Dielectric Relaxation and Polaron Hopping in Cs2AgBiBr6 Halide Double Perovskites

• Published in: Journal of physical chemistry. C 2022-06, Vol.126 (24), p.10199-10208
• Main Authors: Tailor, Naveen Kumar, Parikh, Nishi, Yadav, Pankaj, Satapathi, Soumitra
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.2c02073

The lead-free Cs2AgBiBr6 double perovskites have recently emerged as promising candidates for optoelectronic applications due to their less toxicity and high stability. Although the photophysical study of Cs2AgBiBr6 has been extensively explored, the same for dielectric relaxation and carrier conduction remains elusive. Here, we uncover the dielectric relaxation and charge conduction mechanism in the Cs2AgBiBr6 single crystals using temperature-dependent electrochemical impedance spectroscopy in correlation with modulus spectroscopy. We found that the ionic displacement and space charge polarization are mainly responsible for the dielectric response. A sharp transition frequency as ∼191 Hz for 0 °C to ∼492 Hz for 90 °C was observed in the dielectric constant (ε′) versus temperature study, where dε′/dT exhibits a positive and negative coefficient below and above the transition frequency, respectively. Stevels model analysis suggests that the contribution of traps reduces with the increase in temperature and therefore conduction enhances. This model also confirms the polarons as the main conduction carriers in the Cs2AgBiBr6 semiconductor. The polaron hopping and binding energy were estimated as W H = 0.17 eV and E P = 0.34 eV, respectively. The binding energy of the polaron is larger than the room-temperature thermal energy, indicating the polaron hopping instead of free charge carrier transport as the dominant conduction mechanism. Furthermore, the imaginary part of -Z″ and M″ demonstrates the temperature-activated polaron relaxation from non-Debye type to Debye type process in Cs2AgBiBr6 crystals. Our study provides fundamental insights into the dielectric relaxation behavior needed for developing efficient dielectric switches and unveils the details of the carrier conduction mechanism in lead-free double perovskites.