A Study of Light-Emitting Diodes and Transistors based on Ambipolar Organic Materials

Abstract

This thesis is concerned with the incorporation of a range of fluorescent ambipolar materials into Organic Light-Emitting Diodes (OLEDs) and transistors and the characterisation of their optoelectronic properties.

Initial studies focused on the incorporation of donor-acceptor compounds based on carbazole-fluorene-oxadiazole in OLEDs. Using this approach, a simple, efficient, deep-blue device was realised, with Commission Internationale de l’Eclairage, CIE (x, y) coordinates (0.16, 0.08), very close to the blue standard of (0.14, 0.08) defined by the National Television System Committee. The devices also exhibited one of the highest efficiencies reported for simple deep-blue OLEDs at 4.71%. Further experiments revealed that the efficiency was dependent on the thin film processing technique used in the device fabrication, with thermally evaporated layers of active materials showing enhanced properties, compared to spin-coated films.

The development of OLEDs incorporating structural analogues of the deep-blue emitting carbazole molecule revealed how molecular modification can be used to tune the emission colour of OLEDs, and was found to fit well with the calculated band gap for each molecule. Removing fluorene from the fluorescent materials resulted in a very deep-blue emission, with CIE (x, y) coordinates of (0.16, 0.05), but a simultaneous drop in efficiency. Therefore it was shown that carbazole, not fluorene, was responsible for the deep-blue colour observed during light emission. These experiments revealed that the combination of carbazole separated from oxadiazole (OXD) by the fluorene group is required to achieve high efficiency.

Improvement of efficiency through the chemical addition of OXD groups was also explored as an alternative approach to adding an electron transporting OXD7 layer in the device structure. Addition of two OXD groups was found to result in a complex and more white emission.

Blending of the deep-blue emitting carbazole compound with a yellow-emitting phosphorescent iridium dye produced a single-active-layer white-emitting OLED with CIE (x, y) coordinates (0.30, 0.31), very close to the pure white point of (0.33, 0.33). The efficiency of the device was shown to improve with addition of an OXD7 layer, resulting in simple, white-emitting OLEDs, with an external quantum efficiency of 2.5%. White OLEDs were also tested based on the blending of yellow dye with a carbazole analogue consisting of chemically attached OXD groups, but the efficiency was lower at 0.42%.

Incorporation of the ambipolar donor-acceptor compounds into organic field-effect transistors (OFETs) generally resulted in electrical behaviour that was atypical of a classical transistor, and apparent ambipolar conduction was identified as an artefact due to current from the gate contact leaking to the drain. Further experiments utilised pentacene as a hole transporter to favour ambipolar conduction, but measurements confirmed that the properties of the resulting devices resembled pentacene-only OFETs.

Further study concentrated on devices based on the ambipolar polymer F8BT. The development of in-plane OLEDs was used to characterise the electroluminescent properties of the material. Several contradictions with literature reports were identified during the study of F8BT, including the correct solvent for spin-coating and the effect of annealing temperature. Notably, high temperature annealing was linked to a reduction in F8BT film crystallinity, as identified by Atomic Force Microscopy (AFM) and a resulting instability in electrical characteristics.

Blending of F8BT with an ionic liquid (IL) in a transistor structure produced a device that emitted light, although further study suggested the device in fact behaved as an OLED. However, IL blended with F8BT was successfully used in place of calcium and PEDOT:PSS to produce functioning F8BT OLEDs.