Strain-balanced type-II superlattices for efficient multi-junction solar cells

Abstract

Multi-junction solar cells made by assembling semiconductor materials with different bandgap energies have hold the record conversion efficiencies for many years and are currently approaching 50%. Theoretical efficiency limits make use of optimum designs with the right lattice constant-bandgap energy combination, which requires a 1.0–1.15 eV material lattice-matched to GaAs/Ge. Nevertheless, the lack of suitable semiconductor materials is hindering the achievement of the predicted efficiencies, since the only candidates were up to now complex quaternary and quinary alloys with inherent epitaxial growth problems that degrade carrier dynamics. Here we show how the use of strain-balanced GaAsSb/GaAsN superlattices might solve this problem. We demonstrate that the spatial separation of Sb and N atoms avoids the ubiquitous growth problems and improves crystal quality. Moreover, these new structures allow for additional control of the effective bandgap through the period thickness and provide a type-II band alignment with long carrier lifetimes. All this leads to a strong enhancement of the external quantum efficiency under photovoltaic conditions with respect to bulk layers of equivalent thickness. Our results show that GaAsSb/GaAsN superlattices with short periods are the ideal (pseudo)material to be integrated in new GaAs/Ge-based multi-junction solar cells that could approach the theoretical efficiency limit.

Photocarrier extraction in GaAsSb/GaAsN type-II QW superlattice solar cells

“…Table I displays the parameters for N content x N = 1.2% and Sb concentration y Sb = 3.25%, as experimentally determined for the samples of Ref. 5. This configuration with low N and Sb contents is nearly lattice-matched to GaAs and exhibits very low levels of strain in the constituent layers; the effects of strain are therefore neglected in the model.…”

Tuning the properties of InAs/GaAs quantum dots through a modified capping layer: Application to optoelectronic devices

“…In this line, we have already shown an improved compositional control and crystalline quality in 12-nm period GaAsSb/GaAsN superlattices [269]. Moreover, the type-II band alignment of GaAsSb/GaAsN superlattices provides additional versatility as compared to bulk GaAsSbN.…”

“…The correlation between period thickness, effective band gap and carrier lifetime was investigated in the first series of 200 nm-thick GaAsSb/GaAsN superlattices, which were first structurally analyzed. The alternate introduction of Sb and N in 12 nm-thick period GaAsSb/GaAsN superlattices (SL 12 ) was already found to allow for an accurate control of the Sb and N composition [269], preventing both species from interacting during incorporation to the lattice. Indeed, ternary GaAsSb/GaAs and GaAs/GaAsN superlattices were grown using 3.25% Sb and 1.20% N contents, fulfilling the lattice-matching condition to GaAs, which was satisfied by GaAsSb/GaAsN superlattices grown under the same material source conditions.…”

“…Simulations were performed as a function of the period thickness, for both type-I and type-II superlattice structures. The radiative lifetime of the bulk sample and that reported for an equivalent 12 nm-type-I superlattice [269] are additionally represented. There is a good agreement between the tendencies for experimental radiative lifetimes and the theoretically estimated values for the inverse of the wavefunction overlap (1 ⟨Ψ |Ψ ℎ ⟩ ).…”

“…However, besides the structural and compositional impact, the use superlattice structures may bring a major drawback, which is carrier trapping and recombination in the superlattice, before collection. This would result in a lower effective mobility, degrading carrier transport and resulting in a severe reduction of the quantum efficiency [269,270]. Therefore, both radiative lifetime and carrier transport through the superlattice will be ultimately conditioned by its period, which directly affects the electron and hole wavefunction overlap and the electronic coupling between confined states of subsequent periods giving rise to the formation of minibands.…”