passivating window and back-surface-field layers on either sides of the p-n junctions, and
contacting layers to ensure low resistance contacts to the rest of the circuit. This includes very
highly doped shorting (tunnel) junctions between the subcells of the multijunction device.
Growth of III-V solar cells on silicon has also been demonstrated, but these cells usually show
inferior crystal quality compared with growth on GaAs or germanium. Recently, the first lattice-
matched III-V-on-silicon tandem cell has been demonstrated, with GaNPAs (1.7 eV) as the top
cell (Geisz et al. 2004).
Achieving 40% and greater efficiencies is being pursued by several groups worldwide. One
approach is the use of 2–3% nitrogen in GaxIn1-xAs1-yNy to reduce the band gap to about 1 eV
(for the third junction in a four-junction device). Although 1-eV GaInAsN can be grown lattice-
matched to GaAs, the addition of nitrogen degrades the electrical quality of the GaAs, thus far
preventing the realization of a higher efficiency with a four-junction design. Other potential 1-eV
materials are also being investigated. The use of five- or six-junction cells reduces the amount of
current per junction and provides a pathway for GaInAsN to be a useful addition.
As for other III-V devices, commercial deposition systems are available for growing these
multilayer devices with film thickness uniformities within <1%. Three-junction solar cells based
on GaInP/GaInAs/Ge are commercially available for space applications. The efficiencies of these
cells approach 30% under 1-sun, AM0 (space) conditions. If one were to convert the production
of space cells today (a few hundred kilowatts annually, used with no or very low concentration in
space) to terrestrial concentrator applications at 500–1,000 suns, the terrestrial output of these
cells would be several hundred megawatts in concentrator systems. To date, multijunction cells
have not contributed to the terrestrial PV market, and, at least in the near term, progress in these
high-efficiency solar cells will continue to be driven by growing space-power markets.
NEW TECHNOLOGIES
Dye-sensitized Solar Cells
The dye-sensitized solar cell (O’Regan and Grätzel 1991) has foundations in photochemistry
rather than in solid-state physics. In this interesting device, also called the “Grätzel cell” after its
Swiss inventor, organic dye molecules are adsorbed on a nanocrystalline titanium dioxide (TiO 2 )
film, and the nanopores of the film are filled with a redox electrolyte. The dyes absorb solar
photons to create an excited molecular state that can inject electrons into the TiO 2. The electrons
percolate through the nanoporous TiO 2 film and are collected at a transparent electrode. The
oxidized dye is reduced back to its initial state by accepting electrons from the redox relay via
ionic transport from a metal counter-electrode; this completes the circuit and electrical power is
generated in the external circuit. Dye-sensitized solar cells are extremely attractive because of
the very low cost of the constituent materials (TiO 2 is a common material used in paints and
toothpaste) and the potential simplicity of their manufacturing process. Laboratory-scale devices
of 11% have been demonstrated, but larger modules are less than half that efficient. Stability of
the devices (e.g., dye materials and electrolyte) is an ongoing research issue.