Reverse Dark Current Research Articles

Nanograting p-n Junctions with Enhanced Charge Confinement

Recently, geometry-induced quantum effects in a new quasi-1D system, or nanograting (NG) layers, were introduced and investigated. Dramatic changes in band structure and unconventional photoluminescence effects were found in silicon quantum wells with high-energy barriers. Nanograting metal–semiconductor junctions were fabricated and investigated. Here, we report the latest results on a special type of p-n junction in which the charge confinement of the NG is enhanced. The reverse bias dark current is increased in contrast to the metal–semiconductor junctions. When such a junction works as a photovoltaic cell, NG significantly increases short-circuit current and conversion efficiency without affecting open-circuit voltage. These effects are explained by the formation of geometry-induced excitons. To distinguish exciton formation from G-doping effects, we fabricated NGs in both n-type and p-type top layers and obtained qualitatively the same results. To further verify the excitonic mechanism, we analyzed photoluminescence spectrums previously obtained from NG and other NG-like periodic structures. The collected experimental results and previous findings are well explained by the formation of geometry-induced excitons and corresponding quasi-flat bands. Geometry-induced quantum effects can be used to significantly increase the conversion efficiency of photovoltaic cells and enhance the characteristics of other optoelectronic devices.

Suppressing the Dark Current Under Forward Bias for Dual‐Mode Organic Photodiodes

AbstractTremendous research efforts are developed to suppress the reverse dark current (Jd) and enhance the responsivity of organic photodiodes (OPDs). The functional layers of traditional OPDs usually follow the principle of energy level alignment to make unobstructed photo‐carriers transport under reverse bias, but this inevitably leads to a large forward Jd. Herein, a universal strategy is proposed to manipulate the carrier dynamics and effectively suppress the forward Jd of OPDs, that is, tuning the energy level and electron traps of the anode interface layers (AILs). The bandgap and electron traps of typical organometallic chelate AIL (PEIE‐Co) can be well controlled by adjusting the component ratio of PEIE and metal ions. The wide bandgap increases the carrier injection barrier under reverse and forward bias, endowing OPD with a much lower Jd; the electron traps induce hole tunneling injection by capturing photo‐generated electrons under forward bias, thereby enabling the photomultiplication effect. The obtained OPD exhibits photoconductive/photomultiplication working mode at reverse/forward bias and the specific detectivity approaches ≈1013/1012 Jones, showing promise for adaptively detecting faint and strong light. This study presents an intelligent strategy to achieve dual‐mode OPDs, paving the way for the multifunctional development of photodetectors.

Facile Vertical Structure Broadband Photodetectors Enabled by Polyvinylpyrrolidone-Regulated Perovskite and Near-Infrared-Sensitive Lead Phthalocyanine.

Broadband photodetectors have drawn tremendous attention in many application areas such as imaging, optical communication, and biochemical sensing. Perovskite is a star material with broad spectral absorption, but it is challenging to develop ultraviolet-visible-near-infrared (UV-Vis-NIR) ultra-broadband photodetectors due to the insufficient absorption in the near-infrared region. Moreover, it is difficult to construct a diode-type photodetector with a simple vertical structure based only on perovskite materials. Here, facile vertical structure broadband photodetectors were fabricated based on heterojunctions that were composed of perovskite MAPbI3 films with UV-Vis absorption spectrum and small organic molecule lead phthalocyanine (PbPc) with strong NIR optical absorption, resulting in UV-Vis-NIR ultra-broadband photodetection. The quality of MAPbI3 films was improved by introducing polyvinylpyrrolidone (PVP) modification, and subsequently, the corresponding MAPbI3/PbPc heterojunction-based photodetectors exhibited rectification characteristics and reduced reverse dark currents. When the PVP mass ratio is 1 wt%, the photodetector achieved the best performance that the spectral response uniformity factor was as high as 0.77, the photoresponsivity exceeded 10 A/W, and the photoresponse time was less than 0.5 ms under a light intensity of 0.013 mW/cm2 in the UV-Vis to NIR spectral range. These results are comparable or superior to those of some inorganic, organic, and perovskite photodetectors reported previously. This study would provide an effective strategy to construct high-performance perovskite photodetectors based on a simple vertical structure, paving the way to the realization of UV-Vis-NIR broadband photodetection.

Study on electronic transport performance of Ag-ZnO film by photoassisted conductive atomic force microscopy

Purpose The purpose of this paper is to study the electronic transport performance of Ag-ZnO film under dark and UV light conditions. Design/methodology/approach Ag-doped ZnO thin films were prepared on fluorine thin oxide (FTO) substrates by sol-gel method. The crystal structure of ZnO and Ag-ZnO powders was tested by X-ray diffraction with Cu Kα radiation. The absorption spectra of ZnO and Ag-ZnO films were recorded by a UV–visible spectrophotometer. The micro electrical transport performance of Ag-ZnO thin films in dark and light state was investigated by photoassisted conductive atomic force microscope (PC-AFM). Findings The results show that the dark reverse current of Ag-ZnO films does not increase, but the reverse current increases significantly under illumination, indicating that the response of Ag-ZnO films to light is greatly improved, owing to the formation of Ohmic contact. Originality/value To the best of the author’s knowledge, the micro electrical transport performance of Ag-ZnO thin films in dark and light state was firstly investigated by PC-AFM.

Thickness Study of Ga2O3 Barrier Layer in p-Si/n-MgZnO:Er/Ga2O3/ZnO:In Diode

The p-Si/n-MgZnO:Er/Ga2O3/ZnO:In diodes with different Ga2O3 thicknesses were fabricated through spray pyrolysis deposition at 450 °C with aqueous solutions containing magnesium nitrate, zinc acetate, erbium acetate, gallium nitrate, and indium nitrate precursors. The effects of Ga2O3 layer thickness on the diode properties were investigated. For the deposited films, a combined tiny hexagonal slices and small blocks surface morphology was characterized by scanning electron microscopy for all samples. Diodes were formed after In and Ag deposition on the back side and top side, respectively. The current-voltage characteristics and luminescence spectra are studied. With the increasing of Ga2O3 thickness, the diode forward bias resistance increases while the reverse biased dark current shows the decrease-increase characters. The Er ion corresponded green light emission was characterized for the diode under reverse biased breakdown condition. The increased luminescent intensity with low turn-on current behaviors was characterized by the diode with a Ga2O3 thickness of 4.9 nm. With the diode electrical and luminescence analysis, the effect of the Ga2O3 barrier layer on the diode was discussed. The Ga2O3 barrier layer improves performance for rare earth-related light-emitting devices.

Characterization of defects in CIGSe solar cells through admittance spectroscopy

We use admittance spectroscopy to characterize the energy distribution of defects in CIGSe solar cells before and after annealing to investigate the mechanism of the annealing process improving the performances of solar cells. In this work, we anneal the prepared CIGSe solar cells in compressed air at 150 ℃ for 10 min. We measure dark <i>I-V</i>, <i>C-V</i>, admittance spectra, and illumination <i>I-V</i> tests on CIGSe solar cells before and after annealing to characterize the changes in the performances of solar cells before and after annealing, respectively. The test results of dark <i>I-V</i> characteristics show that the reverse dark current of CIGSe solar cell decreases by about an order of magnitude after annealing, and the ideal factor of the cell also decreases from 2.16 (before annealing) to 1.85 (after annealing). This means that the annealing process reduces the recombination of carriers in CIGSe solar cell. Under reverse bias, the capacitance of CIGSe solar cell is higher than that after annealing, and its <i>C-V</i> characteristics linearly fitted with 1/<i>C</i><sup> 2</sup> <i>vs. V</i>. The fitting results show that the slope of the curve increases after annealing, which means that the annealing process results in a decrease in the free carrier concentration in the absorption layer of CIGSe solar cell, specifically, the carrier concentration contributed by defects after annealing decreases. In addition, the built-in potential before and after annealing of CIGSe solar cell are also obtained through fitting, which are 0.52 V and 0.64 V in value, respectively. The admittance spectrum test results of CIGSe solar cell before and after annealing show that the defect activation energy in the absorption layer significantly decreases after annealing, but the defect concentration remains almost unchanged. The decrease in defect activation energy means that the Shockley Read Hall (SRH) recombination probability of defects in copper indium gallium selenium solar cell decreases. In addition, the test results of the optical <i>I</i>-<i>V</i> characteristics of the battery indicate that the open circuit voltage and parallel resistance of the solar cell significantly increase after annealing, which is consistent with the test results of the dark <i>I-V</i> characteristics, <i>C-V</i> characteristics, and admittance spectroscopy of the solar cell. Therefore, the annealing process of CIGSe solar cells leads to theweakening of the SRH recombination of carriers in the absorption layer of the cell, thereby improving the performance of the solar cell.

Selectively Modulated Photoresponse in Type‐I Heterojunction for Ultrasensitive Self‐Powered Photodetectors

AbstractPhotodetectors based on two‐dimensional (2D) van der Waals heterostructures (vdWHs) have demonstrated great potential in modern nanotechnologies across a wide range of applications. However, due to the severe interface recombination of the photogenerated electron–hole pairs and various absorption edges of constituent layers, they would suffer from low carrier collection efficiency, and the spectral response range of each constituent layer is indistinguishable. Herein, tellurium (Te) nanoflakes with broadband photoresponse are synthesized to construct type‐I InSe/Te vdWHs photodetector, which exhibits an ultralow reverse dark current of 3 × 10−14 A and an ultrahigh current rectification ratio of 108. Moreover, considerable photovoltaic effect of the heterostructure device is observed under illumination, attaining a light on/off ratio of up to 105, a high specific detectivity of 1.77 × 1011 Jones, and a fast response time of 320 µs. Based on type‐I band alignment, the Te layer can collect the photogenerated holes from the InSe layer to suppress the recombination of photogenerated carriers. More importantly, the spectral response of the InSe/Te heterostructure photodetector can be selectively modulated by the InSe layer. This work demonstrates that band alignment engineering of 2D vdWHs holds great potential for developing high‐performance self‐powered photodetectors.

P3HT-rGO composites for High-Performance Optoelectronic Devices

The performance of the polymer-based devices has been believed to be inadequate due to their poor charge transport properties. Therefore, to realize the high-performing device, sometimes it is necessary to blend it with the other materials. Here, the reduced graphene oxide (rGO) was incorporated in the P3HT matrix to improve the charge transport properties of pristine P3HT. Incorporating rGO into the polymer matrix has enhanced the charge carrier mobility and absorbance. As a result, the composite-based photodetectors (PDs) demonstrated substantially higher photocurrent and lower reverse dark current than pure P3HT PDs, yielding a photoresponsivity and detectivity of 0.85 A/W and 4.78 × 1011 Jones, at 2.0 V, respectively. These values are more than one order larger than those obtained with the pure P3HT PDs. Furthermore, impedance measurement showed higher shunt resistance and lower capacitance for the composite-based PDs, verifying the role of rGO in improving the conductivity and suppressing the charge recombination.

In/CdTe/Au p–n junction-diode X/[formula omitted]-ray detectors formed by frontside laser irradiation doping

The fabricated In/p-CdTe/Au diode structures with Schottky (In/p-CdTe) and quasi-ohmic (Au/p-CdTe) contacts were transformed into p–n junction diodes by applying the developed frontside laser irradiation doping technique. The Schottky diodes with In and Au electrodes (both with thickness of 400 nm), deposited on the chemically pre-treated B- and A-face of the detector-grade CdTe(111) single crystals, respectively, were subjected to multiple (from tens to thousands times) irradiation from the In film side with series of nanosecond pulses of a KrF excimer laser. As a result of such irradiation, a thin heavily doped n-type CdTe:In layer was formed, a shallow abrupt p–n junction was created and thus the In/CdTe/Au Schottky diode was transformed into the In/CdTe/Au p–n junction diode. The temperature distributions in the In/CdTe/Au structure under laser irradiation were calculated and the I–V characteristics of the diodes were measured for different energy densities E and number N of laser pulses, respectively. The processes of frontside laser irradiation doping and carrier charge transport mechanisms in the created In/CdTe/Au p–n junction diodes were discussed. The samples with higher performance were obtained when only a part of the In electrode thickness was melted during a single laser action (E∼110 mJ/cm 2) and repeated (N∼300–500) irradiation was employed. The fabricated In/CdTe/Au p–n junction-diode structures with relative low reverse dark currents (∼20–50 nA/cm2) were quite sensitive to X/γ-rays and demonstrated sufficient ability to detect the emission of 241Am, 57Co, and 137Cs isotopes. It has been shown that applying the frontside laser irradiation doping technique to In/CdTe/Au structures, it is possible to transform Schottky diodes with low characteristics into highly rectifying In/CdTe/Au p–n junction diodes which are X/γ-ray detectors with moderate or even high detection parameters.

Conformal MoS2/Silicon Nanowire Array Heterojunction with Enhanced Light Trapping and Effective Interface Passivation for Ultraweak Infrared Light Detection

Abstract2D layered materials have attracted considerable attention for fabricating IR photodetectors. However, performance, especially for weak signal detection, of these IR photodetectors is often limited by the low absorbance of the very thin active materials as well as unsuppressed dark currents. Herein, a photodetector with high sensitivity for ultraweak IR signals based on a conformal MoS2/silicon nanowire array heterojunction with an ultrathin Al2O3 interfacial passivation layer is reported. The conformal light‐trapping nanoarray structure can greatly enhance broadband IR absorption, while the Al2O3 layer can suppress interface carrier recombination effectively and thus lower reverse dark current. The photodetectors exhibit broadband photoresponse ranging from 300 to 1600 nm, low noise current of 0.11 pA Hz−1/2, large responsivity up to 0.61 A W−1 at zero bias voltage, and high specific detectivity of 1011–1012 Jones. Significantly, the devices are capable of detecting ultraweak IR signals (e.g., 100 pW at 808 nm and 3 nW at 1310/1550 nm) with high on–off ratios without applying any external bias. This work proposes a new strategy to enhance light absorption of 2D materials and construct high‐quality junction based on them, which is promising for low‐cost and high‐performance optoelectronic devices.

Water Transfer Printing of Multilayered Near‐Infrared Organic Photodetectors

AbstractTo obtain high‐performance organic near‐infrared (NIR) photodetectors, it is essential to suppress the noise current. Here, high‐performance organic NIR photodetectors based on a multilayered structure fabricated by a water transfer printing strategy are reported. These devices comprise a NIR small molecule electron‐accepting layer (IEICO‐4F) and several repeating polymers stacked electron‐donating layers (PTB7‐Th). These polymer layers are prepared onto water substrate and transferred to the target substrate directly, working as an electron‐donating moiety and also as an electron blocking layer. The water transfer printing fabricated multilayered NIR photodetectors obtain extremely low noise current by suppressing the reverse dark current and decreasing the carriers trapping/de‐trapping. The NIR organic photodiodes with multilayered structure exhibit a noise current as low as about 27 fA Hz−1/2 at −0.5 V, resulting in a specific detectivity of approaching 1012 Jones at the NIR wavelength of 860 nm that is about 3 orders of magnitude higher than that of the control devices with a bulk‐heterojunction structure. Furthermore, the multilayered NIR photodetectors also exhibit a large linear dynamic range, fast response simultaneously. This proposed multilayered structure and the utilization of the water transfer printing method could help create high‐performance organic photodetectors.

Reverse dark current in organic photodetectors and the major role of traps as source of noise

Organic photodetectors have promising applications in low-cost imaging, health monitoring and near-infrared sensing. Recent research on organic photodetectors based on donor–acceptor systems has resulted in narrow-band, flexible and biocompatible devices, of which the best reach external photovoltaic quantum efficiencies approaching 100%. However, the high noise spectral density of these devices limits their specific detectivity to around 1013 Jones in the visible and several orders of magnitude lower in the near-infrared, severely reducing performance. Here, we show that the shot noise, proportional to the dark current, dominates the noise spectral density, demanding a comprehensive understanding of the dark current. We demonstrate that, in addition to the intrinsic saturation current generated via charge-transfer states, dark current contains a major contribution from trap-assisted generated charges and decreases systematically with decreasing concentration of traps. By modeling the dark current of several donor–acceptor systems, we reveal the interplay between traps and charge-transfer states as source of dark current and show that traps dominate the generation processes, thus being the main limiting factor of organic photodetectors detectivity.

Self‐Driven WSe2/Bi2O2Se Van der Waals Heterostructure Photodetectors with High Light On/Off Ratio and Fast Response

AbstractBenefiting from the superior electron mobility and good air‐stability, the emerging layered bismuth oxyselenide (Bi2O2Se) nanosheet has received considerable attention with the promising prospects for electronics and optoelectronics applications. However, the high charge carrier concentration and bolometric effect of Bi2O2Se give rise to the high dark current and relatively slow photoresponse, which severely impede further improvement of the performance of Bi2O2Se based photodetectors. Here, a WSe2/Bi2O2Se Van der Waals p‐n heterostructure is reported with a pronounced rectification ratio of 105 and a low reverse dark current of 10−11 A, as well as an enhanced light on/off ratio up to 618 under 532 nm light illumination. The device also exhibits a fast response speed of 2.6 µs and a broadband detection capability from 365 to 2000 nm due to the efficient charge separation and strong interlayer coupling at the interface of the two flakes. Importantly, the built‐in potential in the WSe2/Bi2O2Se heterostructure offers a competitive self‐powered photodetector with the light on/off ratio above 105 and a photovoltaic responsivity of 284 mA W−1. The WSe2/Bi2O2Se heterostructure shows promising potentials for high‐performance self‐driven photodetector applications.

The Influence of the Conditions of Getter Formation in High-Resistivity Silicon on the Characteristics of PIN Photodiodes

The influence of the conditions of gettering in high-resistivity silicon during fabrication of PIN photodiodes on reverse dark currents has been studied. It is shown that gettering using a combination of phosphorus ion implantation and deposition of a polysilicon film with subsequent doping of the substrate rear side with phosphorus at temperatures below 900°C leads to low values of the reverse dark current and increases the nonequilibrium carrier lifetime.

Comparative study of In/CdTe/Au Schottky- and p–n junction-diode detectors formed by backside laser irradiation doping

Two types of In/CdTe/Au diode structures were fabricated using detector-grade p-CdTe single crystals: (i) by vacuum evaporation of In and Au contacts on the chemically treated (111) surfaces; (ii) by backside laser doping when the In/CdTe structures were irradiated from the crystal side with a wavelength for which the semiconductor was transparent. Thus, diodes with a Schottky contact (In/CdTe) and a p–n junction were obtained, respectively. To prove the doping of the thin region near the In/CdTe interface by In (donor) and study thermal tolerance of both types of In/CdTe/Au diode detectors, room temperature I–V characteristics and 57Co isotope emission spectra were measured before and after annealing of the diodes at temperatures below and above the In melting point. Thermal annealing at lower temperatures led to a slight increase and decrease in reverse dark current of the unirradiated and laser-irradiated samples, respectively. Heating up to 200 °C resulted in a significant increase in reverse current and complete spectra degradation in the samples fabricated without laser processing. After such annealing, the electrical characteristics of the p–n junction diodes, formed by the backside laser doping technique, became optimized and 57Co spectra were almost unchanged. It was supposed that a Schottky barrier at the In/CdTe interface was degraded, while a p–n junction, created in a deeper region of the CdTe crystal, remained functional even after melting and solidifying the In contact.

Влияние условий формирования геттера в высокоомном кремнии на характеристики PIN-фотодиодов

The influence of gettering conditions in high resistivity silicon during the PIN photodiode fabrication process on the reverse dark currents has been studied. It was demonstrated that the getter formation of backside substrate by a combination of phosphorus ion implantation and deposition of polysilicon film followed by phosphorus doping at the temperatures below 900 0C results in reduction of reverse dark current value and increasing of nonequilibrium carrier lifetime.

On the Origin of Dark Current in Organic Photodiodes

AbstractMinimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias ( Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

Impact of average neutron energy on the fast neutron fluency measurement by Np237 fission to capture ratio and reverse dark current of planar silicon detector methods

This work is a subsequent step to study the feasibility of fast neutron fluency measurements using two different complementary methods. Np-237 samples and planar silicon detectors were mounted very close to each other on different sections of a subcritical assembly irradiated with the proton beam of 0,66 GeV (the Quinta assembly at the Joint Institute for Nuclear Research, Dubna, Russia) to provide both samples with the same neutron fluency. We have processed the experimental data of irradiated Np-237 actinide samples and silicon detectors directly placed on two sections of the QUINTA setup without a lead shield-reflector. Applying the try and error method we have found found that the neutron energy for which the ratio of the fission cross section to the capture cross section of the actinide Np-237 from the nuclear data base is equal to the measured ratio of the fissioned and captured actinide isotopes. The retrieved distinct fission and capture cross sections for the distinct neutron energy from the nuclear data base describe the average values. The considered above experimental and earlier obtained data have been shown that the higher is the average neutron energy the smaller is the difference of the neutron fluency measurement between the two methods. This effect has been expected since the silicon detector method efficiently measures the fast neutrons of the energy higher than about 170 keV while the actinide method covers a wider energy range.

Reproducible, High Performance Fully Printed Photodiodes on Flexible Substrates through the Use of a Polyethylenimine Interlayer.

This paper investigates with a statistical analysis the issue of performance reproducibility and optimization in fully inkjet-printed organic photodetectors on flexible substrates. The most crucial process step to obtain reproducible, well performing devices with a high process yield turns out to be the printing of the thin polyethylenimine interlayer used as a surface modifier for the bottom electrode. Controlling solution composition and deposition parameters for this layer, a 57 nA cm-2 mean reverse dark current was achieved, with an outstanding standard deviation as low as 15 nA cm-2, with dramatic improvements in process yield (from less than 20% to over 90%). Device performance in terms of dark currents, EQE (from 50% up to 90% at 525 nm, depending on process), and rectification (ratio between forward current and reverse current over 104 and reaching 105 in the best cases) is among the best for fully printed detectors. Furthermore, the importance of relative humidity control in the deposition environment during the interlayer printing on device characteristics is reported, indicating the processing conditions optimal for scaling to mass manufacturing. The overall interlayer optimization approach was applied to a process using widely adopted materials in the organic optoelectronics field, and thus retains relevance on a broad range.

Фотоприемники с активной областью InGaAs и метаморфным буферным слоем InGaP, выращенные на подложках GaAs

AbstractThe results of studying the reverse dark currents of photodiodes for a wavelength of 1.06 μm grown on a GaAs substrate with the help of an InGaP metamorphic buffer layer are presented. The metalorganic chemical vapor epitaxy (MOCVD) growth technology of InGaP metamorphic buffer layers with a stepwise composition variation is developed. Photodiodes with a photosensitive area of 1 mm in diameter and radiation input through the substrate are fabricated. The photodiode dark current at room temperature and reverse bias of –3 V was 50 nA. It is assumed that the bulk component of the dark current is caused by the tunneling mechanism through the trap levels.