Using high-efficiency single-crystal semiconductor solar cells on the surface to generate electricity, whether it is a single-crystal Si solar cell or a GaAs solar cell, the cost of the battery is too expensive. An alternative is to use a concentrating solar cell power generation system, using cheap concentrating modules to reduce the usage of single crystal semiconductor solar cells to one percent, or even one thousandth.
Table 1[Z] shows the structural parameters of concentrating GaAs solar cells, including P on-N type and N-on-P type. Compared with non-concentrating GaAs solar cells, the structure of concentrating GaAs solar cells has an extra layer under the base layer to reflect secondary carriers back to improve cell efficiency. The concentrating design can not only reduce the amount of solar cells, but also improve the conversion efficiency of solar cells, which is mainly due to the increase of the open circuit voltage Voe and the increase of the fill factor. At present, the highest conversion efficiency of GaAs solar cells is 27.6% (AM 1.5, 255 suns) under concentrated light conditions.
In concentrated solar cell power generation system. Since a single solar cell produces currents of several amperes or more, series resistance has a very large effect on efficiency. Too high series resistance will reduce the fill factor and severely reduce the cell efficiency. The main sources of series resistance include the resistance Re of the grid electrode line, the contact resistance Rs between the metal and the semiconductor (Fig. 1), and the lateral sheet resistance of the current passing through the emitter layer. There are several ways to reduce the series resistance of the battery itself. click here to open to learn more.
(1) Increase the density of grid electrodes. Increasing the density of the grid electrode can simultaneously reduce the grid electrode line resistance, contact resistance and lateral sheet resistance of the entire solar cell, but increasing the grid electrode density often increases the shading area, thus reducing the efficiency of the cell.
(2) Increase the conductivity of the emitter layer. Increasing the conductivity of the emitter layer can reduce the lateral sheet resistance R day. Since N-type GaAs has better conductivity than P-type GaAs, the N on P structure is a better choice.
(3) Reduce contact resistance For concentrating GaAs solar cells, the contact resistance Re must usually be small. A thin layer of alloy, such as Au/Zn/Au or Au/Ge/Ni/Au, is added between the gate electrode and the semiconductor, and with suitable thermal fusion conditions, the contact resistance can be effectively reduced to the required level scope.
Figure 2 shows the electrode design of a concentrator solar cell. The radial grid electrodes guide the current from the central area of the solar cell to the periphery, although the grid electrode density is actually much higher than shown in the figure. In order to reduce the series resistance and reduce the shading area, the shading ratio of the grid electrode will be controlled at 4%~8% as much as possible.
Figure 3 shows the efficiency comparison of Pon-N type and N-on-P type concentrating GaAs solar cells under different concentrating conditions. The dense grid electrode design is used to reduce the shading area and achieve higher efficiency. However, when the concentrated light intensity exceeds 400 times, the efficiency of N-on-P type GaAs solar cells designed with low m-density grid electrodes begins to saturate, while the efficiency of N-on-P type GaAs solar cells designed with high-density grid electrodes begins to saturate.
The efficiency of on-N type GaAs solar cells can continue to increase as the concentration of light increases by a factor of 1000.
This result shows that the design of the grid-like concentrating GaAs solar electric electrode is the key point of the concentrating solar cell, according to the design of the grid-like electrode of the cell. The actual grid concentration magnification optimizes the design of the grid electrode, effectively reducing the density of the grid electrode, which is much higher than the series resistance shown in the figure, and at the same time reducing the shading area as much as possible to increase the photocurrent, in order to improve the efficiency of the battery.