Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

We report the growth and characterization of record‐efficiency ZnO/CdS/CuInGaSe2 thin‐film solar cells. Conversion efficiencies exceeding 19% have been achieved for the first time, and this result indicates that the 20% goal is within reach. Details of the experimental procedures are provided, and material and device characterization data are presented. Published in 2003 by John Wiley & Sons, Ltd.

High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edges relative to the average dangling bond defect energy represented by the Fermi level stabilization energy in In1−xGaxN alloys.