Interlayer Resistance and Edge-Specific Charging in Layered Molecular Crystals Revealed by Kelvin-Probe Force Microscopy

Organic field-effect transistors (OFETs) having an active channel of solution-processed 2,7-dioctyl[1]­benzothieno­[3,2-<i>b</i>]­[1]­benzothiophene (C₈–BTBT) were investigated by Kelvin-probe force microscopy (KFM). We found step-like potential distributions in a channel region, suggesting that the interlayer resistance between the conjugated BTBT core layers is quite high and each conjugated layer is electrically isolated from one another by insulating alkyl chain layers. We also found a noticeable positive charging in the channel region especially at the step edges after the device operation. The observed charging was explained by long-lived positive charges on the trap sites, and the trap density at the step edge was estimated to be on the order of 10¹¹ cm^–2. The KFM measurements suggest that the device performance of the staggered C₈–BTBT OFETs could deteriorate due to the considerably high access resistance, which stems from the high interlayer resistance and/or by the site-specific charge trapping at the contact/semiconductor interface which originates from step edge structures

Characteristic Control of n‑Channel Organic Thin-Film Transistors Using a Dimethyl-Substituted Benzimidazole Dopant

4-(N,N-Dimethylamino)­phenyl-substituted 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N-DMBI-H) has been utilized as a solution-processable n-type dopant in organic electronics. In this study, a dimethyl-substituted N-DMBI-H derivative (DMe-N-DMBI-H), in which two methyl groups are attached at the terminal 5- and 6-positions of the benzimidazole moiety of N-DMBI-H molecule, has been examined to control its electron-donating ability. The effectiveness of DMe-N-DMBI-H as a solution-processable donor dopant has been clarified by evaluating electrical characteristics of DMe-N-DMBI-H-doped PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) thin-film transistors, such as field-effect mobility, gate threshold voltage, and contact resistance. Our electrochemical and electrical characterizations as well as quantum chemical calculations have suggested that DMe-N-DMBI-H works as a donor dopant somewhat stronger than N-DMBI-H

Alkyl and Alkoxyl Monolayers Directly Attached to Silicon: Chemical Durability in Aqueous Solutions

For practical application of self-assembled monolayers (SAMs), knowledge of their chemical durability in acidic or basic solutions is important. In the present work, a series of SAMs directly immobilized on a silicon (111) surface through Si−C or Si−O−C covalent bonds without a native oxide layer were prepared by thermally activated chemical reactions of a hydrogen-terminated Si(111) substrate with linear molecules, i.e., 1-hexadecene, 1-hexadecanol, 1-dodecanol, and n-dodecanal, to investigate the durability of the SAMs to HF and Na2CO3 solutions. While grazing incidence X-ray reflectivity measurements showed that all the as-prepared SAMs had almost the same film density and molecular coverage, keeping the original step and terrace structure of Si(111) as is observed by atomic force microscopy, they gave different degradation behaviors, i.e., pitting and concomitant surface roughening in both solutions. 1-Hexadecene SAM was stable against immersion in both solutions, while the other SAMs were damaged within 60 min, most likely due to the difference in chemical bonding modes at the SAM/Si interface, i.e., Si−C and Si−O−C

Thermal Conversion of Precursor Polymer to Low Bandgap Conjugated Polymer Containing Isothianaphthene Dimer Subunits

Thermal conversion strategy has been utilized in the synthesis of a novel low bandgap polymer containing isothianaphthene (ITN) dimer structure and benzodithiophene (BDT) unit in the backbone (PBIBDT). First, a highly soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]octadiene-fused thiophene dimer and BDT (PPBIBDT) was synthesized by a palladium(0)-catalyzed Stille coupling reaction. Then, heating of the yellow PPBIBDT film spin-coated on a glass plate yielded a dark blue film of PBIBDT that was insoluble in any organic solvents. Thermogravimetric analysis of PPBIBDT showed 14% weight loss with an onset at 230 °C, corroborating the occurrence of the thermally induced retro-Diels–Alder reaction. The PBIBDT film showed red-shifted, broad absorption in the visible and near-infrared regions with a maximum at 706 nm compared to the precursor polymer PPBIBDT with an absorption peak at 445 nm. The introduction of an ITN dimer unit in the backbone lowered the bandgap owing to the stabilized quinoid resonance structure. The field-effect hole mobility of PBIBDT was determined to be 1.1 × 10^–4 cm² V^–1 s^–1 with an on–off ratio of 2.5 × 10², while the PPBIBDT-based device revealed no p- and n-type responses. Organic photovoltaic devices were fabricated based on the planar heterojunction structure of PBIBDT and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and showed a power conversion efficiency of 0.07% under standard AM1.5 sunlight (100 mW cm^–2). These results obtained here will provide fundamental information on the design of thermally induced low bandgap polymers for device applications

Donor–Acceptor Alternating Copolymer Based on Thermally Converted Isothianaphthene Dimer and Thiazolothiazole Subunits

A novel donor–acceptor-conjugated polymer PBITT consisting of isothianaphthene (ITN) dimer donor unit and thiazolothiazole acceptor unit was synthesized by thermal conversion method. First, a soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]­octadiene (BCOD)-fused thiophene dimer and benzodithiophene (PPBITT) was synthesized by palladium(0)-catalyzed Stille coupling reaction. The BCOD moiety underwent the retro-Diels–Alder reaction by the thermal treatment of a red PPBITT film to afford a dark blue film of PBITT that was insoluble in any organic solvents. The optical bandgap of PBITT (1.3 eV) became significantly narrow compared with that of PPBITT (2.1 eV) due to the stabilized quinoid resonance structure of the PBITT main chain. The field-effect hole mobility (μ_h) of PBITT was determined to be 2.2 × 10^–4 cm² V^–1 s^–1 with on–off ratio (<i>I</i>_on/<i>I</i>_off) of 2.5 × 10², whereas the corresponding PPBITT-based device did not show any p- and n-type response. Organic photovoltaic (OPV) devices were fabricated based on the bulk heterojunction film of the polymers and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). The device with the PBITT:PCBM film exhibited higher short-circuit current and lower open-circuit voltage than those of the PPBITT:PCBM-based device, resulting in the comparable power conversion efficiency (∼0.3%). These results obtained here will provide fundamental information on the design of thermally induced donor–acceptor alternating polymers for organic electronics

A Photoconductive, Thiophene–Fullerene Double-Cable Polymer, Nanorod Device

Gold/double-cable copolymer/gold multisegmented nanorods were prepared electrochemically via a template-based method. These “bulk heterojunction” nanorods showed photoconductivity providing us with a platform to study photoinduced charge separation/transport at the nanointerface and begin to think about the rational design of nanoscale solar cells based on such structures