Metamorphic GaInP-GaInAs Layers for Photovoltaic Applications
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Metamorphic GaInP-GaInAs Layers for Photovoltaic Applications A.W. Bett1, C. Baur1, F. Dimroth1 and J. Schöne1,2 1 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg, Germany, [email protected] 2 Technische Fakultät der Christian-Albrechts-Universität zu Kiel, Kaiserstrasse 2, 24143 Kiel
ABSTRACT GaxIn1-xAs and GayIn1-yP layers were grown lattice mismatched to GaAs and Ge by lowpressure metal organic vapor phase epitaxy (LP-MOPVE). These materials are very promising for further increasing the efficiency of monolithic triple-junction solar cells . Different buffer layer structures were realized. Transmission electron microscopy and x-ray diffraction analysis were used to characterize the quality of the crystal . Both linear and step-graded buffers in GaxIn1-xAs were successfully used under an active solar cell structure. GayIn1-yP as buffer material showed a worse performance. Excellent solar cell performance was achieved for lattice mismatched single-, dual- and triple-junction solar cells. INTRODUCTION Monolithic III-V multi-junction solar cells are widely used for power production in space satellites today. The main reasons are the high efficiency close to 30 %, recently achieved under the AM0 solar spectrum [1], and the excellent electron and proton radiation hardness, which is essential in the space environment. Another potential high-volume application for III-V multijunction solar cells is terrestrial concentrators working at high illumination intensities. Recently excellent efficiencies above 37 % under the terrestrial AM1.5 solar spectrum were reported [2,3]. At present, the best III-V solar cells for both terrestrial and space applications are based on the lattice matched material combination of Ga0.51In0.49P/Ga0.99In0.1As/Ge. These monolithic triple-junction solar cell structures are grown by metal organic vapor phase epitaxy (MOVPE). High quality with respect to the electrical performance, i.e mobility, minority carrier lifetime and diffusion length, is obtained in this lattice matched configuration. However, theoretical calculations show that the chosen band-gap combination of the triple-junction cell is not ideal. Higher efficiencies can be achieved by reducing the band-gap of the middle cell. Theoretical calculations have been performed using the program EtaOpt [4]. EtaOpt simulates the radiative limit efficiency. Figure 1 shows the results for such modeling assuming AM0 and 500xAM1.5direct with low optical aerosol value of 0.085 [5] (referred as AOD-spectrum in the following) as illumination conditions. Ge, with a band-gap of 0.66 eV, was assumed as the bottom cell of a monolithic triple-junction solar cell. Figure 1 shows the calculated efficiencies versus the band-gap of the top and the middle cells.
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eta [%]
GayIn1-yP lattice matched to GaxIn1-xAs eta [%] 48 -- 49 47 -- 48 46 -- 47 45 -- 46 44 -- 45 43 -- 44 42 -- 43 41 -- 42 40 -- 41 39 -- 40 38 -- 39 AM0, 1367 W/m² T=298 K
1.8
1.6
1.4
1.2
0.8
1.0
1.2
1.4
Bandgap of middle
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