Light Sources.
Since their inception, electrically driven colloidal quantum-dot light-emitting devices (QD-LEDs) have increased in external quantum efficiency from less than 0.01% to around 20%. The high luminescence efficiency and uniquely size-tunable colour of solution-processable semiconducting colloidal QDs highlight the potential of QD-LEDs for use in energy-efficient, high-colour-quality thin-film display and solid-state lighting applications. The major advantage of such nanostructures is their tunability in terms of size and composition, which allow for a systematic variation of their emission energy. Also, the adoption of a proper architecture has been shown to produce high-efficiency quantum dot LEDs, approaching their theoretical limit, by sandwiching a thin layer of emissive core-shell dots between electron- and hole-transport layers.
Current Projects.
Spectral Diffusion and Charge Transfer Dynamics of InP QDs.
Quantum dots (QDs) have emerged as a leading light-emitting material, with extremely high quantum efficiencies, narrow emission linewidths, synthetic flexibility, and high operating stability. The demand for energy efficient displays utilizing environmentally-benign materials has expanded efforts beyond high-performing yet heavy metal-containing QDs towards less toxic materials with comparable optical properties. We use time-resolved optical microscopy to probe the spectrally-resolved decay dynamics of InP/ZnSe/ZnS QD thin films and light-emitting diodes (QD-LEDs) to reveal the interplay between carrier diffusion, charge transfer, and exciton dissociation in the absence and presence of external fields. We investigate wavelength-dependent energy transfer rates and quantify two energy relaxation rates corresponding to spectrally-distinct populations of mobile and immobile field-screened photogenerated or field-ionized carriers. We also study the photoluminescence (PL) quenching of QD-LEDs in reverse bias towards efficient voltage-controlled optical downconverters, informing the rational design of Cd-free, high-efficiency emitters and devices for next-gen displays.
Anisotropic QD-LEDs.
Quantum Dot Light Emitting Diodes based on Anisotropic Colloidal Heterostructures
ONE Lab: Giovanni Azzellino, Vladimir Bulović
Collaborators: Igor Coropceanu, Moungi G. Bawendi
Additional emergent properties in quantum dot LEDs can be attained by changing the shape of these nanostructured materials, such as by transitioning from spherical quantum dots to elongated structures called nanorods (NRs). A key feature that distinguishes nanorods from quantum dots is that the former emit light with a high degree of linear polarization oriented along their long axis. By orienting nanorods parallel to the substrate, the preferential emission of light perpendicular to the plane of the substrate will automatically reduce outcoupling losses, thus allowing higher external quantum efficiencies than can be achieved with QDs in conventional devices. Moreover, by aligning the nanorods in a particular direction, a simple scheme for the fabrication of polarized LEDs can be developed.
Figure: TEM of self-aligned CdSe-CdS nanorods drop casted on glass
Inkjet Printing of QD-LEDs.
Inkjet Printing High-Resolution Patterning of Quantum Dot LEDs
ONE Lab: Giovanni Azzellino, Vladimir Bulović
Collaborators: Francesca S. Freyria, Igor Coropceanu, Moungi G. Bawendi
The high luminescence efficiency and uniquely size-tunable color of solution-processed semiconducting colloidal quantum dots (QDs) make them promising candidates as optically- and electrically-excited luminophores in energy-efficient, substrate-independent, high-color-quality solid-state lighting and thin-film display technologies. Recent advances in the design of electrically-driven QD-LEDs have pushed their external quantum efficiencies toward 20%, comparable to those of phosphorescent organic LEDs. However, the path of these devices to market is hampered by the difficulty of patterning the emissive quantum-dot layer. Existing prototypes are manufactured by spin-coating the colloidal QDs. Furthermore, the thermal budget of these materials remains low—heating them up decreases their photoluminescence efficiency.
Given all of these constraints, inkjet printing is a good candidate for making patterned QD-LEDs. This technology offers a new and unexplored technique for room-temperature, maskless patterning of quantum dot light-emitting devices.
In this project, we exploit droplet-on-demand inkjet technology to manufacture electroluminescent devices. We adopt both surface treatments and solvent engineering approaches to get rid of the “coffee-stain” effect and to deploy uniform and continuous spots of emissive quantum dots with a lateral resolution of ~10µm, using single-droplet print with a commercial inkjet printer. In addition, with the latest generation of hybrid QD-LED architectures, we show that both visible- and near-infrared-emitting QD-LEDs can be patterned with inkjet technology. We believe this work can open the door to high-resolution QD-LED displays.
Figure: Optical profilometry of an array of infrared core-shell PbS-CdS quantum dots inkjet-printed onto ZnO (scale bar is 50 μm)