Organic-Inorganic Hybrid Semiconductors for Direct X-ray Detection
Creators: name: Nanayakkara, Maheshani Prabodhi Alwis
Publisher: name: University of Surrey
Year: 2022

description: Organic-inorganic hybrid semiconductors such as composites based on high atomic number (Z) bismuth oxide (Bi2O3) nanoparticle (NP) incorporated into an organic bulk heterojunction (BHJ) consisting of p-type Poly(3-hexylthiophene-2,5-diyl) (P3HT) and n-type [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM) are gaining significant attention as an emerging class of materials for direct conversion X-ray detection. This is mainly due to their high sensitivity to X-rays over a broad energy range (from 10 keV to 15 MeV), low voltage operation, and the potential to fabricate conformable detectors using low-cost printing techniques. However, several key challenges remain which impede their further development. These include high dark currents that are typically ×1000 – 10000 higher than the industrial requirements, lack of mechanical design rules for these materials to fabricate curved detectors, and limitations on the transfer and transport of X-ray generated charge carriers. This thesis focuses on addressing the above key challenges. <br>
description: <br>
description: Firstly, the optimization of the design of these NP incorporated BHJ (NP-BHJ) X-ray detectors for the realization of ultra-low dark currents that are &lt;10 pA mm-2, under electric fields as high as ~4 V µm-1 is discussed. Based on in-depth characterization, the enrichment of the hole-selective p-type organic semiconductor near the film surface is demonstrated for the first time. By forming an “inverted” device architecture where the anode contact is formed at the film surface, efficient blocking of charge injection into the device is successfully realized. The resulting detectors demonstrate dose linearity, dose rate linearity, and reproducibility under X-rays from a 70 kV as well as a 6 MV source where an exceptionally high sensitivity of ~1.5 mC Gy-1 cm-2 and &lt;6% variation in angular dependence response is demonstrated. <br>
description: <br>
description: Next, the influence of P3HT molecular weight (MW) on designing curved detectors based on the NP-BHJ detector concept is discussed. Based on the characterization of the mechanical properties of the NP-BHJ layer for different P3HT MWs, the importance of higher P3HT MW in combination with thicker flexible substrates for mechanically robust curved detectors is demonstrated. A stable detector response under X-ray irradiation, for bending radius as low as 1.3 mm is demonstrated with a low variation in detector response (&lt;2.8%) even after 100 repeated bending cycles. These impressive bendability characteristics at higher MW are shown to be due to the higher number of intermolecular interactions and chain entanglements with increasing MW as well as the tendency of longer polymeric chains to form bridging between crystalline domains, all of which ultimately increase the resistance to mechanical failure. <br>
description: <br>
description: Finally, to improve the charge generation, transfer, and transport within such hybrid detectors, a new approach based on incorporating high – Z heteroatoms to the backbone of the p-type polymer in place of NPs is introduced. When irradiated under the X-ray energies ranging from 70 kV to 220 kV, the hybrid detectors fabricated using selenium as the heteroatom are shown to result in X-ray sensitivities comparable to the NP-BHJ X-ray detectors despite their low mass attenuation coefficient compared to NP-BHJ detectors previously developed in this work. These outstanding response characteristics are identified as being due to the lower electron-hole pair creation energy, fast transfer of X-ray generated charge carriers, and improved charge transport. <br>