Understanding the role of ultra-thin polymeric interlayers in improving efficiency of polymer light emitting diodes
- Department of Physics and Centre for Plastic Electronics, South Kensington Campus, Imperial College London, London SW7 2AZ (United Kingdom)
- Department of Physics, University of Bath, Bath BA2 7AY (United Kingdom)
Insertion of ultra-thin polymeric interlayers (ILs) between the poly(3,4-ethylenedioxythiophene):polystyrene sulphonate hole injection and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) light emission layers of polymer light emitting diodes (PLEDs) can significantly increase their efficiency. In this paper, we investigate experimentally a broad range of probable causes of this enhancement with an eye to determining which IL parameters have the most significant effects. The importance of hole injection and electron blocking was studied through varying the IL material (and consequently its electronic energy levels) for both PLED and hole-only diode structures. The role of IL conductivity was examined by introducing a varying level of charge-transfer doping through blending the IL materials with a strong electron-accepting small molecule in concentrations from 1% to 7% by weight. Depositing ILs with thicknesses below the exciton diffusion length of ∼15 nm allowed the role of the IL as a physical barrier to exciton quenching to be probed. IL containing PLEDs was also fabricated with Lumation Green Series 1300 (LG 1300) light emission layers. On the other hand, the PLEDs were modeled using a 3D multi-particle Kinetic Monte Carlo simulation coupled with an optical model describing how light is extracted from the PLED. The model describes charge carrier transport and interactions between electrons, holes, singlets, and triplets, with the current density, luminance, and recombination zone (RZ) locations calculated for each PLED. The model shows F8BT PLEDs have a narrow charge RZ adjacent to the anode, while LG 1300 PLEDs have a wide charge RZ that is evenly distributed across the light emitting layer. Varying the light emitting layer from F8BT to Lumation Green Series 1300, we therefore experimentally examine the dependence of the IL function, specifically in regard to anode-side exciton quenching, on the location of the RZ. We found an exponential dependence of F8BT PLED luminance on the difference, δ, in the highest occupied to lowest unoccupied molecular orbital energy gap between the light emitting polymer and a semiconducting polymeric IL, with δ consequently the most important parameter determining efficiency. Understanding the exponential effect that wider energy gap IL materials have on exciton quenching may allow δ to be used to better guide PLED structure design.
- OSTI ID:
- 22304324
- Journal Information:
- Journal of Applied Physics, Vol. 115, Issue 20; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-8979
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
SUPERCONDUCTIVITY AND SUPERFLUIDITY
CHARGE CARRIERS
COMPUTERIZED SIMULATION
CURRENT DENSITY
DIFFUSION BARRIERS
DIFFUSION LENGTH
EMISSION
ENERGY GAP
INTERACTIONS
LAYERS
LIGHT EMITTING DIODES
MIXING
MOLECULAR ORBITAL METHOD
MOLECULES
MONTE CARLO METHOD
OPTICAL MODELS
POLYSTYRENE
QUENCHING
RECOMBINATION
THICKNESS
VISIBLE RADIATION