Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Bin Kan; Jiangbin Zhang; Feng Liu; Xiangjian Wan; Chenxi Li; Xin Ke; Yunchuang Wang; Huanran Feng; Yamin Zhang; Guankui Long; Richard H Friend; Artem A Bakulin; Yongsheng Chen

doi:10.1002/adma.201704904

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Adv Mater. 2018 Jan;30(3). doi: 10.1002/adma.201704904. Epub 2017 Dec 4.

Authors

Bin Kan¹, Jiangbin Zhang^{2

3}, Feng Liu⁴, Xiangjian Wan¹, Chenxi Li¹, Xin Ke¹, Yunchuang Wang¹, Huanran Feng¹, Yamin Zhang¹, Guankui Long¹, Richard H Friend³, Artem A Bakulin², Yongsheng Chen¹

Affiliations

¹ State Key Laboratory and Institute of Elemento-Organic Chemistry, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, College of Chemistry, Nankai University, Tianjin, 300071, China.
² Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.
³ Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, UK.
⁴ Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai, 200240, China.

PMID: 29205535
DOI: 10.1002/adma.201704904

Abstract

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

Keywords: charge separation; high-performance organic solar cells; low bandgap; small-molecule nonfullerene acceptors.