• Mahapatra, Apurba
• Yadav, Pankaj
• Tailor, Naveen Kumar
• Paul, Rajashik

Abstract

<jats:title>Abstract</jats:title><jats:p>Lead-free metal-halide perovskites have recently appeared as a promising candidate in optoelectronics and photovoltaics because of their non-toxicity, stability, and unique photophysical properties. Much scientific research has been done on optoelectronic characteristics and photovoltaic applications of lead-free perovskites, but the dielectric characteristics and insight into the relaxation phenomenon remain elusive. Here, we study the dielectric relaxation and conduction mechanism in the single crystalline (SC) A<jats:sub>3</jats:sub>Bi<jats:sub>2</jats:sub>X<jats:sub>9</jats:sub> (A = MA<jats:sup>+</jats:sup>/FA<jats:sup>+</jats:sup>) perovskite using temperature-dependent electrochemical impedance spectroscopy in correlation with the modulus spectroscopy. With increasing temperature, the peak of −<jats:italic>Z</jats:italic>″(<jats:italic>ω</jats:italic>) shifts toward a high-frequency regime which specifies the thermally dependent relaxation mechanism in both crystals. The activation energy was estimated as 381 meV for MA<jats:sub>3</jats:sub>Bi<jats:sub>2</jats:sub>I<jats:sub>9</jats:sub> (MBI) crystal and 410 meV for the FA<jats:sub>3</jats:sub>Bi<jats:sub>2</jats:sub>I<jats:sub>9</jats:sub> (FBI) crystal suggesting hopping of mobile ions between lattice sites. The connected orientational polarization with the thermal motion of molecules leads to the enhancement in the dielectric constant (<jats:italic>ϵ</jats:italic>′) with temperature. The <jats:italic>ϵ</jats:italic>″(<jats:italic>ω</jats:italic>) in these crystals shows the significant ionic conductivity with a typical 1/<jats:italic>f<jats:sup>γ</jats:sup></jats:italic> type characteristics (in the low-frequency regime) where <jats:italic>γ</jats:italic> is found to be in the range of 0.93–1.0 for MBI crystal and 0.88–0.98 for FBI crystal. The correlated imaginary part of impedance (−<jats:italic>Z</jats:italic>″) and modulus (<jats:italic>M</jats:italic>″) demonstrate the temperature-activated delocalized relaxation (non-Debye toward the Debye type) in these crystals. Stevels model suggests that the contribution of traps reduces with temperature rise and therefore conductivity enhances. Our study provides a comprehensive analysis and in-depth knowledge about the dielectric and conductivity relaxation mechanism in these lead-free perovskite SCs, which will help to implement efficient energy storage devices using these materials.</jats:p>

Topics

• perovskite
• impedance spectroscopy
• single crystal
• dielectric constant
• activation
• toxicity