Towards Low-Voltage and Bendable X-ray Direct Detectors Based on Organic Semiconducting Single Crystals

Organic semiconductors are promising materials for applications like field effect transistors, solar cells, light-emitting displays, smart tags and molecular sensors. However, as ionizing radiation detectors up to now they have been mainly considered as indirect detectors, i.e. as devices to convert ionizing radiation into visible photons (scintillators), detected by photodiodes. X-rays detectors based on OSSCs of 4-hydroxycyanobenzene (4HCB) have been proven effective under X-ray beams compatible with biomedical applications (energy range of 10-25 keV, dose rates of 0.05 – 120 mGy/s). These devices have a rather fast response (5 ms), they are able to discriminate between different photon energies and they can be made bendable to a radius down to 1 cm. We report the first example of a direct X-ray photons-to-electrical signal detector based on an organic semiconducting single crystal (OSSC), operated at very low voltage, down to 1 V, with stable and linear electrical photo-response to increasing X-rays doses.
The performance of the detector at different photon energies has been investigated using, monochromatic synchrotron light at the SYRMEP beamline of Elettra. Figure 1(a) illustrates the decrease in X-ray sensitivity of the OSSCs-based detectors as the photon energy decreases. In particular, sensitivity changes from 32.1 nC/Gy @ 21 keV to 17.2 nC/Gy @ 10 keV suggest that the 4HCB crystals can be used as active components in energy-discriminating devices. The 4HCB-based device can be responsive to doses as low as 50 μGy/s, as demonstrated by the measurements with 10 keV X-rays reported in Fig. 2(b). The minimum detectable dose rate sensitivity in 4HCB-based devices of 50 μGy/s is in line with the typical values for medical imaging and clinical analysis (presently dose rates around 25 μGy/s are required for such applications). In addition, state-of-the-art detectors based on a-Se, that can be considered a benchmark photoconductor material for dosimetry applications, have a sensitivity of about 0.2 μC/cm2R1, that is not so far from the performance of the here reported 4HCB detectors (about 0.1 μC/cm2R1). The 4HCB-based detector shows a very good stability and repeatability (baseline shift below 5% after up to 50 on/off cycles), as shown in Fig. 1(c).

Figure 1. (a) Energy discriminating capabilities of 4HCB-based devices, at different energies and different dose rates, showing that even changing photon energies the device photoresponse response remains linear.  (b) signal-to-noise ratio of of a 4HCB-based deviceat a dose rate of 0.05 mGy/s. (c) Photoresponse the same detector of Fig. 2b under an on/off switching of a 10 KeV monochromatic photon source, dose rate 40 mGy/s, and 20 V applied bias.

These devices can be fabricated on flexible substrates, to realize bendable detectors (Fig. 2(a,b)). Sample detectors were subjected to continuous changes of the bending radius (from 1 to 100 cm) for up to 100 cycles while a bias sweep between 0 V and 2 V was applied. This repeated bending procedure do not appreciably alter the electrical performance of the devices, neither during the bending operation (Fig. 2(c)) nor after repeated bending cycles.No evidence of degradation neither during nor after repeated bending cycles has been observed. 

Figure 2. (a)  Schematic layout of the flexible X-ray detector fabricated with interdigitated Au electrodes patterned onto a thin and flexible PET substrate. (b) assessment of the bending properties of 4HCB flexible detectors via the imaged tool, capable of bending the device to a radius R. (c) stability of the detectors electrical response for repetitive bending cycles.

The obtained results open the way for breakthrough applications of OSSCs as thin, low-power consumption and potentially bendable X-rays detecting devices, in fields spanning from medical diagnostic, to civil security and industrial applications.

This research was conducted by the following research team:

A. Ciavatti1, E. Capria2, A. Fraleoni-Morgera2,3, G. Tromba2, D. Dreossi2,  P.J. Sellin4, P.Cosseddu5, A.Bonfiglio5, B. Fraboni1

1 Department of Physics and Astronomy, University of Bologna, Bologna, Italy
2 Elettra- Sincrotrone Trieste, Area Science Park, Basovizza, Italy
3 Department of Engineering and Architecture, University of Trieste, Trieste, Italy
4 Department of Physics, University of Surrey, Guildford, Surrey, UK
5 Department of Electrical and Electronic Engineering, University of Cagliari, Cagliari, Italy

Contact person:

Giuliana Tromba: email:
Diego Dreossi: email: 



A.Ciavatti, E. Capria, A. Fraleoni-Morgera, G. Tromba, D. Dreossi, P.J. Sellin, P. Cosseddu, A. Bonfiglio, B. Fraboni, “Towards Low-Voltage and Bendable X-ray Direct Detectors Based on Organic Semiconducting Single Crystals", Advanced Materials 27, 7213 (2015), DOI: 10.1002/adma.201503090

Last Updated on Thursday, 07 April 2016 15:04