Inverted Si:PbS Colloidal Quantum Dot Heterojunction-Based Infrared Photodetector

Dual-Mode Semiconductor Device Enabling Optoelectronic Detection and Neuromorphic Processing with Extended Spectral Responsivity

High-performance semiconductor devices capable of multiple functions are pivotal in meeting the challenges of miniaturization and integration in advanced technologies. Despite the inherent difficulties of incorporating dual functionality within a single device, a high-performance, dual-mode device is reported. This device integrates an ultra-thin Al2O3 passivation layer with a PbS/Si hybrid heterojunction, which can simultaneously enable optoelectronic detection and neuromorphic operation. In mode 1, the device efficiently separates photo-generated electron-hole pairs, exhibiting an ultra-wide spectral response from ultraviolet (265 nm) to near-infrared (1650 nm) wavelengths. It also reproduces high-quality images of 256 × 256 pixels, achieving a Q-value as low as 0.00437 µW cm- 2 at a light intensity of 8.58 µW cm- 2. Meanwhile, when in mode 2, the as-assembled device with typical persistent photoconductivity (PPC) behavior can act as a neuromorphic device, which can achieve 96.5% accuracy in classifying standard digits underscoring its efficacy in temporal information processing. It is believed that the present dual-function devices potentially advance the multifunctionality and miniaturization of chips for intelligence applications.

Indeno[3,2-b]carbazole-Based Small Molecule Layer Enables Optimized Carrier Transport for PbS Quantum Dot NIR Photodetectors

Colloidal quantum dots (CQDs) have garnered considerable attention for photodetectors (PDs), attributable to exceptional photoelectric properties and ease solution-based processing. However, the prevalent use of 1,2-ethanedithiol (EDT) as a hole transport layer (HTL) has limitations, such as energy level discrepancies, requisite oxidation, and intricate multilayer assembly. Organic p-type materials, lauded for their superior attributes and synthetic versatility, are now stepping forward as viable substitutes for conventional EDT HTLs. In this work, we introduced an organic HTL derived from indolo[3,2-b]carbazole, named ZL004, leading to a marked improvement in carrier generation and collection, facilitated by the optimized band alignment and enhanced interfacial charge dynamics. The ZL004-based PDs exhibit a photoresponsivity of 0.45 A/W, a noise current of 1.8 × 10-11 A Hz-0.5, a specific detectivity of 4.6 × 109 Jones, and an expansive linear dynamic range of 107 dB─surpassing EDT-based devices across the board, demonstrating the extraordinary property of organic p-type materials for CQD-based PDs.

High-Performance Self-Powered Quantum Dot Infrared Photodetector with Azide Ion Solution Treated Electron Transport Layer

The demand for self-powered photodetectors (PDs) capable of NIR detection without external power is growing with the advancement of NIR technologies such as LIDAR and object recognition. Lead sulfide quantum dot-based photodetectors (PbS QPDs) excel in NIR detection; however, their self-powered operation is hindered by carrier traps induced by surface defects and unfavorable band alignment in the zinc oxide nanoparticle (ZnO NP) electron-transport layer (ETL). In this study, an effective azide-ion (N3 -) treatment is introduced on a ZnO NP ETL to reduce the number of traps and improve the band alignment in a PbS QPD. The ZnO NP ETL treated with azide ions exhibited notable improvements in carrier lifetime and mobility as well as an enhanced internal electric field within the thin-film heterojunction of the ZnO NPs and PbS QDs. The azide-ion-treated PbS QPD demonstrated a increase in short-circuit current density upon NIR illumination, marking a responsivity of 0.45 A W-1, specific detectivity of 4 × 1011 Jones at 950 nm, response time of 8.2 µs, and linear dynamic range of 112 dB.

Planar Cation Passivation on Colloidal Quantum Dots Enables High-Performance 0.35-1.8 µm Broadband TFT Imager

Solution-processed colloidal quantum dots (CQDs) are promising candidates for broadband photodetectors from visible light to shortwave infrared (SWIR). However, large-size PbS CQDs sensitive to longer SWIR are mainly exposed with nonpolar (100) facets on the surface, which lack robust passivation strategies. Herein, an innovative passivation strategy that employs planar cation, is introduced to enable face-to-face coupling on (100) facets and strengthen halide passivation on (111) facets. The defect density of CQDs film (Eg ≈ 0.74 eV) is reduced from 2.74 × 1015 to 1.04  × 1015 cm-3, coupled with 0.1 eV reduction in the activation energy of defects. The resultant CQDs photodiodes exhibit a low dark current density of 14 nA cm-2 with a high external quantum efficiency (EQE) of 62%, achieving a linear dynamic range of 98 dB, a -3dB bandwidth of 103 kHz and a detectivity of 4.7 × 1011 Jones. The comprehensive performance of the CQDs photodiodes outperforms previously reported CQDs photodiodes operating at >1.6 µm. By monolithically integrated with thin-film transistor (TFT) readout circuit, the broadband CQDs imager covering 0.35-1.8 µm realizes the functions including silicon wafer perspectivity and material discrimination, showing its potential for wide range of applications.

Recent Advances in Low-Dimensional Nanomaterials for Photodetectors

New emerging low-dimensional such as 0D, 1D, and 2D nanomaterials have attracted tremendous research interests in various fields of state-of-the-art electronics, optoelectronics, and photonic applications due to their unique structural features and associated electronic, mechanical, and optical properties as well as high-throughput fabrication for large-area and low-cost production and integration. Particularly, photodetectors which transform light to electrical signals are one of the key components in modern optical communication and developed imaging technologies for whole application spectrum in the daily lives, including X-rays and ultraviolet biomedical imaging, visible light camera, and infrared night vision and spectroscopy. Today, diverse photodetector technologies are growing in terms of functionality and performance beyond the conventional silicon semiconductor, and low-dimensional nanomaterials have been demonstrated as promising potential platforms. In this review, the current states of progress on the development of these nanomaterials and their applications in the field of photodetectors are summarized. From the elemental combination for material design and lattice structure to the essential investigations of hybrid device architectures, various devices and recent developments including wearable photodetectors and neuromorphic applications are fully introduced. Finally, the future perspectives and challenges of the low-dimensional nanomaterials based photodetectors are also discussed.

Understanding Colloidal Quantum Dot Device Characteristics with a Physical Model

Colloidal quantum dots (CQDs) are finding increasing applications in optoelectronic devices, such as photodetectors and solar cells, because of their high material quality, unique and attractive properties, and process flexibility without the constraints of lattice match and thermal budget. However, there is no adequate device model for colloidal quantum dot heterojunctions, and the popular Shockley-Quiesser diode model does not capture the underlying physics of CQD junctions. Here, we develop a compact, easy-to-use model for CQD devices rooted in physics. We show how quantum dot properties, QD ligand binding, and the heterointerface between quantum dots and the electron transport layer (ETL) affect device behaviors. We also show that the model can be simplified to a Shockley-like equation with analytical approximate expressions for reverse saturation current, ideality factor, and quantum efficiency. Our model agrees well with the experiment and can be used to describe and optimize CQD device performance.

A near-infrared photodetector based on carbon nanotube transistors exhibits ultra-low dark current through field-modulated charge carrier transport

Near-infrared photodetectors (NIR PDs) are devices that convert infrared light signals, which are widely used in military and civilian applications, into electrical signals. However, a common problem associated with PDs is a high dark current. Interestingly, gate voltage can regulate carrier migration in the channels. In this study, a PbS quantum dot heterojunction combined with a carbon nanotube (CNT) field effect transistor (FET) is designed and described. Significantly, this NIR PD achieves field-modulated carrier transport in a CNT transistor, in which the dark current is effectively regulated by the gate voltage. In this PD, an ultra-low dark current of 8 pA is obtained by gate voltage regulation. Moreover, the device shows a fast response speed of 6.5 ms and a high normalized detectivity of 4.75 × 1011 Jones at 0.085 W cm-2 power density and -0.2 V bias voltage. Overall, this work details a novel strategy for the fabrication of a PD with an ultra-low dark current based on a FET.

Lead Chalcogenide Colloidal Quantum Dots for Infrared Photodetectors

Infrared detection technology plays an important role in remote sensing, imaging, monitoring, and other fields. So far, most infrared photodetectors are based on InGaAs and HgCdTe materials, which are limited by high fabrication costs, complex production processes, and poor compatibility with silicon-based readout integrated circuits. This hinders the wider application of infrared detection technology. Therefore, reducing the cost of high-performance photodetectors is a research focus. Colloidal quantum dot photodetectors have the advantages of solution processing, low cost, and good compatibility with silicon-based substrates. In this paper, we summarize the recent development of infrared photodetectors based on mainstream lead chalcogenide colloidal quantum dots.

High-Performance and Stable Colloidal Quantum Dots Imager via Energy Band Engineering

Solution-processed colloidal quantum dot (CQD) photodiodes are compatible for monolithic integration with silicon-based readout circuitry, enabling ultrahigh resolution and ultralow cost infrared imagers. However, top-illuminated CQD photodiodes for longer infrared imaging suffer from mismatched energy band alignment between narrow-bandgap CQDs and the electron transport layer. In this work, we designed a new top-illuminated structure by replacing the sputtered ZnO layer with a SnO₂ layer by atomic layer deposition. Benefiting from matched energy band alignment and improved heterogeneous interface, our top-illuminated CQD photodiodes achieve a broad-band response up to 1650 nm. At 220 K, these SnO₂-based devices exhibit an ultralow dark current density of 3.5 nA cm^-2 at -10 mV, reaching the noise limit for passive night vision. The detectivity is 4.1 × 10¹² Jones at 1530 nm. These SnO₂-based devices also demonstrate exceptional operation stability. By integrating with silicon-based readout circuitry, our CQD imager realizes water/oil discrimination and see-through smoke imaging.

Room Temperature Bias-Selectable, Dual-Band Infrared Detectors Based on Lead Sulfide Colloidal Quantum Dots and Black Phosphorus

A single photodetector capable of switching its peak spectral photoresponse between two wavelength bands is highly useful, particularly for the infrared (IR) bands in applications such as remote sensing, object identification, and chemical sensing. Technologies exist for achieving dual-band IR detection with bulk III-V and II-VI materials, but the high cost and complexity as well as the necessity for active cooling associated with some of these technologies preclude their widespread adoption. In this study, we leverage the advantages of low-dimensional materials to demonstrate a bias-selectable dual-band IR detector that operates at room temperature by using lead sulfide colloidal quantum dots and black phosphorus nanosheets. By switching between zero and forward bias, these detectors switch peak photosensitive ranges between the mid- and short-wave IR bands with room temperature detectivities of 5 × 10⁹ and 1.6 × 10¹¹ cm Hz^1/2 W^-1, respectively. To the best of our knowledge, these are the highest reported room temperature values for low-dimensional material dual-band IR detectors to date. Unlike conventional bias-selectable detectors, which utilize a set of back-to-back photodiodes, we demonstrate that under zero/forward bias conditions the device's operation mode instead changes between a photodiode and a phototransistor, allowing additional functionalities that the conventional structure cannot provide.

High-Performance Colloidal Quantum Dot Photodiodes via Suppressing Interface Defects

PbS colloidal quantum dot (CQD) infrared photodiodes have attracted wide attention due to the prospect of developing cost-effective infrared imaging technology. Presently, ZnO films are widely used as the electron transport layer (ETL) of PbS CQDs infrared photodiodes. However, ZnO-based devices still suffer from the problems of large dark current and low repeatability, which are caused by the low crystallinity and sensitive surface of ZnO films. Here, we effectively optimized the device performance of PbS CQDs infrared photodiode via diminishing the influence of adsorbed H₂O at the ZnO/PbS CQDs interface. The polar (002) ZnO crystal plane showed much higher adsorption energy of H₂O molecules compared with other nonpolar planes, which could reduce the interface defects induced by detrimentally adsorbed H₂O. Based on the sputtering method, we obtained the [002]-oriented and high-crystallinity ZnO ETL and effectively suppressed the adsorption of detrimental H₂O molecules. The prepared PbS CQDs infrared photodiode with the sputtered ZnO ETL demonstrated lower dark current density, higher external quantum efficiency, and faster photoresponse compared with the sol-gel ZnO device. Simulation results further unveiled the relationship between interface defects and device dark current. Finally, a high-performance sputtered ZnO/PbS CQDs device was obtained with a specific detectivity of 2.15 × 10¹² Jones at -3 dB bandwidth (94.6 kHz).

Aligned Plasmonic Antenna and Upconversion Nanoparticles toward Polarization-Sensitive Narrowband Photodetection and Imaging at 1550 nm

Lanthanide-doped upconversion nanoparticles (UCNPs) are rising as prospect nanomaterials for constructing polarization-sensitive narrowband near-infrared (NIR) photodetectors (PDs), which have attracted significant interest in astronomy, object identification, and remote sensing. However, polarized narrowband NIR photodetection and imaging based on UCNPs have yet to be realized. Herein, we demonstrate that NIR photodetection and imaging are capable of sensing polarized light as well as affording wavelength-selective detection at 1550 nm by integrating directional-Au@Ag nanorods (D-Au@Ag NRs) with NaYF4:Er3+@NaYF4 UCNPs. Monolayer and large-area D-Au@Ag NRs polarization-sensitive plasmonic antenna films are obtained, and the center of their localized surface plasmon resonance (LSPR) peak is located at around 1550 nm. Experimental and theoretical results reveal that D-Au@Ag NRs have a sharp localized LSPR peak with a dominant scattering cross section. The UCNPs coupled with D-Au@Ag NRs exhibit significantly enhanced and strongly polarization-dependent luminescence with a high degree of polarization (DOP) of 0.72. The first polarization-resolved UC narrowband PD at 1550 nm is achieved, which delivers a DOP of 0.63, a detectivity of 1.69 × 1010 Jones, and a responsivity of 0.32 A/W. Finally, we develop a polarized imaging system for 1550 nm with visual photoelectric detection based on the aforementioned PDs. Our work opens up possibilities for manipulating UC and developing next-generation polarization-sensitive narrowband infrared photodetection and imaging technology.

Quantum-Size-Effect Tuning Enables Narrowband IR Photodetection with Low Sunlight Interference

Infrared photodetection enables depth imaging techniques such as structured light and time-of-flight. Traditional photodetectors rely on silicon (Si); however, the bandgap of Si limits photodetection to wavelengths shorter than 1100 nm. Photodetector operation centered at 1370 nm benefits from lower sunlight interference due to atmospheric absorption. Here, we report 1370 nm-operating colloidal quantum dot (CQD) photodetectors and evaluate their outdoor performance. We develop a surface-ligand engineering strategy to tune the electronic properties of each CQD layer and fabricate photodetectors in an inverted (PIN) architecture. The strategy enables photodetectors with an external quantum efficiency of 75% and a low dark current (1 μA/cm²). Outdoor testing demonstrates that CQD-based photodetectors combined with a 10 nm-line width bandpass filter centered at 1370 nm achieve over 2 orders of magnitude (140× at incident intensity 1 μW/cm²) higher signal-to-background ratio than do Si-based photodetectors that use an analogous bandpass filter centered at 905 nm.

Limiting Factors of Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors

Lead sulfide colloidal quantum dots (PbS CQDs) have shown great potential in photodetectors owing to their promising optical properties, especially their strong and tunable absorption. However, the limitation of the specific detectivity (D*) in CQD near-infrared (NIR) photodetectors remains unknown due to the ambiguous noise analysis. Therefore, a clear understanding of the noise current is critically demanded. Here, we elucidate that the noise current is the predominant factor limiting D*, and the noise is highly dependent on the trap densities in halide-passivated PbS films and the carriers injected from the Schottky contact (EDT-passivated PbS films/metal). It is found that the thickness of CQDs is proportional to their interface trap density, while it is inversely proportional to their minimal bulk trap density. A balance point can be reached at a certain thickness (136 nm) to minimize the trap density, giving rise to the improvement of D*. Utilizing thicker PbS-EDT films broadens the width of the tunneling barrier and thereby reduces the carrier injection, contributing to a further enhancement of D*. The limiting factors of D* determined in this work not only explain the physical mechanism of the influence on detection sensitivity but also give guidance to the design of high-performance CQD photodetectors.

Controlled Crystal Plane Orientations in the ZnO Transport Layer Enable High-Responsivity, Low-Dark-Current Infrared Photodetectors

Colloidal quantum dots (CQD) have emerged as attractive materials for infrared (IR) photodetector (PD) applications because of their tunable bandgaps and facile processing. Presently, zinc oxide is the electron-transport layer (ETL) of choice in CQD PDs; however, ZnO relies on continuous ultraviolet (UV) illumination to remove adsorbed oxygen and maintain high external quantum efficiency (EQE), speed, and photocurrent. Here, it is shown that ZnO is dominated by electropositive crystal planes which favor excessive oxygen adsorption, and that this leads to a high density of trap states, an undesired shift in band alignment, and consequent poor performance. Over prolonged operation without UV exposure, oxygen accumulates at the electropositive planes, trapping holes and degrading performance. This problem is addressed by developing an electroneutral plane composition at the ZnO surface, aided by atomic layer deposition (ALD) as the means of materials processing. It is found that ALD ZnO has 10× lower binding energy for oxygen than does conventionally deposited ZnO. IR CQD PDs made with this ETL do not require UV activation to maintain low dark current and high EQE.

Large Photomultiplication by Charge-Self-Trapping for High-Response Quantum Dot Infrared Photodetectors

PbS colloidal quantum dots (CQDs) are emerging as promising candidates for next-generation, low-cost, and high-performance infrared photodetectors. Recently, photomultiplication has been explored to improve the detectivity of CQD infrared photodetectors by doping charge-trapping material into a matrix. However, this relies on remote doping that could influence carrier transfer giving rise to limited photomultiplication. Herein, a charge-self-trapped ZnO layer is prepared by a surface reaction between acid and ZnO. Photogenerated electrons trapped by oxygen vacancy defects at the ZnO surface generate a strong interfacial electrical field and induce large photomultiplication at extremely low bias. A PbS CQD infrared photodiode based on this structure shows a response (R) of 77.0 A·W^-1 and specific detectivity of 1.5 × 10¹¹ Jones at 1550 nm under a -0.3 V bias. This self-trapped ZnO layer can be applied to other photodetectors such as perovskite-based devices.

Silicon Surface Passivation for Silicon-Colloidal Quantum Dot Heterojunction Photodetectors

Sensitizing crystalline silicon (c-Si) with an infrared-sensitive material, such as lead sulfide (PbS) colloidal quantum dots (CQDs), provides a straightforward strategy for enhancing the infrared-light sensitivity of a Si-based photodetector. However, it remains challenging to construct a high-efficiency photodetector based upon a Si:CQD heterojunction. Herein, we demonstrate that Si surface passivation is crucial for building a high-performance Si:CQD heterojunction photodetector. We have studied one-step methyl iodine (CH₃I) and two-step chlorination/methylation processes for Si surface passivation. Transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements reveal that the two-step passivated Si:CQD interface exhibits fewer trap states and decreased recombination rates. These passivated substrates were incorporated into prototype Si:CQD infrared photodiodes, and the best performance photodiode based upon the two-step passivation shows an external quantum efficiency (EQE) of 31% at 1280 nm, which represents a near 2-fold increase over the standard device based upon the one-step CH₃I passivated Si.

Silicon: quantum dot photovoltage triodes

Silicon is widespread in modern electronics, but its electronic bandgap prevents the detection of infrared radiation at wavelengths above 1,100 nanometers, which limits its applications in multiple fields such as night vision, health monitoring and space navigation systems. It is therefore of interest to integrate silicon with infrared-sensitive materials to broaden its detection wavelength. Here we demonstrate a photovoltage triode that can use silicon as the emitter but is also sensitive to infrared spectra owing to the heterointegrated quantum dot light absorber. The photovoltage generated at the quantum dot base region, attracting holes from silicon, leads to high responsivity (exceeding 410 A·W-1 with Vbias of -1.5 V), and a widely self-tunable spectral response. Our device has the maximal specific detectivity (4.73 × 1013 Jones with Vbias of -0.4 V) at 1,550 nm among the infrared sensitized silicon detectors, which opens a new path towards infrared and visible imaging in one chip with silicon technology compatibility.

A silicon-based PbSe quantum dot near-infrared photodetector with spectral selectivity

Traditional photodetectors usually respond to photons larger than the bandgap of a photosensitive material. In contrast to traditional photodetectors for broad-spectrum detection, the currently reported PbS/PMMA/PbSe CQD silicon-based photodetectors can detect spectrally selective light sources. This is attributed to two layers with specific functions, a filter layer on top and a photosensitive layer in contact with the silicon channel. Each of the target sources of the device has a selectivity factor of more than 10 against non-target sources. The s-PD (selective photodetector) has three significant advantages: the ability to tunably adjust the detectable spectral range by easily adjusting the size of QDs. The second is using a new architecture to achieve a high-performance selective photodetector, and finally, the ease-of-integration with silicon. The above features enable the device to meet the needs of particular fields such as secure communication, surveillance, and infrared imaging.

Integrated Structure and Device Engineering for High Performance and Scalable Quantum Dot Infrared Photodetectors

Colloidal quantum dots (CQDs) are emerging as promising materials for the next generation infrared (IR) photodetectors, due to their easy solution processing, low cost manufacturing, size-tunable optoelectronic properties, and flexibility. Tremendous efforts including material engineering and device structure manipulation have been made to improve the performance of the photodetectors based on CQDs. In recent years, benefiting from the facial integration with materials such as 2D structure, perovskite and silicon, as well as device engineering, the performance of CQD IR photodetectors have been developing rapidly. On the other hand, to prompt the application of CQD IR photodetectors, scalable device structures that are compatible with commercial systems are developed. Herein, recent advances of CQD based IR photodetectors are summarized, especially material integration, device engineering, and scalable device structures.

Decreasing Interface Defect Densities via Silicon Oxide Passivation at Temperatures Below 450 °C

Low-temperature (LT) passivation methods (<450 °C) for decreasing defect densities in the material combination of silica (SiO_x) and silicon (Si) are relevant to develop diverse technologies (e.g., electronics, photonics, medicine), where defects of SiO_x/Si cause losses and malfunctions. Many device structures contain the SiO_x/Si interface(s), of which defect densities cannot be decreased by the traditional, beneficial high temperature treatment (>700 °C). Therefore, the LT passivation of SiO_x/Si has long been a research topic to improve application performance. Here, we demonstrate that an LT (<450 °C) ultrahigh-vacuum (UHV) treatment is a potential method that can be combined with current state-of-the-art processes in a scalable way, to decrease the defect densities at the SiO_x/Si interfaces. The studied LT-UHV approach includes a combination of wet chemistry followed by UHV-based heating and preoxidation of silicon surfaces. The controlled oxidation during the LT-UHV treatment is found to provide an until now unreported crystalline Si oxide phase. This crystalline SiO_x phase can explain the observed decrease in the defect density by half. Furthermore, the LT-UHV treatment can be applied in a complementary, post-treatment way to ready components to decrease electrical losses. The LT-UHV treatment has been found to decrease the detector leakage current by a factor of 2.