III-V Thin-Film Solar Cells Bonded to Si Substrates via Metal Grids
Authors: Hishida Takashi , Liang Jianbo , Shigekawa Naoteru 
Published: December 2020

Ⅲ-Ⅴ on Si multijunction (MJ) solar cells are promising as next-generation solar cells since they can provide high efficiency with low cost in comparison with conventional Si and Ⅲ-Ⅴ MJ cells. In fabricating such III-V/Si MJ cells, Ⅲ-Ⅴ subcells are placed on Si bottom cell by hybrid approaches such as surface-activated bonding (SAB). The bonding interfaces with lower interface resistances are strongly required so as to achieve better performance of hybrid MJ cells.
It was found that the resistance across the directly-bonded III-V/Si interfaces in MJ cells was higher than the resistance in junctions made of heavily-doped substrates because of the thin heavily-doped bonding layers in actual subcell structures. It was reported that indium tin oxide (ITO) films as intermediate layers between III-V and Si subcells played a role of lowering the series resistance of MJs [1]. It was found, however, that the resistance of GaAs//ITO/Si junctions increased by annealing them [2]. Furthermore, the quantum efficiency of Si bottom cells was lowered by inserting the ITO layers because of the free carrier absorption of ITO layers.
These problems are assumed to be avoided by using metal grids in combination with dielectric materials such as SiO2 and SiN. Figure 1(a) shows a schematic process for fabricating Ⅲ-Ⅴ/metal grid/Si MJ cells. Metal grid structures work as ohmic contacts for both of Ⅲ-Ⅴ and Si layers. Dielectric materials, such as SiO2 and SiN, are likely to work as passivation for Si subcells. The metal grids must be slightly thicker than the dielectric materials so as to ensure successful bonding of semiconductor to metal grid. It is also notable that impacts of shadow loss due to the metal grid can be minimized by aligning the emitter contacts on the top cells to the metal grids. We recently fabricated Si//metal grid/Si and GaAs//metal grid/Si junctions by SAB and reported on their electrical characteristics [3]. In this work, we fabricate III-V cells bonded on conductive Si substrates via metal grids and examine their photovoltaic properties.
We prepared n-on-p InGaP/GaAs double-junction (2J) epi layers grown on GaAs substrates. The epi substrates were bonded to p-Si substrates via metal grids using SAB. 2J cells were fabricated by removing the GaAs substrates and forming emitter and base contacts on the top and bottom surfaces, respectively. The top view of the fabricated cells is shown in Fig. 1(b). Note that the emitter contacts were aligned to metal grids. Their current-voltage (I-V) characteristics under the solar irradiance of AM1.5G/one sun, which were obtained by using an in-house solar simulator, are shown in Fig. 1(c). The observed open-circuit voltage and short-circuit current (2.47 V and 10.9 mA/cm2) indicate that the fabricated cells normally work as 2J cells. Although the structure and fabrication process are not yet optimized, the results that we obtained suggests that the bonding via metal grids are promising for fabricating hybrid MJ cells with low series resistance.
[1] Naoteru Shigekawa, Tomoya Hara, Tomoki Ogawa, Jianbo Liang, Takefumi Kamioka, Kenji Araki, and Masafumi Yamaguchi, "GaAs/Indium Tin Oxide/Si Bonding Junctions for III-V-on-Si Hybrid Multijunction Cells With Low Series Resistance", IEEE J. Photovolt. 8 (3), pp. 879-886 (2018).
[2] Tomoya Hara, Tomoki Ogawa, Jianbo Liang, Kenji Araki, Takefumi Kamioka, and Naoteru Shigekawa, "Electrical properties of GaAs//indium tin oxide/Si junctions for III-V-on-Si hybrid multijunction cells", Jpn. J. Appl. Phys. 57, 08RD05 (2018).
[3]Takashi Hishida, Jianbo Liang, and Naoteru Shigekawa, "Low-resistance semiconductor/semiconductor junctions with intermediate metal grids for III-V-on-Si multijunction solar cells", Jpn. J. Appl. Phys. 59, SB, SBBB04 (2020).

Figure 1