1 Introduction

In recent years, organic solar cells (OSCs) have emerged as one of the most promising technologies in photovoltaic research. At present, due to the continuous development of donor and acceptor materials, especially fused ring electron acceptors (FREAs) with A-D-A and A-DA'D-A molecular structures represented by ITIC and Y6, the photoelectric conversion efficiency (PCE) of OSCs has been greatly improved. ) continued to increase, and now has exceeded 18%. Through the study of light absorption capability, energy level, stacking method, etc., researchers have carried out a lot of work aimed at improving the device performance. Among them, side chain engineering is a strategy to further adjust solubility and intermolecular packing by adjusting the position of substituents, the length and volume of alkyl chains, and has been widely used.

With the rapid development of FREAs, non-covalent fused-ring receptors (NFREAs) have also emerged. NFREAs are usually composed of simple aromatic building blocks, which greatly reduces the cost of synthesis. At the same time, the "non-covalent conformation lock" strategy can ensure that the backbone has good coplanarity and rigidity. Therefore, NFREA has better solubility and processing properties than FREAs, and can achieve better molecular crystallization and ordered packing. However, the current PCE of NFREAs still lags far behind FREAs. Therefore, there is an urgent need to develop novel NFREAs to construct more efficient OSCs.

图1:几种不同分子的结构

2 Introduction

Based on the above considerations, recently, the research team of Professor Huang Hui from the University of Chinese Academy of Sciences designed and synthesized a novel NFREA with two terminal side chains on the basis of previous work: NoCA-5. The photophysical results show that the introduction of terminal side chains can improve the rigidity of NoCA-5 compared with NoCA-1 previously developed by the team, and further theoretical calculations show that NoCA-5 has a lower internal recombination energy for electron transfer. When the two NFREAs were blended with the polymer donor J52, the J52:NoCA-5-based blended films exhibited more balanced mobility, lower recombination loss, smaller Urbach energy, and better morphology. As a result, the final device achieved a PCE as high as 14.82%, a certified value of 14.5%, much higher than that of the NoCA-1 based device (11.71%), an open circuit voltage (VOC) of 0.814, and a short circuit current density (JSC) of 26.02 mA cm^{- 2}. Fill factor 69.96%.

图2:两种器件的光伏性能比较

图3:不同器件的形貌表征

3 Summary

In conclusion, this work not only provides a new idea for designing high-performance NFREAs, but also provides a basis for understanding the mechanism of terminal side chain effect. The relevant research results have been published in the top international journal "Angewandte Chemie International Edition", entitled "Side-Chain Engineering for Enhancing the Molecular Rigidity and Photovoltaic Performance of Noncovalently Fused-Ring Electron Acceptors".

Keywords in this paper: organic solar cells, non-fused ring electron acceptors, side chain engineering, J52, NoCA-1, NoCA-5.

4 Materials covered in the text