Interfacial energetics and charge carrier dynamics in organic and hybrid photo electron sensors

通过将光子转化为电荷来获取太阳光是解决能源危机和全球变暖的最有前途的战略之一，它提供了一种清洁的能源。同时，有机半导体在反向偏压下工作时也可以作为波长选择性辐射传感器。虽然传统的太阳能电池和光电探测器仍然依赖于硅和众所周知的无机晶体有序分子结构中的电荷传输机制，但在过去的几十年里，在有利可图的有机半导体制造方面已经取得了实质性的进展。这些材料具有广泛的优点，如基于溶液的廉价加工的可能性，透明度和轻量化，以及可集成和可穿戴设备的灵活性和生物兼容性。有机太阳能电池(OSC)和有机光电探测器(OPD)的能量产生是基于分子的光激发和形成的局域激子沿圆周波共轭结构的跳跃。此外，当今OSCs最常见的有源层是由施主(D)和受主(A)材料组成的互穿混合物，即所谓的体异质结，其中连续的D：A界面为激子分离和电荷提取提供能量补偿。由于有机材料特有的这些现象，OSCs在功率转换效率(PCE)和OPD性能方面仍然落后于硅电池，OPD的性能仍然很难控制，比如探测率和暗电流。然而，如果这些设备能够保证更长的可操作期，它们可以提供可靠的、经济上有利可图的技术。因此，识别和控制效率损失机制以提高其在各种运行条件下的稳定性是该领域面临的关键挑战之一，本项目旨在通过识别效率损失机制来解决这一稳定性问题。我们将集中于一类应用于光伏和光电探测器的分子，其中包括新型非富勒烯受体(NFA)和新型小分子施主材料。由于这些材料提供了丰富的分子设计策略来调整其性质，因此本研究的一个重要目的是利用拉曼光谱振动模式分析、光电子能谱研究和原子力显微镜可视化表面形貌等技术来研究不同分子的光电性质和薄膜形貌。除了整齐材料和相应混合物的表征之外，器件制造是工作的关键部分，该项目的核心目标是了解薄膜和操作器件中的光诱导退化机制，以确定提高OSC寿命和OPD稳健性的方法。通过这样做，我们的目标是确定制造高度稳定的器件的最佳分子工程路线-例如，强调分子的哪些结构属性是关键的，并且有助于避免辐射和非辐射复合的损失机制。在有机半导体方面，实现具有新型非辐射脂肪酸和合适的供体的器件将是一个令人满意的结果，在与实际工作环境相当的光照和热条件下，提供良好的PCE和长期运行稳定性，以证明基于有机的电子如何成为一种可靠的技术，用于即将上市的商业化。

Harvesting solar light by conversion of photons into electric charges is one of the most promising strategy to tackle energetic crisis and global warming providing a clean source of power. At the same time organic semiconductors can also act as wavelength-selective radiation sensors when operated in reverse bias. While traditional solar cells and photodetectors are still relying on silicon and to the well understood mechanisms of charge transport in the ordered molecular structure of inorganic crystals, in the last decades substantial progresses have been achieved for a profitable manufacture of organic semiconductors. These materials offer a wide range of advantages such as the possibility of solution-based and cheap processing, transparency and light weight together with flexibility and biocompatibility for integrable and wearable devices.Energy production in organic solar cells (OSCs) and organic photodetectors (OPDs) is based on the light-excitation of molecules and hopping of the formed localised excitons along the pi-conjugated structure. Moreover, the most common active layer for OSCs nowadays is comprised of an interpenetrating blend of a donor (D) and an acceptor (A) material, the so-called Bulk Heterojunction, where the continuous D:A interface provides the energy offset for exciton separation and charge extraction. All these processes are still not thoroughly understood and they present numerous scientific issues due to the loss mechanisms such as geminate and bimolecular recombination.Because of these phenomena specific to organic materials, OSCs lag behind silicon cells in terms of Power Conversion Efficiency (PCE) and OPD performances are still difficult to control as for detectivity and dark current. However, these devices can offer a reliable and economically profitable technology if they can guarantee an extended period of operativity. Therefore, one of the key challenges in the field is to identify and control the efficiency loss mechanisms in order to improve their stability in various operational conditions.This project aims to tackle this stability issue by identifying the loss mechanisms. We will focus on a specific class of molecules applied in photovoltaics and photodetectors, which will include new Non-Fullerene Acceptors (NFAs) and novel small molecule donor materials.Since these families of materials offer a rich toolbox of molecular design strategies to tailor their properties, an important aim of this study is to investigate optoelectronic properties and thin film morphology of different molecules by techniques like Raman spectroscopy for vibrational mode analysis, Photoemission Spectroscopy for investigation of energetics and Atomic Force Microscopy to visualise surface topography.Alongside the characterisation of neat materials and respective blends, device fabrication represents an essential part of the work, allowing to assess how properties of different species have outcomes on device efficiency.The core objective of the project is the understanding of light-induced degradation mechanisms in thin films and operational devices in order to identify ways to improve the OSC lifetime and OPD robustness. By doing so, we aim at identifying the best molecular engineering routes for the fabrication of highly stable devices - e.g. highlighting which structural properties of molecules are critical and beneficial to avoid loss mechanisms of radiative and non-radiative recombination.With regards to OSCs, the realisation of devices with novel NFAs and suitable donors providing both good PCE and long operational stability under illumination and thermal conditions comparable to the real working environment would be a satisfactory result to demonstrate how organic-based electronics can be a reliable technology for forthcoming commercialisation on the market.