3D Modeling and optimization of Organic Solar Cells Salvatore Sarciá1; Giuseppe Di Lorenzo1; Riccardo Zammataro1 1 Department of Electrical, Electronics and Computer Engineering University of Catania, Italy salvatoresarci@gmail.com Abstract - In this paper a new design process for Organic and an important factors of metallization design [8]. In order to Solar Cells ( O S C s ) are proposed and developed using t h e provide more absorbtion of solar radition graphene c o m m e r c i a l modeling software for technical design and a nanostructures are incorporates from 0.5 to 5 µm in the device simulation tool Comsol vers. 5.3. Comsol was used to determine design or reduced graphene oxide as anode buffer layer is used. the electromagnetic fields on O S C s e l e m e n t interfaces and Graphene has a high optical transmittance, an excellent to optimize aluminum and active layers. A more efficient electron/hole transport properties, superior mechanical stiffness geometry was founded in order to maximize the light and flexibility. The Graphene can be used as transparent trapping and increase the photovoltaic market. The first electrode to retard charge recombination in OPVs .To reduce or experimental results on the new OSCs are reported. retard the carrier recombination we could form a direct channel between anode and cathode creating an alternating of graphene and ITO patterns. Other authors have investigate the Keywords: Industrial lime, treatment, pollution, energy electromagnetic field induced on solar devices in order to recovery,environment, distillation evaluate the interactions of the optical model at several frequencies and the magnetic field using as tool COMSOL to I. INTRODUCTION calculate the electric field effects starting from an organic The third-generation solar cells can be made of low-cost sample. In this work we have presented different three- and sustainable materials such as polymers however with less dimensional models of multilayer bulk hetero- junction organic efficiency than the standard silicon cells mainly limited by the nanoscale solar cells. The 3D simulations of electromagnetic ability of the active layer to absorb light and convert it into fields applied to OSCs presented in this work clearly reveal electricity. A good solution could be investigate new light that the physical mechanisms depend on this particular contact trapping techniques and the impact of geometry in new model design [9]. Using a cross-platform finite element analysis, and architecture design [1]. In organic solar cells (OSCs) the solver and Multiphysics simulation software, we have analyzed geometrical parameters influence significantly the efficiency specific geometrical patterns that could be optimal for and performance. The variation in the length of the devices is capturing and holding light in thin-cell organic solar cells. not negligible affecting the recombination of charge carriers in the organic solar cells. Many authors have demonstrated as the geometry of organic solar cells influences the electrical II. GEOMETRIC MODEL enhancements, the absorption and scattering efficiency of the A solar organic cell is schematized in Fig. 1. In order to particles [2-8]. The authors in describe the effect of investigate the shape in OSCs we proposed new design with geometrical parameters, particularly shape, on optical different geometry types that could lead to more efficient absorption enhancement for thin film solar cells based on organic solar cells. Five main parts are assembled in the crystalline silicon (c- Si) and gallium arsenide (GaAs) using a electronic device such as a rigid glass support, an anode, an rigorous coupled wave analysis (RCWA) method. The light intermediate layer, a photo-active layer and cathode layer [8]. trapping schemes for organic thin film solar cells include The glass support has the peculiarity to be transparent for light. geometric engineering of the structure of the solar cell at the Furthermore, Indium Tin Oxide (ITO)is used as anode or micro and nanoscale, plasmonic structures, and more as positive electrode with a work function greater than metal [3]. reported in [4]. The intermediate layer consists of conductive polymer For specific types of geometrically shaped solar panels a PEDOT: PSS (Poly (3,4 ethylenedioxythiophene) poly simpler method was derived and applied, however with the (styrenesulfonate)) layer deposited directly on anode with the limitations to broaden the applicability of the model to a wider Spin-coater 2.The active material is deposited on PEDOT:PSS range of geometric shapes. layer mainly composed of poly (3-hexylthiophene) (P3HT) and Deepak K. Gupta et al present an efficient manner to max- the fullerene (6,6)- phenyl-C61 butyric acid methyl ester imize the power output with the application of topology (PCBM) responsible for light absorption, charge carrier optimization (TO) to optimize the front metallization patterns production, and carrier separation. Aluminum is evaporated for free-form solar cells that are cells of unconventional shapes used exclusively as cathode. (e.g. hexagonal, leaf-shaped, circular, motorbike fairings etc) The parametric modeling of the OSCs was performed in a with flexible shape added to the aesthetics of the surround- ings similar way to that described by Calì et al. [10,11]. Copyright held by the author(s). 44 The parametric 3D design software of Solidworks has been curvature and thus smaller radius of curvature. Two charged an useful tool to model mechanical and electronic device. spherical conductors of radius R1 and R2 with R2 > R1 connected by a conducting wire. The potentials V1 e V2 have to be the same, the charge on each sphere is in proportion to their radii: V1 = V2 = = (1) Fig. 3. OSC side view Fig. 1. OSC structure Fig. 4. Modelled OSC structures The final charges are then: Fig. 2. PEDOT:PSS structure In our case we have considered the device manufactured This means that the surface charge density of the smaller (Fig.4) in the laboratories in the University of Ben Gurion [1], sphere is larger, i.e. that the charge is more crowded per unit [5]. The thickness of ITO is 70 nm on 0.7 mm thick glass area on the smaller sphere.In particular the ratio between the substrate (12 × 12 mm), 30 nm of PEDOT: PSS, 200nm the charge density Q on the surface of the sphere and the radius R P3HT:PCBM and 90 nm aluminum cathode. In each sample is constant and with the same potential. For smaller surface are inserted four OSCs with different lengths 7.5, 6.5, 5.5 and namely largest curvature, lower surface charge density 4.5 mm. Rendering and 3D visualization problems are avoid distribution is observed than larger surface. increasing the thickness of deposited materials and decreasing The surface charge densities are calculated: the size glass thickness as in Fig. 3. On the basis of OSCs structure, different alternative shapes are proposed for (3) aluminum cathode taking in account the increasing of charge density. The electrons and holes inside the metal react to the electromagnetic excitation inducing surface charges. The local (4) surface charge density increases drastically in geometrical singularity such as tip apex. We have investigated the effects of as resulted: different geometry characterized of several tip apexes to response to electromagnetic excitations. In general, the charge (5) density is concentrated in the region with largest value of 45 been created inscribed in a circumference having a diameter (6) equal to 1376 µm, with the ”tangent arc” function it was possible to realize the curved shape of the sides as shown in Fig. 7. The surface charge density is lower on second sphere. Thus the curvatures and apertures of the tip apex strongly influence the density charges. The effects of different apertures, curva- III. SIMULATION MODEL ture and sharpness of tip are analyzed to enhance the efficiency The types of geometric models proposed of the samples and electromagnetic field in OSCs. To model the cell with the processed in SolidWorks have been imported in COMSOL and rectangular cathode a 2D sketch was created on the upper face then the physical parameters has been set useful for the of the active layer and using the ”rectangle of the corner” simulation of electromagnetic field. We have considered the function, it was possible to draw the cathodes. Through the Maxwell equations with particular referring to the electric field “basic extrusion” function, it was possible to realize the final E and the magnetic flux density B as reported below: form. To create the Arabic cell, a 2D sketch was created on the upper face of the active layer 5. Using the ”spline” function, ∇·E =0 (7) ∇×E= (8) ∇·B =0 (9) Fig. 5. Model of OSCs inspired to Arabic style Fig. 7. Model of OSCs inspired to pentagon shape Fig. 6. Model of OSCs inspired to nine-pointed stars it was possible to draw the curved profile of the cathode. The rectangular part was created using the ”rectangle of the corner” function. Extruding the sketch the shape of an Arabic shape was created. The nine-pointed star shape was also created by a 2D sketch on the upper face of the active layer12. First a circumference with a radius equal to 2245 µm was Fig. 8. Arabian style model drawn using the ”circle” function, then another circle with a radius of 300 µm and a geometric center on the first circumference was drawn. First of all using the ”circular repetition” function and then the ”shorten entities” function, it ∇ × E = µε (10) was possible to obtain the definitive profile The pentagonal shape was configured by means a 2D sketch on the upper face of active layer 6. Using the ”polygon” function a hexagon has 46 The material properties are described using relative perme- The proposed shape of Aluminum cathode are rectangular, ability ε and the relative permittivity µ. In this application RF pentagonal and nine-pointed stars and are simulated using the module is used for Multiphysics simulation models for RF Module of Comsol Multiphysics. The optimal results of wavelengths of the visible spectrum from 400 nm to 700 nm. In electric field are obtained for cathode of 5.5 mm with order to guarantee the simulation phenomena, optical values pentagonal shape. Thus we have demonstrated that the shape of and electric properties of different materials are used as cathode influences the performances of solar cells. reported in [2]. Each sample was exposed to an incident light beam. Considering the discretization by Finite Element Method in order to obtain reliable simulation results, was used “Free mesh” able to automatically thicken free elements on the edges of the organic cell. On the basis of the light incident on device, the electromagnetic field is applied in perpendicular direction on glass substrate chosen as active port for port boundary conditions. The outgoing signal is determined on surface of the aluminum cathode whereby to calculate the electric field, the magnetic field and power electric flow. From obtained results, power electric flow for cathode of 5.5 mm has less electric value considering rectangular shape aluminum than pentagonal shape as reported in Fig. 9.The maximum peak of electric field along the direction of the organic cell is obtained for length of 5.5 mm and has a value of 2.91*10−4 Vm-1. Fig. 11. Electric Field Aluminum pentagonal Shape IV. CONCLUSIONS In this paper the ultrathin organic solar cells costitued by GLASS/ITO/PEDOT: PSS/ PF3HT:PCBM/ Al layers with different geometry of aluminum cathode are investigated. In particular different geometrical models of OSCs are proposed using the 3D design software SolidWorks and processed in COMSOL to simulate the physical phenomena of electro- magnetic fields. Fig. 12. Electric Field Aluminum rectangular Shape In our case based on charge density distribution it is observed that for lower curvature of Aluminum cathode the electric field increases also for lenght dimensions of 5.5 nm. Fig. 9. 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