Photoelectrochemistry of stacked-cup carbon nanotube films. Tube-length dependence and charge transfer with excited porphyrin

Photoelectrochemical solar cells are constructed with stacked-cup carbon nanotubes (SCCNT) on optically transparent electrodes (OTE). Three SCCNT samples with different tube lengths (long type, L-SCCNT; medium type, M-SCCNT; and short type, S-SCCNT) were electrophoretically deposited on OTE/SnO2 electrodes to probe the tube-length dependence of the photoelectrochemical behavior. The maximum incident photon-to-photocurrent efficiency (IPCE) of 19% is attained at an applied bias potential of 0.2 V vs SCE in OTE/SnO2/L-SCCNT. The power conversion efficiencies (η) of SCCNT-modified electrodes increase with increasing tube length. The maximum power conversion efficiency (η) of OTE/SnO2/L-SCCNT electrode is determined to be 0.11%, which is about 6 times greater than that of the OTE/SnO2/S-SCCNT electrode (0.018%). Molecular assemblies composed of S-SCCNT and 5,15-bis(3,5-di-tert-butylphenyl)porphyrin (H2P) prepared in acetonitrile/toluene (5/1, v/v) were deposited as three-dimensional arrays onto nanostructured SnO2 films to further improve the photoelectrochemical response in the visible region. The composite electrode of S-SCCNT−H2P (OTE/SnO2/S-SCCNT−H2P) exhibits an IPCE of 32% under an applied potential of 0.2 V vs SCE. The observed increase in IPCE is greater than that of the additive effects observed from the single component systems (viz., OTE/SnO2/S-SCCNT or OTE/SnO2/H2P). Improved efficiency of the photocurrent generation is ascribed to the ability of SCCNT in promoting photoinduced charge separation by accepting electrons from excited H2P and transporting to the collecting electrode surface.