Geometric Light Trapping in 2D and 3D Structured Small Molecule Organic Solar Cells
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Geometric Light Trapping in 2D and 3D Structured Small Molecule Organic Solar Cells Marcos Soldera1, Emiliano Estrada1 and Kurt Taretto1 1
Departamento de Electrotecnia, Universidad Nacional del Comahue - CONICET, Argentina
ABSTRACT Despite organic solar cells have recently shown remarkable high power conversion efficiencies approaching 10%, further improvements are required to provide a low-cost alternative to inorganic photovoltaics. Optical losses related to insufficient light trapping and parasitic absorption of the contact layers limit drastically the photocurrent delivered by the cells. Textured surfaces, such as V-grooves (2D) and pyramids (3D), can provide better light coupling into the conformally deposited solar cells. In this work, we analyze the enhancement in light absorption in textured solar cells based on copper phtalocyanine (CuPc) and fullerene (C60) on the micro- and submicroscale. The analysis is carried out with the aid of the finite element method in 2D and 3D, taking into account interference as well as reflection and refraction of the incident AM1.5G spectrum. The results show that both type of structured cells perform better than planar cells reaching up to 23% improvement in maximum photocurrent for normal incidence. We also explore the lateral inhomogeneities of the generation rate within the active layers and their potential effect on the exciton collection efficiency. INTRODUCTION Over the last years organic solar cells have proven a continuous advancement to become a low-cost alternative to inorganic photovoltaics, reaching a confirmed power conversion efficiency of 10% [1]. Among the several factors which limit the performance of state-of-the-art organic solar cells, we address the optical losses due to insufficient light trapping and parasitic absorption in the contact layers. Due to the poor electronic quality of organic semiconductors, bilayer organic cells [2] are deposited with layer thicknesses in the range of 20 - 80 nm, which are comparable to the exciton diffusion lengths but an order of magnitude lower than typical absorption lengths [3,4]. This gives rise to incomplete light absorption and a low power conversion efficiency. Geometric light trapping can alleviate in part such weak absorption by modifying the cell geometry through two fundamental concepts. On the one hand, the optical path inside the cell is extended by allowing the radiation to enter the cell at an oblique angle [5,6], and on the other hand, the textured front surface forces the incoming rays to bounce and transmit between the facets several times before they reflect back [7]. In this contribution we investigate the performance of 2D and 3D texturized solar cells based on the material system copper phtalocyanine (CuPc)/fullerene (C60), which is a reference among small molecule organic donor/acceptor heterojunctions. The analysis is carried out with the aid of the finite element method in 2D and 3D, taking into account interference as well as reflection and refraction of the incident AM1.5G spectrum. MODELIN
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