Je-Jung Yun

Photosensitization of nanoporous TiO2 films with natural dye

Dye-sensitized solar cells were assembled by using natural dyes extracted from wormwood, bamboo leaves and red maple leaves as photosensitizers. The Voc values of natural dyes from wormwood, bamboo leaves and maple leaves were 0.67, 0.66 and 0.59, respectively. The ISC values were varied from 2.56 to 5.95 mA cm−2 and the fill factors from 56 to 66%. Based on investigations into the structure and properties of dye molecules, wormwood revealed the best photosensitized effects among several kinds of natural dyes, which is due to the interaction between the carbonyl and hydroxyl groups of anthocyanin and the surface of TiO2; these wormwood extracts act as efficient sensitizers.

Chlorophyll-layer-inserted poly(3-hexyl-thiophene) solar cell having a high light-to-current conversion efficiency up to 1.48%

We report the fabrication of a chlorophyll-layer-inserted poly(3-hexyl-thiophene) (P3HT) solar cell. A significant enhancement in the light-to-current conversion efficiency of up to 1.48% with a fill factor of 0.32 was achieved in a solar cell with a device structure of indium tin oxide anode/poly (3,4-ethylene dioxy-thiophene):poly(styrene sulfonate)/P3HT/chlorophyll/Al cathode under the standard air mass 1.5 irradiation (20mW∕cm2). These results suggest that the generation of an internal electric field is mainly due to a difference between the highest occupied molecular orbital of the P3HT (donor) and the lowest unoccupied molecular orbital of the chlorophyll (acceptor), which permits the transfer of photoinduced electrons from P3HT to chlorophyll.

204% enhanced efficiency of ZrO2 nanofibers doped dye-sensitized solar cells

Zirconia (ZrO2) nanofibers added mesoporous titania (TiO2) photoelectrode has been synthesized for dye-sensitized solar cells to enhance the efficiency of cell. The ZrO2 nanofibers had reduced the resistance of the photoelectrode as well as enhancement of the absorption spectra in the ultraviolet (UV), visible, and near infrared (IR) region. The internal resistance of the photoelectrode is one of the important factor to affects the power conversion efficiency directly. The ZrO2 nanofibers provide the more photon harvest and optimal electron pathway. Finally, about 200% increases in conversion efficiency has been achieved.

PHOTOVOLTAIC PROPERTIES OF TiO2 PHOTOELECTRODE PREPARED BY USING LIQUID PEG-EEM BINDER

We present one of the central and basic factors related to the TiO2 photoanodes of optimal absorption site. Binder is of particular importance for surfaces and interfaces that involve organic dye and TiO2 layer. We introduced monodispersed liquid copolymer binders; poly(ethylene glycol)-ethyl ether methacrylate (PEG-EEM) instead of solid PEG to increase TiO2 electrodes surface area. We attempt to investigate the morphology of the photoanodes and photovoltaic effects using field emission scanning electron microscopy (FE-SEM), BET and photovoltaic properties under illumination with AM 1.5 simulated sunlight. We achieve 167% enhanced power conversion efficiency when the optimal content of liquid PEG-EEM binder was 4 wt.% than that of PEG binder. We show that the performance of Dye-sensitized solar cell (DSSC) can be strongly improved using liquid type binder.

Enhanced stability of organic thin films for electroluminescence by photoirradiation

Enhanced stability against heat and moisture in organic vapor-deposited TPD thin film having hole transport property has been achieved by UV irradiation. The crystallization of TPD film without any treatment was started within a week even at RT and accelerated with increasing temperature under the air. By photoirradiation using UV light, however, the TPD thin film was found to be difficult to form crystallized state and showed good contact with substrate even at 60 °C and in water for more than one month. Furthermore, the molecular interdiffusion by heat between organic layers in Alq3/TPD bilayer was remarkably suppressed by the UV irradiation due to the enhanced stability of the TPD film.

Enhanced efficiency of dye-sensitized solar cells doped with green phosphors LaPO4:Ce, Tb or (Mg, Zn)Al11O19:Eu

We have successfully introduced green phosphors LaPO4:Ce, Tb (G4) or (Mg, Zn)Al11O19:Eu (G2) into TiO2 photoelectrode of dye-sensitized solar cells. The conversion efficiency of the G4-doped device was enhanced by 30% compared with the pristine TiO2 photoelectrode. The green phosphor doped at 5-wt.% ratio contributed to the reduction of resistances of the surface and interface of the photoelectrode and to the great enhancement of the absorption spectrum in UV-visible and near-infrared regions. The internal resistances and absorbance of the photoelectrode directly affect the power conversion efficiency. Green phosphor plays an important role towards the realization of high-efficiency dye-sensitized solar cells.

Electroluminescence of polymer blends composed of a PVK and a copolymer containing SiPh-PPV and MEH-PPV unit

We report on the emission characteristics of the polymeric light-emitting diodes (PLEDs) with the blend of a copolymer containing MEH-PPV and SiPh-PPV unit (m-SiPh PPV-co-MEH PPV) and a polyvinylcarbazole (PVK). According to change of their blending ratio, the PLEDs showed the change of electroluminescence(EL) intensity and EL spectrum. The EL characteristics were improved when the PVK was a small quantity. Most effective PLEDs with m-SiPh PPV-co-MEH PPV copolymer-rich blend for EL enhancement was when the blending ratio of m-SiPh PPV-co-MEH PPV : PVK equals to 70:30 by weight ratio. It is thought as the result of the m-SiPh PPV-co-MEH PPVs dilution effect and the role of the PVK with hole transport ability. On the other hand, we have attempted to tune the EL color of the m-SiPh PPV-co-MEH PPV EL cells by the blending with the PVK. According to increasing of PVK content, the emission peaks of the PLEDs have blue-shifted from red light. White emission which covers the full range of the visible region is realized when the weight ratio of m-SiPh PPV-co-MEH PPV : PVK equals to 4 : 96, which shows the CIE coordinates are x=0.3266 and y=0.3438.

Optoelectric properties in the phosphor-doped polymeric light-emitting diodes

Bluish white light emission was obtained from a phospher doped light-emitting diodes(LEDs) prepared from a poly(9- vinylcarbazole)(PVK) matrix and an inorganic phosphor as a dopant. We used PVK as an emitter and long afterglow phosphor based on Strontium Aluminate as a guest emitter. The turn-on voltage of the phospher doped PVK LEDs is about 6.5 V, which was lower than the undoped PVK with a voltage of 12V. By doping with inorganic phosphor, the maximum luminescence is about much higher than that of the pure PVK LEDs. External quantum efficiency of the doped PVK LEDs was improved by two times compared to that of pure PVK LEDs. An additional emission observed in electrophosphorescence of the doped PVK LEDs at 630 nm is tentatively assigned to energy transfer from PVK to phospher.

The Doping Effect of Long Afterglow Phosphorescent Pigments in the Polymeric Light-Emitting Diodes

We have fabricated polymeric light-emitting diodes(PLEDs), utilizing inorganic phosphor based on Strontium Aluminate as dopant. It was used the two types of polymeric LEDs as follows: 1) ITO/PVK/Al and 2) ITO/phosphor doped PVK/Al. The EL onset voltage is about 6.5 V for the latter device, lower than the former with a voltage of 12 V. By doping with inorganic the phosphor, the maximum luminescence value is about 23 times higher than the pure PVK device.

Bulk Heterojunction Photovoltaic Cells Based On Poly(3-hexyltiophene) and Pyran-4-ylidene-2-cyanoacetate Derivative

The effects of organic dye, pyran-4-ylidene-2-cyanoacetate derivative, and annealing on the dye-doped bulk heterojunction photovoltaics (PV) based with poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-(6,6)C61 (PCBM) have been investigated. The structures of the PV cells were ITO/PSS:PEDOT/P3HT:PCBM (1:1 wt.%)/Al and ITO/PSS:PEDOT/P3HT:PCBM:Pyran-4-ylidene-2-cyanoacetate derivative (1:1:0.5 wt.%)/Al electrode. Using the thermal treatment of the PV cell containing the organic dye at 140°C for 5 min, an increase in power conversion efficiency to 2.0% and external quantum efficiency (IPCE) to 36% are demonstrated.