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The InAs/GaAs quantum dots structure embedded in GaInP/Ga(In)As/Ge triple junction solar cell with and without Ga0.90In0.10As strain reducing layer was investigated. Conversion efficiency of 33.91% at 1,000 suns AM 1.5D with Ga0.90In0.10As strain reducing layer was demonstrated. A 1.19% improvement of the conversion efficiency was obtained via inserting the Ga0.90In0.10As strain reducing layer. The main contribution of this improvement was from the increase of the short-circuit current, which is caused by the reduction of the Shockley–Read–Hall recombination centers. Consequently, there was a decrease in open circuit voltage due to the lower thermal activation energy of confined carriers in Ga0.9In0.1As than GaAs and a reduction in the effective band gap of quantum dots.

In this paper, 3J-IMM solar cells were studied in comparison with 3J-LM solar cells grown by MOCVD. The structural characteristics were studied by HRXRD, HRTEM and SIMS, which showed a high crystal quality of the metamorphic sub-cell. The solar cell performance in terms of I-V curves and EQE was investigated between the different solar cells structures. The current of the IMM solar cell is almost equal to the LM ´monitor solar cell. Under AM 1.5D, 1000 suns conditions, an Isc of 14.29 A, Voc of 3.538 V, FF of 83.9 %, and an efficiency of 42.3 % were achieved for the 3J-IMM solar cell with a chip size of 1.0 cm2, which is an absolute 2.9% higher than that of a 3J-LM solar cell in conversion efficiency. The excess current contribution from the Ge bottom cell in the 3J-LM solar cells is eliminated, accompanied with a 0.3V increase in Voc for the 3J-IMM solar cells.

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 mm20 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 mm2 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 mm0.4 mm. The efficiencies are both about 34.5% and the effects of chromatic aberration can be ignored.

A typical GalnP/GalnAs/Ge tandem solar cell structure operating under AM0 illumination is proposed, and the current-voltage curves are calculated for different recombination velocities at both front and back-surfaces of the three subcells by using a theoretical model including optical and electrical modules. It is found that the surface recombination at the top GaInP cell is the main limitation for obtaining high efficiency tandem solar cells.

This paper introduces in detail the working principle of Si/SiGe Quantum cascade laser(QCL). Appropriate parameters are used to calculate the hole subband structure of Si/Si1-xGex quantum well using a six-band k·p method.The dispersion relation and energy band for different layer thickness and compositions are investigated. Meanwhile,the energy separations between hole subbands in Si/Si1-xGex/Si quantum wells are also analyzed. Finally the calculated results are used for the Si/SiGe QCL design，which will be beneficial to the structure optimization of Si/SiGe QCL.