2, Northwestern University, Evanston, Illinois, United States
Halide perovskites AMX3 have shown tremendous promise for a variety of applications due to their excellent charge transport characteristics, with solar cells based on the flagship material CH3NH3PbI3 attaining efficiencies above 22%. One application which has recently attracted interest is the use of perovskites as semiconductor hard radiation detectors. The first steps toward hard radiation detection have recently been achieved in CH3NH3PbBr3:Cl and HC(NH2)2PbI3. However, the hybrid perovskites have stability issues which have yet to be resolved, and inorganic perovskites such as CsSnBr3 or CsSnI3 have not yet reached the same level of success, with the possible exception of CsPbBr3. Less explored are the defect perovskites, which have compositions A2MX6 or A3M2X9 to allow for the substitution of tetravalent or trivalent metals M in the archetypical AMX3 formula and subsequent charge balancing with ordered vacancies. Recently, exploration of new compositions has included the double perovskites A2M’M’’’X6, in which a +1 and +3 metal M’ and M’’’ form an ordered structure that is a doubling of the AMX3 unit cell.
Among the known perovskite derivatives, the iodide defect perovskites A3M2I9 (A = Cs+, Rb+; M = Sb3+, Bi3+) are of particular interest. They possess the ns2 lone pair that has been a crucial component of the remarkable properties of Pb-based perovskites, and are 2D and 0D perovskite derivatives similarly based on MI6 octahedra with greater stability than the hybrid perovskite compositions. The defect perovskites are suitable candidates for room-temperature radiation detection due to their wide bandgaps (1.9-2.2 eV), high densities, and resistivities above 109 Ωcm.
Here we report on the radiation response of single crystals of the defect perovskites A3M2I9 (A = Cs+, Rb+; M = Sb3+, Bi3+). All four materials respond to 241Am 5.5 MeV alpha-particles in both hole and electron collection configurations, permitting evaluation of the charge transport for each charge carrier. The 241Am response spectra for electrons is used to estimate the electron mobility-lifetime products, which range from 4 x 10-6 cm2V-1 for Rb3Sb2I9 to 5 x 10-5 cm2V-1 in Cs3Bi2I9. Hole responses yield similar values ranging from 3 x 10-6 cm2V-1 in Rb3Sb2I9 to 3 x 10-5 cm2V-1 for Cs3Bi2I9. The rise time of the response pulse in each material is used to estimate the mobility, which are below 10 cm2V-1s-1 for all four defect perovskites. The achievement of alpha particle response is remarkable given the relatively low mobilities measured here, and implies that the lifetime of charge carriers should be quite high. These results are compared with recent photoluminescence measurements indicating potential self-trapping of charge carriers, which would simultaneously reduce the mobility and enhance the lifetimes of charge carriers.