Highly Bright and Efficient Electroluminescence of New PPV Derivatives Containing Polyhedral Oligomeric Silsesquioxanes (POSSs) and Their Blends

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Abstract

A new series of highly bright and efficient poly(p-phenylenevinylene)s (PPV)s based on polyhedral oligomeric silsesquioxanes (POSSs) was synthesized via the Gilch polymerization method. The three POSS-containing PPVs are as follows: POSS05-PPV(containing 5 mol % POSS-appended PPV units), POSS25-PPV, and POSS100-PPV (this is the first π-conjugated polymer composed of 100 mol % POSSs-appended repeating units). These polymers were found to be completely soluble in common organic solvents, and homogeneous thin films of these polymers were fabricated easily by spin-coating onto the quartz glasses and indium tin oxide (ITO) substrates. The POSS-containing PPVs exhibit higher glass transition temperatures (64−77 °C) than that of MEH-PPV (58 °C), indicating that electroluminescence (EL) devices fabricated with these polymers should have good thermal stabilities. The presence of the POSSs in the PPV side chains was found to result in the lowering of the maximum wavelengths of the UV−vis absorption and photoluminescence (PL) spectra, which occurs because of the nanosize effect of the POSS moieties appended to the polymer backbones. As the POSS content ratio in the polymers increases, the maximum wavelengths of the UV−vis absorption and PL emission spectra are increasingly blue-shifted except for those of POSS05-PPV. Light-emitting diodes (LEDs) with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al were fabricated using the novel POSS-containing PPVs. The LED devices based on the POSS-containing PPVs were found to exhibit maximum EL wavelengths ranging from 577 to 543 nm (POSS05-PPV, 577 nm; POSS25-PPV, 575 nm; and POSS100-PPV, 543 nm). The LED devices containing POSS05-PPV or POSS25-PPV emitted orange-red light, whereas that containing POSS100-PPV emitted yellow-orange light. Interestingly, the maximum brightness and luminance efficiency of the POSS25-PPV device were found to be up to 6340 cd/m2 at 13.2 V and 0.26 cd/A at 6190 cd/m2, respectively, which are higher maximum brightness and luminance efficiencies than those of the MEH-PPV device (3880 cd/m2 at 14.8 V and 0.075 cd/A at 3880 cd/m2). The improvements in the EL properties of PPV derivatives that result from the introduction of the POSS moieties into the PPVs and the contribution of the POSS moieties to the conjugated polymer matrixes of EL devices were also reproduced in binary blend systems composed of a POSS-containing PPV (POSS25-PPV) and MEH-PPV. Surprisingly, the luminescence efficiency (0.48 cd/A at 10540 cd/m2) of the binary blend consisting of 5 wt % of POSS25-PPV and 95 wt % of MEH-PPV was found to be enhanced by a factor of 6.4 with a maximum brightness of 11 010 cd/m2 (at 14.3 V). It is likely that the enhanced device performance of POSS25-PPV and the binary blend consisting of 5 wt % of POSS25-PPV and 95 wt % of MEH-PPV are due to the formation of suitable insulation domains of the POSS moieties in the conjugated polymer matrixes, resulting in a balance of charge carriers, that is, of electrons and holes.

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