Formation of high-performance photovoltaic materials from solution

Rahim Munir, KAUST, Aram Amassian, KAUST, and Detlef Smilgies, CHESS, Cornell University

Mixed organic-inorganic halide perovskite materials have been in the spotlight in the photovoltaics research community due to excellent optoelectronic properties and the potential of cost-efficient production via solution processing.

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Figure 1: Stages of film formation and the resulting film thickness as the solvent leave the film for three halide cases, iodide, bromide, and chloride.

Although high power conversion efficiency has been achieved for these materials, the fundamental understanding of solidification of perovskite inks and the reproducibility of related manufacturing processes remains an open question.

CHESS users from KAUST expressed their interest in moving a step forward in answering the profound issues of understanding the solution processing of perovskite materials. Prof. Aram Amassian and Ph.D. candidate Rahim Munir designed the in situ experiments to reveal the surprising information which was previously unknown to the community. Spin coating in combination with in situ time-resolved grazing incidence wide angle scattering (GIWAXS) of different lead halide inks showed the diversity and complexity of the process, including evidence of strong solvent-solute interactions resulting in complexation. The results obtained at CHESS D1 station helped them to understand the linkage between the intermediate precursor phases as well as the morphology and reproducibility of as-cast films which in turn affect the power conversion efficiency of solar cells [1].

Mixed organic-inorganic perovskites are formed from simple lead halide salts mixed with methylammonium iodide. The magic in making good perovskite films lies in finding proper mixing ratios, solvents, and processing conditions. The present study focused on the best-known mixtures in dimethyl formamide (DMF) as the solvent. In-situ GIWAXS revealed a process very similar to sol-gel in which the precursor is a solvent-solute complex which starts off exhibiting gel-like properties and can subsequently crystallize into an intermediate solvated phase, as shown in Figure 2. Depending on the halide used and composition, the precursor can also remain disordered, as confirmed by in situ GIWAXS measurements. The latter also show solvated and disordered precursors to be unstable while the ordered precursors are stabilized. The stability of the precursor is proved to have significant implications with respect to the reproducibility of the manufacturing process, a key concern for organohalide perovskite solar cells. These results of reduced variation of power conversion efficiencies due to stabilization of as cast film can be extended to other solvent systems as well.

Figure 2: Stages of perovskite formation from spin coating to drying and the final heat treatment.

This study will help the community to understand the solution processing of perovskite material and the lessons from these results can be used to fine tune perovskite crystallization and extend these findings to more manufacturing-friendly techniques such as blade coating and slot-die coating. The commercialization of perovskite-based technologies will require high reproducibility of efficient solar cells, and this study provides new insight which should help engineers make a significant leap toward improving device reproducibility.

References:

[1] Rahim Munir, Arif D. Sheikh, Maged Abdelsamie, Hanlin Hu, Liyang Yu, Kui Zhao, Taesoo Kim, Omar El Tall, Ruipeng Li, Detlef-M. Smilgies and Aram Amassian: "Hybrid Perovskite Thin-Film Photovoltaics: In Situ Diagnostics and Importance of the Precursor Solvate Phases", Adv. Mater. online (10.1002/adma.201604113)

Link to research article: http://onlinelibrary.wiley.com/doi/10.1002/adma.201604113/abstract