**8. Novel materials for intermediate band solar cells**

As noted previously, there have been numerous attempts to use established QD material systems for IBSCs. However, these QD systems have had only limited success because their band alignments do not meet the IBSC requirements. Levy *et al.* and Dahal *et al*. have identified exotic material combinations that are more suitable for IBSCs [40, 41]. These material systems are difficult to grow and very few attempts have been made to realize them. Simmonds *et al*. have reported the growth of InAs(Sb) QDs with AlAs0.56Sb0.44 barriers on InP substrates [42]. This system has near ideal bandgaps for IBSCs, and furthermore, InAs(Sb)/AlAsSb QDs exhibit a type II band alignment. This offers strong electron confinement, while the valence band offset at the InAs(Sb)/AlAsSb interface is small (zero for certain As and Sb compositions). Initial attempts were made to develop InAs (no antimony) QDs. However, when InAs was directly grown on AlAsSb barriers, irrespective of good QD morphology, no PL emission was ob‐ served. This was attributed to aluminum diffusion into the QDs. The aluminum diffusion issue was solved by introducing very thin layers of GaAsxSb1-x between the QD and AlAsSb layers. This process resulted in PL emission between 0.7 – 0.9 eV (at 77 K). The GaAsxSb1-x spacer scheme was further optimized by Sun *et al*. to tune the QD morphology and bandgap [43]. Figure 21 (a) shows the schematic of InAs QD using GaAs and GaAsSb cladding layer scheme. Figure 21 (b) shows the atomic force microscope image of InAs/AlAsSb QDs with 5 ML of GaAs below the QDs. These QDs have an areal density 2×1010 cm−2 and are 4.1 nm tall and 33 nm in diameter. The average size of these QDs can be easily controlled, and hence the energies of the quantum confined states, simply by changing the InAs coverage. Power-dependent photoluminescence measurements on these QD samples confirm a type II band alignment (Figure 22). In samples containing GaAs/InAs/GaAsSb QDs, carrier lifetimes as long as 7 ns are measured. This is greater than the lifetimes measured in typical type I QD systems. These longer lifetimes are especially beneficial for efficient carrier extraction, leading to higher IBSC efficiency.

**Figure 20.** External quantum efficiency characteristics of photovoltaic cells fabricated with and without strain compen‐

As noted previously, there have been numerous attempts to use established QD material systems for IBSCs. However, these QD systems have had only limited success because their band alignments do not meet the IBSC requirements. Levy *et al.* and Dahal *et al*. have identified exotic material combinations that are more suitable for IBSCs [40, 41]. These material systems are difficult to grow and very few attempts have been made to realize them. Simmonds *et al*. have reported the growth of InAs(Sb) QDs with AlAs0.56Sb0.44 barriers on InP substrates [42]. This system has near ideal bandgaps for IBSCs, and furthermore, InAs(Sb)/AlAsSb QDs exhibit a type II band alignment. This offers strong electron confinement, while the valence band offset at the InAs(Sb)/AlAsSb interface is small (zero for certain As and Sb compositions). Initial attempts were made to develop InAs (no antimony) QDs. However, when InAs was directly grown on AlAsSb barriers, irrespective of good QD morphology, no PL emission was ob‐ served. This was attributed to aluminum diffusion into the QDs. The aluminum diffusion issue was solved by introducing very thin layers of GaAsxSb1-x between the QD and AlAsSb layers. This process resulted in PL emission between 0.7 – 0.9 eV (at 77 K). The GaAsxSb1-x spacer scheme was further optimized by Sun *et al*. to tune the QD morphology and bandgap [43].

sation layers. Clearly, the devices with strain compensation outperform the device without compensation.

**8. Novel materials for intermediate band solar cells**

270 Solar Cells - New Approaches and Reviews

AlAsSb spacers, and b) atomic force microscopy image of InAs QDs on AlAs0.56Sb0.44 with a five-monolayerthick GaAs cladding layer placed beneath the QDs. The QD density is 2×1010cm−<sup>2</sup> , and they are 4.1 nm tall and 33 nm in diameter [42]. **Figure 21.** a) Schematic of an InAs QD structure with GaAs and GaAs0.95Sb0.05 cladding layers, as well as thin AlAsSb spacers, and b) atomic force microscopy image of InAs QDs on AlAs0.56Sb0.44 with a five-monolayer-thick GaAs clad‐ ding layer placed beneath the QDs. The QD density is 2×1010cm−2, and they are 4.1 nm tall and 33 nm in diameter [42].

**Figure 21**: a) Schematic of an InAs QD structure with GaAs and GaAs0.95Sb0.05 cladding layers, as well as thin

InAs was directly grown on AlAsSb barriers, irrespective of good QD morphology, no PL emission was observed. This was attributed to aluminum diffusion into the QDs. The aluminum diffusion issue was solved by introducing very thin layers of GaAsxSb1-x between the QD and AlAsSb layers. This process resulted in PL emission between 0.7 – 0.9 eV (at 77 K). The GaAsxSb1-x spacer scheme was further optimized by Sun *et al*. to tune the QD morphology and bandgap [43]. Figure 21 (a) shows the schematic of InAs QD using GaAs and GaAsSb cladding layer scheme. Figure 21 (b) shows the atomic force microscope image of InAs/AlAsSb QDs with 5 ML of GaAs below the QDs. These QDs have an areal density 2×1010 cm−<sup>2</sup> and are 4.1 nm tall and 33 nm in diameter. The average size of these QDs can be easily controlled, and hence the energies of the quantum confined states, simply by changing the InAs coverage. Power-dependent photoluminescence measurements on these QD samples confirm a type II band alignment (Figure 22). In samples containing GaAs/InAs/GaAsSb QDs, carrier lifetimes as long as 7 ns are measured. This is greater than the lifetimes measured in typical type I QD systems. These longer To understand the performance and operation of InAs/AlAsSb QD PV cells, an AlAsSb p-i-n solar cell device was fabricated with 10 layers of InAs QDs buried within the optimized cladding layers (similar to schematic in Figure 21(a)). EQE data from an AlAsSb control cell without dots or cladding layers and another cell with cladding layers only is also presented for comparison (Figure 23). The EQE spectra show an extended wavelength response in cases where there are cladding layers and QDs. To date, this is the longest wavelength response reported in any QD PV device. The QD cell shows an extremely broad-band photoresponse up to 1800 nm, consistent with the PL measured from respective devices. Though these results are encouraging, further device optimization will be required to achieve a high efficiency IBSC performance, perhaps including the use of high solar concentrations.

lifetimes are especially beneficial for efficient carrier extraction, leading to higher IBSC efficiency.

IBSC performance, perhaps including the use of high solar concentrations.

To understand the performance and operation of InAs/AlAsSb QD PV cells, an AlAsSb p-i-n solar cell device was fabricated with 10 layers of InAs QDs buried within the optimized cladding layers (similar to schematic in Figure 21(a)). EQE data from an AlAsSb control cell without dots or cladding layers and another cell with cladding layers only is also presented for comparison (Figure 23). The EQE spectra show an extended wavelength response in cases where there are cladding layers and QDs. To date, this is the longest wavelength response reported in any QD PV device. The QD cell shows an extremely broadband photoresponse up to 1800 nm, consistent with the PL measured from respective devices. Though these results are encouraging, further device optimization will be required to achieve a high efficiency

the decay trace at peak wavelength.

**Figure 22:** a) PL peak position at 77 K as a function of the cube root of excitation power for 7 ML and 8 ML InAs QDs with optimized cladding scheme, shown in the inset, and b) time dependent PL decay traces for 8 ML InAs SAQDs at different detection wavelengths. The dashed line is the fitting curve for **Figure 22.** (a) PL peak position at 77 K as a function of the cube root of excitation power for 7 ML and 8 ML InAs QDs with optimized cladding scheme, shown in the inset, and b) time dependent PL decay traces for 8 ML InAs SAQDs at different detection wavelengths. The dashed line is the fitting curve for the decay trace at peak wavelength.

**Figure 23.** External quantum efficiency measured from an InAs/AlAsSb QD PV cell with GaAs and GaAsSb cladding layers, compared to measurements from a control cell and a cell incorporating only the cladding layers (no QDs). The QD cell shows a response up to 1800 nm, consistent with the PL measurements.
