Effects of the Terminal Structure, Purity, and Molecular Weight of an Amorphous Conjugated Polymer on Its Photovoltaic Characteristics

The photovoltaic characteristics of an amorphous polymer containing EDOT and fluorene units were investigated. In particular, the effects of the terminal structure, residual amount of Pd, and molecular weight were systematically investigated. Direct arylation polycondensation of EDOT followed by an established purification method readily afforded polymers with different terminal structures, Pd contents, and molecular weights. Of these factors, the terminal structure of the polymer was a crucial factor affecting the photovoltaic characteristics. For example, the polymer with a Br terminal had a PCE of 2.9% in bulk-heterojunction organic photovoltaics (BHJ OPVs) with a fullerene derivative, whereas the polymer without a Br terminal had a PCE of 4.6% in the same cell configuration. The decreased Pd residues and high molecular weights of the polymers increased the long-term stability of the devices. Moreover, BHJ OPVs containing the high-molecular-weight polymer could be fabricated with an environmentally friendly nonhalogenated solvent

Perovskite Solar Cells Prepared by Advanced Three-Step Method Using Additional HC(NH₂)₂I Spin-Coating: Efficiency Improvement with Multiple Bandgap Structure

In the conventional two-step prepared perovskite solar cells, the CH₃NH₃PbI₃ (MAPbI₃) film usually contains an unreacted PbI₂ at the interface between an electron transport layer (ETL) and a perovskite active layer. To reduce the unreacted PbI₂ in the two-step prepared MAPbI₃ film, we have recently reported a new three-step method, which was realized by an additional MA­(I,Br) spin-coating. Here, we propose an advanced three-step method, viz., an additional HC­(NH₂)₂I (FAI) spin-coating on the two-step prepared MAPbI₃ film. The additional FAI spin-coating formed a FA_<i>x</i>MA_1–<i>x</i>PbI₃ solid solution by the incorporation of FA ion into MAPbI₃. Also, the additional FAI spin-coating yielded a FA_<i>y</i>MA_1–<i>y</i>PbI₃ layer (<i>y</i> > <i>x</i>) by converting the unreacted PbI₂, which resulted in the layered structure with different FA concentrations and hence, with the multiple bandgap structure. The best PCE of 18.1% was achieved by optimizing the FAI spin-coating process

Suppression of Homocoupling Side Reactions in Direct Arylation Polycondensation for Producing High Performance OPV Materials

Suppression of side reactions in C–H direct arylation polycondensation is important for developing this method as a reliable synthetic tool for conjugated polymer materials. To find appropriate reaction conditions for avoiding homocoupling side reactions, two types of reaction conditions were investigated: the direct arylation of electron-rich C–H monomers in <i>N</i>,<i>N</i>-dimethyl­acetamide (DMAc system) and the direct arylation of electron-poor C–H monomers in toluene (toluene system). The investigation reveals that homocoupling side reactions are suppressed under the toluene system. Because the combination of electron-poor C–H monomer (acceptor) and electron-rich C–Br monomer (donor) is applicable to the toluene system, a donor–acceptor polymer without a defect structure can be synthesized under the toluene system. The obtained polymer shows almost same power conversion efficiency (PCE) in bulk-heterojunction OPVs as the same polymer prepared by a conventional method and purified by Soxhlet extraction. These results show that the established direct arylation polycondensation affords a high-quality material in terms of both structural integrity and purity. OPV cells with an optimized device structure result in a maximum PCE of 6.8%

Unique Device Operations by Combining Optical-Memory Effect and Electrical-Gate Modulation in a Photochromism-Based Dual-Gate Transistor

We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers, photochromic spiropyran (SP)-doped poly­(triarylamine) (PTAA) and pristine PTAA, as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect

Electrochemical Generation and Spectroscopic Characterization of Charge Carriers within Isolated Planar Polythiophene

In order to unveil the nature of charge carriers in a doped polythiophene, a sterically isolated polythiophenene, poly­(<b>1EDOT</b>), was electrochemically synthesized on electrodes. Generation of charge carriers was induced upon controlled electrochemical doping and investigated through various spectroscopic methods; the charge carriers were identified based on spin concentration (ESR spectra), aromatic character (Raman spectra), and electronic transition (UV–vis–NIR absorption spectra) of the polythiophene. Peculiarity of this study lies in the fact that the electrochemistry of the poly­(<b>1EDOT</b>) reflects the p-doping process of a single polythiophene wire because interwire interaction (i.e., π–π stacking) is effectively prevented; therefore, the results should be essential and informative to understand polythiophene-based materials and devices. Upon electrochemical doping, ESR active polarons were generated. Further doping concentrated the polarons, which led to the formation of polaron pairs. Eventually, the polaron pairs merged into bipolarons at the doping level of about 30–35%. Such a transformation of charge carriers under different doping levels has been extrapolated from studies using oligomeric model compounds. To the best of our knowledge, this is the first example addressing the nature of the charge carriers generated in a single polythiophene wire by exploiting the unique structure of the isolated polythiophene. Importantly, the comparison of poly­(<b>1EDOT</b>) with common polythiophenes such as poly­(3,4-ethylenedioxythiophene) (i.e., poly<b>EDOT</b>) implied that π–π stacking strongly affects the generation and stability of charge carriers. Furthermore, we found that the polaron pair plays an important role in charge hopping transport in the conduction mechanism

Syntheses and Photovoltaic Properties of Narrow Band Gap Donor–Acceptor Copolymers with Carboxylate-Substituted Benzodithiophene as Electron Acceptor Unit

Stille-coupling of carboxylate-substituted dibrominated benzodithiophene (BDTC) with 2,5-distannylthieno­[3,4-<i>b</i>]­thiophene gave novel donor–acceptor type alternating copolymers, PBDTC-TT, where BDTC works as an electron-accepting unit in the polymers. They showed broad absorption bands from 500 nm to the near-infrared region, optical band gap (<i>E</i>_g) about 1.5 eV, small π-stacking distance (3.6 Å), and good thermal stability. The hole mobilities of PBDTC-TT determined from performance of their organic field-effect transistors were 3.1–6.9 × 10^–4 (cm² V^–1 s^–1). The bulk heterojunction (BHJ) solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC₇₀BM/LiF/Al. A PBDTC-TT device exhibited photocurrent response upon exposure to light with wavelength of 300–900 nm and incident photon to current conversion efficiency over 40% in the range of 400–750 nm. The power conversion efficiency of the best-performed device reached 3.03% with short-circuit current density of 12.54 mA cm^–2, fill factor of 0.48, and open circuit voltage of 0.51 V under illumination of AM 1.5 G/100 mW cm^–2. These results show that the BDTC unit can behave as an electron accepting building block for donor–acceptor type narrow band gap polymers, and these types of polymers can be used as a donor material in the active layer for BHJ photovoltaic cells

Electrochemical Generation and Spectroscopic Characterization of Charge Carriers within Isolated Planar Polythiophene

In order to unveil the nature of charge carriers in a doped polythiophene, a sterically isolated polythiophenene, poly­(<b>1EDOT</b>), was electrochemically synthesized on electrodes. Generation of charge carriers was induced upon controlled electrochemical doping and investigated through various spectroscopic methods; the charge carriers were identified based on spin concentration (ESR spectra), aromatic character (Raman spectra), and electronic transition (UV–vis–NIR absorption spectra) of the polythiophene. Peculiarity of this study lies in the fact that the electrochemistry of the poly­(<b>1EDOT</b>) reflects the p-doping process of a single polythiophene wire because interwire interaction (i.e., π–π stacking) is effectively prevented; therefore, the results should be essential and informative to understand polythiophene-based materials and devices. Upon electrochemical doping, ESR active polarons were generated. Further doping concentrated the polarons, which led to the formation of polaron pairs. Eventually, the polaron pairs merged into bipolarons at the doping level of about 30–35%. Such a transformation of charge carriers under different doping levels has been extrapolated from studies using oligomeric model compounds. To the best of our knowledge, this is the first example addressing the nature of the charge carriers generated in a single polythiophene wire by exploiting the unique structure of the isolated polythiophene. Importantly, the comparison of poly­(<b>1EDOT</b>) with common polythiophenes such as poly­(3,4-ethylenedioxythiophene) (i.e., poly<b>EDOT</b>) implied that π–π stacking strongly affects the generation and stability of charge carriers. Furthermore, we found that the polaron pair plays an important role in charge hopping transport in the conduction mechanism

Electrochemical Generation and Spectroscopic Characterization of Charge Carriers within Isolated Planar Polythiophene

In order to unveil the nature of charge carriers in a doped polythiophene, a sterically isolated polythiophenene, poly­(<b>1EDOT</b>), was electrochemically synthesized on electrodes. Generation of charge carriers was induced upon controlled electrochemical doping and investigated through various spectroscopic methods; the charge carriers were identified based on spin concentration (ESR spectra), aromatic character (Raman spectra), and electronic transition (UV–vis–NIR absorption spectra) of the polythiophene. Peculiarity of this study lies in the fact that the electrochemistry of the poly­(<b>1EDOT</b>) reflects the p-doping process of a single polythiophene wire because interwire interaction (i.e., π–π stacking) is effectively prevented; therefore, the results should be essential and informative to understand polythiophene-based materials and devices. Upon electrochemical doping, ESR active polarons were generated. Further doping concentrated the polarons, which led to the formation of polaron pairs. Eventually, the polaron pairs merged into bipolarons at the doping level of about 30–35%. Such a transformation of charge carriers under different doping levels has been extrapolated from studies using oligomeric model compounds. To the best of our knowledge, this is the first example addressing the nature of the charge carriers generated in a single polythiophene wire by exploiting the unique structure of the isolated polythiophene. Importantly, the comparison of poly­(<b>1EDOT</b>) with common polythiophenes such as poly­(3,4-ethylenedioxythiophene) (i.e., poly<b>EDOT</b>) implied that π–π stacking strongly affects the generation and stability of charge carriers. Furthermore, we found that the polaron pair plays an important role in charge hopping transport in the conduction mechanism

Electrochemical Generation and Spectroscopic Characterization of Charge Carriers within Isolated Planar Polythiophene

In order to unveil the nature of charge carriers in a doped polythiophene, a sterically isolated polythiophenene, poly­(<b>1EDOT</b>), was electrochemically synthesized on electrodes. Generation of charge carriers was induced upon controlled electrochemical doping and investigated through various spectroscopic methods; the charge carriers were identified based on spin concentration (ESR spectra), aromatic character (Raman spectra), and electronic transition (UV–vis–NIR absorption spectra) of the polythiophene. Peculiarity of this study lies in the fact that the electrochemistry of the poly­(<b>1EDOT</b>) reflects the p-doping process of a single polythiophene wire because interwire interaction (i.e., π–π stacking) is effectively prevented; therefore, the results should be essential and informative to understand polythiophene-based materials and devices. Upon electrochemical doping, ESR active polarons were generated. Further doping concentrated the polarons, which led to the formation of polaron pairs. Eventually, the polaron pairs merged into bipolarons at the doping level of about 30–35%. Such a transformation of charge carriers under different doping levels has been extrapolated from studies using oligomeric model compounds. To the best of our knowledge, this is the first example addressing the nature of the charge carriers generated in a single polythiophene wire by exploiting the unique structure of the isolated polythiophene. Importantly, the comparison of poly­(<b>1EDOT</b>) with common polythiophenes such as poly­(3,4-ethylenedioxythiophene) (i.e., poly<b>EDOT</b>) implied that π–π stacking strongly affects the generation and stability of charge carriers. Furthermore, we found that the polaron pair plays an important role in charge hopping transport in the conduction mechanism

Supramolecular Assemblies of Ferrocene-Hinged Naphthalene­diimides: Multiple Conformational Changes in Film States

We design a new naphthalenediimide (NDI) π-system, <b>NDI–Fc–NDI</b>, having a ferrocene linker as a hinge unit and long alkyl chains as supramolecular assembling units. The NDI units are “directionally flexible” in concert with the pivoting motion of the ferrocene unit with a small rotational barrier. The NDI units rotate around the ferrocene unit faster than the NMR time scale in solution at room temperature. UV–vis absorption, synchrotron X-ray diffraction, and atomic force microscope studies reveal that <b>NDI–Fc–NDI</b> forms a fibrous supramolecular assembly in solution (methylcyclohexane and highly concentrated chloroform) and film states, wherein the NDI units are in the slipped-stack conformation. The <b>NDI–Fc–NDI</b> supramolecular assembly in the film state exhibits multiple phase transitions associated with conformational changes at different temperatures, which are confirmed by differential scanning calorimetry, polarized optical microscopy, and temperature-dependent X-ray diffraction. Such thermal transitions of <b>NDI–Fc–NDI</b> films also induce changes in the optical and electronic properties as revealed by UV–vis absorption and photoelectron yield spectroscopies, respectively. The thermal behaviors of <b>NDI–Fc–NDI</b>, realized by the unique molecular design, are considerably different from the reference compounds such as an NDI dimer connected with a flexible 1,4-butylene linker. These results provide us with a plausible strategy to propagate the molecular dynamics of the π-system into macroscopic properties in film states; the key factors are (i) the supramolecular alignment of molecular switching units and (ii) the directional motion of the switching units perpendicular to the supramolecular axis