posted on 2023-12-28, 16:39authored byXueyan Hou, Jack F. Coker, Jun Yan, Xingyuan Shi, Mohammed Azzouzi, Flurin D. Eisner, James D. McGettrick, Sachetan M. Tuladhar, Isaac Abrahams, Jarvist M. Frost, Zhe Li, T. John S. Dennis, Jenny Nelson
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), 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
correlates negatively with electron mobility and organic solar cell
device performance, which we relate to their molecular symmetry, with
a lower symmetry leading to 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.