Solution grown caesium-formamidinium lead halide perovskites for detection of gamma photons
Hybrid formamidinium (FA) lead halide perovskites (FAPbX3, X=I or Br/I) gained considerable popularity due to their excellent performance as photovoltaic and high energy photon-detecting materials [1]. The detection of gamma photons is enabled by high electronic quality of FAPbI3 single crystals (SCs): low noise level and dark current, high mobility-lifetime product (1.8×10−2 cm2 V−1), and high absorptivity of high-energy photons by Pb and I [1]. The difficulties arise from the phase instability of the desired three-dimensional (3D) FAPbI3 cubic perovskite phase that undergoes a phase transition to non-perovskite 1D hexagonal lattice. The reason lies in the large size and spatial geometry of FA cation. The Goldschmidt tolerance factor (GTF) concept is a useful tool in estimation of the compositionally-dependent stability of 3D perovskites with ABX3 general formula and idealized cubic lattice. GTF=(rA+rx)/[√2(rB+rx)], where rA, rB and rx represent the ionic radii of each lattice site constituent (in this case, rFA+=253 pm, rPb2+=119 pm and rI-=220 pm). Stable cubic perovskites typically exhibit a GTF=0.8-1 (GTF=0.987 for cubic FAPbI3 at room temperature). Decreasing the GTF of FAPbI3 can be obtained by replacing FA+ cations by smaller Cs+ ions, and/or by replacing I- anions with smaller Br- ions, likely leading to higher stability.
We will present a facile, inexpensive, solution-phase growth of cm-scale SCs of variable composition CsxFA1-xPbI3-yBry (x=0-0.1, y=0-0.6). Comparing to the parent cubic FAPbI3 compound these SCs show improved phase stability with shelf life (the time before hexagonal phase impurities could be detected) of up to 20 days for quaternary CsxFA1-xPbI3 SCs and of more than 4 months for quinary CsxFA1-xPbI3-yBry SCs [2]. These SCs possess outstanding electronic quality, represented by a high carrier mobility-lifetime product ( up to 1.2x10-1 cm2 V-1) and a low dark carrier density allowing the sensitive detection of gamma radiation. With stable operation up to 30 V, these novel SCs have been used in a prototype of a gamma-counting dosimeter.
Figure 1. Energy resolved spectrum of an 241Am source using perovskite SCs; (b) A photograph of typical 0.5-1 cm CsxFA1-xPbI3-yBry SCs on a millimetre-grid paper; (c) Photoluminescence spectra of ground SCs.
[1] S. Yakunin, D. N. Dirin, Y. Shynkarenko, V. Morad, I. Cherniukh, O. Nazarenko, D. Kreil, T. Nauser, M. V.Kovalenko. Nat. Photon. 2016, 10 (9), 585-589. [2] O. Nazarenko, S. Yakunin, V. Morad, I. Cherniukh, M. V. Kovalenko. NPG Asia Mater., 2017, 9, e373.