Honghui Shen

Plasmonic absorption enhancement in organic solar cells with thin active layers

The influence of silver nanoparticles on light absorption in organic solar cells based on poly(3-exylthiophene):(6,6)-phenyl-C61-butyric-acid-methyl ester is studied by means of finite element method simulations. The metallic nanoparticles are embedded directly inside the active layer. We investigate the enhancement mechanism and the influence of factors such as the spacing between neighboring nanoparticles, the particle diameter, and the coating thickness. The plasmonic resonance of the particles has a wideband influence on the absorption, and we observe a rich interaction between plasmonic enhancement and the absorption characteristics of the active layer material. An enhancement with a factor of around 1.56 is observed for nanoparticles with a diameter of 24 nm and a spacing of 40 nm, bringing the structure to the absorption level of much thicker active layers without nanoparticles. In addition, a significant effect of the particle coating thickness is observed.

Angle insensitive enhancement of organic solar cells using metallic gratings

We explore the optical enhancement of organic photovoltaic cells by incorporating a metallic grating as the back contact. We numerically demonstrate a strongly enhanced light absorption exploiting a complex interplay between multiple electromagnetic wave phenomena, among which surface plasmon polariton (SPP) resonances, waveguide mode resonances, Fabry–Perot modes, and scattering. We focus on a triangular grating structure and describe the particular opportunities to obtain a good angular performance. In addition we introduce a novel multiperiodic geometry that incorporates multiple types of SPP resonances. Our triangular structure shows an increased absorption of 15.6% with the AM1.5G spectrum in the 300–800 nm wavelength range. For the multiperiodic grating case a significant further increase to 20.7% is shown.

Combined plasmonic gratings in organic solar cells

We propose an organic solar cell structure with combined silver gratings consisting of both a front and a back grating. This combination provides multiple, semi-independent enhancement mechanisms which act additively, so that a broadband absorption is obtained. Both gratings couple the incident light into various plasmonic modes, showing a more localized or propagating character respectively. In addition, some modes only appear for tilted incident light, and therefore present a complex angle-dependent behavior. We provide extensive numerical simulations, resulting in an optimized period of 490 nm, with front grating elements of 60 by 10nm and back elements of 60 by 30 nm. With these parameters an integrated absorption enhancement factor around 1.35 is observed, with absorption increasing from 48% to 65% under TM polarized light. In addition, the solar cell with combined gratings is much less sensitive to the angle of incident light than the single grating cases. Furthermore, the grating structure does not have a large influence on the TE polarized light absorption.

Enhanced optical transmission through tapered metallic gratings

To achieve non-resonant behavior, we propose a metallic grating device with linearly tapered slits. The tapering provides a gradual impedance variation from the entrance to the exit of the slits, leading to broadband and wide-angle enhanced transmission in the infrared. In addition, the light is strongly localized and enhanced at the slit exits, in contrast with straight slits. We describe the phenomenon with a transmission line model, which is in accordance with rigorous simulations.

Excitation of multiple dipole surface plasmon resonances in spherical silver nanoparticles.

We observe the appearance of multiple dipole surface plasmon resonances in spherical Ag nanoparticles when embedded in an organic semiconductor that exhibits a highly dispersive permittivity. Comparing the absorption spectra of thin-films with and without Ag nanoparticles reveals the presence of two plasmon peaks. Numerical simulations and calculations based on an electrostatic model allow us to attribute both peaks to dipole resonances, and show that the strong dispersion of the organic permittivity is responsible for this behavior. The presence of these two plasmon resonances was found to enhance the absorption of the organic semiconductor over a broad wavelength range.

Tailored and tapered metallic gratings for enhanced absorption or transmission

Summary form only given. Plasmonic structures, such as metallic gratings and apertures, provide unusual abilities to modify the absorption or the transmission of thin-layered devices. In order to optimize the absorption in thin-film solar cells, we introduce complex grating geometries, implementing multiple periodicities and blazing. These gratings optimize the diffraction efficiency and increase the number of accessible modes. On the other hand, in order to achieve enhanced transmission at infrared wavelengths, we propose tapered apertures. The non-resonant funnelling phenomena provide for broadband and wide-angle focusing opportunities.

Plasmonic absorption enhancement in organic photovoltaics

Organic solar cells have a strong potential for the near future, because their material and fabrication costs can be much smaller than for traditional technologies. However, in order to become commercially viable their efficiency needs to increase. One of the novel, interesting techniques to increase the light absorption is by including plasmonic enhancements [1]. These metallic features instigate strong, local resonances which trap the light in the very thin layers. In this work we report rigorous numerical investigations of organic cells with embedded metallic nanoparticles and with patterned metallic electrodes. The state-of-the-art material P3HT:PCBM is employed as the active layer. We show that in both cases, with particles or with patterned electrodes, a strong enhancement is achievable, with roughly an increase of up to 50% in the solar light absorption. Such enhancements could prove crucial for more efficient energy devices.

Increasing Polymer Solar Cell Efficiency with Triangular Silver Gratings

We investigate strongly enhanced light absorption in a thin P3HT:PCBM solar cell with a triangular silver grating back contact. The correlation between grating and plasmonic absorption spectrum features are identified and studied with rigorous numerics.

Plasmonic nanoparticle enhancement in thin organic solar cells

Thin film solar technologies can potentially provide energy at a very low device cost, because of lower material consumption. However, given their thin absorbing layer there is a need for absorption boosting measures, such as light trapping or management techniques. This problem is particularly important for organic solar cells, since the active layer thickness is often limited by the exciton diffusion length. Here, we examine a recent promising light trapping scheme, which consists of incorporating small metallic nanoparticles into the organic active layer. It turns out that the localized plasmon resonance of the particles can significantly enhance the optical fields and absorption. We examine the potential of this technique in a thin solar cell with P3HT:PCBM as the active material, by performing rigorous numerical simulations. Ultimately, we find that a layer of only 33 nm thick with silver particles of 24 nm diameter, can provide the same solar absorption efficiency as a layer of 61 nm thickness without particles. Such enhancements may pave the way towards cheaper energy harvesting devices.