High-Efficiency Phosphorescent Hybrid Organic−Inorganic Light-
Emitting Diodes Using a Solution-Processed Small-Molecule Emissive
Layer
Changjun Fan, Yong Lei, Zhen Liu, Ruixue Wang, Yanlian Lei, Guoqing Li, Zuhong Xiong,
and Xiaohui Yang *
School of Physical Science and Technology, Southwest University, Chongqing 400715, P. R. China
*
S
Supporting Information
ABSTRACT: The morphology and optical and electrical properties of solution-processed and vacuum-deposited 4,4′,4″-
tris(carbazol-9-yl)triphenylamine (TCTA):2,2′-(1,3-phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) composite
films are investigated. All of the films exhibit smooth and pinhole-free morphology, while the evaporated films possess enhanced
carrier-transport properties compared to solution-processed ones. The close correlation between the carrier-transport feature and
the packing density of the film is established. High-efficiency monochromatic and white phosphorescent hybrid organic−
inorganic light-emitting diodes with solution-processed small-molecule emissive layers are reported: the maximum external
quantum efficiencies of blue, yellow, and red devices are 18.9, 14.6, and 10.2%, respectively; white devices show a maximum
luminance efficiency of 40 cd A
−1
and a power efficiency of 20.8 lm W
−1
at 1000 cd m
−2
. The efficiencies of blue, red, and white
devices represent significant improvement over previously reported values.
KEYWORDS: hybrid organic−inorganic light-emitting diodes, solution-processed small-molecule films, vacuum-deposited films,
carrier-transport property, packing density of the film
1. INTRODUCTION
Hybrid organic−inorganic light-emitting diodes (HyLEDs)
using air-stable metal oxide carrier-injection layers overcome
several issues that may adversely affect the efficiency and
stability of conventional devices such as the sensitivity of a low-
work-function metal cathode- a nd electron-injection layer
toward oxygen and moisture in the ambient and the
interactions between the conducting polymer hole-injection
layer and indium−tin oxide (ITO), providing a novel approach
for the development of new display and solid-state lighting
devices.
1−8
Metal oxides are particularly suitable to work as
carrier-injection layers in HyLEDs because they possess many
appealing properties, for instance, exceptional stability, ease of
synthesis, visible-light transparency, high carrier mobility, and
controllable surface morphology on a nanometer length scale.
In 2006, Morii et al. reported the first HyLED with the
structure of ITO/TiO
2
/poly(9-dioctylfluorene-al t-benzothia-
diazole) (F8BT)/MoO
3
/Au, having a luminance efficiency
(LE) of ca. 0.1−0.3 cd A
−1
.
1
After intensive studies over the
past decade, the LE of HyLEDs based on F8BT has been
increased to 61.6 cd A
−1
.
9
However, compared to conventional
devices, the efficiency and operating stability of HyLEDs
warrant further improvement.
In our opinion, there are several problems to be solved in the
area of HyLEDs, which are listed as the following: (1)
Conjugated polymers such as F8BT
6−11
and phenyl-substituted
poly(phenylvinylene) (SY-PPV)
5,12,13
have been mainly used as
the emissive layer (EML) in HyLEDs. As a result, the device
efficiency is restricted by the singlet-to-triplet exciton formation
ratio. Meanwhile, the low-energy nonemissive triplet excited
states of conjugated polymers signifi cantly quench the
luminescence of phosphorescent emitters, which makes such
polymers unsuitable to work as the host materials for
phosphorescent devices. (2) Widely studied F8BT and SY-
Received: June 29, 2015
Accepted: September 3, 2015
Published: September 3, 2015
Research Article
www.acsami.org
© 2015 American Chemical Society 20769 DOI: 10.1021/acsami.5b05815
ACS Appl. Mater. Interfaces 2015, 7, 20769−20778