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令人兴奋。OSC有潜力成为一种极具成本竞争力、大面积且用途广泛的光伏技术。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，这是一层由供体和受体分子混合组成的层。混合比例在1:4和1:1之间（按重量或体积）被认为需要在供体和受体之间的界面上有效地产生自由电子和空穴，并有效地传输到电极。然而，在2011年，一种新的器件架构被引入：基于富勒烯的OSC，标准受体分子，作为吸收层，只有非常少量（5vol%）的供体分子，但工作得很好。到那时为止，对OSC的传统理解是，这种OSC根本不应该工作，或者至少没有它们那么好；与此同时，它们的效率达到了6%以上。它们的工作机制仍然很不清楚。这些意想不到的结果再次表明，osc领域（很可能是一般的有机电子学领域）有一些令人惊讶的地方，而且它的全部潜力还很难估计。为了支持进一步的长期技术创新，需要进行基础研究。为OSC揭示这种新颖架构的工作机制是这个项目的核心。为了实现这一目标，这种新型结构的有机薄膜和相应的osc将在堆叠和加工条件上有系统的变化。对于装置制备的高控制，将使用纯化小分子的真空处理。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）和目前世界纪录的OSCs的基础，但很少用于OSCs。通过掺杂输运层实现的OSC空穴接触的系统变化，富勒烯与供体的混合比例的变化以及衬底温度的变化，将研究光电压和自由载流子的产生。我将使用傅里叶变换光电流光谱（FTPS）来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层和有机吸收层之间的势垒，以及使用x射线确定混合膜的微观结构，这些都是探测它们迷人的相互作用所必需的。利用单载流子装置和瞬态测量，研究了载流子通过吸收层的输运，特别是空穴输运及其复合动力学。除了工作效率外，这里所研究的太阳能电池本身也可以被认为是非常有趣的。供体分子的高度稀释性质是实验研究供体-受体相互作用的一个极好的模型系统，这是任何OSC的核心，但仍未完全理解。发现这种新型OSC结构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。该项目的结果将刺激新型和更好的材料的开发，使研究人员能够进一步优化这种有前途的高效和稳定的太阳能电池架构，并为有机电子的其他应用探索新的设备概念。

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.