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Investigation of the Ion Migration Mechanism and its Effect on the Slow Response of Perovskite Solar Cells

Ebadi Garjan, Firouzeh | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54390 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nanoscience and Nanotechnology
  6. Advisor(s): Taghavinia, Nima; Mohammadpour, Raheleh
  7. Abstract:
  8. In recent years, organo-metal perovskite solar cells have attracted remarkable attention due to their low cost manufacturing method as well as the rapid growth of efficiency. Despite the fast growing efficiency of organo-metal perovskite solar cells, there are big challenges around their low stability under real operational condition. In addition to extrinsic parameters like oxygen and humidity, intrinsic instability of perovskite rises mainly from ion migration in perovskite film. In order to understand the ion migration and its effect on photovoltaic parameters of the devices, appropriate characterizations and analysis are needed. Since ions are much slower compare to electrons, their response to external perturbation signals (light or applied voltage) expected in different time scales than those for electrons. In this research we employed different techniques to characterize slow response of perovskite solar cells and the underlying phenomena. First, we employed transient photo-voltage rise and decay measurements to investigate the origin of slow asymmetrical rise and decay for three different types of perovskite compounds MAPbI3, MAPb(I0.83Br0.17)3 and Cs0.05 (FAMA) 0.95Pb (I0.83Br0.17)3. The experiments were carried out in the range of 5 to 45 °C and the activation energy of 0.29 eV, 0.34 eV and 0.51 eV for ionic defects was obtained for the above compounds, respectively. Also, an equivalent circuit model was proposed. The main feature of our proposed circuit is that it simultaneously has an electron branch for electronic phenomena and a separate ionic branch to explain ionic phenomena. Based on our experimental results and the values of electronic and ionic components extracted from the proposed equivalent circuit, we explained the asymmetry in the rise and decay profile of the open circuit voltage of the devices. In the second part of this study, by adding bismuth impurities to the perovskite composition, we introduced a system in order to perform a systematic investigation of the low frequency negative capacitance phenomenon in these devices. We used electrochemical impedance spectroscopy technique to study the low frequency response of these cells. We show that negative capacitance in perovskite solar cells is of the same nature as the slow positive response, which was originally ascribed to a ‘giant dielectric constant’, and later to accumulation of electronic and ionic charge at interfaces. However, we find that both positive and negative, low-frequency ‘capacitances’ do not describe charge accumulation, but are a consequence of transient injection currents modified by a slow process
  9. Keywords:
  10. Electromigration ; Perovskite-Based Solar Cell ; Negative Capacitance Circuit ; Photovoltage Decay ; Frequency Ranges

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