Control of Circularly Polarised Electroluminescence in Polymer Light Emitting Diodes

State of the art Organic Light Emitting Diode (OLED) displays rely on a circularly polarised filter to improve contrast by blocking the internal reflection of the ambient light. This cuts out 50 % of light generated by an OLED panel, which limits device efficiency and operational lifetime. Direct circularly polarised (CP) emission from polymer light emitting diodes (PLED) will pass through the filter without a loss of intensity, which will lead to displays that are more efficient with improved operational lifetimes. The key challenge for the application of CPPLED in real world display technologies is to simultaneously achieve high device performance and high electroluminescence dissymmetry (gEL). High gEL relies on the development of materials with strong high photoluminescence dissymmetry(gPL). More importantly, the optimisation of the devices is essential to maintain strong emission dissymmetry without sacrificing the device performance. Firstly, this thesis demonstrates CP-PLED with high gEL by blending aza[6]H helicene in a wide range of polyfluorene polymer including Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT, Chapter 4 and 5), Poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2, Chapter 6) and Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO, Chapter 7). These results show that the blend system of chiral additive and achiral light emitting polymer is a simple and general strategy to achieve CP-PLED with high gEL. Particularly, F8BT devices in Chapter 5 reaches the highest known devices performance amongst all CP-PLED and PFO devices in Chapter 7 reaches the highest device performance of all blue CP-PLEDs. Secondly, this work also demonstrates several novel chiroptical phenomena that have not been observed before. Chapter 4 reports the first gEL and gPL inversion as a function of active layer thickness. Chapter 5 demonstrates the first example in organic system where manipulation of the device architecture inverts the gEL due to the opposite direction of the driving current. Chapter 6 reports the first system where gEL and gPL from the same active layer exhibit opposite handedness. Chapter 7 demonstrates the first chiral β-phase emission. Finally, we show that the original of CP-EL is more complicated than CP-PL and circular dichroism. In the CP-PLED system, the apparent gEL is the result of both intrinsic chirality that is originated from chiral chromophores and extrinsic chiral medium (Chapter 4, 6 and 7). It can be further impacted by the thickness of the active layer, metal layer (Chapter 4) and transport layer (Chapter 5) and inverted by the device structure (Chapter 5).Open Acces