved PL spectroscopy, different nonradiative decay pathways are identified. The dependence of exciton diffusion on energetic disorder and nonradiative decay is discussed. The results suggest that the difference in domain size and order gives rise to significantly enhanced radiative decay from lamellar films as compared to platelets or films formed by spin‐coating.     

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.     

Engineering Perovskite Nanocrystal Surface Termination for Light‐Emitting Diodes with External Quantum Efficiency Exceeding 15%

Hybrid organic–inorganic metal halide perovskites are particularly promising for light‐emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide‐based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5–10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole‐free films. As a result of time‐resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.     

Control of the Surface Composition of a Conducting‐Polymer Complex Film to Tune the Work Function

Control of surface composition of a hole‐injecting conducting polymer complex, poly(3,4‐ethylenedioxy thiophene) (PEDOT) doped with a polystyrene sulfonate (PSS) has been conducted in the spin‐cast films. We found that the work function of the polymeric complex films formed via single spin‐coating can be greatly increased up to 5.44 eV by increasing the surface concentration of the PSS dopant. As a result, we improved the device efficiency and the lifetime of green emitting polymer light‐emitting diodes (PLEDs). This implies that the PSS surface layer of the films spin‐cast from the conducting polymer complexes plays a key role in making high performance PLEDs.     

The Effect of Interfacial Roughness on the Thin Film Morphology and Charge Transport of High‐Performance Polythiophenes

We control and vary the roughness of a dielectric upon which a high‐performance polymer semiconductor, poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (pBTTT) is cast, to determine the effects of roughness on thin‐film microstructure and the performance of organic field‐effect transistors (OFETs). pBTTT forms large, well‐oriented terraced domains with high carrier mobility after it is cast upon flat, low‐surface‐energy substrates and heated to a mesophase. Upon dielectrics with root‐mean square (RMS) roughness greater than 0.5 nm, we find significant morphological changes in the pBTTT active layer and significant reductions in its charge carrier mobility. The pBTTT films on rough dielectrics exhibit significantly less order than those on smooth dielectrics through characterization with atomic force microscopy and X‐ray diffraction. This critical RMS roughness implies that there exists a condition at which the pBTTT domains no longer conform to the local nanometer‐scale curvature of the substrate.     

Significantly Enhanced Photoluminescence of Doped Polymer‐Metal Hybrid Nanotubes

We report on the significantly enhanced photoluminescence (PL) of hybrid double‐layered nanotubes (HDLNTs) consisting of poly(3‐methylthiophene) (P3MT) nanotubes with various doping levels enveloped by an inorganic, nickel (Ni) metal nanotube. From laser confocal microscopy PL experiments on a single strand of the doped‐P3MT nanotubes and of their HDLNTs, the PL peak intensity of the HDLNT systems increased remarkably up to ～350 times as the doping level of the P3MT nanotubes of the HDLNTs increased, which was confirmed by measurements of the quantum yield. In a comparison of the normalized ultraviolet and visible absorption spectra of the doped‐P3MT nanotubes and their HDLNTs, new absorption peaks corresponding to surface‐plasmon (SP) energy were created at 563 and 615 nm after the nanoscale Ni metal coating onto the P3MT nanotubes, and their intensity increased on increasing the doping level of the P3MT nanotube. The doping‐induced bipolaron peaks of the HDLNTs of doped‐P3MT/Ni were relatively reduced, compared with those of the doped‐P3MT nanotubes before the Ni coating, due to the charge‐transfer effect in the SP‐resonance (SPR) coupling. Both energy‐transfer and charge‐transfer effects due to SP resonance contributed to the very‐large enhancement of the PL efficiency of the doped‐P3MT‐based HDLNTs.     

Effect of tantalum doping on the structural and optical properties of RF magnetron sputtered indium oxide thin films

Tantalum doped indium oxide films are prepared by RF magnetron sputtering technique and the films are annealed in air at 300 °C. The effect of Ta doping on the structural, morphological, and optical properties of the annealed films are studied using techniques like X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), EDX analysis, micro-Raman, UV–visible and photoluminescence spectroscopy and electrical measurements. The XRD patterns present a cubic bixbyite structure for all the films with preferred orientation along the (222) plane. The lattice constant estimation presents a reduction in lattice size with Ta doping. The W–H plot shows a tensile strain for all the films and also indicates the presence of strain induced broadening of the XRD peaks. The Raman spectra present all the characteristic modes of In₂O₃ cubic bixbyite phase. FESEM and AFM images show the uniform and dense distribution of smaller grains in the films. All the films show high transmittance (above 85%) in the 400–900 nm region. Electrical measurement shows a systematic increase of carrier concentration and electrical conductivity with increase in Ta doping concentration. Band gap energy increases with increase in Ta doping concentration. All the films show intense PL emission in the UV region.     

Probing the Anisotropic Field‐Effect Mobility of Solution‐Deposited Dicyclohexyl‐α‐quaterthiophene Single Crystals

Measuring the anisotropy of the field‐effect mobility provides insight into the correlation between molecular packing and charge transport in organic semiconductor materials. Single‐crystal field‐effect transistors are ideal tools to study intrinsic charge transport because of their high crystalline order and chemical purity. The anisotropy of the field effect mobility in organic single crystals has previously been studied by lamination of macroscopically large single crystals onto device substrates. Here, a technique is presented that allows probing of the mobility anisotropy even though only small crystals are available. Crystals of a soluble oligothiophene derivative are grown in bromobenzene and drop‐cast onto substrates containing arrays of bottom‐contact gold electrodes. Mobility anisotropy curves are recorded by measuring numerous single crystal transistor devices. Surprisingly, two mobility maxima occur at azimuths corresponding to both axes of the rectangular cyclohexyl‐substituted quaterthiophene (CH4T) in‐plane unit cell, in contrast to the expected tensorial behavior of the field effect mobility.     

Dynamic Doping in Planar Ionic Transition Metal Complex‐Based Light‐Emitting Electrochemical Cells

Using a planar electrode geometry, the operational mechanism of iridium(III) ionic transition metal complex (iTMC)‐based light‐emitting electrochemical cells (LECs) is studied by a combination of fluorescence microscopy and scanning Kelvin probe microscopy (SKPM). Applying a bias to the LECs leads to the quenching of the photoluminescence (PL) in between the electrodes and to a sharp drop of the electrostatic potential in the middle of the device, far away from the contacts. The results shed light on the operational mechanism of iTMC‐LECs and demonstrate that these devices work essentially the same as LECs based on conjugated polymers do, i.e., according to an electrochemical doping mechanism. Moreover, with proceeding operation time the potential drop shifts towards the cathode coincident with the onset of light emission. During prolonged operation the emission zone and the potential drop both migrate towards the anode. This event is accompanied by a continuous quenching of the PL in two distinct regions separated by the emission line.     

Formation of a Ground‐State Charge‐Transfer Complex in Polyfluorene//[6,6]‐Phenyl‐C61 Butyric Acid Methyl Ester (PCBM) Blend Films and Its Role in the Function of Polymer/PCBM Solar Cells

Evidence is presented for the formation of a weak ground‐state charge‐transfer complex in the blend films of poly[9,9‐dioctylfluorene‐co‐N‐(4‐methoxyphenyl)diphenylamine] polymer (TFMO) and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM), using photothermal deflection spectroscopy (PDS) and photoluminescence (PL) spectroscopy. Comparison of this polymer blend with other polyfluorene polymer/PCBM blends shows that the appearance of this ground‐state charge‐transfer complex is correlated to the ionization potential of the polymer, but not to the optical gap of the polymer or the surface morphology of the blend film. Moreover, the polymer/PCBM blend films in which this charge‐transfer complex is observed also exhibit efficient photocurrent generation in photovoltaic devices, suggesting that the charge‐transfer complex may be involved in charge separation. Possible mechanisms for this charge‐transfer state formation are discussed as well as the significance of this finding to the understanding and optimization of polymer blend solar cells.     

A New Approach to Model‐Based Simulation of Disordered Polymer Blend Solar Cells

The 3D nanomorphology of blends of two different (organic and inorganic) solid phases as used in bulk heterojunction solar cells is described by a spatial stochastic model. The model is fitted to 3D image data describing the photoactive layer of poly(3‐hexylthiophene)‐ZnO (P3HT‐ZnO) solar cells fabricated with varying spin‐coating velocities. A scenario analysis is performed where 3D morphologies are simulated for different spin‐coating velocities to elucidate the correlation between processing conditions, morphology, and efficiency of hybrid P3HT‐ZnO solar cells. The simulated morphologies are analyzed quantitatively in terms of structural and physical characteristics. It is found that there is a tendency for the morphology to coarsen with increasing spin‐coating velocity, creating larger domains of P3HT and ZnO. The impact of the spin‐coating velocity on the connectivity of the morphology and the existence of percolation pathways for charge carriers in the resulting films appears insignificant, but the quality of percolation pathways, considering the charge carrier mobility, strongly varies with the spin‐coating velocity, especially in the ZnO phase. Also, the exciton quenching efficiency decreases significantly for films deposited at large spin‐coating velocities. The stochastic simulation model investigated is compared to a simulated annealing model and is found to provide a better fit to the experimental data.