**5. Radiation resistance and space applications of III-V compound single-junction and multi-junction solar cells**

Development radiation-resistant solar cells is necessary for space application because solar cells degrade due to defect generation under radiation environment in space. Recombination centers tend to affect the solar cell performance by reducing the minority carrier diffusion length L in solar cell active layer from a preirradiation value L0 to a post-irradiation value L<sup>φ</sup> through Eq.

$$\left(\mathbf{1}/\mathbf{L}\_{\boldsymbol{\Phi}}\,^{2} - \mathbf{1}/\mathbf{L}\_{\boldsymbol{0}}\right)^{2} = \boldsymbol{\Sigma}\mathbf{I}\_{\text{ri}}\boldsymbol{\sigma}\_{\text{i}}\mathbf{v}\_{\text{th}}\boldsymbol{\mathfrak{q}}/\mathbf{D} = \mathbf{K}\_{\text{L}}\boldsymbol{\mathfrak{q}},\tag{13}$$

where suffixes 0 and φ show before and after irradiation, respectively, Iri is introduction rate of i-th recombination center by electron irradiation, σ<sup>i</sup> the capture cross section of minority-carrier by i-th recombination center, vth the thermal velocity of minority-carrier, D the minority-carrier diffusion coefficient, KL the damage coefficient for minority-carrier diffusion length, and φ the electron fluence. The III-V compound solar cells have better radiation tolerance compared to crystalline Si cells because many III-V compound materials have direct band gap and higher optical absorption coefficient compared to Si with in-direct bandgap. In addition, InP-related materials such as InP, InGaP, AlInGaP, InGaAsP are superior

#### **Figure 17.**

*Calculated depth distribution of carrier collection efficiency in (a) Si, (b) GaAs and (c) InP under 1-MeV electron irradiation, calculated by using our experimental values [40–42] and Eq. (11), and by assuming carrier collection efficiency as a function of exp.(x/L).*

radiation-resistant compared to Si and GaAs and have unique properties that radiation-induced defects in InP-related materials are annihilated under minoritycarrier injection such as light-illumination at room temperature or low temperature of less than 100 K [38, 39].

**Figure 17** shows calculated depth x distribution of carrier collection efficiency in Si, GaAs and InP under 1-MeV electron irradiation, calculated by using our experimental values [40–42] and Eq. (13), and by assuming carrier collection efficiency as a function of exp.(x/L). It is clear from **Figure 17** that GaAs has better radiation-tolerance and InP has superior radiation tolerance compared to Si.

**Figure 18** shows changes in efficiency of Si single-junction, GaAs single-junction and InGaP/GaAs/Ge 3-junction space solar cells as a function of 1-MeV electron

#### **Figure 18.**

*Changes in efficiency of Si single-junction, GaAs single-junction and InGaP/GaAs/Ge 3-junction space solar cells as a function of 1-MeV electron fluence.*

**Figure 19.**

*Historical product efficiency of space solar cells against date of first flight. Open points are for planned products and estimate flight dates.*

## *High-Efficiency GaAs-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94365*

fluence. The InGaP/GaAs/Ge 3-junction solar cells is now mainly used for space as shown below because they are radiation-resistant and are highly efficient compared to Si and GaAs space solar cells [43].

Because GaAs single-junction solar cells and III-V compound multi-junction solar cells have high-efficiency and radiation-resistance compared to Si solar cells, III-V compound solar cells are mainly used in space as shown in **Figure 19** [44].

## **6. Future prospects**

The multijunction solar cells will be widely used in space because of their high conversion efficiency and good radiation resistance. However, in order to apply super-high-efficiency cells widely on Earth, it will be necessary to improve their conversion efficiency and reduce their cost. **Figure 20** summarizes efficiency potential of single-junction and multi-junction solar cells, calculated by using the similar procedure presented in Section 2, in comparison with experimentally realized efficiencies under 1sun illumination. Altough single-junction solar cells have potential efficiencies of less than 32%, 3-junction and 6-junction solar cells have potential efficiencies of 42% and 46%, respectively.

The concentrator PV (CPV) systems [45] with several times more annual power generation capability than conventional crystalline silicon flat-plate systems will open a new market for apartment or building rooftop and charging stations for battery powered electric vehicle applications. Other interesting applications are in agriculture and large-scale PV power plants.

The multi-junction solar cells are greatly expected as high-efficiency solar cells into solar cell powered electric vehicles. **Figure 21** shows required conversion efficiency of solar modules as a function of its surface area and electric mileage to attain 30 km/day driving. A preferable part of the installation is the vehicle roof. Because of space limitation for passenger cars, development high-efficiency solar cell modules with efficiencies of more than 30% is very important as shown in **Figure 21** [46, 47].

#### **Figure 20.**

*Calculated conversion efficiencies of various single-junction, 3-junction and 6-junction solar cells, calculated by using the similar procedure presented in Section 2, in comparison with experimentally realized efficiencies under 1-sun illumination.*

#### **Figure 21.**

*Required conversion efficiency of solar modules as a function of its surface area and electric mileage to attain 30 km/day driving. A preferable part of the installation is the vehicle roof.*

In addition to high-efficiency, cost reduction of solar cell modules is necessary. Therefore, further development of high-efficiency and low-cost modules is necessary.
