Dark Current-voltage Characteristics Research Articles

Investigation of Passivating and Protecting Methods for Multijunction Solar Cells

Methods for passivation and protection of p–n junctions at places of their emergence at the lateral surface of a mesa structure and for sealing-off of multijunction solar cells based on a GaInP/GaAs/Ge structure have been studied. Protective coatings based on silicon nitride and silicone layers were examined by analyzing dark current–voltage characteristics of the solar cells. The distribution of the electroluminescence was analyzed.

Low frequency noise-dark current correlations in HgCdTe infrared photodetectors.

In this paper, low frequency noise and dark current correlation is investigated as a function of reverse bias and temperature for short-wave infrared (SWIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR) HgCdTe homo-junction photodetectors. Modelling of dark current-voltage characteristics shows that the detectors have ohmic-behavior under small reverse bias, thus enabling further analysis of 1/f noise-current dependences with the empirical square-law relation (SI ∼ I2) at different temperature regions. It is found that for the SWIR and MWIR devices, the total 1/f noise spectral density at arbitrary temperatures can be modelled by the sum of shunt and generation-recombination noise as SI(T,f)=[αSHISH2(T)+αG-RIG-R2(T)]/f, with no contribution from the diffusion component observed. On the other hand, for the LWIR device the diffusion component induced 1/f noise that cannot be overlooked in high temperature regions, and a 1/f noise-current correlation of SI(T,f)={αs[IDIFF2(T)+IG-R2(T)]+αSHISH2(T)}/f is proposed, with a shared noise coefficient of αs ≅ 1 × 10-9 which is close to that calculated for shunt noise. The 1/f noise-current correlation established in this work can provide a powerful tool to study the low frequency noise characteristics in HgCdTe-based photodetectors and to help optimizing the "true" detectivity of devices operating at low frequency regime.

A 80 kV gas switch triggered by a 17 μJ fiber-optic laser.

A gas switch triggered by μJ laser pulses was developed. The switch employed a 10 mm-gap GaAs photoconductive semiconductor switch (PCSS) mounted in parallel with one of its two gaps. The dark current-voltage characteristic of the PCSS was obtained, and the gas switch characteristics at different bias voltages were experimentally investigated. The results indicate that the switch can be triggered reliably by using a 17 µJ laser pulse, and the jitter is less than 3 ns at the bias voltage of 80 kV and 60% of the self-breakdown voltage.

Unipolar superlattice structures based on MBE HgCdTe for infrared detection

Mid-wave infrared nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs (013) substrates were fabricated. A wide-gap Hg1-xCdxTe (x = 0.75 and 0.84) and a superlattice of 18 periods Hg0.20Cd0.80Te (9 nm)–HgTe (2 nm) were used as a barrier layer. In a wide temperature range, the mechanisms of the formation of dark current–voltage characteristics of fabricated nBn structures were studied. It was shown that for a barrier with a composition of 0.75, surface leakage currents dominate, and for a barrier with a composition of 0.84, the bulk component of the dark current dominates. For superlattice structures, the bulk current value is greater than for the wide-gap barrier, which is associated with a decrease in the height of the potential barrier for holes. The admittance of metal–insulator-semiconductor (MIS) test systems based on superlattice structures was studied. The features of the capacitance–voltage characteristics of MIS capacitors based on the superlattice nBn structure associated with changes in the differential resistance of the barrier layer at different polarity of the bias voltage are revealed.

Pulse measurements of small area thin film μc-Si:H/ZnO:B photodiodes

We introduce a triggered optoelectronic system operating in a pulse mode in the near infrared and visible spectral range 0.75– eV. The system measures current-voltage (I-V) characteristics in dark and under visible light illumination as well as electroluminescence (EL) spectra of small area thin film photodiodes and light emitting diodes with size below 1 mm2. The usefulness of the setup is demonstrated by measurement of optoelectronic properties of a hydrogenated microcrystalline silicon μc-Si:H)p-i-n diode deposited on a semi-transparent nanostructured ZnO:B electrode. No s-shaped I-Vcharacteristics were observed under white illumination near an open circuit voltage Uoc indicating a negligible charge accumulation near μc-Si:H/ZnO:B interface. The weak infrared EL correlates with the current density.

Estimation of trap energy of fuchsin dye sensitized Organic Photovoltaic Device based on Titanium Dioxide (TiO2)

Recently, dye sensitized photovoltaic cell plays an important role as a renewable source of energy because of low material cost, easy processing techniques. But the organic devices are prone to traps due to which charge carriers get trapped or recombined within the device before collection at the electrodes. So traps limit the device performance. The study on the charge transport mechanism enables us to estimate this trap energy. In this work, we have designed the photovoltaic device and estimated the trap energy of inexpensive water-soluble Fuchsin dye based photovoltaic cells. To design a photovoltaic device we have used ITO as a front electrode and copper as a back electrode. 540mg of TiO2 nanoparticles and 100mg of Fuchsin dye mixed well in N, N-Dimethyl formamide (DMF). A thin layer of this TiO2 nanoparticles and dye has been deposited between two electrode using popular method named doctor’s blade method. A drop of electrolyte made of lithium percolate (LiClO4), ethylene carbonate (EC) and propylene carbonate (PC) solution has been used to obtain better device performance. The analysis of dark current density- voltage (J-V) characteristics using exponential charge transport mechanism of this photovoltaic cell the trap energy has been calculated 0.5399 ± 0.006 eV. Also, we have observed photovoltaic effect of this device. Under illumination of 120 kLUX intensity, maximum short circuited current density is obtained 4.62 μA/cm2 and open circuited voltage is 0.24 Volt.

Estimation of potential induced degradation in solar Mini-modules

This work includes the quantitative determination of the effect of potential induced degradation (PID) on the performance of solar mini-module. The PID effect has been studied experimentally by applying a DC voltage of −1000 V on the solar modules with respect to module frame at room temperature (25 ± 1°C) for different time durations. The effects were analyzed using current-voltage (I-V) measurements (dark and illuminated) before and after the PID test. The dark current-voltage characteristics of the modules showed increasing trend in the leakage current with PID test duration, indicating the voltage stress led to enhanced recombination of the charge carriers. The cell parameters such as open-circuit voltage (Voc), short-circuit current (Isc) and fill-factor (FF) were also analyzed and a severe degradation was noticed in these parameters. For instance, the FF deteriorated from 76.7% to 41.6% in 42 h of PID test with a significant reduction in Voc and Isc. The degradation mechanism was also analyzed using impedance analysis to gain further insights on solar cell junction and surface. The observed PID degradation of solar modules even at room-temperature reveals that the PID effect is very much significant on the performance of the module and need to be addressed for effective deployment of solar technologies in the field.

Effect of strain on the dark current-voltage characteristic of silicon heterojunction solar cells

Anisotropic mechanical strain as low as 0.1% modifies the electronic response of crystalline semiconductor-based devices and in particular affects the performance of solar cells. We measure the dark current-voltage characteristic of silicon heterojunction solar cells under different levels of tensile uniaxial stress and observe a reversible change of the j-V curve with applied strain. Using a two-exponential description of the j-V characteristic to fit our experimental data, we obtain the strain dependence of the diffusion saturation current and find a decrease of about 3% for a tensile strain level of 6.7×10-4. We compare these experiments to a theoretical model that accounts for the effect of strain on the band energy levels, densities of states and mobilities of carriers. The theoretical estimation of the change in saturation current is found to be in reasonable agreement with experimental results.

Synthesis and characterisation of methylammonium lead tri-bromide perovskites thin films by sequential physical vapor deposition

Methylammonium lead tri-bromide (MAPbBr3) thin films were grown by sequential physical vapor deposition of lead(II)bromide and methylammonium bromide (MABr). X-ray diffractograms confirmed the cubic MAPbBr3 structure with Pm3‾m space group. UV–Vis spectra revealed a red shift in absorption onset from 540 to 550 nm as the thickness of MABr increased with an optimum band gap of 2.28 eV, obtain from a Tauc Plot. Field emission scanning electron micrographs showed large pin-hole-free and densely packed grains with average grain size that increased from 217 to 302 nm with the thickness of MABr. Analysis of dark current-voltage characteristics of Au/MAPbBr3/FTO devices revealed ohmic behaviour at low voltages and trap-limited space charge limited current at high voltages. The carrier mobility increased from 1.89 × 10−2 to 1.08 × 10−1 cm2 V−1 s−1 while trap density decreased from 1.89 × 1016 to 1.40 × 1016 cm−3 as the thickness of MABr increased.

Electronic properties and photovoltaic performance of VONc-ZnO hybrid junction solar cells

In this work, we investigated the effect of the additional layer of zinc oxide (ZnO) nanoparticles to the active layer of Vanadyl-2,3-naphthalocyanine (VONc) solar cells. The two diodes were prepared using conventional thermal evaporation technique to produce ITO/VONc and ITO/ZnO/VONc heterojunctions. The change of capacitance-voltage characteristics showed that ITO/ZnO/VONc cell has a built-in potential (0.85 ± 0.01 V) higher than that for ITO/VONc cell (0.74 ± 0.02 V). The dark current-voltage characteristics were recorded for the two diodes in the temperature range of 298–393 K. The rectifier behavior of the two diodes at different temperatures has the same behavior, and ITO/ZnO/VONc cell has a higher rectification ratio. This behavior for two diodes suggests that the dominant forward conduction mechanisms found to be thermionic-emission and space charge limited current mechanisms at low and high applied voltage, respectively. The essential parameters of the diode, such as shunt resistance, series resistance, quality factor, and barrier height, were also calculated. The photovoltaic behavior of two devices has been studied with constant illumination of 80 mW/cm2 at room temperature. The efficiency of the cell is increased ten times by adding ZnO layer.

Response of Waveguide-Integrated Germanium-on-Silicon p-i-n Photodiodes to Neutron Displacement Damage

The effects of 14-MeV neutron displacement damage (DD) on waveguide (WG)-integrated germanium-on-silicon p-i-n photodiodes (PDs) for silicon photonics have been investigated up to the fluences of $7.5\times 10^{12}$ n/cm2 (14 MeV) or $1.4\times 10^{13}\,\,\text{n}_{1\text {-MeV}_{\mathrm{ eq}}}$ /cm2(Si). This article includes the measurements of dark current–voltage characteristics across temperature from 150 to 375 K, measurements of PD junction capacitance, spectral response measurements from 1260 to 1360 nm, and frequency-response measurements. The devices are found to be susceptible to DD-induced carrier removal effects; however, they also continue to operate without meaningful impact to performance for the DD dose levels examined. Since the PD test chips include silicon photonic integrated grating couplers and WGs, which carry the optical signal to the PD, some assessment of the impact of DD on these passive devices can also be inferred. This article does not examine the short-term annealing or transient behavior of the DD, and instead, it has only considered the lasting damage that remains after any initial period of room-temperature annealing.

Investigation of optical properties of QD-InAs/GaAs heterostructures obtained by ion-beam deposition

In this paper, we investigate heterostructures with an array of InAs quantum dots encapsulated by GaAs barrier layers obtained by ion-beam epitaxy. The thickness of the layers was less than 30 nm. It is shown that this technique allows to obtain quantum dots with lateral dimensions up to 50 nm with a height of 10 nm. The density of the obtained array of quantum dots was 109 cm−2. The studies performed using photoluminescence methods revealed the peaks of the main transitions for quantum dots at 1.1 eV (1150 nm) for samples with GaAs barrier, which corresponds to the near-infrared. The width of the main peak of the samples was about 0.2-0.25 eV, which is associated with the dispersion of quantum dots sizes. Dark current-voltage characteristics of the structures proved that the value of dark current density at 90 K is about 10−6 A/cm2. The asymmetry of the dark current curves at positive and negative shifts is determined. The samples also showed that an increase of temperature leads to degradation of characteristics. When the operating temperature rises to 300 K, the density of the dark current changes in the range from 0.1 to 0.01 A/cm2.

Dark current–voltage characteristics of vacuum deposited multilayer amorphous selenium-alloy detectors and the effect of x-ray irradiation

Doped and stabilized amorphous selenium (a-Se) alloys in a multilayer form are currently used as a photoconductor in direct conversion flat panel x-ray imagers in mammography and tomosynthesis. While much progress has been made on the physics of such detectors, there are still unresolved questions on such issues as the principles of operation of the so-called p-i-n detector structure in extinguishing the dark current. The present paper examines dark current transients after the application of a voltage in seven types of a-Se alloy based devices: i-layer, i-n, n-i, i-p, p-i, p-i-n, and n-i-p structures. The substrate was ITO coated glass, the top (radiation receiving) electrode was chromium, and the films were fabricated by vacuum deposition. The nominal device thickness was 200 μm, similar to commercial mammographic detectors. It is shown that n-i, i-n, n-i-p, and p-i-n devices have dark currents less than 1 pA mm−2 at an applied field of 10 V/μm. The dark current in the p-i-n device is the lowest at approximately 0.01 pA mm−2 at an applied field of 10 V μm−1. Experiments have been carried out by subjecting the detector to a staircase voltage-time profile during the voltage application (turn-on) and a staircase voltage ramp-down during the turning-off of the bias voltage. Step-voltage ramp-up and step-voltage ramp-down current transient are typical of expected dark current transient behavior in a semiconductor with traps in which carriers are captured and released from various trap centers in the bandgap. The dark current transients are qualitatively similar to those expected from a capacitor in parallel with a large resistor and both in series with a much smaller resistor. Current transients during charging and discharging experiments were integrated to find how much of the injected charge is released during discharge experiments. It is shown that the majority of this trapped charge is stored in the n-type and p-type blocking layers, near the contacts. The trapped carriers in the i-layer represent a very small portion. The evolution of the dark current upon single and repeated x-ray exposure has also been examined in n-i and p-i-n devices. Right after the cessation of irradiation, there is an excess or residual dark current whose magnitude is roughly ∼20 times higher in p-i-n and about ∼2 times higher in the case of n-i under an exposure of 3.36 R and a mean photon energy of 34.2 keV. The absorbed dose is 1.53 Gy. The excess dark current has a fast decay component with a time constant ∼10 s and a slow component with a time constant ∼100 s. It is shown that the decay in the irradiation induced excess dark current is very similar to the initial dark current. There is no permanent change in the dark current, and within a few hundred seconds, the dark current reaches the same level as that in the unexposed detector. The experimental results in this work highlight the distinct advantages of p-i-n and n-i-type a-Se multilayer structures in x-ray detection applications.

Optoelectronic Properties of Hydrogenated Amorphous Substoichiometric Silicon Carbide with Low Carbon Content Deposited on Semi‐Transparent Boron‐Doped Diamond

Hydrogenated amorphous substoichiometric silicon carbide (a‐Si1−xCx:H, x < 0,1) thin films and diodes with low carbon content are prepared from a mixture of H2, SiH4, and CH4 by plasma‐enhanced chemical vapor deposition at a relatively high temperature of 400 °C on semi‐transparent boron‐doped nanocrystalline diamond (B‐NCD) electrodes with an underlying Ti grid. Vibration spectra indicate that CH4 prevents Si crystallization at elevated deposition temperatures and confirm an increasing carbon content up to x = 0.1 for samples grown with SiH4/CH4 flows up to 1:3. Dark current–voltage characteristics of B‐NCD/a‐Si1−xCx:H diodes show a rectifying ratio of about four orders at ±1 V. However, under white light illumination, an energy conversion efficiency of 4% is limited by a high serial resistivity of the B‐NCD electrode and S‐shaped photocurrent near the open‐circuit voltage.

Automated system for determining parameters of a solar cell model

An automated system is developed, which allows measuring the light and dark current-voltage characteristics (CVC) of a solar cell (SC). It consists of a STM32 based control unit, a digital-to-analog converter, a voltage-current transformation circuit, a current and voltage sensor and a laptop with a LabVIEW graphical programming environment. The digital voltage value from STM32 is transmitted to the AD5624R DAC via the SPI interface. AD5624R is a 12 bit DAC with built-in reference voltage 2,5 V. In this configuration, the accuracy of the voltage control is 1,2 mV and the accuracy of the current setting is 40 μA. INA226 sensor was used to measure the voltage on the sample and the current flowing through it. The current is measured as the voltage drop across the shunt resistor, which is connected to the corresponding sensor inputs, using the built-in 16-bit ADC. A shunt resistor with a resistance of 0.1 Ohm is used, which allows measuring the current up to 800 mA with a resolution of 25 μA. The readings of the sensor were calibrated using the precision voltmeter B7-46. Measurement and data processing is carried out with a virtual device created in the LabVIEW environment. As a microcontroller, the SM32F401RE Nucleo development board was used. This unit provides control of the DAC, the current sensor INA226, connection of the circuit for measuring light or dark CVC, as well as recording and storing the data received from sensors in the computer. By measuring the light and dark current-voltage characteristics it is possible to obtain an approximate solar cell model, the parameters of which depend on the illumination and temperature. Solar cell light CVC is used to determine the photocurrent. By its value at a certain temperature and luminosity, it’s possible to define the photocurrent at different temperatures and luminosity levels. The saturation current and the parameter α of the solar cell are calculated using the dark CVC. With the obtained data, a model of a solar cell is defined, which can be further used to determine the maximum power point at different levels of illumination and temperature. With information about the maximum power point, it is possible to calculate the optimal resistance value for the solar cell and to provide the corresponding control signal to the voltage converter. An approach for calculating the parameters of the SC model from its CVC is described and used to determine the MPP (maximum power point) of the solar cell module. The difference with the experimental data was 6.6%. In further research, the study of methods for calculating the parameters of the SC, which could provide a more accurate model, will be continued.

PEDOT:PSS/GaAs1−xBix organic–inorganic solar cells

We demonstrated that poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/GaAs1–xBix organic–inorganic solar cells, which were fabricated by spin-coating PEDOT:PSS onto GaAs1–xBix thin films grown via molecular beam epitaxy, can capture near-infrared light. The dark current–voltage characteristics of each fabricated solar-cell exhibited diode-like rectification with an ideality factor between 1.5 and 2.0. The open-circuit voltages of the solar cells decreased with increasing the Bi composition in the GaAs1–xBix thin films. The spectral response revealed that the absorption edges of the solar cells were red-shifted as the Bi composition increased, reaching ∼1 eV in the near-infrared region.

Fabrication of grating coupled GaAs/AlGaAs quantum well infrared photodetector on an Si substrate

The grating coupled GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are integrated onto Si substrates using metal wafer bonding and epitaxial lift-off process. The 1 μm depth of hexagonal hole structure of grating was formed. The energy-dispersive x-ray spectroscopy results confirmed that the grating coupled QWIP is successfully mounted on an Si substrate. By evaluating the Raman spectra, PL, and surface roughness of bonded QWIP samples, the authors found that the grating does not induce any change in the optical or structural characteristics of actual QWIP layers. The dark current–voltage characteristics show a nearly identical dark current level between grating coupled QWIP and nongrating QWIP. The photocurrent spectrum shows that the peak photocurrent intensity of grating coupled QWIP is about 16 times higher than that of nongrating QWIP. This indicates that the grating effectively contributes to an increase in the light absorption of QWIP, showing large room for improvement of QWIP performance by further optimization of a grating structure.

Solar Cells Based on Cu(In, Ga)Se2 Thin-Film Layers

This paper presents the results of experimental studies of spectral, dark current–voltage and light load characteristics of the selenide–copper–gallium–indium (Cu(In, Ga)Se2) solar cell. Тhe main fundamental parameters of the photoactive semiconductor layer Cu(In, Ga)Se2, such as the band gap, the resistivity of the layer, the equilibrium majority–carrier concentration, the lifetime and the product μnτn of nonequilibrium minority carriers from the spectral, photoelectric and dark current–voltage characteristics are determined. Based on an analysis of the light-load current–voltage characteristics at various solar radiation powers (50–1000 W/m2), the main parameters of the p–n junction were determined, as well as the nonideality factor and the magnitude of the reverse diode saturation current; a photogeneration mechanism was established in the studied solar radiation range, which had the character of a diffusion mechanism, where carrier recombination in the photoactive layer did not have a significant effect. We found that in conditions of real solar lighting (Рrad = 50–1000 W/m2), the output parameters of the solar cell – short-circuit current, open circuit voltage, the maximum output power increases with Рrad. The fill factor (FF) of the light-current–voltage characteristics has a maximum at Рrad ≈ 200 W/m2, and an efficiency has a maximum value at Рrad ≈ 600 W/m2. The observed dependences of FF and efficiency are explained by the dependence of the series (Rser) and shunt (Rsh) resistance of a solar cell on Prad. To maintain the efficiency of a solar cell based on thin-film layers Cu(In, Ga)Se2, equally high in conditions of increased radiation, as well as in conditions of low solar radiation, it is necessary that Rser decreases and Rsh does not change with Prad.

Hybrid p-Au@PbI2/n-Si heterojunction photodetector prepared by pulsed laser ablation in liquid

In this study, a hybrid Au@PbI2/Si heterojunction photodetector was prepared by laser ablation in liquid technique at different number laser pulses. The electrical properties of Au@PbI2/Si heterojunction were measured as a function of the number of laser pulses. The dark current-voltage characteristics showed that fabricated heterojunctions exhibited rectification properties and the largest value of rectification factor at 5 V bias was ˜35 for heterojunction prepared at 100 laser pulses. Hall effect measurements reveal that all synthesized Au@PbI2 have p-type conduction and its Hall mobility decreases as the number of laser pulses increase. The ideality factor and the turn-on voltage of heterojunctions were determined as a function of the number of laser pulses. Capacitance-voltage characteristics of the heterojunctions confirm that all the heterojunctions are abrupt and the value of built-in-potential was ranged from 0.3 to 0.8 V depending on the number of laser pulses. The effect of the number of laser pulses on the figures of merit of the hybrid Au@PbI2/Si heterojunction photodetector namely; responsivity, external quantum efficiency, detectivity and minority carrier lifetime was studied. The responsivity of the photodetector shows a presence of two response peaks located at visible and near-infrared regions which are related to the absorption edge of Au@PbI2 nanoparticles and bulk silicon. The energy band diagram of hybrid Au@PbI2 nanocomposite under illumination condition prepared at 100 laser pulses was constructed.

Photoelectric Properties of Composite Si Layers with Ag Nanoparticles Obtained by Ion Implantation and Laser Annealing

Structural and photoelectric properties of composite Ag:Si layers formed in the near-surface area of a single-crystal c-Si substrate by a high-dose implantation of Ag+ ions with subsequent pulsed laser annealing (PLA) have been studied. It has been established that, as a result of ion implantation, a maximal concentration of Ag impurity (NAg ~ 4 × 1022 at/cm3) is concentrated near the surface and drops to a level of ~1019 at/cm3 at a depth of ~60 nm. Meanwhile, Ag nanoparticles and silver oxide (Ag2O) inclusions are contained in a formed thin layer of amorphized Si (a-Si). Melting of a near-surface area and diffusion redistribution of the implanted impurity have been achieved in conditions of PLA, which increases Ag concentration near the surface and at a depth of 60 nm. Dark current–voltage characteristics of a junction between a Ag:Si layer and a p-Si substrate showed formation of a diode structure as a result of PLA. Photoconductivity measurements on formed samples demonstrate the presence of a photoresponse (photo-EMF) in the range of wavelengths of 500–1200 nm, which intensity increased for samples that are subjected to PLA when increasing energy density in the pulse. The obtained results demonstrate the potential use of composite Ag:Si layers and the method of their formation in the technology of photodetectors.