Dark Current Measurements Research Articles

Exploration and Optimization of the Polymer-Modified NiOx Hole Transport Layer for Fabricating Inverted Perovskite Solar Cells.

The recombination of charge carriers at the interface between carrier transport layers such as nickel oxide (NiOx) and the perovskite absorber has long been a challenge in perovskite solar cells (PSCs). To address this issue, we introduced a polymer additive poly(vinyl butyral) into NiOx and subjected it to high-temperature annealing to form a void-containing structure. The formation of voids is confirmed to increase light transmittance and surface area of NiOx, which is beneficial for light absorption and carrier separation within PSCs. Experimental results demonstrate that the incorporation of the polymer additive helped to enhance the hole conductivity and carrier extraction of NiOx with a higher Ni3+/Ni2+ ratio. This also optimized the energy levels of NiOx to match with the perovskite to raise the open-circuit voltage to 1.01 V. By incorporating an additional NiOx layer beneath the polymer-modified NiOx, the device efficiency was further increased as verified from the dark current measurement of devices.

Improved temporal performance and optical quantum efficiency of avalanche amorphous selenium for low dose medical imaging.

Active matrix flat panel imagers (AMFPIs) with thin-film transistor arrays experience image quality degradation by electronic noise in low-dose radiography and fluoroscopy. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) (HARP) photoconductor in indirect AMFPI. In this work, we aim to improve temporal performance of HARP using a novel composite hole blocking layer (HBL) structure and increase optical quantum efficiency (OQE) to CsI:Tl scintillators by tellurium (Te) doping. Two different HARP structures were fabricated: Composite HBL samples and Te-doped samples. Dark current and optical sensitivity measurements were performed on the composite HBL samples to evaluate avalanche gain and temporal performance. The OQE and temporal performance of the Te-doped samples were characterized by optical sensitivity measurements. A charge transport model was used to investigate the hole mobility and lifetime of the Te-doped samples in combination with time-of-flight measurements. The composite HBL has excellent temporal performance, with ghosting below 3% at 10mR equivalent exposure. Furthermore, the composite HBL samples have dark current and achieved an avalanche gain of 16. Te-doped samples increased OQE from 0.018 to 0.43 for 532nm light. The addition of Te resulted in 2.1% first-frame lag, attributed to hole trapping within the layer. The composite HBL and Te-doping can be utilized to improve upon the limitations of previously developed indirect HARP imagers, showing excellent temporal performance and increased OQE, respectively.

Preparation of high-performance large-area perovskite solar cells by atomic layer deposition of metal oxide buffer layer

Perovskite solar cells have been widely recognized as the most promising new-type photovoltaic device due to its power conversion efficiency rapidly increasing from 3.8% to over 26% in merely fifteen years. However, the high performances are achieved mainly on small area cells with an active area lower than 0.1 cm<sup>2</sup>. When enlarging the active area of perovskite solar cells, the efficiency falls dramatically. So, how to reduce the gap between performances of small area cells and large area cells gradually becomes a critical point in the path towards the commercialization of perovskite photovoltaic technology. Herein, a strategy of pre-growing thin layer of TiO<sub>2</sub> on a rough FTO substrate by atomic layer deposition method before spin-coating SnO<sub>2</sub> nanoparticles is proposed. Due to the inherent conformal film growth mode of atomic layer deposition, the FTO substrate can be completely covered by TiO<sub>2</sub>, thus preventing the direct contact between local protrusions of FTO and perovskite layer and impeding the current leakage phenomenon, which can be verified by the measurements from X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy, and further proved by the dark current measurement. By using this method, the repeatability and consistency of the small area cell fabrication technology on the same substrate are improved obviously. The improved electron transport process revealed by photoluminescence results and incident light management process revealed by external quantum efficiency results also brings about better solar cell performances. More importantly, highly efficient 0.5 cm<sup>2</sup> large area perovskite solar cells are fabricated through optimization of TiO<sub>2</sub> thickness. When growing 200 cycles TiO<sub>2</sub> (~9 nm in thickness) by using atomic layer deposition technology, the champion large area perovskite solar cell possesses a power conversion efficiency as high as 24.8% (certified 24.65%). The device performances also show excellent repeatability between different fabrication batches. The perovskite solar cell with TiO<sub>2</sub> buffer layer grown by the atomic layer deposition method can still retain over 95% of its initial efficiency after having been stored in a nitrogen atmosphere for 1500 h. The technique proposed in this paper can be helpful in manufacturing perovskite solar cell modules in the realistic photovoltaic market and can be extended to the large area fabrication of other perovskite optoelectronic devices such as light emitting diode, laser and detector.

Defect-engineered graphene-on-silicon-carbide platform for magnetic field sensing at greatly elevated temperatures

High-temperature electrical properties of p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) and high-purity on-axis 4H-SiC(0001) originate from the double-carrier system of spontaneous-polarization-induced holes in graphene and thermally-activated electrons in the substrate. In this study, we pre-epitaxially modify SiC by implanting hydrogen (H+) and helium (He+) ions with energies ranging from 20 keV to 50 keV to reconstruct its post-epitaxial defect structure and suppress the thermally-developed electron channel. Through a combination of dark current measurements and High-Resolution Photo-Induced Transient Spectroscopy between 300 K and 700 K, we monitor the impact of ion bombardment on the transport properties of SiC and reveal activation energies of the individual deep-level defects. We find that the ion implantation has a negligible effect on 6H-SiC. Yet in 4H-SiC, it shifts the Fermi level from ∼600 meV to ∼800 meV below the minimum of the conduction band and reduces the electron concentration by two orders of magnitude. Specifically, it eliminates deep electron traps related to silicon vacancies in the charge state (2-/-) occupying the h and k sites of the 4H-SiC lattice. Finally, we directly implement the protocol of deep-level defect engineering in the technology of amorphous-aluminum-oxide-passivated Hall effect sensors and introduce a mature sensory platform with record-linear current-mode sensitivity of approximately 80 V/AT with -0.03-%/K stability in a broad temperature range between 300 K and 770 K, and likely far beyond 770 K.

Pixel source follower glow in HxRG detector

AbstractThe Atmospheric Remote‐Sensing Infrared Exoplanet Large‐survey (ARIEL) space mission is dedicated to the study of the exoplanets' atmosphere. To do so, the payload module is made of two instruments. The ARIEL InfraRed Spectrometer instruments is one them, it will perform spectroscopic measurements from the near‐infrared domain to the long‐wave infrared domain. The instrument relies on the use of two H1RG detectors. During early phases of instrument development dark current measurements have been done on an 8 m cutoff detector operated in window mode. These measurements revealed an unexpected level of dark current. This observation triggered a joint series of tests at the European Space Agency technical center and at the Astrophysics Department of the Atomic Energy Commission. As demonstrated in this paper, the excess of dark current originates from the collection, by the photosenstive layer, of light emitted by the source follower transistor of the pixel unit cell. This effect is referred as glow. Based on experimental results, this glow is studied as a function of the detector operating conditions (temperature, biases, timing diagram). These results also show that for the range of operating temperatures considered, the dark current performance of the detector is limited by the glow.

Mixed Organic Cations Promote Ambient Light-Induced Formation of Metallic Lead in Lead Halide Perovskite Crystals.

One major concern toward the performance and stability of halide perovskite-based optoelectronic devices is the formation of metallic lead that promotes nonradiative recombination of charge carriers. The origin of metallic lead formation is being disputed whether it occurs during the perovskite synthesis or only after light, electron, or X-ray beam irradiation or thermal annealing. Here, we show that the quantity of metallic lead detected in perovskite crystals depends on the concentration and composition of the precursor solution. Through a controlled crystallization process, we grew black-colored mixed dimethylammonium (DMA)/methylammonium (MA) lead tribromide crystals. The black color is suggested to be due to the presence of small lead clusters. Despite the unexpected black coloring, the crystals show higher crystallinity and less defect density with respect to the standard yellow-colored DMA/MAPbBr3 crystals, as indicated by X-ray rocking curve and dark current measurements, respectively. While the formation of metallic lead could still be induced by external factors, the precursor solution composition and concentration can facilitate the formation of metallic lead during the crystallization process. Our results indicate that additional research is required to fully understand the perovskite precursor solution chemistry.

Surface Modification of ZnO Nanocrystals with Conjugated Polyelectrolytes Carrying Different Counterions for Inverted Perovskite Light-Emitting Diodes.

In this work, bromide ions (Br-) on the conjugated polyelectrolytes (CPEs) were converted to tetrafluoroborate (BF4-) or hexafluorophosphate (PF6-) ions through anion exchange. The three CPEs (PFN-Br, PFN-BF4, and PFN-PF6) were utilized solely for surface modification of zinc oxide nanocrystals (ZnO NCs). The ionic groups on CPEs can form permanent dipoles to facilitate charge injection from ZnO NCs to cesium lead bromide (CsPbBr3) NC emitters, therefore promoting luminescent properties of inverted perovskite light-emitting diodes (PeLEDs). The experimental results reveal that ZnO NC films were smoothened by CPEs that allowed flat deposition of the perovskite active layers; moreover, the improved contact between ZnO and perovskite layers was beneficial for reducing leakage current, as verified in the dark current measurement of devices. In addition, the incorporation of CPEs helped to passivate the defects of ZnO NC films and prolong the carrier lifetime of CsPbBr3 NCs. PeLEDs based on different CPEs were then constructed and evaluated. The device based on PFN-Br showed the highest brightness and current efficiency, and the one based on PFN-BF4 exhibited better current efficiency over PFN-Br under the low current density below 160 mA/cm2. This is the first report using fluorene-based CPEs with Br-, BF4-, or PF6- groups to modify the properties of ZnO and CsPbBr3 NCs for the construction of inverted PeLEDs so far. Our experiments explored new kinds of CPEs on the surface modification of ZnO NCs and device performance of PeLEDs.

Dark Current Measurement and Noise Correction Method for LWIR QWIP Detection System Based on Focal-Plane Temperature

The performance of long-wave infrared (LWIR) quantum well (QWIP) detection systems is seriously affected by the dark current of the detectors. Tiny variations in the focal-plane temperature of the devices cause fluctuations in the dark current, which in turn generate temporal noise. It is difficult to measure the dark current accurately after the detector assembly is packaged. To address the above problems, a QWIP dark current measurement method based on focal-plane temperature is proposed, as well as a method to reduce dark current noise. First, the response model of the LWIR QWIP detection system was established, and the dark current model was introduced. Then, the detection system components were introduced, chiller calibration experiments were carried out, and the dark current values of the QWIP at different temperatures were measured by combining the system design and parameters. Next, the dark current noise correction method was proposed, the target data were collected, and experiments were carried out to correct them. Finally, after the calculation, the temporal noise was reduced by 57.69% after the correction, which is proof of a significant effect. This method can obtain the real-time dark current value by collecting the focal-plane temperature data, and reduce the dark current temporal noise (difficult to eliminate using conventional methods), which is beneficial for promoting the application of QWIPs in LWIR remote sensing detection.

Effects of doping and minority carrier lifetime on mid-wave infrared InGaAs/InAsSb superlattice nBn detector performance

The effect of majority carrier concentration and minority carrier lifetime on the performance of mid-wave infrared ( λ cutoff = 5.5 μ m ) nBn detectors with variably doped InGaAs/InAsSb type-II superlattice absorbers is investigated. The detector layer structures are grown by molecular beam epitaxy such that their absorbing layers are either undoped, uniformly doped with a target density of 4 × 1015 cm−3, or doped with a graded profile, and variable-area mesa detector arrays are fabricated. Each material's temperature-dependent minority carrier lifetime is determined by time-resolved photoluminescence, and majority carrier concentration is extracted from capacitance–voltage measurements. Detector performance is evaluated with dark current and photocurrent measurements, from which quantum efficiency and shot-noise-limited noise-equivalent irradiance are calculated. The two doped detectors have lower dark current densities compared to their undoped counterpart due to the reduction in diffusion current as well as suppression of depletion current. Although both intentionally doped devices exhibit lower minority carrier lifetimes relative to the undoped device, the device with graded doping maintains a comparable quantum efficiency to the undoped device. Ultimately, the graded doping structure exhibits the highest sensitivity with a shot noise-limited noise-equivalent irradiance of 6.3 × 1010 photons/cm2 s in low-background light conditions, within a factor of 4× of an infrared detector pixel with Rule 07 dark current density and unity quantum efficiency. A detailed analysis of the dark current, quantum efficiency, and minority carrier lifetime provides insight into the material and device design factors that must be considered to realize a device with optimal sensitivity.

2 MeV proton irradiation effect on the performance of InAs/GaSb type-II superlattice long-wave infrared detectors.

In this work, we investigated the effect of 2 MeV proton irradiation on the performance of InAs/GaSb type-II superlattice long-wave infrared detectors by combining ground-based irradiation experiments with Stopping and Range of Ions in Matter (SRIM) computer simulations. We found that irradiation with 2 MeV protons significantly increases the dark current density of the detector by more than two orders of magnitude. At the same time, the quantum efficiency decreases by an order of magnitude. Moreover, proton irradiation degrades the device's performance by raising bulk and surface leakage currents. As the mesa size of the device increases, the proportion of surface leakage current in the total dark current drops. When the mesa size is smaller, the detector irradiation damage is greater. In addition, simulations indicate that the amount of damage produced by 2 MeV proton irradiation rises as irradiation fluences increase. The trends of the irradiation damage produced by the SRIM simulation results and the photocurrent measurements after proton irradiation are consistent, whereas the dark current measurements are identical only at small fluences.

Temperature Dependence Study of Electrical and Electro-Optical Performances of Midwave Infrared Ga-Free T2SL Barrier Photodetector

In this paper, we report on temperature dependence performances of a midwave infrared (MWIR) Ga-free InAs/InAsSb type-II superlattice (T2SL) barrier (XBn) photodetector grown by molecular beam epitaxy on n-type GaSb substrate. The T2SL structure, with a 3 µm thick active region, was processed in a mesa device in order to perform dark current measurements and spectral photoresponse as a function of temperature. Analyses of these temperature dependence characterizations help us to improve the design of Ga-free T2SL MWIR XBn detectors.

Using electroluminescence of subcells for obtaining fundamental resistive-less dark IV characteristic of multi-junction solar cells

A new experimental method for obtaining resistive-less dark current-voltage (IV) characteristic of multi-junction solar cells has been developed. The method realizes by measuring dark voltage, dark current, and additional measurements of the electroluminescence (EL) intensity of subcells. It has been shown that in the resistive-less mode (low currents), the dark voltage depends logarithmically on the product of EL intensities of subcells. Experimental definition of this dependence has made it possible to determine the relationship between the intensities and the voltage. The method has been applied to a GaInP/GaAs solar cell, in which three effects have an influence on IV characteristic: the mismatch of photocurrents generated by subcells, the nonlinear series resistance, and counter-electromotive force. It has been experimentally found that the result of the method does not depend on these effects and allows obtaining a fundamental dark resistive-less IV characteristic of multi-junction solar cells.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Enhancing the photovoltaic properties of SnS-Based solar cells by crystallographic orientation engineering

Tin monosulfide (SnS) is a promising light-harvesting material for solar cell applications, owing to its potential for large-scale production, cost-effectiveness, eco-friendly source materials, and long-term stability. However, SnS crystallizes in an orthorhombic structure, which results in a highly anisotropic charge transport behavior. Tailoring the crystallographic orientation of the SnS absorber layer plays a critical role in the enhancement of the transfer of charge carriers and the power conversion efficiency (PCE). By controlling the substrate tilting angle and temperature ramp rate in vapor transport deposition, the crystal growth orientation was tuned to a preferred direction which significantly suppressed the unfavorable (040) crystallographic plane. Through the combination of these two approaches, the PCE could be increased from 0.11% to 2%. The effect of the tilting angle was numerically simulated to investigate its role in controlling the film uniformity and directing the film growth. In addition, the correlation between the texture coefficient of the (040) plane and the charge transport properties was determined by a combination of analytical methods such as device performance studies, electrochemical impedance spectroscopy, along with transient photovoltage, space-charge-limited current, and dark current measurements. These techniques were blended together to prove that the marked improvement in PCE can be ascribed to a reduced charge recombination (in both SnS bulk and interfaces) and an enhanced hole mobility.

Dual-band InGaAs nBn photodetectors at 2 μm

An nBn type InGaAs photodetector structure operating at 2 μm with dual-band operation capability has been numerically designed and experimentally characterized. A compositionally graded and unintentionally doped InGaAlAs layer with a delta-doped nano-layer behaves as the barrier for majority carrier flow. The pixels fabricated with a 20 μm pitch mesa process yielded peak quantum efficiencies of 67% and 53% without anti-reflective coating for the e-SWIR and SWIR sides, respectively. Dark current measurements on a large area pixel yield 3.40 mA/cm2 at 300 K and 0.61 μA/cm2 at 200 K for the e-SWIR side, while 8.05 μA/cm2 at 300 K and 1.09 nA/cm2 at 200 K are obtained for the SWIR side. I–V characteristics analysis performed with pixels having different areas shows that the designed nBn structure has no surface leakage current presenting a potential benefit for dual-band applications requiring mesa structures that usually suffer from surface states for the InGaAs material system. Temperature dependent dark current characterization confirms this result and implies diffusion current dominated dark current.

Study of HgCdTe (100) and HgCdTe (111)B Heterostructures Grown by MOCVD and Their Potential Application to APDs Operating in the IR Range up to 8 µm.

The trend related to reach the high operating temperature condition (HOT, temperature, T > 190 K) achieved by thermoelectric (TE) coolers has been observed in infrared (IR) technology recently. That is directly related to the attempts to reduce the IR detector size, weight, and power dissipation (SWaP) conditions. The room temperature avalanche photodiodes technology is well developed in short IR range (SWIR) while devices operating in mid-wavelength (MWIR) and long-wavelength (LWIR) require cooling to suppress dark current due to the low energy bandgap. The paper presents research on the potential application of the HgCdTe (100) oriented and HgCdTe (111)B heterostructures grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrates for the design of avalanche photodiodes (APDs) operating in the IR range up to 8 µm and under 2-stage TE cooling (T = 230 K). While HgCdTe band structure with molar composition xCd < 0.5 provides a very favorable hole-to-electron ionization coefficient ratio under avalanche conditions, resulting in increased gain without generating excess noise, the low level of background doping concentration and a low number of defects in the active layer is also required. HgCdTe (100) oriented layers exhibit better crystalline quality than HgCdTe (111)B grown on GaAs substrates, low dislocation density, and reduction of residual defects which contribute to a background doping within the range ~1014 cm–3. The fitting to the experimentally measured dark currents (at T = 230 K) of the N+-ν-p-P+ photodiodes commonly used as an APDs structure allowed to determine the material parameters. Experimentally extracted the mid-bandgap trap concentrations at the level of 2.5 × 1014 cm−3 and 1 × 1015 cm−3 for HgCdTe (100) and HgCdTe (111)B photodiode are reported respectively. HgCdTe (100) is better to provide high resistance, and consequently sufficient strength and uniform electric field distribution, as well as to avoid the tunneling current contribution at higher bias, which is a key issue in the proper operation of avalanche photodiodes. It was presented that HgCdTe (100) based N+-ν-p-P+ gain, M > 100 could be reached for reverse voltage > 5 V and excess noise factor F(M) assumes: 2.25 (active layer, xCd = 0.22, k = 0.04, M = 10) for λcut-off = 8 μm and T = 230 K. In addition the 4-TE cooled, 8 μm APDs performance was compared to the state-of-the-art for SWIR and MWIR APDs based mainly on III-V and HgCdTe materials (T = 77–300 K).

Opaque pixel mask with a broadband absorbing metasurface: application to infrared detectors.

We present an imager architecture comprising dark and active pixels allowing the simultaneous measurement of photonic and dark current, which is of particular interest for low-photon-flux astronomical applications. The principle of operation relies on both the total opacity of a thin metallic screen of sufficient area and the anti-reflective properties of well-designed resonant metal-dielectric gratings made on the same screen. The concept is exemplified in the context of cooled HgCdTe hybrid detectors, at short- and long-wave infrared ranges.

Morphology and I-V Characteristics of Electrochemically Deposited Zinc Oxide on Silicon

This paper reported on the electrochemical deposition of zinc oxide (ZnO) on p-silicon (p-Si) (100) substrate in the mixture of 0.1 M of zinc chloride (ZnCl2) and potassium chloride (KCl) electrolyte at a volume ratio of 1:1, 3:1 and 5:1 namely Sample A, B and C. The deposition process was done in room temperature with a current density of 10 mA/cm2 for 30 minutes. Prior to the experiment, all samples were treated by RCA cleaning steps. All samples were characterized using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The results show that all samples have the same morphology of a flake-like structure with different Zn:O ratio that were 2.81, 2.35 and 2.49 for samples A, B and C. The current-voltage (I-V) characteristic graph was obtained by dark current measurement using Keithley SMU 2400 and the threshold voltage (Vth) values were determined at 2.21 V, 0.85 V and 1.22 V for sample A, B and C respectively which correspond with the Zn:O ratio where the highest value of Zn:O ratio can be found in sample A and the lowest in sample B. Based on these results, it shows that electrochemical deposition technique is capable of being used to deposit the flake-like structure ZnO on semiconductor material to form the p-n junction which behaves like a diode. The value of Vth seems to be depended on the ratio between Zn and O. Higher ratio of Zn and O will cause the higher value of intrinsic carrier concentration and built in potential which will increase the Vth value.

Determination of X-ray detection limit and applications in perovskite X-ray detectors

X-ray detection limit and sensitivity are important figure of merits for perovskite X-ray detectors, but literatures lack a valid mathematic expression for determining the lower limit of detection for a perovskite X-ray detector. In this work, we present a thorough analysis and new method for X-ray detection limit determination based on a statistical model that correlates the dark current and the X-ray induced photocurrent with the detection limit. The detection limit can be calculated through the measurement of dark current and sensitivity with an easy-to-follow practice. Alternatively, the detection limit may also be obtained by the measurement of dark current and photocurrent when repeatedly lowering the X-ray dose rate. While the material quality is critical, we show that the device architecture and working mode also have a significant influence on the sensitivity and the detection limit. Our work establishes a fair comparison metrics for material and detector development.

Evaluation of the Radiation Hardness of Photodiodes in 180-nm CMOS Technology for Medical Applications

Radiation-hard photodiode structures implemented in medical applications are designed in 180-nm CMOS technology. Designed photodiodes were tested against total ionizing doses (TIDs) of 100, 200, and 400 Gy(Si), respectively, and they show high stability in terms of dark current characteristics. After TID of 400 Gy(Si), the dark current increased by up to 15%, compared to the unirradiated characteristics values. TCAD electrical simulations were performed and calibrated with the dark current measurements in order to explain the impact of generated defects due to ionizing radiation. Parameters that are used to model TID radiation have been varied in physical boundaries in order to achieve the desired fitting with the measurements. It is shown that due to the filling of acceptor interface traps with electrons, the space charge region extends, but the extension is limited and partially compensated by the fixed positive charges in the silicon nitride layer. The presented photodiodes result in the improved radiation hardness over the design in 350-nm CMOS technology.