Synthesis and Properties of 4,4'-Spiro-bi[cyclopenta[2,1-b;3,4-b']dithiophene]Cored 1,8-Naphthalimide Derivatives for Organic Solar Cells

doi:10.7517/j.issn.1674-0475.2017.04.034

Abstract

Abstract:

With the rapid development of high performance non-fullerene electron acceptor, the power conversion efficiency of polymer solar cells increased dramatically in the last few years, among which structurally non-planar electron acceptors play a very important role. In this paper, a new family non-fullerene electron acceptor based on 1,8-naphthalimide unit with cyclopenta[2,1-b;3,4-b']dithiophene(CPDT-(NMI)₂), 4,4'-spiro-bi[cyclopenta[2,1-b;3,4-b']dithiophene](SCPDT-(NMI)₄) or 4-H-cyclopenta-[2,1-b;3,4-b']dithiophene-4-one (CPDT-O-(NMI)₂) core were synthesized. The absorption and fluorescence spectra, as well as the electrochemical properties of these compounds were characterized. Results showed that SCPDT-(NMI)₄ showed a blue-shifted absorption compared to that of CPDT-(NMI)₂, which is ascribed to the twisted molecular structure of CPDT-(NMI)₂ for its high steric hindrance. While CPDT-O-(NMI)₂ showed two absorption bands with peaking wavelength at 420 nm and 550 nm, which is quite different to that of SCPDT-(NMI)₄ and CPDT-(NMI)₂. Solid state absorption spectra confirm that SCPDT-(NMI)₄ and CPDT-(NMI)₂ showed low intermolecular interaction, which could be attributed to the non-planar molecular structure of these two compounds as well. Fluorescence of the SCPDT-(NMI)₄ film was found to be red-shifted 11 nm over CPDT-(NMI)₂, which is ascribed to the spiro-conjugation effect of SCPDT-(NMI)₄ in excited state. Cyclic voltammetry results demonstrated that LUMO energy levels of these compounds are around -3.5 to -3.8 eV, making them suitable electron acceptor in polymer solar cells. Polymer solar cell using PBDB-T as the electron donor and SCPDT-(NMI)₄ as the electron acceptor showed a high power conversion efficiency of 1.16%, which is much higher than that of CPDT-(NMI)₂ (0.11%). Fluorescence of the blended films confirmed that photon induced electron transfer between PBDB-T and NMIs is not completed, which is believed to be one of the main reason for the low device performance.

Key words: organic solar cells, non-fullerene electron acceptor, photosensitive materials, spiro conjugation

ZHANG Jiabin, MA Yuchao, YI Jinduo, DOU Junyan, WANG Hongyu, MA Changqi. Synthesis and Properties of 4,4'-Spiro-bi[cyclopenta[2,1-b;3,4-b']dithiophene]Cored 1,8-Naphthalimide Derivatives for Organic Solar Cells[J]. Imaging Science and Photochemistry, 2017, 35(5): 736-748.

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