Wonjoo Lee

Enhanced conversion efficiency in dye-sensitized solar cells based on hydrothermally synthesized TiO2-MWCNT nanocomposites.

A 50% enhancement in the conversion efficiency (4.9-7.37%) is realized in dye-sensitized solar cells using hydrothermally synthesized TiO(2)-multiwalled carbon nanotube (MWCNT) nanocomposites as compared to hydrothermally synthesized TiO(2) without MWCNT and Degussa P25. Several characterizations have been employed to reveal the nature of the modification imparted to the MWCNTs under hydrothermal processing conditions and the resulting TiO(2)-MWCNT conjugation through -COOH groups. Efficient charge transfer in the nanocomposite and efficient electron transport by MWCNT (significantly higher incident-photon-to-current conversion efficiency) are suggested to be the possible reasons for the enhancement.

Achievement of 4.51% conversion efficiency using ZnO recombination barrier layer in TiO2 based dye-sensitized solar cells

The authors report the use of chemically deposited ZnO recombination barrier layer for improved efficiency of TiO2 based dye-sensitized solar cells. The ZnO layers of different thicknesses were deposited on spin coated porous TiO2. The presence of ZnO over TiO2 was confirmed by x-ray diffraction, electron dispersive x-ray analysis, and supported by x-ray photoelectron spectroscopy, proved inherent energy barrier between the porous TiO2 electrode and lithium iodide electrolyte. They found that TiO2 based dye-sensitized solar cell with 30nm ZnO layer thickness showed 4.51% efficiency due to the formation of efficient recombination barrier at electrode/electrolyte interface. Further increase in ZnO barrier thickness may leak the electrons injected from the dye due to its low electron effective mass of 0.2me.

Enhanced charge collection and reduced recombination of CdS∕TiO2 quantum-dots sensitized solar cells in the presence of single-walled carbon nanotubes

This paper reports the modification of CdS∕TiO2 quantum-dot sensitized solar cells by using single-walled carbon nanotubes (SWCNTs) on indium-doped tin oxide (ITO) electrodes. The presence of the SWCNT layers on an ITO electrode increased the short-circuit current under the irradiation condition and also reduced the charge recombination process under the dark condition. The power conversion efficiency of CdS∕TiO2 on ITO increased 50.0% in the presence of SWCNTs due to the improved charge-collecting efficiency and reduced recombination.

Enhanced conversion efficiency in dye-sensitized solar cells based on ZnO bifunctional nanoflowers loaded with gold nanoparticles

The ZnO nanoflowers loaded with gold (Au) nanoparticles (NPs) are synthesized by a hydrothermal route using mixed precursors and controlled conditions. The dye-sensitized solar cells based on the ZnO nanoflowers with Au NPs show power conversion efficiency of 2.5%, which is considerably higher than that of ZnO nanoflowers without Au NPs. Detailed characterizations are performed, presented, and discussed.

TiO2 nanotubes with a ZnO thin energy barrier for improved current efficiency of CdSe quantum-dot-sensitized solar cells

This paper reports the formation of a thin ZnO energy barrier between a CdSe quantum dot (Q dots) sensitizer and TiO2 nanotubes (TONTs) for improved current efficiency of Q dot-sensitized solar cells. The formation of a ZnO barrier between TONTs and the Q dot sensitizer increased the short-circuit current under illumination and also reduced the dark current in a dark environment. The power conversion efficiency of Q dot-sensitized TONT solar cells increased by 25.9% in the presence of the ZnO thin layer due to improved charge-collecting efficiency and reduced recombination.

Nanocrystalline CdS-water-soluble conjugated-polymers: High performance photoelectrochemical cells

Enhanced photoelectrochemical cell performance of nanocrystalline CdS-water-soluble conjugated-polymer sensitizers was demonstrated. The water-soluble polymers with quaternary pyridinium salts can be easily layered after dipping nanocrystalline CdS films in the aqueous polymer solution. The 2.37% energy conversion efficiency of CdS-poly(2-ethynyl-N-carboxypropylpyridiniumbromide) (LM 3) conjugated-polymer sensitizer was significantly higher than that of the bare (0.57%) and CdS-poly(2-ethynyl-N-aminopropylpyridiniumbromide) (LM 2) conjugated-polymer sensitizer (2.15%) under AM1.5 condition (80mW∕cm2).

Enhanced photovoltaic performance of Cu-based metal-organic frameworks sensitized solar cell by addition of carbon nanotubes

In the present work, TiO2 nanoparticle and multi-walled carbon nanotubes composite powder is prepared hydrothermally. After doctor blading the paste from composite powder, the resulted composite film is sensitized with Cu-based metal-organic frameworks using a layer-by-layer deposition technique and the film is characterized using FE-SEM, EDX, XRD, UV/Visible spectrophotometry and photoluminescence spectroscopy. The influence of the carbon nanotubes in photovoltaic performance is studied by constructing a Grätzel cell with I3−/I− redox couple containing electrolyte. The results demonstrate that the introduction of carbon nanotubes accelerates the electron transfer, and thereby enhances the photovoltaic performance of the cell with a nearly 60% increment in power conversion efficiency.

Manipulating interfaces in a hybrid solar cell by in situ photosensitizer polymerization and sequential hydrophilicity/hydrophobicity control for enhanced conversion efficiency

The polyacetylene photosensitizer with quaternary pyridinium salts was layered on CdS nanoparticles films by in situ polymerization of 2-ethynylpyridine and 4-bromobutyric acid. The hydrophilic nature of the polyacetylene is shown to enhance the interfacial contact and electrical coupling between hydrophilic CdS and the polymer. The hydrophilicity of the polymer was modified toward hydrophobicity by anion exchange in order to adequately layer the hydrophobic poly(3-hexylthiophene) by spin coating, power-conversion efficiency 1.18% (AM1.5, I=100mW∕cm2).

Facile interfacial charge transfer across hole doped cobalt-based MOFs/TiO2 nano-hybrids making MOFs light harvesting active layers in solar cells

Efficient separation of charges and their mobility are key challenges in metal–organic-framework (MOF) based devices. In the present study, thin films of cobalt-based metal organic frameworks (MOFs) are synthesized using a layer-by-layer technique, and their electrical/optoelectronic properties are studied. The as-prepared MOF films show electrically insulating behavior, which after hole doping demonstrate p-type conduction behaviour. The measured HOMO–LUMO energy states of the MOF films are found to be well matched for sensitizing TiO2, and the photoluminescence quenching experiment demonstrates a facile photoelectron transfer path from the doped frameworks to TiO2. Consequently, the doped MOFs are employed successfully as light harvesting and charge transporting active layers in a fully devised TiO2-based solar cell. Two different organic ligands viz., benzene dicarboxylic acid and naphthalenedicarboxylic acid are used to synthesize two kinds of Co–MOFs having different geometrical dimensions of unit cells and pores, and their influence on hole doping and charge transportation is studied. Under optimized conditions, the Co–MOF based device demonstrates a solar-to-electric energy conversion efficiency of 1.12% with a short circuit current of 2.56 mA cm−2, showing promising future prospects of the application of Co–MOFs in photovoltaic devices. Further, the photovoltaic performance of the Co–MOF based device is comparatively studied with that of the previously reported Cu–MOF and Ru–MOF based similar devices, and the influence of different metal centers of MOFs on their light harvesting performance is discussed.

Enhanced charge-collection efficiency of In2S3∕In2O3 photoelectrochemical cells in the presence of single-walled carbon nanotubes

This letter reports on the efficiency of In2S3∕In2O3 photoelectrochemical cells by enhanced charge collection and reduced recombination reaction in the presence of single-walled carbon nanotubes (SWCNTs) on indium-doped tin oxide substrate. Nanocomposite system as In2S3∕In2O3/SWCNTs was assembled using spray-coating and wet chemical processes. Due to enhanced charge collection and reduced recombination in the presence of SWCNTs, 52.9% increment of power conversion efficiency is achieved compared to those without SWCNTs layers.