Plasmon-assisted solar cells

Because of its ubiquity in electronics, silicon is the favorite semiconductor used in solar photovoltaic cells. But Si is a poor light emitter and absorber, and the efficiency of thin-film Si PV cells is even poorer than that of wafer-based thick cells. An important goal is to make the cells inexpensive by using thin films, but also to make them nicely absorptive. Scientists at the University of New South Wales in Australia have now enhanced the absorption of sunlight using surface plasmons generated on silver nanoparticles deposited on PV cells. SPs are collective oscillations of conduction electrons that can arise when light impinges on a metal particle whose size is on the order of the light’s wavelength. Strong scattering occurs, and the SPs couple to the waveguide modes of Si; the light is effectively trapped. The researchers used this phenomenon in a PV solar cell to increase the cell’s absorption efficiency. To create the particles, they deposited thicknesses of silver that ranged from 10 nm to more than 20 nm; during annealing, the silver coalesced into islands larger than 100 nm across. For 1.25-µm thin-film PV cells the group found a 16-fold absorption enhancement for light at 1050 nm, where Si normally absorbs poorly, and a 33% increase across all wavelengths of light (see the figure). For 300-µm-thick wafer PV cells, plasmon trapping increased full-spectrum absorption by 19%. According to Supriya Pillai, optimizing the nanoparticle size should bring additional improvements. The scientists also used plasmons to increase emission from thin-film LEDs. (S. Pillai et al., J. Appl. Phys. 101 , 093105, 2007 .)