# Polycrystalline Formamidinium Lead Bromide X-ray Detectors
## 1 Introduction  
In recent years, lead halide perovskites (LHPs) have attracted much attention due to their success in various applications such as light emitting devices [1], photovoltaic devices [2], and photodetectors [3]. The generic formula of a LHP is APbX\({}_{3}\), where A can be an inorganic atom such as Cs, or an organic molecule such as methylammonium (MA) [CH\({}_{3}\)NH\({}_{3}^{+}\)], or formamidinium (FA) [HC(NH\({}_{2}\))\({}_{2}^{+}\)]. The species X is a halogen atom such as I, Cl, or Br. Perovskite materials have a three-dimensional network structure of Pb2\({}^{+}\) cations 6 coordinated with a halide species, and surrounded by 12 coordinated cations. Here we report the X-ray detection performance of organic LHP material containing the FA molecule. In FA-based LHPs the strength and the extension of the hydrogen bonds between the atoms (N-H-X) affects the microscopic dynamics of the organic cation [4; 5; 6]. Pressure can alter the orientation of the organic cation and the length of the hydrogen bond between the atoms. At high temperatures, there is some evidence that formamidinium lead bromide (FAPbBr\({}_{3}\)) is more stable in humid and hot conditions when compared to methylammonium lead bromide (MAPbBr\({}_{3}\)) [7; 8]. FAPbBr\({}_{3}\) may also possess some advantages in terms of device performance compared to MAPbBr\({}_{3}\), due to its larger cation size where MA (R = 2.70 A) and FA (R = 2.79 A) [9; 10], and the absence of room temperature phase transitions. In this work we report the performance of polycrystalline FAPbBr\({}_{3}\) used as an energy integrating X-ray detector operating in a photoconductive mode. The polycrystalline perovskite detectors were fabricated using room temperature pressing of perovskite powders at high pressure. Due to the presence of high atomic number elements such as Pb, anda relatively high density of 4.1 g cm\({}^{-3}\), the X-ray detection efficiency of lead halide perovskites is high. Figure 1a shows the calculated X-ray total mass attenuation coefficient as a function of X-ray energy for FAPbBr\({}_{3}\), which is compared to that of two major detector materials cadmium telluride (CdTe) and selenium (Se), modelled using the NIST XCOM photon cross section database [11]. The total mass attenuation coefficient of FAPbBr\({}_{3}\) has a broadly similar energy dependence to that of CdTe, although with some boost at higher energies above the lead K-shell absorption edge (e.g., at energies above 85 keV). The relative X-ray absorption \(\epsilon_{X}(x)\) for a detector of thickness \(x\) is given by  
\[\epsilon_{X}(x)=1-\exp(-\frac{\mu}{\rho}\rho x) \tag{1}\]  
where \(\frac{\mu}{\rho}\) is the mass attenuation coefficient of the detector material, and \(\rho\) is the material density (see Supplementary Information and references [12; 13; 14; 15]). Using Equation (1), Figure 1b compares the relative X-ray absorption for a 1 mm thick detector of FAPbBr\({}_{3}\) with CdTe and Se, showing that FAPbBr\({}_{3}\) has a very similar detection efficiency to CdTe for energies above the Pb K-shell absorption edge. For X-ray energies below the K-shell edge, the relative X-ray absorption of FAPbBr\({}_{3}\) is slightly less than that of CdTe, i.e., approximately 20% less at 50 keV. In photoconductive mode the X-ray sensitivity of the detector is determined by the magnitude of the photocurrent produced by the interaction of X-rays. The sensitivity is controlled by both the relative X-ray absorption and the charge transport performance of the detector material. In polycrystalline perovskite materials, the charge transport performance may be limited by the microstructure of the material, and also by the presence of grain boundaries (see Supplementary Information). Previous reports by [PERSON] et al describe large wafers of MAPbI\({}_{3}\) with a diameter of 1.27 cm and a thickness of 1 mm, produced by applying a pressure of 0.3 GPa to MAPbI\({}_{3}\) microcrystals. These devices showed a sensitivity of 2527 uC Gy\({}^{-1}\)cm\({}^{-2}\) under 70 kVp X-ray exposure [16].