www.MaterialsViews.com COMMUNICATION www.advenergymat.de © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 52 wileyonlinelibrary.com Adv. Energy Mater. 2011, 1, 52–57 Dye-sensitized solar cells (DSCs) have received wide-spread research attention due to their high power conversion efficiency and low cost. [1,2] Improving light harvesting in the 600 – 900 nm wavelength range, where state-of-the-art ruthenium-complex sen- sitizers absorb weakly, is one promising pathway to increasing power conversion efficiency to over 15%. [2] There have been multi-pronged research efforts to increase the light absorption of DSCs by developing strong absorbing dyes and using energy relay dyes. [3–5] Here, we report for the first time the use of plasmonic effects [6,7] to increase the light absorption and efficiency of DSCs. Plasmonic back reflectors, which consist of 2D arrays of silver nanodomes, were incorporated into solid-state dye-sensitized solar cells (ss-DSCs) by nanoimprint lithography. The reflectors enhance absorption through excitation of plasmonic modes and increased light scattering. ss-DSCs with plasmonic back reflec- tors show increased external quantum efficiency, particularly in the long wavelength region of the dye’s absorption band. Con- sequently, ss-DSCs made with ruthenium-complex sensitizers (Z907) and strong-absorbing organic sensitizers (C220) have 16% and 12% higher short-circuit photocurrents, respectively. They achieve power conversion efficiencies of 3.9% and 5.9%, on par with the world record for devices with the same dyes. SS-DSCs use solid-state hole-transport materials (HTM) to replace conventional liquid electrolytes and offer a viable pathway towards higher efficiency because the open-circuit voltage can be tuned by adjusting the highest-occupied molecular orbital (HOMO) of the HTM. [8,9] The use of solid-state materials also solves the potential leakage problems associated with the vola- tility and corrosiveness of liquid electrolytes. An ss-DSC is com- posed of a mesoporous TiO 2 photoanode, which is sensitized with a monolayer of dye, filled with HTM, and capped by reflec- tive metal contacts deposited on top of the active layer. Currently, optimized ss-DSCs are still limited by electron–hole recombina- tion [10] and incomplete pore filling of the HTM, [11,12] such that the optimized active layer is only 2 μm thick, much less than the thickness needed to achieve sufficient light absorption. Efforts to increase light absorption in ss-DSCs have primarily been focused on developing strong absorbing dyes [9] and new TiO 2 nanostruc- tures with high internal surface area for dye adsorption; [13] few studies have attempted to optimize the optical properties of the structure of the ss-DSCs to enable better photon management. The use of plasmonic effects has been proposed as a prom- ising pathway to increase light absorption in active layers of solar cells, [6] and has been demonstrated on several thin-film solar-cell materials such as amorphous silicon, [14–16] gallium arsenide, [17] polymers, [18,19] and dye monolayers on TiO 2 . [20–22] Most of the reported plasmonic solar cells have substantially lower efficiency than state-of-the-art devices made with the same material, because the active layers used are significantly thinner than they normally are in cells. Studies on such thin cells have been of great value as they have unequivocally demonstrated that spectral enhancements in the photocurrent density can be obtained through: 1) excitation of localized surface plasmon resonances of metallic nanoparticles; [23] 2) scattering of light by metallic nanoparticles into dielectric-like waveguide modes of the solar cell; [24] and 3) coupling to propagating surface plasmon polariton (SPP) modes. [25] An important distinction is made here between the excitation localized surface plasmon (SP) resonances that occur when conduction electrons in finite-sized particles are driven into oscillation, and SPPs which are surface electromagnetic waves that propagate along metal surfaces. The efficient excitation of localized SP resonances depends on the geometry, size, shape, and dielectric environment of the metal particles and field enhancements are only produced in very close proximity to the metals ( ∼10 nm). The propagating SPP waves can most efficiently be excited by generating periodic grating structures that allow free space light waves to pick up sufficient in-plane momentum to couple to the shorter wave- length (higher propagation constant) SPP waves. Whereas SPPs have their highest field intensity at the metal/ dielectric interface, they also exhibit a large penetration depth (100 nm – 1 μm) into the dielectric medium adjacent to the metal. [26] For this reason, the excitation of SPP can produce absorption enhancements in thicker active layers in a solar cell. It should also be mentioned that coupling to dielectric waveguide modes can be enhanced by exploiting plasmonic effects; near the surface plasmon frequency of metallic particles, their scattering ability (i.e. cross section) is resonantly enhanced and the scat- tering into both dielectric-like and SPP modes can be increased. Here, we demonstrate for the first time that both the SPP- and scattering-induced effects as mentioned above can be I-Kang Ding, Jia Zhu, Wenshan Cai, Soo-Jin Moon, Ning Cai, Peng Wang, Shaik M Zakeeruddin, Michael Grätzel, Mark L. Brongersma, Yi Cui,* and Michael D. McGehee* Plasmonic Dye-Sensitized Solar Cells DOI: 10.1002/aenm.201000041 I.-K. Ding, Dr. J. Zhu, Dr. W. Cai, Prof. M. L. Brongersma, Prof. Y. Cui, Prof. M. D. McGehee Department of Materials Science and Engineering Stanford University Stanford, CA 94305, USA Email: yicui@stanford.edu; mmcgehee@stanford.edu Dr. S.-J. Moon, Dr. S. M. Zakeeruddin, Prof. M. Grätzel Institut de Chimie Physique École Polytechnique Fédérale de Lausanne 1015 Lausanne, Switzerland N. Cai, Prof. P. Wang State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022, P.R. China