In photovoltaic, multi quantum wells (MQW) allow to tailor the optical absorption. This is particularly interesting in multijunction solar cells  but it also permits to improve the efficiency of a single junction solar cell . 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) . Finally the recombination rate is large and impacts both open-circuit voltage and shortcircuit current. Furthermore, thanks to barriers some minibands can occur . 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.
Benoit Galvani, Fabienne Michelini, Marc Bescond, Masakazu Sugiyama, Jean-François Guillemoles, and Nicolas Cavassilas, "Minibands modeling in strain-balanced InGaAs/GaAs/GaAsP cells," Proc. SPIE 10099, Physics, Simulation, and Photonic Engineering of Photovoltaic Devices VI, 100990X (Presented at SPIE OPTO: February 01, 2017; Published: 23 February 2017); https://doi.org/10.1117/12.2252074.
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