Electrophotonic improvement of polymer solar cells by using graphene and plasmonic nanoparticles

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1 Copyright 2017 by American Scientific Publishers All rights reserved. Printed in the United States of America /2017/7/305/007 doi: /mex Electrophotonic improvement of polymer solar cells by using graphene and plasmonic nanoparticles Ali Department of Electrical Engineering, University of Business and Technology, Jeddah 21432, Saudi Arabia; Department of Engineering Physics, Alexandria University, Alexandria, 21544, Egypt ABSTRACT It is essential to enhance a solar cell performance at near-infrared region which represents almost 40% of sunlight energy. In this paper, an efficient light trapping polymer solar cell which uses plasmonic nanoparticles and antireflection transparent graphene layer is introduced. The shape of the periodic nanostructure of nanocrystalline zinc oxide (nc-zno) grown on its flat surface and the thickness of graphene layer are optimized. Lumerical finite difference time IP: domain (FDTD) On: solution Tue, software 11 Sep 2018 is used 22:12:01 to design and analyze the proposed structure. In addition, electrical andcopyright: optical models American are developed Scientific Publishers to calculate the short circuit current density, fill factor and overall efficiency of the designed Delivered polymer by solar Ingenta cell structure. The distributed gold nanoparticles (Au-NPs) inside the active layer with 41 Au-NPs/unit cell produce the maximum efficiency and short circuit current density, 8.94% and ma/cm 2 respectively, and a high light absorption near-infrared region is obtained. Finally, the electric field distribution inside the solar cell structure is also illustrated in this work. Keywords: FDTD, Plasmonic Nanoparticles, Polymer Solar Cell, Short Circuit Current Density, Overall Efficiency. 1. INTRODUCTION Organic photovoltaic devices (OPVs) have drawn much research interest in the past decades due to their low cost, flexibility, lightweight, large area and its roll-to-roll (R2R) production compatibility. 1 3 However, the lower efficiency and stability are still massive challenges. 4 7 As a result, many techniques have been introduced to enhance the power conversion efficiency (PCE) by optimizing the device structure 8 and using a plasmonic cavity. 9 Surface plasmon localization on metallic nanoparticles is considered one of the efficient technique used to increase the optical absorption of solar cells. 10 Qu et al. improved the optical absorption by distributing silver nanoparticles (Ag-NPs) uniformly at the interface between (PEDOT:PSS) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and (P3HT:PCBM) a.elrashidi@ubt.edu.sa poly(3-hexylthiophene):(6,6)-phenyl-c61-butyric-acid-methyl ester layers. 11 The optical absorption enhancement reached to almost 100% when the Ag-NPs were located in the interface layer. Organic solar cell with a thick active layer with 11% power conversion efficiency was obtained theoretically by Nicla et al. 12 Overall efficiency of a polymer solar cell was improved by using nanoholes photoactive layer to almost 6.7%. 4 The authors compared their model to Tumbleston et al. model, 13 where a photonic crystal photoactive layer was used to reach 5.03% efficiency enhancement. Graphene is a monolayer of carbon atoms packed in two-dimensional (2D) single-atom-thick, which attracted attention in many research areas specially solar cells. 14 Graphene can be used as a highly transparent antireflection coating instead of indium tin oxide (ITO) not only for the remarkable optical, electrical, mechanical and thermal properties but also for the corrosion of ITO due to the chemical interaction with PEDOT:PSS. 15 Mater. Express, Vol. 7, No. 4,

2 Jiang et al. improved the electrical conductivity of where k is the Boltzmann constant and T is the absolute PEDOT:PSS to 35% by adding a reduced graphene to temperature. the solar cell structure. 16 Carrier mobility and collection The fill factor can also be calculated from Eq. (3) as a efficiency were improved by providing additional function of maximum output power P max. charge transport pathways in the hole transport layer when reduced graphene was added. FF = P max In this paper, a polymer solar cell light harvesting was J sc V oc (3) improved specially at near infrared region which accounts By calculating the maximum output power, P max, the overall solar cell efficiency,, can be calculated as a ratio of for almost 40% of the sunlight energy. Short circuit current density (J sc and light absorption in P3HT:PCBM active maximum output power to solar input power. layer were simulated by applying a finite difference time Nevertheless, the optical power absorbed in the active domain (FDTD) method using Lumerical FDTD solutions layer is affected by Au plasmonic nanoparticles distributed software package. Optimization of a periodic nanostructure inside the layer, and depends on the maximum value of shape grown on a nc-zno layer and the radius of Au reflectivity. The transmitted power of plasmonic nanoparticles is affected by NPs shape, relative permittivity of nanoparticles (NPs) distributed inside the active layer are introduced in this work. Different thicknesses and surface the gold nanoparticles, and dielectric function of the surrounded shapes of a graphene layer are also studied. Furthermore, medium. 17 The maximum value of reflectivity is the optical absorption has been enhanced by distributing calculated as in Eq. (4). Au-NPs, with different densities, inside the active layer of the proposed solar cell structure. More sensitive and accurate results can be obtained max = P ( ) d m max 1/2 g d + m max (4) using FDTD simulation software by minimizing the meshing size, increase simulation time and by using a where d is the permittivity of the surrounding medium, 3-D analysis. However, the obtained results can be slightly m is a gold nanoparticles dielectric constant at corresponding max, g is an integer and P is structural periodic- different from the experimental work due to the use of adhesive materials between layersip: which has a considerable effect but not taking into consideration Copyright: in theamerican simula- Scientific ity. Hence, the dielectric permittivity can be expressed by On: Tue, 11 Sep :12:01 using a multi-oscillator Publishers Drude-Lorentz model: 17 tion work. Delivered by Ingenta 2 6 D m = k 2 k 2 + j D k=1 2 2 k 2. THEORETICAL MODEL k (5) In order to calculate the electrical parameters of a polymer where is the Au dielectric permittivity at high frequency, solar cell, a single diode model is considered for the filling D and D are the plasma and collision frequen- actor, FF, by applying the Green empirical expression. 4 cies of the free electron gas, k is the amplitude of Lorentz In the first step, short circuit current density, J sc, is calculated using Eq. (1) by assuming that, an electron will be k is the damping constants for k value from 1 to 6. oscillator, k is the resonance angular frequencies and produced for every incident photon. 13 J sc = q I A d (1) hc where q is the electron charge, h is the Planck s constant, c is the speed of light, I is the standard air mass 1.5 (AM1.5) spectral irradiance and A is the optical absorption. Absorbed optical power in the proposed structure can be obtained using lumerical FDTD solutions by calculating the difference between the optical power at the top and at the bottom of active layer. The open circuit voltage, V oc, of P3HT:PCBM is considered to be a fixed value equal to 0.62 V as it is depends on the energy level of the material. 4 Hence, the fill factor can be calculated using Eq. (2). FF = qv oc/kt ln qv oc /kt qv oc /kt + 1 (2) 3. MATERIALS AND STRUCTURE Lumerical FDTD solutions software, which is an electromagnetic wave solver based on finite difference time domain method, is used in designing and analyzing the proposed polymer solar cell. In this simulation analysis, unit cell dimensions are simulated and optimized to be w = 460 nm (width) and L = 800 nm (length) with Au back contactor thickness h = 500 nm, used as a cathode. A nc- ZnO layer used as an electron transport layer, is grown above Au layer with thickness t1 = 70 nm as illustrated in Figure 1. A periodic nanostructure of nc-zno material can be attached to the surface with periodicity t2 = 400 nm using 2D face centered cube structure. The active layer P3HT:PCBM with thickness h1 = 590 nm is utilized to cover the nanostructures of electron transport layer. The PEDOT:PSS layer is used as a hole transport layer (HTL) with thickness h2 = 50 nm and graphene layer is used as transparent antireflection layer and as an anode contact 306

3 4. RESULTS AND DISCUSSION Fig. 1. Schematic diagram of the proposed structure. To maximize the absorbed power in the active layer, the short circuit current density and consequently the overall solar cell efficiency, four phases are introduced. In the first phase, different shapes of nc-zno nanostructure, grown on nc-zno flat surface, are simulated to get the maximum short circuit current density and high light absorption. Cylindrical, conical, hemispherical, pyramidal, triangular strip and rectangular strip nanostructure shapes have been studied. The radius of the randomly distributed Au-NP in the active layer is optimized in the second phase. In the third phase, thickness and surface roughness of the graphene layer, replaced ITO layer, are also simulated. Finally, the density of Au-NPs distributed in the active layer are studied and consequently electric field distribution is also developed in different layers inside the proposed structure. Fig. 2. Energy levels diagram of all materials for the given structure. Fig. 3. Absorption of different nanostructure shapes versus optical wavelength. 307 with thickness h3, need to be optimized. Plasmonic NPs 4.1. Different nc-zno Nanostructure Shapes are randomly distributed in the active layer with different In order to enhance the light absorbed in the active layer, concentrations. periodic nanostructure of nc-zno material is grown inside A one unit cell has been simulated and the boundthe active layer with height t2 = 400 nm, where the periary conditions are considered a periodic structure in odic structure is a FCC in 2D. Different shapes are used in x-direction and y-direction and perfect matching layer in the simulation process, cylindrical, conical, hemispherical, z-direction. A plane wave source with wavelength band pyramidal, triangular strip and rectangular strip shapes nm and offset time 7.5 fs is used as a light source. Figure 3 shows the absorbed light in the active layer of IP: Tue, 11 Sep :12:01 In addition, the solar generation calculation region ison: given the givenpublishers shapes. The absorption of cylindrical and hemicopyright: in the active layer to calculate the short circuitamerican current Scientific spherical shapes is higher than the other shapes specially Delivered by Ingenta density. at wavelength larger than 650 nm. Energy band diagram illustrates the electrons and holes However, the short circuit current density of the hemitransportation inside the structure. Energy band diagram spherical shape is higher than the cylindrical shape, of the proposed structure is illustrated in Figure 2. As the and ma/cm2 respectively, as given in Table I. Howelectron hole pair is generated in the active layer, electron ever the 7.15 ma/cm2 short circuit current density is transport from the active layer to nc-zno layer and then obtained in case of a flat ZnO surface. Consequently a to the electrode layer. Consequently, the generated hole higher overall efficiency is obtained from the hemisphertransfer from active layer to HTL then to graphene layer. ical shape, 5.71%, and almost 5.7% is obtained from the cylindrical shape compared to 3.68% in case of flat ZnO surface. Therefore, the hemispherical shape is selected as an optimum shape for giving a higher electrical and optical performance than the cylindrical shape.

4 Table I. Short circuit current density and overall efficiency for different nanostructure shapes. Nanostructure shape Table II. Short circuit current density and overall efficiency for different NPs radius. NP radius (nm) Impact of Using Graphene Layer In this section, graphene is used as an electrode and as an antireflection transparent layer replacing ITO layer. Optimum graphene thickness is considered as the main parameter that affects both electrical and optical solar cell properties. Different thicknesses are studied and these thicknesses are 100, 125, 150, 175, 200, 225 and 250 nm Optimization of Metallic Plasmonic NP Radius As shown in Figure 5, the absorption is highly depending Plasmonic Au-NPs, used to enhance the absorption in the on the wavelength band. At thickness 200 nm, the light active layer, 17 are randomly distributed inside the active absorption is high between 550 nm and 630 nm the absorplayer (9 Au-NPs). The transmitted and absorbed power of tion in wavelength band nm of 100 nm thickness plasmonic nanoparticle depends on its shape as given in is more efficient than 200 nm. Eqs. (4) and (5), so the NP radius has a great effect on the To determine which thickness is more convenient, its performance. Figure 4 shows the enhancement On: of the IP: Tue, 11 Sep :12:01 Table III is used to illustrate the short circuit current American light absorption for different Au-NPs Copyright: radii. For radii 12.5, Scientific Publishers density and overall efficiency for the given thickness. 15, 20, 22.5, 25, 30, 35 and 40 nm, the maximumdelivered absorp- by Ingenta As noticed in Table III, the maximum short circuit current tion is obtained at radius 30 nm specially at the longer density is ma/cm2, and the efficiency is 8.31%, both wavelength band nm. Furthermore, the change occurred at graphene thickness 200 nm. So, the optimum of NP radius has a slight effect on the optical absorpthickness of graphene layer is chosen to be 200 nm for its tion below 600 nm and has a higher effect at wavelength high properties. greater than 600 nm. However the short circuit current In addition, to enhance the light harvesting of the density and overall efficiency almost constant for radius graphene layer, different surfaces shapes are compared to more than 30 nm. the flat surface. The light absorption of different surfaces Maximum short circuit current density and overall effiis illustrated in Figure 6, these surfaces shapes are flat, 2 ciency are obtained at NP radius 30 nm, Jsc = 15.4 ma/cm conical, periodical hemispherical and inverted hemispheriand = 7.94% as shown in Table II. So, the optimum cal shapes. As shown in Figure 6, the absorption of the flat Au-NP radius is taken to be 30 nm. surface is typical to inverted hemispherical surface. However, the short circuit current density of the flat surface and Fig. 4. Light absorption of different radius of 9 plasmonic Au-NPs distributed in the active layer. 308 Fig. 5. Absorbed light of different graphene thicknesses.

5 Table III. Short circuit current density and overall efficiency of different graphene thicknesses. Graphene thickness (nm) Fig. 7. Absorption of different Au-NPs density inside the active layer. inverted hemisphere are and ma/cm2 respectively and consequently the overall efficiencies are 8.31% and 8.63% respectively. Hence, the inverted hemispherical graphene surface shape is chosen although it gives a lower absorption in the higher wavelength. Fig. 6. Absorption of different graphene surfaces shapes. Fig. 9. Different positions of electric field layers Influence of Au-NPs Density Fig. 8. Average short circuit current density versus the total number of The effect of plasmonic NPs density distributed in the Au-NPs/ unit cell. active layer is studied in this section. By randomly distributing 9 Au-NPs/unit cell, the NPs is fill almost 2% of Short circuit current density is totally depends on the the available volume of the active layer. We can increase carriers transferred from active layer to ZnO layer then the NPs density by two nanoparticles at each step to end to the ohmic contact. Hence, NPs closer to the ZnO with 45 Au-NPs/unit cell which fill around 9.5% of the nanostructure play the key rule in this case. The averavailable volume. IP: On: Tue, 11 Sep :12:01 age short circuit current density has been simulated using Copyright: Figure 7 shows selected numbers of NPs, the American samples Scientific Publishers five different random distribution models as shown in Delivered are 9, 19, 27, 33, and 41 nanoparticles per unit cell. by Ingenta Figure 8. Hence, an overall conclusion can be reached, The maximum absorption occurs at higher wavelengths, nm, when 41 Au-NPs are distributed in the active layer. Table IV. The electrical parameters of conventional PC, NH and the proposed structure. As illustrated in Figure 8, the average values of the short circuit current density is monotonically increasing with Structure NPs density until reach to the maximum, ma/cm2, Conventional PC at density 41 NPs per unit cell NH The overall efficiency is calculated at different NPs Proposed structure density, where the maximum overall efficiency, 8.94%, is obtained at short circuit current density ma/cm2 and both occurred at 41 Au-NPs/unit cell. The calculated fill factor using Eq. (3) of the proposed structure for 41 Au-NPs is 0.83.

6 the short circuit current density is not stable for NPs density between 9 and 23 which depends on the selected NPs position and how closer to the ZnO nanostructure and then saturated at 17.3 ma/cm2 for NPs density between 25 and 45 NPs/unit cell. As a comparison, the obtained short circuit current density and overall efficiency from the proposed structure is compared to the introduced nanohole, NH, structure 4 and the conventional PC structure 13 as illustrated in Table IV. Conventional PC model introduced a short circuit current density ma/cm2 which improved to be 13 ma/cm2 using NH structure and reaches to ma/cm2 in the (a) proposed model. The overall efficiency is improved to 8.94% instead of 6.71% and 5.03% given by NH and conventional PC structures Electric Field Distribution Electric field distribution is represented at different layers, inside and outside the active layer, as illustrated in Figure 9. The layers are; x1 at the top of active layer, x2, at the bottom of active layer and x3 as a vertical layer. Au-NPs absorb the transmitted light at the top of active layer is illustrated in Figure 10(a). At the bottom of active layer, the light is concentrated in the mid of nanostructure and transferred to the flat surface of nc-zno layer as in Figure 10(b). Finally, an overview of light transmission through the structure is given in Figure 10(c). The electric field distribution shows that, the inverted hemispherical shape in the graphene layer behaves as a nanoantenna which retransmits the light into the solar cell body and then, by the help of Au-NPs, the electric field is transferred and confined into nanostructure of nc-zno layer. This happens due to the contrast of reflective index between the nanostructure and the surrounding medium of active layer. 5. CONCLUSION (b) (c) Optical absorption, short circuit current density, fill factor IP: On: Tue, :12:01 andsep overall efficiency of a polymer solar cell are simulated Copyright: American Scientific Publishers using FDTD method. A major enhancement of the electridelivered bycal Ingenta and optical properties in the active layer is achieved by using graphene layer, 200 nm thicknesses, instead of ITO layer. A hemispherical nc-zno nanostructure grown inside the active layer produce maximum optical and electrical properties among different simulated shapes. Moreover, Au-NPs, 30 nm radius, are randomly distributed inside the active layer with different concentration. The maximum obtained overall efficiency and short circuit current density are 8.94% and ma/cm2 respectively when the plasmonic density is 41 Au-NPs/unit cell. In addition, electric field distribution shows that the inverted hemispherical shape in the graphene layer behaves as a nanoantenna by retransmitting the tapped light from the surface into the polymer solar cell structure. Hence, the light is transferred from the Au-NPs distributed in the active layer to nc-zno nanostructure and then to the Au electrode layer. References and Notes Fig. 10. Electric field distribution at different layers in the solar cell structure, (a) at the top of active layer, (b) at the bottom of active layer and (c) vertical layer S. Rafique, S. Abdullah, M. M. Shahid, M. Ansari, and K. Sulaiman; Significantly improved photovoltaic performance in polymer bulk heterojunction solar cells with graphene oxide; Scientific Reports Journal 7, (2017). 2. S. Rafique, S. Abdullah, W. Mahmou, A. Al-Ghamdi, and K. Sulaiman; Stability enhancement in organic solar cells by incorporating V2 O5 nanoparticles in the hole transport layer; RSC Advances Journal 6, (2016). 3. J. Pan, P. Li, L. Cai, Y. Hu, and Y. Zhang; All-solution processed double-decked PEDOT: PSS/V2 O5 nanowires as buffer layer of high

7 performance polymer photovoltaic cells; Solar Energy Materials and Solar Cells Journal 144, 616 (2016). 4. D. Rahman, M. Hameed, and S. Obayya; Light harvesting improvement of polymer solar cell through nanohole photoactive layer; Optical and Quantum Electronics 47, 1443 (2015). 5. J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. Chen, J. Gao, G. Li, and Y. Yang; A polymer tandem solar cell with 10.6% power conversion efficiency; Nat.. Journal 4, 1 (2013). 6. L. Chen, W. Sha, and W. Choy; Light harvesting improvement of organic solar cells with self-enhanced active layer designs; Opt. Express 20, 8175 (2012). 7. D. Zheng, W. Huang, P. Fan, Y. Zheng, J. Huang, and J. Yu; Preparation of reduced graphene oxide: ZnO hybrid cathode interlayer using in situ thermal reduction/annealing for interconnecting nanostructure and its effect on organic solar cell; ACS Applied Mater. Interfaces 9, 4898 (2017). 8. S. Chou and W. Ding; Ultrathin, high-efficiency, broad-band, omni acceptance, organic solar cells enhanced by plasmonic cavity with subwavelength hole array; Optics Express 21, A60 (2013). 9. B. Zeng, Q. Gan, Z. Kafafi, and F. Bartoli; Polymeric photovoltaics with various metallic plasmonic nanostructures; Journal of Applied Physics 113, (2013). 10. F. Liu, D. Qu, Q. Xu, X. Pan, K. Cui, X. Feng, W. Zhang, and Y. Huang; Efficiency enhancement in organic solar cells with extended resonance spectrum of localized surface plasmon; IEEE Photonics Journal 5, (2013). IP: On: Tue, 11 Sep :12:01 Copyright: American Scientific Publishers Delivered by Ingenta 11. D. Qu, F. Liu, Y. Huang, W. Xie, and Q. Xu; Mechanism of optical absorption enhancement in thin film organic solar cells with plasmonic metal nanoparticles; Optics Express 19, (2011). 12. N. Gasparini, L. Lucera, M. Salvador, M. Prosa, G. Spyropoulos, P. Kubis, H. Egelhaaf, C. Brabec, and T. Ameri, Highperformance ternary organic solar cells with thick active layer exceeding 11% efficiency; Energy Environ. Sci. Journal 10, 885 (2017). 13. J. Tumbleston, D. Ko, E. Samulski, and R. Lopez; Electrophotonic enhancement of bulk hetero junction organic solar cells through photonic crystal photoactive layer; Applied Physics Letter 94, (2009). 14. X. Lina, Z. Zhanga, Z. Yuana, J. Lia, X. Xiaoa, W. Honga, X. Chena, and D. Yua; Graphene-based materials for polymer solar cells; Chinese Chemical Letters 27, 1259 (2016). 15. H. Park, J. Rowehl, K. Kim, V. Bulovic, and J. Kong; Doped graphene electrodes for organic solar cells; Nanotechnology Journal 21, 50 (2010). 16. X. Jiang, Z. Wang, W. Han, Q. Liu, S. Lu, Y. Wen, J. Houc, F. Huang, S. Peng, D. He, and G. Cao; High performance siliconorganic hybrid solar cells via improving conductivity of PEDOT: PSS with reduced graphene oxide; Applied Surface Science Journal 407, 398 (2017). 17. A. ; Investigating the performance of ultra-sensitive optical sensor using plasmonic nanoparticles; Nanosci. Nanotechnol. Lett. 8, 465 (2016). Received: 8 May Revised/Accepted: 17 July