Characterization of thick film poly(triarylamine) semiconductor diodes for direct x-ray detection

Abstract

Thick film ͑Ն5 m thick͒ semiconducting polymer diodes incorporating poly͑triarylamine͒ ͑PTAA͒ have been produced and applied as direct x-ray detectors. Experiments determined that a rectifying diode behavior persists when increasing the thickness of the active layer above typical thin film thicknesses ͑Ͻ1 m͒, and the electrical conduction mechanism of the diodes has been identified. Direct current and photoconductivity measurements on indium tin oxide/ poly͑3,4-ethylenedioxythiophene͒/poly͑styrenesulfonate͒/PTAA/metal diodes confirm that carrier conduction occurs via a Poole-Frenkel mechanism. The energy band structure of diodes ͑having gold or aluminum top electrodes͒ has been elucidated and used to explain the resulting electrical characteristics. Theoretical calculations show that, upon irradiation with x-rays, the diode quantum efficiency increases with increasing polymer film thickness. The diodes produced here display characteristics similar to their thin film analogs, meaning that they may be operated in a similar way and therefore may be useful for radiation dosimetry applications. Upon irradiation, the diodes produce an x-ray photocurrent that is proportional to the dose, thus demonstrating their suitability for direct x-ray detectors. The x-ray photocurrent remains the same in a device after a cumulative exposure of 600 Gy and after aging for 6 months.

Heavy metallic oxide nanoparticles for enhanced sensitivity in semiconducting polymer x-ray detectors

“…Previously, direct detection of electron [13] and alpha (α) particles [14,15] has been achieved using polymers as the active layer. We have recently shown that direct X-ray induced photocurrents can be observed in metal/semiconducting polymer/metal diode structures [7,8,16,17] indicating the feasibility of using conjugated polymers in direct real-time radiation detection applications. The addition of small organic semiconducting molecules to Published in Nanotechnology 23 (2012) 235502 4 the polymer increased the charge-carrier mobility, which resulted in increased X-ray sensitivities [18].…”

Achieving a Stable Time Response in Polymeric Radiation Sensors under Charge Injection by X-rays

“…Figure 1b (30)(31)(32), and the band gap energy, E g , for PTAA was found to be 2.95 eV from previous PL spectroscopy (23). The barrier height for hole injection ( b φ ) at the PTAA/metal interface can be evaluated using the following equation (16):…”

“…The details of the device preparation can be found elsewhere (23). In summary, a 5 wt% solution of PTAA in toluene was spin-cast on top of the ITO providing a single polymeric active 6 layer thickness of either 20 µm or 30 µm.…”

“…In the present work, the interfacial layer of poly (3,4 ethylene dioxythiophene): poly(styrene sulfonate) has been excluded from the diode structure, because it was found to have no effect on electrical properties in the devices. The PTAA films were initially left to dry under atmospheric conditions and then annealed under vacuum at 150°C, which is above the glass transition temperature of the polymer (T g ≈ 103°C) (23), for 12 hours to eliminate any trapped solvent. The thickness profile of the active layer was subsequently measured using a surface profilometer (Dektak, Veeco Instruments).…”

“…The fabrication and characterization of prototype organic semiconductor sensors using thick PTAA films, for use as charge-sensitive, direct-detection X-ray sensors, have already been presented (23,24). Here, we have investigated the effects of the choice of metal for the top electrode by comparing the results obtained from Al, Au, Ni and Pd.…”