Solar-cell-efficiency enhancement using nanostructures

Increasing the efficiency of photovoltaic devices remains a focus for the space-power community, since it impacts mission costs and capabilities. State-of-the-art, high-efficiency III-V semiconductor solar cells use epitaxially grown indium gallium phosphide (InGaP)/gallium arsenide (GaAs)/germanium (Ge) triple junctions (see Figure 1) to achieve >30% efficiency.¹ However, this is not the optimum material combination to reach the highest efficiency, since it is constrained by the Ge-substrate lattice parameter. One approach to circumvent this constraint uses metamorphic epitaxial layers to modify the in-plane lattice parameter without introducing strain-induced structural defects, creating an optimized band-gap combination. The triple junction's serial nature also limits efficiency: the GaAs middle cell is currently the limiting junction.

Figure 1. Epitaxial triple-junction cell and solar-spectrum splitting per junction. ARC: Antireflective coating. InGaP: Indium gallium phosphide. GaAs: Gallium arsenide. Ge: Germanium. AM0: Airmass zero reference solar spectrum (outside the Earth's atmosphere).

Theoretical calculations indicate that the conversion efficiency can exceed 45%, either by extending the spectral bandwidth of the middle cell² or through implementation of an intermediate-band solar-cell concept.³ We examined use of nanostructured indium arsenide (InAs) quantum dots (QDs) within GaAs solar cells to improve efficiency.⁴ InAs QDs are grown epitaxially based on the Stranski-Kranstanow mode, which relies on strain mismatch between the GaAs matrix and InAs to form the QDs. For successive QD layers, compressive stress accumulates, ultimately leading to degraded QD uniformity and device properties. A tensile-strain compensation layer is used to create strain-neutral layer stacks containing as many as 40 periods. Figure 2 shows an atomic-force-microscopy image of InAs QDs on a GaAs surface. The QD density is ~5×10¹⁰/cm² with nominal dimensions of 6×30nm² (height × base). The InAs/GaAs combination is the most extensively studied QD system. Its size and density are intimately tied to the epitaxial growth conditions.

Figure 2. Atomic-force-microscopy image of indium arsenide quantum dots (QDs).

The external quantum efficiency (EQE) indicates the number of electrons extracted from a semiconductor device per incident photon as a function of the photon wavelength. Figure 3(a) shows EQE measurements for QD-enhanced GaAs photovoltaic cells containing 0 to 40 periods of InAs QDs. The EQE drops at the GaAs band edge near 870nm, indicating that low-energy photons in a baseline GaAs cell will not produce any photocurrent. However, QD-enhanced solar cells show subgap response (>870nm), indicating that the QDs produce current under illumination. As the number of QD layers expands, the EQE increases for below-GaAs-bandgap wavelengths, approaching 20% for a 40-period structure, with further increases expected as the number of QD layers is built up.

Figure 3. External quantum-efficiency (EQE) and current density-voltage data shows additional current generation by multiple QD layers within a GaAs solar cell.

Figure 3(b) shows illuminated current density-voltage curves for GaAs solar cells without QDs and with 10- and 40-period strain-balanced QD layers. The baseline GaAs-cell performance exhibits typical values for this type of photovoltaic cell without antireflection coating (short-circuit current density J_sc=24.1mA/cm², open-circuit voltage V_oc=1046mV, and efficiency η=16.2%). Addition of 40 QD layers raises the current density to 26.0mA/cm², which represents an ~8% increase. This contributes to a reduction in open-circuit voltage. Some voltage reduction is expected because of QD band-gap tuning, but this can be minimized with optimized strain balancing.

Using detailed balance theory, we calculate the efficiency of a standard triple-junction cell as a function of the number of QD layers. Starting from an accepted value of 32.5% for no QD layers and using the measured 0.017mA/QD layer increase—see Figure 3(a)—Figure 4 shows the resulting efficiencies. We expect a conversion efficiency of up to 38.7% for 200-period QD-enhanced solar cells.⁴

Figure 4. QD-enhanced solar cells have potential to exceed 38% conversion in multilayer stacks.

In summary, we have developed a good understanding of the relevant interactions between QD epitaxy, structure design, and QD-enhanced solar-cell performance. Calculations indicate that QDs hold considerable promise for achieving enhanced conversion efficiencies over the current state of the art, particularly by increasing the available current. Continued work aims at increasing the additional current generated per QD, which can enable ultimate efficiencies in excess of 40%.