Improving light absorption in organic solar cells by plasmonic contribution David Duche a , Philippe Torchio a,Ã , Ludovic Escoubas a , Florent Monestier a , Jean-Jacques Simon a , Franc - ois Flory b , Ge ´ rard Mathian a a Aix-Marseille University, Institut Mate´riaux Microe ´lectronique Nanosciences de Provence-IM2NP, CNRS-UMR 6242, Domaine Universitaire de Saint-Je ´ro ˆme, Service 231, 13397 Marseille Cedex 20, France b Ecole Centrale Marseille, IM2NP CNRS-UMR 6242, Technopo ˆle de Cha ˆteau-Gombert, 38 rue Fre ´de´ric Joliot Curie,13451 Marseille Cedex 20, France article info Article history: Received 4 December 2008 Received in revised form 20 February 2009 Accepted 25 February 2009 Keywords: Organic solar cells Surface plasmon Absorption enhancement Bulk heterojunction FDTD abstract Plasmonic phenomenon inside the materials composing an organic solar cell based on a photoactive poly(2-methoxy-5-(2 0 -ethyl-hexyloxy)-1,4-phenylenevinylene):(6,6)-phenyl-C 61 -butyric-acid-methyl ester (MEH-PPV:PCBM) bulk heterojunction is studied using Finite Difference Time Domain (FDTD) method calculations and the modeling results are compared with experimental results. Enhanced absorptance of light up to 50% is experimentally obtained in a 50-nm-thick blend layer including spin-coated silver nanospheres with a diameter of 40 nm. FDTD calculations based on the design of 2D-grating of nanoparticles confirm the high values of absorptance. Spatial distributions of electromagnetic field power density in the structures show confinement of the power at the interface or in the vicinity of the nanoparticles depending on the wavelength and on the preferential directions. & 2009 Elsevier B.V. All rights reserved. 1. Introduction In order to produce low-cost energy without green house effect, photovoltaic materials are fabricated in thin film form and strategies are developed to increase light harvesting in the films. New photonic concepts are currently developed by numerous researchers in the photovoltaic domain in order to improve the collection efficiency of light inside cells based on crystalline silicon, thin film silicon, or organic materials: new surface micro- or nanostructuration techniques [1], optimization of the electromagnetic (e.m.) field distribution [2,3], realization of photonic crystals by bulk structuration [4], plasmonic excitation using metallic nanoparticles (NPs) [6–11], etc. A surface plasmon is an optically generated wave, which propagates along a metal/dielectric interface. In tuning the light excitation, a resonance can occur when the frequency of the incident photons equals the collective oscillation frequency of conduction electrons of metallic particles. In photonics, this domain is known as appellation plasmonics. Among a wide field of applications, these properties can be used in the photovoltaic domain in order to improve the photonic absorption, thus the solar cell efficiency, in particular in the spectral bands where materials absorb weakly. Some noble metal particles such as silver or gold can increase absorption in the visible range. A plasmon can be due to the interaction between a metallic surface (non- localized plasmon) or metallic nanoparticles (localized plasmon) and the light, more precisely between charges (conduction electrons) and electromagnetic waves [5]. The aim of our study is to excite localized plasmons by using metallic NPs to trap or confine light inside the photoactive material, or to obtain beneficial resonant internal light scattering on these metallic particles. This excitation depends on several parameters such as nature of the metal, surface density of the particles, their diameter, incident angle of light, wavelength, polarization, etc. Light absorption could then be amplified, in particular in the spectral ranges where the photovoltaic material absorbs weakly only the light. A few years before, thin film amorphous silicon solar cells containing 100-nm-diameter gold nanoparticles have been man- ufactured by Derkacs et al. [6] to engineer the transmission and spatial distribution of the electromagnetic field in visible spectrum inside the a-Si:H layer. Thus, an increase of 8.1% on the short-circuit current density and of 8.3% on the energy conversion efficiency was observed in comparison to the values achieved in reference devices without the gold particles. The authors attributed this improved performance to scattering from surface plasmon polaritons in nearby metallic NPs. Pillai et al. [7] found that surface plasmons can increase the response of crystalline silicon cells over the visible as well as the near- infrared spectra. They reported a significant enhancement of the absorptance for both 1.25-mm-thick film and wafer-based 300- mm-thick crystalline planar silicon structures including silver ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.02.028 Ã Corresponding author. E-mail address: philippe.torchio@univ-cezanne.fr (P. Torchio). Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]] Please cite this article as: D. Duche, et al., Sol. Energy Mater. Sol. Cells (2009), doi:10.1016/j.solmat.2009.02.028