Type-II GaAsSb/GaAsN superlattice solar cells

Trabajo presentado en el SPIE Octo: Physics, Simulation, and Photonic Engineering of Photovoltaic Devices VII, celebrado en San Francisco (California, Estado Unidos), del 27 de enero al 1 de febrero de 2018
Dilute nitride GaAsSbN is an ideal candidate to form the 1-1.15 eV lattice-matched sub-cell that would significantly enhance the performance of 3- and 4-junction solar cells. However, growth problems inherent to this quaternary alloy lead typically to a poor crystal quality that limits its applicability. Better compositional control and crystal quality have been recently reported by growing the material as a GaAsSb/GaAsN superlattice, because of the spatial separation of Sb and N that avoid miscibility problems. Moreover, these structures provide bandgap tunability trough period thickness. Here we study the performance of lattice-matched 1.15 eV GaAsSb/GaAsN type-II superlattice p-i-n junction solar cells with different period thickness and compare them with the bulk and GaAsSbN/GaAs type-I superlattice counterparts. We demonstrate carrier lifetime tunability through the period thickness in the type-II structures. However, the long carrier lifetimes achievable with periods thicker than 12 nm are incompatible with a high carrier extraction efficiency under short-circuit conditions. Only superlattices with thinner periods and short carrier lifetimes show good solar cell performance. Quantum kinetic calculations based on the non-equilibrium Green’s function (NEGF) formalism predict a change in transport regime from direct tunneling extraction to sequential tunneling with sizable thermionic emission components when passing from 6 nm to 12 nm period length, which for low carrier lifetime results in a decrease of extraction efficiency by more than 30%.
J. M. Ulloa acknowledges funding from the Spanish MINECO through project MAT2016-77491-C2-1-R and D. Fuertes Marrón from Comunidad de Madrid (S2013/MAE-2780). U. Aeberhard, D. Fuertes Marrón and J.M. Ulloa acknowledge EU COST Action MP1406 “Multiscale in modelling and validation for solarphotovoltaics (MultiscaleSolar)”.