A team of scientists at the University of California, Berkeley, Lawrence Berkeley National Laboratory, also in Berkeley, and at Cornell University in Ithaca, N.Y., has reported that the bandgap energy of InN is approximately 0.7 eV at room temperature, rather than 2 eV, as suggested in previous studies. Multijunction photovoltaic cells fabricated of InN and GaN to form InGaN ternary alloys with bandgaps of 0.7 to 3.4 eV thus may cover the entire solar spectrum and offer conversion efficiencies of 50 to 70 percent.The researchers investigated ultrapure InN films grown on sapphire by molecular beam epitaxy, rather than by sputtering, as in previous work. They performed optical absorption measurements with an NIR-VIS-UV spectrophotometer and IR reflection experiments with a Fourier transform IR spectrometer. Their results appeared in the Nov. 15 issue of Physical Review B.Because InGaN alloys tolerate relatively large lattice mismatches, the researchers are confident that it should be possible to construct inexpensive photovoltaic cells of the materials, in which the bandgaps of different layers would be tuned to absorb particular components of the solar spectrum. A challenge remains in the development of P-type InGaN alloys.