‘Inorganics-in-organics’ semiconductors for high energy radiation detection.

Abstract

X-ray detectors are an invaluable tool for healthcare diagnostics, cancer therapy, homeland security and non-destructive evaluation among many fields. However, potential uses for X-rays are limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitates thick crystals with dimensions of ~ 1 mm – 1 cm, resulting in rigid structure, high operational voltages and high cost. Semiconducting polymer X-ray detectors are an emerging low cost technology. Their solution processable nature enables the fabrication of such detectors on flexible substrates over large areas using printing and roll-to-roll coating techniques. However, the low atomic number (Z) of organic materials results in low X-ray attenuation and hence, low X-ray sensitivities. This thesis focuses on direct detection of X-rays using organic semiconducting systems incorporating high Z Bi2O3 nanoparticles (NPs). For the work discussed, a thick organic bulk heterojunction (~10 – 20 μm) consisting of the p-type poly(3-hexylthiophene-2,5-diyl) (P3HT) and the n type [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM) are utilised. The effectiveness on the utilisation of a dual carrier system is demonstrated through the fabrication of detectors in a diode architecture by varying the NP loading from 0 – ~50% (by wt.). These hybrid detectors demonstrate sensitivities of 1712 μC mGy-1 cm-3 when irradiated under 50 kV tungsten X-ray source and ~30 and 58 μC mGy-1 cm-3 under 6 and 15 MV X-rays generated from a medical linear accelerator. A flexible detector was fabricated which demonstrated a high sensitivity approaching 300 μC mGy-1 cm-3, highlighting the promise of the technology for dosimetry and imaging in non-planar architectures. These performances are discussed based on the structural and electrical characteristics of the hybrid thick film diodes. Possible mechanisms for the high sensitivity observed are proposed where photoconducting gain, impact ionisation and Mie scattering are identified as central contributors towards the detector sensitivity. Routes to increasing the detector thickness to 100 μm – 1 mm thickness range is demonstrated using a pellet based on powder sintering technique. These P3HT:PC70BM:Bi2O3 detectors operate as photoconductors showing X-ray sensitivities in the region of ~ 160 C mGy-1 cm-3 and a preliminary X-ray imager based on the pellets are fabricated. Finally, potential routes for further improvement of the detector characteristics are discussed.