Chain Conformation Control of Fluorene-Benzothiadiazole Copolymer Light-Emitting Diode Efficiency and Lifetime

Abstract

The β-phase, in which the intermonomer torsion angle of a fraction of chain segments approaches ∼180°, is an intriguing conformational microstructure of the widely studied light-emitting polymer poly­(9,9-dioctylfluorene) (PFO). Its generation can in turn be used to significantly improve the performance of PFO emission-layer-based light-emitting diodes (LEDs). Here, we report the generation of β-phase chain segments in a copolymer, 90F8:10BT, containing 90% 9,9-dioctylfluorene (F8) and 10% 2,1,3-benzothiadiazole (BT) units and show that significant improvements in performance also ensue for LEDs with β-phase 90F8:10BT emission layers, generalizing the earlier PFO results. The β-phase was induced by both solvent vapor annealing and dipping copolymer thin films into a solvent/nonsolvent mixture. Subsequent absorption spectra show the characteristic fluorene β-phase peak at ∼435 nm, but luminescence spectra (∼530 nm peak) and quantum yields barely change, with the emission arising following efficient energy transfer to the lowest-lying excited states localized in the vicinity of the BT units. For ∼5% β-phase chain segment fraction relative to 0% β-phase, the LED luminance at 10 V increased by ∼25% to 5940 cd m–2, the maximum external quantum efficiency by ∼61 to 1.91%, and the operational stability from 64% luminance retention after 20 h of operation to 90%. Detailed studies addressing the underlying device physics identify a reduced hole injection barrier, higher hole mobility, correspondingly more balanced electron and hole charge transport, and decreased carrier trapping as the dominant factors. These results confirm the effectiveness of chain conformation control for fluorene-based homo- and copolymer device optimization.

Blue light-emitting conjugated polymer with low modulus and better EL spectra stability

“…High-efficiency deep-blue emission is one of the primary advantages of polyfluorene-based LCPs. [17][18][19] However, owing to the nature of the wide band gap, devices' color purity is susceptible to internal defects induced by interchain aggregates or device operation. [20][21][22] This is why some reported strategies for stretchable devices inevitably manifested or aggravated defect emission in the electroluminescence (EL) spectra of polyfluorene-based blue light-emitting diodes.…”

Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

“…The loading range selected was guided by prior studies 16,17 that showed successful β-phase formation for 9,9-dioctylfluorene-based copolymers in the presence of bulky co-monomer units for fractions below about 20% and with an eye to avoiding a switch from co-monomer-modulated 9,9-dioctylfluorene emission to comonomer-dominated emission. The general study of absorption and photoluminescence (PL) emission for (100-X)F8:XPyTPA-Pt(acac) polymer films with different PyTPA-Pt(acac) fractions and 9,9-dioctylfluorene conformations (see Supporting Information Figure S1 to S3 for full data sets) was followed by a more detailed study of triplet energy transfer as a function of temperature and conformation for the 5% loading, 95F8:5PyTPA-Pt(acac) copolymer (Figure 1B).…”

Fluorescence Studies on a Thermoresponsive PNIPAM-Polyfluorene Graft Copolymer

“…Polyfluorenes combine high photoluminescence (PL) quantum yields, good thermal stability and good solubility in a variety of solvents. − However, the PL efficiency of these organic conjugated polymers (OCPs) strongly depends on the properties and relaxation processes of the electronic excited states, which has led to considerable efforts in characterizing their behavior in solution. − Moreover, the photophysical properties are strongly determined by chemical structure, molecular weight, the presence of defects, the solvent from which the polymer is processed, and chain conformation and interchromophorics. − In many of these polymers, physical and/or chemical properties change as a response to external environmental stimuli, such as temperature, pH, electrolyte, light, electricity, magnetic field, etc. , In OCPs, the main deactivation excited-state pathway involves optical excitations that migrate along the polymer chain, or between different chains, executing a random walk inside the broadened conjugation length, and therefore, different excitation energies. ,− However, in polyfluorenes an additional competitive deactivation pathway, competitive with intrachain electronic energy transfer, is presented and involves conformational relaxation. ,,− …”