Donghyun Son

Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

The mechanism of light-soaking phenomenon in inverted-type organic solar cells (IOSCs) with a structure of indium-tin-oxide/TiOx/P3HT:PCBM/Au was studied by electron spin resonance (ESR) spectroscopy. Charge accumulation in the cell during UV-light irradiation was observed using ESR, which was clearly correlated with the light-soaking phenomenon. The origin of the charge accumulation is clarified as holes that are deeply trapped at p-type P3HT polymer-chain ends with bromine after hole transfer from the band excitation in the TiOx layer. The holes are considered to be electrostatically attracted to trapped electrons in the TiOx layer after the band excitation. These accumulated charges are the origin of the light-soaking phenomenon. Our results strongly suggest that passivation of the residual OH groups in the TiOx layer is needed to avoid the light-soaking phenomenon by preventing electron trappings, a step that is indispensable in the operation of highly stable IOSCs without UV-light irradiation based o...

Direct Observation of Charge Carriers in Highly Magnesium-Doped Tris(8-hydroxyquinoline) Aluminum Thin Film by Electron Spin Resonance

We report on an electron spin resonance (ESR) study of a tris(8-hydroxyquinoline) aluminum (Alq3) thin film doped highly with Mg. We observed a clear ESR signal of radical anions in the film prepared by coevaporating Alq3 and Mg (Alq3:Mg= 1:3). The g value and peak-to-peak ESR linewidth ΔHpp are obtained as 2.0030 and 2.24 mT, respectively, which coincide well with the reported values of an Alq3 thin film with a low Mg doping concentration. The spin susceptibility of the film was found to obey Curies law, which indicates that no interaction exists between spins of Alq3 anions. The doped charges are found to be localized at deep trapping sites by lineshape analysis and from the temperature dependence of ΔHpp. The doping concentration was evaluated to be 4.3%. This low doping concentration can be explained by the formation of Mg clusters in the film, which was directly confirmed by transmission electron microscopy.

Study on n-type Doped Electron-Transporting Layers in OLEDs by Electron Spin Resonance

We present an electron spin resonance (ESR) study of Cs2CO3 and LiF doped tris(8-hydroxyquinoline) aluminum (Alq3) thin films. The g values of the Cs2CO3 and LiF doped Alq3 thin films were obtained as 2.0040 and 2.0028, repectively. The g value of the LiF doped film was very close to a theoretically calculated one for an isolated Alq3 radical anion, while Cs2CO3 doped film showed a large g shift. This feature can be explained by large spin-orbit interaction caused by heavy Cs atoms. While doping of Cs2CO3 did not showed cathode-metal (Al and Au) dependence, LiF doping required Al as cathode.

Direct observation of UV-induced accumulated charges in inverted-type polymer solar cells with TiOx layer by electron spin resonance

ABSTRACT We present an electron spin resonance (ESR) study of inverted-type polymer solar cells with the structure of indium tin oxide (ITO)/amorphous titanium oxide (TiOx)/regioregular poly(3-hexylthiophene) (P3HT):phenyl-C61-butryic acid methyl ester (PCBM)/Au. Two light-induced ESR signals with narrow and broad linewidth were observed by light irradiation. We found a strong correlation between typical light-soaking effects and the ESR signal with the broad linewidth under UV light irradiation. From the fact that the light-soaking effect requires UV irradiation, the ESR signal with the broad linewidth is found to be related with the light-soaking effect.