Dissertation Defence: Unraveling Degradation Pathways in Perovskite Solar Cells via Impedance Spectroscopy
June 3 at 9:00 am - 1:00 pm
Elnaz Ghahremani Rad, supervised by Dr. Alexander R. Uhl, will defend their dissertation titled “Unraveling Degradation Pathways in Perovskite Solar Cells via Impedance Spectroscopy” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering.
An abstract for Elnaz Ghahremani Rad’s dissertation is included below.
Examinations are open to all members of the campus community as well as the general public. This examination will be offered in hybrid format. Registration is not required to attend in person; however, please email alexander.uhl@ubc.ca to receive the Zoom link for this exam.
Abstract
Solar cells based on hybrid perovskite absorber films have emerged as promising low-cost, highly efficient photovoltaic technology, yet their long-term operational instability continues to hinder commercial deployment. This thesis formulates and applies a set of strategies to evaluate degradation mechanisms in perovskite solar cells, with a particular emphasis on impedance spectroscopy as a diagnostic tool. The objective is to understand how degradation occurs, identify its electrical signatures, and explore approaches to mitigate instability. This study begins with the fabrication and optimization of predominantly inverted perovskite solar cells using both solution- and vacuum-based methods. Various device architectures, absorber compositions, charge transport layers, and electrode materials were investigated to improve reproducibility and performance.
Impedance spectroscopy was employed to probe the evolution of device behaviour during operation and aging under various stress conditions. A guideline on best practices and methodologies for performing impedance spectroscopy on perovskite solar cells has been published, outlining key measurement techniques and providing clear guidance on how to interpret the results. The guideline addresses common challenges in establishing reliable impedance measurements and in interpreting experimental data, particularly uncertainties related to measurement conditions. It outlines practical strategies to minimize device degradation and measurement noise during impedance testing. In addition, dedicated sections on equivalent‑circuit analysis and recent advances in drift–diffusion modelling are included, highlighting methods used to interpret and understand impedance behaviour.
Building on this work, a physical model was developed and employed to separate ion-induced pathways, allowing the extraction of a unified equivalent circuit to interpret the impedance response. It investigates how ionic degradation affects perovskite solar cells and identifies charge collection losses as the dominant degradation mechanism rather than charge recombination losses. These losses correlate with a substantial drop in current density and a transition from normal to inverted hysteresis. A key finding is the emergence of a double-inductor feature in the low-frequency region of the spectra of aged devices, which serves as a clear signature of ion-driven degradation. The results indicate that ion-induced electric field screening may be closely associated with the appearance of this double inductor feature at low frequencies.
To mitigate device degradation under environmental conditions, a low-cost and flexible extrinsic encapsulation strategy was investigated. A polymer-based barrier layer, parylene-C, was deposited via chemical vapour deposition to protect the devices. Stability was examined under two conditions: ambient dark storage and elevated temperature conditions. It was found that parylene-C can significantly extend the operational lifetime of perovskite solar cells, particularly under accelerated aging conditions. Notably, parylene-C showed no chemical interaction with the perovskite layer. Impedance spectroscopy was also used to assess the impact of parylene-C encapsulation on the device’s impedance behaviour.
Overall, this thesis seeks to advance the understanding of degradation pathways in perovskite solar cells and demonstrates the potential of impedance spectroscopy for diagnosing and tracking device stability. The insights gained contribute to the development of more reliable perovskite photovoltaic technologies and support ongoing efforts toward their commercialization.
