Structure-property relationships for the electronic applications of pure bis-adducted isomers of phenyl-C61 butyric acid methyl ester

Structure-property relationships for the electronic applications of pure bis-adducted isomers of phenyl-C61 butyric acid methyl ester

Details

  • Time: 10:00 -11:30 am (Beijing Time)
  • Date: Wednesday, March 6, 2024
  • Venue: MA205
  • Speaker:Prof. John Dennis , Department of Chemistry, Xi’an Jiaotong-Liverpool University
  • Host: Dr. Andrew Fowlie

Abstract

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure and (time-dependent) density-functional-theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM, and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10-2 cm2 V s-1, which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to-date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers corelates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading more amorphous bis-isomers, less energetic disorder and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.

Speaker

Prof. John Dennis gained his D.Phil. from the University of Sussex in 1993 under the supervision Harry Kroto. After publishing 30 papers in 3 years as a graduate student (including three in Nature), all his postdoctoral Research was conducted under internationally competitive personal research fellowships (from the Australian Research Council, The Japan Society for the Promotion of Science and the Alexander von Humboldt Foundation). He then gained tenure at Queen Mary University of London in the Department of Chemistry in 1999 and transferred to the Physics Department at the same institution with a promotion to a professorial position in 2005. He joined the Department of Chemistry at XJTLU in October 2022. Immediately before this he worked as a Professor at Zhejiang University for two years, being brought to China via a 'High-Level Foreign Expert Award' from the Zhejiang Province 1000 Talent Plan in 2020. He has published almost 100 papers on fullerenes that average over 80 citations per paper, generating an h-index of 42.