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Title: Selective triplet exciton formation in a single molecule

Abstract

The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs). According to a simple model based on spin statistics, the injected charges form spin-singlet (S1) excitons and spin-triplet (T 1) excitons in a 1:3 ratio. After the first report of a highly efficient OLED based on phosphorescence, which is produced by the decay of T 1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage. Because T 1 excitons have lower energy than S 1 excitons (owing to the exchange interaction), use of the energy difference could—in principle—enable exclusive production of T 1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope and demonstrate a simple method of selective formation of T 1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high appliedmore » voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T 1 excitons without creating their S 1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T 1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.« less

Authors:
 [1];  [2];  [3];  [1];  [3];  [4];  [4];  [5];  [3]
  1. Inst. of Physical and Chemical Research (RIKEN), Wako (Japan); Univ. of Tokyo, Kashiwa (Japan)
  2. Inst. of Physical and Chemical Research (RIKEN), Wako (Japan); Univ. of California, San Diego, CA (United States); Northwestern Univ., Evanston, IL (United States); Univ. of California, San Diego, CA (United States)
  3. Inst. of Physical and Chemical Research (RIKEN), Wako (Japan)
  4. Univ. of Tokyo, Kashiwa (Japan)
  5. Univ. of California, San Diego, CA (United States)
Publication Date:
Research Org.:
Univ. of California, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division
OSTI Identifier:
1524680
Grant/Contract Number:  
SC0018201
Resource Type:
Accepted Manuscript
Journal Name:
Nature (London)
Additional Journal Information:
Journal Name: Nature (London); Journal ID: ISSN 0028-0836
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; STM; exciton; OLED; phosphorescence; Hubbard NEGF

Citation Formats

Kimura, Kensuke, Miwa, Kuniyuki, Imada, Hiroshi, Imai-Imada, Miyabi, Kawahara, Shota, Takeya, Jun, Kawai, Maki, Galperin, Michael, and Kim, Yousoo. Selective triplet exciton formation in a single molecule. United States: N. p., 2019. Web. doi:10.1038/s41586-019-1284-2.
Kimura, Kensuke, Miwa, Kuniyuki, Imada, Hiroshi, Imai-Imada, Miyabi, Kawahara, Shota, Takeya, Jun, Kawai, Maki, Galperin, Michael, & Kim, Yousoo. Selective triplet exciton formation in a single molecule. United States. doi:10.1038/s41586-019-1284-2.
Kimura, Kensuke, Miwa, Kuniyuki, Imada, Hiroshi, Imai-Imada, Miyabi, Kawahara, Shota, Takeya, Jun, Kawai, Maki, Galperin, Michael, and Kim, Yousoo. Wed . "Selective triplet exciton formation in a single molecule". United States. doi:10.1038/s41586-019-1284-2.
@article{osti_1524680,
title = {Selective triplet exciton formation in a single molecule},
author = {Kimura, Kensuke and Miwa, Kuniyuki and Imada, Hiroshi and Imai-Imada, Miyabi and Kawahara, Shota and Takeya, Jun and Kawai, Maki and Galperin, Michael and Kim, Yousoo},
abstractNote = {The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs). According to a simple model based on spin statistics, the injected charges form spin-singlet (S1) excitons and spin-triplet (T1) excitons in a 1:3 ratio. After the first report of a highly efficient OLED based on phosphorescence, which is produced by the decay of T1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage. Because T1 excitons have lower energy than S1 excitons (owing to the exchange interaction), use of the energy difference could—in principle—enable exclusive production of T1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T1 excitons without creating their S1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.},
doi = {10.1038/s41586-019-1284-2},
journal = {Nature (London)},
number = ,
volume = ,
place = {United States},
year = {2019},
month = {6}
}

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