**4.1. SBQW solar cells layers structure**

the surface deposited nanoparticle quantum wells solar cells, which leads to the short circuit current density and optoelectronic conversion efficiency improved greatly under normal illumination incidence provided by a solar simulator with a Xenon arc lamp. The short circuit current density and voltage and the power output and voltage characteristics are shown in Figure 14 for the multiple quantum wells solar cells deposited SiO2 nanoparticles on the surface before and after. For a SiO2 nanoparticle surface density about 2.1×109 cm−2, the short circuit current density increased 12.9% and maximum power conversion efficiency increased to 17.0%. For Au nanoparticle surface density about 2.7×109 cm−2, the short circuit current density

The conversion efficiency and photocurrent response are improved substantially for the solar cells device structures whose quantum wells region bound with a lower refractive index substrate. A model developed by Soller and Hall [33] shows that when a horizontal electric dipole is located on a silicon insulator substrate, an excess of 80% of the light emitted by the electric dipole is coupled into the waveguide modes of the high refractive index Si insulator layer. [34]The ratio of the power of the electric dipole into waveguide modes fully to the total power of the electric dipole for the solar cells device structure over 600~1200 nm wavelengths occurs with a maximum efficiency no more than 10% in the course of the emission into waveguide, and leaky modes is in the range of 85~90%. This low efficiency is due to the small refractive index contrast to the solar cells device structure and could be improved with greater

Concentrators have the advantage to reduce the cost of photovoltaic systems by collecting the direct photon component with inexpensive lenses. The economic benefits of the concentrators demand a lot of high efficiency solar cells and the application of a nanostructure technology to these photovoltaic materials. GaAs materials provide the highest conversation efficiency in single-junction solar cells in all concentrations. However, the bandgap of GaAs is 1.42 eV and higher than the 1.1 eV of optimal efficiency bandgap at the conditions of high concentration. [35]Strain balanced InGaAs quantum wells in the intrinsic region of GaAs single junction solar cells can extend the absorption edge in substrate devices. [36] The result of increasing short circuit current accompanied with drop in open circuit voltage and increasing efficiency

An optimum of similar band gap can also exist in the case of two junction tandem solar cells. The efficiency record is 30.2% for the InGaP/InGaAs tandem solar cells at 300 times concen‐ tration. [12] However, as shown in Figure 15, the bandgaps combination of 1.8 eV/1.42 eV of an InGaP/GaAs tandem solar cells is significantly higher than the optimum bandgaps combi‐ nation under 500 times concentration. Approaches have being actively pursued to lower the bandgaps of tandem solar cells, including lattice mismatched dilute nitrides InGaP/InGaAs grown on a virtual substrate. [38] These introducing dislocations can lower the voltage of the tandem solar cells, though the efficiency record has achieved about 42 % by the virtual

increased 7.3% and maximum power conversion efficiency increased only 1%.

refractive index. [8]

348 Solar Cells - New Approaches and Reviews

**4. InGaP/GaAs MQWs solar cells**

prevails over comparable conventional solar cells. [37]

Many quantum wells can be grown in the intrinsic layers of the *p-i-n* solar cell without dislocations and relaxation [41] by strain balancing the pressure imposed by the lower lattice spacing of InGaAs quantum wells and the higher lattice spacing of GaAsP barrier materi‐ als. [42]The strain balancing (SB) method and a resultant energy band diagram are shown in Figure 16.

All the solar cells devices were fabricated by metal organic vapour phase epitaxy (MOVPE). The InGaP/GaAs tandem solar cells were grown on the bottom cell where quantum wells in the intrinsic region of it. The second cell was top cell with higher emitter doping and lower In content in quantum wells. In the two cells, the bottom cell was grown by III-V growing technologies. The top cell was grown on a passive Ge substrate according to a conventional

**Figure 16.** The energy band structure of three strain balanced quantum wells in GaAs solar cells

GaAs cell in order to create a control cell device. The control top cell was grown poorly due to an insufficient thickness of the *p-i-n* top cell. A schematic of the InGaP/GaAs tandem solar cells devices structure is shown in Figure17. [12]

**Figure 17.** A cross section of the InGaP/GaAs tandem quantum well solar cells devices structure

All solar cells devices were processed for quantum efficiency measurements, fully metalized devices for dark current measurements, and concentrator devices were prepared from each wafer.
