Enhancement of Porphyrin Photocurrent Based on Plasmonic Cu Light-Harvesting Nanoantenna

Abstract

Introduction Plasmonic metal-nanostructured electrodes have been used for the development of high-performance organic photoelectric conversion systems because they can act as superior light-harvesting nanoantennae. Many previous reports have demonstrated that this optical phenomenon led to a substantial enhancement of the photocurrent from porphyrin dyes. The enhancement mechanism has been suggested to be a significant enhancement of the photoexcitation efficiency of the dyes, which were located within the strong local electromagnetic fields generated via a localized surface plasmon resonance (LSPR) of the metal nanostructures. However, all of the plasmonic electrodes reported so far are composed of expensive metallic species (Au and Ag). We believe that the components of the plasmonic electrodes should be inexpensive and that the enhancement of photoelectric conversion efficiency using “inexpensive plasmonic nano-antenna materials” is an important goal because one of the main advantages of the organic photoelectric conversion systems is low cost due to the use of organic dyes. In this study, we fabricated photoelectric conversion systems consisting of hybrids of porphyrin molecules as a dye and a plasmonic Cu arrayed electrode because the Cu material is significantly cheaper than the traditional plasmonic materials. Experimental section The arrayed electrodes were fabricated by thermally-depositing Cu onto the two-dimensional silica colloidal crystals consisting of silica particles with 160, 260, and 330 nm in diameters.1 Effective suppression of the spontaneous oxidation of the Cu electrodes by protecting with the ultrathin layers consisting of mercaptohexadecanoic acid and the ultrathin sol-gel layers of Ti(O) and polyvinyl alcohol has allowed us to accurately evaluate the Cu nanoantenna effect on the porphyrin photocurrent. The porphyrin monolayers were immobilized onto the surface of the colloidal crystals. Results and Discussions The Cu LSPR could be tuned from visible to near-infrared wavelengths by controlling the underlying silica diameter. The resultant LSPR above 600 nm was efficiently excited where the interband transition is comparatively low. Also, the molecular ultrathin films, which covered the Cu surfaces, effectively suppress the spontaneous oxidation in the air and electrolyte aqueous solution. Photocurrent measurements for porphyrin-immobilized Cu arrayed electrodes (working electrode) were carried out in an aqueous 0.1 M NaClO4 solution containing 5 mM methyl viologen dichloride as an electron acceptor, using a three electrode photoelectrochemical cell. Ag|AgCl (sat. KCl aq.) and platinum wire were used as the reference and counter electrodes, respectively. Photocurrents generated by irradiation of the monochromated light from a Xe lamp were detected. As a result, we have succeeded in efficiently enhancing the photocurrent from the porphyrin up to approximately 54 times by tuning the generating wavelength of Cu nanoanntena to the wavelengths above 600 nm, as compared with those generated from the porphyrin-immobilized Cu planar electrode. In a previous study, we reported that the nanoantenna effect from the Au arrayed electrodes enhanced the porphyrin photocurrent up to approximately 15 times.2 Therefore, this study demonstrates that the plasmonic Cu electrodes, which have combined strong light harvesting antenna effect and low material cost, are very useful for improving the performance of organic photoelectric conversion systems. Therefore, this is an important basic research which can accelerate the practical application of highly-efficient plasmonic photoelectric conversion systems.