Role of Interfacial Charge Transfer State in Charge Generation and Recombination in Low-Bandgap Polymer Solar Cell

The charge carrier dynamics in blend films of poly­[2,6-(4,4-bis­(2-ethylhexyl)-4<i>H</i>-cyclopenta­[2,1-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) was studied by transient absorption spectroscopy in order to address the origin of limited external quantum efficiency (EQE) of this solar cell compared to that of a benchmark solar cell composed of regioregular poly­(3-hexythiphene) (RR-P3HT) and PCBM. Upon photoexcitation, PCPDTBT singlet excitons promptly convert to the interfacial charge transfer (CT) state that is a Coulombically bound charge pair of PCPDTBT polaron and PCBM anion at the heterojunction with almost 100% efficiency in a picosecond. In other words, the exciton diffusion efficiency η_ED and charge transfer efficiency η_CT are 100% in this blend, which are higher than and comparable to those of the RR-P3HT/PCBM solar cell, respectively. On a time scale of nanoseconds, 70% of the PCPDTBT bound polarons are dissociated into free charge carriers, and the others recombine geminately to the ground state through the CT state. The charge dissociation efficiency η_CD = 70% is lower than that of RR-P3HT/PCBM solar cells. The PCPDTBT dissociated polarons recombine bimolecularly on a time scale of nano- to microseconds with a charge lifetime of ∼10^–7 s, which is shorter than that observed for RR-P3HT/PCBM blends. In summary, the lower charge dissociation efficiency and shorter charge lifetime are the limiting factors for the photovoltaic performance of PCPDTBT/PCBM solar cells. Furthermore, the origin of such limitation is also discussed in terms of the charge dissociation and recombination through the interfacial CT state in PCPDTBT/PCBM blends

Superhydrophobic Porous Surfaces: Dissolved Oxygen Sensing

Porous polymer films are necessary for dissolved gas sensor applications that combine high sensitivity with selectivity. This report describes a greatly enhanced dissolved oxygen sensor system consisting of amphiphilic acrylamide-based polymers: poly­(<i>N</i>-(1H, 1H-pentadecafluorooctyl)-methacrylamide) (pC7F15MAA) and poly­(<i>N</i>-dodecylacrylamide-<i>co</i>-5- [4-(2-methacryloyloxyethoxy-carbonyl)­phenyl]-10,15,20-triphenylporphinato platinum­(II)) (p­(DDA/PtTPP)). The nanoparticle formation capability ensures both superhydrophobicity with a water contact angle greater than 160° and gas permeability so that molecular oxygen enters the film from water. The film was prepared by casting a mixed solution of pC7F15MAA and p­(DDA/PtTPP) with AK-225 and acetic acid onto a solid substrate. The film has a porous structure comprising nanoparticle assemblies with diameters of several hundred nanometers. The film shows exceptional performance as the oxygen sensitivity reaches 126: the intensity ratio at two oxygen concentrations (<i>I</i>₀/<i>I</i>₄₀) respectively corresponding to dissolved oxygen concentration 0 and 40 (mg L^–1). Understanding and controlling porous nanostructures are expected to provide opportunities for making selective penetration/separation of molecules occurring at the superhydrophobic surface

Quantification of Amino Groups on Solid Surfaces Using Cleavable Fluorescent Compounds

We quantified amino groups displayed on inorganic and organic surfaces in aqueous solution using different types of cleavable fluorescent compounds and an aldehyde dye. The cleavable fluorescent compounds were designed to bind covalently to amino groups and then liberated under specific conditions. Among the investigated materials, cleavable coumarin was most appropriate for the quantification of amino groups on silica and resin surfaces. The developed method can measure small amounts (∼pmol/cm²) of amino groups on a flat polymeric surface, detecting only amino groups that are exposed to aqueous solution and available for surface immobilization of ligands and biomolecules

Surfactant-Induced Polymer Segregation To Produce Antifouling Surfaces via Dip-Coating with an Amphiphilic Polymer

We propose a rational strategy to control the surface segregation of an amphiphilic copolymer in its dip-coating with a low-molecular-weight surfactant. We synthesized a water-insoluble methacrylate-based copolymer containing oligo­(ethylene glycol) (OEG) (copolymer <b>1</b>) and a perfluoroalkylated surfactant (surfactant <b>1</b>) containing OEG. The dip-coating of copolymer <b>1</b> with surfactant <b>1</b> resulted in the segregation of surfactant <b>1</b> on the top surface of the dip-coated layer due to the high hydrophobicity of its perfluoroalkyl group. OEG moieties of surfactant <b>1</b> were accompanied by those of copolymer <b>1</b> in its segregation, allowing the OEG moieties of copolymer <b>1</b> to be located just below the top surface of the dip-coated layer. The removal of surfactant <b>1</b> produced the surface covered by the OEG moieties of the copolymer that exhibited antifouling properties. Using this strategy, we also succeeded in the introduction of carboxy groups on the dip-coated surface and demonstrated that the carboxy groups were available for the immobilization of functional molecules on the surface

Efficient Charge Generation and Collection in Amorphous Polymer-Based Solar Cells

The charge generation and recombination dynamics in amorphous blend films of poly­[2,7-(9,9-dioctylfluorene)-<i>alt</i>-5,5-(5′,8′-di-2-thienyl-2′,3′-diphenylquinoxaline)] (N-P7) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were comprehensively studied in order to address the origin of the relatively high performance for amorphous polymer-based solar cells. Upon photoexcitation, N-P7 singlet excitons are promptly converted to the interfacial charge transfer (CT) state, which is a Coulombically bound pair of the N-P7 polaron and PCBM radical anion, with 100% efficiency in a picosecond. More than half of the N-P7 polarons in the CT state are dissociated into free carriers, and the rest of them recombine to the ground state in a nanosecond. The dissociation efficiency η_CD is estimated to be 0.65 under the open-circuit condition and is slightly enhanced up to 0.7–0.8 under the short-circuit condition. Such highly efficient dissociation is well explained by considering charge delocalization. The charge collection efficiency η_CC is as high as >0.9 in the thin device but decreases to <0.8 in the thick device, suggesting that bimolecular recombination loss is not negligible in the thick device. Furthermore, the origin of the efficient device performance is discussed in terms of the field dependence and the charge delocalization

Acid-Group-Content-Dependent Proton Conductivity Mechanisms at the Interlayer of Poly(<i>N</i>‑dodecylacrylamide-<i>co</i>-acrylic acid) Copolymer Multilayer Nanosheet Films

The effect of the content of acid groups on the proton conductivity at the interlayer of polymer-nanosheet assemblies was investigated. For that purpose, amphiphilic poly­(<i>N</i>-dodecylacrylamide-<i>co</i>-acrylic acid) copolymers [p­(DDA/AA)] with varying contents of AA were synthesized by free radical polymerization. Surface pressure (π)–area (<i>A</i>) isotherms of these copolymers indicated that stable polymer monolayers are formed at the air/water interface for AA mole fraction (<i>n</i>) ≤ 0.49. In all cases, a uniform dispersion of the AA groups in the polymer monolayer was observed. Subsequently, polymer monolayers were transferred onto solid substrates using the Langmuir–Blodgett (LB) technique. X-ray diffraction (XRD) analyses of the multilayer films showed strong Bragg diffraction peaks, suggesting a highly uniform lamellar structure for the multilayer films. The proton conductivity of the multilayer films parallel to the direction of the layer planes were measured by impedance spectroscopy, which revealed that the conductivity increased with increasing values of <i>n</i>. Activation energies for proton conduction of ∼0.3 and 0.42 eV were observed for <i>n</i> ≥ 0.32 and <i>n</i> = 0.07, respectively. Interestingly, the proton conductivity of a multilayer film with <i>n</i> = 0.19 did not follow the Arrhenius equation. These results were interpreted in terms of the average distance between the AA groups (<i>l</i>_AA), and it was concluded that, for <i>n</i> ≥ 0.32, an advanced 2D hydrogen bonding network was formed, while for <i>n</i> = 0.07, <i>l</i>_AA is too long to form such hydrogen bonding networks. The <i>l</i>_AA for <i>n</i> = 0.19 is intermediate to these extremes, resulting in the formation of hydrogen bonding networks at low temperatures, and disruption of these networks at high temperatures due to thermally induced motion. These results indicate that a high proton conductivity with low activation energy can be achieved, even under weakly acidic conditions, by arranging the acid groups at an optimal distance

Charge Generation and Recombination in Fullerene-Attached Poly(3-hexylthiophene)-Based Diblock Copolymer Films

The charge generation and recombination dynamics in fullerene-attached poly­(3-hexythiophene) (P3HT)-based diblock copolymer were studied in comparison with those in blend films of P3HT and a fullerene derivative (PCBM) in order to understand the potential advantage of diblock copolymer-based polymer solar cells. Upon photoexcitation, P3HT singlet excitons are promptly converted to P3HT polarons with a time constant of ∼30 ps in both P3HT-PCBM diblock copolymer and P3HT/PCBM blend films. This similar charge generation dynamics is indicative of analogous phase-separated morphology both in these films on a scale of nanometers. After the charge generation, a part of polarons in disorder phases geminately recombine to the ground state in diblock copolymer films, while no geminate recombination is observed in blend films. This geminate recombination loss is probably due to defects of phase-separated structures in diblock copolymer films. On the other hand, charge carrier lifetime is as long as 15 μs in diblock copolymer films. Such a long carrier lifetime may result in a relatively high fill factor in P3HT-PCBM copolymer films. Finally, we discuss the overall device performance in terms of phase-separated structures

Hands-Off Preparation of Monodisperse Emulsion Droplets Using a Poly(dimethylsiloxane) Microfluidic Chip for Droplet Digital PCR

A fully autonomous method of creating highly monodispersed emulsion droplets with a low sample dead volume was realized using a degassed poly­(dimethylsiloxane) (PDMS) microfluidic chip possessing a simple T-junction channel geometry with two inlet reservoirs for oil and water to be loaded and one outlet reservoir for the collection of generated droplets. Autonomous transport of oil and water phases in the channel was executed by permeation of air confined inside the outlet reservoir into the degassed PDMS. The only operation required for droplet creation was simple pipetting of oil and aqueous solutions into the inlet reservoirs. Long-lasting fluid transport in the current system enabled us to create ca. 51,000 monodispersed droplets (with a coefficient of variation of <3% for the droplet diameter) in 80 min with a maximum droplet generation rate of ca. 12 Hz using a PDMS chip that had been degassed overnight. With multiple time-course measurements, the reproducibility in the current method of droplet preparation was confirmed, with tunable droplet sizes achieved simply by changing the cross-sectional dimensions of the microchannel. Furthermore, it was verified that the resultant droplets could serve as microreactors for digital polymerase chain reactions. This hands-free technique for preparing monodispersed droplets in a very facile and inexpensive fashion is intended for, but not limited to, bioanalytical applications and is also applicable to material syntheses

Regioselective Synthesis of Eight-Armed Cyclosiloxane Amphiphile for Functional 2D and 3D Assembly Motifs

A crystalline tetramethylcyclotetrasiloxane (TMCS)-derived amphiphile was regioselectively synthesized with eight peripheral hydrophilic amide groups and hydrophobic dodecyl chains by Pt(0)-catalyzed hydrosilylation and amidation reactions. The as-synthesized materials showed ordered lamellar structure formation in the powder form. It also exhibits superior two-dimensional (2D) monolayer formation properties at the air–water interface with unexpectedly high collapse surface pressure and elastic modulus. The monolayers act as two-dimensional building blocks with finely controllable thickness on a several nanometer scale irrespective of the substrate type and properties. The amphiphile forms nanofibers spontaneously by good–poor solvent strategies, which contributes to porous three-dimensional (3D) structures possessing superhydrophobic surface wettability

Fluorescent Ferroelectrics of Hydrogen-Bonded Pyrene Derivatives

Organic materials with diverse molecular designs show multifunctional properties such as coupled ferroelectric, optical, ferromagnetic, and transport properties. We report the design of an alkylamide-substituted pyrene derivative displaying fluorescent ferroelectric properties coupled with electron transport properties. In solution phase, this compound displayed concentration-dependent fluorescence, whereas in xerogels, a fluorescent green organogel (>0.1 mM) and entangled nanofibers were observed. A discotic hexagonal columnar liquid crystalline phase was observed above 295 K due to intermolecular hydrogen bonding and π-stacking interactions. The direction of the hydrogen-bonded chains could be inverted by the application of an external electric field along the π-stacked column, resulting in ferroelectric polarization-electric field (<i>P</i>–<i>E</i>) hysteresis. The local electric field arising from the ferroelectric macrodipole moment arrangement along the π-stacking direction affected the electron transport properties on the π-stack of pyrenes, thus confirming the current-switching phenomena according to <i>P</i>–<i>E</i> hysteresis. We report that multifunctional properties such as ferroelectricity, fluorescence, and electron transport switching were successfully achieved in hydrogen-bonded dynamic π-molecular assemblies