NUMERICAL SIMULATIONS AND MODELING OF GAINP SOLAR CELLS Assaf Ben Or, Peter Fuss-Kailuweit, Simon P. Philipps, Ulrich Fiedeler, Stephanie Essig, Eduard Oliva, Frank Dimroth and Andreas W. Bett Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany assaf.ben.or@ise.fraunhofer.de, Phone: +49(0)761/4588-5627, Fax: +49(0)761/4588-9250 ABSTRACT: GaInP solar cells are a major component of III-V multi-junction high-efficiency solar cells. Due to the structural complexity of these devices numerical modeling is crucial for design optimization, as well as for the understanding of the cell physics. This work presents results based on a 2D model of state-of-the-art single junction GaInP solar cells within the commercial simulation environment Sentaurus TCAD from Synopsys. The model is validated by comparing calculated cell reflection, external quantum efficiency (EQE) and current-voltage (JV) characteristic with experimental data. For this purpose GaInP solar cells with deliberate structural variations were fabricated and characterized. The obtained model provides a platform for cell design and a deeper insight to the cell physics. Here the influence of the base layer properties and the window-emitter interface on the cell characteristics is studied. Keywords: III-V Semiconductors, modelling, interfaces, performance. 1 INTRODUCTION Monolithically stacked multi-junction solar cells based on III–V semiconductor materials, such as lattice-matched or metamorphic GaInP/GaInAs/Ge triple-junction solar cells, are the state-of-the-art approach for high-efficiency photovoltaic energy conversion in space and terrestrial applications [1,2]. They consist of stacked p-n junctions with different bandgap energies to exploit the solar spectrum very profitably. Due to the complex electrical and optical interactions between the different layers, a physically accurate numerical model is helpful to guide the technological approaches. In the past excellent results have been obtained for the modelling of III-V single- junction and dual-junction solar cells using numerical semiconductor simulation tools, e.g. [3-5]. It was shown that the particular physical phenomena in III-V solar cells, such as carrier transport at hetero-interfaces as well as optical interference effects, can be well described. Some publications presented simulation results of GaInP single-junction solar cells or GaInP top cells in multi-junction structures [6-10]. Recently, a study on the influence of the hetero-interfaces on the majority carrier transport processes of GaAs solar cells was published [11]. However, if a validation is performed it is only based on one experimental structure. To improve predictive capabilities of the model, we used several different device structures to validate our model. In this paper we present a GaInP solar cell model which can be used to optimize the cell design and to analyse the influence of material and structure parameters such as doping levels or layer thicknesses on the solar cell performance. By using numerical analysis and different structures of GaInP solar cells, an electro-optical model of a single-junction GaInP solar cell was obtained. Based on the model the External Quantum Efficiency (EQE), reflection, J-V characteristic and the energy band diagrams were calculated, taking into account material and structural parameters. The model is ready to be integrated into a multi-junction solar cell model, which is subject to further work. 2 MODELING APPROACH For the results presented here the simulation environment Sentaurus TCAD from Synopsys is used [12]. We model a two-dimensional symmetry element of the solar cell, which is constructed by a cut through the layers from cap to substrate perpendicular to the grid fingers. The element covers a width of 460 μm corresponding to half of the finger spacing. This ensures that series resistance effects caused by lateral current flow in the device are taken into account. To model the GaInP solar cells two prerequisites must be fulfilled: First, the necessary models describing the occurring physical phenomena need to be implemented and validated. Of particular importance for the GaInP cell modelling are optical generation and recombination of minority carriers, charge carrier transport at hetero- interfaces, including thermionic emission. The optical generation is calculated using the Transfer Matrix Method (TMM) [13]. Second, material parameters such as optical constants, carrier mobilites, bandgap energies, electron affinity and parameters for radiative, Auger, Shockley-Read-Hall as well as interface recombination are required as input for the models for each semiconductor layer in the structure. It is essential to identify valid material parameters of the GaInP solar cell to establish a reliable predictive model. By calculating the optical generation of the complete structure and solving the Poisson, continuity and transport equations for holes and electrons, the code calculates the device characteristics such as the current density–voltage (J–V) characteristics under dark conditions and under illumination, the quantum efficiency, the reflectivity, and internal quantities such as the electric field, the free and trapped carrier concentrations, the electron and hole currents, or the recombination and generation rates. 3 MODEL VALIDATION 3.1 Investigated structures In order to validate the model several single- junction GaInP solar cells with deliberate structural