Investigating Side Chain Mediated Electroluminescence from Carbazole-Modified Polyfluorene

Abstract

In molecular design of electroluminescent (EL) conjugated polymers, introducing a charge transport moiety on a side chain is found to be a promising method for balancing electron and hole fluxes in EL devices without changing the emitting color if there is no interaction between moiety and main chain. In the case of grafting a carbazole (Cz) moiety (hole transporting) on blue emitting polyfluorene, a green emission appears with intensity comparable to the blue emission, which was attributed to a possible interaction between main chain and Cz as previously reported by us. Here, a detailed study of its EL mechanism was carried out by means of time-resolved EL with the assistance of molecular simulation and thermally stimulated current measurements; exploration of how main chain segments interact with the transport moiety was performed. We found the Cz groups in Cz100PF play multiple roles: they act as (1) hole transporter to improve hole injection, (2) hole trapping site for efficient electron-hole recombination to yield blue-emitting excitons, and (3) source of green emission from electroplex formed via electric field-mediated interaction of the Cz/Cz radical cation with an electron in the nearby PF backbone. In combination, these observations suggest that integrated consideration for both intramolecular and intermolecular interactions provides a new route of molecular design of efficient EL polymers.

A Review on the Emitting Species in Conjugated Polymers for Photo‐ and Electro‐luminescence

“…20). 14,20,64 In our recent report, 21 we found an electroplex emission from poly(9,9-bis(N-carbazolyl-decyl)-fluorene) (Cz100PF) (Fig. 21), in which cross recombination of hole in the stacked carbazole/carbazole (Cz/Cz) dimer and electron in the nearby main chain segment under the applied electric field leads to a formation of this emitting species.…”

“…[3][4][5][6][7][8][9][10][11][12][13] However, the dimers formed under electrical excitation, electromer and electroplex, though have been observed in organic small-molecule materials, [14][15][16][17][18] but they are often observed in conjugated polymers. [19][20][21] On the other hand, aggregate, an emitting species with a ground-state p-p interaction, does not mention often in organic small-molecule materials though it also exists, but is very important in conjugated polymer due to its long chain characteristics. 1,[22][23][24][25][26][27][28][29][30][31] Since aggregate is a ground-state emitting species, energy transfer from non-interacting isolated chain segments to aggregates can occur and thus play an important role in PL and EL properties in conjugated polymers.…”

On the formation mechanism for electrically generated exciplexes in a carbazole–pyridine copolymer

“…In this case, they are referred to as electromers or electroplexes. Although there are numerous reports concerning the observation of electroplexes,14–22 surprisingly little is known about their formation pathway.…”

“…When considering the literature, it is noticeable that electroplexes and exciplexes are reported in particular for materials that contain a nitrogen atom in certain moieties. For example, electroplexes are known for combinations of organic semiconductors that contain carbazole‐moieties, amine‐moieties, oxadiazole‐moieties, and other diazole‐moieties 14–22. The same type of materials are also prone to exciplex formation.…”

Polyfluorenes for Device Applications

“…The device emits sky blue light with two characteristic peaks at 420 and 460 nm, as for PFO, and an additional strong broad peak at 525 nm [27]. After further investigation [28], we found that the Cz groups in 15 play multiple roles as: . This efficiency is higher than that of non-end-capped polymer 3 (that is, bromine exists at each chain end) by one order of magnitude and was attributed to an efficient hole trapping at the end-capper groups [due to the lower HOMO level of triarylamine (TAA) endcapper (5.48 eV) than 3 (5.88 eV)], resulting in more chances for holes and electrons to recombine on the main chain than at sites with less efficient emissions, such as aggregates and excimer-forming sites.…”