In these cases cited above, there is no more than 900 nm NIR light upconverted to visible light. In 2018, Strassel and his co-workers extended the NIR spectral peak sensitivity up to 1000 nm, meanwhile, this device is visibly transparent and the innovative use of NIR selective squaraine dye is the highlight of this work [41]. In 2019, the same team reported a solution-processed upconversion device, by adding an electrolyte to the super yellow (SY) emitter (Figure 3e), thereby transforming the OLED into a light-emitting electrochemical cell (LEC), which is more fault-tolerant and has a lower driving voltage than traditional OLED [39]. In the same year, Yeddu et al. raised infrared sensitivity up to 1.1 μm. For such long-wavelength infrared light detection, the detector requires low bandgap organic semiconductors, therefore, they synthesized a novel low bandgap polymer poly 4-(4,8-bis (5-(2-butyloctyl) thiophen-2-yl) benzo [1,2-b:4,5-b ] dithiophen-2-yl)- 6,7-diethyl- [1,2,5] thiadiazol [3,4-g] quinoxaline (PBDTT-BTQ) with strong photoresponse in NIR wavelengths of 700−1100 nm which the previously used Si-based PDs cannot offer. PBDTT-BTQ's infrared absorption is superior to others as shown in Figure 3f [40].

**Figure 3.** Structures and performance characterization of all-organic NIR-to-visible upconversion devices. (**a**) Structure and working mechanism of the upconversion device under dark (top image) and illumination condition (lower image) [35]. Copyright 2018, Materials Horizons. (**b**) Picture of the lineshadow mask and LED in night vision (left picture), and the picture of the line-shaped shadow mask when an obstacle (waste-water-filled cloudy box) was placed between the mask and the camera (right picture) [36]. Copyright 2015, Advanced Material. (**c**) Structure of the upconversion devices based on TADF-OLEDs [37]. Copyright 2017, ACS Photonics. (**d**) Structure of the upconversion device [38]. Copyright 2018, The Journal of Physical Chemistry Letters. (**e**) Structure of the upconversion device that stacks with a LEC or an OLED as visible light-emitting component [39]. Copyright 2019, ACS Applied Materials & Interfaces. (**f**) Comparison of absorption coefficients of Si, P3HT (film), and PBDTT-BTQ (film). Inset shows the molecular structure of PBDTT-BTQ [40]. Copyright 2019, ACS Photonics.

#### 2.2.2. SWIR-to-Visible Upconversion

In 2020, Li and his co-workers reported their efforts in all solution-based process SWIR-to-visible upconversion devices. They fabricated a blend of a diketopyrrolopyrrole– dithienylthieno[3,2-b] thiophene (DPP–DTT) polymer donor and a hetero-polycyclic aromatic compound SWIR dye with a CsPbBr3 perovskite LED. This device can detect infrared light up to 1400 nm and convert it to 516 nm visible green light [42]. The spectral detection of hybrid upconverters is expanded to the SWIR range for the first time. This advanced device can be operated without cooling and was proved to apply in biological fields, such as providing reliable heart rate monitoring. The structure and performance of this upconverter are shown in Figure 4.

**Figure 4.** Structure and performance characterization of all-organic SWIR-to-visible upconversion devices. (**a**) A schematic of the measurement setup. (**b**) The heartbeat waveform measured by the device using NIR (850 nm) and SWIR (1050 nm) light illuminations, this picture shows a heart rate of 82 beats min−1. (**c**) The absorption spectrum of the DPP–DTT: SWIR dye blend layer deposited on glass. The insets in (**c**) are molecular structures of the DPP–DTT polymer and the SWIR dye. (**d**) R(λ) of the organic SWIR PDs, with different BHJ layer thicknesses, as a function of the wavelength. The inset in (**d**) is the structure of this organic SWIR PD. (**e**) R(λ) of an organic SWIR PD as a function of the wavelength with a 200 nm thick BHJ layer operated under different reverse biases. (**f**) R(λ) of an organic SWIR PD, with a 200 nm thick BHJ, measured at different temperatures [42]. Copyright 2020, Advanced Science.

In summary, the infrared-to-visible upconversion devices have developed from the traditional all-inorganic structure to the widely used all-organic structure at present, successfully achieving low-cost infrared-to-visible light upconversion imaging. However, for almost every all-organic upconversion device only the NIR band can be detected, and the upconversion efficiency of the device is very low. Generally, it can only work under the high-power infrared laser, which cannot meet the requirements of practical application.

#### *2.3. Hybrid Organic–Inorganic Upconversion Devices*

Previously, blended organic and inorganic materials were synthesized and manufactured to form devices. The flexibility of organic chemistry allows organic molecules to form with useful luminescent and conducting properties. Due to the stronger covalent and ionic bonds, inorganic materials can be designed to have better thermal/mechanical stability and higher electromigration. Using different emissive organic materials, the wavelengths of OLED emissions can vary across the full visible spectrum. However, inefficient photo carrier injection from inorganic detectors to OLEDs results in low infrared-to-visible upconversion efficiency. Therefore, there is still a lot of work for researchers to do in order to enhance devices' performance.

#### 2.3.1. NIR-to-Visible Upconversion

In 2008, Chen et al., of the University of Waterloo, added a Ti/Au thin film between the InGaAs infrared detector and the OLED, and the conversion efficiency of the integrated upconversion device was improved to 1.5% [43]. Based on this research, in 2010, Chen et al. obtained an optical upconverter by optimizing the structure of an InGaAs/InP heterojunction phototransistor (HPT) with a gain mechanism and integrating it with an OLED [44]. The structure and equivalent circuit diagram of this upconverter are shown in Figure 5a. The optical upconverter not only realizes the conversion of 1.5 μm NIR to green light (520 nm) at room temperature but also achieves the optical power conversion efficiency of 1.55 W/W (corresponding to an external quantum efficiency of 59%).

In 2012, Chen et al., proposed a pixel-less imaging upconversion device, they employed Technology Computer-Aided Design software (TCAD) to simulate the propagation of photo-generated hole carriers in the upconverter, it was found that no matter how large the size of the incident infrared beam is, the concentration distribution of photo-generated holes at the intermediate interface of i-InGaAs/C60 shows that the lateral propagation distance is less than 1 μm [45]. The NIR-to-visible light upconverter was used for pixel-less imaging. The image displayed under the NIR illumination through the shaped hole and the characteristic length of the shaped hole and the image are shown in Figure 5b. From the image they demonstrated, the actual resolution of the device can be calculated. The resulting spatial resolution of 6 μm is smaller than its calculated theoretical value of 1 μm. According to their analysis, the small resolution was mainly due to the limited resolution of digital cameras (~6 μm). However, the infrared pixel-less imaging device achieved a spatial resolution better than 6 μm, which was sufficient for practical applications.

Generally, due to the serial device architecture, the turn-on voltage of the upconversion devices is high. In 2018, Yu et al. reported a low turn-on voltage upconverter, the photocarrier can be efficiently injected into the OLED from the perovskite layer under NIR light, due to the perovskite layer supplied extra voltage to activate the OLED at a 1.9 eV sub-band voltage. On this account, the upconversion device lights up at 1.9 V [46], which is lower than the bandwidth voltage of the emitting layer in the OLED. Further on, Yu et al. demonstrated the pixel-free NIR imaging at a low operation voltage of 3 V which is shown in Figure 5c.

In the same year, Ding et al. reported an NIR-to-visible upconversion strategy based on fully integrated microscale optoelectronic devices, and the dimension of this device is 220 μm × 220 μm (Figure 5d). These devices were implanted in behaving animals, and their effectiveness in photogenetic nerve regulation was proved by in vitro and in vivo experiments. They proposed the theory of the photon–"free electron"–photon process eliminating the limitation of traditional nonlinear upconversion methods [47]. In 2019, Shi et al. reported an ultrafast and low-power optoelectronic NIR-to-visible upconversion device, similar to Ding's theory, they applied the photo–"free electron"–photon concept. Their devices can be excited at low power (1–100 mW/cm2) and the devices' luminescence decay lifetime (Figure 5e) can be adjusted from ~20 to ~200 ns. Such unique characteristic is beneficial for the application of high-flow chemical detection and dynamic biological stimulations [48].

**Figure 5.** Structures and performance characterization of hybrid organic–inorganic NIR-to-visible upconversion devices. (**a**) Structure of the hybrid upconversion device with integrated InGaAs/InP HPT and OLED. The right picture demonstrates a highly reflective metal layer that inserts in the upconverter, and the embedded optical mirror could improve the absorption and emission efficiency [44]. Copyright 2010, Advanced Materials. (**b**) Picture A of the shaped aperture showing the letters "IR". Picture B of the operating device at 10 V with NIR illumination through the aperture. Picture C is the shape aperture design showing the minimal feature size. Image D of the operating device is overlaid with the shape of the aperture indicating the minimum size of the captured function [45]. Copyright 2012, Advanced Material. (**c**) Band diagram and working mechanism of the upconversion device under the dark and under NIR illumination [46]. Copyright 2018, ACS Applied Materials & Interfaces. (**d**) Microscopic image of upconversion devices with red (**left**) and yellow (**right**) emissions under IR excitation [47]. Copyright 2018, PANs. (**e**) Lifetimes and typical excitation power densities of representative upconversion mechanisms, including lanthanide-based, TTA-based, quantum dot–quantum well-based, and optoelectronic-device-based upconversion designs of this reference [48]. Copyright 2019, Photonics Research.

#### 2.3.2. SWIR-to-Visible Upconversion

In 2007, Ban et al. took the lead in proposing an organic–inorganic hybrid upconversion device integrating in series with an InGaAs/InP infrared detector and an OLED and successfully upconverted SWIR light of 1.55 μm that can be converted into green visible light of 520 nm. However, the conversion efficiency of this infrared-to-visible upconversion

device is only 0.7% [49]. The structure and performance of this upconverter are shown in Figure 6a,b.

**Figure 6.** Structures and performance characterization of hybrid organic–inorganic SWIR-to-visible upconversion devices. (**a**) Structure of the inorganic–organic upconversion device. (**b**) Current– voltage–luminance curves of the upconversion device under 1.5 μm infrared illumination (input power density of 1.1 mW/mm2). The inset shows the emission spectrum from the OLED of the integrated upconversion device with a wavelength peak at 520 nm (green light) [49]. Copyright 2007, Applied Physics Letters. (**c**) Schematic diagram of the three configurations of the hole transform layer (HTL) at the semiconductor–organic interface of the integrated devices. (**d**) OLED green emission luminance of the device A, B, C as a function of the device bias. (**e**) Schematic diagram of the crosssection of one organic–inorganic upconverter device. (**f**) Emission spectrum from the OLED of the integrated upconverter, the peak wavelength of this OLED is 520 nm (green light) [50]. Copyright 2007, IEEE Transactions on Electron Devices.

For an OLED device, its structure is generally composed of an anode, hole injection layer (HIL), hole transport layer (HTL), emission layer, electron transport layer (ETL), electron injection layer (EIL), and cathode. Obviously, in addition to the light-emitting layer, other parts will also affect the performance of the device. Ban et al. also compared the effects of different HTL layer structures on the device performance as shown in Figure 6c. The HTL layer of device C has only one layer of N, N -di(naphthalene-l-yl)-N, N -diphenylbenzidine (NPB), device B adds a layer of CuPc under NPB, and device A continues to add a layer of C60 under CuPc [50]. It can be seen from Figure 6d that the organic–inorganic interface has a great influence on the efficient injection of holes, and the turn-on voltage of device C is different from that of devices A and B, because the addition of C60 can reduce the carrier injection barrier. The structure and emission spectrum of this upconverter are shown in Figure 6e,f.

#### *2.4. Colloidal Quantum Dots-Based Upconversion Devices*

From what was discussed above, most all-organic and hybrid upconversion devices can only convert NIR light into visible light, but usually cannot detect short-wave infrared light, and the application range is severely limited. Besides, the upconversion efficiency of the device is very low [29,37,40–42,45,49], for these organic–inorganic upconversion devices, which are limited by the low photon–electron conversion efficiency of the infrared PD part. Generally, infrared-to-visible upconversion devices can only work under the high-power infrared laser, which cannot meet the practical application requirements. Most of the reported infrared upconversion devices need to be fabricated by high-cost vacuum deposition methods [35,37,45,46], and as for inorganic infrared detectors in hybrid upconversion devices, they still need to be fabricated by epitaxial growth, both of these two problems make the fabrication cost of organic–inorganic hybrid upconversion devices still expensive and unsuitable for large-area imaging applications. In addition, the reported infrared upconversion devices have high operating voltages, which are not conducive to the preparation of flexible devices.

Quantum dots (QDs) have narrow emission linewidth and adjustable bandgap, so that tunable infrared response can be realized in a wide spectral range, and it is the reason that QDs have great potential value in developing quantum information and opto-electronic devices. For example, bulk mercury telluride (HgTe) materials are semi-metallic materials with zero bandgap and large exciton Bohr radii. With the help of the "quantum confinement effect", HgTe QDs [8–10,51–56] can theoretically achieve full spectrum coverage in the infrared to terahertz range. Therefore, the application of colloidal QDs (CQDs), in the field of infrared detectors can realize infrared detection in all major infrared bands of NIR, SWIR, and MIR. In addition, the luminescence spectrum of CQDs is extremely narrow, the color saturation and purity are high, and the optical stability is very good, which can be synthesized in a large-scale liquid phase. Therefore, quantum dot LEDs (QLEDs) have excellent performances of a wide color gamut, long life, and low cost.

CQDs, in particular, can be synthesized by solution treatment, and used in photovoltaics, luminescence, and photodetection. Except for the top electrode, the entire infrared upconversion device is prepared by solution method, which greatly simplifies the preparation of the device. Therefore, the infrared upconversion devices based on CQDs, benefiting from the property of CQDs, can be processed by solution method and can be fabricated into flexible devices.

#### 2.4.1. NIR-to-Visible Upconversion

In 2011, Kim and his co-workers reported their achievement of a low-cost upconversion device with infrared sensitivity up to 1.5 μm using PbSe inorganic colloidal nanocrystals as infrared PD. It was the first time that the NIR-to-visible light converter of CQDs' integrated phosphorescent emitting OLED was reported, the structure and schematic diagram are shown in Figure 7a,b. It must be known that all-organic upconversion devices before this work nearly had no infrared sensitivity beyond 1 μm, moreover, there were not any hybrid devices that could reach a 1.3% photon-to-photon conversion efficiency (that this work obtained) in the past. In contrast to previous studies, Kim et al. employed a wide bandgap ZnO layer as a hole blocking layer to improve the on/off ratio of the device, and the dark current of the device was obviously reduced, moreover the signal-to-noise ratio of the device was increased [57].

**Figure 7.** Structures and performance characterization of CQDs based NIR-to-visible upconversion devices. (**a**) Structure of PbSe QD infrared-to-green light upconversion device. (**b**) Schematic energy band diagrams of PbSe QD upconversion devices in the IR illumination [57]. Copyright 2011, Nano Letters. (**c**) Structure of the upconversion device. As shown in the diagram, the colloidal nanocrystals PbS are used as a gate sensitized to the infrared, and an ITO porous layer is used as source electrode of the phototransistor. (**d**) Photo of the sample attached to the measuring case in ambient light (left), and photos of the area of the device (0.04 cm2) with and without infrared lighting (λ = 940 nm), absent ambient light (right) [58]. Copyright 2016, Nature Photonics. (**e**) Diagram showing the cross-section view of a NIR–visible light upconversion device, with a NIR photodiode and a visible light LED based on CsPbBr3 perovskite. (**f**) Molecular structures of DPP–DTT, PC70BM, and COi8DFIC, and (**g**) their corresponding normalized absorbance spectra [59]. Copyright 2018, Advanced Optical Materials. (**h**) The structure of the conversion system and circuit show, schematically, the principle of operation of this upconversion device. (**i**) The photon-to-photon conversion efficiency of the device, under different power densities of NIR, as a function of the applied voltage [60]. Copyright 2020, IEEE Access.

In 2016, Yu et al. reported a novel upconversion light-emitting phototransistor (LEPT) by incorporating a phosphorescent OLED into a phototransistor. To be different from the ascending upconverters with two terminals, this LEPT is a vertical three-terminal phototransistor with an infrared photoactive gate integrated into an OLED, which has highefficiency such as its external quantum efficiency (EQE) of up to 1 × 105% and detectivity of 1.23 × 1013 Jones [58]. The structure of this LEPT is demonstrated in Figure 7c, and a photograph of the sample clamped in the measurement box is shown in Figure 7d. In 2018, Li et al. demonstrated a perovskite LED based on CsPbBr3 with high performance, it can be processed in solution and has a narrow emission spectrum. Their research proved that the NIR-sensitive polymer DPP–DTT blend with a norfullerene acceptor COi8DFIC is an appropriate layer of charge generation layer for the upconversion procession, as a result of its unique combination of high NIR absorption and low visible light absorption (Figure 7e–g) [59]. In 2020, Tang and his co-workers proposed a high conversion efficiency for all QDs-based upconverters (Figure 7h). Former upconversion systems were always manufactured by inorganic semiconductors using wafer fusion technology. However, they were limited by the mismatch of lattice between different semiconductors, leading to poor photonic conversion efficiency. As for the all-organic devices, their efficiency has been improved a lot, but the stability of these upconverters is still a potential issue for their application. In this reference, the use of innately stable, efficient, and high-performance QDs made the device achieve a high photon-to-photon conversion efficiency of 6.3% (Figure 7i), and can be fabricated by solution process [60].

#### 2.4.2. SWIR-to-Visible Upconversion

In 2020, Ning Zhijun's research group from ShanghaiTech University integrated CQD infrared detectors and colloidal QLED for the first time to obtain a CQD infrared-to-visible light upconversion device (Figure 8a). This all-CQDs upconversion device had high photonic conversion efficiency of up to 6.5%, and it can be turned on at a low applied bias of only 2.5 V. Through this upconverter the short-wave infrared (Figure 8b) with a power density of 10–600 mW·cm−2 was converted into visible green light (520 nm) (Figure 8c). Ning's group incorporated silver nanoparticles in the PD's layers that can extract charges (Figure 8d) with the aim of optimizing the carrier tunneling and making the PD layer provide enough photocurrent so that the LED can be turned on successfully. This advanced achievement confirms the application of CQDs in upconversion devices and the feasibility of realizing SWIR upconversion imaging [61].

**Figure 8.** Structures and performance characterization of CQDs based SWIR-to-visible upconversion devices. (**a**) Structure and composition of upconversion devices. (**b**) Photoluminescence spectra of CdSe/ZnS core/shell QDs, with the peak position at 525 nm. Inset: transmission electron microscopy (TEM) of CdSe/ZnS QDs. (**c**) Absorbance of PbS QD thin films, with the exciton peak position at 1500 nm. Inset: TEM of PbS QDs. (**d**) PD energy band graph with Ag nanoparticles in ZnO film. Without Ag nanoparticles (top) or in darkness (middle), the hole transfer from the ITO film to ZnO is blocked by the energy level shift. Under the lighting (bottom), holes enter the ZnO film [61]. Copyright 2020, Nature Electronics.

#### 2.4.3. MIR-to-Visible Upconversion

In 2020, Motmaen reported a novel and high-performance integrated chip which can realize the upconversion process from MIR (3–5 μm) to the green light (523 nm). This upconversion device was fabricated using a doped PbSe layer as a PD, an NPN (*n*-type, *p*-type, *n*-type) bipolar junction transistor (NPN-BJT) as a current amplifier, and an OLED as an emission layer; its structure is shown in Figure 9a, the energy band diagram is shown in Figure 9b. In this new type of upconverter, PbSe QDs were used to realize the photodetection and generate current, then the output electric information can be amplified by NPN-BJT. With the amplified current the LED can be driven and emits visible green light [62]. Under 3 μm MIR light illumination (0.5 mW/cm2) (Figure 9c) and total voltage of 13.5 V, the EQE is calculated to 600% (Figure 9d), such high EQE is mainly thanks to the amplifier function of NPN-BJT.

The liquid semiconductor QDs synthesized by colloidal chemistry can precisely tune the bandgap by adjusting the size, and then achieve tunable infrared photoresponse in a wide spectral range. Therefore, from the above demonstration, it can be seen that the application of CQDs in the field of infrared-to-visible upconversion can realize infrared detection in all main infrared bands of NIR, SWIR, and MIR.

Therefore, after several decades of development, infrared-to-visible upconversion devices have shown a transition trend from traditional all-inorganic, all-organic, and organic–inorganic materials to emerging liquid CQD materials. The development of infrared-to-visible light upconversion devices based on CQDs has become one of the important trends to achieve wide detecting spectrum, high-performance, low-cost, flexible, and large-area infrared imaging in the future. The performance of the upconversion devices in the references discussed above is shown in Table 1.

**Figure 9.** Structures and performance characterization of CQDs-based MIR-to-visible upconversion devices. (**a**) Structure of MIR-to-visible light upconversion device. (**b**) Schematic shows the energy band diagram of the PbSe/ ZnS quantum dots PD and the transition inside PD. The transfer of electrons from the energy levels of the excited state to the continuum by means of the thermal energy of the environment causes a dark current (black circle, Idark). The photons excite electrons from energy levels of ground state to energy levels of excited state, resulting in a photocurrent (red circle, Iphoto). (**c**) The output power density depending on the lighting power density MIR with different wavelengths. (**d**) The efficiency of PbSe upconversion device as function of the applied voltage under MIR light illumination (λ = 3 μm, 0.5 mW/cm2) [62]. Copyright 2020, Scientific Reports.

**Table 1.** Progress in upconversion devices.

1 Materials are mixed together; 2 Materials are integrated but not mixed together; Abbreviations: AI: all-inorganic; AO: all-organic; HOI: hybrid organic–inorganic; Alq3, Alq: tris-8-hydroxyquinolineAluminum; SnPc: tin phthalocyanine; CBP: 4,4-N, N-dicarbazole-biphenyl; Ir(ppy)3: fac-tris(2-phenylpyridinato) iridium (III); PDPP3T: Poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-diox-opyrrolo[3,4-c]pyrrole-1,4-diyl} -alt-{[2,2 :5 ,2-terthiophene]-5,5-diyl}]; PCBM: [6,6]-phenyl-C61-butyric acid methyl ester; Be(pp)2: Bis[2-(2-hydroxyphenyl)-pyridine] beryllium; Ir(ppy)2(acac): bis(2-phenylpyridine) (acetylacetonato) iridium (III); ClAlPc: chloroaluminum phthalocyanine; PC61BM: [6,6]- phenyl-C61-butyric acid methyl ester; 4CzIPN: 1,2,3,5-tetrakis(carbazol-9-yl)-4,6 dicyanobenzene; 2CzPN: 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene; 4CzTPN-Ph: 1,4-dicyano-2,3,5,6-tetrakis (3,6-diphenylcarbazol-9-yl)benzene; 2PXZ-OXD: 2,5-bis(4-(10-phenoxazyl)phenyl)-1,3,4-oxadiazole; SY: fluorescent poly(para-phenylene vinylene) copolymer called super yellow; PBDTT-BTQ: poly 4-(4,8-bis(5-(2-butyloctyl) thiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophen-2-yl) -6,7-diethyl-[1,2,5] thiadiazolo [3,4-g]quinoxaline; C545T: 10-2 benzothiazoly-1,1,7,7–tetramethyl-2,3,6,7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6,7,8-ij] quinolizine-11-one.

#### **3. Challenges and Perspectives**

Although infrared upconversion devices have made many remarkable signs of progress in the past few years and some advantages of them are irreplaceable, there are still some important problems that need further consideration.

Overall, the performance of infrared-to-visible upconversion devices remains to be significantly enhanced to meet application requirements. High conversion efficiency, low turn-on voltage, flexible device, and wide detection band are vital to upconverters. To meet these above challenges, we need to pay more attention to the following aspects.

**Author Contributions:** Conceptualization, X.T., T.R. and G.M.; methodology, M.C.; software, X.T., T.R. and G.M.; validation, X.T., T.R. and G.M.; formal analysis, M.C.; investigation, X.T., T.R. and G.M.; resources, X.T., T.R. and G.M.; data curation, X.T., T.R. and G.M.; writing—original draft preparation, T.R., G.M.; writing—review and editing, X.T.; visualization, X.T., T.R. and G.M.; supervision, X.T.; project administration, X.T.; funding acquisition, X.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China and the National Key R&D Program of China (NSFC No. 62035004, 2021YFA0717600, and NSFC No. 62105022).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

## **Simulation of Monolithically Integrated Meta-Lens with Colloidal Quantum Dot Infrared Detectors for Enhanced Absorption**

#### **Yan Ning, Shuo Zhang, Yao Hu, Qun Hao \* and Xin Tang \***

Department of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China<EMAIL_ADDRESS>(Y.N<EMAIL_ADDRESS>(S.Z<EMAIL_ADDRESS>(Y.H.) **\*** Correspondence<EMAIL_ADDRESS>(Q.H<EMAIL_ADDRESS>(X.T.)

Received: 13 October 2020; Accepted: 4 December 2020; Published: 14 December 2020

**Abstract:** Colloidal quantum dots (CQDs) have been intensively investigated over the past decades in various fields for both light detection and emission applications due to their advantages like low cost, large-scale production, and tunable spectral absorption. However, current infrared CQD detectors still suffer from one common problem, which is the low absorption rate limited by CQD film thickness. Here, we report a simulation study of CQD infrared detectors with monolithically integrated meta-lenses as light concentrators. The design of the meta-lens for 4 μm infrared was investigated and simulation results show that light intensity in the focused region is ~20 times higher. Full device stacks were also simulated, and results show that, with a meta-lens, high absorption of 80% can be achieved even when the electric area of the CQD detectors was decreased by a factor of 64. With higher absorption and a smaller detector area, the employment of meta-lenses as optical concentrators could possibly improve the detectivity by a factor of 32. Therefore, we believe that integration of CQD infrared detectors with meta-lenses could serve as a promising route towards high performance infrared optoelectronics.

**Keywords:** colloidal quantum dots; meta-lens; detectivity; optical concentrator

#### **1. Introduction**

With steady advancement, the maximum absorption wavelength of colloidal quantum dots (CQDs) has been extended from near-infrared to longer wavelength, providing a promising alternative to bulk infrared semiconductors like HgCdTe, InSb and type-II superlattices [1,2]. Among all the colloidal nanomaterials, mercury telluride (HgTe) CQDs have so far demonstrated the highest spectral tunability from the short-wave infrared (SWIR) [3,4] and mid-wave infrared (MWIR) [5–9] to the long-wave infrared (LWIR) [10,11] and terahertz (THz) region [12]. HgTe CQD-based photoconductors [7,8,13], photovoltaics [5,6,9], and phototransistors [14,15] have all been studied. Among them, HgTe CQD photovoltaic detectors have unique advantages like the absence of 1/*f* noise [9] and fast operation speed [5]. SWIR detectors with detectivity up to 5×1010 Jones at room temperature [16], MWIR detectors with background-limited infrared photodetection (BLIP) performance at cryogenic temperature [6], SWIR hyperspectral detectors with 64 channels [17], CQD-silicon dual-channel detectors [18], and SWIR/MWIR dual-band imagers with vertical junctions [19] have all been demonstrated in photovoltaic configuration. Despite all the exciting achievements, most infrared CQD detectors still suffer from one common problem, which is the low absorption rate limited by the film thickness of CQDs. CQD detectors are usually made by layer-by-layer drop-casting, and for an infrared HgTe CQD detector, the regular thickness is ~400 nm and only 20% of incident light can be absorbed in the MWIR region [6]. Further increase of film thickness would lead to cracks and delamination due to mechanical stress from the drying and ligand exchange process [20]. Therefore, to improve the

performance of infrared CQD photodetectors, one needs to find a strategy to enhance light absorption without increasing CQD film thickness.

For infrared detectors, the key figure of merit is detectivity, *D*\*, which can be expressed by

$$D^\* = \frac{\sqrt{A}}{I\_{\text{ll}}} \Re \tag{1}$$

where *A* is the light sensing area, *In* is noise current, and is responsivity, which is related to absorption α by

$$\mathfrak{R} = \frac{\alpha \varepsilon}{hv} \mathfrak{n} \tag{2}$$

where *e* is elementary charge, *h* is the Planck constant, *v* is the frequency of photon, and η is internal quantum efficiency. Therefore, the absorption of CQD film is related to *D\** by

$$D^\* = \frac{\sqrt{A}}{I\_n} \frac{\alpha c}{hv} \mathfrak{n} \tag{3}$$

Strategies of using metallic plasmonic nanostructures, resonant cavities, and photonic crystals have all been reported to boost CQD absorption up to 2–3 times [6,16]. Those structures are fabricated using regular lithographic processes [11] or multi-beam laser interference lithography [21,22], which may change the surface condition of CQDs and introduce noise source into CQD detectors. To optimize *D\**, it is highly favorable to find a way to increase light absorption and decrease noise at the same time. The noise of a CQD photovoltaic detector mainly consists of Johnson noise, which scales with detector electric area *Ae* by a factor of *Ae* 1/2 [18]. Employment of optical concentrators, therefore, seems to be a suitable method to address such a challenge. With light concentrators, the electric area of a detector can be decreased while maintaining the same responsivity by improving light absorption, leading to enhanced detectivity. Compared to three-dimensional (3D) microlenses or microspheres, meta-lenses can be fabricated in scalable ways and can provide functionalities like controlling the phase, amplitude, and polarization of the incident light by simply altering the shape and the size of each meta-unit [23,24]. HgCdTe and GaSb detectors with meta-lenses have been reported. However, such a concept has rarely been investigated for infrared CQD photodetectors.

Here, we report a simulation study of CQD infrared detectors with a monolithically integrated meta-lens as light concentrator. The target spectral range is MWIR (3–5 μm). The design of the meta-lens in the center of MWIR (4 μm) is discussed. The simulation results showed that light intensity in the focused region is ~20 times higher than that of the incident light. More importantly, we find that the chromatic aberration of the meta-lens might unintentionally help the development of spectrally selective infrared detectors, which can be used to distinguish wavelength information of incident light. Besides simulation of the meta-lens, full device stacks were also simulated, and results showed that with a meta-lens, absorption of 80% can be achieved even when the electric area of the CQD detector was decreased by a factor of 64. Therefore, we believe that integration of CQD infrared detectors with a meta-lens could serve as a promising route towards high performance infrared optoelectronics.

#### **2. Modeling and Simulation**

COMSOL Multiphysics is used to conduct the simulation in our study. To save computation resources, two-dimensional (2D) simulation is used as shown in Figure 1. The light is introduced from the "incident port" with controlled power of 1 W. A listening port is placed on the top of the model. The left and right sides are set as "Scattering Boundary Condition" to avoid light reflection. The substrate material is chosen to be Al2O3 with a refractive index of 1.6 + 0*i*. The meta-lens units are made of silicon with a refractive index of 3.5 + 0*i*. For HgTe CQDs, the refractive index is 2.3 + 0.1*i* [6]. The rest of the model is set to be air with a refractive index of 1 + 0*i*. The absorption of CQDs is calculated by comparing the incident power and outlet power. The wavelength of incident light is

swept from 1.5 to 5.0 μm to calculate the spectral absorption. In out simulation, the diameter of the meta-lens is set to be 200 μm to match with our commonly used size of detector in experimental tests. The phase profile and meta-lens design are generated by using open source software called MetaOptics (version v2) [25]. In our design, we use an unpolarized plane wave with uniform intensity distribution across the meta-lens region. To lower fabrication complexity, a pillar structure with periodicity of 2 μm and height of 6 μm was used. Complete phase shift of 0–2π can be realized by changing the diameter from 0.6 to 1.7 μm. Further shrinking the size of the meta-lens (periodicity, diameter, and height) should still work as long as the light can still be focused onto the CQD detector.

**Figure 1.** Simulation model of CQD detector integrated with a meta-lens.

#### **3. Results and Discussions**

The proposed design of CQD photodetectors with a monolithically integrated meta-lens is illustrated in Figure 2a. Pillar structures made of silicon were selected as the material to construct the meta-lens unit. The incident light illuminated on the meta-lens can be concentrated and focused into the CQD detector. As shown in the cross-sectional illustration in Figure 2b, the electric area of the CQD detector is much smaller than the optical area of the meta-lens. To design a functional meta-lens, the first step is to calculate the required phase profile based on the lens equation

$$\phi(\mathbf{x}, y) = \frac{2\pi}{\lambda} n\_m (f - \sqrt{\mathbf{x}^2 + y^2 + f^2}) \tag{4}$$

where λ is the targeted wavelength, *nm* is the index of refraction of the medium, and *f* is the focal length. For a regular imaging lens, the phase profile for λ = 4 μm and *f* = 300 μm is shown in Figure 2c (black curve). For a meta-lens, the continuous phase profile modulation is achieved by periodically changing the effective index of refraction of the medium, which can be done by tuning the height and diameter of the silicon pillar in the meta-lens unit. The red curve in Figure 2c shows the required phase profile of a meta-lens, based on which the meta-lens design was generated using open source software called MetaOptics. In our design, a pillar structure with periodicity of 2 μm and height of 6 μm was used. By changing the diameter from 0.6 to 1.7 μm, a phase shift of 0–2π was covered. Figure 2d shows the 2D phase profile mapping and structure design of a meta-lens.

**Figure 2.** (**a**) Illustration of meta-lens integrated with CQD detectors. Incident light is collected and focused into CQD detectors with a small electric area. (**b**) Cross-sectional illustration of the meta-lens integrated with the CQD detector. Light illuminated onto the optical area is focused to the CQD detector with electric area *Ae*. (**c**) Phase profile of a regular lens (black curve) and meta-lens (red). (**d**) Design of meta-lens using pillar structures. The diameter and focal length are 200 and 200 μm, respectively. Inset is a representative 2D phase mapping used to generate the meta-lens design.

To verify the optical function of the meta-lens design, the light intensity distribution after passing through the meta-lens is simulated. Various values of numerical aperture (NA) were used and the results showed that the incident light could be successfully concentrated and focused into a smaller spot and that the focal length could be precisely tuned by changing the geometrical parameters of the meta-lens (Figure 3a). For a meta-lens with diameter of 200 μm, the diameter of the focused region is around 10–20 μm, depending on the NA (Figure 3b). Overall, the enhancement ratio of light intensity in the focused region is ~20, which is similar to previously reported experimental values [26].

Besides simulation of the meta-lens with different NAs, chromatic aberration was also investigated. For the meta-lens with a NA of 0.43 and targeted wavelength of 4 μm, we calculated the intensity distribution for incident light with different wavelengths. Chromatic aberration was clearly found, as shown in Figure 4a. As the wavelength deviates from the targeted center wavelength of 4 μm, the focal spot obviously shifts, moving the focused light away from the designed position where the CQD detector would be located. Chromatic aberration limits the operation spectral ranges of the meta-lens and is hard to correct. However, such properties could be potentially used to develop infrared detectors with spectral selectivity (Figure 4b). Spectral selectivity presents in all three meta-lenses (2.5, 4.0, and 4.8 μm). Away from the targeted wavelength, the enhancement ratio decreases rapidly. The simulation results clearly demonstrate the versatility and flexibility of meta-lenses.

**Figure 3.** (**a**) COMSOL simulation of light intensity distribution with various numerical apertures (NAs). (**b**) Enhancement ratio profile across the focused area (white dashed line in Figure 2a).

**Figure 4.** (**a**) Simulated light intensity distribution of a meta-lens with NA of 0.43 with light at different wavelength. (**b**) Spectral enhancement factors across the focused area for meta-lens designed for 2.5, 4.0, and 4.8 μm (dashed line in Figure 3a).

Finally, we simulated the full CQD device stacks with a meta-lens using COMOSL. The simulation model is shown in Figure 5a. The incident light is first focused by the meta-lens and then absorbed by the CQD layers. The diameter of the meta-lens is 200 μm and the width of the CQD detector is swept from 5 to 30 μm with the same thickness of 400 nm. As shown in the simulated light intensity distribution mapping, most of the incident light can be focused onto the CQD detectors by tuning the geometrical parameters of the meta-lens (Figure 5b). With the width of the CQD detectors increasing from 5 to 30 μm, the absorption gradually increases from 10% to 80% and saturates at a width of 25 μm (Figure 5c). Compared to the reference detector with a width of 200 μm without a meta-lens, it is impressive that 4 times more light can be absorbed by a smaller detector at the targeted wavelength of ~4 μm. Based on Equation (3) and previous discussion, higher absorption leads to higher responsivity (×4) and smaller detector results in smaller noise current (×0.125). Therefore, the employment of meta-lenses as optical concentrators could improve the detectivity by a factor of 32. Previous experimental studies of HgCdTe with meta-lenses reported an enhancement ratio of ~5.5, demonstrating that the concept of integration of meta-lenses with infrared detectors should work [26]. The current study mainly focuses on simulation and theoretical calculation, and experimental tests would be conducted to verify such an enhancement factor in the future.

**Figure 5.** (**a**) Simulation model used to calculate the absorption of CQD detector with meta-lens. (**b**) Simulated light intensity distribution of light passing through meta-lens. (**c**) Simulated absorption of CQD detector with width of 5 to 25 μm with meta-lens and simulated absorption from a reference detector with a width of 200 μm (no meta-lens).

#### **4. Conclusions**

In conclusion, our simulation results show that integration of meta-lenses with CQD detectors is a promising way to improve detector detectivity by increasing absorption and decreasing noise at the same time. Full device stack simulation shows that, with meta-lenses, a high absorption of 80% can be achieved even when the electric area of the CQD detectors is decreased by a factor of 64. With meta-lenses as optical concentrators, the detectivity can be improved by a factor of 32. By drop-casting HgTe CQDs on a commercial read-out integrated circuit, a previous study demonstrated that a MWIR thermal camera with a noise equivalent differential temperature (NEDT) of 102 mK [27] showed a simple fabrication route to low-cost thermal cameras with CQDs. Therefore, we believe that integration of CQD infrared detectors with meta-lenses could serve as a promising route towards both high performance single-element detectors and focal plane arrays.

**Author Contributions:** Conceptualization, X.T.; Methodology, Y.N., S.Z., and Y.H.; Supervision, X.T. and Q.H.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Science Foundation of China (NSFC) (62035004).

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

#### **References**

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## *Article* **Simulation and Design of HgSe Colloidal Quantum-Dot Microspectrometers**

**Chong Wen 1, Xue Zhao 1, Ge Mu 1, Menglu Chen 1,2,3 and Xin Tang 1,2,3,\***

**Abstract:** In recent years, colloidal quantum dots (CQD) have been intensively studied in various fields due to their excellent optical properties, such as size-tunable absorption features and wide spectral tunability. Therefore, CQDs are promising infrared materials to become alternatives for epitaxial semiconductors, such as HgCdTe, InSb, and type II superlattices. Here, we report a simulation study of a microspectrometer fabricated by integrating an intraband HgSe CQD detector with a distributed Bragg reflector (DBR). Intraband HgSe CQDs possess unique narrowband absorption and optical response, which makes them an ideal material platform to achieve high-resolution detection for infrared signatures, such as molecular vibration. A microspectrometer with a center wavelength of 4 μm is studied. The simulation results show that the optical absorption rate of the HgSe CQD detector can be increased by 300%, and the full-width-at-half-maximum (FWHM) is narrowed to 30%, realizing precise regulation of the absorption wavelength. The influence of the incident angle of light waves on the microspectrometer is also simulated, and the results show that the absorption rate of the HgSe quantum dot detector is increased 2–3 times within the incident angle of 0–23 degrees, reaching a spectral absorption rate of more than 80%. Therefore, we believe that HgSe CQDs are a promising material for realizing practical HgSe microspectrometers.

**Keywords:** colloidal quantum dots; distributed bragg reflector; microspectrometer

#### **1. Introduction**

Spectral detection is the core technology used in the fields of physicochemical property analysis, chemical composition analysis, and environmental monitoring [1–4]. It also shows great potential in emerging artificial intelligence networks, autonomous driving, and machine vision [5–7]. A spectrometer is generally composed of dispersive optics, a detector system, and an optical path adjustment system. An important development direction of the spectrometer is the miniaturization of spectrometers. Micro-spectrometers with a small size, low energy consumption, and low cost have become promising alternatives in many spectral detection fields. Currently, micro-spectrometers are mainly made by micro-nano optical processing or micro-electro-mechanical system (MEMS) technology, including photonic crystal arrays [8–10], arrayed waveguide gratings [11], planar etched gratings [12], micro-Michelson interferometers [13], and narrowband optical filters [14,15]. The use of non-silicon-based photodetectors is another way to fabricate spectrometers, including graphene detectors [16], organic photodetectors [17], and colloidal quantum dot detectors (CQDs) [18]. Compared with MEMS and etched gratings, spectrometers fabricated with detectors capable of detecting specific wavelengths have the advantages of compact structure and system stability.

**Citation:** Wen, C.; Zhao, X.; Mu, G.; Chen, M.; Tang, X. Simulation and Design of HgSe Colloidal Quantum-Dot Microspectrometers. *Coatings* **2022**, *12*, 888. https:// doi.org/10.3390/coatings12070888

Academic Editor: Aomar Hadjadj

Received: 8 April 2022 Accepted: 13 June 2022 Published: 22 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Infrared detectors play a key role in spectrometers. Colloidal quantum dots (CQD) have been intensively studied in various fields due to their excellent optical properties, such as size-tunable absorption features and wide spectral tunability. Therefore, CQDs are promising infrared materials to become alternatives for epitaxial semiconductors, such as HgCdTe, InSb, and type II superlattices. At present, the tunable bands of the developed CQDs have covered the short-wave infrared (SWIR) [19], mid-wave infrared (MWIR) [20], long-wave infrared (LWIR), and terahertz bands [21,22]. Photodetectors based on quantum dots for photoconductivity [23], photovoltaics [20,24], and phototransistors have been investigated [25]. Benefiting from the ease of processing and spectral tunability of CQDs, filter-free narrowband photodetectors [26], double-endedSWIR/MWIR dual-band detectors [27], and SWIR hyperspectral sensors have been investigated [28].

Among narrow-band optoelectronic materials, mercury selenide (HgSe) CQDs have thus far demonstrated the only Lorentzian lineshape of optical absorption in mid-wave infrared (MWIR), which is suitable for spectrally selective detection [29,30]. Despite the exciting properties of HgSe CQDs, most infrared CQD detectors still suffer from a common problem, a low absorption rate limited by the CQD film thickness [24]. CQD detectors are usually made by layer-by-layer drop-casting, and for an infrared CQD detector, the typical thickness is ~400 nm, and only 20% of incident light can be absorbed [31]. Another problem that needs to be improved is that HgSe CQDs still have a relatively broad FWHM. In this work, high-performance HgSe CQD microspectrometers are designed by integrating a HgSe CQD sensor and a resonant cavity (RC) composed of distributed Bragg reflectors (DBR). Finite element analysis of HgSe CQD microspectrometer detectors was systematically conducted.

The simulation results show that the spectrometer has strong wavelength selectivity, high absorption rate up to 91%, and extremely narrow full width at half maximum down to 0.2 μm in MWIR. The wavelength selection of the absorption peak can be achieved by adjusting the thickness of the optical cavity. Therefore, the integration of the narrowband CQD material HgSe with the resonator can achieve precise spectrally selective detection. It can be concluded that the spectrometer has the advantages of adjustable wavelength, narrow half-peak, high spectral resolution, high spectral responsivity, small size, and low cost.

#### **2. Modeling and Simulation**

COMSOL Multiphysics is used to conduct the simulation in our study. The proposed design of a CQD microspectrometer with a resonant cavity is illustrated in Figure 1a. A sapphire (Al2O3) with high transmittance in the infrared band is used as the substrate, and Ti3O5 and SiO2 are periodically deposited on it as the DBR. The optical spacer layer and CQD detector were fabricated on the DBR and capped with insulating layers and gold film. The DBR is composed of alternating layers of two materials, each of which has a different refractive index. The interference effect between reflected waves makes the reflection window of the DBR depend on wavelength. To save computation resources, a two-dimensional (2D) simulation is used, as shown in Figure 1b. There is only one physical field in the model, a simple parallel light field entering the device from the entrance port. The light is introduced from the "incident port" with a controlled power of 1 W. The wavelength of incident light is swept from 2.5 to 10.0 μm to calculate spectral absorption.

The listening port is placed at the bottom of the model. Considering that the size of the model is much smaller than the size of the actual device, we set the left and right sides as "continuous periodic boundary conditions" to avoid the influence caused by the limited width of the model. Below the incident port is the substrate, and the material of the substrate is chosen to be Al2O3 with a refractive index of 1.6 + 0i because of its low absorption for infrared light.

**Figure 1.** Architecture of the HgSe CQD microspectrometer. (**a**) Schematic of the CQD microspectrometer, consisting of a CQD sensor and filters. The optical spacer layer is under the DBR, and the quantum dot layer is sandwiched between the optical spacer layer and the optical reflection layer. (**b**) Simulation model of a CQD microspectrometer with a resonant cavity. (**c**) Spectral absorption plots of HgSe CQDs (red line), simulated spectral absorption plots (black lines). (**d**) TEM of HgSe CQDs.

The DBR is composed of alternating layers of high and low refractive index dielectric films. The material of the high refractive index layer of the DBR is selected as Ti3O5 with a refractive index of 2.35 + 0i, and the material of the low refractive index layer is selected as SiO2 with a refractive index of 1.4 + 0i. The optical spacer layer is used to regulate the thickness of the resonator cavity, and the material is also SiO2. HgSe CQD were used as the photosensitive layer of the micro-spectrometer, and the refractive index was obtained by fitting the actual absorption spectrum of HgSe CQD, which had an absorption peak with a central wavelength of 4.19 μm in the range of 1.67–10 μm, as shown in Figure 1c,d. The mean size of the CQDs is 4.7 nm, and the size distribution is uniform, with a standard deviation of 0.5 nm. The bottom of the model is the gold film, as the total reflection layer of light waves, which, together with the DBR and the optical spacer layer, constitutes an optical resonant cavity. The rest of the model is set to air with a refractive index of 1 + 0i. In the simulation, the width of the 2D model is 0.4 μm, the thickness of the Al2O3 substrate is 1.1 μm, the thickness of the HgSe CQD film is set to 0.4 μm which is the thickest thickness of the active layer in most CQD detectors [31], and the thickness of the DBR dielectric film layer is determined by [32]:

$$\mathbf{n}\_{\rm H}\mathbf{d}\_{\rm H} = \mathbf{n}\_{\rm L}\mathbf{d}\_{\rm L} = \lambda\_0/4 \tag{1}$$

where nH and nL are the refractive indices of the DBR high and low refractive index dielectric layers, dH and dL are the thicknesses of the two dielectric layers, and λ0 is the center wavelength of the DBR reflection window. In the design, we use unpolarized plane waves with uniform intensity distribution. To reduce manufacturing complexity, the spectrometer uses a vertically stacked architecture. The absorption, wavelength of the absorption peak, and FWHM can be regulated by adjusting the thickness of the optical spacer layer.

#### **3. Results and Discussion**

When light passes through the structure, reflected waves are generated between multiple interfaces. The bandwidth of the reflection window of the DBR is given by [33,34]:

$$
\Delta\lambda\_0 = \frac{4\lambda\_0}{\pi} \arcsin\left(\frac{\mathbf{n}\_{\rm H} - \mathbf{n}\_{\rm L}}{\mathbf{n}\_{\rm H} + \mathbf{n}\_{\rm L}}\right) \tag{2}
$$

where λ0 is the center wavelength of the bandwidth. Another important factor determined by the DBR is the reflectivity of the resonator top mirror. The reflectivity R of the distributed DBR is given by:

$$\mathcal{R} = \left(\frac{1 - \left(\frac{\mathbf{n\_H}}{\mathbf{n\_L}}\right)^2 \frac{\mathbf{n\_H^2}}{\mathbf{n\_n}\mathbf{n\_b}}}{1 + \left(\frac{\mathbf{n\_H}}{\mathbf{n\_L}}\right)^2 \frac{\mathbf{N\_H^2}}{\mathbf{n\_n}\mathbf{n\_b}}}\right)^2\tag{3}$$

where nH and nL are the refractive indices of the high refractive index layer and the low refractive index layer, na and nb are the refractive index of substrates on both sides of the DBR, and N is the number of layers of the DBR. For the resonant cavity, continuous resonance wavelength modulation is achieved by changing the optical length of the layer of the DBR and cavity, which can be done by tuning the thickness of the optical spacer layer and the layer of the DBR. In our design, the thickness of the DBR dielectric layer needs to match the absorption wavelength of the quantum dot narrowband material, so the absorption rate and FWHM of the photodetector source layer can be regulated by simply changing the number of DBR layers and the thickness of the optical isolation layer.

By introducing spectrally flat light as an illumination source, the optical field distributions in the resonant cavity are simulated, as shown in Figure 2. The core of the optical field is located on the boundary between the active layer and the optical spacer layer, which may be due to the absorption of photons by the active layer. Since the active layer placed in the resonant cavity is a narrowband CQD material, unwanted resonances and broadening FWHM of peaks are avoided, allowing tunable narrowband photodetection. The simulated absorption of the microspectrometer further demonstrates the accurate spectral tunability of our device.

The quantum efficiency depicting the absorption of a photodetector to optical signals is also analyzed. At zero bias, assuming that the optical active layer is defect free, η can be calculated using:

$$\eta = (1 - \mathbb{R}) \left( 1 - e^{-\alpha \mathbf{d}} \right) \tag{4}$$

where α(h*ν*) is the absorption coefficient, d is the thickness of the active layer, and R is surface reflection of the photodetectors. In our design, the total quantum efficiency ηtot of photodetector with RC is [35,36]:

$$\eta\_{\rm tot} = \frac{1 + \mathcal{R}\_2 \mathbf{e}^{-\alpha \mathbf{d}}}{1 - 2\sqrt{\mathcal{R}\_1 \mathcal{R}\_2} \mathbf{e}^{-\alpha \mathbf{d}} \cos\left(\frac{4n\pi}{\lambda} \mathbf{L} + \psi\_1 + \psi\_2\right) + \mathcal{R}\_1 \mathcal{R}\_2 \mathbf{e}^{-2\alpha \mathbf{d}}} \eta \tag{5}$$

where R1 is the corresponding power reflectivity of the top mirror composed of DBR of RC, R2 is the corresponding power reflectivity of the bottom reflective mirror composed of gold reflective film of RC, L, and n are the thickness and refractive index of the optical spacer layer, ψ1, ψ2 are the phase shift caused by the top and the bottom mirror of the resonant cavity to the reflected light, and λ is the wavelength of incident light. The improvement in the reflectivity of the top mirror increases the resonant effect of the resonator and hinders light from entering the resonator, so the quantum efficiency of the photodetector is greatly reduced when the number of DBR layers increases, which is proved in our simulation. In truth, in our simulations, when the number of layers of the DBR reaches three, the absorption rate of the CQD detector drops sharply. The full quantum efficiency is more dependent on the αd of the optoelectronic material. Limited by the layer-by-layer dropcasting method of the CQD detector, a further increase of film thickness would lead to cracks and delamination due to mechanical stress from the drying and ligand exchange process [31]. Additionally, the variation in the thickness of the optical isolation layer affects the periodic shift of the central wavelength of the spectral absorption peak.

The HgSe active layer in the device is replaced by a virtual broadband material whose imaginary part of the refractive index is constant, independent of the wavelength of light, unlike HgSe CQDs. It can be considered a broad-spectrum optoelectronic material with the same absorption for different wavelengths of light, and the results are shown in Figure 3. For broad-spectrum optoelectronic materials, the harmonic density and order of the spectral absorption peaks gradually increase, and the side mode suppression ratio also decreases gradually when the wavelength decreases and the thickness of the optical spacer layer increases. In our design, the absorption suppression of the harmonic sub-peaks of the resonant cavity comes from the unique intraband transition characteristics of HgSe CQDs as the active layer material. A unique intraband transition property of HgSe CQDs is that electronic transitions occur between the first two conduction band states, 1S e and 1P e of HgSe CQDs. When the incident photon energy Ephoton in the infrared matches the intraband energy gap Eintra, the light absorption reaches a maximum value, and if the energy of the incident photon is higher or lower than Eintra, the light absorption drops sharply, which creates the Lorentzian lineshape of the absorption properties of HgSe CQDs.

**Figure 3.** Spectral absorption of photodetectors with resonant cavities constructed from narrowband (HgSe, black line) and broadband (red line) optoelectronic materials at optical spacer layer thicknesses of 0.6 (**a**), 0.8 (**b**), and 1.0 (**c**) μm.

According to existing experimental conclusions, the influence of the size and size of the CQDs on the energy band can be further learned. The absorption peaks of HgSe redshift with increasing size, which is controlled by the reaction conditions for synthesizing CQDs [37,38]. By controlling the reaction time and temperature, the energy gap of the CQDs becomes smaller, and as a result, the Fermi level is in the Se state, defined as the two electrons in the conduction band of each CQD being in the Se state and the Pe state being empty. Compared to other electron doping densities, the dark current of HgSe film is reduced [39,40]. On the other hand, the uneven distribution of quantum dot size will also have an impact on carrier mobility. The carrier mobility of the HgSe film can be measured and calculated by the FET circuit, and the mechanism is explained by Marcus theory [40]:

$$\mu\_{\rm EFT} = \text{Conexp}\left(-\left(\gamma + \Delta \mathbf{G}\right)^2 / 4\gamma \mathbf{k}\_{\rm B} \mathbf{T}\right) \tag{6}$$

where Con is the constant, ΔG is the energy difference caused by uneven size of CQDs, T is the temperature in Kelvin, kB is the Boltzmann constant. γ is the recombinant energy of the polarization effect of the medium around the carrier [41]. γ is obtained by:

$$\gamma = \frac{\mathbf{e}}{4\pi\varepsilon\_0} \left(\frac{1}{\mathbf{a}} - \frac{1}{2(\mathbf{D} + \mathbf{l})}\right) \left(\frac{1}{\varepsilon\_1} - \frac{1}{\varepsilon\_{\text{st}}}\right) \tag{7}$$

where e is the elementary charge, ε0 is the vacuum permittivity, D is the radius of the CQDs, l is the spacing between CQDs, ε1 is the optical dielectric constant of the matrix surrounding CQDs, εst is the static dielectric constant. The intraband transition property makes HgSe CQDs an excellent optoelectronic material for detecting specific wavelengths of infrared. By tuning the size of the HgSe CQDs, the Eintra and the corresponding peak absorption wavelength can be controlled. This design can modify the reflection window of the DBR to match the specific wavelength detectable by HgSe by adjusting the optical length of the DBR dielectric thin-film layer. The resonance enhancement effect of the resonant cavity is used to improve the absorption rate and fine-tune the absorption wavelength of the HgSe CQD detector. Previous studies have already demonstrated that photo-excited carriers through intraband transition can be collected to generate photocurrents. The application of the intraband transition for photoconductors was first demonstrated in the 3–5 μm range of infrared [42].

To verify the optical functionality of the DBR design, the spectral absorption of the CQD detector in the single-band microspectrometer was simulated using a one-layer and two-layer DBR, as shown in Figure 4. The number of layers of DBR is determined by the number of high and low refractive index film layer pairs. When the resonant wavelength of the resonant cavity accurately matches the absorption peak of the CQD active layer, the absorption rate of the CQD detector increases 2.8 times. The FWHM is reduced to 18% and 31%, respectively, in the application of one layer and two layers of DBR. There is an approximate linear relationship between the central wavelength of the absorption peak and the thickness of the optical spacer layer, indicating that the central wavelength of the absorption peak can be precisely tuned by changing the thickness of the optical spacer layer (Figure 4d). Compared with one-layer DBR, the FWHM of the absorption peak of the CQD detector with two layers of DBR is narrower, but the peak absorption oscillates when the optical isolation layer is thickened, which may indicate that increasing the number of layers of DBR will produce more interference when the optical length of the resonant cavity increases.

Overall, the performance of the CQD detector was significantly increased after adopting the resonator with one layer of DBR. The absorption of the CQD detector was increased 2–3 times, and the FWHM was narrowed to 30% of the original. The central wavelength of the absorption peak can be tuned in the range of 0.7 μm, which is consistent with previously reported studies [26].

In addition to modeling the cavity length of the resonator, the influence of the angle of incidence was also studied. For a cavity with an optical spacer layer of 0.82 μm (absorption > 90%, Figure 5a), we simulated spectral absorption of the CQD detector with incident light from 0 degrees to 90 degrees, and the modeling of the incident angle of the light wave is shown in Figure 6a. The simulation results are shown in Figure 6b,c. The results show that within the incident angle of 0–25 degrees, the absorption of the CQD detector is still increased 2–3 times, and the FWHM is narrowed by 30%, but the center wavelength is shifted by 0.3 μm at most (Figure 6d–f). After the incident angle exceeds 25 degrees, the absorption rate of the CQD detector drops sharply, and the center wavelength of the peak of absorption produces a large periodic change.

**Figure 4.** Absorption of the CQD microspectrometer. (**a**,**b**) Absorption of a CQD microspectrometer with different optical spacer thicknesses with one layer of DBR. (**c**,**d**) Absorption of a CQD microspectrometer with different optical spacer thicknesses and two layers of DBR.

**Figure 5.** Effect of resonant cavity composed of DBR on CQD detector. (**a**) Absorption of the CQD detector with one layer of DBR, two layers of DBR, and no DBR (Ref) for the thickness of the optical spacer layer is 0.82 μm. Absorption (**b**), FWHW (**c**), and center wavenumber (**d**) of absorption peak of the CQD detector at different thicknesses of the optical spacer layer with one layer (red line), two layers (blue line) of DBR, and no DBR (black line).

**Figure 6.** Influence of incidence angle on mincrospectrometer performance. (**a**) Simulation model of a CQD microspectrometer with an incident angle. Absorption of a CQD microspectrometer with different incident angles in one layer of DBR (**b**) and two layers of DBR (**c**). Absorption (**d**), center wavenumber (**e**), and FWHW (**f**) of absorption peak of the CQD detector in the range of incident angle of 0–45 degrees with one layer (red line), two layers (blue line) of DBR, and no DBR (black line).

#### **4. Conclusions**

In conclusion, our design and simulation confirm that the wavelength-specific detection of HgSe for infrared forms a good match with the reflection window formed by DBR and the resonance wavelength of the resonant cavity, which can further obtain a lower FWHM and a higher absorption rate. The detector assumes the function of a single-band spectrometer in the range of MWIR according to the simulation results, and it is possible to fabricate a snapshot HgSe CQD microspectrometer by integrating multiple photodetectors capable of detecting various infrared regions by simply adjusting the cavity length of the resonant cavity. We believe that the simulation results have important implications for microspectrometers. In addition, the effect of enhancing the light absorption of the active layer with this device structure should be applicable to thinner film-like detectors.

**Author Contributions:** Conceptualization, X.T., C.W. and G.M.; methodology, M.C.; software, C.W.; validation, X.T., C.W., G.M. and X.Z.; formal analysis, X.Z. and M.C.; investigation, X.T., C.W. and G.M.; resources, X.T., C.W. and G.M.; data curation, X.T., C.W., X.Z. and G.M.; writing—original draft preparation, C.W. and G.M.; writing—review and editing, X.T. and M.C.; visualization, X.T., C.W. and G.M.; supervision, X.T.; project administration, X.T.; funding acquisition, X.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China: 2021YFA0717600; National Natural Science Foundation of China: NSFC No. 62035004; National Natural Science Foundation of China: NSFC No. 62105022.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

## *Article* **Simulation of Resonant Cavity-Coupled Colloidal Quantum-Dot Detectors with Polarization Sensitivity**

**Pengfei Zhao 1,†, Ge Mu 1,†, Menglu Chen 1,2,3 and Xin Tang 1,2,3,\***

**Abstract:** Infrared detectors with polarization sensitivity could extend the information dimension of the detected signals and improve target recognition ability. However, traditional infrared polarization detectors with epitaxial semiconductors usually suffer from low extinction ratio, complexity in structure and high cost. Here, we report a simulation study of colloidal quantum dot (CQD) infrared detectors with monolithically integrated metal wire-grid polarizer and optical cavity. The solution processibility of CQDs enables the direct integration of metallic wire-grid polarizers with CQD films. The polarization selectivity of HgTe CQDs with resonant cavity-enhanced wire-grid polarizers are studied in both short-wave and mid-wave infrared region. The extinction ratio in short-wave and mid-wave region can reach up to 40 and 60 dB, respectively. Besides high extinction ratio, the optical cavity enhanced wire-grid polarizer could also significantly improve light absorption at resonant wavelength by a factor of 1.5, which leads to higher quantum efficiency and better spectral selectivity. We believe that coupling CQD infrared detector with wire-grid polarizer and optical cavity can become a promising way to realize high-performance infrared optoelectronic devices.

**Keywords:** polarization; colloidal quantum dots; detectivity; optical cavity

#### **1. Introduction**

Polarization imaging technology plays an increasingly important role in target recognition and detection. The polarization state of light results from the interaction between light and objects at the microscopic level. When the surface topography, states and observation orientation are different, the specular polarization information of the object will change accordingly [1,2]. Therefore, when the temperature difference between the observed objects and the surrounding environment is relatively small, infrared thermal imaging fails to give high-contrast images, whereas polarization imaging can provide a new information dimension for target discrimination with high contrast between the targets and the surrounding background [3,4]. Therefore, polarization imaging technology has become an powerful imaging technology besides traditional intensity imaging and spectral imaging, which has a wide prospective application in military infrared anti-counterfeiting equipment [5], remote sensing, astronomical observation [6] and medical diagnosis [7].

Traditional polarization imaging techniques are mainly divided into time-sharing polarization imaging [8,9], amplitude polarization imaging and aperture polarization imaging. However, they rely on moving optics, registration and sophisticated calibration procedures, which greatly increase their complexity and cost.

With the increasing demands for high sensitivity, low cost and scalable polarized infrared detector equipment, there is a trend to directly couple the polarizers such as metal wire-grid polarizer with infrared detector on pixel level. High sensitivity, compact structure

**Citation:** Zhao, P.; Mu, G.; Chen, M.; Tang, X. Simulation of Resonant Cavity-Coupled Colloidal Quantum-Dot Detectors with Polarization Sensitivity. *Coatings* **2022**, *12*, 499. https://doi.org/ 10.3390/coatings12040499

Academic Editor: Alicia de Andrés

Received: 28 February 2022 Accepted: 5 April 2022 Published: 7 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and wide bandwidth have been demonstrated with different processing technology [10,11]. Despite their high performance, these detectors are still limited by the high cost of epitaxial semiconductors [12]. It is advantageous to find a method that has both high polarization sensitivity and low cost. With the continuous progress, the maximum absorption wavelength of colloidal quantum dots (CQDs) has been extended from near infrared to longer wavelength, which provides a promising alternative for bulk infrared semiconductors such as HgCdTe, InSb and type II superlattice. Among all colloidal nano materials, mercury telluride (HgTe) CQDs has shown the highest spectral tunability so far; from short-wave infrared and mid-wave infrared to long-wave infrared and terahertz (THz) regions, we can control the reaction time and temperature to obtain colloidal quantum dots with different particle sizes for different infrared regions [13]. The solution workability and broad spectral tunability of colloidal quantum dots inspire a variety of inexpensive, high-performance optoelectronic devices [14]. Therefore, coupling metal nanowire-grid polarizer with colloidal quantum dot detector seems to be a suitable method to solve this challenge.

In this work, a high-performance polarization-sensitive infrared detector is designed by integrating HgTe CQDs photovoltaic detector with wire-grid polarizer and optical cavity. Finite element analysis of HgTe CQDs polarization-sensitive detectors was systematically conducted covering short-wave infrared and mid-wave infrared. The simulation results show that this detector has high polarization selectivity in both short-wave infrared and mid-wave infrared. The extinction ratio in short-wave and mid-wave region can reach up to 40 and 60 dB, respectively. Besides high extinction ratio, the optical cavity enhanced wiregrid polarizer could also significantly improve light absorption at resonant wavelength by a factor of 1.5, which leads to higher quantum efficiency and better spectral selectivity.

#### **2. Modeling and Simulation**

The structure of the proposed HgTe CQDs detector with resonant cavity-coupled wire-grid polarizer is illustrated in Figure 1a. Distributed Bragg reflectors (DBRs) are set on an Al2O3 substrate, on top of which subwavelength wire-grid polarizers are added. The reflectivity of DBR is determined by the number of the alternating layers and their difference in refractive index [15]. The polarization selectivity of the metal wire grid is originated from the structural asymmetry. The transverse wave (TE polarized light) in which the electric vector is parallel to the direction of the wire grids can excite the electrons to oscillate along the grid, so TE polarized light will be reflected, and the transverse magnetic wave (TM polarized light) with electric vector perpendicular to the wires cannot excite free electron oscillations and therefore shows high optical transmission [12]. On top of the metal wire-grid polarizer, HgTe CQDs photodiodes are constructed with indium tin oxide and gold as bottom and top contacts. The DBR and top gold contact forms a resonant cavity.

**Figure 1.** Architecture of resonant cavity-coupled Colloidal quantum-dot detectors with polar ization sensitivity. (**a**) Structure schematic and (**b**) simulation model of Resonant cavity-coupled Colloidal quantum-dot detectors with polarization sensitivity. Electric field distribution of (**c**) TM waves and (**d**) TE waves.

COMSOL Multiphysics is used to conduct the simulation in our study. To save computation resources, a two-dimensional (2D) simulation is used, as shown in Figure 1b. The light is incident from the bottom with controlled power of 1 W. The left and right sides are set as "Periodic boundary condition". The substrate material is chosen to be Al2O3 with a refractive index of 1.3 + 0i. Low refraction SiO2 material with a refractive index of 1.4 + 0i and high refractive index Si material with a refractive index of 3.42 + 0i are selected for Bragg mirror, and SiO2 material is also selected as optical spacer. For HgTe CQDs, the refractive index is 2.3 + 0.1i [16], and the rest of the model is set to be air with a refractive index of 1 + 0i. The wire-grid polarizer and the top electrode are set as perfect electrical conductors (PEC). The electric field distribution of TM wave and TE wave obtained by simulation is shown in Figure 1c,d, and it can be seen that the structure has strong polarization selectivity. The index of refraction of CQDs films is measured before simulation. Based on the measured index of refraction, the simulation fraction *fsimulation* can be estimated by using different effective medium approximations, including the Maxwell-Garnett (MG) model and Bruggeman (BG) model:

$$\text{MGmodel}: \frac{\varepsilon\_{eff} - \varepsilon\_{\text{CQD}}}{\varepsilon\_{eff} + 2\varepsilon\_{\text{CQD}}} = (1 - f\_{\text{simulation}}) \frac{\varepsilon\_{\text{m}} - \varepsilon\_{\text{CQD}}}{\varepsilon\_{\text{m}} + 2\varepsilon\_{\text{CQD}}} \tag{1}$$

$$\text{BGmodel}: f\_{\text{simulation}} \frac{\varepsilon\_{\text{CQD}} - \varepsilon\_{\text{eff}}}{\varepsilon\_{\text{CQD}} + 2\varepsilon\_{\text{eff}}} + (1 - f\_{\text{simulation}}) \frac{\varepsilon\_m - \varepsilon\_{\text{eff}}}{\varepsilon\_m + 2\varepsilon\_{\text{eff}}} = 0 \tag{2}$$

where *εeff* is the effective dielectric constant, which is obtained by the measured index of refraction n, *εCQDs* is the dielectric constant of CQDs, and *εm* is the dielectric constant of medium. For HgTe, *εCQDs* is taken as 15.1. *εm* is taken to be 1, since the CQDs are surrounded by air. According to the thickness of colloidal quantum dot film, the approximate refractive index is 2.3 + 0.1i; in previous studies, this value has also been proved to be reliable [14,16].

#### **3. Results and Discussions**

*3.1. Effects of Period and Duty Cycle*

The center wavelength *λcenter* of Bragg mirror defined the operation spectral ranges of resonant cavity, and it can be expressed as

$$
\lambda\_{center} = 4n\_ld\_l = 4n\_hd\_{h\prime} \tag{3}
$$

where *n*1 and *nh* are the refractive index of the two materials, and *d*1 and *dh* are the thickness of the two materials. Bragg mirrors are formed by two alternating layers with different refractive index. By changing the thickness of each layer in the simulation, the reflection window can be controlled. When the response of the detector is in short-wave infrared, the center wavelength is set to be 2 μm, so the thickness of SiO2 and Si is calculated to be 340 nm and 140 nm, respectively. When the detector works in the mid-wave region, the center wavelength is about 4 μm, and the thickness of SiO2 and Si is set to be 700 nm and 280 nm [17].

For wire-grid polarizer, the period and duty cycle are two key parameters that influence its spectral extinction ratio [18]. The duty cycle *f* is defined as: *f* = *b*/*p*, where *p* is the period of the wire grid, and b is the width of wire grid. The polarization performance of polarizer is usually characterized by extinction ratio, that is, ER = 10 log(TM/TE). In our simulation, the extinction ratio is defined by the absorption ratio of CQDs under TM and TE wave incidence.

When short-wave infrared plane waves are incident and the period of metal wire grid increases from 0.6 μm to 1.0 μm, the spectral absorption characteristic curves of HgTe CQDs is simulated, as shown in Figure 2a. The corresponding extinction ratio of absorption is shown in Figure 2b. The HgTe CQDs demonstrate clear polarization-dependent absorption. With decreasing period from 1.0 to 0.6 μm, the extinction ration increases by almost three orders of magnitude. Duty cycle of wire grid is another important structural parameter. When the plane wave is incident vertically, the absorption characteristic curves of HgTe CQDs with wire grids with different duty cycles are simulated in the short-wave region (Figure 2c). When the duty cycle of the grid increases from 0.3 to 0.7, the absorption rate of TM wave decreases gradually and redshifts, whereas that of TE decreases significantly, and the extinction ratio increases (Figure 2d). In the mid-wave region, when the period increase from 0.6 μm to 1.0 μm, the absorption characteristic curve of HgTe CQDs with metal wire grid is simulated, as shown in Figure 3a,b. When duty cycle changes from 0.3 to 0.7, the absorption rate of TM wave in the mid-wave region redshifts. (Figure 3c). The simulation results show that the extinction ratio of the CQDs detector in the mid-wave region is much larger than that in the short-wave region (Figure 3d). Thus, the detector will be more suitable in the mid-wave region than the short-wave region according to the simulation results.

**Figure 2.** (**a**) Influence of different periods on polarization performance and absorption rate in shortwave region. (**b**) Extinction ratio for different periods in short-wave region. The thickness of metal wire grid is 100 nm. Duty cycle *f* is 0.5. The thickness of SiO2 is 340 nm. The thickness Si is 140 nm. (**c**) Influence of different duty cycle on polarization performance and absorption rate in short-wave region. (**d**) Extinction ratio for different duty cycle in short-wave region. Thickness of metal wire grid is 100 nm. The period *p* is 0.8 μm. The thickness of SiO2 is 340 nm. The thickness Si is 140 nm.

The extinction ratio in short-wave and mid-wave region according to our simulation results can reach 40 and 60 dB, respectively, through coupling the single-layer wire grid and Bragg mirror, which are higher than that of other similar structures [10,19,20]. Although the extinction ratio using multi-layers wire grid is larger than ours, the complicated structure is difficult for mass production [12]. Compared with previous studies, our structure has considerable extinction ratio on the basis of its simpler structure, which makes the detector easier to manufacture. Therefore, the coupling of colloidal quantum dot detector and nano

metal wire grid will be a promising method for the development of high-performance polarization photodetectors.

**Figure 3.** (**a**) Influence of different periods on polarization performance and absorption rate in midwave region. (**b**) Extinction ratio for different periods in mid-wave region. The thickness of metal wire grid is 100 nm. Duty cycle *f* is 0.5. The thickness of SiO2 is 700 nm. The thickness Si is 280 nm. (**c**) Influence of different duty cycle on polarization performance and absorption rate in mid-wave region. (**d**) Extinction ratio for different duty cycle in mid-wave region. The thickness of the metal wire grid is 100 nm. The period *p* is 0.8 μm. The thickness of SiO2 is 700 nm. The thickness of Si is 280 nm.

#### *3.2. Effects of Optical Spacer Thickness*

The optical spacer above the wire-grid polarizer is composed of SiO2. The role of the resonator is to select the light with a desired wavelength and suppress the light with other wavelength [21]. Therefore, the desired wavelength of TM absorption peak can be selected by adjusting the thickness of the SiO2 layer. The wavelength of the peak absorption for a resonant cavity can be expressed by

$$
\lambda = \frac{2\pi l \sqrt{n^2 - \sin^2 \theta}}{\pi m - \phi} \tag{4}
$$

where *l* is the optical path, *n* is the index of refraction of the optical spacer, *θ* is the angle of incidence, *m* is the order of interference, and *φ* is the phase shift of light between the two reflective surfaces. Therefore, by changing the thickness of the optical spacer (SiO2), the spectra can be tuned accordingly.

In the short-wave region and mid-wave infrared, the absorption curves of TM wave and TE wave with various thickness of the optical spacer are shown in Figure 4a,b. The black curve is the reflection window of the distributed Bragg mirror, and the TM absorption peak redshifts with the increased thickness of the optical spacer. For short-wave infrared HgTe detectors, the TM absorption peak decreases gradually with the decrease in wavelength, whereas the magnitude of absorption remains almost unchanged in the mid-wave infrared region. After the addition of optical spacer, the CQD detectors remain sensitive to the polarization direction with high absorption under TM incidence in short-wave region and mid-wave region (Figure 4c,d).

**Figure 4.** (**a**) TM wave and TE wave absorption curves with different resonant cavity thickness and the reflection window of Bragg mirror in short-wave region. (**b**) TM wave and TE wave absorption curves with different resonant cavity thickness and the reflection window of Bragg mirror in midwave region. (**c**) Single peak of TM and TE in short-wave region. (**d**) Single peak of TM and TE in mid-wave region.

#### *3.3. Effects of Plasmon Resonance by Wire Grid*

Compared to HgTe detectors without resonant cavity, the cavity can significantly improve light absorption at targeted wavelength, resulting in the enhancement of quantum efficiency [22]. Moreover, our simulation shows that the metal wire grid provides surface plasmon resonance effect which leads to further increase in the infrared absorption of detector. The absorption of light at different polarization angles by the CQDs infrared detector with resonant cavity and wire-grid polarizer in the short-wave region and midwave infrared is shown in Figure 5a,b. The integration of HgTe CQD photovoltaic detector, wire-grid polarizer and resonant cavity can not only provide polarization-sensitivity, but also significantly increases CQDs film absorption.

**Figure 5.** (**a**) Absorption effects of resonator and metal grid on different polarization angles in shortwave region. (**b**) Absorption effects of resonator and metal grid on different polarization angles in mid-wave region.

It can be seen that the proposed resonant cavity-enhanced wire-grid polarizers not only show excellent polarization selectivity, but also give enhanced absorption with high spectral selectivity, which provides a promising method to improve the spectral resolution of infrared polarization detector.

#### **4. Conclusions**

In conclusion, we proposed the design of HgTe CQDs photodetectors with resonant cavity-enhanced wire-grid polarizer, providing a theoretical study for designing low-cost polarization infrared detectors in the future. The simulation results show that the detector can work in both short-wave region and mid-wave region with high extinction ratio up to 40 dB and 60 dB. Our simulation shows that the metal wire grid provides surface plasmon resonance effect which leads to further increase in the infrared absorption of detector. Benefitting from CQDs' solution processibility, HgTe CQDs could be directed integrated with the wire-grid polarizers. Therefore, we believe that the simulation results have important implications for the focal plane array polarization imaging.

**Author Contributions:** Conceptualization, X.T., P.Z. and G.M.; methodology, M.C.; software, P.Z.; validation, X.T., P.Z. and G.M.; formal analysis, M.C.; investigation, X.T., P.Z. and G.M.; resources, X.T., P.Z. and G.M.; data curation, X.T., P.Z. and G.M.; writing—original draft preparation, P.Z. and G.M.; writing—review and editing, X.T. and M.C.; visualization, X.T., P.Z. and G.M. supervision, X.T.; project administration, X.T.; funding acquisition, X.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China and National Key R&D Program of China (NSFC No. 62035004, 2021YFA0717600 and NSFC No. 62105022).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

**Xue Zhao 1, Ge Mu 1, Xin Tang 1,2,3 and Menglu Chen 1,2,3,\***

**Abstract:** In this paper, we investigate an intraband mid-infrared photodetector based on HgSe colloidal quantum dots (CQDs). We study the size, absorption spectra, and carrier mobility of HgSe CQDs films. By regulating the time and temperature of the reaction during synthesis, we have achieved the regulation of CQDs size, and the number of electrons doped in conduction band. It is experimentally verified by the field effect transistor measurement that dark current is effectively reduced by a factor of 10 when the 1Se state is doped with two electrons compared with other doping densities. The HgSe CQDs film mobility is also measured as a function of temperature the HgSe CQDs thin film detector, which could be well fitted by Marcus Theory with a maximum of 0.046 ± 0.002 cm2/Vs at room temperature. Finally, we experimentally discuss the device performance such as photocurrent and responsivity. The responsivity reaches a maximum of 0.135 ± 0.012 A/W at liquid nitrogen temperature with a narrow band photocurrent spectrum.

**Keywords:** intraband photodetector; colloidal quantum dot; HgSe; mid-infrared

#### **1. Introduction**

In recent years, colloidal quantum dots (CQDs) devices have gradually been widely used in imaging, remote sensing, gas detection, medical diagnosis, and epidemic prevention and control [1–4]. The performance of CQDs photodetectors has been comparable to commercial InGaAs photodiodes but order-of-magnitude reduction in cost [5]. Compared traditional infrared photodetectors, CQDs photodetectors theoretically offer the advantages of higher operating ambient temperatures, smaller dark currents, faster response performance [6], and lower fabrication cost [7].

The materials selected for the infrared photodetectors with interband transition [8,9] are usually limited in semimetal or ternary alloy with a small gap [10]. However, the intraband CQD devices, utilizing optical transitions from lowest electron state labelled 1Se and the next level 1Pe, hold unique promise to extend the wavelength from near- and mid-infrared to long-wave infrared (Supplementary Materials Figure S1). What is more, intraband photodetectors promise a better photoelectric performance [11,12] due to the low conduction band density which inhibit the process of Auger relaxation [13–16]. For example, the Auger relaxation coefficient is at least three orders smaller in intraband HgSe CQDs compared with the bulk materials with the same energy gap [17]. Hence, the study of CQDs intraband photodetectors has important research significance. The air stable n-type mercury (ii) selenide (HgSe) CQDs provide an excellent platform for studying intraband photodetectors [18,19].

In current stage, due to the limited study on intraband materials, the intraband photodetectors based in HgSe CQDs have excessive dark current compared to interband CQD photodetectors at the same wavelength. The solution is to precisely adjust the number of electrons in 1Se [11,20]. The unpaired electrons in Se state or Pe state would result

**Citation:** Zhao, X.; Mu, G.; Tang, X.; Chen, M. Mid-IR Intraband Photodetectors with Colloidal Quantum Dots. *Coatings* **2022**, *12*, 467. https://doi.org/10.3390/ coatings12040467

Academic Editor: Ana-Maria Lepadatu

Received: 28 February 2022 Accepted: 23 March 2022 Published: 30 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in large dark current. The dark current is minimal when 1Se is fully filled which is two electrons in conduction band per CQD (Supplementary Materials).

To address these challenges, in this paper, we provide a way to control the doping inside the HgSe CQDs, which is characterized by optical and transport measurements. Then, we build the HgSe CQDs intraband photodetector in mid-infrared (Mid-IR) and further analyze its photocurrent, and responsivity.

#### **2. Materials and Methods**

In a glove box with a nitrogen environment, 27.2 mg of mercury chloride (HgCl2) and 4 mL of oleylamine (OAm) were placed in a 40 mL vial, and were heated at 100 ◦C for 1 h, and then were heat-balanced at 115 ◦C (or 120 ◦C) for half an hour. A total of 126 mg (0.1 mmol) of selenium urea was dissolved in 10 mL of OAm and heated at 180 ◦C under nitrogen for 2 h to form a brown transparent liquid. A total of 1 mL of this solution was taken and injected into the mercury chloride/oleylamine (HgCl2/OAm) solution, and was reacted at 115 ◦C for 3–8 min. We removed the reaction solution out of the glove box and used cold water to end the reaction. After the end of the reaction, the HgSe CQDs solution was placed in a centrifuge tube. A total of 1 mL of 1-dodecyl mercaptan (DDT), 20 drops of didecyldimethylammonium bromide (DDAB), and 15 mL of methanol (MeOH) were added to the centrifuge tube. The solution in the centrifuge tube was centrifuged at 7500 r.p.m for 6 min. After centrifugation, the supernatant was decanted, and the remaining pellet was dissolved in chlorobenzene and filtered through a disposable filter.

The photocurrent spectrum of the material with Nicolet iS20 FTIR (Thermo Fisher Scientific, Waltham, MA, USA) was measured. The FTIR implemented 500,000:1 signal-tonoise ratio and 0.25 cm−1 spectral resolution.

Photoconductance spectra was measured with a standard Thermo Scientific Nicolet iS20 FTIR Spectrometer. The internal light was reflected in the sample with a gold mirror. The samples were biased witha3V battery in series. The current across the sample was sent to transimpedance amplifier Femto-200 with a gain of 10,000 before being sent back to the FTIR input.

#### **3. Results and Discussion**

#### *3.1. Absorption Spectra*

The Transmission Electron Microscope (TEM) image of HgSe CQDs with a synthesis temperature of 115 ◦C and a synthesis time of 5 min is shown in Figure 1a. The average size of CQDs is 4.7 nm and the size distribution is uniform with the standard deviation of 0.5 nm (Supplementary Materials Figure S2). As can be seen in Figure 1b, the size of the CQDs can be varied by changing the synthesis temperature and time [21,22]. Changing the size of CQDs can achieve the movement of the absorption spectrum. The intraband absorption peak of HgSe CQDs is between 2000–3000 cm−1, and the absorption peak will be redshifted by increasing the reaction time. The relationship between CQDs of different sizes and energy bands is shown in Figure 1c. When the size of CQDs is ''small", the Fermi level is lower than Se states, and the HgSe CQDs are defined as "n− type". When the size of CQDs is ''medium", the energy gaps become smaller, and the band shift. As a result, the Fermi level is in Se state, which we define as two electrons in conduction band per CQD [17]. When the size of CQDs is ''large", the Fermi level is higher than Se states, and the HgSe CQDs are defined as "n+ type". By changing the size of CQDs, the position of the absorption peaks in intraband and interband can be changed in Figure 1b. HgSe intraband photodetector can specifically detect signals in the Mid-IR range, possessing potential application in gas detection such as CO2 with a signature absorption of 2400 cm−1.

**Figure 1. Characterization of HgSe CQDs materials.** (**a**) TEM images of HgSe CQDs with a particle diameter of 4.7 nm and standard deviation of 0.5 nm. (**b**) Absorption spectra of HgSe CQDs. (**c**) Diagram of the relationship between the size of CQDs and energy bands.

#### *3.2. Transport Properties of HgSe CQDs*

Transport properties such as carrier type, carrier density, and carrier mobility greatly affect the photodetector performance. We can judge the performance of the material by measuring the carrier type and mobility of the HgSe film [23]. This information is given by the field effect transistor (FET) measurement. HgSe CQDs film is spin-coated on 300 nm SiO2/Si substrate, which has a layer of interdigited electrodes. A solid ligands exchange process with an ethanedithiol/hydrochloric acid (EDT/HCl) solution is followed. In this process, short ligands would displace the long chain OAm ligands between CQDs. After the ligand exchange, we rinse the film with isopropanol (IPA).

The HgSe/EDT CQD film is measured by a field effect transistor circuit. Whether CQDs are "n− type" or "n+ type" can be determined by the slope sign of the FET transfer curves around zero gate potential. A schematic diagram of the FET structure is shown in Figure 2a.

**Figure 2. Transport study on HgSe CQDs.** (**a**) Schematic diagram of the field effect pipe structure. (**b**) Carrier mobility at different temperatures and mobility fitted with Marcus theory as a function of temperature. (**c**) FET source-drain current as a function of voltage for HgSe/EDT at 80 K.

For CQDs photodetectors, dark current and noise are important figures of merit. The shot noise has a great influence on the CQDs photoconductivity detectors, and the formula for shot noise is shown in Equation (1):

$$
\dot{a}\_n = \sqrt{4e I\_{dark}} \tag{1}
$$

where *e* is the electron charge, *Idark* is the dark current of the photodetector.

The dark current is an important factor affecting the noise in Equation (1) and the root cause is the thermally excited electron-hole pairs. Therefore, the dark current has an exponential relationship with the temperature [24]. For photodetectors with intraband transition, the dark current was minimal at low temperature [25]. Therefore, after using liquid nitrogen to cool down to 80 K, the dark current is effectively reduced. The curve of FET source-drain current as a function of voltage for HgSe/EDT at 80 K is shown in Figure 2c.

In addition to cooling down to reduce the dark current, the effect of the dark current can also be reduced by adjusting the number of electrons filled in the 1Se band. By varying the temperature and time of synthesis, the number of electrons filled by HgSe CQDs in the 1Se band can be regulated. In Figure 2c, when the number of doped electrons is two, the dark current is minimal. Compared with "n− type" or "n+ type" CQDs, the CQDs with fully doped Se state have a ten-fold reduction in dark current.

The mobility of HgSe/EDT thin films can be calculated through the FET circuit, and the mobility calculation formula is as follows [26]:

$$
\mu\_{FT} = \frac{L}{W \mathbb{C}\_i V\_D} \bullet \frac{\Delta I\_D}{\Delta V\_G} \tag{2}
$$

Among them, the thickness of silica (SiO2) is 300 nm, *Ci* = 1.15 × 10−4 *F*/*m*2 is the capacitance, *L* = 10 *μm* is the gap, *W* = 1000 *μm* × 50 is total channel width, "50" is the number of channels, and *VD* = 1 *V* is drain voltage. Δ*ID* Δ*VG* is the slope of the transfer current and the gate voltage. After calculation, the mobility of HgSe films exchanged with EDT can reach 0.046 ± 0.002 cm2/Vs at room temperature.

Figure 2b shows the mobility curve with temperature. The low temperature limits the movement of the carrier, and the carrier mobility decreases to 0.012 ± 0.001 cm2/Vs in Figure 2b. The increase in temperature will lead to less binding of the carrier and an upward trend in the mobility rate. After 260 K, the migration rate's growth rate slows down. The highest mobility is 0.046 ± 0.002 cm2/Vs at room temperature. The trend of migration change can be explained by Marcus theory:

$$
\mu\_{FET} = \text{Conxp}(-\left(\gamma + \Delta \mathbf{G}\right)^2 / 4\gamma k\_B \mathbf{T})\tag{3}
$$

where *Con* is the constant, ΔG the energy difference caused by uneven size of CQDs, *kB* = 1.380649 × 10−23 *J K* is the Boltzmann constant. The polarization effect of the medium around the carrier is given by the recombinant energy *γ* [27]. *γ* is expressed by the following equation:

$$\gamma = \frac{e}{4\pi\varepsilon\_0} (\frac{1}{a} - \frac{1}{2(a+b)}) (\frac{1}{\varepsilon\_1} - \frac{1}{\varepsilon\_{st}}) \tag{4}$$

where *e* = 1.6 × 10−19*C* is the elementary charge, *ε*0 = 8.85 × 10−12 *F m* is the vacuum permittivity, *a* = 2.3 *nm* is the radius of the CQDs, b = 1 *nm* is the spacing between CQDs, *ε*1 = 6.8 is the optical dielectric constant of matrix surrounding CQDs which is measured by the ellipsometry, *εst* = 10.9 is the static dielectric constant. We can get *γ* = 16 *meV*, *G* = 16.9 *meV* and *Con* = 0.09 by the Marcus fitting calculation. The disorder energy is similar to the values in the previous reference [26].

#### *3.3. Photocurrent*

The photocurrent and dark current curves of the HgSe CQDs intraband photodetector are shown in Figure 3a. As shown in Figure 3a, the *Iphotocurrent Idarkcurrent* = 10% when the bias voltage is ±3 V. Compared with previous studies [28], our work effectively improves the light–dark current ratio of pure HgSe quantum dots thin films. Figure 3b shows the photocurrent spectrum of the HgSe detector at 80 K. The photocurrent spectrum of the HgSe intraband photodetector is narrow at 80 K and the photoconduction is from 2000–3000 cm−1 in Figure 3b. The HgSe photodetector has a narrow photoresponse, making it possible to react more sensitively to change in the outside world. And the HgSe intraband transition photodetector has an obvious photocurrent at low temperatures, so it can be widely used in harsh low temperature environments for detection.

The responsivity is calculated by measuring the photocurrent of the HgSe photoconductor device. The right-inset graph in Figure 3b shows the responsivity of HgSe thin films as a function of temperature (Supplementary Materials). It can be seen from the figure that the responsivity reaches a maximum of 0.135 ± 0.012 A/W at 90 K. It gradually heats up after 90 K, and the responsivity gradually decreases.

HgSe CQDs intraband photodetectors have narrow band spectral response in 2000–3000 cm−1, which can be used in gas detection. Due to the detection capability of the intraband photodetectors in the infrared field, it can be applied to the fields of infrared imaging, target recognition, and radar detection, and other fields. Through the study of HgSe CQDs intraband photodetectors, we found that HgSe CQDs intraband photodetectors have better photoelectric performance at low temperatures. Therefore, HgSe CQDs intraband photodetectors can be applied to infrared detection in a low temperature environment. HgSe CQDs intraband photodetectors have broad application prospects.

#### **4. Conclusions**

In summary, this paper mainly introduces the intraband photodetector based on HgSe CQDs. Its intraband absorption spectrum is between 2000–3000 cm−1. We provide a method to achieve accurate electron doping through repeated experiments to suppress the dark currents. When the fully doped Se state is obtained experimentally, the effect of the dark current is minimal. Compared with "n− type" and "n+ type" CQDs, our dark current is effectively reduced by a factor of 10 measured by the field effect transistor. The mobility of HgSe/EDT films measured by FET can reach 0.046 ± 0.002 cm2/Vs. We experimentally calculate the responsivity of HgSe CQDs to be 0.135 ± 0.012 A/W. Through the study of the performance of the intraband infrared photodetector, the selection of infrared photodetector materials can be broadened, which is of great significance for infrared detection.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/coatings12040467/s1 [29,30]. Figure S1. Bands diagram; Figure S2. TEM image and size analysis of the 5 mins sample.

**Author Contributions:** Conceptualization, X.Z., X.T. and M.C.; Formal analysis, X.Z. and X.T.; Funding acquisition, M.C.; Investigation, G.M.; Methodology, X.Z.; Project administration, M.C.; Supervision, X.T. and M.C.; Validation, M.C.; Visualization, X.T.; Writing—original draft, X.Z.; Writing—review & editing, X.Z., G.M., X.T. and M.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is funded by NSFC 62105022, Beijing Nova Program Z211100002121069.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

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