Elucidating Structure Formation in Highly Oriented Triple Cation Perovskite Films

Abstract

Metal halide perovskites are an emerging class of crystalline semiconductors of great interest for application in optoelectronics. Their properties are dictated not only by their composition, but also by their crystalline structure and microstructure. While significant efforts are dedicated to the development of strategies for microstructural control, significantly less is known about the processes that govern the formation of their crystalline structure in thin films, in particular in the context of crystalline orientation. This work investigates the formation of highly oriented triple cation perovskite films fabricated by utilizing a range of alcohols as an antisolvent. Examining the film formation by in situ grazing-incidence wide-angle X-ray scattering reveals the presence of a short-lived highly oriented crystalline intermediate, which is identified as FAI-PbI 2 -xDMSO. The intermediate phase templates the crystallization of the perovskite layer, resulting in highly oriented perovskite layers. The formation of this dimethylsulfoxide (DMSO) containing intermediate is triggered by the selective removal of N,N-dimethylformamide (DMF) when alcohols are used as an antisolvent, consequently leading to differing degrees of orientation depending on the antisolvent properties. Finally, this work demonstrates that photovoltaic devices fabricated from the highly oriented films, are superior to those with a random polycrystalline structure in terms of both performance and stability.

Solvent–antisolvent interactions in metal halide perovskites

“…These three factors were empirically found to influence the quality of the perovskite films, and consequently the performance of perovskite solar cells. 57,67,72 Importantly, a study by Taylor et al demonstrated that these factors have significant implications for the choice of optimal processing conditions of the layers. 72 There, the 14 investigated antisolvents were categorised into type I, type II and type III based on the optimal antisolvent application rate that led to high solar cell performance (Fig.…”

“…For example, we recently demonstrated that the occurrence of a templating intermediate can influence grain orientation and consequently performance and stability but varies strongly based on the antisolvent used. 67 …”

“…It has been shown by us that photovoltaic devices fabricated from highly oriented MHP structures are superior in stability and performance as compared to those with random polycrystalline structures. 67 Recently, we reported that the formation of highly oriented triple cation perovskite films can be triggered by the use of alcoholic antisolvents. For example, Fig.…”

“…Additional experiments revealed that the intermediate phase consists of a FAI-PbI 2 - x ·DMSO complex, that forms as a consequence of the preferential removal of DMF from DMF:DMSO host solvent mixtures during the solvent-engineering process when using alcoholic antisolvents. 67 …”

“…As suggested by Telschow et al , the similar solubility of antisolvents reduces the effectivity of selective removal of the host solvent accelerating the saturation of the precursor solution and preventing formation of the intermediate phase that promotes a higher degree of crystallinity. 67 These recent reports highlight the complexity of solvent–antisolvent interactions involved in the process for removing undesired solvents and retaining the antisolvents that aid promoting higher efficiency MHP-based PV devices. Thus, a more well-defined methodology must be proposed in order to fully take advantage of the HSP theory towards improving PV devices.…”