The degradation of poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] thin films studied by capacitance-voltage analysis and attenuated total reflection infrared spectroscopy

The degradation of poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), affected by the presence of oxygen and moisture traps, is a major concern in the use of this electroluminescent polymer in light emitting diodes. This paper reports on a novel way of studying presence of oxygen in MEH-PPV through the Metal-Insulator-Semiconductor (MIS) Capacitance-Voltage (C-V) analysis, and the Attenuated Total Reflection Infrared (ATR-IR) spectroscopy technique. The presence of oxygen detected by ATR-IR has also been verified by X-ray photoelectron spectroscopy (XPS). 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. Moreover, the observed extension in the weak inversion region became more pronounced with aging in ambient environmental conditions. This increase in the weak inversion region is attributed to electron trapping by oxygen to form negative ions in the MEH-PPV layer. An increase in the trapped charge density of MEH-PPV from 1.2×10+11 cm−2 to 1.7×10+11 cm−2 has been observed. ATR-IR spectroscopy shows the formation of carbonyl peak at 1651 cm−1 with aging, which is due to the presence of oxygen. Atomic concentration table from XPS analysis shows the increase in the oxygen concentration of the MEH-PPV polymer with aging, further supporting the ATR-IR results. Both the C-V analysis and ATR-IR spectroscopy are powerful tools for investigating the degradation of MEH-PPV polymer.