Light Concentration in Polymer Bulk Heterojunction Solar Cells with Plasmonic Nanoparticles

Light Concentration in Polymer Bulk Heterojunction Solar Cells with Plasmonic Nanoparticles Jinfeng Zhu* a, Baoqing Zeng* a, Richard S. Kim b, Zhe Wu a, a School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China; b National Research Council, Air Force Research Laboratory, 41 Avionics Circle Wright-Patterson AFB, OH 45433, USA ABSTRACT We investigate the light concentration effect of localized surface plasmon resonance by embedding a layer of silver nanoparticles in the low band gap polymer bulk heterojunction solar cells. Particle electromagnetic interaction is demonstrated by using the 3-dimensional finite-difference time-domain computational method. This nanostructure exhibits broadband optical absorption enhancement and weak dependence on incident light polarization. The optical concentration mechanism is discussed by near-field distribution analysis. This method can be used to optimize the design of plasmonic organic solar cells for high energy conversion efficiency. Keywords: organic solar cell, bulk heterojunction, low band gap, nanoparticle, surface plasmon, light concentration 1. INTRODUCTION Due to their flexibility, light weight, cheapness, polymer solar cells (PSCs) have attracted more and more attention within the past few years [1]. Their power conversion efficiency is poorer than silicon solar cells, since the typical electron-hole diffusion lengths in PSCs are is about ~0nm, but their active layers are usually required to be ~100nm thick to absorb sufficient sunlight []. Although various methods, such as introducing bulk heterojunction and low band gap materials, have been put forward to alleviate this contradiction, a smaller thickness of active layer with adequate absorption is still quite essential to get higher efficiencies. Up to date, there has been considerable interest in overcoming weak absorption of photovoltaic thin films through plasmonic nanostructures by means of far field scattering, light concentration and surface plasmon polariton [3]. Very recently, some groups have contributed to the experimental research using light concentration [4]. Meanwhile, a systematic electromagnetic modeling is required to interpret the mechanism and further optimize the light concentration in plasmonic PSCs [5].. THEORY.1 Modeling Our electromagnetic modeling focuses on the light concentration effect of localized surface plasmon resonance on the hybrid PSCs by embedding a monolayer of metallic nanopartilces. As shown in Fig. 1, the nanoparticles are assumed to be silver spheres with a periodic hexagonal distribution, which is a common self-assembly nanostructure in chemical and material engineering. The nanoparticles are adhered to the poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) transparent anode and embedded in the active layer. The plasmonic active layer consists of poly[,6-(4,4- bis-(-ethyl-hexyl)-4h-cyclopenta[,1-b;3,4-b ]dithiophene)-alt-4,7-(,1,3-benzothiadiazole)]/[6,6]-phenyl-c71-butyric acid methyl ester (PCPDTBT/PC 70 BM, 1:3 in weight) and silver nanoparticles. The optical modeling of the materials requires the complex refractive index or complex dielectric function.. Optical properties of active layer PCPDTBT/PC 70 BM is a low ban gap material with optical dispersion. Its refractive index is extracted from Ref. 6. Its refractive index is shown in Fig.. * Prof. Zeng: bqzeng@uestc.edu.cn; J. Zhu: nanoantenna@hotmail.com. Photonics and Optoelectronics Meetings (POEM) 011: Optoelectronic Devices and Integration, edited by Erich Kasper, Jinzhong Yu, Xun Li, Xinliang Zhang, Jinsong Xia, Junhao Chu, Zhijiang Dong, Bin Hu, Yan Shen, Proc. of SPIE Vol. 8333, 83331C 01 SPIE CCC code: 077-786X/1/$18 doi: 10.1117/1.914545 Proc. of SPIE Vol. 8333 83331C-1

Fig. 1 Schematic of the plasmonic hybrid PSC. The radius of the silver nanoparticles is R, and the periodic spacing between them is P. The thickness of hybrid active layer is d=40nm. The thicknesses of aluminum cathode and PEDOT/PSS anode are 100nm and 0nm, respectively.. 0.6.0 0.4 n 1.8 k 1.6 0. 1.4 0.0 400 500 600 700 800 900 1000 Free Space Wavelength (nm) Fig. Real and imaginary parts of the refractive index as a function of free space wavelength for the bulk heterojunction blend PCPDTBT/PC 70 BM (1:3)..3 Dielectric functions of silver nanoparticles In order to combine with the ultra-thin polymer bulk heterojunction, smaller silver nanoparticles with diameters from 5~0nm are used. The size-dependent dielectric function of the metallic particles can be expressed as below [7], p 1 1 ω p Γ( R) Γ i +Γ ( R) ω +Γ +Γ ( R) +Γ εω (, R) = εb( ω) + ω ( ) + ( ) ω ω ω ω where ε b (ω) denotes bulk dielectric function [8], and ω p is bulk plasmon frequency. The bulk relaxation frequency Г =ν F /d, ν F is the Fermi velocity, and d is the electron mean free path. The radius-dependent relaxation frequency Г(R)= Г +Aν F /R, and A is a particle-shape-dependent parameter. For the calculation of spherical silver particles, we have the dielectric functions shown in Fig. 3 assuming ν F =1.39 10 6 m/s, d=5nm, A=0.75, and ћω p =3.8eV. (1) Proc. of SPIE Vol. 8333 83331C-

ε 5 4 3 1 ε 1 ε 1 0-10 -0-30 -40-50 -60 1. 1.6.0.4.8 3. 3.6 Energy (ev) Bulk R=5nm R=10nm R=15nm R=0nm 0 1. 1.6.0.4.8 3. 3.6 Energy (ev) Fig. 3 Real (inset) and imaginary parts of silver dielectric constants as functions of photon energy and particle size..4 Numerical method We assume all refractive media are isotropic in order to simplify our investigation. The simulations are performed by a 3-dimentional finite-difference time-domain computational electromagnetic method [9]. In this modeling, periodic boundary conditions and electromagnetic symmetries are assumed due to the hexagonal nanoparticle periodicity, and perfectly matched layers are used as artificial absorbing layers to simulate optical open boundary conditions. Both x- and y-polarized light are normally incident on the anodic surface. The optical absorption in different active layers is calculated and compared. The absorption in the active layer for a normally incident plane wave can be calculated by P = ω ε E d ' () V V where E is the local, simulated electric field and V is the volume of active layer. 3. LIGHT CONCENTRATION BY PLASMONIC EFFECTS 3.1 Absorption spectra of active layer Generally, the absorbance spectra of the pristine and hybrid active layers with the same thickness are compared in Fig. 4. As an organic semiconductor, the low band gap material PCPDTBT/PC 70 BM typically has a low absorption coefficient. Therefore, as the thickness of its pristine active layer decreases, the drop in its dominant absorption band (360~900nm) is particularly significant, especially in the red and near infrared band. The enhanced absorption peak of hybrid active layers induced by particle plasmons is observed at around =604nm. Furthermore, the absorption enhancement extends to wavelengths beyond the surface plasmon resonance, and enhancing levels are varied according to wavelength changes. This broadband absorption enhancement is significant in the solar spectrum application. Proc. of SPIE Vol. 8333 83331C-3

Absorbance 1.0 0.8 0.6 0.4 0. 0.0 Pristine Hybrid 400 500 600 700 800 900 Free Space Wavelength (nm) 1.6 1.4 1. 1.0 0.8 0.6 0.4 0. Solar Irradiance (W/m /nm) Fig. 4 Absorbance spectra of x-polarized incident light for the bare and hybrid active layers. The solar irradiance spectrum is also shown. 3. Optical field concentration The electric field distributions of the orthogonal polarized incident light at the wavelength 604nm are compared in Fig. 5 (a) and (b). The electric field is concentrated between nanospheres mainly along the direction of light polarization, and absorbance spectra of orthogonal polarized light almost totally overlap (the spectrum of y-polarized light not shown here). So the hexagonal array nanoparticles show very weak absorption dependence on light polarizations due to the perfect electromagnetic symmetry of the two-dimensional lattice, which is beneficial for the unpolarized sunlight. Thus, we can simplify our modeling by only taking x-polarized spectra into account for all discussions. 10 nm Fig. 5 Electric field profiles of x-y cross sections cutting through a spherical nanoparticle in hybrid active layers. x-polarized incident light (a) at λ=604 nm, (c) at λ=54nm, (d) at λ=68nm; y-polarized incident light (b) at λ=604 nm. In the profiles, E 0 and H 0 are the original electric and magnetic fields of incident light source, R=15 nm and P=5 nm. In the color bar, we use normalized the electric field value, and E max is the maximum electric field in the profile. Further investigation about the light concentration mechanism can be done through analyzing the field profiles. The absorbance enhancement changes with wavelengths, due to the variation of PCPDTBT/PC 70 BM and silver dielectric properties, as illustrated in Fig. (a), (c) and (d). Optical light field is less concentrated by silver nanoparticles in Proc. of SPIE Vol. 8333 83331C-4

PCPDTBT/PC 70 BM at λ=54nm than at λ=68nm, as seen in Fig. (c) and Fig. (d). These small particles have a very low albedo, and the interspace among the nanoparticles is filled with lossy bulk heterojunction blend. The interspace can be considered as plasmonic lossy cavities separated by near-field metallic mirror nanoantennas. According to Mie theory and taking the electromagnetic interaction of nanoparticles into account, one can optimize the maximum light absorption in OPVs by engineering particle shapes, sizes, surrounding dielectric environment and particle spatial distributions. The near-filed enhancement becomes limited because of the loss in absorbing medium cavities, whereas, in the dominant absorption band of PCPDTBT/PC 70 BM, most of the enhanced absorption takes place in bulk heterojunction blend without significant absorption loss of metallic nanospheres. The simulation results also show that the absorbed light energy concentrated closed to the hole conductor PEDOT/PSS, and this might facilitate hole extraction and reduce carrier recombination in active layers [10]. 3.3 Optimization of nanoparticle spacing The light concentration factor F c for hybrid active layers is defined as below, F c = λ λ1 λ λ1 Ph S P0 Pp S P 0 AM1.5 AM1.5 dλ dλ where λ 1 ~λ denotes the dominant absorption band of PCPDTBT/PC 70 BM, and S AM1.5 (λ) is AM 1.5 solar irradiance spectrum. P 0 (λ), P h (λ) and P p (λ) are incident light power, absorption power of hybrid active layer and pristine active layer, respectively. We show the optimization of the light concentration factor with a series of periodicity and particle diameter in Fig. 6. It indicates that when the nanospheres are very far from each other, F c is very small, approaching 1.0. When particles are far enough from each other, the filed scattered by one sphere shows the property of evanescent wave which is so weak in the vicinity of other spheres compared to the exciting field, that the electromagnetic interaction between nanospheres can be neglected. As the distance decreases, F c gets larger till it reaches its maximum at certain spacing. The maximum F c is 1.94 (D=10nm, P=13nm) as seen in Fig. 6. As particles get closer to each other, the scattering near-filed around one anther gets larger and comparable to the exiting field and the particles become electromagnetically coupled. If the distance becomes smaller than the optimum spacing, the factor acutely drops because most of the filed is mainly concentrated in between the nanospheres, and the reduction of bulk heterojunction absorber volume fraction in hybrid layers decreases its total absorption even though some localized field is greatly enhanced. Besides, it is observed that for increasing D, the concentration factor peak shifts to larger spacing because increasing the sphere diameter tends to raise the scattering cross section [7]. (1).0 1.8 F c 1.6 1.4 D=5nm D=10nm 1. D=15nm D=0nm 1.0 5 10 5 50 75 P (nm) Fig. 6 Light concentration factor as a function of the periodicity P and particle diameter D in PCPDTBT/PC 70 BM bulk heterojunction blend. Proc. of SPIE Vol. 8333 83331C-5

4. CONCLUSION We proposed incorporating metallic nanoparticles close to the transparent anodic electrode in the OPV cells. The plasmonic enhancement effects on the absorbance of active layers are investigated by the finite-difference time-domain method. The nanostructure shows the property of broadband optical absorption enhancement with a weak dependence on polarization of incident light. The light concentration is optimized by tuning metallic nanosphere sizes and periods. Our research provides a guide for the optical design of using plasmonic effects to improve the performance of polymer solar cells. REFERENCES [1] J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Nguyen, M Dante, and A. J. Heeger, Efficient tandem polymer solar cells fabricated by all-solution processing, Science, Vol. 317, no. 5835, pp. -5 (007) [] S. Günes, H. Neugebauer, and N. S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev., Vol. 107 (4), pp. 134 1338 (007) [3] H. Atwater and A. Polman, Plasmonics for improved photovoltaic devices, Nature Materials, Vol. 9, pp. 05-13 (010) [4] W. Yoon, K. Jung, J. Liu, T. Duraisamy, R. Revur, F. L. Teixeira, S. Sengupta, and P. R. Berger, Plasmonenhanced optical absorption and photocurrent in organic bulk heterojunction photovoltaic devices using selfassembled layer of silver nanoparticles, Sol. Energy Mater. Sol. Cells, Vol. 94 (), pp. 18-13 (010) [5] J. Zhu, M. Xue, H. Shen, Z. Wu, S. Kim, J. Ho, A. Hassani-Afshar, B. Zeng and K. L. Wang, Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres, Appl. Phys. Lett., Vol. 98, pp. 151110 (011) [6] G. Dennler, K. Forberich, T. Ameri, C. Waldauf, P. Denk, C. J. Brabec, K. Hingerl, and A. J. Heeger, Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence, J. Appl. Phys., Vol. 10, pp. 13109 (007) [7] U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters, (Springer, Berlin, Germany, 1995) [8] E. D. Palik, Handbook of Optical Constants of Solids, (Academic, New York, USA, 1985) [9] K. Yee, "Numerical solution of initial boundary value problems involving Maxwell's equations in isotropic media," IEEE Trans. Antennas Propag., Vol. 14, pp. 30 307 (1966) [10] S. Kim, J. Zhu, H. Shen, M. Xue, K. L. Wang, Z. Yu, L. Li, J. Park Q. Pei and G. Park, "Plasmonic organic Solar cell and its absorption enhancement analysis using cylindrical Ag nano-particle model based on finite difference time domain (FDTD)," CLEO: 011 - Laser Applications to Photonic Applications, Baltimore, MD, USA (011) Proc. of SPIE Vol. 8333 83331C-6