1 Introduction
In recent years, organic solar cells (OSCs) have attracted more and more attention due to their low cost, light weight, solution processability, and high flexibility, and have been rapidly developed. At present, researchers have developed various donor-acceptor materials with excellent properties. Among them, non-fullerene acceptors have played a key role in enhancing photoelectric conversion efficiency (PCE), especially fused ring electron acceptors (FREAs) based on A-D-A and A-DA'D-A structures can provide strong absorption while improving Due to the aggregation and delocalization of π electrons, the highest PCE of FREAs-based devices is now over 18%. Despite the excellent performance of such devices, their large-scale commercial applications are limited due to their complex synthetic routes, low yields, and high costs.
Compared with FREAs, the synthetic route of non-covalent fused-ring electron acceptors (NFREAs) is relatively simple, high yield and low cost. By introducing a non-covalent conformational structure, NFREAs have comparable planarity to FREAs, as well as nearly equally excellent light absorption capacity and charge transport mobility. Not long ago, the research group of Professor Huang Hui from the University of Chinese Academy of Sciences reported a new type of NFREA, named BTzO-4F, which is composed of S---O intramolecular non-covalent interaction and benzotriazole moiety, and finally obtained achieved a PCE of 13.8%. However, to date, PCEs based on NFREAs devices still lag far behind those based on FREAs. Therefore, there is an urgent need to develop new methods to construct high-performance NFREAs.
Figure 1: Molecular structures and properties of three novel NFREAs
2 Introduction
Based on the previous work, recently, Prof. Hui Huang's research group designed and synthesized a series of novel NFREAs with gradually increasing degree of fluorination through a simple two-step method, namely BN-0F, BN-2F and BN-4F. In these novel acceptor molecules, differential fluorination of end-groups can precisely tune electronic properties, molecular stacking, charge transport, film morphology, and energy loss, thereby affecting the open-circuit voltage (VOC), short-circuit current density, and short-circuit current density of the final device. (JSC) and fill factor (FF) for fine-tuning. Specifically, the researchers paired the polymer donor J52 with BN-0F, BN-2F and BN-4F to prepare the corresponding OSCs devices, and found that with the increase of the degree of fluorination, the VOC of the three devices gradually decreased , which is consistent with the change in the LUMO energy level. However, the JSCs increased from 21.91 mA cm-2 to 25.25 mA cm-2 and then 25.76 mA cm-2, respectively, while the FFs all remained above 60%. More importantly, the PCE of the J52:BN-2F-based device is as high as 14.53%, which is the highest efficiency of the current NFREAs-based device.
Figure 2: Photovoltaic performance testing of different devices
In addition, the researchers also used atomic force microscopy and transmission electron microscopy to study the surface and bulk morphologies of the blend films of the three devices. The results show that all these blend films exhibit very smooth and clear nanoscale morphologies. Two-dimensional grazing-incidence wide-angle X-ray scattering characterization shows that the crystallinity of the blend films is gradually enhanced, which in turn leads to an increase in the electron mobility. These results can illustrate that fine-tuning the degree of fluorination of the π-extended end group can tune the light-harvesting ability, energy level change and charge transport properties of the acceptor molecule compared to the commonly used end group (INCN). At the same time, the morphology of the blend films can also be made more precise, which is beneficial to the realization of high-performance OSCs.
Figure 3: Blend film morphologies of different devices
3 Summary
In conclusion, this work not only designed and synthesized a series of novel simple and high-performance NFREAs, but also made a great contribution to promoting the development of NFREAs-based OSCs. The relevant research results have been published in the top international journal "Angewandte Chemie International Edition", entitled "High-performance Unfused-Ring Electron Acceptors for Organic Solar Cells Enabled by Noncovalent Intramolecular Interactions and End Group Engineering".
4 Materials covered in the text
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1887136-01-7 | ICB 2109805-70-9 |
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ICBF | ICB-2F |
