Kyungkon Kim

High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1- phenyl-(6,6)C61 blends

The effects of annealing and fullerene loading in regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) based bulk heterojunction photovoltaics have been investigated. Under specific loading and annealing conditions, a combination of morphological and electronic factors can be brought to play to achieve optimal filling factors, open-circuit voltage (Voc), and short-circuit current density (Jsc). We demonstrate that this occurs at surprisingly low loadings of PCBM and annealing temperatures nearing the melting point of the polymer. Further, we report power conversion efficiencies approaching 5% in the P3HT:PCBM system.

Roles of donor and acceptor nanodomains in 6% efficient thermally annealed polymer photovoltaics

The authors have fabricated thin film polymer photovoltaics using 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 within regioregular poly(3-hexylthiophene) bulk heterojunction absorbing layers. Using thermal annealing at temperatures approaching the glass transition temperature, they have examined the formation of nanodomains within the matrix. These domains modify charge transport pathways in such a way as to allow for the efficient use of thicker absorbing layers. This results in a nearly 20% gain in overall performance for this polymer system with external power efficiencies exceeding 6%.

Selective positioning of organic dyes in a mesoporous inorganic oxide film

Although sequential adsorption of dyes in a single TiO(2) electrode is ideal to extend the range of light absorption in dye-sensitized solar cells, high-temperature processing has so far limited its application. We report a method for selective positioning of organic dye molecules with different absorption ranges in a mesoporous TiO(2) film by mimicking the concept of the stationary phase and the mobile phase in column chromatography, where polystyrene-filled mesoporous TiO(2) film is explored for use as a stationary phase and a Brønsted-base-containing polymer solution is developed for use as a mobile phase for selective desorption of the adsorbed dye. By controlling the desorption and adsorption depth, yellow, red and green dyes were vertically aligned within a TiO(2) film, which is confirmed by an electron probe micro-analyser. The external quantum efficiency (EQE) spectrum from a solar cell with three selectively positioned dyes reveals the EQE characteristics of each single-dye cell.

Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C60 bulk heterojunction photovoltaic devices

We obtained improved poly(3-octylthiophene) (P3OT)/C60 bulk heterojunction photovoltaic devices by doping with stable and highly electrically conductive Au and Ag nanoparticles. Doped devices showed 50–70% improved efficiency, with the Ag nanoparticles exhibiting the greatest increase in efficiency. We suggest that a dominate mechanism for the efficiency enhancement of doped photovoltaic device is the improved electrical conductivity through the introduction of “dopant” levels within the band gap of the P3OT.

A highly efficient organic sensitizer for dye-sensitized solar cells

We have synthesized a highly efficient organic dye for a dye-sensitized solar cell; the overall solar-to-energy conversion efficiency was 9.1% at AM 1.5 illumination (100 mW cm(-2)): short-circuit current density (J(sc)) = 18.1 mA cm(-2), open circuit photovoltage (V(oc)) = 743 mV and fill factor (ff) = 0.675.

High performance organic photosensitizers for dye-sensitized solar cells

We report highly efficient organic photosensitizers containing pi-conjugated alkoxy-substituted oligophenylenevinylene linkers with electron donor-acceptor units for dye-sensitized solar cells. TA-DM-CA showed an overall solar-to-energy conversion efficiency of 9.67% at AM 1.5 illumination (100 mW cm(-2)).

Thienyl-substituted methanofullerene derivatives for organic photovoltaic cells

A series of thienyl-substituted methanofullerenes as electron acceptors for bulk-heterojuction solar cells with poly(3-hexylthiophene) (P3HT) were synthesized and characterized with respect to electrochemical and photophysical properties. The first one-electron reduction potentials of the higher adducts are shifted toward more negative values by ∼100 mV as compared to the monoadduct. As a result, the solar cells composed of the bisadduct (2) and trisadduct exhibit a larger open-circuit voltage (Voc) than the solar cell composed of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as they have higher LUMO energy levels. Photophysical studies on spin-coated films by femtosecond laser flash photolysis indicate that ultrafast electron transfer from the P3HT donor polymer to all of the fullerene derivatives occurs to form the bound radical pair (BRP) state. No decay of the BRP state of P3HT:thienyl-substituted monoadduct (1) films was observed in the time range of 3 ns. The AFM investigation on P3HT:1 and P3HT:higher adduct films after thermal annealing showed fine donor and acceptor domains and larger domains, respectively. The bulk-heterojunction (BHJ) solar cells based on P3HT:1 exhibited a power conversion efficiency (PCE) of 3.97%, which is comparable with that of the P3HT:PCBM cell. The P3HT:2 based cell showed a PCE value of 1.72% with a higher open-circuit voltage of 0.72 V.

Formation of efficient dye-sensitized solar cells by introducing an interfacial layer of long-range ordered mesoporous TiO2 thin film.

Long-range ordered cubic mesoporous TiO 2 films with 300 nm thickness were fabricated on fluorine-doped tin oxide (FTO) substrate by evaporation-induced self-assembly (EISA) process using F127 as a structure-directing agent. The prepared mesoporous TiO 2 film (Meso-TiO 2) was applied as an interfacial layer between the nanocrystalline TiO 2 film (NC-TiO 2) and the FTO electrode in the dye-sensitized solar cell (DSSC). The introduction of Meso-TiO 2 increased J sc from 12.3 to 14.5 mA/cm (2), and V oc by 55 mV, whereas there was no appreciable change in the fill factor (FF). As a result, the photovoltaic conversion efficiency ( eta) was improved by 30.0% from 5.77% to 7.48%. Notably, introduction of Meso-TiO 2 increased the transmittance of visible light through the FTO glass by 23% as a result of its excellent antireflective role. Thus the increased transmittance was a key factor in enhancing the photovoltaic conversion efficiency. In addition, the presence of interfacial Meso-TiO 2 provided excellent adhesion between the FTO and main TiO 2 layer, and suppressed the back-transport reaction by blocking direct contact between the electrolyte and FTO electrode.

Highly durable and flexible dye-sensitized solar cells fabricated on plastic substrates: PVDF-nanofiber-reinforced TiO2 photoelectrodes

In this study, we developed a novel nanostructured polymer nanofiber/TiO2 nanoparticle composite photoelectrode with high bendability by a spray-assisted electrospinning method. The composite film is used as the photoelectrode in plastic dye-sensitized solar cells (DSCs). The polymer/TiO2 composite photoelectrode has a structure similar to that of a fiber-reinforced composite; the matrix of the composite photoelectrode contains TiO2 nanoparticles, and PVDF nanofibers are embedded in this matrix. Compared to conventional DSCs, composite-based DSCs show outstanding bending stability because the polymer nanofibers prevent delamination of the electrode by relieving the external stress and effectively retarding crack generation and propagation. Moreover, the efficiency of the cell containing composite electrodes is comparable to that of a cell containing only TiO2, suggesting that the proposed PVDF-nanofiber-reinforced photoelectrode is a promising candidate for a bendable photoelectrode in high-efficiency flexible plastic DSCs.

Synthesis and luminescence properties of four-armed conjugated structures containing 1,3,4-oxadiazole moieties

New 9,10-bis(phenylethynyl)anthracene cored, four-armed conjugated structures in which the 1,3,4-oxadiazole ring is utilized to link the surface alkyl tail groups, were synthesized and the luminescence properties of thin films prepared therefrom were studied. It was found that the four-armed compound (ANTH-OXA6t-OC12) with the dodecyloxy surface group is a high glass transition temperature (Tg: 211 °C) material and exhibits good solubility in common organic solvents and, surprisingly, results in good quality thin films by spin-coating from solutions. In contrast, the compound with tert-butyl surface groups shows poor solubility and gives films of inferior quality. When the films were exposed to UV light of 335 nm, they emitted mainly strong red light (500–750 nm) with much weaker emission at 375–450 nm. The (phenylethynyl)anthracene core is the only red light emitting chromophore in the structure, and the diphenyl oxadiazole moieties are the shorter wavelength light emitters. In contrast, an electroluminescence (EL) device having the configuration of ITO/PEDOT-PSS/ANTH-OXA6t-OC12/Li ∶ Al emits only red light with an external quantum efficiency of 0.02% indicating that the outer diphenyl oxadiazole moieties act only as electron transporters.