Organic semiconducting single crystals as novel room-temperature, low-cost solid-state direct x-ray detectors

The detection of ionizing radiation is an important task for a number of technologically and socially relevant activities, ranging from environmental monitoring to industrial, security, and health applications. As such, a large variety of detectors for ionizing radiations have been developed in the past, based either on gas-filled containers (such as ionization chambers or Geiger counters), on solid or liquid materials capable of producing visible photons upon exposure to the radiation (scintillators), or on gaseous, liquid, or solid materials (or a combination of them) capable of visualizing the track of ionizing particles (cloud chambers, bubble chambers, spark chambers, silicon detectors, etc.). Ionizing radiation detectors may also be categorized as being “direct” or “indirect” detectors. A “direct detector” is one in which the incoming ionizing radiation is transduced by a sensor directly into an electrical signal. In an “indirect detector,” the transduction occurs in a two-step process, the first step being performed by a first sensor (for example, a scintillator) and the second step being performed by a second sensor (for example, a photodiode). Indirect detectors are therefore constituted of two (or more) sensors, complemented by a signal amplifying/processing unit. This approach induces a loss of information in the 244process, especially at low radiation doses, and, most importantly, a rather complex and fragile device structure. State-of-the-art direct detectors are made with inorganic semiconductors, such as silicon, cadmium telluride, and gallium nitride, in which the incoming radiation directly generates electron-hole pairs that constitute the collected output electrical signal. However, some of these materials (e.g., Si, Ge) have less than ideal bandgap widths and show high dark currents, making them unattractive candidates for room-temperature operation, while others contain a large number of defects and electrically active impurities that affect the recombination and collection processes of the photogenerated carriers, as well as detector stability under irradiation and polarized operation. In general, to achieve a high radiation-detection efficiency it is necessary to have materials that exhibit a very low dark current (<10-7 A), and thus possess a high resistivity (>109 Ocm). Large-area x-ray-detecting panels for digital imaging have been developed for medical applications, but are mostly based on the “indirect detection” approach, with amorphous silicon (or other inorganic semiconductors) coated by a scintillator material. The complexity of these devices and the high fabrication cost of detector-grade inorganic semiconductors limit their application in other fields. The main drawbacks of such large-area detectors are the poor conversion efficiency and signal-to-noise ratio typical of indirect detecting systems, plus the high cost, not taking into account the high complexity of operation (requiring trained operators). Therefore, alternative materials and technologies enabling the fabrication of direct detectors on large areas and at acceptable prices have to be sought. © 2015 by Taylor & Francis Group, LLC.