The potential for new 4‐, 5‐, and 6‐junction solar cell architectures to reach 50% efficiency is highly leveraging for the economics of concentrator photovoltaic (CPV) systems.The theoretical performance of such next‐generation cells, and experimental results for 3‐ and 4‐junction CPV cells, are examined here to evaluate their impact for real‐world solar electricity generation. Semiconductor device physics equations are formulated in terms of the band gap‐voltage offset Woc  (Eg/q) − Voc, to give a clearer physical understanding and more general analysis of the multiple subcell band gaps in multijunction cells. Band gap‐voltage offset is shown experimentally to be largely independent of band gap Eg for a wide range of metamorphic and lattice‐matched semiconductors from 0.67 to 2.1 eV. Its theoretical Eg dependence is calculated from that of the radiative recombination coefficient, and at a more fundamental level using the Shockley‐Queisser detailed balance model, bearing out experimental observations. Energy production of 4‐, 5‐, and 6‐junction CPV cells, calculated for changing air mass and spectrum over the course of the day, is found to be significantly greater than for conventional 3‐junction cells. The spectral sensitivity of these next‐generation cell designs is fairly low, and is outweighed by their higher efficiency. Lattice‐matched GaInP/GaInAs/Ge cells have reached an independently confirmed efficiency of 41.6%, the highest efficiency yet demonstrated for any type of solar cell. Light I‐V measurements of this record 41.6% cell, of next‐generation upright metamorphic 3‐junction cells with 40% target production efficiency, and of experimental 4‐junction CPV cells are presented. Copyright © 2010 John Wiley & Sons, Ltd.

Multijunction III-V concentrator cells of several different types have demonstrated solar conversion efficiency over 40% since 2006, and represent the only third-generation photovoltaic technology to enter commercial power generation markets so far. The next stage of solar cell efficiency improvement, from 40% to 50%-efficient production cells, is perhaps the most important yet, since it is in this range that concentrator photovoltaic (CPV) systems can become the lowest cost option for solar electricity, competing with conventional power generation without government subsidies. The impact of 40% and 50% cell efficiency on cost-effective geographic regions for CPV systems is calculated in the continental US, Europe, and North Africa. We take a systematic look at a progression of multijunction cell architectures that will take us up to 50% efficiency, using modeling grounded in well-characterized solar cell materials systems of today's 40% cells, discussing the theoretical, materials science, and manufacturing considerations for the most promising approaches. The effects of varying solar spectrum and current balance on energy production in 4-junction, 5-junction, and 6-junction terrestrial concentrator cells are shown to be noticeable, but are far outweighed by the increased efficiency of these advanced cell designs. Production efficiency distributions of the last five generations of terrestrial concentrator solar cells are discussed. Experimental results are shown for a highly manufacturable, upright metamorphic 3-junction GaInP/GaInAs/Ge solar cell with 41.6% efficiency independently confirmed at 484 suns (48.4 W/cm 2 ) (AM1.5D, ASTM G173-03, 25 C), the highest demonstrated for a cell of this type requiring a single metalorganic vapor-phase epitaxy growth run.

We report on the fabrication and optical characterization of photonic superlattices comprised of sequentially grown stacks of colloidal photonic crystals. The superlattice periodicity induces the formation of minibands due to folding of the photonic band structure. This represents the first instance in which midgap states have been incorporated into a colloidal photonic crystal via a specifically engineered structural modification.

We have carried out a systematic temperature-dependent study of intersubband absorption in InAs/AlSb quantum wells from 5 to 10 nm well width. The resonance energy redshifts with increasing temperature from 10 to 300 K, and the amount of redshift increases with decreasing well width. We have modeled the transitions using eight-band k⋅p theory combined with semiconductor Bloch equations, including the main many-body effects. Temperature is incorporated via band filling and nonparabolicity, and good agreement with experiment is achieved for the temperature dependence of the resonance.

We have measured the picosecond time-resolved cyclotron resonance of photogenerated transient carriers in undoped InSb/Al 0.09 In 0.91 Sb quantum wells by two-color pump-probe spectroscopy in a magnetic field. The strong conduction-band nonparabolicity of InSb causes the average cyclotron mass of the electrons, which we monitor directly in time, to decrease as the electrons relax towards the band edge. In addition, the nonparabolicity results in multiple resonances due to the strongly energy-dependent mass and g factor, allowing us to determine the time evolution of the Fermi-Dirac distribution function for the excited carriers in quantizing magnetic fields.

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We employ a cavity-length study to determine the temperature variation of the internal loss and gain per unit current density in a ten-stage interband cascade laser that operated cw up to 269K with an emission wavelength of 4.05μm. The characteristic temperature for the gain per unit current density is 39K, which is slightly lower than T0 of the threshold current and is consistent with dominance by Auger recombination. The internal loss for the 150-μm-wide mesa devices increased from 11cm−1 at 78Kto28cm−1 at 275K.