Introduction: The use of carbon dragon complexes to prepare a new organic solar cell interface layer material, the resulting device photoelectric conversion efficiency of up to 18%.
1. Preface
Organic solar cells (OSCs) show very promising application prospects in flexible, printable and translucent devices, while the development of photoelectric conversion efficiency (PCE) and stability remains an important factor restricting the actual industrial production of OSCs. A large number of people have been explored on the photovoltaic performance of devices, and their methods mainly focus on molecular design, device structure modification and construction of active layers. By matching innovative molecular designs with suitable feeders, Y6 and its derivatives have opened up a new world, with efficiencies based on non-fullerene OSCs already exceeding 18%. In addition to molecular innovation, interface engineering can also improve the performance of devices.
Interface engineering refers to improving the performance of the device by inserting a layer or two of the intermediate layers between the active layer and the cathode or anode, and further adjusting the morphology or function of the electrode. At present, there are the following main types of interface materials: metal oxides (MO), such as the commonly used ZnO and MoOx, etc.; alcohol-soluble polymers and small molecules, such as amino PDINO and PFN-Br; low-reactive metals and metal salts or composites, such as Ca and LiF; carbon-based materials such as fullerene, graphene and their derivatives. Thermal evaporation and orthogonal solvent spin coating are the two most commonly used methods of dealing with these interface materials, and since most interface materials are alcohol-soluble small molecules and polymers, solvent spin coating is considered more cost-effective than thermal evaporation due to its printability.
Figure 1: Schematic diagram of the structure of the related molecules and devices
2. Introduction
In view of the above, recently, the research team of Professor He Feng, Professor Xia Haiping of Southern University of Science and Technology, and Professor Tan Yuanzhi of Xiamen University cooperated to synthesize a new alcohol-soluble metal nanographene through efficient and specific reactions using carbon dragon complexes and hexabenzoanthyne-based derivatives as raw materials. As a result, the degree of dπ–pπ conjugate of the reaction products was greatly improved, resulting in a series of new cathode intermediate layer (CIL) materials: HBC-H, HBC-P, HBC-S. These transition metal-based large metal aromatic systems with nanographene centers have comprehensive properties such as enhanced carrier transport and excellent intermolecular interactions.
Figure 2: Comparison of device PV performance based on three different CIL materials
The researchers further applied the synthesized material to the PM6:BTP-eC9-based device and studied the effect of different CIL materials on photovoltaic performance, and found that when HBC-S was used as the CIL material, the efficiency of the device increased from 16% to more than 18%, mainly due to the significantly improved short-circuit current density (JSC) of 26.51mA cm-2 and the fill factor (FF) of 79.22%. In order to study the properties of these CIL materials, the researchers used single crystal X-ray diffraction analysis, optical testing, cyclic electrochemical testing and theoretical calculations to conduct in-depth research on the dπ–pπ conjugate system of these new CIL materials, and characterized their surface contact by morphological characterization and electrochemical impedance spectroscopy. The study found that the large conjugated backbone formed after the introduction of conjugated side chains can enhance intramolecular charge transfer and provide more orderly intermolecular electron transport, ultimately obtaining a lower and more suitable LUMO energy level than the receptor, making it easier to carrier injection. After HPC-S treatment, the interface contact is good, the morphology is regular, and the interface resistance is reduced, which greatly hinders the carrier recombination.
Figure 3: Characterization of electrochemical properties of molecules of different CIL materials
3. Summary
In summary, the work not only innovatively synthesizes new CIL materials with highly dπ–pπ conjugate systems using carbon dragon derivatives, but also offers great potential for the development of the next generation of high-performance OSCs. The research results have been published in the international authoritative academic journal Advanced Materials, entitled "Nanographene–Osmapentalyne Complexes as a Cathode Interlayer in Organic Solar Cells Enhance Efficiency over 18%".
Keywords of this article: organic solar cells, interface engineering, charge transport, carbon dragon compounds, cathode intermediate layer, interface layer materials, HBC-P, HBC-H, HBC-S.
4. Relevant materials involved in the text
PM6：1802013-83-7
BTP-eC9:2598965-39-8
PTB7-Th：1469791-66-9
PC71BM：609771-63-3
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https://doi.org/10.1002/adma.202101279