Recently, the demand for sensors that can detect light in the shortwave infrared (SWIR) region has increased rapidly in fields such as automation [1]. Traditionally, semiconductors such as InGaAs [2] and Ge [3] are used for this purpose. These semiconductors are produced through epitaxial growth; however, this method is time-consuming and expensive [4]. Colloidal quantum dots (CQDs) offer a promising alternative. Their bandgap can be controlled by changing the physical size [5], making them suitable for various optoelectronic devices [6]. Additionally, CQDs can be synthesized in solution, requiring simpler reactors for synthesis, which results in significant cost savings compared with conventional semiconductor epitaxial growth [7]. Therefore, PbS CQDs provide an optimal alternative to traditionally grown semiconductors for SWIR sensing.

To fabricate a photodiode (PD) using CQDs, it is important to increase the conductivity of the CQD thin film [8]. This is achieved by replacing native ligands with shorter ligands [9,10]. Although these surface ligand treatments control the electrical properties, they also affect the optical properties. Therefore, when fabricating CQD-based SWIR PDs, it is essential to manage both the CQD size for light absorption and the optical design of the PD.

In this study, we fabricated a PD using CQDs tailored to absorb light at approximately 1500 nm. Next, we analyzed the dependence of the external quantum efficiency (EQE)—an indicator that quantifies the degree to which light is converted into electrons—on the thickness of the active layer. Through our investigations, we discovered that these changes originated from the optical interference effects within the CQD PD. By evaluating the optical properties of the conductive CQD thin films, the factors affecting their PD optical properties were identified, and a PD design method was proposed.

The chemical materials used in this study were purchased from Sigma-Aldrich Chemical Co. and Tokyo Chemical Industry Co. and used without further purification. To synthesis PbS CQDs, lead acetate trihydrate [Pb(Ac)₂] (1.15 g), oleic acid (2.1 mL), and octadecene (ODE) (30 mL) were added to a three-neck flask and degassed at 100 °C for 6 h under vacuum conditions. After the degassing, bis(trimethylsilyl) sulfide [(TMS)2S] in ODE was injected into the three-neck flask with a lead oleate solution under vigorous stirring. PbS CQDs with absorption exciton peaks at 1500 nm were injected via the multiinjection method at 120 °C [11]. When the target wavelength was reached and the reaction was completed, the crude solution was cooled to room temperature. The crude solution was washed twice using acetone and methanol. Subsequently, it was dispersed in octane at a concentration of 50 mg/mL.

To fabricate conductive CQD films, ammonium iodide (0.174 g) and lead iodide (0.552 g) were dissolved in dimethylformamide (10 mL) for solution-phase ligand exchange [12]. After the purified PbS CQDs were prepared at a concentration of 10 mg/mL, they were mixed with the iodide solution for 3 min. After the solution-phase ligand exchange, the PbS CQDs were precipitated with toluene and dried in a vacuum oven. The dried PbS CQD powder was produced as a thin film using a solvent mixture of butylamine (BTA) and difluoropyridine (DFP). The dried PbS CQD powder, which had a bandgap of approximately 1.35 eV, was well dispersed in BTA. Conversely, PbS CQD powder with a bandgap of 0.8 eV exhibited poor dispersion in BTA. To address this issue, a mixture of DFP and BTA at a 1:1 volume ratio was used to enhance the dispersion. Owing to the high dielectric constant of DFP, this mixture facilitated improved dispersion of the PbS CQD powder. A PbS CQD thin film with the desired thickness was fabricated using the PbS CQD powder in a mixed solvent of BTA and DFP.

The patterned indium tin oxide (ITO) substrates were cleaned via sonication in ethanol, acetone, and deionized water. A zinc oxide (ZnO) layer was fabricated on the patterned ITO substrate via rotation at 3,000 rpm for 30 s and annealing at 50 °C for 12 h [13]. Subsequently, a 100–350-nm-thick conductive CQD (PbS-I) layer was spincoated onto the ZnO substrate using PbS CQD powder and a mixed solution of BTA and DFP. A 20-nm-thick ethanedithiol-passivated PbS CQD layer was coated onto the substrate [14], and gold electrodes were deposited via thermal evaporation.

The absorption spectra were measured using an ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu, UV2600). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were obtained using FEI Tecnai G2 and ZEISS EVO 10 instruments, respectively. The complex optical indices were measured using an ellipsometer (ELLIPSO Technology). The light absorption of the active layer of the CQD PD was calculated using complex optical indices and the transfer matrix method (TMM). The EQE spectra were measured using a xenon lamp, monochromator, and lock-in amplifier in the alternating-current mode at 25 Hz.

3.1. EQE of PbS CQD PDs

Figure 1(a) shows the absorption spectrum of the PbS CQD, with the first exciton peak at 1511 nm. The inset of Fig. 1(a) shows a TEM image indicating that the average particle size was 5.54 nm. To fabricate a SWIR PD with these CQDs, conductive CQD thin films were fabricated by exchanging native ligands with iodide ligands [14]. This ligand exchange was achieved via a solution-phase process [12]. The PD was fabricated by depositing a conductive CQD film on a glass/ITO (150 nm)/ZnO (150 nm) substrate, as shown in the inset of Fig. 1(b). Figure 1(b) presents the EQE spectra of the fabricated CQD PD. The EQE changes were analyzed according to the thickness of the active layer. A fringe pattern appeared owing to the optical interference effect of the multilayer thin film. Typically, a thickness of >1 μm is required for sufficient light absorption in the SWIR region. However, the EQE at a wavelength of 1550 nm for the CQD thin film prepared at a concentration of 150 mg/mL exceeded those for thick films produced at concentrations of 300, 350, and 400 mg/mL. This is explained as follows: in films with a concentration above150 mg/mL, it exceeds the thickness that can effectively collect charges excited by light. However, considering that the PD fabricated at 500 mg/mL exhibited the highest EQE, this phenomenon cannot be solely attributed to electrical limitations.

Figure 1. (a) UV-vis absorption spectrum of PbS CQDs in solution. TEM image, with the average size of CQDs shown in the inset. (b) EQE spectra of PbS CQD PDs with respect to the PbS CQD active layer thickness. The architecture of the PbS CQD PDs is shown in the inset.

3.2. Optical properties of PbS CQD films

To investigate the optical properties of the PbS-CQD PDs, we examined the thickness and optical characteristics of the conductive PbSCQD thin film. Figure 2(a) shows the thickness of the formed thin film with respect to the concentration of the PbS CQD powder, as assessed through cross-sectional SEM analysis. Notably, the thickness of the CQD thin film increased with the concentration of PbS CQD powder. The results of the ellipsometry analysis of the complex refractive index of the conductive PbS CQD thin film are presented in Fig. 2(b). The process of exchanging native ligands with atomic ligands to enhance the conductivity led to a larger proportion of high refractive indices. Under the effective medium approximation, CQD thin films composed of highly conductive short ligands or atomic ligands exhibit an increase in refractive index as the inorganic content per unit volume increases. In particular, iodide-passivated PbS CQD thin films exhibit high refractive indices of >2.8 in most wavelength ranges. Additionally, increasing the conductivity by reducing the distance between the CQDs increases the extinction coefficient, owing to an increase in the number of CQD particles per unit volume [15]. Increasing the extinction coefficient not only increases the absorption but also increases the Fresnel reflection at the ZnO and CQD thin-film interface owing to the increased refractive index [13]. This enhanced Fresnel reflection increased the degree of light interference within the active layer of the PbS CQD PDs.

Figure 2. (a) Thickness of conductive PbS CQD thin films with different concentrations of CQD powder. The inset shows a cross-sectional SEM image. (b) Complex optical refractive index of a conductive PbS CQD film measured via ellipsometry.

3.3. Optical interference in PbS CQD PDs

When native ligands are exchanged to increase the conductivity of the CQD film, an increase in the refractive index results in light interference effects. This optical interference can enhance or hinder light absorption at certain wavelengths, regardless of the intrinsic light absorption properties of the CQD. To investigate this phenomenon, we evaluated the light absorption with respect to the active-layer thickness of the PbS CQD PDs using the TMM [16], as shown in Fig. 3(a). At wavelengths of <800 nm, most of the light was absorbed. However, at longer wavelengths—particularly those exceeding 1000 nm—the optical interference determined whether the absorption increased or decreased with an increase in the thickness of the active layer. When the CQDs were passivated with iodide for higher conductivity, their refractive index increased, causing interference effects on the PDs. Increasing the thickness of the PbS CQD thin film to 125 nm increased the light absorption at 1525 nm. However, when the film thickness reached 250 nm, the absorption decreased, and beyond 250 nm, the absorption increased again, as shown in Fig. 3(b). The EQE data of the PbS CQD PDs at a wavelength of 1525 nm extracted from Fig. 1(b) are shown as red dots in Fig. 3(b), and they exhibit a similar trend to the calculated optical absorption. These findings indicate that the size control of CQDs and optical design of PDs can be performed simultaneously to control the optical absorption region of CQD PDs.

Figure 3. (a) Contour plot showing the absorption of the PbS CQD active layer calculated using the TMM with respect to the thickness of the PbS CQD layer and the wavelength of incident light. The first absorption exciton peak at 1,525 nm is indicated by the red dotted line. (b) Calculated absorbance (solid line) of the PbS CQD layer at a wavelength 1,525 nm with respect to the thickness of the PbS CQD layer and the measured EQE (red dots) of the CQD PDs.

In our study on sensors in the SWIR region using CQDs, we observed that the EQE and optical absorption properties of the PD depend on the thickness of the CQD active layer. Enhanced conductivity was achieved through a ligand exchange process during the PD fabrication. This resulted in optical interference within the PD owing to the high refractive index of the conductive CQD thin film. Thus, to effectively detect light in the SWIR region, the PD design must account for this optical interference and allow tunable absorption through control of the CQD size.

This research was supported by the Basic Science Research Program (Nos. 2022R1C1C1004981 and 2021R1C1C1005093) through the National Research Foundation of Korea and Grant RS-2022- 00144108 funded by the Ministry of Trade, Industry, and Energy of the Korean government.

The authors declare no conflicts of interest.