Jea Uk Lee

Fabrication of Highly Conductive and Transparent Thin Films from Single-Walled Carbon Nanotubes Using a New Non-ionic Surfactant via Spin Coating

Oligothiophene-terminated poly(ethylene glycol) was synthesized and used as a non-ionic and amphiphilic surfactant for fabricating high-quality single-walled carbon nanotube (SWCNT) films by a simple spin coating method. The absence of charge repulsion between SWCNT/surfactant complexes successfully leads to formation of a dense network of SWCNTs on the substrate through a single deposition of spin coating. When the SWCNT film was treated with nitric acid and thionyl chloride after washed with dichloromethane and water, a high-performance SWCNT film with the sheet resistance of 59 ohm/sq and the transparency of 71% at 550 nm was successfully obtained. Since the SWCNT film exhibits a high value of σ(dc)/σ(ac) (∼17) and excellent dimensional stability after releasing from the substrate, the film can be used as a transparent electrode in flexible optoelectronic devices.

Synthesis and photophysical property of well-defined donor–acceptor diblock copolymer based on regioregular poly(3-hexylthiophene) and fullerene

A new, well-defined diblock copolymer (P3HT-b-C60) based on regioregular poly(3-hexylthiophene) (P3HT) and fullerene was synthesized. First, regioregular P3HT was synthesized through Grignard metathesis polymerization, and then methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) were copolymerized by using an end-functionalized P3HT as a macroinitiator for the atom transfer radical polymerization to yield a diblock copolymer (P3HT-b-P(MMA-r-HEMA)). A fullerene derivative functionalized with carboxylic acid, [6,6]-phenyl-C61-butyric acid (PCBA), was then chemically linked to the HEMA unit in the second block (P(MMA-r-HEMA)) to produce a diblock copolymer with the second block containing fullerenes. Annealing thin films of the copolymer revealed nanometer-scale phase separation, a more suitable morphology for enabling excitons generated in the P3HT domain to more efficiently reach the donor–acceptor interface, relative to simple blends of P3HT and C60. As a result, photoluminescence of the P3HT-b-C60diblock copolymer in the films showed a complete quenching of photoluminescence of P3HT, which is indicative of charge transfer between P3HT and fullerene.

Synthesis of C60-end capped P3HT and its application for high performance of P3HT/PCBM bulk heterojunction solar cells

A new C60-end capped poly(3-hexylthiophene) (P3HT-C60) was synthesized via a simple three-step process, and used as a compatibilizer for P3HT/PCBM composite for the purpose of controlling the morphology of P3HT/PCBM composite film, and thus improving the long-term thermal stability of solar cell performance. When a small amount of P3HT-C60 was added to P3HT/PCBM, the bicontinuous and nanometre-scale film morphology was developed and preserved for 2 h of annealing at 150 °C. Furthermore, the addition of P3HT-C60 as a compatibilizer suppressed large-scale phase separation of P3HT/PCBM composite even after prolonged annealing time (8 days), and as a result, the P3HT/PCBM/P3HT-C60 bulk heterojunction solar cells exhibited the excellent long-term thermal stability of device performance.

Degradation and stability of polymer-based solar cells

Stability of polymer solar cells (PSCs) is critically important for PSCs to be commercialized. The performance deterioration of PSCs arises mainly from macrophase separation of the finely tuned nanoscale morphology of donor–acceptor blends, photo-degradation of active layer materials, and oxidative degradation of donor polymers due to diffusion of oxygen and water molecules from the interlayer/electrode. In this article, the degradation mechanisms of various types of active layer materials are discussed and the methods how to protect the active layer materials from degradation to stabilize the device performance of PSCs are extensively discussed based on recent publications.

Morphology control of a polythiophene-fullerene bulk heterojunction for enhancement of the high-temperature stability of solar cell performance by a new donor-acceptor diblock copolymer.

A well defined diblock copolymer (P3HT-b-C(60)) based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was synthesized via two controlled polymerization steps and used as a compatibilizer for the P3HT/PCBM blend, which has widely been used as an active layer in bulk heterojunction polymer solar cells. The addition of a small amount of P3HT-b-C(60) results in not only the reduction of phase size of P3HT/PCBM blend but also the suppression of macrophase separation for long-time thermal annealing owing to the preferential location of the diblock copolymers at the interface between P3HT and PCBM phases. The morphology change with the annealing time is closely related to the change of the power conversion efficiency (PCE) of solar cells: the PCE of P3HT/PCBM greatly decreases with increasing annealing time while the addition of P3HT-b-C(60) significantly reduces the decrease of PCE for long-time thermal annealing.

Facile Method to Functionalize Graphene Oxide and Its Application to Poly(ethylene terephthalate)/Graphene Composite

Graphene oxide (GO) prepared in bulk quantities by oxidation of graphite with strong oxidants contains many hydrophilic groups, such as hydroxyl, epoxy, and carboxyl acid. We present a method to efficiently convert these hydrophilic groups into alkyl and alkyl ether groups by a one step reaction of bimolecular nucleophilic substitution with alkyl bromide. The functionalized graphene oxide (fGO) can be homogeneously dispersed as exfoliated monolayers in various organic solvents without degradation of size and shape of graphene oxide sheet. The degree of substitution reaction of each hydrophilic group in GO with alkyl bromide is quantitatively determined by comparing the deconvoluted O 1s X-ray photoelectron spectrum of GO with that of fGO. Addition of a small amount of fGO in poly(ethylene terephthalate) (PET) improves remarkably tensile and gas barrier properties of PET/fGO composite due to homogeneous dispersion of fGO sheets in PET matrix.

Direct exfoliation of graphite using a non-ionic polymer surfactant for fabrication of transparent and conductive graphene films

A high-yielding dispersion of graphene at high concentration in solvent is critical for practical applications. Herein, we demonstrate the formation of a stable dispersion of pristine graphene in ethanol by exfoliating graphite flakes into individual graphene layers using a non-ionic polymer surfactant under bath-type sonication. Oligothiophene-terminated poly(ethylene glycol) was synthesized and used as a non-ionic and amphiphilic surfactant for exfoliating graphite into graphene. A high-quality graphene film was fabricated from the exfoliated graphene solution by the vacuum filtration method. TEM and SEM reveal that the size of exfoliated graphene flakes is larger than 1 μm. When the graphene film was treated with nitric acid and thionyl chloride after washing with solvent, the film showed high performance with a sheet resistance of 0.3 kΩ sq−1 and a transparency of 74% at 550 nm.

Efficiency enhancement of P3HT/PCBM bulk heterojunction solar cells by attaching zinc phthalocyanine to the chain-end of P3HT

A new solution processable zinc phthalocyanine dye (ZnPc), as an interface modifier between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in bulk heterojunction solar cells, was successively synthesized and linked to the chain-end of P3HT through the formation of a coordination complex. ZnPc dye molecules do not aggregate but preferentially locate at the interface between P3HT and PCBM, and thus contribute to the photocurrent generation by both direct photo-excitation and enhancement of charge transfer between P3HT and PCBM. To localize the zinc phthalocyanine dyes at the donor–acceptor interface more effectively, another new organic dye molecule, fullerene-functionalized zinc phthalocyanine (ZnPc-C60) was also synthesized and linked to the chain-end of the P3HT, where ZnPc-C60 contributes not only to the photocurrent generation by direct photo-excitation, but also lowers the interfacial tension, resulting in the reduction of the domain size and the suppression of the macrophase separation of the P3HT/PCBM blend for prolonged thermal annealing. This leads to higher device efficiency with 20% enhancement of the short circuit current and to enhancement of long-term thermal stability of device performance as compared to that of the reference P3HT/PCBM device.

Supramolecular Assembly of End-Functionalized Polymer Mixtures Confined in Nanospheres

Supramolecular assembly of functionalized polymers, capable of forming block copolymer-like molecular clusters, has emerged as a promising alternative for creating nanoscopically ordered structures. Here, we demonstrate that nanospheres, which have intriguing internal nanodomains and controllable surface functionality, can be fabricated by supramolecular assembly of two complementarily end-interacting species of mono-end-functionalized polymers using the self-organized precipitation (SORP) method. An exotic internal morphology, hierarchically organized structure of perforated spherical layers, was formed inside the nanosphere prepared from the stoichiometric mixture of the end-functionalized polymers, which is due to the formation of diblock-like supramolecules and their packing frustration in the spherically confined nanospace. When the mixing ratio of the two end-functionalized polymers differs from the stoichiometric ratio, the nanoparticle surface is enriched with an excess of unpaired functionalized groups, which therefore provides us with a useful way to precisely control the surface functionality of the nanoparticles.

Synthesis of fluorinated amphiphilic triblock copolymer and its application in high temperature PEM fuel cells

New fluorinated amphiphilic triblock copolymers composed of fluorinated poly(arylene ether) and sulfonated poly(styrene-co-acrylonitrile) were synthesized through condensation polymerization followed by controlled radical polymerization, for application in polymer electrolyte membrane fuel cells. The morphological structure of the triblock copolymers was investigated as a function of hydrophilic block length, and was related to the proton conductivity and durability of the copolymer membranes. Unlike commercially available Nafion, which shows a decrease of the proton conductivity at high temperatures above 80 °C, the newly synthesized membranes exhibit high proton conductivity and continuous increases in proton conductivity with increasing temperature from 30 °C to 100 °C. They also show excellent thermal, oxidative and hydrolytic stabilities.