Soonchul Kang

Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.

Light Harvesting and Energy Transfer in Multiporphyrin‐Modified CdSe Nanoparticles

Multiporphyrin-modified CdSe nanoparticles (CdSe-H2P) were prepared to elucidate the interaction between chromophores and luminescent semiconducting nanoparticles in the excited and ground states. The CdSe-H2P nanoparticles were obtained by place-exchange reactions of hexadecylamine-thiophenol-modified CdSe nanoparticles with porphyrin alkanethiols in toluene. The number of porphyrin molecules on the surface of a single CdSe nanoparticle increased with increasing reaction time to reach a saturated maximum of 21. The porphyrins as well as the core in CdSe-H2P can absorb UV/Vis radiation. Steady-state emission and emission-lifetime measurements reveal efficient energy transfer from the CdSe excited state to the porphyrins in the CdSe-H2P nanoparticles. The resulting porphyrin excited singlet state is not quenched by the CdSe core. These unique properties are in sharp contrast with those of multiporphyrin-modified metal and silica nanoparticles. Thus, semiconducting nanoparticle-multiporphyrin composites are highly promising as novel artificial photosynthetic materials.

meso-3,5-Bis(trifluoromethyl)phenyl-substituted expanded porphyrins: synthesis, characterization, and optical, electrochemical, and photophysical properties.

Trifluoroacetic acid-catalyzed condensation of pyrrole with electron-deficient and sterically hindered 3,5-bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso-3,5-bis(trifluoromethyl)phenyl-substituted expanded porphyrins including [22]sapphyrin 2, N-fused [22]pentaphyrin 3, [26]hexaphyrin 4, and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20-tetrakis(3,5-bis(trifluoromethyl)phenyl)porphyrin 1. These expanded porphyrins are characterized by mass spectrometry, (1)H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO-LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.

Local stoichiometry in amorphous supramolecular composites analyzed by solid-state C13 nuclear magnetic resonance

Solid-state nuclear magnetic resonance (NMR) has been applied to “amorphous” active layers consisting of donor-acceptor self-assembled composites in organic solar cells. Several stoichiometric supramolecular complexation states as well as the charge-transfer states are revealed by the solid-state NMR, which have been difficult to access by conventional spectroscopy. The spectra show clear correlation between local self-assembled supramolecular structures and the organic solar cell performances.

Paramagnetism enhancement by in situ electrochemical hole doping into a Prussian Blue thin film

A Prussian Blue (PB) film formed on an indium tin oxide substrate was incorporated in an electrolysis cell with an ionic-liquid midlayer (N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide) as an electrolyte. The ionic-liquid electrolysis cell mounted on a SQUID magnetometer was examined under DC bias voltage up to 4.0 V by injecting the charge carriers electrochemically into the PB film. Electrochemical hole doping increased the FeIII/FeII ratio in the PB film, resulting in a remarkable enhancement of magnetization accompanied by quasi-reversible electrochromism.