Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties

The
synthesis and properties of the hybrid organic/inorganic germanium
perovskite compounds, AGeI3, are reported (A = Cs, organic
cation). The systematic study of this reaction system led to the isolation
of 6 new hybrid semiconductors. Using CsGeI3 (1) as the prototype compound, we have prepared methylammonium, CH3NH3GeI3 (2), formamidinium,
HC­(NH2)2GeI3 (3), acetamidinium,
CH3C­(NH2)2GeI3 (4), guanidinium, C­(NH2)3GeI3 (5), trimethylammonium, (CH3)3NHGeI3 (6), and isopropylammonium, (CH3)2C­(H)­NH3GeI3 (7) analogues. The crystal structures of the compounds are classified
based on their dimensionality with 1–4 forming 3D perovskite frameworks and 5–7 1D infinite chains. Compounds 1–7, with the exception of compounds 5 (centrosymmetric)
and 7 (nonpolar acentric), crystallize in polar space
groups. The 3D compounds have direct band gaps of 1.6 eV (1), 1.9 eV (2), 2.2 eV (3), and 2.5 eV (4), while the 1D compounds have indirect band gaps of 2.7
eV (5), 2.5 eV (6), and 2.8 eV (7). Herein, we report on the second harmonic generation (SHG) properties
of the compounds, which display remarkably strong, type I phase-matchable
SHG response with high laser-induced damage thresholds (up to ∼3
GW/cm2). The second-order nonlinear susceptibility, χS(2), was determined
to be 125.3 ± 10.5 pm/V (1), (161.0 ± 14.5)
pm/V (2), 143.0 ± 13.5 pm/V (3), and
57.2 ± 5.5 pm/V (4). First-principles density functional
theory electronic structure calculations indicate that the large SHG
response is attributed to the high density of states in the valence
band due to sp-hybridization of the Ge and I orbitals, a consequence
of the lone pair activation.