eV Sub-cell Research Articles

Performance assessment of a triple-junction solar cell with 1.0 eV GaAsBi absorber

Group III–V semiconductor multi-junction solar cells are widely used in concentrated-sun and space photovoltaic applications due to their unsurpassed power conversion efficiency and radiation hardness. To further increase the efficiency, new device architectures rely on better bandgap combinations over the mature GaInP/InGaAs/Ge technology, with Ge preferably replaced by a 1.0 eV subcell. Herein, we present a thin-film triple-junction solar cell AlGaAs/GaAs/GaAsBi with 1.0 eV dilute bismide. A compositionally step-graded InGaAs buffer layer is used to integrate high crystalline quality GaAsBi absorber. The solar cells, grown by molecular-beam epitaxy, achieve 19.1% efficiency at AM1.5G spectrum, 2.51 V open-circuit voltage, and 9.86 mA/cm2 short-circuit current density. Device analysis identifies several routes to significantly improve the performance of the GaAsBi subcell and of the overall solar cell. This study is the first to report on multi-junctions incorporating GaAsBi and is an addition to the research on the use of bismuth-containing III–V alloys in photonic device applications.

Diluted nitride type-II superlattices: Overcoming the difficulties of bulk GaAsSbN in solar cells

We demonstrate type-II GaAsSb/GaAsN superlattices (SL) as a suitable structure to form the lattice-matched 1.0–1.15 eV subcell that would allow the implementation of the optimum monolithic multi-junction solar cell design. The separation of Sb and N atoms during growth leads to an improved composition homogeneity and a lower defect density than in the bulk GaAsSbN counterparts. The type-II band alignment SLs provide long radiative lifetimes that facilitate carrier collection as compared to equivalent type-I SLs. Moreover, the radiative lifetime can be controllably tuned through the period thickness, which is not possible in type-I SLs. A reduced period thickness results in enhanced absorption due to increased wavefunction overlap, as well as in a change in the transport regime from diffusive to quasiballistic, providing improved carrier extraction efficiency. As a result, the short period SL single-junction solar cells show an enhanced power conversion efficiency of 134% over the equivalent bulk devices.

Designing of 1 eV GaNAs/GaInAs superlattice subcell in current-matched four-junction solar cell**Project supported by the National Natural Science Foundation of China (No. 61274134) and the International Cooperation Program of Suzhou, China (No. SH201215).

A reasonably-thick GaNAs/GaInAs superlattice could be an option as a roughly 1 eV subcell to achieve high-efficiency multi-junction solar cells on a lattice-matched Ge substrate. A detailed consideration of a high-efficiency design for a GaInP/GaAs/1 eV/Ge device is presented. Calculations have been done for this structure to obtain the confined energies of the electrons and holes by utilizing the Kronig-Penney model, as well as the absorption coefficient and thereby the external quantum efficiency. The effect of well layers, GaNAs or GaInAs, on the absorption and photocurrent density under the AM 1.5 condition is discussed in order to realize a requirement of current matching in the four-junction solar cells. The management of these considerations implies the feasibility of the GaNAs/GaInAs superlattice subcell design to improve the overall conversion efficiency of lattice matched GaInP/GaAs/1 eV/Ge cells.

Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell

Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs0.94Bi0.06 yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.