Inkjet-Printed Quantum Dot–Polymer Composites for Full-Color AC-Driven Displays

Adv. Mater. 2009, 21, 1–5 2009 WILEY-VCH Verlag Gmb We demonstrate print-deposition of high resolution, patterned, multicolored thin films of luminescent colloidal quantum dot (QD)-polymer composites and use the printed patterns in fabricating robust, bright, full-color AC-driven displays. The benefits of AC electroluminescent (EL) displays include simple, low-cost fabrication and high reliability; however, finding efficient and stable phosphors for full-colored displays remains a major challenge. Today, red-, green-, and blue-light-emitting (RGB) phosphors for AC-EL comprise different material systems and can have luminous efficiencies that span an order of magnitude, rendering manufacture of a multicolor display with balanced RGB components difficult. While there has been substantial research on stacked and patterned devices, one of the most feasible strategies for realizing RGB AC-EL displays is color filtering of white, ZnS-based phosphors. Filtering, however, wastes up to 90% of the output optical power to achieve color saturation, which requires that the display be operated at ten times video brightness in order to meet the RGB color standard. This results in greater power consumption, faster pixel degradation, and shorter display lifetimes. To achieve full-color AC-EL displays, in this study we use colloidally synthesized QDs to print patterns of robust, solution-processable, luminescent light-converting thin films for AC-EL displays. QDs offer narrow-band luminescence that can be tuned across the visible spectrum by varying their size and chemical composition. This property motivated the use of QD luminescent centers in thin-film light-emitting diode (LED) structures, in which QDs were electrically excited to demonstrate low-power, color-saturated devices, with side-by-side patterning of RGB pixels enabled by microcontact printing of the QD layers. In alternate device designs, QDs can also be optically excited, where, for example, the small Stokes shift of redand green-light-emitting QDs enables their optical excitation by blue light. This property of QDs led to the demonstration of optical down-conversion using blue GaN LEDs to excite QDs in polylaurylmethacrylate (PLMA), and the generation of point sources of saturated-color light. In the present work, we demonstrate a planar, full-color AC-EL display that comprises luminescent thin films of QDs that absorb blue electroluminescence from a commercial phosphor powder and then emit photons at a longer wavelength characteristic of the QD band gap. Emission and absorption spectra for the QDs and EL phosphors