Mishra Aditya, Hope Michael A, Almalki Masaud, Pfeifer Lukas, Zakeeruddin Shaik Mohammed, Grätzel Michael, Emsley Lyndon
Laboratory of Magnetic Resonance, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Laboratory of Photonics and Interfaces, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
J Am Chem Soc. 2022 Aug 24;144(33):15175-15184. doi: 10.1021/jacs.2c05316. Epub 2022 Aug 12.
Surface and bulk molecular modulators are the key to improving the efficiency and stability of hybrid perovskite solar cells. However, due to their low concentration, heterogeneous environments, and low sample mass, it remains challenging to characterize their structure and dynamics at the atomic level, as required to establish structure-activity relationships. Nuclear magnetic resonance (NMR) spectroscopy has revealed a wealth of information on the atomic-level structure of hybrid perovskites, but the inherent insensitivity of NMR severely limits its utility to characterize thin-film samples. Dynamic nuclear polarization (DNP) can enhance NMR sensitivity by orders of magnitude, but DNP methods for perovskite materials have so far been limited. Here, we determined the factors that limit the efficiency of DNP NMR for perovskite samples by systematically studying layered hybrid perovskite analogues. We find that the fast-relaxing dynamic cation is the major impediment to higher DNP efficiency, while microwave absorption and particle morphology play a secondary role. We then show that the former can be mitigated by deuteration, enabling H DNP enhancement factors of up to 100, which can be harnessed to enhance signals from dopants or additives present in very low concentrations. Specifically, using this new DNP methodology at a high magnetic field and with small sample volumes, we have recorded the NMR spectrum of the 20 nm (6 μg) passivating layer on a single perovskite thin film, revealing a two-dimensional (2D) layered perovskite structure at the surface that resembles the = 1 homologue but which has greater disorder than in bulk layered perovskites.
表面和体相分子调节剂是提高混合钙钛矿太阳能电池效率和稳定性的关键。然而，由于它们的浓度低、环境异质性以及样品质量小，按照建立构效关系的要求，在原子水平上表征它们的结构和动力学仍然具有挑战性。核磁共振（NMR）光谱已经揭示了关于混合钙钛矿原子水平结构的大量信息，但NMR固有的低灵敏度严重限制了其用于表征薄膜样品的效用。动态核极化（DNP）可以将NMR灵敏度提高几个数量级，但迄今为止用于钙钛矿材料的DNP方法仍然有限。在这里，我们通过系统地研究层状混合钙钛矿类似物，确定了限制钙钛矿样品DNP NMR效率的因素。我们发现快速弛豫的动态阳离子是提高DNP效率的主要障碍，而微波吸收和颗粒形态起次要作用。然后我们表明，可以通过氘代减轻前者的影响，实现高达100的H DNP增强因子，这可以用于增强来自极低浓度的掺杂剂或添加剂的信号。具体而言，在高磁场和小样品体积下使用这种新的DNP方法，我们记录了单个钙钛矿薄膜上20 nm（6 μg）钝化层的NMR光谱，揭示了表面的二维（2D）层状钙钛矿结构，其类似于n = 1同系物，但比体相层状钙钛矿具有更大的无序度。
