Strong Induced Chiroptical Effects in Light Emitting Polymer Blends

Current OLED displays rely on a circularly polarised (CP) filter to enhance contrast by
trapping ambient light inside the display. This means that 50% of the randomly polarised
light emitted from each OLED pixel never leaves the screen, halving display efficiency and
decreasing operational lifetime. To overcome this, we pair a chiral small molecule with a
non-chiral device optimised polymer, which allows for CP-dependent applications while
retaining the performance properties of the original polymer. Previously circularly polarised
polymer emission has been achieved by the propagation of linearly polarised light through
cholesteric stacks, where light becomes circularly polarised as it moves through an entire
pitch. For the first time, we demonstrate a system where CP electroluminescence arises due
to both intrinsic emission from chiral dipoles and extrinsic propagation through a cholesteric
stack. Remarkably, this competition results in an inversion of the handedness of the CP EL
emission as a function of film thickness. We compare how the chemical structure of the
polymer and post-deposition processing impacts the chiroptical response of the resulting
device in an effort to provide a set of design rules for future high performance CP-OLEDs.