Direct p-doping of Li-TFSI for efficient hole injection: Role of polaronic level in molecular doping

Kiwoong Kim, Junkyeong Jeong, Minju Kim, Donghee Kang, Sang Wan Cho, Hyunbok Lee, Yeonjin Yi

Research output: Contribution to journalArticlepeer-review

9 Citations (Scopus)


Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) has been popularly employed as an efficient p-dopant that increases the conductivity of a hole transport layer (HTL) in perovskite solar cells and dye-sensitized solar cells. However, the working mechanism of the Li-TFSI dopant is a long-standing question. The hygroscopicity of Li-TFSI makes it difficult to isolate the exact doping mechanism. In this study, we unveil the role of Li-TFSI in the p-doping to the N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) HTL. A series of systematic in situ measurements using ultraviolet and inverse photoelectron spectroscopy reveal that electron transfer from NPB to Li-TFSI occurs due to the lower-lying negative polaronic level of Li-TFSI rather than the positive polaronic level of NPB. The hole injection barrier between NPB and indium tin oxide is significantly reduced with Li-TFSI doping, enhancing the device performance of hole-only devices and organic light-emitting diodes dramatically. With excessive dopants, however, the agglomerative property of Li-TFSI became dominant, decreasing the doping efficiency. These results provide robust guidelines for developing an efficient doping method for a molecular system with high conductivity.

Original languageEnglish
Pages (from-to)565-571
Number of pages7
JournalApplied Surface Science
Publication statusPublished - 2019 Jun 30

Bibliographical note

Funding Information:
This study was supported by the National Research Foundation of Korea [NRF- 2018R1D1A1B07051050 , 2018R1A6A1A03025582 , 2017R1A2B4002442 , and 2017R1A5A1014862 (SRC program: vdWMRC center)], Samsung Display Company, and an Industry-Academy joint research program between Samsung Electronics and Yonsei University .

Publisher Copyright:
© 2019 Elsevier B.V.

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Condensed Matter Physics
  • Physics and Astronomy(all)
  • Surfaces and Interfaces
  • Surfaces, Coatings and Films


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