Dark Current Measurements Research Articles

Effects of thermal annealing on the diode properties and space charge limited conduction of the physical vapor-deposited n-ZnS/p-Si(100) heterojunctions

The effects of thermal annealing on the device performance of the as-grown n-ZnS/p-Si(100) deposited via home-made physical vapor deposition machine was examined through the analysis of the I-V characteristics at room temperature under dark current measurements. Both as-grown and thermally annealed samples were found to exhibit a rectifying diode-like behavior and revealing that under reverse bias, avalanche of leakage current flow were observed at voltages greater than -15 V in the as-grown sample. An almost completely disappearing leakage current flow and a dramatic increase of forward current were exhibited after thermal annealing. Exposure of the samples to different wavelengths of light (200-800 nm) showed the thermally annealed sample to have a promising photodetection property in the UV (∼339 nm) and in some wavelengths in the visible range (500, 550 and 600 nm). The current transport of the as-grown and thermally annealed n-ZnS/p-Si(100) heterojunctions fabricated were dominantly space-charge limited conduction with possible presence of defect states in the surface and in the bulk of the material.

Electrical Properties of Bulk, Non-Polar, Semi-Insulating M-GaN Grown by the Ammonothermal Method

Bulk, non-polar M-GaN grown by the Ammonothermal method was studied using dark current versus temperature measurements, photocurrent spectra, photoinduced current transient spectroscopy (PICTS), high temperature current-voltage (I-V), and low-frequency capacitance-voltage (C-V). These measurements show the samples are semi-insulating p-type, with the Fermi level pinned near Ev + 0.9 eV. The concentration of uncompensated acceptors pinning the Fermi level is low, ∼1014 cm−3, while the deep trap spectra are dominated by hole traps with ionization energies close to 0.7 eV, 0.9 eV, and 1.2 eV. The former determine the temperature dependence of photocurrent, while the 0.9 eV defects are the ones pinning the Fermi level. The excellent structural properties and high photosensitivity of the non-polar ammono-grown GaN indicate they may have applications as for radiation detectors and as substrates for nonpolar AlGaN/GaN enhancement mode high electron mobility transistors.

Non-Isothermal Modeling of Dark Current-Voltage Measurements of a CIGS Solar Cell

Dark current density-voltage measurements were performed in a temperature range 110–290 K in both forward and reverse bias on a 0.5 cm2 solar cell sample to determine current and voltage losses in a CIGS solar cell. For the first time a JD − V model applicable in a wide temperature range was constructed by adding non-ideality terms to the Shockley diode equation and comparing the experimentally acquired data to the model simulation results. The shunt current was modeled assuming space charge limited current by a uniform distribution of trap levels. A total of only 9 adjustable parameters in the final JD − V model sufficed to simulate all experimental data simultaneously, i.e., no re-estimation had to be performed at different temperature levels. The estimates for the diode ideality factor n = 1.8 and the diode activation energy ΔE = 1.20 eV coincide with values typically obtained for CIGS solar cell dominated by recombination in the absorber. The quality of the CIGS solar cell used in this study was also confirmed by the low back contact activation barrier of 52 meV and the high shunt resistance. We demonstrate that this flexible model can be used to make an initial prediction of the resistive losses and non-idealities in thin film solar cells.

High temperature GaAs X-ray detectors

Two GaAs p+-i-n+ mesa X-ray photodiodes were characterized for their electrical and photon counting X-ray spectroscopic performance over the temperature range of 100 °C to –20 °C. The devices had 10 μm thick i layers with different diameters: 200 μm (D1) and 400 μm (D2). The electrical characterization included dark current and capacitance measurements at internal electric field strengths of up to 50 kV/cm. The determined properties of the two devices were compared with previously reported results that were made with a view to informing the future development of photon counting X-ray spectrometers for harsh environments, e.g., X-ray fluorescence spectroscopy of planetary surfaces in high temperature environments. The best energy resolution obtained (Full Width at Half Maximum at 5.9 keV) decreased from 2.00 keV at 100 °C to 0.66 keV at –20 °C for the spectrometer with D1, and from 2.71 keV at 100 °C to 0.71 keV at –20 °C for the spectrometer with D2. Dielectric noise was found to be the dominant source of noise in the spectra, apart from at high temperatures and long shaping times, where the main source of photopeak broadening was found to be the white parallel noise.

Nanostructured NiOx-coated TiO2 electrodes for dye-sensitized solar cells

NiOx nanostructures are employed to modify the nanoporous TiO2 photoanode for dye-sensitized solar cells with the surface sol-gel process by multiple spin coating. X-ray photoelectron spectroscopy confirms that the chemical states for the as-prepared NiOx nanostructures are more diverse than those derived by magnetron sputtering. Combined with SEM-EDX results, the structure of NiOx layer cannot be interpreted a uniform “thin film”, but as incomplete coating with a few particulate nickel oxides. The results of dark current measurement and electrochemical impedance spectroscopy analysis suggest that an energy barrier is formed at the interface of electrode and electrolyte by the coated NiOx layers. The basic NiOx-coated surface favors dye attachment, leading to a dramatic enhancement in the amount of adsorbed dye onto the TiO2 surface. It also demonstrates that an energy conversion efficiency of 6.64% could be achieved for the NiOx-coated TiO2 electrode with three spin-coated layers of NiOx which accounts for a 27% increment in energy conversion efficiency over that for the unmodified TiO2 electrode.

Temperature and Injection Dependence of Photoluminescence Decay in Midwave Infrared HgCdTe

Photoluminescence decay (PLD) measurements have been performed on mid-wave infrared (MWIR) Hg-vacancy p-doped HgCdTe samples at temperatures ranging from 85 K to 330 K. The doping level is $$p_{0} = 6 \times 10^{15} \;{\hbox{cm}}^{ - 3}$$ at 80 K and the cut-off wavelength is $$\lambda_{\rm{c}} = 4.2\;\upmu{\hbox{m}}$$ at 300 K. The PLD signal has been fitted with a photo-injection level dependent model in order to estimate the contributions from the different recombination mechanisms to the total minority carrier lifetime. Shockley–Read–Hall centers lying in the bandgap at 25 meV from the conduction or the valence band has been found to limit the minority carrier lifetime from 85 to at least 200 K. The value of the Auger 1 lifetime coefficient is extracted from the first instants of signal decay for each temperature and reaches $$G_{\rm{eei}} n_{i}^{ - 3} = 5 \times 10^{ - 26} \;{\hbox{cm}}^{6} \;{\hbox{s}}^{ - 1}$$ at 85 K. The temperature evolution of the different contributions to the lifetime are in accordance with dark current density measurements in HgCdTe photodiodes.

Low dark current in mid-infrared type-II superlattice heterojunction photodiodes

A mid-infrared (MWIR, 3–5μm) InAs/GaSb type-II superlattice (T2SL) photodiode device with very low dark current is presented. The novel heterojunction device is compared to a conventional pn-homojunction device. Photodetectors with reduced dark current allow an increased operating temperature and thus to lower the cooling requirements for high performance infrared imaging applications. We report on a dark current reduction by a factor of more than 100 at a typical operation voltage of −100mV at 77K, which was realized merely by device design. This measured dark current is the lowest reported to our knowledge for T2SL-detectors operating in the 3–5μm range. At the same time, the photo current signal is unaffected by the heterostructure design over the entire bias voltage range.

Dark current and photoresponse characteristics of extended wavelength infrared photodetectors

The dark current and spectral photoresponse thresholds of a semiconductor photodetector are normally determined by the minimum energy gap (Δ) of the material, or the interfacial energy gap of the heterostructure. In this manuscript, we discuss the performance of an asymmetric p-GaAs/AlxGa1-xAs heterostructure-based infrared photodetector, which shows an extended wavelength threshold beyond the limit set by Δ. The measured dark current was found to agree well with fits obtained from a 3D carrier drift model using the designed value of Δ ∼ 0.40 eV (∼3.1 μm). In contrast, the spectral photoresponse showed extended wavelength thresholds of ∼68 μm, ∼45 μm, and ∼60 μm at positive, zero, and negative biases, respectively, at 5.3 K. For a reference (symmetric) photodetector, the dark current was fitted with the designed value of Δ ∼0.30 eV, and excellent agreement was obtained for both the measured dark current and spectral response. This underlies the advantage of using asymmetric infrared photo-detector designs, in which an extension to the detected wavelengths can be obtained with little compromise to the dark current characteristics.

Transparent Conductive Oxide-Less Dye-Sensitized Solar Cells Consisting of Dye-Cocktail and Cobalt Based Redox Electrolyte

Thin Ti-metal protected stainless steel metal mesh coated with mesoporous TiO2 as flexible photoanode has been used to fabricate back contact transparent conductive oxide-less dye-sensitized solar cells. TiO2 nanoparticle having a particle size of 15–20 nm sensitized with dye cocktail of two indoline dyes D-205 and D-131 were first utilized owing to their complementary light harvesting properties. Short-circuit photocurrent density (Jsc) for the dye cocktail combination of D-205 and D-131 (1:1) was found to be increased due to the increased photon harvesting in the 400–500 nm mainly associated with the contribution from D-131 dye. In addition, the electron recombination was suppressed when dye cocktail was employed as confirmed by the dark current measurement leading to higher open-circuit voltage (Voc). The enhanced Jsc accompanied with increased Voc resulted in to an improved efficiency of 3.59% for this cocktail combination. To enhance the efficiency even further, we have utilized TiO2 nanoparticle having a larger particle size of 30 nm facilitating the mass transport of the bulky [Co(bpy)3]3+/2+ redox species. In order to enhance the photon harvesting window TiO2 nanoparticles were sensitized with porphyrin dye (YD2-o-C8) along with different dye cocktail combinations with another complementary dye (Y123). Utilization of a dye cocktail of YD2-o-C8 and Y123 (4:1) led to improved photoconversion efficiency of 5.25% under simulated solar irradiation.

Thin Al1-x Ga x As0.56Sb0.44 diodes with extremely weak temperature dependence ofavalanche breakdown.

When using avalanche photodiodes (APDs) in applications, temperature dependence of avalanche breakdown voltage is one of the performance parameters to be considered. Hence, novel materials developed for APDs require dedicated experimental studies. We have carried out such a study on thin Al1–xGaxAs0.56Sb0.44 p–i–n diode wafers (Ga composition from 0 to 0.15), plus measurements of avalanche gain and dark current. Based on data obtained from 77 to 297 K, the alloys Al1−xGaxAs0.56Sb0.44 exhibited weak temperature dependence of avalanche gain and breakdown voltage, with temperature coefficient approximately 0.86–1.08 mV K−1, among the lowest values reported for a number of semiconductor materials. Considering no significant tunnelling current was observed at room temperature at typical operating conditions, the alloys Al1−xGaxAs0.56Sb0.44 (Ga from 0 to 0.15) are suitable for InP substrates-based APDs that require excellent temperature stability without high tunnelling current.

Method of Measuring Geometric Noise in a Matrix of Photoreceptors Using an Intelligent System of Detecting Obstacles in the Path of a Train

We present an empirical method of measuring geometric noise in matrix photoreceptors included in an intelligent system developed for detecting obstacles in the path of a train. A reference calibration field uses thermal images of crossties. Measurement of geometric noise sensitivity and dark current is based on selected levels of homogeneity in the reference image. The experimental results demonstrated the advantage of the proposed method compared to a conventional two-point calibration.

Test of the HAPD light sensor for the Belle II Aerogel RICH

The Aerogel Ring-Imaging Cherenkov detector (ARICH) is being installed in the endcap region of Belle II spectrometer to identify particles from B meson decays by detecting the Cherenkov ring image from aerogel radiators. To detect single photons, high-sensitive photon detector which has wide effective area (∼70mm × 70mm), a Hybrid Avalanche Photo Detector (HAPD), has been developed in a collaboration with Hamamatsu K.K. The HAPD consists of hybrid structure of a vacuum tube and an avalanche photodiode (APD). It can be operated in 1.5T magnetic field of the spectrometer and withstands the radiation levels expected in the Belle II experiment. There are two steps of electric pulse amplification: acceleration of photo-electron in electric field in the vacuum tube part and electron avalanche in the APD part resulting in total gain of order 105. For the ARICH, we use 420 HAPDs in total. Before installing them, we performed quality assessment studies such as measurements of dark current, noise level, signal-to-noise ratio and two-dimensional scan with laser illumination. We also measured quantum efficiency of the photocathode. During the HAPD performance tests in the magnetic field, we observed very large signal pulses which cause long dead time of the readout electronics in some of the HAPDs. We have carried out a number of studies to understand this phenomenon, and have found a way to mitigate it and suppress the degradation of the ARICH performance. In this report, we will show a summary of the HAPD performance and quality assessment measurements including validation in the magnetic field for all of the HAPDs manufactured for the ARICH in the Belle II.

A Novel Read-Out Electronics Design Based on 1-Bit Sigma-Delta Modulation

The conventional front-end electronics for PET imaging consist of an energy circuit and a timing circuit. A single channel in front-end electronics typically requires several amplifiers, an ADC and a TDC. In this paper, we present a novel front-end electronic design using 1-bit sigma-delta ( $\boldsymbol {\Sigma }$ - $\boldsymbol {\Delta }$ ) modulation and an FPGA. The new design requires only one analog amplifier per channel. The output of the analog amplifier is read directly by the FPGA. Both the energy and timing calculation are implemented in FPGA firmware. The scope of this paper is to introduce the novel design in detail and to evaluate its performance in energy and dark current measurements. Simulink simulations were performed to validate the design with ideal components. A one-channel prototype circuit was built to assess the design with real components. The prototype circuit was tested with different input signals, including test pulses, pulse signals from a PMT detector, DC current signals and dark current signals from an SiPM sensor. Both the simulation and experimental results show that the method is inherently stable and has excellent accuracy and linearity in energy and dark current measurements. The prototype analog board was built with discrete components cost about $ 0.5 in total. The power consumption was about 20 mW. We conclude that the new method provides a cost-efficient and power-efficient way to accurately measure the energies of analog pulses and dark currents from detectors. The timing performance of this method is currently under evaluation.

Comparison of two fast neutron fluence measurement methods based on Np-237 fission-to-capture ratio measurement (spectral index) and a reverse dark current measurement in a planar silicon detector

The idea of the first method is to search the neutron energy for the ratio of fission cross section to capture cross section of the selected actinide isotope from the nuclear data base that is equal to the measured ratio of the fissioned and captured actinide isotope Np-237.The idea of the second method consists in the measurement of the reverse dark current increase, which is linearly proportional to neutron fluence, induced by the fast neutron irradiation in planar silicon detectors.Np-237 samples and planar silicon detectors were placed inside a subcritical assembly (the Quinta assembly at the Joint Institute for Nuclear Research, Russia) very close to each other assuming that in both samples the same neutron fluence should pass.We concluded that minor actinide samples and planar silicon detectors can be used as neutron fluence detectors especially in the high neutron energy range, where measurements are difficult.Considering the importance of high energy neutron measurement in the ADS (Accelerator Driven System), actinide and silicon detectors could be a very useful tool.

High Performance CMOS Light Detector with Dark Current Suppression in Variable-Temperature Systems.

This paper presents a dark current suppression technique for a light detector in a variable-temperature system. The light detector architecture comprises a photodiode for sensing the ambient light, a dark current diode for conducting dark current suppression, and a current subtractor that is embedded in the current amplifier with enhanced dark current cancellation. The measured dark current of the proposed light detector is lower than that of the epichlorohydrin photoresistor or cadmium sulphide photoresistor. This is advantageous in variable-temperature systems, especially for those with many infrared light-emitting diodes. Experimental results indicate that the maximum dark current of the proposed current amplifier is approximately 135 nA at 125 °C, a near zero dark current is achieved at temperatures lower than 50 °C, and dark current and temperature exhibit an exponential relation at temperatures higher than 50 °C. The dark current of the proposed light detector is lower than 9.23 nA and the linearity is approximately 1.15 μA/lux at an external resistance RSS = 10 kΩ and environmental temperatures from 25 °C to 85 °C.

Temperature dependence of an AlInP 63Ni betavoltaic cell

In this paper, the performance of an Al0.52In0.48P 63Ni radioisotope cell is reported over the temperature range of −20 °C to 140 °C. A 400 μm diameter p+-i-n+ (2 μm i-layer) Al0.52In0.48P mesa photodiode was used as a conversion device in a novel betavoltaic cell. Dark current measurements on the Al0.52In0.48P detector showed that the saturation current increased increasing the temperature, while the ideality factor decreased. The effects of the temperature on the key cell parameters were studied in detail showing that the open circuit voltage, the maximum output power, and the internal conversion efficiency decreased when the temperature was increased. At −20 °C, an open circuit voltage and a maximum output power of 0.52 V and 0.28 pW, respectively, were measured.

A Novel Gel Electrolyte Based on Polyurethane for Highly Efficient in Dye-sensitized Solar Cells

A structurally interconnected block copolymer was facilely prepared by the polymerization of poly(oxyethylene)-segmented diol (PEG2000 and PPG1000) and Isophorone diisocyanate (IPDI), followed by a late-stage curing to generate urethane group cross-linked gels. The gel structure, with multiple functionalities including poly(oxyethylene) segments and urethane linkers were characterized by Fourier transform infrared spectroscopy. The gel-like copolymer was used to absorb a liquid electrolyte; formation of 3D interconnected nanochannels, as could be observed by field emission scanning electronic microscopy has confirmed this absorption of the liquid electrolyte by the copolymer. This elastomeric copolymer was used as the matrix of a polymer gel electrolyte (PGE) for a dye-sensitized solar cell (DSSC), which shows extremely high photovoltaic performance (soaking for 1 h in the electrolyte). In particular, the PGE containing 65 wt% of the liquid electrolyte renders a power conversion efficiency of 7.68 % for its DSSC, with a short-circuit photocurrent density of 16 mA cm−2, an open-circuit voltage of 0.73 V, and a fill factor of 0.65. Electrochemical impedance spectra, and dark current measurements were used to substantiate the explanations of the photovoltaic parameters.

Influence of an Al-blended TiO2 photoanode on the photovoltaic properties of n-DSSCs

ABSTRACTThe performance of n-type dye-sensitized solar cells (n-DSSCs) based on a Co complex redox electrolyte were investigated with a photoanode composed with mixer consists of both TiO2 and AlCl3. The surface of TiO2 films coated on FTO glass substrates was amended by soaking them in a TiCl4:AlCl3 blended aqueous solution at different molar ratios, and calcining them to yield the surface-modified photoelectrodes (Ti:Al/TiO2). An enhancement in the power conversion efficiency 2.02% of the resulting n-DSSCs with Ti:Al (9:1)/TiO2 compared to the reference cell by the plain TiO2. The dark current measurements, incident photon-to-current conversion, and efficiency electrochemical impedance spectroscopy (EIS) revealed that the combination of a mixer onto the TiO2 film led to the delay of the charge recombination among the photo-injected electrons and Co complex redox electrolyte, resulting in an enhanced short-circuit current and open-circuit current, compared to device without surface modification.

(Invited) Development of the Hybrid Solar Cells Using Copper Iodide As a Hole Transport Material

Developing highly efficient and low-cost solar cells as an alternative to conventional silicone solar cell has been attracted much interest. In this regard, the dye-sensitized solar cell (DSC) and perovskite solar cell (PSC) has been intensively studied as a promising system due to its unique operating mechanism to result in reasonably high energy conversion efficiency and its much lower fabrication cost. We have developed a dye-sensitized solid-state cells (DSSCs) using p-CuI or p-CuSCN as a hole-conductor. Although the DSSC is free from the problems associated with the liquid electrolyte, it suffers from its own problems, such as oxidation of CuI under illumination, incomplete pore-filling, loosening of contacts between TiO2 surface and CuI crystallites, and so on. The deposition of CuI inside the voids and on top of the surface of the nanocrystalline TiO2 electrode is achieved by the so called “solution casting” in which the hole conductor (CuI) is dissolved in a suitable solvent and filled into the voids while heating the film to evaporate the solvent. The incomplete pore-filling is a major problem associated with this type of solar cells. Electrodeposition of CuI within the pores of interconnected TiO2 particles has been investigated by the current pulse and continuous current electrodeposition techniques, both in the dark and under illumination. It has been found that the solar cells fabricated using the electrodes prepared by pulse electrodeposition, under illumination, give a collection efficiency of 3.28% with N-719 dye, whereas those prepared by continuous electrodeposition give only 0.75%, both in the absence of triethylamine hydrothiocyanate (THT, a crystal growth inhibiter), under simulated AM 1.5 illumination. CuI films were also grown by the combination of pulse electrodeposition (without THT) followed by solution casting. The cells prepared by these electrodes give even higher collection efficiency of 3.85% compared to 3.38% of the films prepared by only solution casting of CuI in the presence of THT. The major drawback of CuSCN as p-type semiconductor in this solar cell is its poor hole conductivity (10-2 S m-1) and consequent slower rate of reduction of oxidized dye molecules thus allowing the recombination of electrons injected to conduction band of TiO2 with the oxidized dye molecules and/or with holes in CuSCN. We investigated a versatile procedure adopted for significantly enhancing the p-type conductivity of cuprous thiocyanate to result in significantly improved, controllable and tunable performance of the TiO2/N719 Dye/CuSCN all-solid state solar cell. Another approach for DSSCs is focused on TiO2/Sb2S3/CuI/Au-FTO solar cell which utilizes Sb2S3 as an extremely thin light absorbing layer and CuI as a hole conducting material. The solar cell shows very poor performance of lower open-circuit voltage of 0.256 V, a short-circuit current density of 9.50 mA cm-2 and a fill factor of 0.39 under 1 sun illumination for a cell with active surface area of 0.25 cm-2. This is due to the recombination of photo-generated electrons with the holes of the highly conducting CuI. This recombination can be overcome to a certain degree by applying a blocking layer on the Sb2S3 layer to cover the voids in the Sb2S3 layer. The application of N719 dye on the Sb2S3 layer as a recombination blocking layer enhances the conversion efficiency of the solar cell by a factor of 2.5 due to the enhancement of all the solar cell parameters to result in an open circuit voltage of 0.37, short-circuit current density of 13.4 mA cm-2, and a fill factor of 0.50. IPCE and dark current measurements reveal that the main role of the dye on the Sb2S3 layer is the suppression of recombination the electrons with the holes in CuI. Interactions of the NSC groups in the N719 dye with CuI are facilitating the transfer of the photo-generated holes in Sb2S3 to the CuI layer for efficient transport towards the counter electrode. In addition to DSCs, perovskite solar cells (PSCs) have been developed intensively because of rapid increase in its efficiency. PSC has been considered as a kind of derivative of DSCs at first, but recent studies uncovered its unique characteristics different from DSCs. We have also developed PSCs using CuI as a hole transport material. In early studies, application of CuI to PSC gave only poor efficiencies (1-2 %). Recently remarkable improvement has been achieved by introducing carbon nanotube and controlling conditions of CuI film formation. We have successfully fabricated PSCs around 10% of efficiency. The contact between perovskite and CuI seems to play essential role for fabricating efficient PSCs.

Effect of Reaction Time and Temperature on Chemical, Structural, Optical, and Photoelectrical Properties of PbS Thin Films Chemically Deposited from the Pb(OAc)2–NaOH–TU–TEA Aqueous System

Lead sulfide (PbS) thin films have been deposited on float glass substrates by the chemical bath deposition technique using a Pb(CH3COO)2–NaOH–(NH2)2CS–N(CH2CH2OH)3 definite aqueous system. The chemical and structural characteristics, as well as the variation of the optical and photoelectrical properties, were studied as functions of reaction time and temperature. For this purpose, the following characterization techniques were employed: x-ray diffraction analysis, x-ray photoelectron spectroscopy, ultraviolet–visible–near infrared spectrophotometry, and dark and light current measurements. Based on the results, it was observed that increase in the reaction temperature increased the deposition rate of the PbS thin film (associated with the cubic crystalline structure); increase of this parameter from 40°C to 70°C (with reaction time of 60 min) led to an increase of the thickness from ∼129 nm to ∼459 nm and the crystallite size (D) from 15.3 nm to 20.2 nm; on the other hand, increase in temperature decreased the energy bandgap (Eg) from 1.66 eV to 0.51 eV and the relative photosensitivity factor (Sph) from 0.468 to 0.032. A similar effect was obtained with increase of the reaction time for given temperature.