Quantum wells in concentrator solar cells

The perennial challenge of solar energy conversion is to generate significant quantities of power at a reasonable cost. As part of a CO₂ reduction strategy, the International Energy Agency recently set a goal of attaining 1TW of peak power capacity from solar electricity by 2050.¹ All photovoltaic technology sectors must grow rapidly to meet this goal. Yet solar panel manufacturing already uses more silicon than the entire microelectronics industry. In addition, though the technology can scale to terawatt levels,² expanding the manufacturing infrastructure is very capital-intensive.

Concentrator photovoltaic systems may provide an alternative solution. By using mirrors or lenses, they focus sunlight onto small, highly efficient solar cells. This shifts the manufacturing burden from semiconductors to metal and glass, materials that have more established manufacturing industries.³ Moreover, recent improvements in concentrator cell efficiencies suggest that this approach may be cost-effective and rapidly scalable.

We recently demonstrated a single junction quantum well solar cell with an efficiency of 27.3% (see Figure 1), the highest value for any nanostructured solar cell to date. It is also very close to the highest efficiency recorded for a single junction cell (27.8%).⁴ The device is made of a p-i-n structure with a gallium arsenide phosphide (GaAsP) indium gallium arsenide (InGaAs) multi-quantum well stack grown in the i-region. The lower band-gap InGaAs layer is compressively strained, while the GaAsP barrier layer is under tensile strain (see Figure 2). Therefore, a judicious choice of composition and layer thickness results in a stack of quantum wells where each GaAsP/InGaAs bilayer exerts no net force on neighboring layers.⁵

By incorporating strained semiconductors into a solar cell without introducing structural defects, we can adjust the absorption threshold for the solar cell. The single junction cell has an absorption edge at 1.33eV, which is fundamentally better matched to the solar spectrum than a 1.42eV gallium arsenide cell. The low defect density allows cells to become increasingly radiatively efficient at concentrator intensities, exhibiting photon recycling effects that further boost efficiency.^6,7

Figure 1. Efficiency for the GaAsP quantum well solar cell approaches the gallium arsenide (GaAs) single junction record.

Figure 2. The strain-balanced material combines thin layers of alternating tensile and compressively strained semiconductor. The resulting heterostructure is locally strained but exerts no net force on the substrate.

Adjusting the absorption threshold becomes particularly important when fabricating highly efficient, multi-junction solar cells. Here the broad solar spectrum is absorbed using a series- connected stack of subcells with different bandgaps. While this structure can lead to very high efficiencies, it requires careful control of the absorption threshold of each subcell. The series connection ensures that the lowest photocurrent in the stack will limit the photocurrent in the entire cell.

By growing defect-free, strain-balanced stacks of material, we can adjust the absorption threshold of the junctions without relaxing the semiconductor lattice or growing optically thin junctions. This strain-balanced approach makes double junction efficiencies greater than 34% and triple junction efficiencies up to 42% under solar concentration feasible. Work is under way to achieve these goals.

K. W. J. Barnham pioneered the quantum well solar cell at Imperial College London and founded the Quantasol company, where he is now chief technology officer.