91̽��

Abstract:

Combination of a cryogenic ion-trap machine, operated at 4.7 K, with the free-electron-laser FELIX allows the first experimental characterization of the unusually bright antisymmetric stretch (ν₃) and π-bending (ν₂) fundamentals of the He–X⁺–He (X = H, D) chromophore of the in situ prepared HHe_n⁺ and DHe_n⁺��(n = 3–6) complexes. The band origins obtained are fully 91̽��ed by first-principles quantum-chemical computations, performed at the MP2, the CCSD(T), and occasionally the CCSDTQ levels employing extended basis sets. Both the experiments and the computations are consistent with structures for the species with n = 3 and 6 being of T-shaped C_2v and of D_4h symmetry, respectively, while the species with n = 4 are suggested to exhibit interesting dynamical phenomena related to large-amplitude motions.

Abstract:

Metal halide perovskites have demonstrated extraordinary potential as materials for next-generation optoelectronics including photovoltaics and light-emitting diodes. Nevertheless, our understanding of this material is still far from complete. One remaining puzzle is the phenomenon of perovskite “photo-brightening”: the increase in photoluminescence during exposure to light in an ambient atmosphere. Here, we propose a comprehensive mechanism for the reactivity of the archetypal perovskite, MAPbI3, in ambient conditions. We establish the formation of lead-oxygen bonds by hydrogen peroxide as the key factor leading to perovskite photo-brightening. We demonstrate that hydrogen peroxide can be applied directly as an effective “post-treatment” to emulate the process and substantially improve photoluminescence quantum efficiencies. Finally, we show that the treatment can be incorporated into photovoltaic devices to give a 50 mV increase in open-circuit voltage, delivering high 19.2% steady-state power conversion efficiencies for inverted perovskite solar cells of the mixed halide, mixed cation perovskite FA0.83Cs0.17Pb(I0.9Br0.1)3.