Degradation of MEH-PPV due to Oxygen/Moisture Traps through C-V Analysis and Attenuated Total Reflection IR Spectroscopy
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I11.5.1
Degradation of MEH-PPV due to Oxygen/Moisture Traps through C-V Analysis and Attenuated Total Reflection IR Spectroscopy R. K. Khillan, Y. Su* and K. Varahramyan Louisiana Tech University, Ruston, LA 71272. Email: [email protected], [email protected], [email protected]
ABSTRACT
We present the studying of oxygen and moisture traps in MEH-PPV through the MIS Capacitance – Voltage (C-V) analysis, and the Attenuated Total Reflection Infrared (ATR IR) spectroscopy technique. The presence of oxygen studied by ATR IR has also been verified by optical images from high resolution optical microscope. In quasi-static C-V measurements of the MIS (Al/MEH-PPV/p-Si) capacitors made, an extension of the weak inversion region was measured before strong inversion, which becomes more pronounced with aging. This increase in the weak inversion region is attributed to electron trapping by oxygen to form negative ions in the MEH-PPV layer. ATR IR spectroscopy shows the formation of carbonyl peak at 1651 cm-1 with aging, which is due to the presence of oxygen. Both the C-V analysis and Attenuated Total Reflection IR Spectroscopy are powerful tools for investigating the degradation of MEH-PPV polymer.
INTRODUCTION
Lifetime and efficiency of the Polymer Light Emitting Diodes remain to be improved for PLEDs to compete with the conventional semiconductor devices. MEHPPV is an electroluminescent polymer most widely used for the fabrication of PLEDs1-3. The degradation of MEH-PPV is a major factor for short lifetime and low efficiency of PLED. The presence of oxygen and moisture traps enhances the degradation of MEHPPV.
I11.5.2
Techniques as Deep Level Transient Spectroscopy (DLTS) and Thermally Stimulated Current (TSC) are normally used to identify the origins and states of traps in polymer layers. Campbell et al4 used the DLTS technique to study traps in the poly (pphenylene vinylene) based Schottky diodes. Recently Tseng et al5 have reported molecular oxygen and moisture as traps in MEH-PPV through Field Induced Thermally Stimulated Current (FI-TSC) and Photoexcitation Thermally Stimulated Current (PETSC). In this paper the electrical and chemical changes in MEH-PPV polymer leading to device degradation are studied. The oxygen and moisture traps in MEH-PPV are characterized through C-V analysis, ATR IR spectroscopy, and optical microscopy.
EXPERIMENTAL METHODS
A 0.6% solution of MEH-PPV in p – Xylene was spin-coated on p-type silicon substrates, resulting in 80 nm thick MEH-PPV film. The samples were kept in vacuum for 15 hrs for complete drying of the MEH-PPV layer. Subsequently, 150nm thick aluminum was thermally evaporated on the MEH-PPV film at 10-6 Torr. The capacitance – voltage measurements of the resulting MIS (Al/MEH-PPV/ p-Si) devices were conducted using a Keithley electrical probe station. The thickness measurements were done using Tencor Alfa step profilometer. The ATR Infrared spectroscopic measurements were performed using Thermo Nicolet Nexus 470 FTIR. The optical images were obtained using high resolution OLYMPU
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