Design of 3-dimensional Textured Organic Solar Cells
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1168-P05-17
Design of Three-Dimensional Textured Organic Solar Cells Kanwar S. Nalwa1 Sumit Chaudhary1 Department of Electrical and Computer Engineering, Iowa state University, Ames, IA-50011.
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ABSTRACT Organic materials are attractive for photovoltaics primarily because of the prospect of high throughput solution-processible manufacturing using roll-to-roll or spray deposition. In the formation of polymer based photovoltaic devices, the aspect that limits the power conversion efficiency is the bottleneck between short diffusion lengths of the excited states (excitons) in polymers, in the range of 10-20 nm. Optical absorption length, which is in the range of 50-200 nm and much larger than the exciton diffusion length, poses the limit on charge generation and collection. It is important to achieve complete optical absorption in active layers much thinner than optical absorption length to minimize losses due to recombination of charge carriers. Previously, light trapping techniques have been coupled with organic solar cell but without significant success. In this paper, three-dimensional sub micron grating structure is analyzed using finite element method (FEM) simulations for finding the optical absorption in different layers of solar cell to optimize the photonic concentrator effect of the grating structure. The energy dissipation of electromagnetic field in the active layer is studied as a function of active layer thickness, grating pitch and height. The superiority of grating structure in terms of light trapping feature as compared to planar geometry is clearly demonstrated by simulation results. INTRODUCTION Polymer based photovoltaic devices (PPVD) have gained a lot of attention in the last decade due to their flexibility, solution-processibility, and potential low cost and roll-to-roll manufacturing capability. They have an active layer where a donor is blended with an acceptor material. The interface between these donors and acceptors acts as sites where dissociation of excitons leads to free electrons and holes. The acceptors are often the fullerenes or the derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) having better miscibility in organic solvents. For the poly(3-hexylthiophene) based PPVD, highest efficiencies reaching up to 5% have been reported [1-3]. The performance of these devices is limited by several factors. The high energy band gap of the polymer materials poses a serious limitation on the capability to harvest lower energy photons from sunlight. Moreover the charge carrier mobility of the materials is also moderate which makes it necessary to keep the thickness of the active layer low. Thus there is a conflict between higher optical absorption from thicker layer and efficient transport of carriers, which has to be considered carefully to determine the optimal thickness of active layer. This conflict can be resolved using a 3d-textured grating shaped solar cell geometry which according to the simulations reported, demonstrate higher absorption as compared to the planar solar cells
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