Multifunctional Molecular Modulation for Perovskite Solar Cells: How Molecular Engineering and Solid-State NMR Lead to High Performance and Stability

Some of the key challenges for hybrid perovskite solar cells are their limited stability, scalability, and molecular level engineering. In our Nature Communications article, we demonstrate how molecular engineering of multifunctional molecular modulators (MMMs) and using solid-state nuclear magnetic resonance (NMR) to investigate their role in double-cation pure-iodide perovskites can lead to stable, scalable, and efficient perovskite solar cells.

Oct 25, 2018
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This blog post was written by Jovana V. Milic and Dominik J. Kubicki.

Hybrid organic-inorganic perovskite solar cells are one of the most promising thin-film photovoltaic technologies to date. Despite their remarkable solar-to-electric power conversion efficiency, particularly for the perovskite compositions containing triple-cation and mixed-halide formulations, the main challenges associated with their limited stability, scalability, and molecular level engineering are still unresolved. There has been an ongoing effort to overcome some of these limitations by using organic molecules as additives to the already complex perovskite composition. However, the microscopic role of the additives in the perovskite structure is mostly speculative as the structure-property relationships are poorly understood. Our objective is to tackle these challenges through rational molecular design in conjunction with solid-state nuclear magnetic resonance (NMR), as a unique technique for probing interactions within the perovskite material at the atomic level.

We focused our attention on a simplified yet thermally stable double-cation pure-iodide composition. Furthermore, we designed a series of organic compounds equipped with specific functional groups that should enable them to act as molecular modulators (MMs), interacting with the perovskite through distinctive types of noncovalent interactions, such as hydrogen bonding or metal coordination. While interaction with the perovskite surface via hydrogen bonding can beneficially affect the material crystallinity and, consequently, its electronic quality, coordination to the metal cation sites could ensure passivation of some of the possible structural defects, such as undercoordinated metal ions.

Indeed, upon applying molecular modulation, the interaction with the perovskite surface resulted in improved film morphologies, whereas metal coordination reduced the level of defects acting as centers for nonradiative charge recombination, which can be detrimental for the device performance. Combining the two traits provides the opportunity to simultaneously address the performance and stability, which has led us to develop a multifunctional molecular modulator (MMM) with the capacity to interact with the perovskite surface and suppress the defects through metal coordination. Moreover, we have shown that upon linking the two functions, the MMM adopts a unique tautomeric form, which leads to an additional hydrogen bonding site, while remaining sufficiently hydrophobic to increase the material resistance to environmental conditions. Finally, a closer look into the properties revealed improved crystallinity of the perovskite material, as the MMM proved to act as a structure-directing agent, altering the crystallinity by inducing larger grain formation and higher quality of the material. As a result, the corresponding solar cells demonstrated a remarkable performance with efficiencies exceeding 20% for large areas above 1 cm2 (the active area of typical laboratory cells is approximately ten times smaller). Furthermore, the rewarding performance came alongside operational stability even under ambient conditions.

We are therefore excited to apply this approach to more advanced MMMs for stable, scalable, and efficient perovskite solar cells in the future.  We hope that this work will stimulate wider application of atomic-level characterization techniques, such as solid-state NMR, for analyzing the structure-activity relationships in hybrid perovskites and unraveling the mechanistic aspects of their photovoltaic operation.

For more details, please refer to our Nature Communications article at https://www.nature.com/articles/s41467-018-06709-w.


Jovana V. Milic

Scientist, EPFL

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