Dark Current Density Research Articles

Bi2S3Electron Transport Layer Incorporation for High-Performance Heterostructure HgTe Colloidal Quantum Dot Infrared Photodetectors

Infrared photodetectors (PDs) based on epitaxial semiconductors occupy the majority of the market, but their high cost from material growth and device integration limits their application fields. Colloidal quantum dots (CQDs) provide a high potential candidate for infrared PDs due to their unique infrared sensitivity, tunable physical and chemical properties, and good compatibility with readout integration circuits. In particular, HgTe CQD PDs have demonstrated a wide detection range from short-wave to long-wave infrared. Although significant progress has been achieved in HgTe CQD infrared PDs, they are still in the primary stage of QD synthesis and prototype device fabrication. Here, we develop a new p–i–n photodiode from the traditional p–i device structure. Bismuth sulfide (Bi2S3) films were adopted as the electron transport layer, which could favor uniform absorber deposition, superior charge extraction, and suppressed interfacial loss. The Bi2S3-based photodiodes have achieved a room-temperature dark current density as low as 1.6 × 10–5 A/cm2 at −400 mV. Furthermore, their specific detectivity (D*) achieves ∼1011 Jones at room temperature.

Excess PbBr2 Passivation of Large PbS Colloidal Quantum Dots to Reduce Dark-Current Density for Near-Infrared Detection

Small-size $\mathrm{Pb}\mathrm{S}$ colloidal quantum dots (CQDs) have achieved excellent performance in photoelectric conversion devices through the ligand-exchange method of mixed lead-halide passivation. However, with the increase of $\mathrm{Pb}\mathrm{S}$ CQD diameter, the proportion of (100) facets on the CQD surface increases and the original proportion of mixed lead-halide ligand cannot passivate (100) facets completely, which will introduce deep defects and deteriorate device performance. Here, we demonstrate an excessive ${\mathrm{Pb}\mathrm{Br}}_{2}$ concentration ligand strategy to sufficiently passivate large-size $\mathrm{Pb}\mathrm{S}$ CQDs with an absorption peak at 1300 nm. The first-principles calculation results suggest that ${\mathrm{Br}}^{\ensuremath{-}}$ can passivate (100) facets more efficiently compared with ${\mathrm{I}}^{\ensuremath{-}}$. With the increase of ${\mathrm{Pb}\mathrm{Br}}_{2}$ concentration (0--0.464 mmol/mL), both optical and electrical measurements imply that defects are effectively passivated, while carrier lifetime increases and dark-current density decreases. Finally, a device with specific detectivity of $5.22\ifmmode\times\else\texttimes\fi{}{10}^{12}$ Jones is obtained. This passivation strategy can also be used in other large-size $\mathrm{Pb}\mathrm{S}$ CQDs (diameter >4 nm) to realize a better device performance.

Preferred Acceptor-Domain Positioning via a Phase-Switched Bulk Heterojunction for Self-Powered Photodetector and Photovoltaic Applications

In this study, a phase-switching process is introduced to a PM6:Y6-based active layer to implement a uniform surface with preferred Y6 acceptor-domain positioning and to minimize solution loss owing to the use of medium molds. By designing the process to fit the surface properties of a small-molecule Y6 acceptor-based bulk heterojunction, a sustainable and highly reproducible phase-switched active layer that can be formed at the desired location without wasting materials is successfully formed over an area of 2.25 cm2. It improved charge transport ability and suppressed surface defects owing to a molecular orientation favorable for efficient charge separation and transport. Furthermore, adding chloronaphthalene induced the domain growth of Y6 acceptors with dense molecular packing, which enhanced the effects of the phase-switching process. Uniformly distributed performance in the phase-switched active layer via the phase-switching process was demonstrated in fabricated large active-area devices. Furthermore, the process was utilized in a PM6:Y6-based self-powered photodetector, which improved detectivity owing to the suppression of the dark current density. Therefore, the phase-switching process is an important technology for forming high-quality thin films composed of small-molecule acceptors for efficient self-powered photodetector and photovoltaic applications.

Investigation of a p-i-n photodetector with an absorbing medium based on InGaAs/GaAs quantum well-dots

The static and dynamic characteristics of waveguide photodetectors with an absorbing region based on InGaAs/GaAs quantum well-dots were studied at room temperature. The absorption band of InGaAs/GaAs quantum well-dots is in the spectral range from 900 to 1100 nm. The waveguide photodetectors have a width of 50 μm and a length of the absorbing region from 92 μm to 400 μm. A low dark current density (1.1 and 22 μA/cm2 at -1 and -20 V) and cut off frequency of 5.6 GHz, limited by the time constant of a parasitic equivalent electric RC-circuit, were obtained. Keywords:waveguide photodetector, modulation frequency, quantum well-dots, integrated photonics.

Исследование p-i-n-фотодетектора с поглощающей средой на основе InGaAs/GaAs квантовых яма-точек

The static and dynamic characteristics of waveguide photodetectors with an absorbing region based on InGaAs/GaAs quantum well-dots were studied at room temperature. The absorption band of InGaAs/GaAs quantum well-dots is in the spectral range from 900 to 1100 nm. The waveguide photodetectors have a width of 50 µm and a length of the absorbing region from 92 µm to 400 µm. A low dark current density (1.1 и 22 μA/cm^2 at -1 и -20 V) and cut off frequency of 5.6 GHz, limited by the time constant of a parasitic equivalent electric RC circuit, were obtained.

Efficient hole extraction and dark current suppression in organic photodetectors enabled by atomic-layer-deposition of ultrathin Co3O4 interlayers

Ultra-thin ALD Co3O4 electron blocking layer in fullerene and non-fullerene based OPDs is shown to effectively reduce dark current density and promote photogenerated charge extraction with the photodetection wavelength range from visible to NIR.

Modelling and Performance Analysis of CuPc and C60 Based Bilayer Organic Photodetector

An optoelectronic device model for organic photodetector based on bilayer structure has been presented. Drift-diffusion and optical-generation model from Synopsys tool have been incorporated and its optoelectronics behavior has been discussed. The model shows an outstanding rectifying behavior under dark condition due to the different work function of the electrodes. Photocurrent density of 6.64 mA/cm2 is found under the illumination of 3 W/cm2. To analyze rectifying behavior of current density-voltage characteristics of the organic photodetector, the curve has been fitted with the Shockley equation. The enhancement of ideality factor of diode current under illumination from that of dark current at forward bias is attributed to enhancement of recombination loss due to generation of photo-carrier and injection of carriers from electrodes. Almost equal probability of photocurrent spectra in the entire spectral region indicates equal probability of exciton generated and dissociated at the interface between CuPc and C60 layers. The detectivity of the proposed photodetector is calculated and it is in order of 1010 Jones at 650 nm due to high dark current density and recombination loss. The presence of interface trap density and large transport distance give evidence of low response speed in the device.

Vertical Architecture Solution-Processed Quantum Dot Photodetectors with Amorphous Selenium Hole Transport Layer

Colloidal quantum dots (CQDs) provide wide spectral tunability and high absorption coefficients owing to quantum confinement and large oscillator strengths, which along with solution processability, allow a facile, low-cost, and room-temperature deposition technique for the fabrication of photonic devices. However, many solution-processed CQD photodetector devices demonstrate low specific-detectivity and slow temporal response. To achieve improved photodetector characteristics, limiting carrier recombination and enhancing photogenerated carrier separation are crucial. In this study, we develop and present an alternate vertical-stack photodetector wherein we use a solution-processed quantum dot photoconversion layer coupled to an amorphous selenium (a-Se) wide-bandgap charge transport layer that is capable of exhibiting single-carrier hole impact ionization and is compatible with active-matrix readout circuitry. This a-Se chalcogenide transport layer enables the fabrication of high-performance and reliable solution-processed quantum dot photodetectors, with enhanced charge extraction capabilities, high specific detectivity (D* ∼ 0.5–5 × 1012 Jones), fast 3 dB electrical bandwidth (3 dB BW ∼ 22 MHz), low dark current density (JD ∼ 5–10 pA/cm2), low noise current (in ∼ 20–25 fW/Hz1/2), and high linear dynamic range (LDR ∼ 130–150 dB) across the measured visible electromagnetic spectrum (∼405–656 nm).

Improved Operation Stability of Colloidal Quantum Dots Photodiodes via Suppressing Ion Migration

Solution-processed colloidal quantum dots (CQDs) photodiodes are compatible for monolithic integration with silicon-based readout circuit, enabling high-performance and low-cost infrared imaging. However, operation stability of CQDs photodiodes has rarely been explored, which is one of the main obstacles for CQDs imaging commercialization. Here, we reveal that the performance deterioration of the CQDs photodiodes under strong electric field at high temperature is mainly attributed to ion migration in CQDs film, clearly clarified by aging photoconductors based on each functional layer of CQDs photodiodes. The halide ion migration through vacancy increases the defect density of the CQDs layer and blocks device interfaces, resulting in increased dark current density and postponed saturation voltage. We introduce polyimide (PI) into CQDs film to efficiently block the pathway for halide ion migration. As a result, the CQDs photodiodes with PI achieve greatly improved operation stability over 27 h under 104 V/cm electric field at 85 °C.

Diffusion of phosphorus in technology for manufacturing silicon p-i-n photodiodes

Comparative characterization of phosphorus diffusion from planar sources and liquid-phase diffusion by using PCl3 in technology for manufacturing silicon p-i-n photodiodes was carried out. The quantitative analysis of dislocations formed when using different variants and modes of diffusion has been performed. The influence of dislocation number on the dark current density and responsivity of photodetectors has been studied. A table has been given for estimation of surface resistance with account of colour inherent to phosphorosilicate glass after doping phosphorus into the surface layer.

Identification of the Origin of Ultralow Dark Currents in Organic Photodiodes.

Organic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density Jd . Recently, using thermal admittance or spectral photocurrent measurements, Jd has been attributed to thermal charge generation mediated by mid-gap states. Here, we report the temperature dependence of Jd in state-of-the-art OPDs with Jd down to 10-9 mA cm-2 at -0.5V bias. For a variety of donor-acceptor bulk-heterojunction blends we find that the thermal activation energy of Jd is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5eV, but higher than expected for mid-gap states. Ultra-sensitive sub-bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor-acceptor interface mediated by intra-gap states near the band edges. This article is protected by copyright. All rights reserved.

Simulation Study on the Structure Design of p-GaN/AlGaN/GaN HEMT-Based Ultraviolet Phototransistors.

This work investigates the impacts of structural parameters on the performances of p-GaN/AlGaN/GaN HEMT-based ultraviolet (UV) phototransistors (PTs) using Silvaco Atlas. The simulation results show that a larger Al content or greater thickness for the AlGaN barrier layer can induce a higher two-dimensional electron gas (2DEG) density and produce a larger photocurrent. However, they may also lead to a larger dark current due to the incomplete depletion of the GaN channel layer. The depletion conditions with various Al contents and thicknesses of the AlGaN layer are investigated in detail, and a borderline between full depletion and incomplete depletion was drawn. An optimized structure with an Al content of 0.23 and a thickness of 14 nm is achieved for UV-PT, which exhibits a high photocurrent density of 92.11 mA/mm, a low dark current density of 7.68 × 10-10 mA/mm, and a large photo-to-dark-current ratio of over 1011 at a drain voltage of 5 V. In addition, the effects of other structural parameters, such as the thickness and hole concentration of the p-GaN layer as well as the thickness of the GaN channel layer, on the performances of the UV-PTs are also studied in this work.

Effect of Cyano Substitution on Non‐Fullerene Acceptor for Near‐Infrared Organic Photodetectors above 1000 nm

AbstractNear‐infrared organic photodetectors (NIR OPDs) comprising ultra‐narrow bandgap non‐fullerene acceptors (NFA, over 1000 nm) typically exhibit high dark current density under applied reverse bias. Therefore, suppression of dark current density is crucial to achieve high‐performance of such NIR OPDs. Herein, cyano (CN) with a strong electron‐withdrawing property is introduced into alkoxy thiophene as a π‐bridge to adjust its optoelectronic characteristics, and the correlation between dark current density and charge injection barrier is investigated. Compared with their motivated NFA (COTH), the novel CN‐substituted NFAs, COTCN and COTCN2, exhibited deeper‐lying highest occupied molecular orbital energy levels and narrower optical bandgap (<1.10 eV), owing to the strong inductive and resonance effect of CN. The dark current and total noise currents are minimized as the number of substituted CN increases because of the larger hole injection barrier. Consequently, PTB7‐Th:COTCN2 exhibited the best shot‐noise limited detectivity (D*sh, 1.18 × 1012 Jones) and total noise detectivity (D*n, 1.33 × 1011 Jones) compared with those of PTB7‐Th:COTH (D*sh, 2.47 × 1011 Jones and D*n, 1.96 × 1010 Jones).

High-performance extended short-wavelength infrared PBn photodetectors based on InAs/GaSb/AlSb superlattices

High performance short-wavelength infrared PBn photodetectors based on InAs/GaSb/AlSb superlattices on GaSb substrate have been demonstrated. At 300 K, the device exhibits a 50% cut-off wavelength of ∼ 2.1 μm as predicted from the band structure calculation; the device responsivity peaks at 0.85 A/W, corresponding to a quantum efficiency (QE) of 56% for 2.0 μm-thick absorption region. The dark current density of 1.03 × 10−3 A/cm2 is obtained under 50 mV applied bias. The device exhibits a saturated dark current shot noise limited specific detectivity (D*) of 3.29 × 1010cm⋅Hz1/2/W (at a peak responsivity of 2.0 μm) under –50 mV applied bias.

High‐Performance Organic Photodetectors Using SnO2 as Interfacial Layer with Optimal Thickness

A photodiode‐type photodetector with bulk heterojunction is built with an ITO/SnO2/P3HT:PC61BM/MoO3/Al structure. By evaluating SnO2 interfacial layer thicknesses of 15, 19, 22, 26, and 29 nm, the effect of thickness on the photodetector response is investigated. The results show that the carrier extraction can reach saturation at −1 V. In addition, the device with interlayer thickness of 22 nm achieves the lowest dark current density (1.6 × 10−6 A cm−2), highest specific detectivity (1.3 × 1011 Jones), widest linear dynamic range (LDR, 43.7 dB), and fastest response (raising/falling time of 1.81/2.22 μs). Hence, the performance of the organic photodetectors can be greatly improved by adopting the proper thickness of the SnO2 interfacial layer.

Quasi-3-dimensional simulations and experimental validation of surface leakage currents in high operating temperature type-II superlattice infrared detectors

The surface leakage in InAs/GaSb type-II superlattice (T2SL) is studied experimentally and theoretically for photodiodes with small sizes down to 10 × 10 μm2. The dependence of dark current density on mesa size is studied at 110 and 200 K, and surface leakage is shown to impact both generation–recombination (GR) and diffusion dark current mechanisms. A quasi-3-dimensional model to simulate the fabrication process using surface traps on the pixel's sidewall is presented and is used to accurately represent the dark current of large and small pixels with surface leakage in the different temperature regimes. The simulations confirmed that the surface leakage current has a GR and diffusion component at low and high temperature, respectively. Finally, the surface leakage current has been correlated with the change in minority carrier concentration at the surface due to the presence of donor traps.

Dual-band and binary response of high-performance photodetectors with bithiophene-difluoroquinoxaline based copolymers via side chain engineering towards secure optical communication

In present work, three bithiophene-difluoroquinoxaline (TQ) random copolymers (PTQ1-PTQ3) with the similar conjugated skeleton and different substituted side chains such as alkyl, phenyl, and ester groups are designed and synthesized using the direct C–H/C–H coupling polymerization method. All polymers exhibit intense UV absorbance and shallow highest occupied molecular orbital (HOMO) levels, and their photophysical properties could be effectively manipulated by engineering the TQ side chains. The PTQ3, which contains two phenyl groups on 2- and 3-position of 6,7-difluoroquinoxaline units and two dioctyl groups on 4- and 4′- position of 2,2′-bithiophene units, displays intramolecular charge transfer (ICT) character, optimized energy levels, and high hole mobility of 6.4 × 10−4 cm2 V−1 s−1. Furthermore, these polymers blending with small molecular acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetralcis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d']-s-indanone[1,2-b:5,6-b']-dithiophene (ITIC) are evaluated as photosensitive layers in polymer photodetectors (PPDs) for light detection. All devices show the dual-band photoresponse and self-powered effect. Especially, PTQ3-based device exhibits an extremely low dark current density of 8.1 × 10−10 A cm−2, a superior external quantum efficiency (EQE) of 33.9% and a high photo-detectivity exceeding 6.0 × 1012 Jones under 365 nm at 0 V, outperforming the reported thiophene-quinoxaline based PPDs. Moreover, an interesting binary response of optimal PPD has been demonstrated by imposing small external bias voltages, which is conducive to the transmission of multi-encoding information. Then, we also propose a feasible encryption scheme for optical communication by modulating the UV and red dual-band response of the PPD. The results illustrate a viable design strategy for side chain engineered bithiophene-difluoroquinoxaline based polymers and provide some design guidelines for high-performance PPDs applied in secure optical communication.

Improvement of Dynamic Performance and Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors by Incorporating Conjugated Polymers.

Colloidal quantum dots (CQDs) have a unique advantage in realizing near-infrared (NIR) photodetection since their optical properties are readily tuned by the particle size, but CQD-based photodetectors (QPDs) presently show a high dark current density (Jd) and insufficient dynamic characteristics. To overcome these two problems, we synthesized and introduced two types of conjugated polymers (CPs) by replacing the p-type CQD layer in the QPDs. The low dielectric constant and insulating properties of CPs under dark conditions effectively suppressed the Jd in the QPDs. In addition, the energy-level alignment and high-hole mobility of the CPs facilitated hole transport. Therefore, both the responsivity and specific detectivity were highly enhanced in the CP-based QPDs. Notably, the dynamic characteristics of the QPDs, such as the -3 dB cut-off frequency and rising/falling response times, were significantly improved in the CP-based QPDs owing to the sizable molecular ordering and fast hole transport of the CP in the film state as well as the low trap density, well-aligned energy levels, and good interfacial contact in the CP-based devices.

Optimization of thermal annealing of zinc oxide films for enhancing performances of near-infrared organic photodetectors

In this paper, we report the effect of annealing time of hole-blocking ZnO layers prepared by a sol-gel process on the performance of near-infrared organic photodetectors (NIR-OPDs). We investigated changes in the dark current density (JD) values of NIR-OPDs depending on the different annealing time of 10, 20, 30, and 60 min. From the devices based on 20 min-annealed ZnO layers, we attained the lowest JD of 1.10 × 10−8 A/cm2 and the highest specific detectivity (D*) of 6.90 × 1012 Jones in response to 850 nm NIR light. Morphology characterization using atomic force microscopy (AFM) revealed that the 20 min annealed ZnO layer demonstrated the lowest surface rms roughness. The lowest series resistance (RS) and the highest shunt resistance (RSH) obtained from the 20 min annealed ZnO film provided evidence of the improved interface between ZnO and photoactive layers, which enabled the efficient suppression of JD and optimized D* values. Our results suggest that the control of annealing time for ZnO film can be a simple and effective strategy to enhance the performance of various types of OPDs based on ZnO layers.

Theoretical and Experimental Investigation of Barrier‐Energy‐Dependent Charge Injection Mechanisms in Organic Photodetectors

AbstractCharge injection is known as the major source of dark current under an applied reverse bias, which directly influences the performance of organic photodetectors with diode architecture. However, it is unclear which of various contributions, such as electron flow through the junction, shunt leakage, thermionic emission, and tunnelling, are dominant. This study investigates the thermionic emission and tunneling models to describe the origin of experimentally measured dark current generated in an organic photodetector. To elucidate the dominant mechanism, the barrier energies at anodic contacts are set from 0.6 to 1.0 eV using photosensitive layers composed of different acceptors. A linear relation is found between the natural logarithm of the dark current density under reverse bias and the square root of the barrier height, which strongly suggests direct tunneling as dominant mechanism for dark current injection. This conclusion is strengthened by temperature dependent dark current analysis. Further knowledge of the dominant mechanism by charge injection can help devise an effective strategy to suppress dark current for effective organic photodetector device implementation.