Structures based on gaas(sb)(n) semiconductor alloys for high efficiency multi-junction solar cells

Resumen

III-V multi-junction solar cells (MJSCs) have hold record conversion efficiencies for many years, which is currently approaching 50 %. Theoretical efficiency limits are calculated using optimum designs with the right lattice constant-bandgap energy combination, which requires a 1.0–1.15 eV bandgap semiconductor material lattice matched to GaAs/Ge. Insertion of such a material layer in a 4-junction MJSC could lead to an efficiency of 60 % under solar concentration, which would represent a significant breakthrough in photovoltaics. Therefore, 1.0–1.15 eV bandgap materials that can be grown lattice-matched to GaAs/Ge are being nowadays intensively researched. Dilute nitrides, such as Ga1 xInxAs1 yNy or GaAs1 x ySbxNy, are the most suitable candidates: the introduction of a small amount of N in the GaAs matrix sharply reduces the bandgap energy, and at the same time the lattice constant can be adjusted to that of GaAs/Ge. In particular, GaAs1 x ySbxNy has many potential advantages over Ga1 xInxAs1 yNy, such as promoting a more efficient N incorporation and reducing the formation of N-related defects. However, quaternary dilute nitrides, even GaAs1 x ySbxNy, unavoidably suffer from inherent material problems that seriously degrade carrier dynamics, which are likely the reason for the lack of success of the GaAs1 x ySbxNy based solar cells up to now. In this Thesis, we demonstrate that the substitution of the conventional quaternary alloy by a strain-balanced GaAs1 xSbx/GaAs1 yNy superlattice (SL) with a type-II band alignment is a suitable approach to form the lattice-matched 1.0–1.15 eV subcell to be implemented in the optimum monolithic multi-junction solar cell design. The spatial separation of Sb and N atoms avoids the ubiquitous growth problems, providing an accurate composition control and improving the crystal quality. Moreover, these new structures allow for additional control of the effective bandgap through the period thickness. The type-II band alignment provides long carrier lifetimes, which are also tunable down to the values of the bulk alloys by reducing the period thickness. A reduced period thickness in type-II SLs also results in enhanced absorption due to increased wavefunction overlap, as well as in a change in the transport regime from diffusive to quasi-ballistic, which provides improved carrier extraction efficiency. For the “low” N and Sb contents required in the ~1.15 eV structures, single-junction SL solar cells do not overcome the equivalent bulk devices (the latter having double amount of N and Sb) in terms of power conversion efficiency (PCE). Nevertheless, for the higher N and Sb contents required in the ~1.0 eV structures, the SL approach is advantageous in terms of solar cell PCE. Indeed, 3 nm period SL solar cells show an enhanced PCE of 134 % over the equivalent bulk devices. The improvement is attributed to a reduced non radiative recombination and an improved composition homogeneity in the SLs. To fully exploit the potential of type-II SLs in photovoltaics, an adequate rapid thermal annealing (RTA) cycle might be applied to the structures. The RTA is shown to reduce the density of N-induced sub-bandgap radiative states, which seems to be the main reason for the enhancement of the open-circuit voltage (VOC) observed after RTA, particularly in the “low” N content ~1.15 eV structures. The results suggest that radiative recombination in a broad band of deep defect states is a source of VOC degradation in as-grown GaAs(Sb)(N)-based solar cells. In solar cells with higher N and Sb contents and ~1.0 eV bandgap, not only VOC but also short-circuit current density (JSC) is strongly increased after RTA, resulting in substantial enhancements of the PCE. The large increase of JSC after RTA in ~1.0 eV samples is particularly relevant since it could help to provide current matching when integrated in a MJSC.