Application of SC-Simul for Numerical Modeling of the Opto-Electronic Properties of Heterojunction Diodes
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Application of SC-Simul for Numerical Modeling of the Opto-Electronic Properties of Heterojunction Diodes R. Brüggemann, M. Rösch, S. Tardon, G.H. Bauer Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany Email: [email protected] ABSTRACT We apply the publicly available device modeling tool SC-Simul for simulating experiments with user-defined heterojunction diodes to discuss the role of the electric field in solar cells. For amorphous silicon/crystalline silicon heterodiodes, the role of interface defects, an amorphous silicon buffer layer and low-cost crystalline silicon is studied by simulation of current-voltage characteristics and photoluminescence. Photoluminescence is sensitive to the minority carrier density in the volume of the device and can be used to monitor minority carrier properties in these diodes. INTRODUCTION Device modeling is helpful in gaining further insight into device characteristics as it gives access to internal variables. The simulation program SC-Simul allows versatile modeling of the electronic and opto-electronic properties of semiconductor layers and devices [1], both for stationary and time-dependent modeling of different experiments with user-defined devices. Applying this program, we discuss the role of the electric field in charge separation in solar cells by the construction of purpose-built devices. The second part of the paper is devoted to amorphous silicon / crystalline silicon (a-Si:H/c-Si) heterojunction solar cells and the analysis of current-voltage characteristics and photoluminescence (PL) properties. We investigate the role of an intrinsic amorphous-silicon buffer layer, the influence of interface states and the application of low-cost low-lifetime crystalline silicon in such solar cells. DEVICE MODELING: SC-SIMUL PROGRAM DESCRIPTION The program solves Poisson’s equation, the continuity equations for electrons and holes, and the current transport equations including drift, diffusion, and thermionic emission over barriers, if present, in the valence and the conduction band. Both stationary and time-dependent calculations can be performed. The program code in Fortran uses both adapted algorithms and the software library package LSODI. More details are given by Rösch [2]. A graphical user interface assists with the modification of device and material parameters and offers first-hand information by graphical representation of the most important output variables like band structure and distributions of currents and recombination rates. Generally, different device structures can be designed in SC-Simul like a-Si:H pin diodes or a-Si:H/c-Si heterodiodes [3-7] with specific characteristics of the different layers like electron affinity, band gap, Fermi level or doping density and a distribution of states in the band gap. Free
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carriers interact with states in the gap by capture into and emission from these states. From these rates, the recombination rate is calculated. Luminescence is deduced from the local ra
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