Basic Research Needs for Solar Energy Utilization - Office of ...

Concentrators
The key elements of a concentrator PV system are low-cost concentrating optics, low-cost
mounting and tracking systems, and high-efficiency (and relatively low-cost) solar cells. The
large-scale manufacturability of all components has already been demonstrated, including
27%-efficient silicon cells (up to 400 suns concentration) and 28%-efficient GaAs cells.
Concentrator systems using point-focus Fresnel lenses (up to 400 suns) have been routinely
fabricated. Module efficiencies of up to 20% have been demonstrated by commercially made
25%-efficient silicon solar cells. Recent progress in multijunction, III-V-based solar cells for
space applications has led to looking at their terrestrial potential in concentrating applications.
An efficiency of 37.3% (at up to 600 suns intensity) has been achieved recently for a
GaInP2/GaInAs/Ge triple-junction structure (King et al. 2004).
Most of today’s remote and distributed markets for PV systems are not suitable for concentrator
systems. Concentrator systems use only direct (rather than diffuse or global) solar radiation;
therefore, their areas of best application are more limited than those for flat plates (e.g., in the
southwest United States). A brief summary of ongoing research in silicon and III-V-based
concentrator cells is provided below. There is also ongoing research to improve the long-term
reliability of concentrator systems and to develop standard tests for concentrator cells and
systems.
Silicon Concentrator Cells. To achieve the highest efficiencies, the cells are generally
fabricated on float-zone silicon wafers. For concentrations of less than about 30 suns, the grid
design of the 1-sun cell may be modified by increasing the number of grid lines to reduce seriesresistance
losses. Prismatic covers can divert sunlight away from the grid lines to mitigate the
loss of photocurrent from high grid coverage. For high concentrations (>200 suns), the backpoint-contact
silicon cell (Mulligan et al. 2004) has a higher efficiency, partly because of no grid
obscuration. Another approach is to fabricate “dense array” modules for use in reflective dish
concentrators. These consist of interconnected silicon concentrator cells on single wafers, with
several wafers connected in the module (typically 25 × 25-cm size).
Multijunction Concentrator Cells. The devices with highest efficiency are based on III-V
materials, consisting of crystals grown from the elements within groups III and V of the periodic
table, such as gallium, indium, palladium, and arsenic. Solar cells based on III-V materials are
usually grown by metalorganic chemical vapor deposition (MOCVD) or by molecular beam
epitaxy (MBE). Growth by MOCVD is typically accomplished between 600 and 700°C with a
growth rate of 1-10 µm/hr. Growth temperatures and growth rates for MBE are usually
somewhat lower. Both growth methods use a single-crystal substrate as a template for the
epitaxy. For easiest growth, the substrate has a lattice constant that is equal to that of the desired
epitaxial layers (i.e., lattice-matched). However, lattice-mismatched (or “metamorphic”)
structures are also used, as these allow combinations of band gaps that can be better optimized to
the solar spectrum.
The configuration of the record-efficiency, three-junction device is
Ga0.44In0.56P/Ga0.92In0.08As/Ge. The solar cell structures include the active p-n junctions,
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