Organic single crystals as low power, room temperature electrical X-ray detectors

Organic semiconductors are promising materials for several applications, ranging from thin film transistors (TFT), to light emitting diodes (LED), to solar cells and sensors.  As detectors for ionizing radiation, organic semiconductors have so far received less attention, their main use being in scintillators, which convert ionizing radiation into visible photons, and in photodiodes, which turn visible photons coming from a scintillator into an electrical signal.  Only few examples of direct conversion of the ionizing radiation into an electrical signal have so far been reported, and they were always related to semiconducting or conducting polymer thin films, or charge-transfer conducting organic crystals, based on the presence, inside the detector, of metallic electrodes exposed to the ionizing radiation.
Researchers of the Dept. of Physics of the University of Bologna, Dept. of Materials and Industrial Technologies Engineering of the University of Trento, Dept. of Electric and Electronic Engineering of the University of Cagliari and of Sincrotrone Trieste, have recently demonstrated that organic semiconducting single crystals (OSSCs) can be used as efficient direct x-ray detectors.
This was achieved by investigating the photoelectrical response of devices based on OSSCs (from two different molecules: 4-hydroxycyanobenzene (4HCB) (Fig. 1a, b) and 1,8-naphthaleneimide (NTI, Fig. 1e, f)) exposed to an x-ray beam in air, at room temperature and under ambient light, in the configuration sketched in Fig. 1c, g.  Devices based on 4HCB showed a significant increase in photocurrent along both the planar and vertical axes (although to different extents, i.e. in an anisotropic way).  Interestingly, the normalized photocurrent (I_ON-I_OFF)/I_OFF vs V curve presents a maximum at rather low voltages for both axes, suggesting that real devices may be operated at voltages as low as 50 V, hence with low power requirements (Figure 1d).  A similar behaviour under x-ray irradiation has been observed for NTI crystals, whose needle-like shape allowed to create an electrical contact and probe them only in the axial direction (Fig. 1g, h).
Upon repeated x-rays beam on/off cycles (Fig. 2a), even for different bias voltages, no hysteresis nor appreciable current drifts were observed for neither of the 4HCB- and NTI-based devices.  The response time, less than 70 ms, is remarkably fast for organic electronic devices.  Moreover, at all the tested biases the OSSCs-based detectors were found to display a rather linear response with respect to the delivered dose rate (Fig. 2b).