Effect of chromatic aberration on performance of concentrated multi-junction solar cells and their optimization

In order to investigate the influence of the chromatic aberration on the performance of multi-junction solar cells, the performance of the triple-junction GaInP/GaInAs/Ge solar cell under high concentration condition is investigated by a three-dimensional (3D) model based on distributed circuit units. Moreover, the effects of chromatic aberration on the performance of solar cells with different sizes are studied by analyzing the distributions of the voltage, the dark current and the transverse current in each layer. It is indicated that the photo-generated current is mismatched in local region of multi-junction solar cell, which is caused by chromatic aberration. However, the mismatched photo-generated current can be compensated for by the form of transverse current, and the current can be better matched when the size of solar cell is reduced. When the size of solar cell is as big as 20 mm x 20 mm, the mismatched photo-generated current is large, so are the transverse current and the dark current. But the transverse current is far less than the dark current, only 12% of the mismatched photo-generated carriers can flow from the edge to the center of the cell through the transverse resistance between the sub-cells, the rest of the photo-generated carriers are lost in the form of dark current, and the cell is in a state of current mismatching. Finally, the chromatic aberration gives rise to a reduction in the short-circuit current density, and the efficiency is only 94% as high as that of non-chromatic aberration. When the size of the cell decreases, the mismatched photo-generated current and the transverse current also decrease gradually, but the dark current caused by the chromatic aberration exponentially decreases more quickly, and the ratio of the transverse current to the mismatched photo-generated current increases gradually. Therefore, the overall state of the current mismatching is alleviated, and the short-circuit current density is increased gradually. Moreover, when the size of solar cell is 2 mm x 2 mm, the transverse current is much larger than the dark current, 99.98% of the mismatched photo-generated carriers can be compensated for in the form of transverse current. Although the photo-generated current of the cell is mismatched in local region, the overall is still in the state of current matching. The short-circuit current densities with and without chromatic aberration are equal, but the filling factor is reduced due to the transverse resistor. When the size of cell is further reduced, the mismatched photo-generated current is very small, and the influence of the transverse series resistance decreases gradually. Therefore, the value of the filling factor gradually approaches to the value without chromatic aberration. Furthermore, the performance of solar cell with and without chromatic aberration is nearly the same when the size of solar cell is as small as 0.4 mm x 0.4 mm. The efficiencies are both about 34.5% and the effects of chromatic aberration can be ignored.