Plasmonic Efficiency Enhancement of High Performance Organic Solar Cells with a Nanostructured Rear Electrode

Abstract

An essential aspect of reducing the cost of photovoltaic devices lies in enhancing their effi ciency without increasing their thickness. To that end, several absorption enhancement strategies have been proposed, including photonic crystals, [1][2][3] optical cavities, [ 4 , 5 ] and plasmonic nanostructures such as metal nanoparticles, gratings, and nanostructured electrodes. [6][7][8] By employing metal nanostructures, an effi ciency enhancement of Si, [ 9 , 10 ] III-V, [ 11 , 12 ] dye-sensitized, [ 13 , 14 ] and organic [15][16][17] solar cells has been reported. This effi ciency enhancement is generally attributed to an increased light absorption in the solar cell active layer originating from surface plasmon resonances that lead to a strong near fi eld enhancement as well as highly effi cient light scattering. [ 18 , 19 ] Components of the scattered light can propagate to the far fi eld, whereas the enhanced near fi eld decays within distances smaller than the wavelength ( λ ) of the incident light from the nanostructure surface.A common approach to exploit surface plasmons in solar cells is to place metal nanostructures adjacent to the active layer, which allows for light trapping by coupling the light that is scattered by the nanostructures into guided modes. Organic solar cells offer the additional possibility to blend metal nanoparticles into the active layer, which most effi ciently exploits near fi eld enhancement. Employed in optimized devices, this approach however suffers from several drawbacks such as the impact on the active layer morphology and the tendency of nanoparticles to aggregate when blended in a polymer host. [ 20 ] Moreover, the exploitation of plasmonic near fi eld enhancement is challenging as the nanostructures have to be suffi ciently isolated to prevent charge trapping [ 15 ] and exciton quenching, [ 21 ] while still allowing for absorption enhancement. [ 22 ] In general, it is benefi cial to place metal nanostructures on the rear side of the solar cell. This is mainly because at the front of the cell they could give rise to destructive Fano interference and would parasitically refl ect and absorb a signifi cant amount of incident light before it reaches the active layer. [ 7 , 23 ] For Si solar cells, an effi ciency enhancement by a nanostructured metal rear electrode has been reported. [ 9 , 24 ] For organic solar cells, however, the processing usually begins with the transparent electrode, such that the nanostructures have to be applied after the deposition of the organic active layer, without compromising its optical and electrical properties. This is quite challenging as organic materials are readily damaged by the harsh processing conditions often involved in nanofabrication or nanopatterning. As a result, the nanostructures are usually placed at the front side of organic solar cells. [25][26][27] Further, while plasmonic rear side features have been considered, the approaches do not allow for demonstrating devices with state-of-the-art effi ciencies. [ 28 , 29 ] Here,...

Plasmonic electrodes for bulk-heterojunction organic photovoltaics: a review

“…Patterns can be fabricated using large-area, nanofabrication methods such as nanoimprint lithography (NIL), 30,36,70,54 nanosphere lithography, 33,40 and nanotemplate-directed fabrication. 41,42 Roll-to-roll processes that incorporate NIL are currently under development and could enable high-throughput integration of plasmonic back electrodes.…”

“…34 Figures reproduced with permission, courtesy of (a) Ref. 33 into BHJ-OPV layers, as there are a number of recent reviews on this subject [e.g., (Refs. [10][11][12][13][14][15]].…”

“…We will not review the plasmonic effects of discrete metallic NPs distributed can have strongly enhanced electric fields near the surface of the nanostructure, leading to enhanced absorption near these electromagnetic "hot spots." 33 (b) Time-averaged magnetic field amplitude (jH y j) distribution, showing that surface plasmon polaritons (SPPs) can hybridize with Bloch modes in a metallic grating structure to greatly enhance the optical path length in the OPV. 32 (c) Time-averaged magnetic field intensity (jH y j 2 ) distribution at the SPP resonance wavelength from a nanohole array; the spatial mode profile is plotted in the right panel of (c).…”

“…[10][11][12][13][14][15]]. There are three primary ways in which metallic nanostructured electrodes can be placed inside a PV device: (1) on the back metallic electrode; [31][32][33][34][35][36][37][38][39][40][41][42] (2) on the front/transparent electrode; [43][44][45][46][47] or (3) as a charge transport interlayer inside of the device stack [48][49][50][51] (see Fig. 3).…”

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

“…[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] These include, for instance, simple (low-cost) thermal annealing of metal films 8,9 or dispersion of pre-synthesized metal nanoparticles, 10,11 both of which resulted in successful demonstrations of plasmonic light trapping. However, it should be pointed out that the metal nanostructures fabricated by these approaches are usually challenging to match to the theoretical models.…”