Hybrid Charge-Transfer Semiconductors: (C7H7)SbI4, (C7H7)BiI4, and Their Halide Congeners

Abstract

Hybrid metal halides yield highly desirable optoelectronic properties and offer significant opportunity due to their solution processability. This contribution reports a new series of hybrid semiconductors, (C7H7)MX4 (M = Bi 3+ , Sb 3+ ; X = Cl -, Br -, I -), that are composed of edge-sharing MX6 chains separated in space by -stacked tropylium (C7H7 + ) cations; the inorganic chains resemble the connectivity of BiI3. The Bi 3+ compounds have blue shifted optical absorptions relative to the Sb 3+ compounds that span the visible and near-IR region. Consistent with observations, DFT calculations reveal that the conduction band is composed of the tropylium cation and valence band primarily the inorganic chain: a charge-transfer semiconductor. The band gaps for both Bi 3+ and Sb 3+ compounds decrease systematically as a function of increasing halide size. These compounds are a rare example of charge transfer semiconductors that also exhibit efficient crystal packing of the organic cations, thus providing an opportunity to study how structural packing affects optoelectronic properties. Introduction.Hybrid perovskites and their derivatives have emerged as highly efficient solution processable semiconducting materials, making them attractive for numerous applications such as light-emitting devices, photovoltaics, photodetectors, and radiation detectors. 1-8 These compounds combine many of the desirable properties of traditional inorganic semiconductors such as high carrier mobilities and absorption coefficients with the processability of organic electronics. 6,[9][10] However, many questions remain as to how the inorganic framework can influence or couple to the electronic behavior of the organic components. In the realm of hybrid perovskites, much focus has been on compounds containing small organic cations such as methylammonium or formamidinium. [11][12][13][14][15] Unlike smaller cations, organic cations with delocalized electrons can have electronic states near the valence and conduction band of the inorganic lattice, allowing for charge-transfer from the inorganic to organic subunits.

Additive-assisted synthesis and optoelectronic properties of (CH3NH3)4Bi6I22

“…In addition, we note that other recently reported iodobismuthates also have weak or negligible PL emission at room 15 temperature. 47,49,50 Based on the temperature dependent PL measurements, we plotted in Fig S6,…”

(C2H5NH3)2BiCl5−xBrx perovskites containing 1D chains: effect of Br substitution on their structural and optical properties

“…Haloantimonates(III) and halobismuthates(III) having the general formula R a M b X 3b+a (where R = organic cations, M = Sb 3+ /Bi 3+ and X = Cl, Br or I) have been widely studied because of their easy synthesis, enhancement in various properties and unique optoelectronic properties. [19][20][21][22][23] These lead-free organic-inorganic mixed halide perovskites exist in various structural dimensions including 0D, 1D, 2D and 3D architectures. [24][25][26][27] Among them, (C 2 H 5 NH 3 ) 2 BiX 5 (X = Cl, Br) compounds are halobismuthates composed of 1 ∞ [BiX 5 ] 2− 1D chains.…”