#### Theory of Multijunction Solar Cells

Before discussing multi-junction solar cells, let’s review the basic reasons for the energy conversion limitations of single-junction solar cells. Consider an ideal solar cell made of a material with a semiconductor bandgap. When the photon energy is incident inside the cell, the photon will be absorbed and converted into electrical energy, but the electrical energy converted at this time has only excess energy, which will become heat dissipation. And when h v < E, the photons will not be absorbed into electricity. And only in the case of hν>E, the efficiency of converting light energy into electrical energy can reach the highest. It should be noted that even in the case of hv = E, the maximum energy conversion efficiency is still below 100%.

The range of the solar spectrum is very wide, ranging from 0 to 4 eV, so the conversion efficiency of single-junction solar cells to sunlight is naturally much lower than that of single-frequency light. A simple idea to solve this problem is to divide the solar spectrum into several bands according to its energy, and each band uses a solar cell with a suitable energy bandgap to convert light energy into electrical energy, so as to improve the efficiency of converting light energy into electrical energy. E.g. Divide the solar spectrum into blue bands hv1 , hv2 , hv3 , hv4 , hv4 , hv5 , where hv1 <hv2 <hv3 and the photons in these spectral ranges will use three different band gaps of the solar energy Eg1 = hv1 , Eg2 =hv2, Eg3 =hv3 to convert light energy. If the number of spectral bands can be divided into more, higher energy conversion efficiency can theoretically be achieved.

The scholar Henry has calculated under the conditions of A Ml. 5 and 1 sun. Energy conversion efficiency limit of multijunction solar cells. When the number of junctions of the solar cell is 1, 2, 3 and 36, the highest energy conversion efficiencies are 37%, 50%, 56% and 72%, respectively. When the energy conversion efficiency is changed from a single-junction solar cell to a 2-junction solar cell, the increase is most obvious, and the improvement of the energy conversion efficiency of more junction pairs becomes limited. This calculation is good news in some ways, because it is very difficult to actually make a solar cell with 4 or 5 junctions, and if a multi-junction solar cell is to really improve the energy conversion efficiency, each It is very important whether the energy band gap of a sub cell material is selected correctly.