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

通过将光子转换为电荷来收集太阳光是解决能源危机和全球变暖的最有前途的策略之一，提供清洁的电力来源。同时，有机半导体在反向偏压下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无机晶体的有序分子结构中的电荷传输机制，但在过去的几十年中，有机半导体的有利可图的制造已经取得了实质性的进展。有机太阳能电池（OSC）和有机光电探测器（OPD）中的能量产生是基于分子的光激发和形成的局域激子沿着π共轭结构的跳跃。此外，现今用于OSC的最常见的活性层由供体（D）和受体（A）材料的互穿共混物组成，即所谓的体异质结，其中连续的D：A界面提供用于激子分离和电荷提取的能量补偿。由于有机材料特有的这些现象，OSC在功率转换效率（PCE）方面落后于硅电池，而OPD性能在探测率和暗电流方面仍然难以控制。然而，如果这些设备能够保证延长的操作时间，则它们可以提供可靠且经济上有利可图的技术。因此，该领域的关键挑战之一是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。本项目旨在通过识别损失机制来解决这一稳定性问题。我们将重点介绍应用于光致发光和光探测器的一类特定分子，其中包括新型非富勒烯受体（NFA）和新型小分子供体材料。由于这些材料家族提供了丰富的分子设计策略工具箱，可以定制其性能，本研究的一个重要目的是利用拉曼光谱等技术研究不同分子的光电性质和薄膜形貌用于振动模式分析，用于能量学研究的光电子能谱和用于表面形貌可视化的原子力显微镜。除了纯材料和相应混合物的表征外，器件制造是工作的重要组成部分，允许评估不同物种的特性如何影响设备效率。该项目的核心目标是了解光-在薄膜和操作设备中的诱导退化机制，以确定提高OSC寿命和OPD鲁棒性的方法。通过这样做，我们的目标是确定用于制造高度稳定的器件的最佳分子工程路线-例如，突出分子的哪些结构特性对于避免辐射和非辐射复合的损失机制是至关重要和有益的。具有新型NFA和合适供体的器件的实现提供了良好的PCE和在光照和热条件下的长期操作稳定性与真实的工作环境相当将是一个令人满意的结果，以证明有机基电子器件如何能够成为市场上即将到来的商业化的可靠技术。

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.