On the nature of plasmon-induced photocurrent enhancement in Bacteriochlorophyll c sensitized solar cells: Towards red light harvesting

Bacteriochlorophyll c (BChl c) is a naturally occurring pigment which, as a monomer, exhibits strong absorption in the red region (600–700 nm), where the dyes used in the most efficient solar cells absorb only weakly. Herein, we investigate the plasmonic enhancement of photoconversion efficiencies in BChl c sensitized solar cells using gold nanoparticles of different morphologies. Using cyclic voltammetry, a favourable band offset (~0.72 eV) between the lowest unoccupied molecular orbital of BChl c and the conduction band edge of titania photoanode was established. Further, gold nanorods with a localized surface plasmon resonance (SPR) frequency overlapping the absorption maxima of BChl c at ~670 nm were used, and compared with gold nanospheres with a SPR maximum in a region (~525 nm) where BChl c has no absorption. Gold nanorod incorporated solar cells displayed the highest photoconversion efficiency with more than 30% increase in photocurrent, compared to devices containing nanospheres and only BChl c. The results are explained using boundary element method simulations, which show a significantly enhanced scattering cross-section and local electric field in the vicinity of gold nanorods compared to the nanospheres. Electrochemical impedance spectroscopy studies indicate reduced charge transport resistance in both gold nanorod and nanosphere incorporated devices.