Double perovskite radiation sensors. Crystal growth and characterisation of single crystals of Cs${}_2$AgBiBr${}_6$ and layered (BA)${}_2$CsAgBiBr${}_7$

Low-temperature solution grown single crystals of double perovskite Cs${}_2$ AgBiBr${}_6$ semiconductor sensor material and its two-dimensional organic-inorganic derivative (BA)${}_2$CsAgBiBr${}_7$ manifest the great potential for application in radiation detection and imaging. The purity and crystallinity of Cs${}_2$AgBiBr${}_6$ single crystals with cubic symmetry ($Fm\overline{3}m$) have been corroborated by the powder XRD measurement, while the single crystal XRD measurements reveal the dominant {111} lattice planes parallel to the sample surface. A wider range of lower resistivity values (10${}^6$ -10${}^9$ Ωcm) were obtained from the I-V measurements, whereas the van der Pauw method yielded 1.55 x 10${}^9$-6.65 x 10${}^{10}$ Ωcm values, which are typically higher for the Ag-Cs${}_2$AgBiBr${}_6$ than for the carbon paint-Cs${}_2$ AgBiBr${}_6$ contacts. Charge-carrier mobility values for the n-type Ag-Cs${}_2$AgBiBr${}_6$ contacts (0.34-13.48 cm${}^2$V${}^{-1}$s${}^{-1}$) that were calculated from the Hall-effect measurements and estimated from the space-charge-limit current (SCLC) method (Ag-Cs${}_2$AgBiBr${}_6$ 0.58-4.54 cm${}^2$V${}^{-1}$s${}^{-1}$ and carbon-Cs${}_2$AgBiBr${}_6$ 1.90-4.82 cm${}^2$V${}^{-1}$s${}^{-1}$) including the density of trap states (10${}^9$-10${}^{10}$ cm${}^{\text{–}3}$) are comparable to the literature values. Single crystals of the 2D, layered (BA)${}_2$CsAgBiBr${}_7$ double perovskite with monoclinic symmetry (P2${}_1$/m) have the dominant {001} lattice planes parallel to the sample surface. The composition-structure-property relationship and, thus, the effect of the organic barrier layer along the <001> direction is reflected in the higher resistivity values of 4.19 x 10${}^{11}$-2.67 x 10${}^{12}$ Ωcm determined from the I-V measurements, which were conducted along the <001>, compared to the lower van der Pauw resistivity values of 1.65-9.16 x 10${}^{10}$ Ωcm calculated from measurements perpendicular to the <001>. The density of trap states and charge-carrier mobility calculated using the SCLC method are 1.12-1.86 x 10${}^{11}$ cm${}^{\text{–}3}$ and 10${}^{-5}$-10${}^{-4}$ cm${}^2$V${}^{\text{–}1}$s${}^{\text{–}1}$, respectively. These samples are of slightly lower quality compared to those in the literature. The key electrical parameters and X-ray photoresponse measurements indicate that the Cs${}_2$AgBiBr${}_6$ and (BA)${}_2$CsAgBiBr${}_7$ single crystals satisfy requirements for radiation sensors. This study has generated a more comprehensive information on microstructural features and data on the key electrical performance parameters. It contributes to better understanding of the crystal growth and charge-carrier transport in single crystals of double perovskite radiation sensors and their application in X-ray devices utilised in medical diagnostics and security screening.