We numerically investigate the impact of electron–phonon
scattering on the optical properties of a perovskite material (CH3NH3PbI3). Using nonequilibrium Green
function formalism, we calculate the local density of states for several
values of the electron–phonon scattering strength. We report
an Urbach-like penetration of the density of states in the band gap
due to scattering. A physical analytical model allows us to attribute
this behavior to a multiphonon process. Values of Urbach energy up
to 9.5 meV are obtained, meaning that scattering contribution to the
total experimental Urbach energy of 15 meV is quite important. We
also show that the open-circuit voltage Voc, for a solar cell assuming such a material as an absorber, depends
on the scattering strength. Voc loss increases with the scattering strength, up to 41 mV.
Finally, an unexpected result of this study, is that the impact of
electron–phonon scattering on Urbach tail and Voc increases with the phonon energy.
This low value in perovskite (8 meV) is therefore an advantage for
photovoltaic applications.
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 10 4 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 10 7 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.
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