excerpts from
Colloidal quantum dots (QDs) exhibit unique properties such as size-controllable color tunability by the quantum confinement effect, narrow emission linewidth and low-cost solution-based processing, which make QD a rising candidate material for optoelectronic device such as lasers, photodetectors, solar cells and light-emitting diodes (LEDs) [1–3]. Among these applications, colloidal QDs based LEDs (QD-LEDs) has been investigated popularly because it’s potential impacts to the display and lighting industry. Many research groups have attempted to enhance device performance using various QD compositions and device structures [4–6]. While the performance of QD-LEDs using II-VI semiconductor cadmium selenide (CdSe) colloidal QDs has been upgraded dramatically, a prominent future task is the substitution of Cd-containing QDs by less toxic materials. There have been only a few studies on QD-LEDs using Cd-free QDs [7–12]. Indium phosphide (InP) among III-V semiconductor nanocrystals is the most promising materials for Cd-free QD-LEDs owing to less ionic lattice, reduced toxicity and wide emission spectrum tunability covering the range of visible light [13–15]. The electroluminescent (EL) performance of QD-LEDs using InP QDs can be improved by the optimization of device structure and materials for the carrier balance in the multi-layered LED structure. However, the reported EL performance is insufficient and there are still many problems left such as EL quenching by Auger recombination, efficiency roll-off and device degradation, which limits the performance of QD-LEDs [16].
In this work, efficient Cd-free QD-LEDs were fabricated using InP/ZnSe/ZnS multishell QDs synthesized by the heating-up method [17]. The thicker ZnS shell of multishell QDs was adapted to improve the device efficiency, and the optoelectronic characteristics of QD-LEDs were investigated. Moreover, a serious QD charging problem was found in EL stability measurement, however it was also suppressed by the QD system of thicker ZnS shell to some extent.
The performance of our InP-based QD-LEDs can be matched to that of traditional Cd-based QD- LEDs, which shows the great potential for the Cd-free future information displays. With further investigation, the performance of InP-based QD-LEDs would be more improved by optimizing the core/multishell structure of QDs and charge balance in multi-layered system.
.......anyway one can get an inkling as to why InP/ZnS and why more research
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