LUMO levels of non-fullerene acceptors in solid phase for organic solar cells
Ai Sugie a, Weining Han b, Hiroyuki Ichikawa a, Hiroyuki Yoshida c d
a Graduate School of Science and Engineering, Chiba University, Japan
b Graduate School of Advanced Integration Science, Chiba University, Japan
c Graduate School of Engineering, Chiba University, Japan
d Molecular Chirality Research Center, Chiba University, Japan
Poster, Ai Sugie, 143
Publication date: 23rd October 2018

 In recent years, attention has been drawn to non-fullerene acceptors in organic solar cells (OSCs). The non-fullerene acceptors have such advantages as the wide-range of tunability of the energy levels, the high light-harvesting efficiency by the acceptors, and the low cost of fabrication over the fullerene counterparts. For designing and fabricating efficient OSCs, the LUMO levels of non-fullerene acceptors is indispensable information. Usually, the reduction potential measured in solution using the cyclic voltammetry (CV) is used to estimate the LUMO level. However, the thin films of acceptors are used in OSCs and the LUMO levels in the solid state are known to depend on the crystal structure, molecular orientation, etc. Therefore the LUMO levels should be determined in the solid state.

 In principle, the best method for measuring the LUMO levels of solid samples is inverse photoelectron spectroscopy (IPES), which can be regarded as the inversion process of photoelectron spectroscopy (PES) widely used for the HOMO levels. Although the conventional IPES has insufficient precision of the energy levels due to the serious sample damage and the low energy resolution, these issues have been solved in low-energy inverse photoelectron spectroscopy (LEIPS) by lowering the energies of electrons and photons involved in the IPES process [1,2]. Using LEIPS, the LUMO levels can be determined with the precision similar to the HOMO levels by PES.

 In this study, we apply LEIPS to the prototypical non-fullerene acceptors, ITIC, O-IDTBR, IEICO, and PTCDI derivatives. In contrast to the fullerene acceptors, these non-fullerene acceptors often possess large quadrupole moments leading to molecular-orientation dependence of the energy levels [3]. As PTCDI has particularly a large quadrupole moment, we made systematic study on the relation between the energy levels and molecular orientations for the PTCDI derivatives. Based on the thus-determined LUMO levels, the open-circuit voltages of various OSCs consisting of donor polymers and non-fullerene acceptors taken from the literatures are discussed and compared with the results of fullerene acceptors reported previously [4].

We would like to thank Prof. Osaka of Hiroshima University for supplying the sample materials.

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