This image shows novel cadmium selenide (CdSe) quantum dots with ligand enhancement chemistry. The vials on the left contain quantum dots; the vial on the right contains solvent without quantum dots. (Credit: Delaina Amos)
Quantum dots—portions of matter that has electronic properties intermediate between those of bulk semiconductors and those of discrete molecules—are being studied for use in solar cells, transistors, diode lasers and light emitting diodes. A new promising quantum dots application involves combining them with the organic light-emitting diodes (OLEDs). These “hybrid” OLEDs, also called quantum dot LEDs (QD-LEDs), will have increased efficiency and a larger range of colors than conventional ones.
An OLED is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current. This layer of organic semiconductor is situated between two electrodes. Generally, at least one of these electrodes is transparent.
OLEDs are used to create digital displays in devices such as television screens, computer monitors, mobile phones, handheld games consoles and tablets. OLED displays can even be fabricated on flexible plastic substrates leading to the possibility of new applications such as roll-up displays embedded in fabrics or clothing. A major area of research is the development of white OLED devices for use in solid-state lighting applications.
However, commercial manufacturing of the OLEDs is still a complex and expensive process. To make it cheaper and easier, scientists at the University of Louisville in Kentucky are working on new materials and manufacturing techniques like quantum dots and inkjet printing.
According to Delaina Amos, professor at the University of Louisville and principal investigator of the team’s efforts, expense of materials and manufacturing processes has been a major barrier to using OLEDs in everyday lighting devices.
To inexpensively apply the quantum dots to their hybrid devices, the Louisville researchers use inkjet printing, popular in recent years as a way to spray quantum dots and OLED materials onto a surface with great precision. But unlike other groups experimenting with this method, Amos’ team has focused on adapting the inkjet printing technique for use in a commercial setting, in which mass production minimizes expense and translates to affordable off-the-shelf products. “We are currently working at small scale, typically 1 inch by 1 inch for the OLEDs,” Amos says. “The process can be scaled up from here, probably to 6 inches by 6 inches and larger.”
“There’s a reason you don’t see OLED lights on sale at the hardware store,” says Amos, though she adds that they do find uses in small devices such as cameras, photo frames, and cell phone displays. To bring their QD-LEDs closer to becoming market-ready as household lighting appliances, Amos and her team have been synthesizing new, less expensive and more environmentally friendly quantum dots. The team has also modified the interfaces between the quantum dots and other layers of the OLED to improve the efficiency with which electrons are transferred, allowing them to produce more efficient light in the visible spectrum.
In addition to their higher efficiency, wider range of colors, and ability to be applied to flexible surfaces, Amos’ QD-LEDs also use low-toxicity materials, making them potentially better for the environment. “Ultimately we want to have low cost, low toxicity, and the ability to make flexible devices,” Amos says. The team has recently demonstrated small working devices, and Amos adds that she hopes to have larger devices within the next several months.