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/SnO ₂ 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/SnO ₂/L-SCCNT. The power conversion efficiencies (η) of SCCNT-modified electrodes increase with increasing tube length. The maximum power conversion efficiency (η) of OTE/SnO ₂/L-SCCNT electrode is determined to be 0.11%, which is about 6 times greater than that of the OTE/SnO ₂/S-SCCNT electrode (0.018%). Molecular assemblies composed of S-SCCNT and 5,15-bis(3,5-di-tert-butylphenyl)porphyrin (H ₂P) prepared in acetonitrile/toluene (5/1, v/v) were deposited as three-dimensional arrays onto nanostructured SnO ₂ films to further improve the photoelectrochemical response in the visible region. The composite electrode of S-SCCNT-H ₂P (OTE/SnO ₂/S-SCCNT-H ₂P) 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/SnO ₂/S-SCCNT or OTE/SnO ₂/H ₂P). Improved efficiency of the photocurrent generation is ascribed to the ability of SCCNT in promoting photoinduced charge separation by accepting electrons from excited H ₂P and transporting to the collecting electrode surface.