Numerical 3D-Simulation of Micromorph Silicon Thin Film Solar Cells
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Numerical 3D-Simulation of Micromorph Silicon Thin Film Solar Cells Stefan Geißendörfer1, Karsten von Maydell1 and Carsten Agert1 1 NEXT ENERGY ͼ EWE-Forschungszentrum für Energietechnologie an der Carl von Ossietzky Universität , Carl-von-Ossietzky-Str. 15, 26129 Oldenburg, Germany ABSTRACT In this contribution 1, 2 and 3-dimensional simulations of micromorph silicon solar cells are presented. In order to simulate solar cells with rough interfaces, the surface topographies were measured via atomic force microscopy (AFM) and transferred into the commercial software Sentaurus TCAD (Synopsys). The model of the structure includes layer thicknesses and optoelectronic parameters like complex refractive index and defect structure. Results of the space resolved optical generation rates by using of the optical solver Raytracer are presented. The space resolved optical generation rate inside the semiconductor layers depends on the structure of the transparent conductive oxides (TCO) interface. In this contribution the influence of different optical generation rates on the electrical characteristics of the solar cell device are investigated. Furthermore, the optical and electrical results of the 1D, 2D and 3D structures, which have equal layer thicknesses and optoelectronic parameters, are compared. INTRODUCTION For silicon based thin film solar cells, TCO’s like SnO2:F or ZnO:Al can be used as front contact. The propagation of the incident light inside the solar cell depends on the TCO roughness and, therefore, the quantity of photo generated charge carriers. Many theoretical and experimental investigations of the short circuit current enhancement due to surface roughness are published e.g. [1,2] and the topic of several research groups is the improvement of the so-called light trapping effect. One strategy to calculate the rate of photo generated charge carriers is to create virtual device structures in two or three dimensions. Therefore, a measured TCO surface topography has to be transferred to the simulation software. After construction of the virtual stack according to the data of experimentally deposited layers, different optical solvers can be used to calculate the local optical generation rate G(x,y,z). The finite-difference-time-domain method (FDTD) solves the Maxwell equations in time and space and considers interference effects of the incident light. Previews work using this method for silicon thin film solar cells was published by Lacombe et al [3]. However, the optical solver Raytracer, which calculates the light beam propagation depending on incidence angle to device interfaces and refractive indices of interface materials, can also be used in the case of rough interfaces. After modeling of the light beam propagation through the cell stack, the absorption will be calculated by the Lambert-Beer law. With that method it is not possible to regard interference effects. Though, in solar cell devices with good light trapping concepts and the corresponding TCO roughness the interference effects will become negligible.
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