Enhancement of X-ray detection by single-walled carbon nanotube enriched flexible polymer composite
© Han et al.; licensee Springer. 2014
Received: 28 July 2014
Accepted: 2 November 2014
Published: 12 November 2014
Although organic-based direct conversion X-ray detectors have been developed, their photocurrent generation efficiency has been limited by recombination of excitons due to the intrinsically poor electrical properties of organic materials. In this report, we fabricated a polymer-based flexible X-ray detector and enhanced the X-ray detection sensitivity using a single-walled carbon nanotube (SWNT) enriched polymer composite. When this SWNT enriched polymer composite was used as the active layer of an X-ray detector, it efficiently separated charges at the interface between the SWNTs and polymer, preventing recombination of X-ray-induced excitons. This increased the photocurrent generation efficiency, as measured from current-voltage characteristics. Therefore, X-ray-induced photocurrent and X-ray detection sensitivity were enhanced as the concentration of SWNTs in the composite was increased. However, this benefit was counterbalanced by the slow and unstable time-dependent response at high SWNT concentrations, arising from reduced Schottky barrier heights between the active layer and electrodes. At high SWNT concentration, the dark current also increased due to the reduced Schottky barrier height, leading to decrease the signal-to-noise ratio (SNR) of the device. Experimental results indicated that 0.005 wt.% SWNT in the composite was the optimum composition for practical X-ray detector operation because it showed enhanced performance in both sensitivity and SNR. In mechanical flexibility tests, the device exhibited a stable response up to a bending radius of 0.5 cm, and the device had no noticeable change in diode current after 1,000 bending cycles.
Since their development, radiation detectors have been widely used in various fields such as crystallography, medical imaging, and security due to their ability to inspect visually opaque objects [1–3]. Conventionally, the active layers of X-ray detectors have been composed of inorganic materials such as cadmium telluride (CdTe), silicon carbide (SiC), and amorphous selenium (a-Se), since they offer high energy resolution, high detection efficiency, and room-temperature operation [4, 5]. However, inorganic-based X-ray detectors also suffer from some critical drawbacks in terms of large-scale fabrication, high cost, and fragile property. Especially, it is difficult to make flexible devices based on inorganic materials, and although flexibility can be achieved by using a thin inorganic layer, this reduced the active layer thickness and thus sacrifices absorption ability .
In recent decades, organic-based semiconducting materials have been widely used as active layers of various electronic devices such as photovoltaic devices , light-emitting diodes , and thin-film transistors  due to their relatively low cost, availability for large-area fabrication, and mechanical flexibility . However, there have been only a few scientific publications on the use of semiconducting organic materials for direct detection of X-rays. Boroumand et al. reported the first direct X-ray detection of X-ray-induced photocurrents in thick films of conjugated polymers using poly[1-methoxy-4-(2-ethylhexyloxy)-phenylenevinylene] (MEH-PPV) and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) . Intaniwet et al. reported a direct X-ray detector using blends of polymer poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene to increase the transport of holes . Despite these efforts, however, the X-ray-induced output photocurrents of such devices are still limited due to the intrinsically low electron carrier mobility of the organic materials .
Herein, we demonstrated a polymer-based flexible X-ray detector and enhanced the X-ray detection sensitivity using single-walled carbon nanotubes (SWNTs). Because a SWNT has extremely high electron mobility and greater electron affinity than a p-type semiconducting polymer, the X-ray-induced excitons generated in the SWNT enriched polymer composite can be effectively separated and guided toward their respective electrodes without recombination. The composite was coated onto a poly(ethylene terephthalate) (PET) substrate to fabricate the active layer of the device, and homogenous dispersion of the SWNTs was confirmed through optical microscope and scanning electron microscope (SEM) images. Current-voltage (I-V) characteristics and X-ray-induced photocurrents of the devices were measured, and we verified that increasing the concentration of SWNTs in the composite layer not only enhanced the X-ray-induced photocurrent and X-ray detection sensitivity but also reduced the response speed and stability of the device. The optimum SWNT concentration was determined in consideration of both sensitivity and signal-to-noise ratio (SNR) of the device. Since SWNTs have exceptional mechanical properties and polymers are composed of cross-linked molecules, mechanical flexibility of the X-ray detector was achieved without noticeable degradation.
Metallic SWNTs (Hanwha Nanotech, Daejeon, Korea) with 1 to 1.2 nm in diameter, 5 to 20 μm in length, and 70 wt.% purity were used in this work. Poly(styrene-b-paraphenylene) with polyphenylene rich in 1,4-addition (PS-b-PPP) was synthesized via dehydrogenation of poly(styrene-b-1,4-cyclohexadiene) (Polymer Source, Dorval, Canada). The p-type semiconducting polymer, ‘Super Yellow’ (SY, Merck, Darmstadt, Germany), was used without purification.
Preparations of solutions of SWNT enriched polymer composite
The detailed sample preparation process of composite solutions is described elsewhere . In brief, we prepared 1 mg/mL of PS-b-PPP solution in toluene. Then, 1 mg/mL of SWNT solution in PS-b-PPP/toluene was prepared by adding SWNTs into PS-b-PPP/toluene. The mixture was horn-sonicated for 5 min (VC 750, Sonics & Materials, Newtown, CT, USA), followed by a 10-min bath sonication (NXP-1002, Kodo Technical Research, Hwaseong, Korea). SWNT solutions with various concentrations were made by diluting the 1 mg/mL SWNT solution in PS-b-PPP/toluene with pure toluene. SYs (10 mg/mL) were directly dissolved into diluted SWNT solutions to prepare the SWNT/SY composite solution. The resulting SWNT concentrations ranged from 0 to 0.1 wt.% for the SY polymer. The whole process was carried out under room temperature.
Fabrications of SWNT enriched polymer composite-based flexible X-ray detector
A 5-μm-thick SWNT enriched polymer composite film was prepared by successive drop casting onto PET substrates with predefined Au electrodes (60 nm). The coated films were stored at room temperature for an hour until toluene fully evaporated. Finally, LiF (5 nm) and Al (60 nm) were sequentially deposited on the composite layer by thermal evaporation. The defined active area of the devices was 3 × 3 mm2.
I-V characteristics in the dark and under X-ray illumination were measured using a Keithley 2400 SourceMeter (Keithley Instruments Inc., Cleveland, OH, USA). X-ray photocurrent measurements were carried out with an 8.06-keV Kα X-ray generated from a copper target X-ray tube. The X-ray beam was irradiated directly onto and through the Al top electrode at room temperature. The morphology of the composite layer was characterized by a field-emission scanning electron microscope (JSM-7001f, JEOL, Tokyo, Japan).
Results and discussion
Figure 1c,d illustrates the charge separation mechanism in the devices. It has been reported that when a pure p-type conjugated polymer is used as the active layer for an X-ray detector, it produces low photocurrent because of its low electron mobility and high recombination rate; this mechanism is illustrated in Figure 1c . As illustrated in Figure 1d, after SWNTs are added to the active layer, charge separation can easily occur at the SWNT-polymer interface. Electrons move into the SWNTs and holes move into the polymer because the SWNTs have a greater electron affinity than the polymer . This effective charge separation prevents the recombination of charges, enhancing the X-ray-induced photocurrent .
Photocurrents and enhancements of the devices
SWNT concentration (wt.%)
Figure 3a,b,c illustrates the relationship between the photocurrent and applied X-ray dose rate under the reverse bias voltages of 60 V, 90 V, and 120 V, respectively, with figure insets showing the time-dependent device responses. In this experiment, the shutter of the X-ray source was alternately opened and closed for periods of 30 s with a fixed dose rate of 7 mGy/s. Similar to inorganic X-ray detectors, our devices showed linear relationships between the photocurrent and dose rate following the Fowler model .
It should be noted that the X-ray-induced photocurrent of the 0.010 wt.% SWNT device increased suddenly at -120 V (Figure 3c). Furthermore, the time-dependent response of this device showed a non-saturating behavior, and it was enhanced as reverse bias voltage increased. This unexpected behavior of the device can be explained through band diagram analysis. Figure 3d represents the band diagram of the fabricated X-ray detector structure. The work function of the top (LiF/Al) and bottom (Au) electrodes is 4.2 and 5.1 eV [20, 21], respectively. The lowest unoccupied molecular orbital and highest occupied molecular orbital levels of SY are 2.8 and 5.0 eV, respectively [22, 23]. When the pure p-type polymer is used as the active layer of the X-ray detector, the Schottky barrier heights are 2.3 eV for electrons and 0.8 eV for holes. Adding SWNTs (which has a work function of 4.5 eV)  in the active layer reduced the Schottky barrier heights for electrons and holes to 0.6 and 0.3 eV, respectively (Figure 3d). This changed the contacts from Schottky to near-Ohmic, which supply space-charge-limited current (SCLC) . According to a previous report, the generation of SCLC influences the effective barrier height of a device and causes the formation of slower and non-saturating transients in the device . We confirmed that such phenomenon was even more pronounced according to the increment of SWNT concentration.
In summary, we fabricated a polymer-based flexible X-ray detector and enhanced the performance using a SWNT enriched polymer composite as an active layer. The X-ray-induced photocurrent and the sensitivity were enhanced as the SWNT concentration of the composite layer increased. However, at high SWNT concentration, the speed and stability of the response decrease due to the reduction in the Schottky barrier height. The optimum SWNT concentration was determined in consideration of both sensitivity and SNR. With the SWNT enriched polymer composite active layer, we demonstrated the mechanical flexibility of the device which shows a stable X-ray response. We expect that our device will be used in medical imaging and non-destructive analysis as a next-generation flexible X-ray detector.
This work was supported by the IT R&D program of MOTIE/KEIT [10040052, Development of X-ray Detector Sensor and ROIC on Photon Counting Method for 16bit High Resolution Dynamic Image Processing]. This work also supported by the Priority Research Centers Program thorough the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0006689).
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