In 1996, Kondow et al. demonstrated the epitaxial growth of 1.0 eV band gap GaInNAs material with lattice matching to GaAs substrate, and applied it to fabricating infrared laser [46]. Since then diluted nitride GaInNAs materials have been widely used in heterojunction bipolar transistors (HBTs) [47] and lasers [48], where GaInNAs HBTs base layer can reduce the open voltage and run under low working voltage. These features of diluted nitride GaInNAs materials are also useful in wireless communications and power amplifier applications. GaInAsN is a direct band-gap semiconductor material, which can change its band-gap by adjusting the component content of nitrogen and indium while keeping its lattice constant matching to conventional substrate materials such as GaAs and Ge. These advantages bring great potential for using 1.0 eV subcell in a high efficiency multijunction solar cell [49].

The Ga_1-xIn_xN_yAs_1-y is used as a sub-cell material for GaInP/GaAs/GaInNAs/Ge four-junction solar cell by NREL [50]. When y = 0.3x, the lattice constant of Ga_1-xIn_xN_yAs_1-y matches GaAs and Ge, which is an ideal material to construct a GaInP/GaAs/GaInNAs/Ge (1.88/1.42/1.05/0.67 eV) four-junction solar cell with band-gap matching. Figure 1A shows the representative GaInNAs devices structure grown by MOVPE. The device is grown with dimethylhydrazine (DMHy) as the nitrogen source. At the same time, the experimental results showed that the remaining factor of GalnAsN cell efficiency is 0.93 and 0.89 after 5 × 10¹⁴ and 1 × 10¹⁵ e/cm² electron fluence of 1 MeV electrons irradiation, respectively [51]. The specific degradation of the device parameters is summarized in Table 1. The results showed that this type of cell structure possesses superior radiation resistance comparing to the traditional lattice matched multi-junction solar cell.

However, difficulties in epitaxial growth of diluted nitride materials have prevented its further development. It is also found that the diffusion length of minority carriers is short, the internal quantum efficiency is low for the GaInNAs subcell, and these poor minority carrier transmission characteristics resulted in lower current and voltage. Since the current of a multi-junction solar cell is determined by the smallest sub-cell, the low current in GaInNAs subcell has a significant impact on the overall cell performance [51]. The quality of GaInNAs solar cells was improved by fabrication of p-i-n structure cells [52], annealing [53], doping Sb in GaInNAs material [54], and changing substrate epitaxial orientation [55]. Miyashita et al. demonstrated a solar cell with p-i-n (p-GaAs/i-GaInAsN (Sb)/n-GaAs) structure, and tested the cell under the AM1.5 spectrum. The short-circuit current density (J _sc ) reached 21.5 mA/cm², the open-circuit voltage (V _oc ) reached 0.42 V, and the filling factor (FF) reached 0.71 [56]. They further optimized the content of Sb and found that the content of Sb was less than 1%, which is more beneficial to improve GaInAsN crystal quality and device performance [57]. Han et al. found that GaAsN material grown by (311) B substrate epitaxy could not only improve the incorporation efficiency of nitrogen, but also effectively enhance the carrier lifetime of the material [58]. Although the theoretical conversion efficiency of GaInNAs multijunction solar cell could reach 41%, the actual growth problem still needs to be overcome, which needs to be focused on as a development goal in the future.

According to the theoretical calculation, the optimum bandgap energy for top and bottom subcell for a tandem multijunction solar cell is 1.65–1.8 eV and 1.0–1.5 eV, respectively, and the conversion efficiency of this structure reaches 32.5% under 1-sun AM0 spectrum [59]. Typically, the III-V compound material based multijunction solar cells are fabricated by MOVPE or molecular beam epitaxy (MBE) techniques, where the lattice matching and energy matching between subcells is a critical problem. The presence of mechanical stacks makes this feasible and allows to use of lattice- and current-mismatched combinations of semiconductor materials. By using the mechanical stacking method, III-V compound materials can be stacked together regardless of their bandgap energy and lattice constants. This technique can reduce the production cost significantly, and the substrate removal process greatly reduces the weight of solar cells, which is useful if used in space solar cell applications [60–62].

Still, the utilization of multiple substrates requires the removal of substrates, thereby, it makes the solar cell fabrication process complicated and affects the interface quality [63]. At the same time, a complicated design requirement becomes an obstacle for large-scale applications of mechanical stacked solar cells in space [64]. Since each subcell in a multi-junction solar cell consists of a p-n junction, if individual subcells directly stacked together in series, it will form a reverse p-n junction between subcells which block the current flow. Therefore, interconnecting individually processed solar cells keeping both electrical and optical properties still is a technical challenge need to consider with high conductivity and low transmission loss [65]. This problem can be solved by adding a tunnel junction between the subcells [66]. The 30% conversion efficiency of III‐V//Si multi-junction solar cells using smart stack technology have been reported [67]. A schematic diagram of InGaP/AlGaAs//Si triple-junction solar cell is shown in Figure 1B. A “smart stack,” “areal current matching,” and “solar concentration” two-terminal GaAs/Si tandem solar cell, having potential to achieve an efficiency of approximately 30%, are proposed and the indoor experiment proves its feasibility, although the experiment result is lower than the simulation [64]. These types of mechanical stacking processes are quite flexible and allow different cells to be combined, and consequently reduces the fabrication cost and increases the cell efficiency.

In multi-junction solar cells, the lattice dislocations induced by the lattice mismatch during the epitaxial growth reduce the material quality and deteriorates the device performance. The wafer bonding technique, which refers to the physical integration of the two different materials, overcomes this issue very well and allows the formation of a monolithic multijunction solar cell structure without both electrical and optical losses, regardless of the different lattice constant of subcells [68]. As early as 1986, Lasky et al. demonstrated surface treated silicon wafer bonding at room temperature and obtained very good bonding strength through high-temperature annealing [69]. The freedom of material selection in the process of semiconductor device design is greatly improved because the bonding technology can realize the lamination structure of materials with different thermal expansion coefficient and lattice constant, and can limit the dislocation and defect to the area near the bonding interface. Figure 2A shows the structure of GaInP/GaAs//InGaAsP/InGaAs wafer-bonded four junction solar consists of GaInP/GaAs and InGaAsP/InGaAs dual junction cells, whereas Figure 2B shows the degradation of InGaAsP and InGaAs subcell electrical properties under 1 MeV electron irradiation. The result shows that the bonding interface has no effects on overall cell performance and radiation resistance, the main degradation happened in the third and fourth subcell, and InGaAsP subcell has a superior radiation resistance than InGaAs cell owing to In-P bonds [70].

At present, the wafer bonding technology has been studied extensively and widely used in the structural design and integration field of microelectronics and optoelectronics. A variety of bonding methods have been developed, however, they can be mainly divided into two categories: direct bonding and intermediate-layer bonding [71]. The direct wafer bonding process includes cleaning and activating two polished wafers and sticking them together at room temperature. Heterogeneous integration of multi-junction solar cells has to meet three stringent conditions: good mechanical strength at the bonding interface, high optical transmittance, and low resistivity. It does not work without each condition being met. Intermediate-layer bonding technology introduces a layer with good ductility and adhesion to alleviate stress and improve the bonding interface [72]. GaInP/GaAs//GaInAsP/GaInAs four-junction wafer-bonded concentrator solar cells with an efficiency of 46% at 508 suns have been reported [73]. It demonstrates that the wafer bonding is a feasible method to combine lattice mismatched different III-V compound materials and realize high efficiency multijunction solar cell structure. The main challenge of wafer bonding technology is the preparation of the bonding surface which should have a low surface roughness to ensure high electrical conductivity.

Another approach of solving current matching issue in multijunction solar cell is the metamorphic growth method, which involves applying a compositionally graded buffer (CGB) layer between the lattice mismatched subcells [74]. The main purpose of using a CGB layer is to distribute the strain relaxation layers into a lattice constants gradually changed thick buffer layer instead of growing a highly lattice mismatched layer onto its under layer [75]. According to the growth method, the metamorphic growth technique can be divided into two types: IMM [76] and upright metamorphic (UMM) [77]. In the IMM method, for example GaInP/GaAs/InGaAs triple junction solar cell, a CGB layer is inserted between the GaAs and InGaAs subcells and the growth direction of solar cell epilayers is in the inverse direction, in order to delay the strain relaxation in CBG layer at the later stages of growth process. However, the IMM method requires an additional step of substrate lift-off before solar cell device fabrication processes. On the other hand, in the UMM method, the growth direction is from bottom subcell to top subcell, so it requires an even higher quality CBG layer to control the strain relaxation. Figures 3A,B show the general structure of IMM and UMM cells, respectively.

Comparing to traditional lattice matched (LM) solar cells, metamorphic multijunction solar cells are aiming to achieve current matching between its subcells and expected to have higher conversion efficiency. Experimentally, NREL has shown that IMM solar cells have a conversion efficiency of 40.8% for triple-junction solar cells at high concentration [15] and 47.1% efficiency for 6 J solar cells at 143 suns concentration [8]. Furthermore, IMM cell can reduce the cell weight by removing the substrate and increase the mass power ratio. IMM cells also can be utilized to develop flexible solar cells for any non-flat surfaces. These high efficiency, flexible, and lightweight properties of IMM multi-junction solar cells make them the most promising for space applications [ [14, 78, 79]]. For UMM multijunction solar cell, the conversion efficiency exceeded 31% under one Sun AM0 spectrum compared to traditional lattice matched solar cell conversion efficiency is limited to 30% [80]. The critical point of increasing UMM solar cells is improving the quality of CGB layer to suppress lattice dislocation and threading dislocations induced by strain relaxation. However, the fabrication process of UMM solar cells is similar to the matured fabrication technology of LM cells, therefore, this advantage makes the UMM multi-junction solar cell utilization possible for potential applications [77].

The reported experimental efficiency, theoretical limits, and their advantages of all above mentioned solar cells in Lattice Matched GaInNAs Multi-Junction Solar Cells, Mechanically Stacked Solar Cells, and Wafer Bonded Multijunction Solar Cells are presented in Table 2. According to the Shockley-Queisser balance model, the theoretical efficiency limit of single-junction, triple-junction, and four-junction solar cells are 33.5%, 56% and 62%, respectively [85]. Along with the rapid development of new materials and high quality fabrication technologies, all these solar cells could achieve higher efficiency and better performance. Furthermore, UMM and IMM solar cell efficiencies tend to surpass LM cells even more as their manufacturing technology continues to be innovated and developed, and they are expected to become the next generation space solar cells.

Messenger et al. investigated the electron and proton irradiation effects of double-junction GaAs/Ge solar cell with different energies [88]. The cell performance is degraded with an increase of the particle fluence under both electron and proton irradiation, as shown in Figure 4. Additionally, electron irradiation with greater incident energy resulted in more severe deterioration of cell performance for the same irradiation fluence. For proton irradiation, on the other hand, low energy protons produced a bigger reduction in cell performance when the proton energy is in the range of 0.2–9.5 MeV. Proton irradiation experiments on GaInP/GaAs/Ge, GaAs/Ge solar cells with proton energy of less than 200 KeV are reported and it was found that the low energy particle irradiation caused defects in different subcells and different regions in the tandem multi-junction solar cell [23, 89].

The radiation-induced defects in the base and the emitter layers form non-radiative recombination centers and capture photo-generated electron-hole pairs before they are collected by the junction region, and eventually reduce the short circuit current. On the other hand, the radiation-induced damages in the junction region mainly cause the degradation of open circuit voltage by introducing deep energy levels into the band gap and accelerating the recombination of valence band holes and conduction band electrons [89].

The effects of 40, 100, 170 keV energy proton irradiation on GaAs/Ge cells with different fluences were discussed, the degradation of spectral response results indicated that the largest damages are caused by 170 keV protons, and the lowest by 40 keV protons, in the long wavelength range (720–900 nm). The degradation effect on the normalized maximum power (P _max) is the largest for the 170 keV protons and lowest for 100 keV protons because irradiation with 170 keV protons produces the most severe damage in the junction region of the cells. The short circuit current decreases with increasing proton energy under the energies of 40–170 keV proton irradiation and the degradation extent of V _oc is the largest for 170 keV proton because they produce defects with deep energy levels in the space charge region, accelerating the recombination of electrons and holes, which is also the reason for the significant decline in P _max for solar cell [23].

Sharps et al. investigated the electron and proton radiation effects of GaInP/GaAs/Ge solar cells with different energies [91]. Same as that of GaAs/Ge double-junction solar cell, the P _max of solar cells declined with the increase of the particle fluence for the same energy. For 1–12 MeV electron irradiation, the degradation of cell performance is severer for higher energy electrons for same irradiation fluence, as shown in Figure 5A. The degradation of P _max is 9 and 13% for 1 MeV electrons at fluences of 5 × 10¹⁴ and 1 × 10¹⁵ e/cm². However, for proton irradiation, the degradation of cell performance is bigger for lower energy protons in the energy range of 50–200 keV, as shown in Figure 5B. The main reason for this is that less relative damage occurs for energies below 200 keV, and lower energies would have more of an effect on the emitter as compared with the base region. At the same time, GaInP top cell and GaAs middle cell current matching point was studied in more detail by quantum efficiency measurements. It was found that the crossover from GaInP current-limited subcell to GaAs current-limited subcell occurs at 2 × 10¹⁵ e/cm² for the 1 MeV electrons [90]. This result indicated that advanced triple junction solar cells with current matching at end of life (EOL) can be achieved by reducing the amount of beginning of life (BOL) current mismatch, which is helpful for designing high efficiency radiation hardened space solar cells.

Wang et al. investigated the electron irradiation effects of GaInP/GaAs/Ge solar cell with 1.0–11.5 MeV energy electron beams [91]. The degradation of normalized P _max of GaInP/GaAs/Ge solar cell under different fluence of electron irradiation is shown in Figure 6A, whereas the degradation of external quantum efficiency (EQE) changes irradiated with 1.8 MeV electron beam with different fluences are shown in Figure 6B. The degradation effects of electron irradiation of the P _max and the EQE of LM solar cells increase with the increase of irradiation fluences and electron energy, as observed in GaAs/Ge double-junction cell. Figure 6C exhibits that the EQE of GaAs middle cell degrades more than GaInP top cell under same irradiation fluence which is indicating the radiation resistance of GaInP/GaAs/Ge triple-junction cells is dominated by the GaAs middle cell.

By taking advantages of bandgap tailoring in III-V compound materials and using a CGB layer, IMM multijunction solar cells can be adjusted to its output parameters suitable for BOL or EOL requirements. Takamoto et al. studied the radiation effects of these two types of IMM GaInP/GaAs/InGaAs space solar cells under 1 MeV electron irradiation, where the electrical parameters of both types solar cells declined with the increase of electron irradiation fluence [92]. In this study, the electrical parameters V _oc , I _sc (short-circuit current) and η decayed o 89.3%, 96.3%, 84.1% and 88.6%, 98.6%, and 79.5% of its original values for one group of cells have higher efficiency at BOL and another group of cells have higher efficiency at EOL, respectively, when the electron irradiation fluence reaches 1 × 10¹⁵ cm⁻². These details of degradation of electrical parameters of these two types of cells are summarized in Table 3.

Imaizumi et al. studied the radiation response of In_0.5Ga_0.5P, GaAs, In_0.2Ga_0.8As, and In_0.3Ga_0.7As single-junction solar cells, whose materials are also used as component subcells of inverted metamorphic triple-junction solar cells, and results show that the photo-generation current in the InGaAs bottom subcell of InGaP/GaAs/InGaAs IMM3J cells was severely damaged under the electron and proton radiation, which can be attributed to the stronger decrease of minority-carrier diffusion length in InGaAs compared with that in InGaP and GaAs subcells after irradiation [93]. By comparing the irradiation resistance of two InGaAs cells (In_0.2Ga_0.8As and In_0.3Ga_0.7As cells), GaAs and InGaP cells, it was found that radiation resistance of these two InGaAs cells is approximately equivalent to InGaP and GaAs cells from the initial material qualities. However, the InGaAs cells show lower radiation resistance especially for the I _sc comparing to InGaP and GaAs cells due to the bigger decrease of minority-carrier diffusion length in InGaAs materials. And the InGaP and two InGaAs cells exhibited equivalent radiation resistance of V _oc , but with different degradation mechanisms.

Zhang et al. investigated the 1 MeV electron radiation effects of IMM GaInP/GaAs/InGaAs solar cells by electrical properties, spectral response, and photoluminescence (PL) signal amplitude analysis [94]. The results show that the electrical parameters of IMM solar cell decrease continuously with an increase in the electron fluence same as traditional LM GaInP/GaAs/Ge solar cells. As shown in Figure 7, P _max illustrates the maximum degradation compared to V _oc and I _sc , and, V _oc is degraded more compared to I _sc . This phenomenon is explained as V _oc is the sum of three series sub-cell voltages, where the I _sc is the smallest current produced in three series sub-cells. The In_0.3Ga_0.7As bottom subcell exhibited most severe damage in the irradiated IMM triple junction cells due to the drastic degradation of the effective minority carrier lifetime (τ _eff ) of In_0.3Ga_0.7As subcell than that of GaAs subcell. Therefore, the radiation hardness of IMM GaInP/GaAs/InGaAs solar cell is mainly determined by the InGaAs subcell.

The equivalent fluence method was proposed by Tada et al. from Jet Propulsion Laboratory, California Institute of Technology [96, 97]. The key point of this approach is the corresponding relative damage coefficient, which is related to the radiation damage effects caused by different types of charged particles with a different energy to relative damage coefficients.

In the first step, the critical fluence ( ϕ ) of electron or proton irradiation, which is corresponding to the electrical properties of the solar cell degraded to a specified level of its original value (such as 75% of I _sc0 , V _oc0 , P _max0 ), has to be determined according to the experimental results [98]. Then, the relative damage coefficient (RDC) of different energy electron and proton regarding 1 MeV electron and 10 MeV proton is calculated. The ratio of the critical fluence for 1 MeV electrons to the critical fluence for other electron energies is taken as a measure of the RDC of electrons, and similarly, the relative damage coefficients of different proton energies are normalized regarding the 10 MeV proton critical fluence for RDC of protons, as shown in the following equations [88]: RD C x → 1 = ϕ e ( x M e V ) ϕ e ( 1 M e V ) (1) RD C x → 10 = ϕ p ( x M e V ) ϕ p ( 10 M e V )   (2) where ϕ e   and ϕ p are the critical fluence for electrons and protons.

The next step is using the orbital environment parameters to calculate the corresponding relative damage coefficients for omnidirectional particles on bare cells from the measured values for normally incident particles. By substituting the electron and proton environment parameters under consideration into following integration equations, one can obtain the equivalent 1 MeV electron fluence for the mission in question. ϕ 1 MeV,electron,electrons = ∫ d ϕ e ( E ) d E D e ( E ) d E (3) ϕ 1 MeV,electron,protons = D p e ∫ d ϕ p ( E ) d E D p ( E ) d E (4) where ϕ e ( E ) and ϕ p ( E ) is the particle fluence of electron and proton, respectively at energy E. D e ( E ) and D p ( E ) are the relative damage coefficient of electron and proton, respectively. D p e is the proton to electron damage equivalency ratio, which converts 10 MeV proton fluence to an equivalent 1 MeV electron fluence. The calculation result of Eqs. 3, 4 are the 1 MeV electron fluence normally incident on solar cells that will cause same damage as the selected omnidirectional spectrum. The result of calculated relative damage coefficient for omnidirectional proton irradiation and electron irradiation of GaAs/Ge solar cells are shown in Figures 8A,B.

In the equivalent fluence method, it requires a sufficient measurement of data to calculate the RDC and generate a detailed degradation curve, as shown Figure 4, which shows eight proton energies (0.05, 0.1, 0.2, 0.3, 0.5, 1.3, and 9.5 MeV) and four electron energies (0.6, 2.4, and 12 MeV). Another relatively simple way of predicting the degradation of a solar cell performance regarding a given electron or proton fluence is using a semi-empirical equation [99]: P ϕ P 0 = 1 − C ⋅ ln ( 1 + ϕ ϕ x )   (5) where P ₀ and P ϕ are the output power (also can be replaced with I _sc and V _oc ) of the solar cell before and after irradiation with different irradiation fluences ϕ , respectively. C and ϕ x are the fitting parameters for the same structure solar cell using a large amount of experimental data upon specific irradiation particles.

The displacement damage dose method was initiated by Naval Research Laboratory (NRL) [100]. The key point of this method is finding the non-ionizing energy loss (NIEL) values for different materials. By using the NIEL method, the radiation fluence of particles is converted into displacement damage dose (DDD), and the degradation curve of the electrical parameters of solar cells with the change of DDD can be obtained.

The NIEL values of different materials upon different particles with different energies can be calculated by using MULASSIS software [101] or the following equation [102]: NIEL ( E ) = n ⋅ ∫ T d Q m a x ( d σ ( θ , E ) d Q ) E ( Q ) ⋅ G ( Q ) ⋅ Q ⋅ d Q  (6) where n is the atomic density of the target material, T d is the threshold energy to displace atom, Q m a x is the maximum energy that can be given to a recoil nuclei by an incident particle of a given energy E, G(Q) is the energy partition function and (dσ/dQ) _E the differential interaction cross section. Figure 9A shows the calculated results of electron and proton NIEL for GaAs with the energy range from zero up to 200 MeV. The DDD induced by irradiation particles can be calculated by the following equation using particle fluence:       D d ( E ) = ϕ ( E ) ⋅ NIEL ( E ) ⋅ [ NIEL ( E e ) NIEL ( E 1 M e V ) ] n − 1 ( p r o t o n   n = 1 , e l e c t r o n   1 < n < 2 ) (7) where D _d (E) is the DDD, ϕ ( E ) is the particle fluence, and NIEL is the non-ionizing energy loss value of the target material. When calculating D _d of proton irradiation, n value is 1, and the n value of electron irradiation is in the range of 1 to 2. Using NIEL of the same particle with different energies, the relative damage coefficient for different energies of a charged particle, i.e. proton, the irradiation can be obtained from using the following equation [99]:   D x → 10 = NIEL ( x M e V ) NIEL ( 10 M e V ) (8) where D _x→10 is the X MeV proton to 10 MeV proton relative damage coefficient, and NIEL is the corresponding non-ionization damage energy for different energies. Electron to proton irradiation damage equivalency factor Rep can be calculated by following equation [99]: R e p = 1 n ∑ n D j D i ( n = 1,2,3 ⋯ n ) (9) where Rep is electron to proton irradiation equivalent damage coefficient, D _j is the actual DDD values by using electron irradiation and D _i is the corresponding fitting values of DDD. By considering Rep degradation curves of electrical parameters of solar cells against the irradiation fluence can be described into a single curve against the D _d . For example, Figure 9B shows the re-plot of Figure 4 using NIEL method. The DDD of solar cells in complex electron and proton environment can be calculated by Eq. 10, and applied to evaluate the degradation curve of the electrical performance of space solar cells with the DDD.       D d = ∫ d ϕ ( E e ) d E p ⋅ NIEL ( E p ) d E p + R e p ∫ d ϕ ( E e ) d E e ⋅ NIEL ( E e ) [ NIEL ( E e ) NIEL ( 1 M e V ) ] n − 1 d E e (10)

Besides, predicting the degradation of a solar cell performance regarding a given electron or proton fluence can also be conducted by using the following semi-empirical equation: P m a x P 0 = 1 − C ⋅ ln ( 1 + D d D x ) (11) where P _max and P ₀ are the output power (also can be I _sc or V _oc ) of solar cell before and after irradiation, D d is the given DDD regarding to the given irradiation fluence, respectively. C and D _x are the fitting parameters obtained by on-ground experimental data. Eq. 4 is more versatile than Eq. 5 for different kinds of particles by using D d as variable for given irradiation condition.

Comparing to the equivalent fluence method, the NIEL approach requires fewer experimental measurements to successfully predict the radiation damage of different particles at different energies and is easier to implement. Only taking few energies of proton and electron irradiation test data, one can obtain the degradation trends of the electrical properties a solar cell under different particles with different energies and different fluences. Messenger et al. compared the equivalent fluence method and the displacement damage dose method in details and concluded that the key of the displacement damage dose method was NIEL, while the equivalent fluence method needed a lot of experimental data [88]. The displacement damage dose method was successfully applied for predicting on-board solar cell measurements for GaAs/Ge and CIS solar cells at 500 × 67,300 km near equatorial orbit, shown in Figure 10.

There is no doubt that space solar cells should move toward higher efficiency, low cost and better radiation resistance. In this direction, many types of new technologies are trying to solve these problems. Currently, LM triple-junction solar cells are the main stream in space applications. In theory, the solar cell has more junction number has higher efficiency, but it is difficult to increase the number of cell junctions in real cell fabrication. The theoretical limit of N-junction (N for the infinite) solar cells conversion efficiency can reach 68.2% [3]. But the cell fabrication becomes more difficult when the number of junctions is increased, especially for more than 5–6 junctions. The efficiency of multi-junction solar cells from single-junction to six-junction is presented in Table 4. Besides, apart from the high cost of III-V materials, the price of GaAs is ten times than that of Si, the growth of III-V materials requires expensive equipment, and hence, the production cost of multijunction solar cells is very high and mostly used in space applications now. Therefore, in the future, the development trends of III-V multijunction solar cells are low-cost, high efficiency, high radiation resistance and simple fabrication method.