# Polycrystalline Formamidinium Lead Bromide X-ray Detectors
## 3 Results and Discussion
### Photoluminescence Studies  
The PL behaviour of FAPbBr\({}_{3}\) pellets as a function of the pressure used during their fabrication were investigated by acquiring room temperature PL spectra of a set of five pellets. The PL measurement was carried out using a continuous-wave green laser with a wavelength of 520 nm. Each of the pellets was prepared under identical conditions, apart from the pressure used during pressing. The PL study shows three different aspects of the impact of pressure on FAPbBr\({}_{3}\) pellets in terms of changes of the peak intensity, peak line width, and bandgap evolution. As has been previously reported for organic lead halide perovskites, the near band-edge room temperature PL emission is extremely bright [22], which was observed in our samples both for the as-grown crystals and for the pellet devices. Figure 3a shows the broad emission with a peak emission wavelength at approximately 565 nm, consistent with the presence of the bromine halide anion.  
Figure 3a,b show the decrease in PL intensity as a function of increasing pressure, indicating an increase in non-radiative processes in the near-surface region of the material since the penetration depth of the incident laser is <100 nm [23]. For pellets formed at higher pressures the surfaces showed a mirror-like finish, consistent with a reduction in porosity and increased material density, which may also contribute to the reduced photoluminescence intensity.  
The inset data in Figure 3a shows the dramatic increase in PL intensity when the 12 MPa pellet was annealed at 145 \({}^{\circ}\)C in the air for 15 min. All pellets showed a similar significant increase in PL intensity after annealing in air, which may be attributed to the presence of oxygen atoms during the annealing process acting as a passivator in perovskite materials. The role of oxygen passivation of surface defects in perovskites has been discussed by [PERSON] et al. [24] where O\({}_{2}\) atoms were found to passivate surface traps caused by lead dangling bonds which attenuated the non-radiative recombination rate and increased the PL intensity.  
Figure 2: **(a)** Image of 1 cm FAPbBr\({}_{3}\) pellets showing colour changes as a function of increasing pressure. **(b)** FAPbBr\({}_{3}\) density as a function of pressure. **(c–e)** AFM topography imaging of FAPbBr\({}_{3}\) pellets with pressure of 12, 74, and 124 MPa. **(f)** Mean roughness of FAPbBr\({}_{3}\) pellet as a function of pressure.  
The corresponding linewidth of the PL spectrum was seen to widen by 25% with increasing pressure, as shown in Figure 3c, due to the formation of a secondary PL peak at slightly shorter wavelength. Other studies of bandgap changes in lead halide perovskites have been reported, for example in CsPbBr3 where the bandgap energy increased at very high pressures >1 GPa due to lattice amorphization [25; 26]. Reports of high pressure studies of FAPbBr3 nanoparticle showed phase transitions and changes in bandgap energies at pressures in the range 1-4 GPa [27; 28; 29]. However these effects were observed at pressures significantly greater than those used in our study. We attribute the observed band gap increase in our samples to changes in microstructure as a function of increased pressure.