**4. Historical progress and key issues for high-efficiency III-V compound multi-junction solar cells**

While single-junction cells may be capable of attaining AM1.5 efficiencies of up to 30-32% as shown in **Figure 2**, the multi-junction (MJ) structures [26, 27] were recognized early on as being capable of realizing efficiencies of up to 46% as shown below. **Figure 11** shows the principle of wide photo response using MJ solar cells for the case of a triple-junction cell. Solar cells with different bandgaps are stacked one on top of the other so that the cell facing the Sun has the largest bandgap (in this example, this is the InGaP top cell). This top cell absorbs all the photons at and above its bandgap energy and transmits the less energetic photons to the cells below. The next cell in the stack (here the GaAs middle cell) absorbs all the transmitted photons with energies equal to or greater than its bandgap energy, and transmits the rest downward in the stack (in this example, to the Ge bottom cell). As shown in **Figure 12**, the spectral response for an InGaP/GaAs/Ge monolithic, two-terminal triple-junction cell shows the wideband photo response of multijunction cells. In principle, any number of cells can be used in tandem.

As a result of research and development, high-efficiencies have been demonstrated with III-V multi-junction solar cells: 37.9% under 1-sun and 44.4% under

#### **Figure 11.**

*Principle of wide photo response by using a multijunction solar cell, for the case of an InGaP/GaAs/ Ge triple-junction solar cell.*

**Figure 12.** *Spectral response for an InGaP/GaAs/Ge monolithic, two-terminal three-junction cell.*

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

concentration for 3-junction cells [28] and 39.2% under 1-sun, 47.1% under concentration for 6-junction solar cells [7]. **Figure 13** shows historical record-efficiency of III-V multi-junction (MJ) and concentrator MJ solar cells in comparison with 1-sun efficiencies of GaAs and crystalline Si solar cells, along with their extrapolations [3].

**Table 3** shows key issues for realizing super high-efficiency MJ solar cells. The key issues for realizing super-high-efficiency MJ solar cells are (1) sub cell material selection, (2) tunnel junction for sub cell interconnection, (3) lattice-matching, (4) carrier confinement, (5) photon confinement, (6) anti-reflection in wide wavelength region and so forth. For concentrator applications by using MJ cells, the cell front contact grid structure should be designed in order to reduce the energy loss due to series resistance (resistances of front grid electrode including contact resistance, rear electrode, lateral resistance between grid electrodes) by considering shadowing loss attributed to grid electrode, and tunnel junction with high tunnel peak current density is necessary. Because cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells in order to reduce losses of electrical connection and optical absorption, effectiveness of double hetero structure tunnel diode is also presented in this chapter.

Selection of sub-cell layers by considering optimal bandgap and lattice matching of materials is one of key issues for realizing super high-efficiency MJ cells. **Table 4** shows one example for selection of top cell material and comparison of InGaP and AlGaAs as a top cell material. InGaP that has better interface recombination velocity, less oxygen-related defect problems and better window material AlInP compared to those of AlGaAs has been proposed as a top cell material by NREL group [29]. As described above, InGaP materials are now widely used as front widow and back surface filed layers of solar cells instead of AlGaAs.

**Figure 14** shows the connection options for two-junction cells: the two cells can be connected to form either two-terminal, three-terminal or four-terminal devices. In a monolithic, two-terminal device, the cells are connected in series with an optically transparent tunnel junction intercell electrical connection. In a twoterminal structure, only one external circuit load is needed, but the photocurrents in

#### **Figure 13.**

*Historical record-efficiency of III-V multi-junction (MJ) and concentrator MJ solar cells in comparison with 1-sun efficiencies of GaAs and crystalline Si solar cells, along with their extrapolations.*


#### **Table 3.**

*Key issues for realizing super-high-efficiency III-V compound multi-junction solar cells.*


#### **Table 4.**

*Comparison of InGaP and AlGaAs as a top cell material.*

the two cells must be equal for optimal operation. Key issues for maximumefficiency monolithic cascade cells (two-terminal multijunction cells series connected with tunnel junction XE "tunnel junctions") are the formation of tunnel junctions of high performance and stability for cell interconnection, and the growth of optimum bandgap top- and bottom-cell structures on lattice-mismatched substrates, without permitting propagation of deleterious misfit and thermal stress-induced dislocations.

As shown in **Table 3**, cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells. DH structure has been found to effectively prevent from impurity diffusion from tunnel junction and high tunnel peak current density has been obtained by the authors [30, 31]. **Figure 15** shows annealing temperature (equivalent to growth temperature of top cell layers) dependence of tunnel peak current densities for double hetero structure tunnel diodes. X is the Al mole fraction in AlxGa1-xAs barrier layers [30, 31]. It has also been found that the impurity diffusion from the tunnel junction is effectively suppressed by the wider bandgap material tunnel junction with wider bandgap material-double hetero (DH) structure [32]. These results are thought to be due to the lower diffusion coefficient for impurities in the wider band gap materials such as the AlInP barrier layer and InGaP tunnel junction layer [32].

As a result of developing high performance tunnel junction with high tunnel peak current density, high efficiency MJ solar cells have been developed [30, 33, 34]. *High-Efficiency GaAs-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94365*

**Figure 14.**

*Schematic diagrams of various configurations of two-junction cells.*

**Figure 15.**

*Annealing (growth) temperature dependence of tunnel peak current densities for double hetero structure tunnel diodes. X is the Al mole fraction in AlxGa1-xAs barrier layers.*

**Figure 16** shows a structure and light-illuminated (AM1.5G 1-sun) I-V characteristics of InGaP/GaAs/InGaAs 3-junctuon solar cell. 37.9% efficiency under AM1.G 1-sun and 44.4% under 300-suns concentration have been demonstrated with InGaP/GaAs/InGaAs 3-junction solar cell by Sharp [35]. Spectrolab has achieved 38.8% efficiency under 1-sun with 5-junction solar cells [36]. FhG-ISE has demonstrated 46.0% under 58-suns concentration with 4-junction solar cells [37]. Most recently, 39.2% under AM1.5 1-sun and 47.1% under 144-suns have been realized with 6-junction cell by NREL [7].

**Figure 16.** *A structure and light-illuminated I-V characteristics of InGaP/GaAs/InGaAs 3-junctuon solar cell.*
