Significant progress has been achieved on lead halide perovskites in opto-electronic
applications, with 22.1% power conversion efficiency reached in solar cells and 11.7% external
quantum efficiency (EQE) in light-emitting diodes (LEDs). Since the first report of cadmium
selenide (CdSe) quantum dot (QD) LED, various candidates have been considered as potential
emitting materials such as cadmium sulphide (CdS) and cadmium telluride (CdTe). However,
it is only recently that the use of lead halide perovskites in the form of QDs emerged. Cesium
lead halide (CsPbX3, X = Cl, Br, and I) QDs were successfully synthesized by hot injection
method and exhibited promising properties such as high quantum yields (50−90%), short
radiative lifetime (1−29 ns), tunable emission wavelength through the entire visible spectra,
and narrow emission (FWHM ~20 nm). With such properties, the interest in CsPbX3 QDs grew
considerably and their incorporation in LEDs was shortly reported. While the performances of
CsPbX3 QD LEDs have been rapidly improved, with EQE reaching 6.3%, there are several
issues which cause instability of the devices. Most importantly, the widely used organic charge
transport layers have poor stability and cannot efficiently protect the perovskite layer from
moisture in the atmosphere.
In this research, we chose to approach the issue by focusing on metal oxide semiconductors
as charge transport layers. It is known that metal oxides have much higher carrier mobility
through intrinsic or extrinsic doping and excellent stability compared to organic materials.
Therefore, metal oxide charge transport layers play an important role in the effective protection
of the perovskite layer but also possess the adequate energy-levels and charge transport
properties to improve the efficiency. Here, we investigate the light-emission characteristics and
stability of CsPbX3 QD LEDs by using nickel oxide (NiO), molybdenum oxide (MoO3) as the
hole transport layer and zinc oxide (ZnO), titanium oxide (TiO2) as the electron transport layer.
In addition, CsPbX3 QDs were synthesized by the supersaturated recrystallization method. This
method allows the synthesis to be performed in air and at room temperature, as opposed to the
hot injection process which used controlled atmosphere and higher temperatures. The CsPbX3
(X = Cl, and Br) QDs exhibited tunable emission wavelengths between 400 and 520 nm, with
full width half maximum (FWHM) between 15 and 20 nm. Furthermore, we offer important
energy band engineering guidelines for device design with a view to accomplishing both highly
efficient and stable opto-electronic applications.