The chemistry and device physics of organic solar cells based on non-fullerene acceptors

“该项目属于EPSRC太阳能技术、光电器件和电路以及能源应用材料研究领域。有机太阳能电池（OSCs）由于其重量轻、溶液可加工性、灵活性和半透明性，有可能成为下一代可再生能源收割机。最近发明的创纪录的高性能非富勒烯熔接环电子受体（FREAs）将单电池器件的功率转换效率（pce）提高到19%以上。通常在有机光伏（OPV）中，激子（束缚电子和空穴）本质上是光产生的，因为光活性层具有低介电常数，导致激子具有高结合能，因此导致器件性能较差。因此，将激子分离成自由载流子需要供体和受体分子之间的异质结。然而，据报道，这种异质结会导致界面不稳定并限制PCE，因此这项工作将只关注单组分同质结osc。整个博士项目将研究的FREAs是Y6， COTIC-4F和COTIC-4Cl。Y6分子是在PCE中表现出加速的常见FREAs之一，它具有受体-供体-受体（a -d -a）结构，由一个核心，两个电子接受末端部分和溶解性烷基取代基组成。Y6已被证明可以在没有异质结的情况下产生自由载流子（而不是激子），这给了高效异质结器件的可能性带来了希望。COTIC-4F/4Cl是一种新型窄带隙非富勒烯受体，具有A-D-D-D-A结构，可以增强分子内电荷转移，并将光学带隙降低到1.10 eV。由于太阳辐射强度的50%位于近红外区域，因此需要具有较低的光学带隙来收集太阳辐射。到目前为止，还没有报道这些窄带隙受体是否也能在整齐薄膜中产生自由载流子。因此，这项工作将研究整齐薄膜中的电荷动力学，并成功地制造出单组分同质结器件。另一个活跃的研究领域将是在COTIC-4F/4Cl光活性层中引入掺杂剂以改善OSCs的电荷输运特性。随着掺杂过程产生大量的自由载流子，器件性能增强的形态学影响，如优化的结晶度和降低的陷阱密度可以同时发生。通过在有源层上同时掺杂p型和n型，目的是形成一个p-i-n结，使载流子能够有效地向金属触点传输。frea允许当前PCE的增长，但仅适用于溶液处理系统。真空处理的OSCs比溶液处理的OSCs具有更高的形态稳定性，这可能是由于侧链降解的敏感性，以及分子在膜生长过程中发现接近平衡结构。真空镀膜工艺的主要优点是价格低廉，可以快速涂覆大面积的表面。再加上最小的材料消耗、低温加工和与柔性基板的兼容性，这可能使OSCs成为世界上最便宜的电力来源。Y6太大，无法真空处理，因此，通过合成去除笨重的烷基侧链，应该使其足够小，可以真空处理。”

"This project falls within the EPSRC Solar technology, Optoelectronic Devices and Circuits, and Materials for Energy Applications research areas. Organic solar cells (OSCs) have the potential to be next-generation renewable energy harvesters due to their lightweight, solution processability, flexibility and semi-transparency. Recent inventions of record high performance non-fullerene fused ring electron acceptors (FREAs) have increased power conversion efficiencies (PCEs) to over 19% in a single cell device. Conventionally in organic photovoltaics (OPV), excitons (bound electrons and holes) are intrinsically photogenerated due to the photoactive layer having a low dielectric constant causing excitons with high binding energies and therefore leading to poorer device performance. Thus, separating excitons into free charge carriers requires a heterojunction between donor and acceptor molecules. However, this heterojunction has been reported to cause instabilities at the interface and limits the PCE, therefore this work will solely focus on single-component homojunction OSCs. The FREAs that will be investigated throughout the PhD project is Y6, COTIC-4F and COTIC-4Cl. The Y6 molecule is among the common FREAs that has demonstrated an acceleration in PCE. It has an acceptor-donor-acceptor (A-D-A) structure consisting of a core, two electron accepting terminal moieties and solubilising alkyl substituents. Y6 has proven to intrinsically generate free charge carriers (rather than excitons) without a heterojunction giving hope to the possibility of efficient homojunction devices. COTIC-4F/4Cl are novel narrow bandgap non-fullerene acceptors containing an A-D-D-D-A structure that can enhance intramolecular charge transfer and also lower the optical bandgap to 1.10 eV. As 50% of solar radiation intensity lies in the near infrared region, possessing a low optical bandgap is therefore desirable to harvest solar radiation. As of yet, it has not been reported whether these narrow bandgap acceptors can also intrinsically generate free charge carriers in neat films. Thus, this work will research the charge dynamics in neat films and if successful single component homojunction devices will be fabricated. Another active area of research will be the introduction of dopants to the COTIC-4F/4Cl photoactive layer to improve the charge transport properties of OSCs. Along with a significant number of free charge carriers generated by the doping process, device performance-enhancing morphological impacts such as optimised crystallinity and reduced trap density can occur concurrently. By simultaneously performing both p and n-type doping to the active layer, the aim is to form a p-i-n junction that will enable an efficient transport of charge carriers towards the metal contacts. FREAs have permitted the current growth in PCE, but only for solution-processed systems. Vacuum processed OSCs were found to have a higher morphological stability than solution processed OSCs which could be due to susceptibility of side chain degradation, and molecules finding near equilibrium structures during film growth. The key advantages of vacuum coating processes are that they are inexpensive and fast to coat large surface areas. Coupling this with minimal material consumption, low temperature processing and compatibility with flexible substrates, this could potentially make OSCs the cheapest source of electricity in the world. Y6 is too large to be vacuum processed thus, by synthetically removing the bulky alkyl side chains should make it small enough to be vacuum processed."