Enhancing quantum efficiency of MEH‐PPV through the reduction of chain aggregations by thermal treatments

Abstract

The quantum efficiencies of photoluminescence (PL) and electro‐luminescence (EL) of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) were significantly increased by heat treatments under vacuum with further removing the undissolved portion. The UV–vis absorption was found to decrease with heating time, while PL intensity increased. The maximum PL quantum yield was 6.5 times that of the untreated MEH‐PPV, which was attributed to the reduction of chain aggregations and the interruption of conjugation length. The maximum EL quantum yield of their prepared ITO/PANI/MEH‐PPV/Ca/AL light emitting diodes (PLED) was 46 (at 3 V) times that of the untreated sample. A typical turn‐on voltage of 2.5 V for MEH‐PPV PLED was able to decrease to 1 V after heat treatments, which was believed to result from the decrease of cis linkages in the polymer chains as revealed by the 1H NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1705–1711, 2005

Origin of the methylene bonds in poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] prepared according to Gilch's method: novel applications

“…For MEH‐PPV(I), the broken polymer segments during thermal treatment at 200 °C were highly reactive and tended to chemically bond to the adjacent polymer chains, provoking a reduction of chain aggregation. As a result, the PL quantum efficiency in film form was increased to ∼6 times that of untreated MEH‐PPV 16…”

“…In our previous study, we have mistakenly attributed the peak at δ = 6.6–6.7 ppm for the MEH‐PPV prepared from monomer I to the formation of cis ‐vinylene bonds during Gilch polymerization 15. As the resulting polymer was subjected to thermal treatment at 200 °C for 1–2 h, half of the methylene bonds were broken as indicated by the decrease of the peak at δ = 6.6–6.7 ppm 16. In this study, we have also found that the MEH‐PPV(I) began to degrade at ∼200 °C, but that the major degradation took place at 367 °C (Fig.…”

“…Film samples were prepared by dissolving the thermally treated polyblend in toluene at room temperature, and then spin‐casting onto a glass plate. The preparation procedure of PLED device has been given elsewhere 16. The EL spectrum of thermally treated polyblend was recorded on a SPEX1403 fluorescence spectrophotometer at a forward bias of 15 V.…”

“…This bond has been mistakenly assigned to the cis defect in our previous studies because of the similarity between their 1 H‐NMR spectra 15. Through thermal annealing of MEH‐PPV in order to break the methylene bonds at 200 °C, the extent of chain aggregation was significantly reduced, so that the maximum quantum yield of photoluminescence (PL) was increased to six times that of untreated MEH‐PPV 16…”

Properties of polymer light‐emitting diodes coated on surface‐treated ITO/glass substrates

“…Recently, polymer light‐emitting diodes (PLEDs) have attracted much attention for applications in large size flexible displays1–3 because of their simple structure and facile fabrication using soluble coating methods4, 5 (spin coating or ink‐jet printing). The PLED is the current operational device which emits light through the recombination of excitons injected from the anode and cathode electrodes.…”