Control of Circularly Polarised Electroluminescence in Polymer Light Emitting Diodes
File(s)
Author(s)
Wan, Li
Type
Thesis
Abstract
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).
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).
Version
Open Access
Date Issued
2020-08
Date Awarded
2020-11
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Campbell, Alasdair
Fuchter, Matthew
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)