A Study on Improving the Sensitivity of Indirect X-ray Detectors by Adding Hybrid Perovskite Quantum Dots

: In this paper, we demonstrate the enhancement in detection sensitivity of an indirect X-ray detector based on poly(3-hexylthiophene) (P3HT) and fullerene derivatives [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM) by adding perovskite quantum dots (PeQDs). The weight ratio of P3HT and PC 71 BM was ﬁxed at 1:1 (20 mg/mL in chlorobenzene), and different amounts of FAPbBr 3 PeQDs of (0–3) mg were added to the P3HT:PC 71 BM active layer solution. The experimental results show that the detector using P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg) achieved a sensitivity of 2.10 mA/Gy · cm 2 . To further improve the sensitivity, a ligand exchange experiment was performed on the P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg) detector. Under the condition of 12 h ligand exchange time, the detector with P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg) showed the highest sensitivity of 2.26 mA/Gy · cm 2 , which was increased by 28% compared to the pristine detector with a P3HT:PC 71 BM active layer.


Introduction
Recently, research has been conducted on the synthesis of various nanocrystals (NCs) that can be applied to optoelectronics and photonic devices [1][2][3][4]. The NCs with different structures and dimensions are attracting considerable attention because of their different electrical and optical properties. The structures and dimensions of the NCs can be adjusted as needed, and they are divided into zero-dimensional (0D) quantum dots (QDs) [5,6], one-dimensional (1D) nanorods (NRs) [7], nanowires (NWs) [8], and two-dimensional (2D) nanoplatelets (NPLs) [9,10]. The different structures of NCs have their own advantages. For example, 0D QDs are physically and chemically stable, while 1D NRs are long continuous structures with reduced ligand-induced resistance due to minimizing charge transfer between cores [11]. The 2D NPLs induce a strong quantum confinement effect in one direction to increase quantum efficiency [12]. Among zero-dimensional QDs, ABX 3 (A = Cs, formamidinium (FA), methylammonium (MA), B = Pb, Sn, and X = Cl, Br, I) perovskite quantum dots (PeQDs) have been synthesized under simpler process conditions compared to cadmium-based QDs, and appropriate optical properties can be obtained by tuning the material composition during synthesis [13][14][15][16][17][18]. PeQDs has a relatively high carrier mobility due to the perovskite crystal structure that promotes charge transport [19] and has excellent optical properties due to its narrow full width at half maximum (FWHM) [20] and high-photoluminescence quantum yield (PLQY) [21]. In addition, compared to the bulk structure, it is suitable for flexible device applications in the same manner as other zero-dimensional QDs [22][23][24]. Therefore, PeQDs can be applied in various fields, because it is easy to optimize the energy bandgap between materials, and they offer many structural advantages [25,26].
The PeQDs are being applied in various fields, such as displays [27], sensors [28], and memory devices [29]. In addition, bulk-type perovskite materials have the advantages of long carrier diffusion length, low defect density and high carrier mobility, so they are applied to solar cells [30,31], detectors [32,33] and phototransistors [34]. Among these, the X-ray detector is being used as the core of non-destructive testing, and the higher the sensitivity per area, the easier it is to apply, so research in the field is expanding [35][36][37][38][39]. Xray detectors are divided into two types: direct detection and indirect detection, depending on whether or not a detector is coupled to the scintillator. Direct detection is a method by which X-rays irradiated to a photoconductor layer without a scintillator are directly converted into charges and collected on electrodes. Indirect detection is a method by which a photodetector is coupled with a scintillator, and irradiated X-rays are converted into visible photons by the scintillator, to generate charges in the photodetector active layer. Although the direct detection method has the advantage of high resolution, it introduces design restrictions due to its thick thickness and high voltage. On the other hand, the indirect detection method has low resolution, but it is cheap and can be operated at low voltage, so it is being applied in a relatively wide variety of fields.
In this paper, we fabricated detectors using an organic active layer with added organicinorganic NCs, composed of electron donor conjugated polymer poly(3-hexylthiophene) (P3HT) and electron acceptor fullerene derivatives [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM) with the addition of formamidinium lead trihalide perovskite quantum dots (FAPbBr 3 PeQDs) (Qdot TM ABX3-530, Quantum Solutions, Southampton, UK) as active layer. Figure 1a shows a schematic of the proposed detector. In the device structure, poly(3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was used as a hole transport layer (HTL) to facilitate hole transport. The P3HT:PC 71 BM active layer improved the absorbance of the detector by adding FAPbBr 3 PeQDs. Figure 1b shows the corresponding energy level and carrier generation-collection process of the proposed detector. The detector without a scintillator operated as a photodetector, and short-circuit current density (J SC ) and series resistance (R S ) were extracted from the J−V curve of the photodetector. The electrical properties evaluated without the scintillator could be used to predict the detector performance in advance. The incident visible photons generated by the CsI(TI) scintillator formed excitons in the P3HT:PC 71 BM active layer and were separated into electrons and holes at the heterojunction. The added FAPbBr 3 PeQDs improved detector sensitivity by facilitating visible photon absorption and carrier generation. The amount of charge collected on the electrode could be used to determine the radiation parameters, such as the collected current density (CCD), dark current density (DCD), and sensitivity.  Figure 2 shows the fabrication process of the detector using P3HT:PC 71 BM active layer added with FAPbBr 3 PeQDs. Here, the average size of FAPbBr 3 PeQDs was 9.6 nm. First, the indium tin oxide (ITO)-patterned glass was cleaned in an ultrasonic bath, and it was then sequentially treated for 5 min with acetone, methanol, and isopropyl alcohol (IPA). Next, it was baked in a vacuum oven at 100 • C for 10 min, and the glass surface was changed from hydrophobic to hydrophilic by UV treatment for 15 min. To form HTL, PEDOT:PSS blended with dimethylformamide (DMF) at a ratio of 50 wt.% was spin-coated onto the cleaned glass substrate at 3000 rpm for 30 s, and it was baked at 160 • C for 30 min on a hot plate. For the active layer, the mixture of P3HT and PC 71 BM of weight ratio 1:1 was dissolved in chlorobenzene (20 mg/mL) containing various amounts of FAPbBr 3 PeQDs. A solution of FAPbBr 3 PeQDs dispersed in toluene was mixed with acetonitrile at a ratio of 5:4 and purified by centrifugation to obtain FAPbBr 3 PeQDs powder. The amount of FAPbBr 3 PeQDs was measured by analytical balance (AS 82/220.R2, Radwag, Radom, Poland). The FAPbBr 3 PeQDs were added in P3HT:PC 71 BM solution with (0, 1, 2, and 3) mg weight amount, and stirred for 3 h at 60 • C. The P3HT:PC 71 BM:FAPbBr 3 active layer solution was spin-coated on the HTL at 700 rpm for 30 s and baked at 120 • C for 10 min. Then the LiF/Al (0.5 nm/120 nm) cathode was deposited on the active layer using a thermal evaporator under a pressure of 10 −7 torr. Finally, the fabricated detector was sealed with a cover glass in a glove box under nitrogen conditions.  Figure 3 shows the experimental set-up and equipment layout for extracting the characteristics of the detector. The performance evaluation of the detector is performed separately from solar parameter measurement using a solar simulator (XES-40S2-CE, San-Ei Electric, Osaka, Japan) and X-ray parameter measurement using an X-ray generator (AJEX 2000H, AJEX Meditech, Suwon, South Korea). First, the solar parameters, such as J SC and R S , of the photodetector without the scintillator were evaluated. The photodetector was exposed to a 100 mW/cm 2 xenon lamp filtered with AM 1.5 G, and the J−V characteristics were measured using a source meter (Keithley 2400, Tektronix, Beaverton, OR, USA). By extracting the solar parameters, the characteristics of the photodetector could be analyzed, and the radiation characteristics of the detector combined with photodetector and scintillator could be determined. In the next step, the radiation parameters of the detector were evaluated. The detector combined with scintillator (J13113, Hamamatsu Photonics, Shizuoka, Japan) was placed at a distance of 30 cm from the X-ray generator. The operating conditions of the X-ray generator were fixed at 80 kVp and 60 mAs, and the exposure time was 1.57 s. The charge generated by X-ray exposure was collected by applying a −0.6 V bias voltage between the anode and cathode of the detector. The CCD, which defined when X-rays were irradiated, and the DCD, which defined when X-rays were not irradiated, were calculated by Equations (1) and (2), respectively: CCD [µA/cm 2 ] = Collected current during X-ray ON/Exposed time · Exposed detector area (1) DCD [µA/cm 2 ] = Collected current during X-ray OFF/Exposed time · Exposed detector area (2) At the same time, the absorbed dose was measured by using an ion chamber (Capintec CII50, Mirion Technologies, Florham Park, NJ, USA), and the value was 3.34 mGy. The sensitivity is defined as the rate of charge generated in the absorbed dose, which was calculated by Equation (3):

Results and Discussion
3.1. Experiment on P3HT:PC 71 BM Active Layer with Different Amounts of FAPbBr 3 PeQDs Figure 4a shows the absorption spectra of the pristine P3HT:PC 71 BM and P3HT:PC 71 BM with different amounts of FAPbBr 3 PeQDs by using UV/Vis spectrophotometry (OPTIZEN 2120UV, Mechasys, Daejeon, Korea). The absorbance of active layers tended to increase as the amount of FAPbBr 3 PeQDs added increased within the range of (0-3) mg, and the highest absorbance was observed under the P3HT:PC 71 BM:FAPbBr 3 PeQDs (3 mg) film condition due to the high extinction coefficient of FAPbBr 3 PeQDs added to the P3HT:PC 71 BM active layer and the absorbance of the P3HT:PC 71 BM:FAPbBr 3 PeQDs film being higher than that of the pristine P3HT:PC 71 BM film. In addition, Figure 4a shows the emission peak (560 nm) of the CsI(TI) scintillator that was measured when X-rays were irradiated. Comprehensive results show the absorbance of the P3HT:PC 71 BM:FAPbBr 3 PeQDs active layer has a peak similar to the emission peak (560 nm) of the CsI(TI) scintillator. Figure 4b shows the J−V curves of the photodetector based on the P3HT:PC 71 BM active layer with different amounts of FAPbBr 3 PeQDs under AM 1.5 G conditions with an intensity of 100 mW/cm 2 .  Table 1 lists the measured parameters J SC and R S . The photodetector based on the pristine P3HT:PC 71 BM exhibits a J SC of 9.59 mA/cm 2 and R S of 324.58 Ω. After the incorporation of varying amounts of FAPbBr 3 PeQDs, the performance of the detectors with FAPbBr 3 additive active layer was improved. This is because the FAPbBr 3 PeQDs dispersed in P3HT:PC 71 BM improve the performance of the detector by increasing photon absorption. The performance of the device with FAPbBr 3 PeQDs (1 mg) exhibited a J SC of 11.05 mA/cm 2 and R S of 262.79 Ω. However, when the amount of FAPbBr 3 PeQDs was increased to 3 mg, it was estimated that the performance of the device was significantly reduced, due to the formation of trap sites for charge recombination. As mentioned in the experimental set-up, the X-ray generator was set up under the same conditions, and the photodetector and CsI(TI) scintillator were combined to measure the amount of charge generated during X-ray irradiation. Regarding the stability upon X-ray exposure, we referred to a study on the stability measurement of P3HT:PC 61 BM detector [40]. The exposed X-ray dose rate was~100 mGy/s for 1 h, and a 2.3 % decrease in photocurrent was observed after X-ray exposure. Figure 5a shows the charge density in the X-ray ON state and X-ray OFF state. The X-ray exposure time and measurement distance were fixed at 1.57 s and 30 cm, respectively. Referring to Equations (1)-(3), CCD, DCD, and sensitivity were calculated. Figure 5b   To understand the effect of FAPbBr 3 PeQDs incorporation on the morphology of the P3HT:PC 71 BM films, we used atomic force microscopy (AFM) to measure the morphology of the pristine P3HT:PC 71 BM and P3HT:PC 71 BM with different amounts of FAPbBr 3 PeQDs films. Figure 6 shows the AFM images of the corresponding samples. Table 2 shows the surface characterization parameters, such as the average surface roughness (R a ) and the roughness peak-to-peak (R max ) of P3HT:PC 71 BM films with different amounts of FAPbBr 3 PeQDs. Since FAPbBr 3 PeQDs are nanoparticles, the R a and R max tend to increase in proportion to the amount added. The increase in absorbance of the active layer in Figure 4a was explained by the R a . As R a of the P3HT:PC 71 BM with different amounts of FAPbBr 3 PeQDs active layer increases, the absorption area increases, resulting in effective visiblephoton absorption. The carriers generated by the additional photon absorption were increased, and the absorbance was improved, compared to the pristine P3HT:PC 71 BM active layer. However, when the proportion of FAPbBr 3 PeQDs increases in the P3HT:PC 71 BM active layer, they aggregate to form a large structure, interfering with exciton generation and charge transport [38]. As a result, the performance of the P3HT:PC 71 BM:FAPbBr 3 PeQDs (2 mg) was lower than that of the P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg). In the case of the P3HT:PC 71 BM:FAPbBr 3 PeQDs (3 mg), the performance was reduced compared to that of the pristine P3HT:PC 71 BM, because it was significantly affected by PeQDs aggregation.
The detectors were characterized under dark conditions to clarify the potential charge transport phenomenon. Figure 7a shows the log J−V curves of the detectors with the pristine P3HT:PC 71 BM and P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg). The carrier mobility could be calculated from the space-charge-limited-current model (SCLC) model using the Mott-Gurney law equation [41]: where µ is the mobility, J is the current density, L is the thickness of the active layer, V a is the applied voltage, ε 0 (=8.854 × 10 −14 F/cm 2 ) is the product of the free-space dielectric constant, and ε r is the relative permittivity of the active layer. The thickness of the active layer was measured by surface profiler (KLA-Tencor AS-500), and the relative permittivity of the mixture according to the blending ratio of the P3HT:PC 71 BM:FAPbBr 3 PeQDs was calculated using the volume ratio and molecular density. As a result, the detector with the P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg) obtained the highest mobility of 1.41 × 10 −4 cm 2 /V·s, which is improved by 119% compared to the detector with the pristine P3HT:PC 71 BM. In addition, the defect densities of the detectors were calculated using Equation (5) [42]: where V TFL is the trap-filled limit (TFL) voltage from the plots, and q (1.602 × 10 −19 C) is the elementary charge of electron. Under the condition of P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg), the detector showed the lowest defect density of 1.60 × 10 16 cm −3 .

Experiments on FAPbBr 3 PeQDs Ligand Exchange
To ensure optimal charge transport conditions in the active layer based on the P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg), we performed ligand exchange experiments. Although the oleic acid and oleylamine (OA/OAm) ligands can prevent aggregation between cores in solution and remain dispersed, the poor conduction of their long chains interferes with charge transport [43]. Here, the OA/OAm ligands of the FAPbBr 3 PeQDs were exchanged with pyridine ligand. That is because pyridine is a short-chain ligand with high conductivity, which improves the charge transport path [44]. At the same time, we also performed experiments to confirm the optimal ratio of ligands by changing the ligand exchange time. Ligand exchange experiments were conducted under conditions of (0, 6, 12, and 18) h, and the active layer was fixed with P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg). FAPbBr 3 PeQDs-covered OA/OAm ligands were dispersed in toluene and mixed with pyridine in a volume ratio of 5:2, and ligand exchange was performed at 30 • C for 0, 6, 12, and 18 h, respectively. Ligand-exchanged FAPbBr 3 PeQDs were mixed with P3HT:PC 71 BM and fabricated and evaluated as individual devices. For further confirmation of the optimal performance of the detector under the different ligand exchange times, thermogravimetric analysis (TGA) measurement was performed for FAPbBr 3 PeQDs with (0 and 12) h ligand exchange time. Figure 8a shows the TGA graphs. The temperature change proceeds from the boiling point of OA (286 • C), OAm (384 • C), and pyridine (116 • C) to 500 • C, and the weight loss of the OA/OAm-and pyridine-capped FAPbBr 3 PeQDs continues. The total weight loss of FAPbBr 3 PeQDs at 0 h was 44.0%, showing the ligand of OA/OAm. The curve for FAPbBr 3 PeQDs for 12 h showed that OA/OAm was exchanged for pyridine. A weight loss of 14.9% occurred from the boiling point of pyridine (116 • C) to an intermediate plateau (316.8 • C), which is the ligand of pyridine. We compared the ligand surface coverage of FAPbBr 3 PeQDs using TGA graphs [45]. Under the 12 h condition, the surface coverage of OA/OAm was reduced by 30.5% compared to the 0 h condition, and the surface coverage of pyridine was 80.2% higher than that of OA/OAm. Figure 8b shows the normalized surface coverage:  Figure 9a,b show the evaluation parameters (R S , mobility, defect density, CCD − DCD, and sensitivity) of the detectors prepared at different exchange times that were obtained. When the ligand exchange time was 12 h, the optimized detector showed the lowest R S and the highest sensitivity. Compared to the devices with OA/OAm ligands, R S was 250.05 Ω, which was reduced by 11.01%, and the sensitivity was 2.26 mA/Gy·cm 2 , which was increased by 7.84%. However, the radiation properties of the X-ray detector were decreased under the condition of 18 h, because the large structure of the aggregated FAPbBr 3 PeQDs was defective, resulting in increased defect density and R S and decreased mobility.

Conclusions
In this paper, we studied the characteristics of an organic X-ray detector in which the active layer is composed of P3HT, PC 71 BM, and FAPbBr 3 PeQDs. The performance of the detector was improved by adding FAPbBr 3 PeQDs to the P3HT:PC 71 BM active layer constituting the bulk-heterojunction. First, the amount of FAPbBr 3 PeQDs added to the pure P3HT:PC 71 BM detector was fixed at (0, 1, 2, and 3) mg, and the absorbance of the active layer was measured. As the amount of FAPbBr 3 PeQDs increased, the absorbance of the active layer tended to increase linearly. As a result, the sensitivity of the detector based on the P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg) was 2.10 mA/Gy·cm 2 , which was increased by 18.64%. Solar and radiation parameters of the detectors with the P3HT:PC 71 BM:FAPbBr 3 PeQDs active layer were optimized by the addition of 1 mg FAPbBr 3 PeQDs, compared to the pristine P3HT:PC 71 BM detector, due to the increased J SC and mobility. Second, to further improve the performance under the conditions of P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg), an experiment was conducted in which the OA/OAm ligand was replaced by a pyridine ligand. The short-chain ligand pyridine promoted charge transport, compared to the long-chain ligand of OA/OAm. The ligand exchange experiment was conducted under conditions of (0, 6, 12, and 18) h, and the surface coverage ratio of the short-chain ligand was increased to efficiently transport charges. Under the condition of 12 h, the optimized detector showed the highest sensitivity of 2.26 mA/Gy·cm 2 , which, compared to the P3HT:PC 71 BM:FAPbBr 3 PeQDs (1 mg, 0 h), was increased by 7.84%.

Conflicts of Interest:
The authors declare no conflict of interest.