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

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 latticematched
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 latticematched
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