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PbS QDs were synthesized using a colloidal method [11]. First, a sulfur precursor solution was prepared by dispersing 1 mmol of sulfur powder (S) in 2.4 mL of oleylamine (OLA) and stirring the mixture at room temperature for 30 min under a nitrogen atmosphere. Separately, a lead precursor solution was prepared by dissolving 1.5 mmol of lead(II) chloride (PbCl₂) in 5 mL of OLA and stirring under nitrogen for 30 min at room temperature, followed by heating to 160^oC for 1 h. To remove residual impurities, we degassed the lead precursor solution under a vacuum at 120^oC for 15 min. The reaction temperature was then adjusted to 110^oC, and 225 µL of the S–OLA solution together with 274 µL of trioctylphosphine (TOP) were swiftly injected into the PbCl₂–OLA solution under a nitrogen flow. After 30 min of reaction, the mixture was cooled to room temperature. The resulting product was washed by adding methanol and toluene to the crude solution in a 4:1:1 ratio, followed by centrifugation at 3000 rpm for 5 min to isolate the PbS QDs. Finally, the purified QDs were redispersed in toluene to obtain a PbS colloidal solution with a concentration of 20 mg/mL^-1.

The PbS QD composite paste was prepared using conductive polymers as charge‐transport materials. PCBM was employed as the electron-transport layer and P3HT as the hole-transport layer [15,19]. PbS QD solution (1 mL) dispersed in toluene at a concentration of 20 mg/mL^-1 was mixed with PCBM and P3HT at various ratios, as summarized in Table 1. The total amount of PCBM and P3HT was adjusted to be greater than, equal to, or less than the amount of PbS QDs to investigate the effect of composition. For comparison, an additional paste containing only PbS QDs without electron or hole transport layers was also prepared. To increase the viscosity required for screen-printing fabrication, we added 10 mg of ethyl cellulose as a rheology modifier. The mixture was stirred in air at 60^oC for 3 h and then additionally stirred at room temperature for 12 h.

The SWIR photodetector was fabricated using a screen-printing technique [19]. As the substrate, 5 mm × 5 mm alumina plates were sequentially cleaned with acetone, methanol, and isopropyl alcohol (IPA). Interdigitated (IDE) silver electrodes were then deposited onto the cleaned substrates by screen printing with silver paste. The prepared PbS QDs composite ink was subsequently printed through a shadow mask defining a 3 mm × 3 mm active area onto the silver IDE electrodes, according to the desired composition conditions. After printing, the devices were annealed at 95^oC for 30 min to remove residual solvent, yielding photodetectors with a 9 mm² sensing region.

Fig. 1. 
Schematic diagram of the fabrication of a SWIR photodetector based on a PbS QDs composite.

The characteristics of the synthesized PbS QDs were evaluated using absorbance spectroscopy and XRD. As shown in Fig. 2(a), the QDs exhibited a distinct absorption peak at 1533 nm with a full width at half maximum (FWHM) of 102 nm [5,6,17]. This narrow bandwidth confirms the ability of the QDs to selectively detect IR light in the 1533 nm region, which lies within the eye-safe range, and suggests that photodetectors based on these QDs can achieve high selectivity and excellent photo response.

Fig. 2. 
(a) Absorbance and (b) XRD analysis characteristics of synthesized PbS QDs.

For structural analysis, thin films were prepared by spin-coating a 20 mg mL^-1 PbS QD solution onto 15 mm × 15 mm glass substrates at 1500 rpm for 30 s, followed by annealing at 95^oC for 30 min. As shown in Fig. 2(b), the XRD pattern displayed diffraction peaks such as (111), (200), and (220) near 25.9^o, 39.7^o, and 42.8^o, consistent with reported PbS crystal planes, confirming the successful synthesis of well-crystallized PbS QDs [11,17].

The electrical and photoresponse characteristics of the fabricated PbS QD-based SWIR photodetectors were evaluated in an optical chamber. Each device was mounted with probe tips, and measurements were performed using an SMU (B2902A, Keysight) under an applied bias of 3 V [12-15]. An IR light source (Thorlabs, SLS202L/M) provided illumination to compare current variations under illuminated and dark conditions. Three types of SWIR photodetectors were characterized under identical conditions, and both the light current (under IR illumination) and dark current (without illumination) were measured. As shown in Fig. 3, Sample 2, which was prepared with PCBM (10 mg) and P3HT (10 mg) in a total amount equal to that of the PbS QDs, exhibited the highest current change of approximately 440 µA. The graphs in Fig. 3(b) and 3(d) were enlarged to evaluate the response and recovery times, and the times required to reach 90% of the total current increase after the light was turned on and 90% of the total current decrease after the light was turned off were calculated. Consequently, the response and recovery times were determined to be approximately 0.42 and 0.75 s, respectively. This result indicated that electrons and holes generated in the PbS QDs were efficiently extracted through PCBM and P3HT, which have high charge mobilities, producing large current modulation and a rapid photo response [20-22]. When the combined concentration of PCBM and P3HT exceeded that of the PbS QDs, the density of QDs in the active layer decreased, lowering the IR absorption efficiency. The reduced QDs density also promoted defect formation and localized oxidation, which introduced trap states and enhanced charge recombination, yielding a markedly smaller current change of only ~0.412 µA. Conversely, when the combined concentration of PCBM and P3HT was lower than that of the QDs, the charge mobility was insufficient, resulting in diminished current modulation and slower response [7,18,19]. A reference device composed exclusively of PbS QDs, without electron- and hole-transport layers, exhibited a current change of only ~0.35 µA, slow response, and high noise. This behavior is attributed to poor film uniformity and limited charge-transport pathways in the absence of the conductive polymers that enhance film adhesion and facilitate carrier extraction [6,11,13]. The photoresponse of each device was quantified as the percentage change in current relative to the dark current, calculated using equation (1) [12,14,22].

Fig. 3. 
Current characteristics of SWIR photodetectors under light source irradiation according to PbS QDs composite mixing conditions (a) Sample 1, (b) Sample 2, (c) Sample 3, and (d) Sample 4.

Fig. 4. 
Enlarged graphs of (a) Samples 2 and (b) Sample 4 for the calculation of response and recovery times.

As presented in Fig. 5, the device of Sample 2 exhibited a photoresponse of 277%, approximately 27 times higher than that of Sample 4, which showed the lowest value. Furthermore, the response time of Sample 2 was about 20 times faster than that of Sample 4.

Fig. 5. 
Photoresponse and response time characteristics of the fabricated SWIR photodetectors.