# Polycrystalline Formamidinium Lead Bromide X-ray Detectors
## 3 Results and Discussion
### X-ray Sensitivity  
In order to assess the X-ray sensitivity and performance of the pellet devices, the samples were irradiated with a 40 kVp X-ray beam, where the beam current was controlled  
Figure 5: (**a**) Dark current density of the pellets as a function of pressure before and after annealing process. (**b**) The dark current, measured at 5 V, as a function of pressure, both with and without pellet annealing. (**c**) The resistivity variation with different pressures before and after annealing. to vary the dose rate (air equivalent) from 1.4 to 14.0 mGy s\({}^{-1}\). The radiation dose rate on the samples was calibrated using an ionisation chamber. The devices' response was demonstrated by turning the incident X-ray on and off with various dose rates. The X-ray sensitivity of the device was calculated from the gradient of the photocurrent density as a function of dose rate. Figure 6a shows the measured X-ray photocurrent as a function of increasing dose rate for a set of three FAPbBr\({}_{3}\) pellets produced with increasing pressure. The devices' X-ray sensitivity shows a significant increase as the forming pressure is increased, with the sensitivity at 10 V bias increasing from 18.0 \(\upmu\)C Gy\({}^{-1}\) cm\({}^{-2}\) for the 12 MPa pellet, to 168.6 \(\upmu\)C Gy\({}^{-1}\) cm\({}^{-2}\) for the 124 MPa pellet. Although there is a small increase in pellet density as the pressure is increased, shown by Figure 2b, any resulting increase in the X-ray absorption efficiency is not the dominant cause of the higher detector sensitivity. In fact, these results confirm that the increased X-ray sensitivity is predominantly due to the improved charge transport properties of the material formed at higher pressures caused by the observed changes in microstructure of increased grain size and decreased porosity. The influence of post-processing annealing of the pellets on X-ray sensitivity was also studied, with the annealing carried out on pellets at 145 \({}^{\circ}\)C in ambient air for 15 min. Figure 6b shows the increase is X-ray photocurrent due to annealing for a pair of pellets that were formed with a pressure of 12 MPa. The annealed device showed a significant increase in X-ray sensitivity to 46.6 \(\upmu\)C Gy\({}^{-1}\) cm\({}^{-2}\), compared to the 12 MPa non-annealed pellet. All the pellets formed at different pressures showed increased sensitivity after annealing (see Supplementary Information), although the relative improvement was much less significant for those pellets formed at higher pressure. The observed improvement in X-ray sensitivity  
Figure 6: (**a**) Photocurrent vs. dose rate response for devices formed with various pressures. (**b**) Comparison of X-ray photocurrent response of non-annealed and annealed FAPbBr\({}_{3}\) pellets formed at a pressure of 12 MPa. (**c**) Time response to X-rays using a mechanical X-ray chopper for the FAPbBr\({}_{3}\) device formed at 75 MPa. after annealing is again consistent with improved charge transport due to the increase in grain sizes and improved microstructure within the material. The X-ray response time was measured for the 75 MPa FAPbBr\({}_{3}\) annealed pellet using a 10 V bias voltage, with a mechanically-chopped X-ray beam with a dose rate of 17 mGy s\({}^{-1}\). Figure 6c shows the resulting photocurrent signal which has an on/off response time of 3.1 ms, which is limited by the speed of the mechanical shutter. Therefore the response speed of the perovskite detector is faster than our instrument limit. This work has demonstrated the X-ray detection performance of polycrystalline pressed FAPbBr\({}_{3}\) pellets as X-ray detectors, achieving a maximum sensitivity of 169 \(\upmu\)C Gy\({}^{-1}\) cm\({}^{-2}\) at 40 keV for a device with a thickness of 2 mm. This measured X-ray sensitivity is 8 times greater than typical values for a 200 \(\upmu\)m thick amorphous selenium X-ray detector [30], and comparable with the sensitivity of a 350 \(\upmu\)m thick polycrystalline CZT X-ray detector [31].