Plasmons in solar energy conversion

Abstract

Harnessing the oscillation of electrons on conductor surfaces can effectively enhance the performance of organic photovoltaic cells.The need for affordable and large-scale, carbon-neutral energy sources has fueled a search for revolutionary concepts in solarto-electricity energy conversion. Studies often focus on doubling or tripling the conversion efficiency using third-generation concepts that circumvent traditional 1 and inch closer to thermodynamic limits. 2 Alternatively, researchers focus on making solar cells not quite as efficient but as cheap as paint. 3 Such low-cost options are usually obtained by employing organic semiconductors or dye-sensitized systems. 4 A common thread in all these systems is the tradeoff between efficient light absorption and charge collection. Cells engineered to absorb as much light as possible exhibit decreased efficiency, because increased path lengths strongly increase energy losses by recombination of oppositely charged electrical carriers. Photovoltaics engineered to exhibit less recombination absorb little light. A sweet spot can be found that has limited efficiency. To surpass this practical limit, new materials and approaches are needed.A new way to increase solar-energy conversion is through surface plasmons, i.e., collective surface oscillations of conducting electrons in metal nanostructures that tend to trap optical waves near their surface. They enhance optical processes in their vicinity and are widely used in Raman spectroscopy, where enhancement factors can be many orders of magnitude. 5 Analogous to Raman scattering, they enhance optical absorption, allowing for development of solar cells that circumvent the tradeoff between optical thickness and carrier transport. Yet, because strong recombination can occur at metal surfaces in contact with the active layer of a solar cell, attempts at this approach have not been very successful, except for some cases where they were used deliberately as recombination sites. 6 We have recently succeeded in circumventing these issues by employing buffer layers between the plasmonically active material and the active layer of the solar cell. 7,8 We also employed a hybrid approach, where plasmonic effects potentially enable third-generation solar-energy conversion. 9 Figure 1 shows one of the possible approaches to using plasmonic enhancement in organic solar cells. We vapor deposit silver particles on a transparent conducting front contact of indium tin oxide (ITO). We subsequently spin a layer of transparent conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), on top of this, followed by the active layer. Upon inclusion of silver particles, these cells Continued on next page

FDTD modeling to enhance the performance of an organic solar cell embedded with gold nanoparticles

“…One of the demonstrated architectures is by having the plasmonic element in the PEDOT:PSS layer [4,5]. Another possible architecture is to deposit the plasmonic element within the photoactive layer [6,7] but since this layer is usually composed of a random intermixing of p-n junction, the presence of the electrically conducting element would often lead to substantial charge recombination and internal short [7]. Notably, the plasmonic enhancing effect has also been achieved by employing a patterned metallic layer as part of the counter electrode [8][9][10].…”