Nucleation or Coarsening—How Additives Actually Govern Perovskite Grain Size

Perovskite solar cells currently enter a stage, where market introduction is within reach. For serious upscaling, control over the quality of the perovskite material is most critical. To this end, the community currently relies heavily on laboratory experience, engineering, and fine-tuning approaches. A key challenge is the controlled growth of perovskite crystallites while processing thin films. Several strategies are currently in use, such as anti-solvent or additive engineering. While the impact of these strategies is well-evidenced in the resulting layers, the underlying mechanisms that govern the crystallization process are still subject to a vigorous debate. A frequently cited theory is that the nuclei for the perovskite crystallization evolve from intermediate solvate clusters that might form colloids that act as seeds[1,2] As of yet, however, insights that unambiguously link the complex formation in the precursor inks to the perovskite formation are lacking.<br/><br/>In this work we propose an alternative hypothesis on the mechanism by which most popular crystallization agents mediate the grain formation in the final perovskite layer and explain, why solid predictions based on nucleation are hard if not impossible. We begin our study with thiourea, an additive which we previously utilized successfully for a plethora of different perovskite compositions[3,4]. Specifically, we probe the precursor stage and transition over in-situ investigations during thin film deposition to the final films and their implementation into perovskite solar cells. While we are able to confirm and complement reports in literature on colloidal lead-complexes in the perovskite precursor ink, that grow in size with increasing concentration, we present strong evidence, that the modification of the lead complex formation or a retardation of the nucleation process might not be the dominant mechanism. We show, that the key impact of Thiourea and other popular additives unfolds during the annealing step, when solvent removal, nucleation and initial perovskite crystallization has already taken place. We model the process of grain growth by phase field simulation, using a coarsening growth process limited by the mobility of the perovskite constituents that in turn is mediated by the specific crystallization agent. We substantiate the general validity of our insights for various iodine-based perovskite compositions and several popular additives. Hereby we find the proposed mechanism to be not only viable to explain the effect of additives, but also to compare the additive approach to several post-processing approaches like thermal hot-pressing, where the mobility of the constituents is manipulated by other means.[5]<br/>Our finding provides a decisive piece to complement the nucleation theory and to bridge the gap between the precursor phase and the final perovskite film, constituting a crucial step beyond heuristics and towards actual additive- and crystallization-engineering.<br/><br/>References:<br/>[1] Ahlawat P, et al. Chem. Mater. 32, 529 (2020).<br/>[2] Flatken MA, et al. J. Mater. Chem. A 9, 13477 (2021)<br/>[3] Brinkmann et al. Nature 604, 280 (2022)<br/>[4] Brinkmann et al. ACS Appl. Mater. & Interf. 11, 40172 (2019)<br/>[5] Pourdavoud et al. Adv. Mater. 29, 1605003 (2017)