Magnetic Resonance Tools for Solar Materials

自旋过程对有机太阳能电池和发光二极管的效率非常重要，但它们的作用在很大程度上仍未被探索。由于弱的自旋-轨道耦合，有机材料具有严格的自旋选择规则，因此光激发物种的性质可以根据特定的自旋构型而发生很大的变化。此外，在过去的几年里，有机-无机钙钛矿杂化材料的发展取得了巨大的进步，这使得制造性能卓越的光电子器件成为可能。有人提出，自旋自由度也与这些材料中的光物理过程有关。因此，在非常不同的背景下，人们积极地研究激发态的自旋性质。在这个项目中，来自巴黎(巴黎-南德大学)、凡尔赛(圣昆廷大学)、格勒诺布尔(内埃尔研究所)、拜罗伊特(拜罗伊特大学[UBAT])和柏林(柏林自由大学[柏林自由大学])的科学家共同努力，我们将重点研究两个方面：单态激子裂变，即单态激子分裂成两个能量更低的三态激子，并在几层混合有机钙钛矿中进行自旋相关的过程，一直到单层极限。我们将探索利用宽带和时间分辨光学检测磁共振光谱作为一种强有力的方法来建立总自旋S=2(五重态)的单重态裂变形成的双激子态的微观性质。这将使我们能够表征双激子态中束缚三重态的微观位置，它们相互作用的强度，以及它们的荧光光谱和动力学性质。ODMR实验的极限将被推到单对三联体检测，以便观察在整体测量中被遮盖的效应。这些实验产生的详细物理图像将作为双激子态电子结构参数的定量分子水平表征的基础，这将在该项目的框架内进行。与太阳能电池和上转换相关的材料具有复杂的形貌，这在单晶的宏观实验中是无法探索的。因此，我们计划开发一个基于微量荧光的ODMR实验。这一进展还将允许在几乎未知的化学气相沉积生长的单分子层和几层微米尺寸的单晶片中探测甲基卤化铅的自旋性质，甲基卤化铅是一种有前途的太阳能电池材料。总而言之，火星计划将开发原创的自旋敏感方法，以探测出现在激子裂变系统中的光激发态的性质，以及对基础光电子学及其应用具有广泛影响的新型材料。

Spin processes are highly important for the efficiency of organic solar cells and light-emitting diodes, yet their role remains largely unexplored. Due to the weak spin-orbit coupling, organic materials possess strict spin-selection rules and the properties of photoexcited species can therefore vary drastically, depending on the particular spin configuration. In addition, over the last years there has been tremendous progress on the development of hybrid organic-inorganic perovskite based materials, which now enables the fabrication of optoelectronic devices with remarkable performance. It has been suggested that the spin degree of freedom is relevant for photo-physical processes in these materials as well. Thus, spin properties of excited states are actively investigated in very different contexts. In this project, which is a collaborative effort involving scientists from Paris (Université Paris-Sud), Versailles (Université Versailles Saint-Quentin), Grenoble (Institut Néel), Bayreuth (Universität Bayreuth [UBAT]) and Berlin (Freie Universität Berlin [FU Berlin]), we will focus on two aspects: singlet exciton fission where a singlet exciton splits into two triplet excitons of lower energy and spin-dependent processes in few layer hybrid-organic perovskites down to the monolayer limit. We will explore the use of broadband and time-resolved optically detected magnetic resonance (ODMR) spectroscopy as a powerful method to establish the microscopic nature of bi-exciton states with total spin S = 2 (quintets) formed through singlet fission. This will allow us to characterise the microscopic positions of bound triplets in bi-exciton states, the strength of their interaction, as well as their fluorescence spectrum and kinetic properties. The limits of the ODMR experiment will be pushed to single geminate triplet-pair detection in order to observe effects obscured in ensemble measurements. The detailed physical picture emerging from these experiments will serve as the basis for a quantitative molecular-level characterisation of the electronic structure parameters of bi-exciton states, which will be carried out within the framework of the project. The materials relevant for solar cells and up-conversion have a complex morphology which cannot be probed in macroscopic experiments on single crystals. We are thus planning to develop a microfluorescence-based ODMR experiment. This development will also allow probing spin-properties of Methylammonium lead halide, a promising solar cell material, in the almost unexplored limit of chemical vapour deposition grown monolayers and few layer single crystal flakes of micrometer sizes. All in all, the MARS project will develop original spin-sensitive methods to probe the properties of photo-excited states that appear in exciton fission systems and novel materials with broad impact on fundamental optoelectronics and its applications.