Voltage-induced infrared absorption from polymer field-effect transistors

Voltage-induced infrared absorption spectra from the top- and bottom-contact field-effect transistors fabricated with n-Si, SiO2, and poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as a gate electrode, an insulator, and a semiconductor, respectively, have been measured in a transmission-absorption configuration by the FT-IR difference-spectrum method. The observed voltage-induced infrared bands have been attributed to positive carriers (polarons) injected into the MEH-PPV layer by the application of minus gate bias. The cross section of the doping-induced 1510-cm−1 band has been obtained to be 7.7 × 10−17 cm2 for the MEH-PPV films doped electrochemically with ClO4−. The observed intensities of the voltage-induced 1510-cm−1 band have been converted to carrier sheet densities by this cross section. The carrier sheet density induced by field effect shows a saturation effect as the gate voltage increases for the top- and bottom-contact devices. The number of carriers injected in the top-contact device is larger than that in the bottom-contact device.