Unravelling the working mechanisms of homoeopathic organic solar cells

有机太阳能电池(OSC)是一个高度活跃的跨学科研究领域，汇集了化学家、物理学家、材料科学家和工程师的专业知识。这项研究不仅在基础科学方面令人兴奋，而且在对经济和社会的潜在积极影响方面也是令人兴奋的。OSC有潜力成为一种极具成本竞争力的、大面积和多功能的光伏技术。学术和工业研究已经产生了超过10%的效率，并使OSC接近商业化。直到最近，用于所有高效OSC的结构都是基于体异质结的，体异质结是由施主和受体分子的混合物组成的一层。在1：4和1：1(重量或体积)之间的混合比例被认为是有效地在施主和受主之间产生自由电子和空穴以及有效地传输到电极所必需的。然而，在2011年引入了一种新的器件结构：在富勒烯的基础上提出了一种标准受体分子作为吸收层的OSC，它只有很少的给体分子(5vol%)，但工作非常好。直到那时，对OSC的传统理解是，这种OSC根本不应该工作，或者至少不应该像他们那样工作；同时，他们正在达到6%以上的效率。他们的工作机制仍远未被理解。这些意想不到的结果再次表明，有机半导体领域(以及最有可能的有机电子产品)存在一些令人惊讶的东西，其全部潜力仍然很难估计。为了进一步支持长期的技术创新，需要进行基础研究。揭示这种新型结构的OSC的工作机理是本项目的核心，为了实现这一目标，将在堆栈和工艺条件下系统地改变这种新型结构的有机薄膜和相应的OSC。为了高度控制器件的制备，将使用真空处理纯化的小分子。与其他方法的关键区别在于，这将与分子掺杂的概念相结合。目前，尽管这种方法是所有商用有机发光二极管(OLED)和当前世界记录的OSC的基础，但在OSCS中很少使用这种方法。通过系统地改变OSC的空穴接触，改变富勒烯和施主的混合比例以及改变衬底温度，我们将研究光电压和自由电荷载流子的产生。我将使用傅里叶变换光电流谱(FTPS)测量电荷转移态的能量，使用阻抗光谱、FTPS和电流-电压测量来量化空穴接触和有机吸收层之间的势垒，以及使用X射线确定混合薄膜的微结构，所有这些都是探索它们迷人的相互作用的关键。我们将使用单载流子器件和瞬变测量来研究电荷载流子输运，特别是空穴输运，通过吸收层及其复合动力学。除了高效工作外，这里研究的太阳能电池本身也可以被认为是非常有意义的。供体分子高度稀释的性质是一个很好的模型系统，可以用来实验研究供体-受体相互作用，这是任何OSC的核心，至今仍未完全了解。发现这种新型结构的OSC的工作机制也将有助于回答为什么富勒烯是如此特殊和成功的受体分子的问题。该项目的成果将刺激新的和更好的材料的开发，使研究人员能够进一步优化这一有望用于高效和稳定的太阳能电池的结构，并为有机电子的其他应用探索新的器件概念。

Organic solar cells (OSC) are a highly active, interdisciplinary field of research drawing together the expertise of chemists, physicists, material scientists and engineers. The research is exciting not only in terms of fundamental science, but also in terms of potential positive impact on the economy and society. OSC have the potential to become a very cost-competitive, large area and versatile photovoltaic technology. Academic and industrial research have produced efficiencies exceeding 10% and brought OSC close to commercialisation.Until recently, the architecture used for all efficient OSC was based on the bulk heterojunction, a layer consisting of a mixture of donor and acceptor molecules. A mixing ratio between 1:4 and 1:1 (by weight or volume) was thought to be required for an efficient generation of free electron and holes at the interface between donor and acceptor, and for efficient transport to the electrodes. However, in 2011, a novel device architecture was introduced: OSC on the basis of fullerenes, the standard acceptor molecules, as absorbing layer were presented that only have a very small amount (5vol%) of donor molecules, yet worked very well. Up to then, the conventional understanding of OSC was that such OSC should not work at all, or at least not as well as they do; meanwhile they are reaching efficiencies of more than 6%. Their working mechanism is still far from understood. These unexpected results again show that the field of OSCs (and most likely organic electronics in general) holds some surprises and that its full potential is yet hard to estimate. To underpin further long-term technological innovations, fundamental studies are required. Unravelling the working mechanism of this novel architecture for OSC is the core of this project.To achieve this goal, thin organic films and corresponding OSCs of this novel architecture will be made with systematic variations in the stack and processing conditions. For high control of the device preparation, vacuum processing of purified small molecules will be used. The key difference to other approaches is that this will be combined with the concept of molecular doping. Presently, this method is rarely used in OSCs, despite being the basis of all commercial organic light emitting diodes (OLED) and the current world record OSCs.Through systematic variations of the OSC hole contact, here realised with doped transport layers, and varying mixing ratios of fullerene and donor and changing substrate temperature, the generation of photovoltage and free charge carriers will be investigated. I will measure the energy of the charge transfer states using Fourier-transform photocurrent spectroscopy (FTPS), quantify the barrier between the hole contact and the organic absorber layer using impedance spectroscopy, FTPS, and current-voltage measurements, as well as determine the microstructure of the mixed films using X-rays, all essential to probe their fascinating interplay. The charge carrier transport, in particular the hole transport, through the absorbing layer and its recombination dynamics will be studied using single-carrier devices and transient measurements. In addition to working efficiently, the solar cells investigated here can be considered of great interest in their own right. The highly diluted nature of the donor molecules is an excellent model system to experimentally study donor-acceptor interactions, something that is central to any OSC and still not fully understood. Discovering the working mechanisms of this novel architecture for OSC will also help to answer the question of why fullerenes are such special and successful acceptor molecules. The results of this project will stimulate the development of novel and better materials, enable researchers to further optimise this promising architecture for efficient and stable solar cells as well as explore new device concepts for other applications of organic electronics.