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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 (ION-IOFF)/IOFF 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). 

Figure 1:  (a) Micrograph of a 4-hydroxycyanobenzene (4HCB) crystal; (b) structural formula of the 4HCB molecule; (c) sketch of the investigated device based on a 4HCB single crystal, where the three crystallographic axes a, b and c are evidenced with blue, red, and green arrows, respectively; (d) plot of the x-ray response of a 4HCB crystal along the three different axes at different voltages; (e) micrograph of 1,8-naphthalendiimide (NTI) crystals; (f) structural formula of the NTI molecule; (g) sketch of the investigated device based on an NTI single crystal; in this case only the main crystal axis, along the length of the needle, was tested; (h) plot of the x-ray response of an NTI crystal at different voltages.

Figure 2:  (a) switching response under x-rays beam of a 4HCB-based device at different voltages; (b) plot of the device response (ΔI = ION-IOFF) vs. dose rate along the vertical axis of a 4HCB-based device, held at different operating voltages, with the metal (Ag) electrodes exposed to the x-rays beam; (c) plot of the device response ΔI vs. the applied voltage for 4HCB-based devices. Black squares: metal (Ag) electrodes shielded from the x-rays beam; red circles: the same device after irradiation with an overall x-rays dose of 2.1 kGy; blue triangles: device tested after an aging period of 1 month; green stars: response of a 4HCB-based all-organic device, contacted with PEDOT:PSS electrodes; (d) micrograph of an all-organic device based on 4HCB.

To assess the radiation hardness under x-rays of the tested OSSCs, a 4HCB-based detector was exposed to a massive amount of radiation (yielding a total dose of 2.1 kGy).  After that, the device was again tested under an on/off switching x-rays beam, showing that the previous heavy x-rays exposure did not affect the overall device performance (Fig. 2c, black squares: response before the x-rays high dose exposure; red circles: response after the exposure). In addition, the overall device performances did not vary even for aged devices (Fig. 2c, blu triangles).
Finally, the intrinsic response of the OSSC to the x-rays was measured by realizing a fully organic, flexible detector, using poly(dimethylsiloxane), PDMS, as a substrate, and PEDOT:PSS, a conducting polymer blend, as the electrodes material.  The performances of this device were comparable to those of another one fabricated using Ag electrodes shielded from x-rays (thus not contributing with secondary electrons to the overall detection mechanism; see Fig. 2c, black squares vs. green stars).
All-organic devices (like those shown in Fig. 2d), thanks to their low cost, ease of fabrication and potentially widespread availability, can represent a very interesting technological development in the field of ionizing radiation detectors.

 

This research was conducted by the following team:

  • Beatrice Fraboni, Andrea Ciavatti , Francesco Merlo, Luca Pasquini, Anna Cavallini – Dipartimento di Fisica, Università di Bologna, Italy
  • Alberto Quaranta - Dipartimento Ingegneria dei Materiali e Tecnologie Industriali (DIMTI), Università di Trento, Italy
  • Annalisa Bonfiglio - Dipartimento Ingegneria Elettrica ed Elettronica, Università di Cagliari
  • Alessandro Fraleoni-Morgera - Sincrotrone Trieste – Organic OptoElectronics Lab
B.F. conceived and supervised the x-rays detection experiments. B.F. and A.F.M interpreted the data and prepared the manuscript. A.F.M. grew the 4HCB and NTI crystals. A.Cia. and F.M. fabricated the devices and carried out the measurements. L.P. supervised the x-ray irradiation. A.B. supervised the fabrication of the all-organic devices. A.Cav. and A.Q. gave conceptual advice.

Reference

Beatrice Fraboni, Andrea Ciavatti , Francesco Merlo, Luca Pasquini, Anna Cavallini, Alberto Quaranta, Annalisa Bonfiglio, and Alessandro Fraleoni-Morgera, “Organic Semiconducting Single Crystals as Next Generation of Low-Cost, Room-Temperature Electrical X-ray Detectors”,
Advanced Materials, 24, 2289 (2012), doi: 10.1002/adma.201200283.

 

Last Updated on Thursday, 30 August 2012 15:46