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

有机电子领域在过去5年中继续取得巨大的技术和科学进步，并已成为一个重要的产业。2012年，有机/可印刷电子产品的全球市场达到120亿美元，其中约一半是有机发光二极管（OLED）显示器。磷光红色和绿色以及荧光蓝色OLED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，例如照明，非常需要更简单的器件架构和更便宜的材料，以满足苛刻的成本目标。此外，有机场效应晶体管（OFFET）现在被用于商业应用，包括柔性有源矩阵电子纸显示器。对于具有更高载流子迁移率（>10-50 cm 2/Vs）和电稳定性的有机半导体，仍然存在重要的需求，以实现更宽范围的应用。此外，基于分布式供体-受体异质结的有机太阳能电池已经实现了性能的稳步改善，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，但有机光电池（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。在过去两年中取得的两个非常重要的突破有可能改变这一点：（i）我们在剑桥的团队已经证明了通过使用单线态裂变，太阳能电池的量子效率达到200%，这为太阳能收集开辟了全新的架构。(ii)基于混合卤化物钙钛矿的混合有机-无机异质结太阳能电池已经显示出效率高达16- 17%的出乎意料的性能，部分地通过长激子/电荷扩散长度和钙钛矿材料中的低能量无序来实现。这一发现可能会提供一种太阳能电池技术，在几年内可以与硅竞争。在这个稳步发展的科学和技术领域，我们确定了三个壮观的和意想不到的发现，创造了不连续的进步的机会。这些是我们计划的重点：（i）波函数离域/相干性-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不再主导传输物理的水平。非常出乎意料的是，这可以在低温加工的非结晶材料中发现。激发态波函数的相关相干性和离域性使非共价材料和异质结中的远程电子转移成为可能;（ii）有机-无机异质结-牛津大学对卤化铅钙钛矿的研究揭示了低温处理的无机半导体，其在本体性质和与有机半导体的界面上都具有出乎意料的清洁性质。了解为什么可以避免这些界面上的电子缺陷/陷阱状态将成为该计划的主要部分。(iii)自旋-有机材料独特的自旋物理学为控制传统无机半导体中不可用的电子过程提供了新的途径。特别地，单重态激子裂变成一对三重态激子的过程提供了克服太阳能电池中的肖克利-奎塞尔（SQ）效率极限的潜力。利用这些现象需要包括有机和无机半导体的混合系统。我们的计划赠款建立在最近的突破基础上，并围绕有机和钙钛矿半导体的波函数离域工程。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梯式变化。

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.