The room-temperature third order nonlinearities in GaAs/AlGaAs multiple quantum wells have been studied using reflection Z-scan technique. Band-filling effect is considered to be the major nonlinear mechanism of the nonlinear absorption. A model to calculate the absorption coefficient of quantum wells in the nonlinear regime is presented.

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Large-area solar cells are more susceptible to imperfect surfaces, such as edges or cracks. Numerical two-dimensional methods are developed and applied to study the effects of the imperfect surface in solar cells. The two-dimensional effects investigated in this work reveal the lateral conduction of solar cells, which causes inconsistent potentials and a deviation of photocurrent. The mask effects from distributed series resistance on luminescence techniques, such as photoluminescence and electroluminescence, are also explored. Lateral conduction hides the information of the real distribution of defects, and numerical methods are applied to eliminate the effect. In multijunction solar cells, lateral conduction in the interlayer leads to the complementarity between the luminescence of top and bottom cells. Furthermore, the distributed series resistance also causes a knee in the I–V curves, resulting in a decrease in the efficiency of the cell. The research in this work demonstrates that different categories of defects show different influences on solar cells, and the effect is more obvious on the open-circuit voltage rather than the short-circuit current. The study of distributed series resistance and lateral conduction in this work not only gives an explanation of experimental data but also provides instructions for the developments of large-area solar cells.

The performance of metal-organic vapor phase epitaxy (MOVPE) grown upright metamorphic (UMM) AlGaInP/AlGaInAs/GaInAs/Ge quadruple junction (QJ) solar cells have been investigated. Metamorphic (MM) epitaxy is achieved through compositionally graded buffer (CGB) layer varying in lattice constants between Ge and Ga0.8In0.2As. High-resolution X-ray diffraction (HRXRD) was used to study the relaxation of strain which is about 98%. Threading dislocation density (TDD) at about 3 × 105 cm−2 was estimated from cathodoluminescence (CL) images. Incorporation of oxygen is effectively suppressed by elevated growth temperature and phosphine flow, which can decrease the nonradiative recombination rate in active layers. Systematic spectra response measurements were used to analyze the comprehensive effect of reverse breakdown (RBD) and luminescence coupling (LC) in multijunction solar cells, including bias light- and voltage-modulated analysis and corresponding analytical models.

Increasing the conversion efficiency of a triple-junction GaInP/GaAs/Ge solar cell has been a challenge for a long time. One of the difficulties is the mismatched currents among three subcells because the middle cell has the lowest photocurrent while the bottom cell has the highest photocurrent. Some efforts have been taken to improve the problem, such as the growth of metamorphic layers, four junctions, increasing the bandgap of the top cell and reducing the thickness of the top cells. This paper presents a new effort to increase the photocurrent of the middle cell by introducing multi-quantum wells (MQWs) into its intrinsic region because the multi-quantum wells can lower the bandgap of the middle cell and keep good material quality by reaching balanced strain in epitaxial layers. Experimental results have shown that the triple junction solar cells with the multi-quantum wells can reach the conversion efficiency of 30.89% (AM0), higher than that of control cells. Including the multi-quantum wells can increase the short-circuit currents and decrease the opencircuit voltage. The influences of the multi-quantum wells on the solar cells are also discussed.