**3.3. GaAs/GaInNAs QWSC and SLSC**

**Figure 11.** Modelled and experimental quantum efficiency versus wavelength for 5 well qt1897b sample from the Quantum photovoltaics Group of Imperial College. The inset shows the wavelength range dominated by the QW

**Figure 12.** Contour plot for conversion efficiency as function of Bragg reflectivity and quantum well number. P

absorption with and without influence of DBR.

172 Solar Cells - New Approaches and Reviews

composition y = 0.09, In composition x = 0.17 and *LW* = 9.6 nm

A recent alloy able to be used as well material in GaAs p-i-n solar cell is the GaInNAs. In order to investigate the *GaAs*/*Ga*1−*<sup>x</sup> InxNyAs*1−*<sup>y</sup>* QWSC conversion efficiency, small nitrogen concentrations were considered to modify the quantum well depth. The lattice matching condition to GaAs is met if the Indium and Nitrogen concentrations satisfy the relation *x* = 2.85*y*. The electron and hole concentrations, in GaAs base and emitter regions are 1.8 <sup>×</sup> <sup>10</sup>17*cm*−<sup>3</sup> and 2.3 <sup>×</sup> 1018*cm*−3, respectively, while their widths are 0.15*µ<sup>m</sup>* (p-region), 0.60*µm* (i-region), and 0.46*µm* (n-region). A 40 nm *Al*0.8*Ga*0.2*As* window layer was incorporated into the p- region to reduce front surface recombination and a 70 nm MgF:SiN layer as ARC was used.

**Figure 14.** Contour plot of GaAs/GaInNAs QWSC efficiency as a function of the quantum well width and the nitrogen composition.

GaInNAs parameters were taken from reference [29]. High values of conversion efficiency are reached up to 3% N composition [30, 31], as depicted in Figure 14. The maximum values of efficiency are obtained for a narrow region around 1% of nitrogen composition and narrow quantum well widths. These N compositions correspond to shallow quantum wells, where the carrier generation overcomes the recombination. Also, for these N fractions a second quantum level appears in the heavy hole band slightly increasing photon absorption.

When the quantum wells are deeper (nitrogen percent increases) the carrier recombination increases and the conversion efficiency drops. In Figure 14 is shown the conversion efficiency as function of quantum well width. For 1% nitrogen composition, the conversion efficiency is almost insensitive to quantum well width due to a compensation effect, as there is a trade-off between quantum well width and quantum well number. Wider quantum wells absorbs more photons, but the amount quantum wells in the intrinsic region (0.6*µm*) is smaller. On the other hand, narrow quantum wells have small photon absorption but it is possible to insert more of them in the intrinsic region. For other nitrogen compositions higher than 1%, the efficiency drops when the quantum well width increases because the carrier recombination is higher.

To study the case of GaAs/GaInNAs SLSC, keeping the other device parameters identical as it was defined for QWSC, the condition for resonant tunneling were calculated by the

matching condition to GaAs is met if the Indium and Nitrogen concentrations satisfy the relation *x* = 2.85*y*. The electron and hole concentrations, in GaAs base and emitter regions are 1.8 <sup>×</sup> <sup>10</sup>17*cm*−<sup>3</sup> and 2.3 <sup>×</sup> 1018*cm*−3, respectively, while their widths are 0.15*µ<sup>m</sup>* (p-region), 0.60*µm* (i-region), and 0.46*µm* (n-region). A 40 nm *Al*0.8*Ga*0.2*As* window layer was incorporated into the p- region to reduce front surface recombination and a 70 nm MgF:SiN

**Figure 14.** Contour plot of GaAs/GaInNAs QWSC efficiency as a function of the quantum well width and the nitrogen

GaInNAs parameters were taken from reference [29]. High values of conversion efficiency are reached up to 3% N composition [30, 31], as depicted in Figure 14. The maximum values of efficiency are obtained for a narrow region around 1% of nitrogen composition and narrow quantum well widths. These N compositions correspond to shallow quantum wells, where the carrier generation overcomes the recombination. Also, for these N fractions a second quantum level appears in the heavy hole band slightly increasing photon absorption.

When the quantum wells are deeper (nitrogen percent increases) the carrier recombination increases and the conversion efficiency drops. In Figure 14 is shown the conversion efficiency as function of quantum well width. For 1% nitrogen composition, the conversion efficiency is almost insensitive to quantum well width due to a compensation effect, as there is a trade-off between quantum well width and quantum well number. Wider quantum wells absorbs more photons, but the amount quantum wells in the intrinsic region (0.6*µm*) is smaller. On the other hand, narrow quantum wells have small photon absorption but it is possible to insert more of them in the intrinsic region. For other nitrogen compositions higher than 1%, the efficiency drops when the quantum well width increases because the carrier recombination

To study the case of GaAs/GaInNAs SLSC, keeping the other device parameters identical as it was defined for QWSC, the condition for resonant tunneling were calculated by the

layer as ARC was used.

174 Solar Cells - New Approaches and Reviews

composition.

is higher.

**Figure 15.** A variably spaced multiple quantum well (cluster) which enhances the resonant tunneling between adjacent wells.

well-known transfer matrix method without back-scattered wave. A variably spaced multiple quantum well or superlattice was considered. Figure 15 illustrates this particular superlattice unit that we have been refering as a cluster, in which the resonant tunneling character were obtained. The resonance takes place for an electric field of 12 kV/cm, which was obtained accounting uniform doping levels at the p- and n-regions, and intrinsic region width 0.60*µm*. We have used a fixed field, which is a parameter in our model.

**Figure 16.** Contour plot of GaAs/GaInNAs SLSC efficiency as a function of the cluster width and number.

To study the GaAs/GaInNAs SLSC performance, it was considered a cluster composed of ten variably spaced multiple quantum wells optimized to maximise the resonant tunneling between adjacent wells. The GaInNAs quantum wells contains of 1% nitrogen composition and the GaAs barriers are 1 nm wide. A series of clusters are inserted in the intrisic region, independent from each other, in such a way that there is no coupling between neighboring clusters.

Figure 16 exhibits a contour plot of the conversion efficiency for the SLSC as a function of the cluster width and the number. If we compare these results with Figure 15 is evidenced that maximum SLSC conversion efficiencies are higher than those of QWSC by 3%. The AB contour in Figure 16 represents highest conversion efficiency obtained in the calculations. It can be also evidenced that the conversion efficiency rises as the width and the number cluster also increase, due to higher photon absorption.
