**4.2. SBQW solar cells simulation results**

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

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

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

devices structure is shown in Figure17. [12]

350 Solar Cells - New Approaches and Reviews

The external quantum efficiency of the two tandem solar cells devices was characterized as described in [43], their results are shown in Figure 18. Left curves which are from top to bottom in Figure 18 are the top cell of first cell, the control top cell and the top cell of second cell with quantum wells, respectively, right curves which are from top to bottom in Figure 18 are the bottom cell of first cell, the control bottom cell and the bottom cell of second cell with quantum wells, respectively. The lower In content of InGaAs in the second cell with quantum wells improves their band gap. Thus the exciton absorption peaks of the second cell at 922 nm compared to 932 nm for the first cell.

**Figure 18.** The external quantum efficiency of the tandem control cells and quantum wells cells devices

The Shockley injection currents and Shockley-Read-Hall dark currents of two tandem solar cells devices are simulated using drift diffusion model. The radiative component of the dark currents is calculated from the generalized Planck formula with no free parameters. [44] The conversion efficiency of first cell was independently measured 22.1 ± 0.7 % under low aerosol optical depth, as is shown in Figure 19. The light current curves of top and bottom cell have been constructed by subtracting the dark current of each cell by using the short circuit current measurement at Fraunhofer. These currents weren't mismatch between the top cell and bottom cell calculated by internal quantum conversion efficiency measurement.

Because the top cell emitter doping is lower, the conversion efficiency of first cell is 27.2%. Extrapolating the model for the dark current to higher concentrations, the conversion efficiency of first cell with no series resistance losses would have achieved 29.8% under low aerosol optical depth. The second cell was grown with the higher top cell emitter doping to overcome

**Figure 19.** Tandem devices dark current model results relative to the light current curve measured at Fraunhofer

the series resistance of the first cell. The quantum well band gap was increased to counter the production of current in the quantum wells solar cells. Concentrator measurements have been performed on the second solar cells. The results in Table 1 show the conversion efficiencies recorded for both solar cells devices.

The superior photoelectric performance of the quantum wells control solar cells devices in Table 1 could be illustrated by function of the Xenon spectrum wavelengths and the resultant short circuit currents intensity in Figure 20. In the second solar cells devices, the current intensity of limiting bottom cell is improved in the condition of the Xenon spectrum illumi‐ nation, the current is matched with the control solar cells devices under low aerosol optical depth. The top cell of the second solar cells limited and controlled performs of the quantum wells solar cells devices, because the top cell spectral response can extend to longer wave‐ lengths.


**Table 1.** The measured photoelectric performance of the quantum wells control solar cells devices.

Figure 21 shows the quantum wells solar cells could perform over the control solar cells under concentrator spectrum and could perform better over the top solar cells with a larger spectral response. The control solar cells were grown on Ge substrate but the quantum wells solar cells were grown on GaAs substrate. It is known that growth of InGaP would give rise to a higher order degree in the arrangement of In and Ga atoms which could lower the InGaP band gap. [45]The most likely cause is the discrepancy between the top solar cells and optimized design of the InGaP/GaAs quantum wells tandem solar cells.

Solar Cells with InGaN/GaN and InP/InGaAsP and InGaP/GaAs Multiple Quantum Wells http://dx.doi.org/10.5772/58899 353

**Figure 20.** The Xenon spectrum used to characterize the second solar cells alongside a concentrator spectrum

the series resistance of the first cell. The quantum well band gap was increased to counter the production of current in the quantum wells solar cells. Concentrator measurements have been performed on the second solar cells. The results in Table 1 show the conversion efficiencies

**Figure 19.** Tandem devices dark current model results relative to the light current curve measured at Fraunhofer

The superior photoelectric performance of the quantum wells control solar cells devices in Table 1 could be illustrated by function of the Xenon spectrum wavelengths and the resultant short circuit currents intensity in Figure 20. In the second solar cells devices, the current intensity of limiting bottom cell is improved in the condition of the Xenon spectrum illumi‐ nation, the current is matched with the control solar cells devices under low aerosol optical depth. The top cell of the second solar cells limited and controlled performs of the quantum wells solar cells devices, because the top cell spectral response can extend to longer wave‐

**Device Fill Factor (%) Efficiency (%)**

**Table 1.** The measured photoelectric performance of the quantum wells control solar cells devices.

Figure 21 shows the quantum wells solar cells could perform over the control solar cells under concentrator spectrum and could perform better over the top solar cells with a larger spectral response. The control solar cells were grown on Ge substrate but the quantum wells solar cells were grown on GaAs substrate. It is known that growth of InGaP would give rise to a higher order degree in the arrangement of In and Ga atoms which could lower the InGaP band gap. [45]The most likely cause is the discrepancy between the top solar cells and optimized design

The second solar cell 81.5 30.4 The control solar cell 81.7 31.6

of the InGaP/GaAs quantum wells tandem solar cells.

recorded for both solar cells devices.

352 Solar Cells - New Approaches and Reviews

lengths.

**Figure 21.** Short circuit currents under a xenon lamp spectrum and calculated from spectral response curves.

**Figure 22.** Efficiency predictions under the assumption of additives and a low aerosol optical depth spectrum

To investigate further the performance of the quantum wells solar cells and control solar cells, the conversion efficiency and dark currents have been combined under the assumption of additives and a low aerosol optical depth spectrum, as shown in Figure 22. The red dots show an improved tandem solar cells structure where the top control solar cell with high disorder is grown on the quantum well bottom solar cell. Such solar cells structure should achieve a conversion efficiency of over 34%.
