Spin Dynamics in Multilayer Organic Photovoltaics and Organic Light-Emitting Diode Films'

我的项目是化学和物理之间的合作，旨在为有机光伏(OPV)和有机发光二极管(OLED)等多层有机器件开发新的高性能有机材料。我们还将首次探索使用铁磁共振光谱(FMR)来表征这些设备的自旋态。自旋翻转机制对于设计高效的有机材料用于可再生能源应用至关重要。自旋单重态和自旋三重态激发态的相互作用直接影响着光伏发电和有机发光二极管的光电转换性能。然而，仍有各种因素显著制约着这些设备的效率。例如，单p-n结光伏电池的最大效率被Shockley-Queisser极限限制在33.7%以下，而OLED中单态产生的内部量子效率由于自旋禁忌辐射从三重态到单重态的衰减而被限制在25%以下。例如，某些有机半导体的独特性质可以通过单重态裂变来克服单结无机光伏的局限性。单态裂变是一个高能的单态激子转化为两个三态激子的过程，每个三态激子携带大约一半的能量。将这种有机半导体耦合到低带隙无机半导体上，可以在单结中制造两个带隙的OPV，原则上将具有比传统光伏更高的效率。另一方面，可以考虑在有机发光二极管中使用显示热激活延迟荧光(TADF)的荧光发射体。通过设计在S1和T1能级之间具有较小能量差的分子，小的能隙可能使系统间发生反向交叉，其中T1中的激子在热激活过程中转换为S1。一旦处于S1态，激子将能够通过荧光衰变回到S0基态。使用TADF机制，可以实现100%的内部量子效率，希望TADF将允许创建稳定和高效的有机发光二极管。因此，我的项目的第一阶段将是有机材料的开发，重点是高度共轭系统，如预计将经历单重态裂变的并苯衍生物。一旦新的有机材料被合成，它们将受到铁磁共振光谱的影响，以表征它们的自旋态。FMR探头通过测量相邻铁磁薄膜自旋源中磁化的进动阻尼，自旋翻转和反向系统间交叉过程。它将使我们能够直接获得上述基本有机自旋过程的速率。虽然铁磁共振很少用于有机材料，但这种方法在研究有机聚合物和有机发光器件方面有很大的潜力，因为它可以应用于整个多层体系的探测。我们希望通过开发用于OPV和OLED的高性能材料，这些器件的广泛适用性意味着我们的技术发展可以给社会带来积极的环境影响。

My project is a collaboration between Chemistry and Physics, which aims to develop new high-performing organic materials for multilayer organic devices such as organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). We will also, for the first time, explore the use of ferromagnetic resonance spectroscopy (FMR) to characterise the spin states of these devices.Spin-flip mechanisms are crucial to the design of efficient organic materials for renewable energy applications. The interplay of spin-singlet and spin-triplet excited states mediates the performance of solar-to-electricity energy generation in photovoltaics and electricity-to-light energy conversion in OLEDs. However, there are various factors which are still significantly restricting the efficiency of these devices. For example, the maximum efficiency for a single p-n junction photovoltaic cell is limited to below 33.7% by the Shockley-Queisser limit, whereas the internal quantum efficiency of singlet-state generation in OLEDs is limited to only 25% due to the spin-forbidden radiative decay from the triplet to singlet state.Promising solutions to the problems mentioned above have been proposed. For example, the unique properties in certain organic semiconductors can be utilised to overcome the limitations of single-junction inorganic photovoltaics via singlet fission. Singlet fission is a process by which a high-energy singlet exciton is converted into two triplet excitons, each carrying about half the energy. Coupling such an organic semiconductor to a low band gap inorganic semiconductor allows fabrication of a two-bandgap OPV in a single junction, which will in principle have a higher efficiency than conventional photovoltaics. On the other hand, the use of fluorescence emitters which exhibit thermally activated delayed fluorescence (TADF) in OLEDs could be considered. By designing molecules with a small energy difference between the S1 and T1 levels, the small energy gap may enable reverse intersystem crossing to occur, where excitons in T1 are converted to S1 in a thermally activated process. Once in the S1 state the excitons will be able to decay back to the S0 ground state via fluorescence. Using the TADF mechanism, internal quantum efficiencies of 100 % can be achieved and it is hoped that TADF will allow the creation of a stable and high efficiency OLEDs.The first stage of my project will, therefore, be the development of organic materials, focusing on highly conjugated systems such as acene derivatives that are predicted to undergo singlet fission. Once the new organic materials have been synthesised, they will be subjected to FMR spectroscopy to characterise their spin states. FMR probes spin flip and reverse intersystem crossing processes by measuring the precessional damping of magnetisation in an adjacent ferromagnetic thin-film spin source. It will give us direct access to the rates of the fundamental organic spin processes described above. Although FMR has rarely been used on organic materials, this approach has great potential in studying OPVs and OLEDs because it can be applied to probe the entire multilayer system. We hope that by developing high-performing materials for OPVs, and OLEDs, the broad applicability of these devices means that our technological developments could bring positive environmental impacts to society.