Drift-diffusion modelling of perovskite solar cells: pushing the boundaries of the charge transport model

Abstract

Over the first decade of their development, perovskite solar cells (PSCs) have proven to be among the most promising and exciting photovoltaic (PV) technologies, with the potential to offer cheap renewable energy on the global scale required to combat the climate crisis. While their huge potential has been recognised, to realise it will require significant developments in many areas, such as stability, manufacturing consistency and reliance on toxic materials. In order to engineer cells to overcome these challenges, the physical processes that govern their operation must be well-understood. Characterisation measurements of PSCs often produce results that confound interpretation based on understanding of existing PV technologies, presenting a significant modelling challenge.
A mixed ionic-electronic drift-diffusion model in which a single mobile ion species is present in the perovskite has been widely accepted and shown to accurately replicate the PSC across a range of external conditions. However, many experimental phenomena are not yet understood in the context of this model, leading to many suggestions of alterations or extensions to the model in order to capture them. The aim of this thesis is to assess whether these additions are in fact necessary by exploring previously unknown behaviour of the model and considering the effect of the proposed alterations on device-level modelling. This question is addressed in two parts. Firstly, non-Boltzmann statistics are considered as an augmentation of the model in the transport layers that sandwich the perovskite. Both numerical and asymptotic methods are employed to solve the augmented model and the effect on device-level performance is assessed for a variety of characterisation techniques, finding that the effects are only quantitative. The second part of the thesis concerns experimental phenomena that have previously been thought to be unobtainable without additions to the model, namely inverted hysteresis and mid-frequency features in electrochemical impedance spectra. In both cases, the experimental phenomena are shown to exist within the already accepted model given certain conditions on the material parameters are met. Specifically, both can appear in the model when an insufficient band offset between the perovskite and one transport layer promotes large carrier injection into the perovskite, affecting the electric potential solution in that layer. A new reduced order model, termed the modified surface polarisation model (mSPM) is derived in this regime and validated against numerical solutions to obtain the first systematic explanation of the underlying physical processes. Furthermore, the mSPM allows simple qualitative recognition of these signatures in real devices and can be used to identify dominant recombination sources and bottlenecks in device efficiency without the requirement of any modelling expertise, aiding the engineering of efficient and stable cells.

More information

Identifiers

ORCID for Will Clarke: orcid.org/0000-0002-1629-9698

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

ORCID for Giampaolo D'Alessandro: orcid.org/0000-0001-9166-9356

Catalogue record

Export record