Organized assemblies of single wall carbon nanotubes and porphyrin for photochemical solar cells: Charge injection from excited porphyrin into single-walled carbon nanotubes

Photochemical solar cells have been constructed from organized assemblies of single-walled carbon nanotubes (SWCNT) and protonated porphyrin on nanostructured SnO₂ electrodes. The protonated form of porphyrin (H₄P²⁺) and SWCNT composites form 0.5-3.0 μm-sized rodlike structures and they can be assembled onto nanostructured SnO₂ films [optically transparent electrode OTE/SnO₂] by an electrophoretic deposition method. These organized assemblies are photoactive and absorb strongly in the entire visible region. The incident photon to photocurrent efficiency (IPCE) of OTE/SnO₂/SWCNT-H₄P ²⁺ is ∼13% at an applied potential of 0.2 V versus saturated calomel electrode. Femtosecond pump-probe spectroscopy experiments confirm the decay of the excited porphyrin in the SWCNT-H₄P²⁺ assembly as it injects electrons into SWCNT. The dual role of SWCNT in promoting photoinduced charge separation and facilitating charge transport is presented.