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). >

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