We have investigated how to optimize the efficacy and angle dependence of emission of top-emissive organic light-emitting diodes (OLEDs) based on metal foil substrates with the aim of creating efficient flexible devices for lighting and signage applications. By systematically varying the device architecture we were able to tune the optical microcavity which exists within the device structure and observe the change in performance. We paid particular attention to the effects of the metal foil roughness. We have seen that by changing the layer thickness of the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) [PEDOT:PSS] in the device, and the substrate reflectivity and roughness we can obtain efficacies not too far from those achieved for standard bottom-emissive devices on glass substrates made with the same emitter. The angle dependence of luminance can be tuned from pointing in the forward direction via Lambertian to having a maximum at around 65°, and we have used optical modeling to help us find the optimum device structure. We conclude that (rough) metal foils are a realistic possibility for making flexible OLEDs and have demonstrated large area (up to 12 cm × 12 cm), thin film encapsulated, flexible devices.
Understanding of the charge transport properties is of great importance for the operation and the efficiency of polymer based light-emitting diodes (LEDs). We investigate the charge transport in hole-only diodes based on poly(p-phenylene vinylene) (PPV) as function of temperature T, charge carrier density p and electric field E. At low voltages the hole mobility is independent on the electric field and charge carrier density. At high voltages both the charge-carrier density and electric-field dependence of the mobility have to be taken into account to describe the hole transport in polymer LEDs.
The hole transport in various poly(p-phenylene vinylene) (PPV) derivatives has been investigated in hole-only diodes as function of temperature T and applied electric field E. A difference of three decades has been found in the hole mobility between a random copolymer with asymmetric sidechains and a PPV-derivative with symmetric sidechains. The temperature dependence of the hole mobility has been analysed within the correlated Gaussian disorder model. The large differences in the mobility values of these PPV derivatives are governed by a strong decrease of the energetic disorder. The high mobility PPV-based polymers are interesting candidates for being used as hole transport layers in heterojunction light-emitting diodes.
The hole transport in the amorphous poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10-PPV) and in the more ordered poly[2,5-bis(3’,7’-dimethyloctyloxy)-p-phenylene vinylene] (OC10C10-PPV) has been investigated both in field-effect transistors (FETs) and light-emitting diodes (LEDs) as function of temperature and applied voltage. From J-V measurements on LEDs a difference of 15x has been found in the hole mobility between OC1C10-PPV and OC10C10-PPV. In FETs the dependence of the field-effect mobility on the carrier density is much stronger in OC1C10-PPV than in OC10C10-PPV. These differences in the mobility in both FETs and LEDs are determined by the difference in microscopic transport parameters between the two materials, which results from a different ordering in the polymeric film of the PPV derivatives. Due to their specific chemical composition OC1C10-PPV is an amorphous polymer and the transport is the same in all directions, while OC10C10-PPV is more ordered and the transport shows anisotropy between sandwich and in-plane devices.