# Polycrystalline Formamidinium Lead Bromide X-ray Detectors
## 4 Conclusions  
We have demonstrated the high sensitivity of FAPbBr\({}_{3}\) polycrystalline X-ray detectors based on polycrystalline material produced using the relatively low-cost method of solution growth followed by mechanical grinding and pressing of pellets. Solution growth of millimetre-sized FAPbBr\({}_{3}\) single crystals using the ITC method is a straightforward synthesis technique that produced perovskite crystals with good charge transport properties. The materials show a remarkable degree of robustness since the charge transport properties are largely preserved after mechanically grinding the crystals into powder. The use of a pellet press with pressures in the range 12-124 MPa produces a high density polycrystalline material, with further optimisation possible using a post-pressing annealing step.  
Our work has demonstrated the performance of X-ray sensors fabricated from polycrystalline FAPbBr\({}_{3}\) pellets which have dark current values in the range 10-100 nA, depending on applied bias voltage, and with X-ray sensitivities for 2 mm thick pellets which are broadly similar to that of single crystal material, with sensitivity values measured up to 169 \(\upmu\)C Gy\({}^{-1}\) cm\({}^{-2}\). The time response of these devices has been demonstrated to be faster than 3 ms, which is currently the measurement limit of our system. Further improvements in the reproducibility and sensitivity of these devices may also be possible through further optimisation of the material preparation methods and by adopting a higher-temperature sintering technique for fabrication of the pellets.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3990/app12042013/s1](https://www.mdpi.com/article/10.3990/app12042013/s1)]([https://www.mdpi.com/article/10.3990/app12042013/s1](https://www.mdpi.com/article/10.3990/app12042013/s1)), Figure S1: X-ray energy spectrum of the Amptek Mini-X2 X-ray tube; Figure S2: Influence of pellet annealing on the measured X-ray sensitivity; Table S1: Summary of mean attenuation length and K-shell escape peaks, calculated using NIST XCOM and GEANT4 Monte Carlo.  
Conceptualization, P.S., R.M. and D.W.; methodology and investigation, S.A.; SEM, AFM and XRD measurements J.G. and J.R.; writing--original draft preparation, S.A., P.S.;  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **X-ray Sensitivity \(\upmu\)C Gy\({}^{-1}\) cm\({}^{-2}\)** & **Reference** \\ \hline FAPbBr\({}_{3}\) (2 mm) & 169 & this work \\ a-Se (200 \(\upmu\)m) & 20 & [30] \\ poly CZT (350 \(\upmu\)m) & 200 & [31] \\ Cs\({}_{2}\)AgBiBr\({}_{6}\) (1 mm) & 250 & [17] \\ MAPbI\({}_{3}\) (1 mm) & 2500 & [16] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of X-ray sensitivity of X-ray detectors fabricated from polycrystalline perovskite and traditional semiconductor materials.  
writing--review and editing, all authors ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]). All authors have read and agreed to the published version of the manuscript.
* [PERSON] acknowledges studentship support from Al-Baha University, Saudi Arabia. J.G. is funded by the Defense Threat Reduction Agency (DTRA) under Basic Research contract HDTRA1-18-1-0019. J.R. is funded by DTRA as part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA) under contract number HDTRA1-20-2-0002.
* Not applicable.
* We are grateful for technical support at University of Surrey from [PERSON], [PERSON], [PERSON], [PERSON], [PERSON].
* The authors declare no conflict of interest.