1. Employing ∼100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and Charge-Transfer Excited State.
- Author
-
Li, Weijun, Pan, Yuyu, Xiao, Ran, Peng, Qiming, Zhang, Shitong, Ma, Dongge, Li, Feng, Shen, Fangzhong, Wang, Yinghui, Yang, Bing, and Ma, Yuguang
- Subjects
- *
ORGANIC light emitting diodes , *FLUOROPHORES , *CHARGE transfer , *EXCITED states , *QUANTUM efficiency , *ELECTROLUMINESCENCE - Abstract
In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron-hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor-acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA-NZP as the light-emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep-red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m−2 and a maximum external quantum efficiency (ηext) of ∼2.8%. The HLCT state is a new way to realize high-efficiency of EL devices. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF