Nanowerk Spotlight) Many researchers are investigating the development of flexible solar cells in hopes of improving efficiency and lowering manufacturing costs. Organic solar cells hold the potential for integration into building facades and windows, due to their optical translucency and ability to be manufactured on large areas at high-throughput.
As an important member of the organic photovoltaics (OPV) family, polymer solar cells draw the most research interest, due to the relatively high power conversion efficiency (PCE) achieved (read more: “The state of nanoimprinted polymer organic solar cell technology”).
However, compared to the high efficiencies (>10%) of inorganic solar cells, the best polymer solar cells (6-7%) still show a lower efficiency. Organic solar cells are regarded as an emerging technology to become one of the low-cost thin-film alternatives to the current dominating silicon photovoltaic technology, due to their intrinsic potential for low-cost processing (high-speed and at low temperature).
However, it is generally believed that the PCE needs to be improved to above 10% in order for organic solar cells to become truly competitive in the marketplace. Currently, the best reported PCE, achieved in laboratories, lies in the range of 6.7% to 7.6% for molecular, and 8.3% to 10.6% for polymeric OPVs.
A recent review article in Advanced Materials (“Plasmonic-Enhanced Organic Photovoltaics: Breaking the 10% Efficiency Barrier”) looks at the recent progress on plasmonic-enhanced OPV devices using metallic nanoparticles, and one-dimensional (1D) and two-dimensional (2D) patterned periodic nanostructures. The authors, Qiaoqiang Gan from The State University of New York, Filbert Bartoli from Lehigh University, and Zakya Kafafi from Northwestern University, discuss the benefits of using various plasmonic nanostructures for broad-band, polarization-insensitive and angle-independent absorption enhancement, and their integration with one or two electrode(s) of an OPV device.
2D nanoaperture arrays with high-order symmetries: SEM images of periodic and quasi periodic 2D nanoaperture arrays milled in a 300 nm-thick silver film. The right insets show the 2D discrete Fourier transform power spectra for each array. The left inset in (b) shows a high-resolution cross sectional SEM image of a polymer filled hole in the triangular array. (© 2011, AIP)
The authors focus on two main research areas: the broadening of the absorption band for an OPV material and its extension to the NIR region, achieving polarization insensitive/independent plasmonic nanostructures; and omnidirectional absorption enhancement as well as the integration of the metallic plasmonic nanostructure(s) with one or two of the electrode(s) of an OPV device. The two main patterned nanostructures frequently used to introduce plasmonic enhancement in OPVs are randomly distributed metallic nanoparticles, and 1D and 2D periodic nanopatterned arrays.
Metallic nanoparticles are probably the most popular and extensively employed nanostructures for enhancing the performance of PV devices, due to their relative ease of fabrication (see for instance: “Gold nanoparticles boost organic solar cell efficiency”). Design considerations using metallic nanoparticles of different materials, concentrations, shapes, sizes, and distributions have been introduced in various layers and interfaces within the devices. One option introduces metallic nanoparticles outside the active layer(s) of the OPV device.
A strong localized plasmon field enhancement and/or increased light scattering when metallic nanostructures were placed outside the active light-harvesting layer result in an enhanced PCE. Embedding metallic nanoparticles inside the active layers of OPVs exploits the strongly confined field of the localized surface plasmon resonance and more efficient light scattering within the active layers.
It is generally believed that small metallic nanoparticles (usually <20 nm in diameter) can act as sub-wavelength antennas in which the enhanced near-field is coupled to the absorbing OPV layer(s), increasing its effective absorption cross-section; while large nanoparticles (>40 nm in diameter) can be used as effective subwavelength scattering elements that significantly increase the optical path length of the sunlight within the active layers.
High-Order Symmetric Plasmonic Nanostructures
Although the synthesis of plasmonic metal nanoparticles seems relatively simple, it is quite challenging to control their size, shape, and monodispersity using solution processing or vacuum thermal evaporation or electrodeposition. Periodically patterned metallic nanostructures offer another approach to enhance the optical absorption of the organic active light-harvesting layers in OPVs. By properly designing plasmonic nanostructures, light can be effectively coupled to surface plasmon polaritons modes which are strongly confined at the metal surface next to the thin active layer. Both 1D and 2D periodic metallic nanostructures have been explored in various OPV designs to achieve unique and remarkable features. The review summarizes important results from numerous studies on these nanostructures.
Plasmonic nanostructures for omnidirectional absorption: (a) SEM image of fabricated crossed trapezoid arrays and (b) a single unit cell of crossed trapezoid where the scale bars are 500 and 100 nm, respectively; (c) schematic drawing of a periodically crossed trapezoid array with the angle of incidence shown as θ; (d) extinction plotted as a function of wavelength and angle of incidence (note that broadband extinction is preserved even at higher angles of incidence). (© 2011, NPG)
The authors conclude that, in order to achieve further progress with OPVs, a renewed focus on the science and technology of nanophotonics for light management and trapping offers the potential of achieving higher-efficiency devices. Plasmonic strategies offer several unique features and are one of the most promising solutions for enhancing the OPV optical absorption and device performance. By incorporating plasmonic nanostructures in the front and back metallic electrodes of an OPV device, it is possible to achieve broadband, polarization- and angle-independent absorption enhancement.
This plasmonic-assisted OPV has the potential to significantly surpass the 10% PCE barrier if the enhanced optical absorption can be transferred into excitons and separated photo-generated charge carriers which are efficiently collected at the respective electrodes.
By Michael Berger. Copyright © Nanowerk