Minibands modeling in strain-balanced InGaAs/GaAs/GaAsP cells

Abstract

In photovoltaic, multi quantum wells (MQW) allow to tailor the optical absorption. This is particularly interesting in multijunction solar cells [1] but it also permits to improve the efficiency of a single junction solar cell [2]. This approach is efficient thanks to the strain-balanced materials which, at a well under compressive strain, associates a barrier under tensile strain. This permits to consider a large number of wells while preventing the formation of dislocations during crystal growth. On the other hand, the use of barriers is a drawback for the collection of the photo-generated carriers and more generally for the electronic transport quality in the MQW. Indeed, since transport is a succession of thermal escape, assisted tunnel escape and, at best, direct tunneling across a barrier, the average carrier velocity is low (of about 104 cm s-1) [3]. Finally the recombination rate is large and impacts both open-circuit voltage and shortcircuit current. Furthermore, thanks to barriers some minibands can occur [4]. The wave functions of carriers in minibands are Bloch waves, meaning that propagation is efficient. Our theoretical study, based on quantum simulation (Green functions formalism) in InGaAs/GaAs/GaAsP cells, sheds light on minibands in which the average velocity of carriers is around 107 cm s-1. However, we also show that, without an adapted design, such minibands are inefficient since they connect only a few wells. We will present some improvements related to the distance between barriers and the positioning of the MQW inside the cell.