Lattice Expansion in Rb-Doped Hybrid Organic–Inorganic Perovskite Crystals Resulting in Smaller Band-Gap and Higher Light-Yield Scintillators

Two-dimensional hybrid-organic–inorganic perovskite (2D-HOIP) lead bromide perovskite crystals have demonstrated great potential as scintillators with high light yields and fast decay times while also being low cost with solution-processable materials for wide energy radiation detection. Ion doping has been also shown to be a very promising avenue for improvements of the scintillation properties of 2D-HOIP crystals. In this paper, we discuss the effect of rubidium (Rb) doping on two previously reported 2D-HOIP single crystals, BA2PbBr4 and PEA2PbBr4. We observe that doping the perovskite crystals with Rb ions leads to an expansion of the crystal lattices of the materials, which also leads to narrowing of band gaps down to 84% of the pure compounds. Rb doping of BA2PbBr4 and PEA2PbBr4 shows a broadening in the photoluminescence and scintillation emissions of both perovskite crystals. Rb doping also leads to faster γ-ray scintillation decay times, as fast as 4.4 ns, with average decay time decreases of 15% and 8% for Rb-doped BA2PbBr4 and PEA2PbBr4, respectively, compared to those of undoped crystals. The inclusion of Rb ions also leads to a slightly longer afterglow, with residual scintillation still being below 1% after 5 s at 10 K, for both undoped and Rb-doped perovskite crystals. The light yield of both perovskites is significantly increased by Rb doping with improvements of 58% and 25% for BA2PbBr4 and PEA2PbBr4, respectively. This work shows that Rb doping leads to a significant enhancement of the 2D-HOIP crystal performance, which is of particular significance for high light yield and fast timing applications, such as photon counting or positron emission tomography.


Rietveld Refinements
The Rietveld program Fullprof 7 was selected to analyse the data in this study. The profile function of a Thompson-Cox-Hastings pseudo-Voigt function was used. The background function was the sixth order of polynomials. The results are shown in Supplementary Figures 1 and 2 while the parameters are shown in Supplementary Table 1.
Supplementary Figure S 2: RbPb 2 Br 5 crystals as comparisons. a) Images of crystals, b) Reitveld refinements of XRD spectrum using reference from, 2 The lattice parameters are shown in Supplementary  Table S1. c) Photoluminescence (PL) and absorption spectra, d) Temperature-dependent Radioluminescence (RL) and light yield as function of temperature (inset).

Absorption Curve fitting
The fit was performed by using Elliot formalism. 8 In principle, the contributions to the absorption coefficient (α) can be defined from free carriers (continuum) (α c ) and excitons (α ex ).
Where the frequency dependence of P cv is approximated as a constant and related to the interband transition matrix element,hω is the photon energy, θ(hω -E g ) is the Heaviside step function, x is defined as R ex /(hω − E g ), and δ denotes a delta function. R ex is exciton Ry- Compound

Temperature-dependent RL fitting
The fit was carried out according to the model proposed by Shibata et al.: 9 where D is the negative thermal quenching coefficient which describes the contribution from thermally excited electrons, C is the thermal quenching coefficients related to non-radiative

Afterglow fitting
The afterglow curves in Figure 4a were fitted with two exponential decay model. The parameters are shown in Supplementary Table 4 Supplementary

Glow curve fitting
For the quantitative analysis, we deconvolute the glow curves into k glow peaks, based on the classic Randall-Wilkins equation: 5,6 where T is the temperature, β is the heating rate, k B is the Boltzmann constant, n 0 i is the initial trap concentration, V is the crystal volume, E i is the trap depth, and σ i is the frequency factor of each component. The unit-less n 0 i V or A i is used to compare afterglow of different crystals. From the fits of Supplementary Eq. S4 to Supplementary Fig. 7, we obtain parameters as shown in Supplementary Table 5.

Supplementary Table S 5:
Parameters of the thermoluminescence (TL) peak fitting, where T max is temperature where the maximum of the peak occurs, E is the trap depth, n 0 is the trap concentration and σ is the frequency factor.

Scintillation decay fitting
Scintillation decay curves in Supplementary Fig. 9 were fitted with three exponential decay