Stability and degradation mechanisms of blue light-emitting organic semiconductors and devices

Abstract

There has been a significant interest in polymer light-emitting diodes (PLEDs) due to their potential applications in efficient displays and large area light sources at low cost. In comparison to their red and green counterparts, the relatively poor operational stability of blue PLEDs is the biggest challenge faced in the commercialisation of PLED devices. This thesis is concerned with the stability issues of blue light-emitting fluorene-based conjugated polymers and small molecules towards highly efficient and longer lived blue PLEDs by elucidating underlying degradation mechanisms.

Poly(9,9-di-octylfluorene) (F8) is known to have different morphological phases including amorphous, liquid crystalline, crystalline and a phase with an additional degree of ordering termed the β-phase. The effects of different side groups in polyfluorenes are studied to understand their influence on structural and optoelectronic properties. The employment of bulky side-groups restricts the formation of the liquid crystalline and beta phases observed in F8. The photoluminescence quantum yield is larger (from ~35 % to 50 %) for polyfluorene chains with bulkier side-groups and their excitons are found to be longer lived (from ~0.35 ns to 0.45 ns), due to reduced non-radiative decay rate resulted from weaker intermolecular interaction.

UV excitation is used as a tool to investigate the photo-stability of blue light-emitting polymers. The photo-stability of different phases of F8 and F8 derivatives are studied under ambient and inert conditions. UV exposure in air causes disorder in the F8 polymer chains due to the formation of defect sites such as fluorenone. Interestingly, the stability of different phases vary and the results show crystalline and beta phases to be most stable, with least degradation in their optoelectronic properties due to their structural ordering. The formation of these ordered phases increases the density of the film thereby decreasing the degradation caused by penetration of oxygen molecules. On the other hand, after UV exposure in nitrogen, there are indications that high energy conformational states of polymer are more reactive to photoexcitation and side-chains are still susceptible to degradation as cross-linking occurs in all films. Polyfluorene with different side-groups attached exhibit a greater stability to UV photoexcitation, as evidence from optical spectroscopy, although these molecules initially have a more disordered structure. This leads to the conclusion that photo-stability is dependent on the chemical structure of the side-groups and this is proven by degradation of the polymers in dilute solutions.

The optoelectronic properties and photo and electrical stability of a series of poly-(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (F8-PFB random copolymer) consisting of different fractions of F8 and PFB units are examined. A Raman spectroscopy study reveals that the PFB units are the reactive site for UV degradation in these copolymers, hence, decreasing the fraction of PFB units improves the copolymer photo-stability. The formation of di-radicals on the PFB unit is considered to be the cause of luminescence quenching. By combining knowledge obtained from homopolymer and copolymer degradation; a highly efficient (>3 cd/A), deeper blue CIE colour coordinate (0.14, 0.12) and longer-lived device is demonstrated using a model 5 mol % PFB containing random copolymer.

Finally, small molecule fluorene stability is studied to determine degradation mechanisms that could be applied to polyfluorene systems. The F8 trimer displays ketone defects when degraded in air. The crystallised F8 and fluorene trimers with bulkier side-groups demonstrate stability greater than that of amorphous phase of F8 trimer. Evidence of chain scissioning after photoexcitation is observed and attributed to the cause of reduced photoluminescence found after degradation in blue light-emitting fluorenes.