Electronic structure of Ge1-x-ySixSny ternary alloys for multijunction solar cells

C. I. VENTURA; J. D. QUERALES FLORES; J. D. FUHR; R. A. BARRIO

PROGRESS IN PHOTOVOLTAICS

JOHN WILEY & SONS LTD

Lugar: LOndres; Año: 2015 vol. 23 p. 112 - 118

1062-7995

ernary group-IV alloys have a wide potential for applications in infrared devices and optoelectronics. In connection with photovoltaic applications, they are among the most promising materials for inclusion in the next generation of high-efficiency multijunction solar cells, because they can be lattice matched to substrates as GaAs and Ge, offering the possibility of a range of band gaps complementary to III?V semiconductors. Apart from the full decoupling of lattice andband structures in Ge1?x?y Six Sny alloys, experimentally confirmed, they allow preparation in a controllable and large range of compositions, thus enabling to tune their band gap. Recently, optical experiments on ternary alloy-based films, photodetectors measured the direct absorption edges and probed the compositional dependence of the direct gap. The nature of the fundamental gap of Ge1?x?y Six Sny alloys is still unknown, as neither experimental data on the indirect edges nor electronic structure calculations are available, as yet. Here, we report a first calculation of the electronic structure of Ge1?x?y Six Sny ternary alloys, employing a combined tight-binding and virtual crystal approximation method, which proved to be useful to describe group-IV semiconductor binary alloys. Our results confirm predictions and experimental indications that a 1 eVband gap is indeed attainable with these ternary alloys, as required for the fourth layer plan to be added to present-day record-efficiency triple-junction solar cells, to further increase their efficiency, for example, for satellite applications. When lattice matched to Ge, we find that Ge1?x?y Six Sny ternary alloys have an indirect gap with a compositional dependence reflecting the presence of two competing minima in the conduction band.