Wen-Chang Chen

Improved Charge Transport and Absorption Coefficient in Indacenodithieno[3,2‐b]thiophene‐based Ladder‐Type Polymer Leading to Highly Efficient Polymer Solar Cells

A novel ladder-type donor (IDTT) is developed by substituting the two outward thiophenes of the IDT donor with two thieno[3,2-b]thiophenes. The polymer derived from this donor possesses longer effective conjugation and better planarity, which improves electron delocalization along the polymer backbone and charge mobility. The polymer solar cell device using PIDTT-DFBT shows a high power conversion efficiency of 7.03% with a large open-circuit voltage of 0.95 V without using any additives or post-solvent/thermal annealing processes.

Effective interfacial layer to enhance efficiency of polymer solar cells via solution-processed fullerene-surfactants

Two methanol-soluble fullerene surfactants have been developed as interfacial layers for cathodes in polymer solar cells. These surfactants facilitate the tuning of cathode work function and extraction of electrons that significantly enhance open-circuit voltage and photocurrent generation. The performance of bulk heterojunction solar cells based on using these surfactant-modified cathodes improved significantly to afford high power conversion efficiencies (as high as 6.63% for a Ag cathode).

Selenophene-DPP donor–acceptor conjugated polymer for high performance ambipolar field effect transistor and nonvolatile memory applications

New donor–acceptor conjugated copolymer, PSeDPP, consisting of selenophene (Se) and 3,6-dithiophen-2-yl-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP), was developed for high performance ambipolar field-effect transistors (FETs) and nonvolatile memory applications. The maximum absorption peak and optical band gap of PSeDPP thin film were observed at 849 nm and 1.29 eV, respectively, indicating the strong intramolecular and intermolecular charge transfer. The polymer crystallinity and FET mobility was significantly enhanced as increasing the solvent boiling point and thermal treatment based on the results from four processing solvents of chloroform, chlorobenzene, o-dichlorobenzene and 1,2,4-trichlorobenzene, as evidenced by TEM, XRD, and AFM. The PSeDPP-based FET processed from1,2,4-trichlorobenzene exhibited a dense nanofiber morphology with lamellar chain packing, leading to the relatively high hole and electron mobility up to 1.62 and 0.14 cm2 V−1 s−1, respectively. PSeDPP was also found to exhibit the first transistor memory characteristics for ambipolar conjugated polymers. The retention time of the FET-based nonvolatile memory devices could maintain the high- and low-conductance states longer than 104 s, and the on-off current ratios of 103–104 at the read voltage of 0 V. These results revealed that PSeDPP had potential applications for flexible electronic device applications.

Donor–acceptor polymers for advanced memory device applications

Organic materials for memory devices have attracted significant scientific interest recently due to the advantages of rich structure flexibility, low cost, solution processability, and three-dimensional stacking capability. This minireview highlights the recent developments in donor–acceptor polymers for resistive switching memory device applications including conjugated polymers, functional polyimides, non-conjugated pendent polymers, and polymer composites. It emphasizes the structure-memory characteristic relationship of donor–acceptor polymers for advanced memory device applications.

Transparent organic-inorganic hybrid thin films prepared from acrylic polymer and aqueous monodispersed colloidal silica

Highly transparent hybrid films containing nano-sized silica domain were synthesized from acrylic polymer and aqueous monodispersed colloidal silica (CS) with a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA). The silica content in the hybrid thin films was varied from 0 to 50 wt.%. The experimental results showed that the silica particle size in the precursor solution and the hybrid films was varied from 20 to 40 nm. It could be controlled by the mole ratio of MSMA to silica. The results of scanning electron microscope (SEM), transmission electron microscope (TEM), and elemental analysis support the above conclusion. The prepared hybrid films showed high film uniformity and optical transparence. The thermal stability of the prepared hybrid films increased with the increasing silica content. The refractive index decreased linearly with the increasing silica fraction in the hybrid films. The experimental results suggest that the hybrid thin films have potential applications as passive films for optical devices.

Cesium carbonate as a functional interlayer for polymer photovoltaic devices

The device characteristics of polymer solar cells with cesium carbonate (Cs2CO3) as an electron-injection interlayer have been investigated. It is found that the insertion of Cs2CO3 at the cathode interface improves the device power conversion efficiency from 2.3% to 3.1%. In order to further understand the mechanism, the interfacial interaction between the active organic layer and the cathode was studied by x-ray photoemission spectroscopy (XPS). The results of XPS measurement indicate the fact that a portion of electrons transfer from the interlayer into the organic layer, resulting in n-type doping. The n-doping effect enhances the efficiency of electron injection and collection. Further, the maximum open-circuit voltage (Voc) was determined from its temperature dependence. For the device with Cs2CO3, the maximum Voc is extremely close to the corresponding value of the energy difference between the highest occupied molecular orbital of the electron donor and the lowest unoccupied molecular orbital of t...

The structures and properties of hydrogen silsesquioxane (HSQ) films produced by thermal curing

The structures and properties of hydrogen silsesquioxane (HSQ) films produced by curing were studied in the temperature range of 240–340 °C for various curing times. The experimental results show that the transformation of the cage structure to the network structure is the major reaction in the studied temperature range. The cage–network transformation can be explained by two-stage zero order kinetics. The rate constant of the first stage is 10–35 times more than that of the second stage. The activation energy and frequency factor of the cage–network transformation are 64.63 kJ mol−1 and 2.76 × 104 s−1 for the first stage while those of the second stage are 38.59 kJ mol−1 and 4.11 s−1, respectively. The difference is probably because the network structure of the second stage limits the structural transformation and results in a small frequency factor. The porosity of the cured HSQ films increases rapidly with curing time in the first 10 min and then slowly for the remaining time. The FE-SEM (Field Emission-Scanning Electron Miscroscope) result suggests the formation of nano-pores in the cured film. The evolution of porosity is probably due to the outgassing of the reaction side-product (SiH4), the trapped solvent (4-methylpentan-2-one) or the cage/network transformation. The last two factors contribute significantly as shown by the refractive index results of the cured films. The increasing film thickness with increasing curing time and temperature indicates the evolution of porosity in the HSQ film.

Polymeric charge storage electrets for non-volatile organic field effect transistor memory devices

In this review, we present the effects of the chemical structure and composition of polymer or composite electrets on tuning the memory characteristics of the non-volatile organic field effect transistor (OFET) memory devices, including surface polarity, π-conjugation length, architecture, donor–acceptor strength, and interface energy barrier. The recent progress in developing polymer-based charge storage electrets is highlighted in order to provide insights into understanding the operation mechanism and the molecular design–memory properties relationship, as well as improving the overall performance of OFET memory devices.

High refractive index polyimide–nanocrystalline-titania hybrid optical materials

In this study, a new synthetic route was developed to prepare polyimide–nanocrystalline-titania hybrid materials with a relatively high titania content (up to 90 wt%). A soluble polyimide with carboxylic acid end groups (6FDA–6FpDA–COOH) was first synthesized from 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′-(hexafluoroisopropylidene) dianiline (6FpDA), and 4-aminobenzoic acid (4ABA). Such end groups could undergo an esterification reaction with titanium butoxide and provide organic–inorganic bonding. A homogeneous hybrid solution was obtained through the mole ratio of titanium butoxide/carboxylic acid, water/acid content, and a mixed solvent system. HRTEM and XRD results indicated the formation of nanocrystalline-titania domains of around 3–4 nm in the hybrid films. The spin-coated hybrid films had relatively good surface planarity, high thermal stability, tunable refractive index (1.571 < n < 1.993), and optical transparency in the visible range. The achieved refractive index of 1.943 is the highest among the polymer–titania based hybrid materials as far as we know. A three-layer anti-reflection coating based on the prepared hybrid films was designed and possessed a reflectance of less than 0.5% in the visible range. Our study demonstrates a new route for preparing high refractive index polyimide–nanocrystalline-titania hybrid materials for potential optical applications.

Large third‐order optical nonlinearities in organic polymer superlattices

Large second hyperpolarizabilities, 〈γxxxx〉, have been measured in solutions of two recently prepared organic polymer superlattices. These compounds have period aromatic and quinoidal structures and differ from each other by a side group substituent. The values observed are 1.6×10−29 esu for the parent copolymer and 3.7×10−30 esu for its acetoxy derivative. The corresponding values of χ(3)xxxx for the solid phase are estimated to be 2.7×10 −7 and 4.5×10−8 esu. Time‐resolved degenerate four‐wave mixing measurements, made at 532 nm, showed that the dynamics of the parent copolymer were faster than the 25 ps resolution of the instrument while the derivative exhibited an additional slower response component.