On the formation mechanism for electrically generated exciplexes in a carbazole-pyridine copolymer

• Published in: Journal of polymer science. Part B, Polymer physics Vol. 50; no. 5; pp. 361 - 369
• Main Authors: Hayer, Anna, Van Regemorter, Tanguy, Höfer, Bianca, Mak, Chris S. K., Beljonne, David, Köhler, Anna
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01.03.2012; Wiley
• Subjects: Applied sciences; Carbazoles; charge transfer; Copolymers; Electrical, magnetic and optical properties; Exact sciences and technology; Excitation; light-emitting diodes (LED); Luminescence; Mathematical analysis; Organic polymers; photophysics; Physicochemistry of polymers; Precursors; Properties and characterization; Quantum chemistry; Segments; UV-vis spectroscopy; Exciplex; Quantum chemistry; Carbazole derivative copolymer; Photoluminescence; UV-vis spectroscopy; Experimental study; Modeling; charge transfer; Luminescence decay; Conjugated copolymer; luminescence; Optical properties; Aromatic copolymer; Pyridine derivative copolymer; Alternating copolymer; light-emitting diodes (LED); photophysics; Formation mechanism; Optical absorption
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.23011

Although carbazole‐containing copolymers are frequently used as hole‐transporting host materials for polymer organic light‐emitting diodes (OLEDs), they often suffer from the formation of undesired exciplexes when the OLED is operated. The reason why exciplexes sometimes form for electrical excitation, yet not for optical excitation is not well understood. Here, we use luminescence measurements and quantum chemical calculations to investigate the mechanism of such exciplex formation for electrical excitation (electroplex formation) in a carbazole–pyridine copolymer. Our results suggest that the exciplex is formed via a positively charged interchain precursor complex. This complex is stabilized by interactions that involve the nitrogen lone pairs on both chain segments. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 Understanding the formation of exiplexes is essential to improving device efficiency of organic light‐emitting diodes (OLEDs). Using a copolymer model compound, the formation of exciplexes in operating OLEDs is shown to proceed by a two‐step mechanism associated with nitrogen containing moieties. A hole localizes on the carbazole moiety and forms a charged inter‐chain precursor complex, resulting in exciplex formation when an electron gets trapped on the precursor complex. Quantum chemical calculations demonstrate the role of a specific nitrogen‐–nitrogen interaction in this process.