Laminated solar energy battery generally have 10~30 layers, or even more, and their stacking is not simply overlapping, but in the process of stacking, many technical conditions need to be met, such as bandgap matching, lattice matching, optical matching, etc.
(1) Band gap matching Band gap matching is to determine the point position of the segmented spectrum. Ideally, each band is divided into the same number of photons. However, because the intrinsic absorption of photons in each sub-cell is not complete, the resulting electron-hole pairs have different composite losses before forming the photocurrent output, which cannot be simply completed. In addition, for materials, although we can obtain adjustable band gap materials through multiple alloys, not all band gaps can be achieved, and not all achieved materials are suitable for making solar energy battery. In addition to the bandgap matching of each sub-cell, the total current decrease caused by lattice matching will still occur. Bandgap matching is very important in designing laminated solar energy battery.
In the study of GaInP/GaAs/Ge (2-terminal) current matching, people try to add In component to GaAs to adjust the band gap of medium battery from 1.42eV to 1.23eV, which is conducive to increasing the current of medium battery, and make GaInP/GaInAs/Ge triple-junction stacking battery. At the same time, it can basically ensure that the GaInP and GaInAs lattice match. However, it is severely mismatched with Ge substrate, and the mismatch rate is close to 1%. Theoretically, the highest efficiency can be achieved. However, due to lattice mismatch, the short-circuit current decreases, which makes it difficult for the battery efficiency to exceed the GaInP/GaInAs/Ge triple junction with perfectly matched lattice.
(2) Lattice matching
Lattice matching means that there are fewer defects in the crystal structure of the epitaxial film and between the epitaxial layer and the substrate, so as to ensure the lifetime of photogenerated carriers and obtain larger photocurrent output. The design principle of current matching requires that the sub-cell has a band gap matching with the spectrum and the number of cell junctions. On the other hand, each sub-cell is required to have a large photocurrent output, which requires a good lattice structure in each sub-cell and between sub-cells, which is the lattice matching design requirements to consider. The lattice mismatch will bring more structural defects to the device, and these defects will bring more recombination centers, leading to the recombination of N-P pairs and reducing the photocurrent density. In the GaInP/GaAs/Ge (2-terminal) current matching study, the lattice constant of substrate Ge (5.6578 A) is larger than that of GaAs (5.6232 A) at room temperature. In order to achieve lattice matching, a certain fraction of In (1%, mass fraction) is usually added to GaAs to reduce the adaptation between the medium cell and the Ge substrate, so as to reduce the interface structural defects and the resulting internal defects of the medium cell, so as to improve the photocurrent output of the medium cell.
Similarly, the top cell uses GalnP alloy. By controlling the composition of In, the lattice can be adjusted to match the middle cell. However, the adoption of alloy compounds changes the crystal constant and also the band gap of the crystal. When the three sub-cell lattices are perfectly matched, the band gap no longer keeps the ratio of 1.85 eV/1.42 eV/0.65eV (300K), and the band gaps of the top and middle cells will be reduced again. In such a bandgap structure, the deviation from the current matching requirement is larger than that of the bandgap matching requirement, and the overall efficiency of the battery cannot be improved. Another idea is to make GalnP/GaAs two-junction series and GalnP/GaAs/GalnAs three-junction series laminated solar energy battery by using GaAs or GalnAs with smaller lattice constants as substrates. It is widely used when the price of GaAs single crystal becomes low. For the epitaxial layer with large mismatch, the strain layer can also be passed through the multi-step growth method to reduce the growth defect density. When lattice mismatch is large, step growth is used, as shown in Figure 1.