Polarized-Emitting Quantum Dot LEDs

H. Schlicke, C. Schloen, C.C. Delgadillo, S. Becker, T. Jochum, H. Weller, J. Niehaus
Fraunhofer Center for Applied Nanotechnology CAN,
Germany

Keywords: QLED, LED, quantum dots, polarized emission, polarization, quantum rods, color conversion

Summary:

Linearly polarized light is essential for various technical applications, especially in display technology. Commonly, linearly polarized light is generated by an unpolarized-emitting light source and a subsequent polarizer. While this method yields high degrees of polarization it is accompanied by a significant loss of energy/light intensity, as polarizing filters block light having undesired polarization directions. Upon excitation, anisotropic quantum dots - specifically elongated quantum rods (QRs) - exhibit a strongly polarized emission aligned with their long axis. Individual QRs can reach degrees of polarization of 0.8 (contrast ratios of 90%). However, their use in macroscopic optical components is not straightforward as a long range alignment of the individual QRs has to be achieved to obtain polarized emission from the bulk material. In this contribution we present a method for the deposition of oriented films of highly luminescent QRs based on alignment in electric fields. Our method allows for a lateral variation of the polarization axis, i.e., patterning of the polarization direction. The obtained aligned QR films can serve as polarized color converters for the efficient conversion of blue or short-wavelength unpolarized light into a strongly polarized emission (photoluminescence). Further we show that such films of aligned QRs can be employed as emitter layers in quantum dot light emitting diodes (QLEDs). In these novel polarized-emitting quantum dot LEDs (PEQLEDs) charge carriers are injected into the aligned QR layer and their recombination induces a linearly polarized electroluminescence. PEQLEDs combine the advantages of the emerging QLED technology, i.e., a high color purity and a virtually continuous tunability of the emission wavelength with direct polarized emission. The latter significantly reduces the energy loss in subsequent polarizers or ideally renders them obsolete for certain applications. Here, we report PEQLEDs showing so far unmatched degrees of polarization, which are highly interesting for applications in 3D projection or other display technologies requiring linearly polarized light, such as e.g. head-up displays (HUDs).