Abstract
In the light-emitting devices (LEDs) based on the π-conjugated polymers, the relationship between the quantum efficiency and the balance of hole ( μ h) and electron ( μ e) mobility has been investigated. In order to measure the μ h and μ e of the LEDs based on π-conjugated polymers, we fabricated the hole transport device (HTD) and the electron transport device (ETD) by using various metal electrodes with different work functions. For the materials of light emitting layer, we synthesized poly[2-( N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV) and poly[2-{4-[5-(4- tert-butylphenyl)-1,3,4-oxadiazolyl]-phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (OxdEH-PPV) with electron-rich groups. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), which is well known material for the polymer-based LED, was synthesized for the reference. We measured the current density vs. applied field ( J– E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J– E curves were analyzed by using the space charge limited conduction (SCLC) model. Based upon the SCLC model, μ h and μ e of MEH-PPV sample was measured to be ∼10 −6 cm 2/V s and ∼10 −8 cm 2/V s, respectively. For CzEH-PPV and OxdEH-PPV samples with electron-rich groups, μ h was similar to μ e with 10 −10–10 −11 cm 2/V s. The μ h and μ e of CzEH-PPV and OxdEH-PPV samples was lower than that of MEH-PPV sample, but more balanced. The quantum efficiency of the LED by using CzEH-PPV or OxdEH-PPV materials was ∼10 times higher than that prepared from MEH-PPV. The balance of the μ h and μ e plays an important role for the quantum efficiency. We analyze the balance of the μ h and μ e and the relatively low mobilities of CzEH-PPV and OxdEH-PPV samples in terms of the heavier effective mass due to the asymmetric dipole distribution in the side chains. The results of photocurrent of the systems qualitatively agreed with the result of the electrical measurements. From AC impedance measurement of the LEDs, we observed that the relaxation time of MEH-PPV was shorter than that of OxdEH-PPV sample because of the higher mobility of MEH-PPV sample.
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