Effect of Molecular Weight and Conformation on Photoluminescence Quantum Yield of Fluorene/poly(2-methoxy-5-(2^′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH−PPV) Copolymers

Abstract

Improving the photoluminescence quantum yield of conjugated polymer films is very important for color-conversion materials and polymer-based photo-electronic devices. We investigated the effect of the molecular weight and conformation of fluorene/MEH−PPV copolymers on the photoluminescence quantum yield of conjugated polymer films. The films cast from high molecular weight copolymers had high quantum yields (61.2 ± 5.0%) and flexible-coil conformation. The quantum yields of low molecular weight copolymers were significantly lower following film formation (26.8 ± 0.05%) and rod-like conformation. Large reduction in the 0−0 emission band was observed following quantum yield reduction, suggesting increased energy transfer rate. The low molecular weight polymers were prone to easy π-π stack structure formation due to their stretched conformation. This structure promoted the energy transfer rate between polymers and thus the energy that comes to quenching sites increased, leading to the reduction in quantum yield. Although the reduction in the quantum yield of conjugated polymer films is generally attributed to aggregate formation, the red-shift in the absorbance and photoluminescence spectra following film formation, which indicate aggregate formation, was small and uncorrelated with the quantum yield.

Recent Advances in Polymer Organic Light-Emitting Diodes (PLED) Using Non-conjugated Polymers as the Emitting Layer and Contrasting Them with Conjugated Counterparts

“…The number-average molecular weight (M n ) is a critical property that impacts the practical applications of the polymer. According to the wellknown Mark-Houwink equation 123 :…”

“…124 In addition, high-molecular-weight alternating copolymer of fluorene and MeH-PPV showed a much higher U PL of 61.2% than the low molecular weight analogue (U PL : 26.8%) whose short polymer chains were more rod-like, which resulted in more pronounced p-p stacking and hence greater energy transfer to the quenching site. 123 Under certain circumstances, a high-molecularweight polymer is not necessarily superior to a low-molecular-weight one, but workers can make a wise use of their different physical properties. For instance, Al-Attar and Monkman fabricated an allsolution processed OLED device, in which a lowmolecular-weight poly(N-vinylcarbazole) doped with Ir(ppy) 3 as the emitting layer, was spun-coated on a high-molecular-weight poly(N-vinylcarbazole) which functioned as the hole transport electron blocking layer.…”

Effect of molecular weight on conformational characteristics of poly(3-hexyl thiophene)

“…The conformational characteristics of polymers depend on the molecular weight . Most previous studies used batch‐type measurement to investigate the conformation of P3HT, and the information obtained from such measurement reflects only the average molecular weight of the samples.…”

“…In this study, we focused on the effect of molecular weight and investigated the conformational characteristics of P3HT. We used an integrated system consisting of a gel permeation chromatograph (GPC), an SLS unit, and a viscometer . A notable advantage of this method is that it provides information on the conformational characteristics of polymers with a wide range of molecular weights.…”

“…As the molecular dimensions are small relative to the wavelength of the incident light, the scattered light intensity has little angle dependence. As in previous work, we used SLS to determine the absolute molecular weight of the polymers and used viscosity measurement to obtain information on the dimensions and conformation of the polymers.…”