3D Simulation of Ion Migration within the Microstructure of Perovskite Solar Cells
Waldemar Kaiser a, Nga Phung b, Antonio Abate b, Alessio Gagliardi a
a Department of Electrical and Computer Engineering, Technical University of Munich
b Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Straße, 16, Berlin, Germany
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Waldemar Kaiser, presentation 124
DOI: https://doi.org/10.29363/nanoge.hopv.2019.124
Publication date: 11th February 2019

Perovskite solar cells (PSCs) have gathered a large interest in the photovoltaic community due to their remarkable optoelectronic properties [1]. Low-temperature processed PSCs show sharp optical absorption edges, low bulk recombination, high charge carrier diffusion lengths, and a high light power conversion efficiency. Major remaining challenges with PSCs are the content of toxic lead, hysteresis effects, and the long-term stability of the material.

Although the quality of thin film PSCs has been enhanced successfully, grain boundaries (GBs) are still present within fabricated thin film devices. The perovskite film microstructure has a significant impact on the performance of the PSCs. Existing studies provided evidence for a strong correlation between ion migration and grain boundaries and emphasized the significance for stability and hysteresis effect during device operation [2,3]. Despite the rapid progress in the understanding of the role of interfaces within perovskite thin films [4,5], several aspects, such as the role of interfaces on the ion dynamics and charge carrier recombination, need both further experimental and theoretical investigations. Present numerical models are mainly based on 1D drift-diffusion simulations, which allow a simplified analysis of trap states [6, 7] and grain boundaries [8]. Ion densities covering a range from 1015 cm-3 [8] up to 1019 cm-3 [7,8] have been used in the numerical analysis to fit the experimental results. As the migration of ions and the distribution of trap states strongly depend on the microstructure of the  PSCs, a 3D numerical device model is required to study the relation between the grain boundaries and the PSC performance.

In this work, we present a joint theoretical-experimental investigation on the role of the microstructure of the perovskite thin film on the device performance. We develop a 3D kinetic Monte Carlo (kMC) model to simulate the PSCs including realistic grain morphologies, the dynamics of ions and photo-generated charge carriers, and recombination processes. The large difference in the diffusivity of ions and photo-generated charge carriers is handled by a multi-timescale approach, which solves the dynamics of ions and charge carriers separately, while the Coulomb interaction between ions and charge carriers is accounted for by an electrostatic background potential. This allows to perform transient device simulations despite the slow motion of ions. The kMC model is used to analyze the time-dependent response of the device performance as a function of charge accumulation within grain boundaries. We study the impact of (i) ion migration and (ii) trap states for different grain sizes and structures on the jV-characteristics of the PSC. The observed results are used to analyze transient current measurements of devices with different grain sizes. We observe that the screening of ions can be reduced by the presence of GBs, with the drawback of increased trap-assisted recombination at the GBs.

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