Hybrid Perovskite Phase Transition and Its Ionic, Electrical and Optical Properties
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Hybrid Perovskite Phase Transition and Its Ionic, Electrical and Optical Properties Md Nadim Ferdous Hoque1, Nazifah Islam1, Kai Zhu2, Zhaoyang Fan1 1 2
Electrical & Computer Engineering, Texas Tech University, Lubbock, Texas, United States National Renewable Energy Laboratory, Golden, Colorado, United States
ABSTRACT Hybrid perovskite solar cells (PSCs) under normal operation will reach a temperature above ~ 60 °C, across the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI3). Whether the structural phase transition could result in dramatic changes of ionic, electrical and optical properties that may further impact the PSC performances should be studied. Herein, we report a structural phase transition temperature of MAPbI3 thin film at ~ 55 °C, but a striking contrast occurred at ~ 45 °C in the ionic and electrical properties of MAPbI3 due to a change of the ion activation energy from 0.7 eV to 0.5 eV. The optical properties exhibited no sharp transition except for the steady increase of the bandgap with temperature. It was also observed that the activation energy for ionic migration steadily increased with increased grain sizes, and reduction of the grain boundary density reduced the ionic migration. INTRODUCTION In spite of recent tremendous progress of PSCs leading toward efficiency over 22%, there are lots of issues regarding long term stability of these solar cells [1]. Moreover, MAPbI3 has a tetragonal-cubic phase transition temperature in the range of 42-57 °C [2]. This transition needs to be addressed, since on a hot summer day a device can easily reach this temperature [3]. Specifically, electronic, and ionic migration characteristics under normal solar cell operating conditions have drawn special attention [4-8]. Ionic migration is attributed to the hysteresis phenomenon as well as gradual degradation of MAPbI3 to MA+ and PbI2, which becomes more prominent under illumination [6]. Moreover, recent development in surface characterization by different types of atomic force microscopy (AFM) has enabled more in-depth research at nanoscale for the charge transport mechanisms [9, 10]. However, there is still missing links between grain size and the ionic activation energy for the intrinsic MAPbI3 films across the phase transition. In this work, we have focused on the phase transition property of MAPbI3. Structural, electrical and optical measurements were conducted to characterize the tetragonal-cubic structural phase transition and the related ionic/electrical and optical property changes. We also address the grain size effect on the ionic activation energy. EXPERIMENT The MAPbI3 thin film was fabricated by spin-coating of MAPbI3 precursor either on glass or FTO coated glass depending on the type of experiments. The film was deposited using antisolvent method that includes three steps of spin-coating with speeds of 500, 3500, and 5000 rpm for 3, 10, and 30 seconds, respectively [11]. For obtaining shiny and continuous film, 600 µL of toluene, used as an anti-solvent, was dropp
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