Modelling the impedance response of Perovskite solar cells

Abstract

Perovskite solar cells (PSCs) are a promising new photovoltaic technology that have the potential to provide low-cost renewable energy on a global scale. Whilst significant problems, particularly in terms of stability and toxicity, remain to be overcome before this technology can be exploited on a wide scale, the full potential of PSCs cannot be reached without a correct description of their fundamental workings. Impedance spectroscopy (IS) is a simple and powerful technique that can be used to probe the fundamental properties and behaviour of PSCs both in situ and in the lab. As such, it can be used to asses changes in device behaviour associated with degradation. However, the results obtained using this technique are poorly understood because of the additional complexity of device behaviour that ensues from ion motion in the perovskite. In this thesis, a method of simulating impedance spectroscopy measurements is developed based on a drift-diffusion modelling approach, that includes the effects of both mobile ions and charge carriers. The impedance simulations reproduce the features and trends observed in experiment, enhancing our understanding of their physical origin. Systematic asymptotic methods are applied to the drift-diffusion model and used to obtain an analytic simplified model of the impedance response of a PSC. This analytic model consists of simple relations for the key features of impedance spectra of a PSC. Excellent agreement is demonstrated between the analytic model and the more complex drift-diffusion model from which it is derived, in a regime where the applied voltage is close to the open-circuit voltage, including the maximum power point. This analytic model is compared to the impedance response of an RC-RC circuit and is used to derive expressions for resistances and capacitances of the cell in terms of physical parameters in the drift-diffusion model. It is found that the shape of the impedance spectra obtained near open-circuit can be related to the dominant source of recombination loss in the cell. An ideality factor (termed the electronic ideality factor) nel is identified that can be obtained through measurements of the high frequency impedance. Unlike the standard ‘ideality factor’ that is typically measured and misinterpreted, nel is not dependent on the ionic properties of the cell and serves as an analogue for the ideality factor in conventional solar cells in which mobile ions are absent. Determination of the electronic ideality factor allows the dominant recombination mechanism within the PSC to be identified. The methods developed in this thesis to interpret impedance spectra provide much needed insight into the impedance response of PSCs and allow physical parameters to be extracted from the data. This enables researchers to more effectively employ IS, characterise key properties and identify losses, helping to develop more efficient and stable PSCs.

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Identifiers

ORCID for Laurence John Bennett: orcid.org/0000-0002-0152-1401

ORCID for Giles Richardson: orcid.org/0000-0001-6225-8590

ORCID for Nicola E Courtier: orcid.org/0000-0002-5714-1096

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