## 5 Conclusions and Discussion  
The non-uniform irradiance, the mismatch between the focusing spectrum and the spectrum of multijunction cell design and the increase in cell temperature are the key factors affecting the photoelectric performance of the multijunction cell. The coupling effect of three factors intensifies its negative impact. Through the optical model, the light intensity and spectral characteristics under the concentration of the Fresnel lens are calculated, and an optical-electrical-thermal coupling model under non-uniform illumination is established to analyze the coupled influence on the cells. The results show that optimizing the placement position of the multijunction cell in the direction of the optical axis can effectively alleviate the negative effects of optical-electrical-thermal coupling caused by non-uniformity of focusing, spectral mismatch and increasing temperature. The main conclusions are as follows:  
1. Non-uniformity of concentration: The dispersion effect causes the irradiance concentrated, the spectrum distribution, energy and non-uniformity change significantly at different positions along the optical axis. Moreover, the energy proportion of short, medium and long wavelengths on the cell surface does not change synchronously thus causing the spectrum mismatch (including on the focal plane). For a \(Z\) value smaller than the focal distance, the loss of medium and long wavelengths mainly exists. For a \(Z\) value larger than the focal distance, the loss of short wavelengths mainly occurs. The non-uniformity of short, medium and long bands is the largest at 110.5 mm, 111.4 mm and 112.3 mm, respectively, and the non-uniformity of all wave bands is the largest at 111.8 mm, which is the result of the synthesis of the three bands. 2. Output performance of multijunction cells: The non-uniformity of the focusing causes the photoelectric performance of the cell placed near the focal plane to decline the most. The optical-electrical-thermal coupling caused by the temperature rise aggravates the negative impact at the focal plane. Based on the numerical analysis of the coupling model and experiments, it was recommended that optimizing the placement position of the multijunction cell in the direction of the optical axis can effectively alleviate the  
Figure 13: Variation in output parameters in the optical axis direction. (**a**) Short-circuit current; (**b**) Open-circuit voltage; (**c**) Filling factor; (**d**) Output power. negative effects of optical-electrical-thermal coupling caused by non-uniformity. In the passive heat dissipation condition of 500 times geometric concentration, compared with the cell placed on the on-focus position, the output power of the solar cell placed 2 mm in front of the focal plane increases by 35% and cell temperature decreases by 15%. The proposed coupled model gives the effects of optical-electrical-thermal coupling on the output performance and the temperature of the multijunction solar cells. However, the simulated value of efficiency decline on the focal plane is higher than those in our experiments. It may result from several factors not modeled, such as the limit of tunneling current of tunnel diodes. However, the coupled effect of multiple physical fields is very common because the non-uniform illumination is a frequent occurrence during the process of power generation. Overall, the simulated trends agree with the experiment result. It can be highlighted that the optical-electrical-thermal coupling effect is strongly affected by non-uniform illumination caused by the dispersion effect. The negative effects caused by non-uniform focusing near the focal plane are exacerbated by the coupled effect, which is caused by the non-uniformity of focusing, spectral mismatch and rising temperature. Therefore, optimizing the distance of the cell on the optical axis is essential for effectively alleviating the negative impact of coupling without extra cost. The choice must be carried out through experimental procedures rather than simulation-only approaches.