Proceedings Article | 20 November 2007
KEYWORDS: Organic light emitting diodes, Molecules, Quantum efficiency, Crystals, Luminescence, Electroluminescence, Argon, Molecular interactions, Chromophores, Aluminum
For stable and efficienct organic light-emitting diodes, it is essential to find molecules with
high photoluminescent efficiency, little self-quenching and balanced charge transporting properties.
Recently, we've designed and synthesized some highly emissive naphtho[2,3-c][1,2,5]thiadiazole
(NTD) derivatives and naphtho[2,3-c][1,2,5]selenadiazole (NSeD) derivatives with unusual
ambipolar transporting properties.
The ambipolar transporting properties of the NTDs were explained by Marcus theory with
carrier reorganization energies and charge-transfer integrals. We obtained high quality single
crystals of 4,9-di(biphenyl-4-yl)-naphtho[2,3-c][1,2,5]thiadiazole (NTD02) and
4,9-bis(4-(2,2-diphenylvinyl)phenyl)-naphtho[2,3-c][1,2,5]thiadiazole (NTD05). They have
disordered NTD rings' orientation with the opposite directions in the center of the molecule
because of NTD's planar configuration and the single-bond connection with the phenyl
substituents. The packing structure of NTD02 shows the planar arrangement of NTD rings,
forming a "charge transporting channel". Quantum calculation also confirms that the π-π stacking
interaction in NTD derivatives benefits the charge transporting via intermolecular hopping on
NTD rings. The hole and electron mobilities of NTD05 are 7.16×10-4 cm2/V·s and 6.19×10-4
cm2/V•s at an electronic field E = 2.0×105 V/cm, respectively. The hole mobility of NTD05 is
close to that of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'-diamine (NPB) and the
electron mobility of NTD05 is two orders-of-magnitude higher than that of
tris(8-hydroxyquinoline) aluminum (Alq3). For the NTD derivatives, NTD05 also shows the best
performance in non-doped OLEDs. CIE coordinates of (0.65, 0.35) and a peak efficiency of 2.4%
are achieved for a double layer OLED with NPB as the hole transporting layer and NTD05 as the
emitting layer. Moreover, we get ultimate red emission with CIE coordinates of (0.71, 0.29) for
some of the NSeD based non-doped OLEDs.
