APC000149Characteristics of high efficiency InGaP InGaAs double junction solar cells grown on GaAs s

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INTRODUCTION
Metamorphic or lattice mismatched III-V semiconductor materials offer ability for solar cell (SC) to absorb solar energy efficiently enabling new bandgap combinations in multijunction solar cells and enhancing conversion efficiency. luosoGaosoP material which has a wideband gap of about 1.86 eV is suitable for the top cells of high efficiency multi-junction solar cells. The tandem combination of an optically thin InGaP top cell and a GaAs bottom cell has a theoretical efficiency of 34% [1]. The advantages of this combination are both lattice parameters and photogeneration current of these cells can be matched, a two-terminal monolithic tandem cell can be successfully obtained by using tunnel junction for connecting these cells [2]. High efficiency of 29.5% but small area of 0.25 cm^ InGaP/GaAs tandem cells have already been reported [3] and some researches [2-4] have been done in order to achieve high efficiency of over 30%. However, theoretical calculations reported higher conversion efficiency for tandem solar cells by thinning of the top cell for current matching condition or the bandgap of the top and bottom cells should be lowered to capture and convert more high energy photons which are wasted as excess photogenerated current in the substrate [5, 6]. The ternary lUxGai.xAs has a lower bandgap than GaAs (between 1.425eV for GaAs and 0.354eV for InAs) and a larger lattice constant with increasing In-content in the ternary composition. For lower bandgap InGaAs, there is no lattice matched substrate material available. Therefore, the theoretically ideal In^Gai-^As material has to be grown lattice mismatched to the GaAs substrate. Figure 1 shows that the luo soGao soP/GaAs tandem cell, which can be grown lattice-matched to a GaAs or Ge substrate is not the best material combination for the highest conversion efficiency solar cell. Unfortunately, there are problems in growing lattice mismatched material; the formation of misfit and threading dislocations that affect the Schockley-Read-Hall recombination centers, reducing minority-carrier lifetime, voltage, and current collection in lattice mismatched solar cells [7]. The graded buffer layers are necessary to be employed between the active cell layer and the substrate to confine most of those dislocations to the substrate-buffer interface [8] resulting in very low defect density in the active cell region. High efficiency can be obtained with perfect buffer layers.
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CPl 169, International Workshop on Advanced Material for New and Renewable Energy edited by L. T. Handoko and M. R. T. Siregar 2009 American Institute of Physics 978-0-7354-0706-0/07/$25.00
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Bafid Gap Bottorn Cell [eV} FIGURE 1. Calculated efficiencies of monolithic tandem cells versus band gaps of the top and bottom cells for a 500XAM1.5D illumination. Our goal in this report is to study the effect of lowering bandgap energy of the bottom cell for higher conversion efficiency. To avoid high dislocation density, we just decreased bandgap energy of the bottom cell from 1.425eV to 1.387eV corresponding to bandgap of GaAs and Gao gvsino 025AS, and lattice mismatch between IUQ o25Gao 975AS and GaAs is low of about 0.18%. Conversion efficiencies of these solar cells were recorded to be 24.37% and 25.11% (AMI.5, 1 sun, 25°C) for the IUQsoGaosoP/GaAs (sample #1) and IUQsoGaosoP/Inoo25Gao975As (sample #2) solar cells, respectively.
Characteristics of high efficiency InGaP/InGaAs double junction solar cells grown on GaAs substrates
H.P.T. Nguyen, K.H. Kim, H. Lim, and J.J. Lee
Division of Electrical and Computer Engineering, AJou University, Suwon 443-749, Korea
Abstract. In this paper, we report on the conversion efficiency improvement in InosoGaosoP/InxGai.xAs tandem solar cells by employing metamorphic InGaAs bottom cell instead of lattice matched GaAs cell. Ino joGao soP/Ino o25Gao 975AS and Ino 5oGao 5oP/GaAs double junction solar cells were grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) on GaAs substrates. High-resolution transmission electron microscopy (HR-TEM) measurement reveals the dislocation in the Ino o25Gao 975AS layer which is caused by the lattice mismatch between Ino o25Gao 975AS subcell and GaAs substrate. Conversion efficiencies of these cells were measured to be 24.37% and 25.11% (AMI.5, 1 sun, 25°C) for the Ino 5oGao 5oP/GaAs and Ino soGao soP/Iuo o25Gao 975AS solar cells, respectively. The details about the solar cell characteristics will be discussed in the presentation Keywords: tandem solar cell, dislocation, InGaP, InGaAs PACS: 84.60.Jt; 61.72.Ff, 61.72.Hh; 78.55.Cr