A Kelvin Probe Force Microscopy Study of the Photogeneration of Surface Charges in All‐Thiophene Photovoltaic Blends

Light‐induced generation of charges into an electron acceptor–donor phase‐segregated blend is studied. The blend is made of highly ordered nanoscopic crystals of 3″‐methyl‐4″‐hexyl‐2,2′:5′,2″:5″,2?:5?,2″″‐quinquethiophene‐1″,1″‐dioxide embedded into a regioregular poly(3‐hexylthiophene) matrix, acting as acceptor and donor materials, respectively. Kelvin probe force microscopy investigations reveal a tendency for the acceptor nanocrystals to capture the generated electrons whereas the donor matrix becomes more positively charged. The presence of particular positively charged defects, i.e., nanocrystals, is also observed within the film. The charging and discharging of both materials is studied in real time, as well as the effect of different acceptor–donor ratios. Upon prolonged thermal annealing at high temperatures the chemical structure of the blend is altered, leading to the disappearance of charge separation upon light irradiation. The obtained results allow a better understanding of the correlation between the nanoscopic structure of the photoactive material and solar‐cell performance.     

Microscopic Morphology of Polyfluorene–Poly(ethylene oxide) Block Copolymers: Influence of the Block Ratio

In this paper, we describe the synthesis and characterization of poly(9,9′‐dioctylfluorene)–poly(ethylene oxide) (PF‐PEO) block copolymers with different block ratio and molecular architectures (diblock or triblock copolymers). Tapping‐mode atomic force microscopy is used to investigate the relationship between the molecular structure and the microscopic morphology of thin deposits. Copolymers with a low average volume ratio of PEO (f_EO from 0.1 to 0.3) exhibit a well‐defined organization into nanoribbons. A model of chain packing is proposed; these structures arise from the interplay of π–π interactions between conjugated PF segments and the interactions of PEO with the mica substrate surface. For copolymers with higher average volume ratio of PEO (f_EO > 0.4), the organized structures disappear and lead to untextured aggregates, probably because long‐range, regular π–π stacking of the segments can no longer take place. We also observe that the nature of the solvent from which deposits are grown and the substrate polarity have a strong impact on the microscopic morphology.     

Nanoscale Characterization of the Morphology and Electrostatic Properties of Poly(3‐octylthiophene)/Graphite‐Nanoparticle Blends

Mixtures of poly(3‐octylthiophene) (P3OT) with graphite nanoparticles have been investigated by scanning force microscopy (SFM) techniques. The morphology as well as the mechanical and electrical properties of the blends has been characterized at the nanoscale level as a function of the carbon nanoparticle content in the blend. An increase in the concentration of carbon nanoparticles results in an increase in the surface roughness of the blend and the appearance of distinct regions with well‐defined electrical and mechanical properties. At intermediate concentrations (5–10 wt % of carbon nanoparticles), the samples show pure P3OT regions, as well as round regions containing a mixture of the polymer and carbon nanoparticles, while at higher concentrations (> 15 wt %), the entire sample is composed of this mixture. The interface between the two regions has been studied by electrostatic scanning force microscopy (ESFM) as a function of the applied tip–sample voltage. ESFM provides evidence for the creation of new electronic states at the heterojunction. The observed results can be qualitatively explained in terms of the electronic properties of the individual molecular components, P3OT, functionalized graphite nanoparticles, and their corresponding heterojunction. The implications of these results for organic polymer solar cells are also discussed.     

Quantitative Measurement of the Local Surface Potential of π‐Conjugated Nanostructures: A Kelvin Probe Force Microscopy Study

We describe a systematic study on the influence of different experimental conditions on the Kelvin probe force microscopy (KPFM) quantitative determination of the local surface potential (SP) of organic semiconducting nanostructures of perylene‐bis‐dicarboximide (PDI) self‐assembled at surfaces. We focus on the effect of the amplitude, frequency, and phase of the oscillating voltage on the absolute surface potential value of a given PDI nanostructure at a surface. Moreover, we investigate the role played by the KPFM measuring mode employed and the tip–sample distance in the surface potential mapping by lift‐mode KPFM. We define the ideal general conditions to obtain a reproducible quantitative estimation of the SP and we find that by decreasing the tip–sample distance, the area of substrate contributing to the recorded SP in a given location of the surface becomes smaller. This leads to an improvement of the lateral resolution, although a more predominant effect of polarization is observed. Thus, quantitative SP measurements of these nanostructures become less reliable and the SP signal is more unstable. We have also devised a semi‐quantitative theoretical model to simulate the KPFM image by taking into account the interplay of the different work functions of tip and nanostructure as well as the nanostructure polarizability. The good agreement between the model and experimental results demonstrates that it is possible to simulate both the change in local SP at increasing tip–sample distances and the 2D potential images obtained on PDI/highly oriented pyrolytic graphite samples. These results are important as they make it possible to gain a quantitative determination of the local surface potential of π‐conjugated nanostructures; thus, they pave the way towards the optimization of the electronic properties of electroactive nanometer‐scale architectures for organic (nano)electronic applications.     

Conductivity of SU‐8 Thin Films through Atomic Force Microscopy Nano‐Patterning

Processing flexibility and good mechanical properties are the two major reasons for SU‐8 extensive applicability in the micro‐fabrication of devices. In order to expand its usability down to the nanoscale, conductivity of ultra‐thin SU‐8 layers as well as its patterning by AFM are explored. By performing local electrical measurements outstanding insulating properties and a dielectric strength 100 times larger than that of SiO₂ are shown. It is also demonstrated that the resist can be nano‐patterned using AFM, obtaining minimum dimensions below 40nm and that it can be combined with parallel lithographic methods like UV‐lithography. The concurrence of excellent insulating properties and nanometer‐scale patternability enables a valuable new approach for the fabrication of nanodevices. As a proof of principle, nano‐electrode arrays for electrochemical measurements which show radial diffusion and no overlap between different diffusion layers are fabricated. This indicates the potential of the developed technique for the nanofabrication of devices.     

Electrochromic Window Based on Conducting Poly(3,4‐ethylenedioxythiophene)–Poly(styrene sulfonate)

A short survey of technological aspects of electrochromism with various electroactive species is given. Different approaches with inorganic and organic materials have been pursued in the past. So far widespread usage of this technology for large area applications has not been achieved. Nevertheless one major technical product, self‐darkening rear‐view mirrors for cars, is already well established. This article reviews some research results on electroactive polythiophenes, especially poly(3,4‐alkylenedioxythiophenes). Some promising results with the commercially available electrically conducting polymer Baytron P (PEDT/PSS) are presented. It is demonstrated that an all solid‐state electrochromic multilayer assembly based on a polymeric electrochromic material might be close to technical realization. The coating of large area substrates with aqueous poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) dispersion can be a way to an economically viable product.     

Ambipolar Charge Transport in Films of Methanofullerene and Poly(phenylenevinylene)/Methanofullerene Blends

Herein, we report experimental studies of electron and hole transport in thin films of [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) and in blends of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) with PCBM. The low‐field hole mobility in pristine MDMO‐PPV is of the order of 10^–7 cm² V^–1 s^–1, in agreement with previous studies, whereas the electron mobility in pristine PCBM was found by current‐density–voltage (J–V) measurements to be of the order of 10^–2 cm² V^–1 s^–1, which is about one order of magnitude greater than previously reported. Adding PCBM to the blend increases both electron and hole mobilities, compared to the pristine polymer, and results in less dispersive hole transport. The hole mobility in a blend containing 67 wt.‐% PCBM is at least two orders of magnitude greater than in the pristine polymer. This result is independent of measurement technique and film thickness, indicating a true bulk property of the material. We therefore propose that PCBM may assist hole transport in the blend, either by participating in hole transport or by changing the polymer‐chain packing to enhance hole mobility. Time‐of‐flight mobility measurements of PCBM dispersed in a polystyrene matrix yield electron and hole mobilities of similar magnitude and relatively non‐dispersive transport. To the best of our knowledge, this is the first report of hole transport in a methanofullerene. We discuss the conditions under which hole transport in the fullerene phase of a polymer/fullerene blend may be expected. The relevance to photovoltaic device function is also discussed.     

A Novel Photoscissile Poly(ethylene glycol)‐Based Hydrogel

A novel eight‐branched poly(ethylene glycol), PEG, macromer having a nitrocinnamate moiety as a pendant group was synthesized and found to form a photoscissile hydrogel upon exposure to 365 nm radiation in the absence of photoinitiators or catalysts. The processes of photocrosslinking and photocleavage were clearly characterized by environmental scanning electron microscopy (ESEM).     

Inside Front Cover: Quantitative Measurement of the Local Surface Potential of π‐Conjugated Nanostructures: A Kelvin Probe Force Microscopy Study (Adv. Funct. Mater. 11/2006)

Kelvin probe force microscopy provides quantitative insight into the electronic properties of thin molecular layers, as shown by the results of P. Samorì and co‐workers on p. 1407. In the cartoon shown in the inside front cover, a scanning charged tip probes the local surface potential of a self‐assembled layer, inducing charge polarization into a nanoscale “effective area”. These measurements make it possible to unravel the interplay between structural and electronic properties of molecule‐based materials and devices. We describe a systematic study on the influence of different experimental conditions on the Kelvin probe force microscopy (KPFM) quantitative determination of the local surface potential (SP) of organic semiconducting nanostructures of perylene‐bis‐dicarboximide (PDI) self‐assembled at surfaces. We focus on the effect of the amplitude, frequency, and phase of the oscillating voltage on the absolute surface potential value of a given PDI nanostructure at a surface. Moreover, we investigate the role played by the KPFM measuring mode employed and the tip–sample distance in the surface potential mapping by lift‐mode KPFM. We define the ideal general conditions to obtain a reproducible quantitative estimation of the SP and we find that by decreasing the tip–sample distance, the area of substrate contributing to the recorded SP in a given location of the surface becomes smaller. This leads to an improvement of the lateral resolution, although a more predominant effect of polarization is observed. Thus, quantitative SP measurements of these nanostructures become less reliable and the SP signal is more unstable. We have also devised a semi‐quantitative theoretical model to simulate the KPFM image by taking into account the interplay of the different work functions of tip and nanostructure as well as the nanostructure polarizability. The good agreement between the model and experimental results demonstrates that it is possible to simulate both the change in local SP at increasing tip–sample distances and the 2D potential images obtained on PDI/highly oriented pyrolytic graphite samples. These results are important as they make it possible to gain a quantitative determination of the local surface potential of π‐conjugated nanostructures; thus, they pave the way towards the optimization of the electronic properties of electroactive nanometer‐scale architectures for organic (nano)electronic applications.     

Patterned Color and Fluorescent Images with Polydiacetylene Supramolecules Embedded in Poly(vinyl alcohol) Films

Poly(vinyl alcohol) (PVA) films embedded with functional polydiacetylene (PDA) are efficiently prepared for color and fluorescence imaging. Intensely blue films are obtained by mixing and drying solutions containing PDA vesicles and PVA. A blue‐to‐red color transition is observed upon heating the polymer films. In addition, selective UV irradiation (through a photomask) of PVA films containing diacetylene monomer results in the generation of micropatterned color (without heating) and both color and fluorescent images (after heating the films at 120 °C for 10 s). Patterned two‐color (blue and red) images in the polymer film are readily obtained by a sequential process of photomasked irradiation, heating, and unmasked irradiation.     

Enhancement of Modulus,Strength, and Toughness in Poly(methyl methacrylate)‐Based Composites by the Incorporation of Poly(methyl methacrylate)‐Functionalized Nanotubes

Poly(methyl methacrylate) (PMMA)‐functionalized multiwalled carbon nanotubes are prepared by in situ polymerization. Infrared absorbance studies reveal covalent bonding between polymer strands and the nanotubes. These treated nanotubes are blended with pure PMMA in solution before drop‐casting to form composite films. Increases in Young's modulus, breaking strength, ultimate tensile strength, and toughness of ×1.9, ×4.7, ×4.6, and ×13.7, respectively, are observed on the addition of less than 0.5 wt % of nanotubes. Effective reinforcement is only observed up to a nanotube content of approximately 0.1 vol %. Above this volume fraction, all mechanical parameters tend to fall off, probably due to nanotube aggregation. In addition, scanning electron microscopy (SEM) studies of composite fracture surfaces show a polymer layer coating the nanotubes after film breakage. The fact that the polymer and not the interface fails suggests that functionalization results in an extremely high polymer/nanotube interfacial shear strength.     

A Morphological Model for the Solvent‐Enhanced Conductivity of PEDOT:PSS Thin Films

The well‐known enhanced conductivity of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films that is obtained by addition of high‐boiling solvents like sorbitol to the aqueous dispersion used for film deposition is shown to be associated with a rearrangement of PEDOT‐rich clusters into elongated domains, as evidenced from STM and AFM. Consistently, temperature dependent conductivity measurements for sorbitol‐treated films reveal that charge transport occurs via quasi 1D variable range hopping (VRH), in contrast to 3D VRH for untreated PEDOT:PSS films. The typical hopping distance of 60–90 nm, extracted from the conductivity measurements is consistent with hopping between the 30–40 nm sized grains observed with scanning probe microscopy.     

Synthesis and Patterning of Luminescent CaCO3–Poly(p‐phenylene) Hybrid Materials and Thin Films

Nature employs specialized macromolecules to produce highly complex structures and understanding the role of these macromolecules allows us to develop novel materials with interesting properties. Herein, we report the role of modified conjugated polymers in the nucleation, growth, and morphology of calcium carbonate (CaCO₃) crystals. In situ incorporation of sulfonated poly(p‐phenylene) (s(PPP)) into a highly oriented calcium carbonate matrix is investigated along with the synthesis and patterning of luminescent CaCO₃–PPP hybrid materials. Functionalized PPP with polar and nonpolar groups are used as additives in the mineralization medium. The polymer (P1) with polar groups give iso‐oriented calcite crystals, whereas PPP with an additional alkyl chain (P2) results in vaterite crystals. The crystallization mechanism can be explained based on self‐assembly and aggregation of polymers in an aqueous environment. Such light‐emitting hybrid composites with tunable optical properties are excellent candidates for optoelectronics and biological applications.     

Single‐Molecule Tracking of Fibrinogen Dynamics on Nanostructured Poly(ethylene) Films

Using fibrinogen (Fg) protein as a probe molecule, mapping using accumulated probe trajectories (MAPT) is performed on nanostructured melt‐drawn high‐density poly(ethylene) (HDPE) films composed of well‐oriented crystalline patches separated by amorphous regions. The spatially grouped molecular trajectories allow for identification of regions with distinct surface properties (i.e., crystalline vs. amorphous) while simultaneously determining the characteristic dynamic protein behavior within those regions. In the presence of solution with a sufficiently high Fg concentration, discrete patches of a dense, ordered protein layer form (presumably on crystalline HDPE regions), leading to a dramatic rise in the surface residence time (by more than two orders of magnitude) of molecules incorporated into the film. Within this ordered Fg layer, individual molecules exhibit slow anisotropic lateral diffusion; the mobility is restricted by the nanostructure boundaries of the underlying HDPE. On HDPE films at low Fg surface coverage, or on films that have been rendered hydrophilic with Ar plasma, short surface residence times and fast, isotropic diffusion are observed. These results demonstrate the ability of spatially resolved single‐molecule tracking to provide mechanistic information about biomolecule‐surface interactions in a highly heterogeneous environment.     

Percolation Mechanism and Characterization of (CdO)y(ZnO)1–y Thin Films

(CdO)_y(ZnO)_1–y thin films have been prepared by the sol–gel process, based on precursor solutions used separately for such oxides. The Cd/(Cd + Zn) atomic ratio in solution ranged from 0 to 0.32. These compositions were selected on the basis of an observed abrupt fall, of ca. four orders of magnitude, in the resistivity of the films within this range. Such a resistivity drop, with a threshold value of around y = 0.17, is consistent with a percolation mechanism in a three‐dimensional, random, two‐phase system composed of isotropic, sphere‐like, conducting CdO regions embedded in a highly resistive ZnO matrix. Optical measurements show that the films are highly transparent, above 90 % transmission, for wavelengths ≥600 nm. The optical absorption edge shifts to longer wavelengths as the Cd content in the film increases. On the basis of the percolation mechanism observed in the multicomponent system (CdO)_y(ZnO)_1–y, possible future pathways are proposed for the design and construction of highly efficient, transparent, conducting oxides.     

All‐Inorganic CsPbI3 Perovskite Phase‐Stabilized by Poly(ethylene oxide) for Red‐Light‐Emitting Diodes

Despite the excellent photoelectronic properties of the all‐inorganic cesium lead iodide (CsPbI₃) perovskite, which does not contain volatile and hygroscopic organic components, only a few CsPbI₃ devices are developed mainly owing to the frequent formation of an undesirable yellow δ‐phase at room temperature. Herein, it is demonstrated that a small quantity of poly(ethylene oxide) (PEO) added to the precursor solution effectively inhibits the formation of the yellow δ‐phase during film preparation, and promotes the development of a black α‐phase at a low crystallization temperature. A systematic study reveals that a thin, dense, pinhole‐free CsPbI₃ film is produced in the α‐phase and is stabilized with PEO that effectively reduces the grain size during crystallization. A thin α‐phase CsPbI₃ film with excellent photoluminescence is successfully employed in a light‐emitting diode with an inverted configuration of glass substrate/indium tin oxide/zinc oxide/poly(ethyleneimine)/α‐CsPbI₃/poly(4‐butylphenyl‐diphenyl‐amine)/WO₃/Al, yielding the characteristic red emission of the perovskite film at 695 nm with brightness, external quantum efficiency, and emission band width of ≈101 cd m^?2, 1.12%, and 32 nm, respectively.     

Understanding the Beneficial Role of Grain Boundaries in Polycrystalline Solar Cells from Single‐Grain‐Boundary Scanning Probe Microscopy

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single‐crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high‐resolution mapping of electronic properties of single GBs and grain surfaces in PX p‐CdTe/n‐CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography‐related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier‐collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n‐CdS/p‐CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl₂ heat treatment used to optimize these cells in the production process.     

PEO作阴极修饰层提高有机光伏电池的稳定性

采用聚氧化乙烯(PEO)作为聚合物太阳能电池的阴极修饰层,以P3HT:PCBM为活性层制备了聚合物本体异质结太阳能电池。考察了PEO的厚度对器件光伏性能及稳定性的影响。比较了加入PEO修饰层前后器件的稳定性,研究了采用PEO修饰层前后器件电阻的差异。结果表明:加入PEO作为阴极修饰层后器件的光电性能(JSC,VOC,FF,PCE)均有明显提高,而器件的串联电阻Rs则有了明显降低。没有阴极修饰层的器件的初始光电转换效率为1.92%,90 h后衰减为初始值的5%;而加入PEO修饰层后初始光电转换效率为3.36%,90 h后仅衰减为初始值的20%,光电转换效率提高了75%,稳定性提高了3倍。