- Problems of I-V tandem solar cells
At present, the three-junction GaAs solar cell is mainly used in space power supply, and all countries in the world hope to apply the cell to the ground. Representative research institutions and companies mainly include Emcore, Spectrolab, Entech, Flatcon, Fraunhofer ISE, Azur, Japan Energy, Sharp, etc. in the United States. Compared with space, the ground spectrum due to the absorption of sunlight by the atmosphere makes the spectrum reaching the earth’s surface, the spatial spectrum distribution and intensity different, and the density of the ground solar spectrum is smaller, which requires the structure of the ground tandem solar cell. The necessary design and adjustment are made, that is, the updated requirements for the structure of the stacked battery are put forward. The main problems of concentrating solar cells are as follows.
①The current of the middle cell is too small, which limits the further improvement of the overall cell efficiency. Therefore, for the GaAs-based triple-junction stacked GaInP/GaAs/Ge (2-terminal) solar cell, the band gaps of each sub-cell are 1.85eV/1.42eV, respectively. /0.65eV, in triple-junction stacked GaInP/GaAs/Ge (2-terminal) cells, the currents are 19mA/cm2, 14mA/cm2, and 25mA/cm2, respectively. Obviously, the current of the GaAs cell is smaller than the current of the top cell and the low cell, which limits the current of the overall cell, so the current matching becomes an important factor restricting the efficiency improvement of the GaInP/GaAs/Ge stacked thin film cell.
②The increase of the concentration multiple will double the short-circuit current, which will increase the resistance power loss of the entire solar cell, which will obviously affect the filling factor of the solar cell.
③ Under the working conditions of high-rate concentrator, the current increases significantly, so that the tunnel junction cannot meet the requirements of large current, and the peak current of the tunnel junction needs to be increased.
- Basic principles of I-V tandem solar cells
Single-junction solar cells can only absorb sunlight in a specific spectral range, and their conversion efficiency is not high. Multi-junction solar cells are prepared by using materials with different band gap widths Eg. The structure of I-V tandem solar cells is shown in Figure 1. They are stacked from top to bottom according to the size of Eg to form a multi-junction tandem solar cell. Battery. Tandem solar cells can selectively absorb and convert different subdomains of the solar spectrum, respectively, and achieve a substantial improvement in the photoelectric conversion efficiency of solar cells.
The usage of multi-junction tandem solar cells can be divided into 2-terminal, 3-terminal and 4-terminal according to the number of terminals. For the 4-terminal structure, each sub-battery is relatively independent and has its own output terminal. Therefore, there is no limit to the polarity, current or voltage of the sub-cells. However, this structure has the problem of difficult connection, and requires a very complicated connection structure and process. The 3-terminal structure requires a contact layer to realize the intermediate output terminal. The advantage is that there is no matching limit on the photocurrent, but the problem with this structure is that the polarities of the subcells are reversed. The series structure in the 2-terminal structure has strict requirements on the photocurrent, and the polarities of the sub-cells must be consistent. However, the 2-terminal structure battery has very important advantages: the sub-cells can be effectively connected through a single high-quality tunnel junction, and the battery only needs to make metallized electrodes on the upper and lower surfaces as the output terminals, and the process can be completely compatible with the single-junction junction. The battery is as simple to integrate as a component.