Control of spin and coherence in electronic excitations in organic and hybrid organic/inorganic semiconductor structures

在过去的5年里，有机电子领域继续取得了巨大的技术和科学进步，并形成了一个重要的产业。2012年，全球有机/可印刷电子产品市场规模达到120亿美元，其中约一半是有机发光二极管(OLED)显示器。红色、绿色以及蓝色荧光有机发光二极管的效率已经接近其理论最大值；要实现这一点，需要复杂的多层结构。对于未来的OLED应用，如照明，需要更简单的器件架构和更便宜的材料来满足苛刻的成本目标。此外，有机场效应管(OFET)现在正被用于商业应用，包括灵活的有源矩阵电子纸显示器。人们仍然需要具有更高载流子迁移率(&gt；10-50 cm2/vs)和电稳定性的有机半导体，以实现更广泛的应用。此外，基于分布式施主-受主异质结的有机太阳能电池在性能上也取得了稳步的改善，目前报道的最佳单结电池的功率转换效率为10-11%。然而，尽管进行了大量的研究工作，有机光伏(OPV)的性能/效率和由此产生的电力成本仍然无法与晶体硅太阳能电池竞争。过去两年中取得的两项重大突破有可能改变这一点：(I)我们剑桥的团队通过使用单态裂变展示了太阳能电池200%的量子效率，这开启了太阳能收集的全新架构。(Ii)基于混合卤化物钙钛矿的有机-无机混合异质结太阳能电池表现出意想不到的性能，效率高达16%-17%，部分原因是钙钛矿材料中较长的激子/电荷扩散长度和低能量无序。这一发现可能会提供一种太阳能电池技术，在几年内实际上可以与硅竞争。在这个稳步发展的科学和技术领域中，我们发现了三个令人惊叹和意想不到的发现，它们创造了不连续进步的机会。这些是我们计划的重点：(I)波函数离域/相干-我们感到惊讶的是，共轭聚合物中的能量无序程度现在可以降低到它不再主导传输物理的水平。令人非常意想不到的是，在低温加工的非晶态材料中可以发现这一点。激发态波函数的相关相干和离域使电子能够在非共价材料和异质结中进行远程电子转移；(Ii)有机-无机异质结--牛津大学关于卤化铅钙钛矿的工作揭示了低温处理的无机半导体在体属性和与有机半导体的界面上都具有出人意料的清洁性质。了解为什么有可能避免这些界面上的电子缺陷/陷阱状态，这将是该方案的主要部分。(3)自旋--有机材料独特的自旋物理提供了控制电子过程的新途径，这是传统无机半导体所不具备的。特别是，单态激子裂变为一对三态激子的过程提供了克服太阳能电池中Shockley-Queisser(SQ)效率限制的可能性。利用这些现象需要有机和无机半导体组成的混合系统。我们的计划拨款建立在最近的突破基础上，并围绕着在有机和钙钛矿半导体中实现波函数离域的工程。它将带来有机和无机大面积电子领域的范式转变，并实现器件性能的阶段性变化。

The field of organic electronics has continued to make great technological and scientific progress over the last 5 years and has given rise to a significant industry. The worldwide market for organic/printable electronics reached $12 billion in 2012, about half of which were organic light-emitting diode (OLED) displays. The efficiency of phosphorescent red and green as well as fluorescent blue OLEDs is already close to their theoretical maximum; to achieve this requires complex multilayer architectures. For future OLED applications, such as lighting, there is an important need for simpler device architectures and cheaper materials to meet demanding cost targets. Also organic field-effect transistors (OFETs) are now being used in commercial applications, including flexible active-matrix electronic paper displays. There continues to be an important need for organic semiconductors with higher carrier mobilities (>10-50 cm2/Vs) and electrical stability to enable a wider range of applications. Also organic solar cells based on distributed donor-acceptor heterojunctions have achieved steady improvements in performance with power conversion efficiencies of 10-11% now being reported for the best single-junction cells. However, in spite of intense research efforts the performance/efficiency and resulting cost of electricity of organic photovoltaics (OPV) is still not competitive with crystalline silicon solar cells. Two very significant breakthroughs made in the last two years have the potential to change this: (i) Our group in Cambridge has demonstrated 200% quantum efficiency in solar cells through the use of singlet fission, which opens up completely new architectures for solar energy harvesting. (ii) Hybrid organic-inorganic heterojunctions solar cells based on mixed halide perovskites have shown unexpected performance with efficiencies up to 16-17%, achieved in part through long exciton/charge diffusion lengths and low energetic disorder in the perovskite materials. This discovery may provide a solar cell technology that could realistically be competitive with silicon in a few years time. Within this steadily advancing field of science and technology we identify three spectacular and unanticipated discoveries that create the opportunity for discontinuous advances. These are the focus of our programme: (i) Wavefunction delocalisation / coherence - We have been surprised that the degree of energetic disorder in conjugated polymers can now be reduced to levels at which it is no longer dominating the transport physics. It is very unexpected that this can be found in low-temperature processed non-crystalline materials. The associated coherence and delocalisation of excited state wavefunctions enables long-range electron transfer in non-covalent materials and heterojunctions; (ii) Organic-inorganic heterojunctions - The Oxford work on lead halide perovskites reveal low-temperature processed inorganic semiconductors with unexpectedly clean properties both in the bulk properties and also at interfaces with organic semiconductors. Understanding why it is possible to avoid electronic defect/trap states at these interfaces will form a major part of the programme. (iii) Spin - The unique spin physics of organic materials offers novel routes for controlling electronic processes that are not available in conventional, inorganic semiconductors. In particular, the process of singlet exciton fission to a pair of triplet excitons offers the potential of overcoming the Shockley-Queisser (SQ) efficiency limit in solar cells. The exploitation of these phenomena requires hybrid systems comprising both organic and inorganic semiconductors. Our programme grant builds on recent breakthroughs and is centered around the engineering of wavefunction delocalisation in organic and perovskite semiconductors. It will bring about a paradigm shift in the field of organic and inorganic large-area electronics and achieve step-changes in device performance.