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UBC Theses and Dissertations

Characterization of poly(3-hexylthiophene) based Schottky diodes Dimopoulos, Alexandros Ioannis


This thesis describes the fabrication and electrical characterization of Schottky diodes based on the polymer semiconductor poly(3-hexylthiophene). Printed electronics may not be able to benefit from high vacuum processing, either for economic or technical reasons. The aim was to observe the effects on performance when Schottky diodes were built at atmospheric pressure. 200 nm thick films of poly(3-hexylthiophene) were formed on glass substrates by spinning a 1 wt% polymer solution in chloroform. Vacuum deposited aluminum and gold where used for the Schottky and ohmic contacts respectively. Two types of diodes were manufactured. One type (Au bottom) had its Schottky junction formed by evaporating aluminum onto the polymer under high vacuum. The other (Al bottom) had its Schottky junction formed by depositing the polymer onto aluminum at atmospheric pressure. The final yield of usable devices was 35% for Au bottom and 22% for Al bottom. The hole density and bulk mobility were derived from both DC and AC measurements. The bulk mobility was found to range from 2×10⁻⁵ cm²V⁻¹s⁻¹ to 6×10⁻⁵ cm²V⁻¹s⁻¹. The hole density was determined to be between 5×10¹⁶ cm⁻³ and 3x10¹⁷ cm⁻³. DC measurements showed that Au bottom devices had a current rectification ratio of 2×10⁴ at ±2 V, 100 times greater than Al bottom devices. The space charge limited current (SCLC) had to be considered to successfully model the DC behaviour. The small signal behaviour was modeled with a 2nd order series/parallel circuit, which was determined through impedance spectroscopy. Small signal performance of both device types was predicted to be poor. The corner frequency was determined to be less than 100 Hz for Al bottom devices, and less than 1 kHz for Au bottom devices. Large signal frequency performance of the diodes was tested with a half-wave peak rectifier. The maximum operating frequency was measured to be 40 kHz for Au bottom devices and 10 kHz for Al bottom devices.

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