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

**Table 2** shows major losses, their origins and key technologies for improving efficiency [6]. There are several loss mechanisms to be solved for realizing highefficiency III-V compound single-junction solar cells. (1) bulk recombination loss, (2) surface recombination loss, (3) interface recombination loss, (4) voltage loss, (5) fill factor loss, (6) optical loss, (7) insufficient –energy photon loss. Key


#### **Table 2.**

*Major losses, their origins of III-V compound cells and key technologies for improving efficiency.*

technologies for reducing the above losses are high quality epitaxial growth, reduction in density of defects, optimization of carrier concentration in base and emitter layers, double-hetero (DH) structure junction, lattice-matching of active layers and substrate, surface and interface passivation, reduction in series resistance and leakage current, anti-reflection coating, photon recycling and so forth.

Solar cell efficiency is dependent upon minority-carrier diffusion length (or minority-carrier lifetime) in the solar cell materials as shown in **Figure 5**.

Radiative recombination lifetime τrad is expressed by

$$
\boldsymbol{\pi}\_{\text{rad}} = \mathbf{1}/\text{BN} \tag{9}
$$

where N is the carrier concentration and B is the radiative recombination probability. The B value for GaAs reported by Ahrenkiel et al. [19] is B = 2 X 10�<sup>10</sup> cm3 /s. Effective lifetime τeff is expressed by

$$\mathbf{1}/\tau\_{\rm eff} = \mathbf{1}/\tau\_{\rm rad} + \mathbf{1}/\tau\_{\rm nonad} \tag{10}$$

where τnonrad is non-radiative recombination lifetime and given by

$$\mathbf{1}/\tau\_{\text{nonrad}} = \sigma \mathbf{v} \mathbf{N}\_{\text{r}} \tag{11}$$

where σ is capture cross section of minority-carriers by non-radiative recombination centers, v is minority-carrier thermal velocity, and Nr is density of nonradiative recombination center.

Therefore, improvement in crystalline quality and reduction in densities of defects such as dislocations, grain boundaries and impurities that act as nonradiative recombination centers are very important for realizing high-efficiency solar cells.

In this chapter, analytical results for historical progress in efficiency of GaAs single-junction solar cells are shown. **Figures 6** and **7** show analytical results for progress in ERE and resistance loss of GaAs single-junction solar cells.

The first GaAs solar cells reported by Jenny et al. [20] were fabricated by Cd diffusion into an n-type GaAs single crystal wafer. Efficiencies of 3.2-5.3% were quite low due to deep junction. Because GaAs has large surface recombination velocity S of

**Figure 5.** *Minority-carrier diffusion length dependence of GaAs solar cell characteristics.*

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

**Figure 6.** *Analytical results for ERE progress of GaAs single-junction solar cells.*

**Figure 7.** *Analytical results for resistance loss progress of GaAs single-junction solar cells.*

around 1 107 cm/s [6, 21], formation of shallow homo-junction with junction depth of less than 50 nm is necessary to obtain high-efficiency. Therefore, hetero-face structure AlGaAs-GaAs solar cells have been proposed by Woodall and Hovel [22] and more than 20% efficiency has been realized [22] in 1972 as shown in **Figure 1** as a result of ERE improvement from 10<sup>8</sup> % to 0.05% as shown in **Figure 6**. Doublehetero (DH) structure AlGaAs-GaAs-AlGaAs solar cell with an efficiency of 23% has been realized by Fan's group in 1985 [23] as a result of ERE improvement from 0.05% to 1.4% as shown in **Figure 6**. Now, DH structure solar cells are widely used for highefficiency III-V compound solar cells including GaAs solar cells.

**Figure 8** shows device structures of GaAs solar cells developed historically. As mentioned above and shown in **Figure 8**, device structures of GaAs cells were improved from homo-junction, to heteroface structure, finally to DH structure. Now, InGaP layer is mainly used as front window and rear back surface field (BSF) layers instead of AlGaAs layer. The reasons are explained in the part of multijunction solar cells.

**Figure 9** shows the chronological improvements in the efficiencies of GaAs solar cells fabricated by LPE (Liquid Phase Epitaxy), MOCVD (Metal-Organic Chemical Vapor Deposition) and MBE (Molecular Beam Epitaxy). LPE was used to fabricate AlGaAs-GaAs heteroface solar cells in 1972 because it produces high-quality

#### **Figure 8.**

*Device structures of GaAs solar cells developed historically.*

**Figure 9.**

*Chronological improvements in the efficiencies of GaAs solar cells fabricated by the LPE, MOCVD and MBE methods.*

epitaxial film and has a simple growth system. Homo-junction structure and heteroface structure GaAs solar cells shown in **Figure 8** were fabricated by LPE. However, it is not as useful for devices that involve multilayers because of the difficulty of controlling layer thickness, doping, composition and speed of throughput. Since 1977, MOCVD has been used to fabricate large-area GaAs solar cells by using DH structure shown in **Figure 8** because it is capable of large-scale, large-area production and has good reproducibility and controllability.

Regarding the differences of surface recombination velocities in semiconductor materials, differences of point defect behavior are thought to be one of the mechanisms. For example, because nearest-neighbor hopping migration energies (0.3 eV and 1.2 eV) of VIn and VP in InP [24] are lower than those (1.75 eV) of VGa and VAs in GaAs, better surface state may be formed on InP surface compared to GaAs surface.

In addition to improvement in surface recombination loss, as a result of technological development, resistance loss has been improved as shown in **Figure 7**. In parallel, bulk recombination loss and interface recombination loss have been

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

**Figure 10.** *Correlation between ERE and interface recombination velocity in InGaP single-junction solar cells.*

improved as shown in **Figure 6**. Recently, efficiency of GaAs solar cells reached to 29.1% [2] by realizing ERE of 22.5% as a result of effective photon recycling [1].

Lattice mismatching also degrades solar cell properties by increase in interface recombination velocity as a result of misfit dislocations and threading dislocations generation. By using interface recombination velocity SI as a function of lattice mismatch (Δa/a0) for InGaP/GaAs heteroepitaxial interface [25], lattice mismatch (Δa/a0) dependence of interface recombination velocity (SI) is semi-empirically expressed by [16].

$$\mathbf{S\_{l}}\,\mathrm{[cm/s]} = \mathbf{1.5} \times \mathbf{10}^{8} \Delta\mathbf{a}/\mathrm{a\_{0}}\tag{12}$$

As one of example for effects of interface recombination loss upon solar cell properties, analytical results for correlation between ERE and interface recombination velocity in InGaP single-junction solar cells are shown in **Figure 10**.
