Polyfluorenes often show both excimer and aggregate formation upon thermal annealing or when current is passed through them. Excimer formation involves the generation of dimerized units of the polymer which emit light at lower energies than the polymer itself. This hinders the use of polyfluorenes for most applications, including light-emitting diodes (LED). When excimer or aggregate formation occurs this lowers the efficiency of the LEDs by decreasing the efficiency of charge carrier recombination. Excimer formation also causes a red shift in the emission spectrum.
Polyfluorenes can also undergo decomposition. There are two known ways in which decomposition can occur. The first involves the oxidation of the polymer that leads to the formation of an aromatic ketone, quenching the fluorescence. The second decomposition process results in aggregation leading to a red-shifted fluorescence, reduced intensity, exciton migration and relaxation through excimers.
Researchers have attempted to eliminate excimer formation and enhance the efficiency of polyfluorenes by copolymerizing polyfluorene with anthracene and end-capping polyfluorenes with bulky groups which could sterically hinder excimer formation. Additionally, researchers have tried adding large substituents at the nine position of the fluorene in order to inhibit excimer and aggregate formation. Furthermore, researchers have tried to improve LEDs by synthesizing fluorene-triarylamine copolymers and other multilayer devices that are based on polyfluorenes that can be cross-linked. These have been found to have brighter fluorescence and reasonable efficiencies.
Aggregation has also been combated by varying the chemical structure. For example, when conjugated polymers aggregate, which is natural in the solid state, their emission can be self-quenched, reducing luminescent quantum yields and reducing luminescent device performance. In opposition to this tendency, researchers have used tri-functional monomers to create highly branched polyfluorenes which do not aggregate due to the bulkiness of the substituents. This design strategy has achieved luminescent quantum yields of 42% in the solid state.
This solution reduces the ease of processability of the material because branched polymers have increased chain entanglement and poor solubility.
Another problem commonly encountered by polyfluorenes is an observed broad green, parasitic emission which detracts from the color purity and efficiency needed for an OLED.
Initially attributed to excimer emission, this green emission has been shown to be due to the formation of ketone defects along the fluorene polymer backbone (oxidation of the nine position on the monomer) when there are incomplete substitution at the nine positions of the fluorene monomer. Routes to combat this involve ensuring full substitution of the monomer’s active site, or including aromatic substituents. These solutions may present structures that lack optimal bulkiness or may be synthetically difficult.