Advances in physicochemical characterization of lead-free hybrid perovskite [NH3(CH2)3NH3]CuBr4 crystals

Abstract

To support the development of eco-friendly hybrid perovskite solar cells, structural, thermal, and physical properties of the lead-free hybrid perovskite [NH3(CH2)3NH3]CuBr4 were investigated using X-ray diffraction (XRD), differential scanning calorimetry, thermogravimetric analysis, and nuclear magnetic resonance spectroscopy. The crystal structure confirmed by XRD was monoclinic, and thermodynamic stability was observed at approximately 500 K without any phase transition. The large changes in the 1H chemical shifts of NH3 and those in C2 close to N are affected by N–H∙∙∙Br hydrogen bonds because the structural geometry of CuBr4 changed significantly. The 1H and 13C spin–lattice relaxation times (T1ρ) showed very similar molecular motions according to the Bloembergen–Purcell–Pound theory at low temperatures; however, the 1H T1ρ values representing energy transfer were about 10 times lesser than those of 13C T1ρ. Finally, the 1H and 13C T1ρ values of [NH3(CH2)3NH3]MeBr4 (Me = Cu, Zn, and Cd) were compared with those reported previously. 1H T1ρ was affected by the paramagnetic ion of the anion, while 13C T1ρ was affected by the MeBr4 structure of the anion; 13C T1ρ values in Me = Cu and Cd with the octahedral MeBr6 structure had longer values than those in Me = Zn with the tetrahedral MeBr4 structure. We believe that these detailed insights on the physical properties will play a crucial role in the development of eco-friendly hybrid perovskite solar cells.