A nanostructured approach to semiconductor device development is based on the fact that the electrical, optical, mechanical, and even thermal properties of these materials can be controlled by changing the particle size(ref. 1). When carriers are confined in dimensions comparable to the Bohr exciton radius of the bulk semiconductor, their energy states become quantized. These nanoscale pieces of semiconductor are called quantum dots (QDs) or “artificial atoms” because of their discrete-like energy states. In the same way that bringing a number of atoms together in a solid results in “energy bands,” bringing a number of QDs together in an array also results in bands.
A groundbreaking theoretical study by Luque and Marti (refs. 2 and 3) predicted that a single intermediate electronic band created by QDs would offer a conversion efficiency of 63.2 percent when it was inserted into an ordinary solar cell. This greatly exceeds the maximum conversion efficiency (i.e., 31 percent) for a single-junction device (refs. 4 and 5), and it is approximately a factor of 2 better than current state-of-the-art space solar cells. In addition to efficiency enhancement, QD cells hold the promise of improved radiation tolerance and temperature coefficients (refs. 5 to 7).
QD structures can be fabricated using a self-assembly technique called the Stranski-Krastanov growth mode. This technique takes advantage of the strain energy generated from the lattice mismatch between the host material and the QD material to transition from two-dimensional layer-by-layer growth to three-dimensional “island” growth. A series of QD layers with intermediate cladding layers can be grown to produce self-organized three-dimensional arrays.
Left: Lattice-matched triple-junction solar cell; AR, antireflective. Right: Crystal growers’ chart with a dashed arrow indicating a lattice-matched triple-junction cell on germanium.
Long description of figure 1.
The current state of the art in space photovoltaics is the lattice-matched, triple-junction solar cell. It is essentially three different cells grown epitaxially with connecting tunnel junctions between. The preceding illustration shows the basic device structure layer compositions and doping for a lattice-matched, triple-junction solar cell. The lattice-matched approach puts a constraint on the available bandgaps (see the preceding graph) and, therefore, how the solar spectrum is divided between the junctions. The cell is optimized through current-matching of the individual junctions. In this particular design, the middle indium gallium arsenide (InGaAs) junction is the current-limiting and, therefore, the efficiency-limiting junction. Incorporating an InAs quantum dot array can lower the effective bandgap of the middle cell, providing subgap absorption and improving the cell’s short-circuit current.
An alternative to the ordinary lattice-matched approach to multijunction solar cells is the metamorphic, or lattice-mismatched, approach. This approach also can benefit from the introduction of QD arrays. In a metamorphic triple-junction cell, the InGaAs junction (bottom cell) of the three-cell stack is the current-limiting junction. This situation is further exacerbated when these devices are used in space, because the bottom junction is the one most affected by radiation degradation.
Atomic force micrographs of InAs QDs. Left: Grown at 450 °C at 1.0 anstroms/sec on GaAs. Right: Grown at 480 °C at 0.34 angstroms/sec on 1.2-eV metamorphic In0.13Ga0.87As grown at 675 °C on GaAs.
This year, Glenn researchers demonstrated the ability to grow both the lattice-matched and mismatched InAs on InGaAs using Stranski-Krastonow growth (see the photomicrographs). Cross-sectional analysis of the atomic force micrograph structures yielded an average diameter of 20±5 nm and height of 5±1 nm (see the following graph on the left), yielding an effective bandgap of 1.04 eV for a bulk bandgap of InAs of 0.38 eV and an electron effective mass of 0.023. The graph on the right shows the spectral response of a nonoptimized InGaAs device in which InAs dots were added to the depletion region. The small peak beyond 1.2 eV confirms some subgap conversion that which was not seen in the non-QD-containing control cell.
Left: Cross section of the atomic force micrograph shown in the preceding figure yielding an approximate InAs QD height of 5.0 nm. Right: Spectral response of an InGaAs solar cell with a 10-layer InAs QD array.
Long description of figure 3.
Find out more about Glenn’s Photovoltaics and Space Environments Branch: http://www.grc.nasa.gov/WWW/5000/pep/photo-space/
Glenn contact:
Dr. Sheila G. Bailey, 216-433-2228, Sheila.G.Bailey@nasa.gov
Rochester Institute of Technology contact:
Dr. Ryne P. Raffaelle, 585-475-5149, rprsps@rit.edu
Authors:
Dr. Sheila G. Bailey, David M. Wilt, Dr. Ryne P. Raffaelle, and Dr. Samar Sinharoy
Programs/Projects:
Glenn’s IR&D and SBIR, ESR&T ICP
Last updated: October 16, 2006
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