Room-Temperature Wafer Bonded Multi-Junction Solar Cell Grown by Solid State Molecular Beam Epitaxy
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Room-Temperature Wafer Bonded Multi-Junction Solar Cell Grown by Solid State Molecular Beam Epitaxy Shulong Lu1, Shiro Uchida2 1 Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, P. R. China 2 Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan ABSTRACT We studied the InGaP/GaAs//InGaAsP/InGaAs four-junction solar cells grown by molecular beam epitaxy (MBE), which were fabricated by the novel wafer bonding. In order to reach a higher conversion efficiency at highly concentrated illumination, heat generation should be minimized. We have improved the device structure to reduce the thermal and electrical resistances. Especially, the bond resistance was reduced to be the lowest value of 2.5 × 10-5 Ohm cm2 ever reported for a GaAs/InP wafer bond, which was obtained by the specific combination of p+-GaAs/n-InP bonding and by using room-temperature wafer bonding. Furthermore, in order to increase the short circuit current density (Jsc) of 4-junction solar cell, we have developed the quality of InGaAsP material by increasing the growth temperature from 490 °C to 510 °C, which leads to a current matching. In a result, an efficiency of 42 % at 230 suns of the four-junction solar cell fabricated by room-temperature wafer bonding was achieved. INTRODUCTION Multi-junction solar cells (SCs) are very promising devices for achieving high conversion efficiencies [1-5]. In order to obtain higher efficiencies, it is necessary for the four-junction solar cell to have an optimum bandgap combination such as 1.9 / 1.4 / 1.0 / 0.7 eV, which leads to current matching. In previous work, inverted metamorphic structure with lattice mismatched Ga1xInxAs bottom junctions was incorporated into four-junction solar cell [4]. However, the efficiencies in this structure were lower than theoretically predicted efficiencies due to dislocations of the strained layers. Direct wafer-bonding offers the possibility of integrating lattice-mismatched subcells from different substrates into a single solar cell [5-8]. This technique may allow to attain a wafer connection between GaAs and InP without electrical and optical losses, and a 46.0% efficiency at 514suns has been reported for a four-junction solar cell fabricated by wafer bonding [5]. This was achieved by a GaInP/GaAs top tandem cell grown on GaAs substrate bonded to an InGaAsP/InGaAs bottom tandem cell grown on InP. However, this device could be further improved, because its bond interface resistance was reported to be 0.10 Ohm cm2, a value that is relatively high compared with the values of 10-5 to 10-6 Ohm cm2 obtained from conventional metal ohmic contacts. Bonded multi-junction solar cells are expected to exhibit especially low bond interface resistances, because these devices operate at high current densities in the range of 5–10 A/cm2 under concentrated sunlight illumination. In the present study, we demonstrated the method to reduce the thermal and electrical resistances in order to obtain a higher efficiency. Especially, we describ
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