Doh C. Lee, Korea Advanced Institute of Science and Technology (KAIST)

Semiconductor nanocrystal quantum dots (QDs) represent an important class of chemically processible nanomaterials. Their use in solar energy conversion and light-emitting applications has garnered attention, as their moderate synthesis temperature and solution-processibility offer opportunities to address constraints on alternative technologies such as scalability. Additionally, by tuning the size, shape, and composition of colloidal nanocrystals, we can exploit quantum confinement effects, e.g., the size-tunable energy gap and discrete energy states. In this presentation, I will discuss strategies (i) to modify the surface of QDs to stabilize their optoelectronic properties and (ii) to design heterostructure QDs with suppressed multi-exciton Auger recombination and enhanced electroluminescent efficiency.

PbSe QDs have received deserving attention because of their interesting new photophysical properties, such as increased carrier multiplication efficiency [McGuire et al., Acc. Chem. Res. 2008, 41, 1810], retarded intraband relaxation [Wehrenberg et al., J. Phys. Chem. B 2002, 106, 10634] and long-lived hot carriers [Tisdale et al., Science 2010, 328, 1543]. However, integration of PbSe QDs into functional devices has been impeded due to their well-reported instability in ambient conditions: for example, PbSe QD-based transistors degrade in air in a matter of seconds! Simple mixing of halide salts, e.g., NH4Cl, with PbSe QD crude solution does the trick and halide-treated PbSe QDs are stable for weeks in solution. In addition, films and devices made with halide-treated PbSe QDs show unprecedentedly stable operation. In this presentation, I will discuss the role of halide salts in stabilizing PbSe QDs by monitoring X-ray photoelectron spectroscopy data and calculating bonding energies of Pb-halide atomic layer via density-functional theory.

We also designed type-I giant QDs core/shell structures with photoluminescence quantum yield close to unity: we prepared a series of highly luminescent CdSe/Zn1-XCdXS core/shell type-I QDs with CdSe core radius of 2.0 nm but different Zn1-XCdXS shell thicknesses (4.5 nm  total radius  8.3 nm) by the layer-by-layer deposition method. Systematic investigation of QD-based light-emitting diodes (QLEDs) and spectroscopic analysis of QD films suggest that thick-shell QDs exhibit reduced Auger-type decay rate and suppressed energy transfer within QD solids. The photophysical changes are responsible for alleviated efficiency roll-off and improved stability in QLEDs. The findings are highlighted in device characteristics: high device efficiency (peak EQE ~ 7.4 %) and the record-high brightness (> 100,000 cd/m2) along with improved device stability. The thick-shell approach represents a simple yet novel structural design of core/shell heterostructure QDs to engineer the optical properties of QD solids, and thus offer a rational guideline for the practical use of QLEDs in displays, lightings and laser applications.