Unique Design and Performance Parameters of Multijunction GaInP/GaAs/Ge Solar Cells
This high-efficiency, multijunction GaInP/GaAS/Ge cell with heterostructure configuration consists of three subcells, namely, the GaInP subcell, the GaAs subcell, and the Ge subcell. Each subcell has its own unique material characteristics, which contribute to the overall external quantum efficiency and conversion efficiency of the entire solar cell. The subcell structures are grown by the MOVPE technology with precise control over the layer thickness, composition, and doping levels of the films. Significant performance improvements are achieved with heterojunction structures incorporating various semiconductor layers and interfaces of appropriate materials and layer thicknesses. Improvement in conversion efficiency is strictly due to higher open-circuit voltage and short-circuit current, improved fill factor, reduced reflection and recombination losses, enhanced crystal quality of the epitaxial layers, and optimization of design parameters for the GaInp and GaAs subcells. Electrical power-generating capability and conversion efficiency of the space-based solar cells at the end of life (EOL) are dependent on the cell area, doping levels, subcell design parameters, antireflection coated radiation shield thickness, and power-generating capability and conversion efficiency at the beginning of life (BOL). The bottom germanium subcell (Figure 6.8) in the GaInP/GaAs/ Ge multijunction solar cell increases the overall voltage output of the tandem cell, because it is in series with the top two subcells, namely, the GaInP and GaAs subcells. Most of the photons in the incident solar spectrum with energy greater than 1.424 eV bandgap of the GaAs layer are absorbed by GaInP and GaAs subcells. However, the germanium layer still has ample photogenerated current in the 0.67-eV bandgap region of the material, nevertheless, the Ge subcell does not limit the multijunction current capability.
Germanium semiconductor is used as a substrate for both the DJ and TJ solar cells because of minimum material cost and surface hardness. These two properties
are considered most vital for high-efficiency, low-cost, space-based solar cells. Note low fabrication cost, high conversion efficiency and surface hardness are the principal design requirements for the electrical power generation equipment aboard communications satellites or surveillance spacecrafts. Important properties of germanium and silicon widely used as substrate materials are summarized in Table 6.6.
Note higher material hardness, lower resistivity, and lower material cost compared to silicon are the principal reasons for selecting the germanium semiconductor material in fabrication of high-efficiency, low-cost space-based solar cells.