Abstract
Plasmonic structures of FTO/TiO2/NPs-Ag and FTO/NPs-Ag/TiO2 electrodes were fabricated by sputter technology and the sol–gel & spin coating procedure. These electrodes with similar optical absorptions in the visible region enhanced by the surface plasmon resonance of silver nanoparticles have different photovoltaic properties, revealing that the significant design can be used to identify the favorably enhanced direction of plasmonic structure. In the FTO/TiO2/NPs-Ag, a 60% enhancement in photocurrent and an improvement in photovoltage were observed and the increased incident photon to photocurrent efficiency (IPCE) was consistent with the enhanced absorption spectrum. The enhanced photocurrent was mainly attributed to plasmonic scattering to elongate optical path lengths by light trapping, leading to the enhanced photocurrent response in the whole visible region in comparison with the standard titania electrode. In addition, the novel plasmonic structures of sandwiched TiO2/NPs-Ag/TiO2 electrodes were fabricated to solve the fluctuation in the TiO2 conduction band at the TiO2/Ag interface because in-plane surface plasmon nanoparticles of silver are perpendicular to the direction of generated electrons moving toward the collection electrode, which can also improve light trapping to enhance the dye molecular absorption. The improvement of the incident photon to photocurrent efficiency (IPCE) spectrum corresponding to the strong absorption and damping reflection indicated light trapping of plasmonic structure to elongate the optical pathways of photons. As a result of improved dye absorption, about 23% enhancement in photocurrent density has been achieved. In comparison of the enhancement in FTO/TiO2/NPs-Ag, more light trapped close to photocurrent collecting electrode provides better charge-collection and light harvesting efficiencies. The long-term stability of DSSC with plasmonic structure has been studied. We found the enhanced stability in photocurrent is continuously degraded with time while standard TiO2 shows a very weak dependence on time. Some of the NPs obviously lost some volume and a number of small NPs were newly formed near the big NPs-Ag during the ambient white light exposure, attributed to enhanced Ag dissolution and shorter transport of Ag+ which recombine with the electrons in the TiO2 conduction band. These results exhibit that nanoporous TiO2 film with Ag nanoparticles is the light-sensitive electrode and this study is potentially applicable to new kinds of solar cells.