Combining light-harvesting with detachability in high-efficiency thin-film silicon solar cells

Correlated organic-inorganic motion enhances stability and charge carrier lifetime in mixed halide perovskites

Organic cations are believed to have little influence on the charge carrier lifetime in hybrid organic-inorganic perovskites. Experiments defy this expectation. We consider formamidinium lead iodide (FAPbI₃) doping with and without Br as two prototypical systems, and perform ab initio time-domain nonadiabatic (NA) molecular dynamics simulations to investigate nonradiative electron-hole recombination. The simulations demonstrate that correlated organic-inorganic motion stabilizes the lattice and inhibits nonradiative charge recombination in FAPbI₃ upon Br doping. Br doping suppresses the rotation of FA and the vibrations of both organic and inorganic components, and leads to hole localization and the extent of localization is enhanced upon thermal impact, notably reducing the NA coupling by decreasing the overlap between the electron and hole wave functions. Doping also slightly increases the bandgap for further decreasing NA coupling and enhances the open-circuit voltage of perovskite solar cells. The small NA coupling and large bandgap beat the slow coherence loss, delaying electron-hole recombination and extending the charge carrier lifetime to 1.5 ns in Br-doped FAPbI₃, which is on the order of 1.1 ns in pristine FAPbI₃. The obtained time scales are in good agreement with experiments. Multiple phonon modes, including those of both the inorganic and organic components, couple to the electronic subsystem and accommodate the excess electronic energy lost during nonradiative charge recombination. The study reveals the unexpected atomistic mechanisms for the reduction of electron-hole recombination upon Br doping, rationalizes the experiments, and advances our understanding of the excited-state dynamics of perovskite solar cells.

NaCl substrates for high temperature processing and transfer of ultrathin materials

Ultrathin materials often require high temperatures for growth and processing, which cannot be withstood by the substrate underneath. For example, polymers are widely used as a supporting layer but unfortunately have low strain-point temperatures. This is the case of polyethylene terephthalate (PET) which has glass transition and melting temperatures of 76 and 250 °C, respectively. In this paper we propose to use polished salt, a material that can withstand high temperatures during fabrication and, at the same time, can be sacrificed during the transfer onto the final substrates. More specifically, we demonstrate thermal dewetting of Au ultrathin metal films and growth of MoS₂ on NaCl at 750 and 650 °C, respectively, and subsequent transfer onto PET films, after which the salt is easily dissolved by water. We believe that the proposed technique can be extended to fabrication of other ultrathin materials, e.g. graphene, as well as final substrates for a wide range of applications, including flexible electronic and optoelectronic devices.