## 1 Introduction  
With the development of photovoltaic technology, many cell technologies such as crystalline silicon cells (c-Si), thin-film cells [1; 2], multijunction cells and emerging PV technology [3; 4; 5; 6] were born and play import roles. Compared with other technologies, multijunction solar cells are commonly used in concentrated photovoltaic (CPV) technology for its special advantages such as reducing cell area, high conversion efficiency and withstanding high temperature [7]. In order to enhance the spectral response, its development has changed from three junction, four junction to even more junctions [8; 9]. The latest record of multijunction solar cell has reached 47.1% and the latest record of module efficiency is 38.9% [9; 10; 11]. Multijunction cells are usually composed of III-V compound semiconductors. GaAs-based compounds and InP-based compounds are commonly used. During the processing of power generation, the photoelectric performance of the solar cell is affected by many factors such as non-uniformity of concentration [12; 13], spectral response [14] and temperature [7; 15; 16; 17; 18; 19], which may cause obvious losses of photoelectric conversion of solar cells. [PERSON] et al. [12] summarized nine factors that mainly lead to the decline in power generation efficiency and concluded that the non-uniformity of illumination will bring a decline of 4% of the system efficiency. The increase in cell temperature decreases efficiency by 7% and the spectral response cause an efficiency loss of 2%. [PERSON] et al. [20] established a circuit model to study the effect of the concentrating cell under non-uniform illumination and indicated that there is still lateral carrier flow which makes the open circuit voltage lower than that under uniform illumination. [PERSON] et al. [21] further studied and explored the current flow law on the surface of multijunction cells in open circuit state under non-uniform illumination. Based on the simulation results, the paper proposed to increase the density of grid lines in the central area of the solar cells to reduce the decline in filling factor (FF) and conversion efficiency caused by non-uniformity of illumination. [PERSON] et al. [22] explored the influence of spectral mismatch on the photoelectric performance of CPV cells. They artificially constructed a spectral non-uniform state by placing different filters at both ends of the cell surface and found that the greater the distance between the two regions, the lower the filling factor and open-circuit voltage. [PERSON] et al. [23] explored the effect of cell temperature and concluded that the accumulation of thermal energy raises the temperature of the solar cells which will lead to a reduction in the cell conversion efficiency in turn and mechanical failures on the long-term operation of the cell. In the outdoor application of high-concentration photovoltaic (HCPV) cells, multi-junction solar cells are usually placed at the design focal plane of Fresnel lens. However, not only the irradiance of illumination on the cell surface is non-uniform, but also the spectrum of concentrating illumination is non-uniform. Our team's previous research found that the characteristics of focusing along the optical axis have a great impact on the photoelectric conversion efficiency and the output performance of III-V multijunction concentrating photovoltaic cells at the focal plane is not the best [24]. When testing the newly developed cells, [PERSON] et al. [25] also found that the output current decreased slightly in the focal plane of the Fresnel lens and the filling factor fluctuated significantly in different optical axis positions. [PERSON] et al. [26] analyzed the influence of dispersion effect on the output characteristics of multijunction cells and proposed that the negative influence of dispersion effect would be relieved by reducing the cell size.