Light-emitting diodes (LEDs) containing semiconducting nanocrystals called quantum dots are ideal for applications such as large-panel displays and solar cells thanks to their high efficiency and colour purity. To date, the chief drawback of these quantum-dot LEDs has been their toxicity, since most contain cadmium or other heavy metals. Now, however, a team of researchers at Samsung in South Korea has engineered cadmium-free light emitters with an efficiency, brightness and lifetime comparable to those of their environmentally unfriendly predecessors.
Quantum dots (QDs) emit light via a process known as radiative recombination. When an electron in the valence energy band within the QD absorbs a photon and moves to the conduction band, it leaves behind an electron vacancy, or hole. The excited electron and hole then recombine, releasing a photon.
The first photoluminescent QDs contained cadmium (Cd) and were made by coating the semiconducting nanocrystals with organic molecules. In later versions, researchers surrounded the semiconductor core with a shell of a different semiconducting material with a large bandgap (that is, a large energy difference between the valence and conduction bands). In such a structure, the bandgap prevents electrons and holes at the core from escaping to the QD’s external surface, making the device intrinsically photoluminescent.
A promising alternative
Indium phosphide (InP)-based QDs are promising alternatives because their photoluminescence quantum yield – the number of photons emitted by the QD, divided by the number absorbed – is high, at 93%. Even so, the light-emitting performance of (InP)-based LEDs has lagged those of their Cd-containing cousins for reasons that are thought to derive from structural defects in the material. These defects reduce the external quantum efficiency (the number of photons exiting the LED, divided by the number of charges injected into it) of InP devices to just 12.2%.
A team led by Eunjoo Jang of the Samsung Advanced Institute of Technology in Suwon overcame this problem by synthesizing QDs from a uniform InP core, a thick inner shell of zinc selenide (ZnSe) and a thin outer shell of zinc sulphide (ZnS). The researchers made their QDs in a two-step process, using hydrofluoric acid to etch out the oxidized core surface during the initial growth of the ZnSe shell, then annealing the core/shell interface at 340 °C. This approach eliminates the efficiency-sapping defects at the interface, they say.
Suppressing undesirable effects
The resulting structures have a highly symmetric spherical shape, with a potential energy profile that gradually increases with distance from the centre. This so-called soft confinement potential reduces the rate of Auger recombination, which occurs when the energy of the photoexcited electron-hole pair is transferred to another electron or hole without a photon being emitted. The uniform shape of the QDs also reduces the number of cavities or sharp corners on the surface of the QD or at the core-shell interface, which helps to suppress Auger recombination still further.
To evaluate the light-emitting properties of their QDs, the Samsung team used them to make LEDs into which they injected electrons and holes. The measured external quantum efficiency for their device was 21.4%, the theoretical maximum for QD-based LEDs. The devices also boasted a maximum brightness of 100,000 candelas/m2 and a lifetime of around a million hours at 100 candelas/m2.
These values are comparable to those of state-of-the-art devices containing cadmium, the researchers say. As such, Jang says that the Samsung team’s InP-based QD-LEDs could soon be useable in next-generation commercial displays, with TV displays being one of the most promising immediate applications.
“Each of the values we measured is a record for cadmium-free QD-LEDs,” she tells Physics World. “Our results clearly show that InP can overcome previous concerns that QDs based on this material are difficult to fabricate perfectly – especially for applications that require a very high external quantum efficiency, like displays.”
The researchers detail their work in Nature.