According to the design of the stacked structure battery, we can find that each sub-battery is connected in series, and the conclusion is obtained from both theoretical and practical measurements: the total output voltage of the battery V=V1+V2+…+Vn, and the total current I, affected by The minimum current Imin of the sub-battery is limited, and I is approximately equal to Imin.
The current matching design principle of tandem solar cells is to carry out current matching design according to the actual photogenerated current generated by each sub-cell, so that their currents are equal in size, so as to avoid the limitation of the total output current of the minimum sub-current cell, that is, the design purpose is to make I1=I2=…=In=I total, so that the design can maximize the output power and conversion efficiency of tandem solar cells.
(1) Optical matching Optical matching is to adjust the thickness of the sub-cell. For the sub-cell with large output current, especially the top cell, the method of thinning design is adopted to reduce the effective intrinsic absorption depth and transmit some high-energy photons. Let the lower-level sub-battery absorb it to improve the output photo-generated current of the sub-battery and achieve the purpose of current matching. Although the principle of optical matching is simple, it is extremely important and applicable. All front-stage batteries have invalid extrinsic absorption, which is mostly released as heat, which increases the operating temperature of the battery and reduces the actual efficiency of the battery. Battery operation constitutes a hazard. Through reasonable design, the actual output power of the battery can be effectively improved.
(2) We can make ideal assumptions for current matching design: under the condition of AM1.5, when only considering the generation of electron-hole pairs, the band gap of each sub-cell material is ideal enough, and the spectrum is divided according to the ideal band gap of each sub-cell into the same number of segments as the number of sub-cells. Each sub-cell performs effective intrinsic absorption, completely absorbs the photons in the corresponding band, and excites electron-hole pairs to form the same photocurrent output. All sub-cells output completely equal currents, and there is no invalid extrinsic absorption in the whole process. occur. For two-junction tandem solar cells, when the spectrum is divided according to Eg2=1.75eV, Eg1=1.13eV, 1λ(um)=1.24/Eg(eV), considering that each segment of the spectrum is effectively absorbed to generate photocurrent, at 1sun The theoretical efficiency of this two-junction solar cell is 38%, which is much higher than the theoretical efficiency of the current single-junction GaAs solar cell of 29%. For an actual battery, it is impossible for each sub-battery to effectively absorb all the photons in each segment of the spectrum, and the electron-hole pairs generated by each sub-battery are not all output as photocurrents. The actual output photocurrent differs from the number of absorbed photons, making the actual efficiency much lower than the limit efficiency.
(3) Three aspects that can be further improved ① Add a structure with a one-junction band gap close to leV between the second junction and the third junction to form a four-junction or more junction cell structure design, as mentioned earlier More The research path of junction solar cells. Further breakthroughs need to be made through in-depth research on materials and growth techniques. ②At present, the addition of In in GaAs cells is often used to form GalnAs cells, which reduces the band gap of the cells and achieves precise matching with the lattice of the Ge substrate. However, in order to make the GalnP top cell match its lattice, the In composition must be increased, which reduces the band gap of the top cell and affects the effective use of solar energy by the triple junction cell. ③ Adopt the lattice of GaAs medium cell (or GalnAs) that does not change, such as using a special medium cell structure similar to quantum dots, to realize the red shift of the band gap of the medium cell, and reduce the band gap to achieve matching with the solar spectrum, that is, quantum dot battery.