Organic Semiconducting Single Crystals as Novel Room-Temperature, Low-Cost Solid-State Direct X-Ray Detectors

The detection of ionizing radiations is a relevant task for a number of technologically and socially relevant activities that span 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, either based on gas-filled containers (like 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 combinations of them) capable of visualizing the track of ionizing particles (cloud chambers, bubble chambers, spark chambers, silicon detectors, etc). Ionizing radiation detectors may be categorized also on their 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-steps 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 hence constituted by two (or more) sensors, complemented by a signal amplifying/processing unit. This approach induces a loss of information in the process, especially at low radiation doses, and most importantly a rather complex and fragile device structure. The state-of-the-art direct detectors are made by inorganic semiconductors, such as silicon, cadmium telluride, and gallium nitride, where the incoming radiation directly generates electron-hole pairs that constitute the collected output electrical signal. However, some of these materials have less than ideal band gap widths and show high dark currents, making them unattractive candidates for room temperature operation (e.g. Si, Ge), while other still contain a large number of defects and electrically active impurities that affect the recombination and collection processes of the photogenerated carriers, as well as the 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 cm).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 of application. The main drawbacks of such large area detectors are the poor conversion efficiency and signal-to-noise ratio typical of the 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 for. Organic materials have recently attracted major attention because they demonstrated, in the case of other electronic/optoelectronic devices (e.g. transistors and light emitting devices), room temperature operation, low environmental impact, low fabrication costs and the possibility of fabricating transparent and flexible devices, allowing unprecedented and integrated device functions and architectures [1-6]. As detectors of ionizing radiation, organic semiconductors have so far been mainly proposed in the indirect conversion approach, either as scintillators [7,8] or as photodiodes, [9-11] which detect visible photons coming from a scintillator and convert them into an electrical signal. A few examples of organic materials used for the direct detection of radiation have been recently reported in the literature and refer to semiconducting-or conducting-polymer thin films, or charge-transfer conducting organic crystals. Interestingly, it has been shown that thin films based on semiconducting polymers can withstand high X-rays doses without significant material degradation [12]. However, to obtain this result, the polymers must be properly encapsulated to protect them from the environmental oxygen and water, otherwise significant degradation of the device occurs rapidly [13]. It has to be noted that an effective encapsulation of semiconducting polymers-based devices leads to markedly increased production costs and device geometry complexity. A few thin film, organic-based direct detectors have been reported recently, but they rely on the presence in the detector of metallic electrodes or intentionally introduced metallic or semimetallic nanoparticles, that act as sourced of secondary photoelectrons, to produce reasonable photoconversion efficiencies . [12, 14-20]. We recently showed that organic semiconducting single crystals (OSSCs) can be used as effective direct X-ray detectors. In particular, devices based on solution-grown OSSCs (from two different molecules: 4-hydroxycyanobenzene - 4HCB - and 1,8-naphthaleneimide –NTI) (Figure 1) have been fabricated and operated in air, under ambient light and at room temperature, at voltages as low as few tens of volts, delivering well reproducible performances and a stable linear response to the X-ray dose rate, with notable radiation hardness and resistance to aging [21]. The role of high-atomic-number components (e.g., metals) in the device response has been elucidated, evidencing the intrinsic response to X-rays of the organic crystals, which allowed the fabrication of well-performing all-organic and optically transparent devices. It is important here to remember how the use of single crystals has been fundamental to the development of semiconductor microelectronics and solid-state science. Whether based on inorganic [22] or organic [23,38] materials, the devices that show the highest performance rely on single-crystal interfaces, thanks to their nearly perfect translational symmetry and exceptionally high chemical purity. Moreover, organic single crystals show band-like transport behavior [24] and top performing rubrene single crystals (vapour-grown, only a few microns thick) have been recently reported to reach hole mobilities of 40 cm2/Vs [25] and electron mobilities up to 11 cm2/Vs , measured in a field effect transistor configuration, which allows to control and amplify the current flowing through the crystal [26,27]. The same crystals, when sufficiently thin (i.e., lower than 3 microns), are reportedly conformable/flexible [28] The observed performance indicates that OSSCs are very promising candidates for a novel generation of low-cost, room-temperature X-ray detectors, that will allow to develop unprecedented applications in the radiation detector research field, mostly complementary to the high performance ones presently based on inorganic semiconductors.