Study of an indirect-type X-ray detector fabricated with organic semiconductor materials
Introduction
▷ Conventionally, X-ray detector have been fabricated with inorganic
materials due to its high detection efficiency. However, inorganic-based
X-ray detector has demerits of large-area fabrication, high cost, and
mechanical robustness.
▷ Organic-based X-ray detector could be a breakthrough for X-ray
imaging because it has advantages such as easy fabrication, scalability,
low cost, and flexibility.
< Fig. 1. Schematic diagram of the indirect-type organic X-ray detector (left) and energy band diagram (right) >
▷ The characteristics of the detector was examined via indirect detection
method. To convert X-ray into visible photon, a scintillator was coupled
with the detector.
▷ The active layer was fabricated using a blend of the conjugated polymer
P3HT and the fullerene derivative ICBA.
Device Preparation
▷ Cleaning ITO-patterned glass substrate.
▷ PEDOT:PSS (Hole Transport Layer) was
spin-coated on the ITO-patterned glass.
▷ Subsequently, P3HT:ICBA solution was
spin-coated by changing the spin-rates
(500, 700, 900, and 1100 rpm).
▷ To make a P3HT:ICBA solution, P3HT
and ICBA were dissolved in
chlorobenzene with different blending
ratios of 3:2, 1:1, and 2:3.
▷ Finally, Aluminum cathode was formed
by thermal evaporation and the device
was encapsulated by using UV sealant.
< Fig. 3. Process of device fabrication >
< Fig. 2. Layout (left) and the real picture (right) of the fabricated device. >
Experimental Set-up
▷ The intrinsic properties of
detector was measured by solar
simulator (XES-40S2-CE).
▷ I-V characteristics was obtained
under 100mW/cm2 illumination
with AM 1.5 G filters.
▷ With scintillator, generated
charges by irradiated X-ray were
collected.
▷ The condition of X-ray generator
was fixed at 80kVp and 63mAs.
▷ Collected charges were
measured by electrometer
< Fig. 4. Solar simulator for measuring intrinsic properties>
< Fig. 5. Experimental set-up for measuring generated charge by irradiated X-ray >
▷ Based on the measured charge values, the collected charge density
(CCD), dark current density (DCD), and sensitivity were calculated using
equations as follows.
Results and Discussion
▷ In order to understand the inherent properties of the organic detector,
the current-voltage (I-V) curves were measured under illumination from
a solar simulator.
▷ Fig. 6 shows the J-V characteristics of the detector with various active-
layer conditions.
▷ The condition of 3:2 blending ratio and spin-rate 900 rpm shows the
highest efficiency and current density of 3.06 % and 8.01 mA/cm2,
respectively.
< Fig. 6. I-V characteristics of the detector as a function of blending ratio and spin-rate >
< Fig. 7. Emission spectrum of scintillator and the absorption spectrum of P3HT:ICBA. >
< Table 1. Device parameters as condition of active-layer >
▷ For the indirect detection method,
CsI(Tl) scintillator was chosen due to
its maximum emission spectrum at
550nm, which was well-matched to
the absorptioin spectrum of
P3HT:ICBA (refer to Fig. 7).
< Fig. 8. Amount of collected charge during the X-ray ON an OFF conditions (left) and trend of detector parameters as a function of applied bias (right). >
▷ Fig. 8 (left) shows the amount of collected charges during the X-ray On
and OFF condition.
▷ Fig. 8 (right) shows the calculated parameters as applied bias.
Conclusion
▷ In this study, we investigated an indirect-type organic-based X-ray
detector as a candidate for the flat-panel radiation detector.
▷ The performance of the detector was evaluated by measuring the
collected charge. Collected charge density (CCD), dark current density
(DCD), and sensitivity were calculated.
▷ In order to enhance the performance, the optimal process condition by
varying the blending ratio and spin-rate was tested.
< Fig. 9. Correlation between intrinsic data (left) and X-ray parameters (right). >