Temperature Dependence Of Current Density Research Articles

Fabrication of n-Si/n-Ga2O3 heterojunctions by surface-activated bonding and their electrical properties

We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated bonding. The energy barrier heights (qϕb) at the Si/Ga2O3 interface for different reverse voltages (Vrev) were derived from temperature-dependent current density–voltage (J–V–T) characteristics. With shifting Vrev to the negative direction, qϕb gradually decreased and reached a constant value due to negatively charged interface states. The conduction band offset at the heterointerface was estimated to be 0.18 eV from the Vrev dependence of qϕb. The qϕb calculated from a capacitance of the heterojunction at thermal equilibrium was larger than those derived from the J–V–T characteristics, attributing to spatially inhomogeneous qϕb caused by the non-uniform distribution of the charged interface states. The density of shallow interface states was also extracted from the reverse J–V–T characteristics, which was estimated to be about 6 × 1012 cm−2 eV−1.

Effect of Na-doped Mo layer as a controllable sodium reservoir and diffusion barrier for flexible Cu(In,Ga)Se2 solar cells

To overcome the absence of sodium (Na) and to prevent undesired impurity diffusion from stainless steel (STS) substrates, a bi-layer molybdenum (Mo) structure consisting of a 1-μm-thick Mo and Na-doped Mo (Mo:Na) with variable thickness was utilized as the back contact layer in copper–indium–gallium–selenide (CIGS) solar cells. Na ions were supplied to the CIGS absorber layer from the Mo:Na layer, and the amount of Na ions was controlled by changing the thickness of Mo:Na layer. The Na doping to the CIGS layer is verified by the X-ray photoelectron spectroscopy (XPS) and the dynamic secondary ion mass spectroscopy (D-SIMS). The recombination process in the bulk CIGS and the CdS/CIGS junction has been analyzed by temperature dependent current density–voltage (J–V–T) measurement. The D-SIMS showed that the amount of Na ions in the CIGS layer grown on Mo/Mo:Na/STS is comparable with that in the conventional CIGS film deposited on Mo coated soda-lime glass. Moreover, the bi-layer Mo also acts as a diffusion barrier layer against the Fe atoms, which deteriorate the quality of CIGS. The higher content of Na ions and the lower content of Fe ions in the CIGS layer lowered the recombination activation energy, series resistance and ideality factor, leading to improved junction quality. As a result, the short circuit current density and power conversion efficiency of the CIGS on the Mo and 600-nm-thick Mo:Na were improved by 8.1 and 69.4%, respectively, compared with those of CIGS solar cells fabricated without a Mo:Na layer.

Phase evolution during annealing of low-temperature co-evaporated precursors for CZTSe solar cell absorbers

Systematic investigations into the phase evolution during reactive annealing of copper–zinc–tin–selenide (CZTSe) precursors for the fabrication of kesterite solar cell absorber layers have been paramount in understanding and suppressing the formation of secondary phases that deteriorate device performance. In this study, the phase evolution during annealing of low-temperature co-evaporated CZTSe precursors is investigated. A detailed analysis of films selenized at different temperatures is used to reveal the possible reaction pathway of CZTSe formation. Utilizing a combination of x-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy, it is shown that CZTSe formation starts by Cu out-diffusion to the surface and Cu–Se phase formation at a temperature of 350 °C. An intimate mixing of binaries and ternaries during low-temperature selenization is observed. On the contrary, only binaries are observed at high-temperature selenization. This suggests that the CZTSe formation pathway involves reaction schemes where (i) a competition between binary and ternary phases dominates at low-temperature and (ii) binary reactions dominate the process at high temperatures. However, the number of binary phases decreases with increasing selenization temperature until they become undetectable by XRD and Raman spectroscopy at a temperature of 540 °C (selenization time 10 min). Utilizing the presented selenization conditions, prototype solar cells with an efficiency of up to 7.5%, an open-circuit voltage of 407 mV, and a fill factor of 59%, could be demonstrated. The temperature-dependent current density–voltage characteristics indicate that the performance of the prototype devices is limited by bulk Schottky–Read–Hall recombination.

Anomalous diameter dependent electrical transport in individual CuO nanowire

Cupric oxide (CuO) nanostructure arrays have been extensively investigated for solar energy harvesting, electrochemical energy storage, chemical sensing, field-effect transistors, etc. Although most of these applications depend on the collective behavior of an array of such structures, analysis of electrical transport in a single nanostructure, which are the building blocks, is essential for understanding both the fundamental aspects and device performance. Here we report the electrical conduction mechanism in thermally grown single CuO nanowire (NW), which reveals that the current density has an anomalous dependence on the diameter of the NWs—decreasing with an increase in diameter. An analysis of the electrical behavior at room temperature shows that the current density in CuO NWs has different slopes in different regions of the applied bias indicating distinct types of charge transport, which are characterized as near Ohmic (lower voltage), trap controlled, and space charge limited conduction (higher applied voltage). Further, the trap density and activation energy are calculated from the temperature-dependent current density data, which shows higher values (9.38 × 1015cm−3, 79.4 meV) in thicker NWs compared to that in the thinner ones (3.96 × 1015 cm−3, 63.9 meV). Investigation of the NWs with Raman and photoluminescence spectra establishes the presence of Cu2O phase in thicker NWs, which act as hole traps to hinder the charge transport in p-type CuO and resulting in lower conductivity at higher diameters. This study helps to design and fabricate prototype nanodevices with desired conductivity based on CuO NWs.

Organic heterojunctions of phthalocyanine-reduced graphene oxide above percolation threshold for photovoltaic application

In this article, non-fullerene, cost effective, and highly soluble zinc tetra-tert-butyl phthalocyanine-reduced graphene oxide (ZnTTBPc-RGO) heterojunction have been used as an active layer in organic photovoltaic devices. The RGO content in the active material is varied from 0.25–0.5 volume fractions which are above the percolation threshold of 0.167 volume fraction of RGO. Temperature dependent current density–voltage characteristics show a transition from Ohmic conduction to space charge limited current (SCLC) conduction at higher applied bias voltages (≥1 V). This enables us to extract charge carrier mobility values of the heterojunction at different temperatures using a trap-free SCLC model. A donor–acceptor solution processable photovoltaic device with 0.33 vol.% RGO content of active layer show the highest power conversion efficiency of 1.02%, highest recombination resistance of 48.65 kΩ, and maximum carrier mobility of 4.32 × 10−4 cm2V−1s−1 among the three devices studied. 80 h exposure to ambient conditions see a ∼40% decay of power conversion efficiency. Quantitative analysis on the density of trap states indicates that the best power conversion efficiency and the highest charge carrier mobility correlate well with a minimum of the trap density and a maximum of recombination resistance for these devices.

Degradation of electrical characteristics in low-bandgap polymer solar cells associated with light-induced aging

Abstract As the efficiency of organic solar cells has been consistently improved, interest is being raised to understand degradation mechanisms inside the solar cells. Among many factors that affect solar cell deterioration, light is the most crucial factor in the degradation mechanism. Here we report degradation under the continuous light-induced conditions for PTB7:PC70BM organic solar cells during the first 24 h with maintaining the temperature at 25 °C. It revealed 30% of initial performance drop after 24 h of light-induced aging, mostly through a decrease in JSC and FF. The morphological and electrical characteristics of the active layer and devices were investigated by atomic force microscopy, impedance spectroscopy, and temperature-dependent current density–voltage characteristics to figure out the origin of the photo-induced degradation. As a result, the light-induced traps were found to be a primary cause of loss. Furthermore, this trap formation was observed with an energy of 78 meV leading to trapped charge limited conduction properties and electrical degradation of solar cells.

Space charge limited conduction in pulsed laser deposited BaTiO3/LaNiO3 hetero-junctions

We report the charge transport mechanism in BaTiO3 (BTO) thin film deposited on LaNiO3 (LNO) buffer layer using pulsed laser deposition technique thus, forming the metal – insulator – metal junction (Au/BTO/LNO). The temperature dependent Current density- Voltage (J-V) characteristics were measured, analyzed and compared with the La0.67Ca0.33MnO3 (LCMO) buffered BTO film (Au/BTO/LCMO). Although the mechanism was found to be space charge limited conduction (SCLC) as in case of BTO/LCMO but the absence of Ohmic region in Log J-Log V plot indicates higher injection rate. Various parameters such as trap density, activation energy, and ratio of free to trapped carriers (θ) were extracted out from the fitted J-V plot and subsequently their temperature dependence was studied and compared with BTO/LCMO. Higher current density, lower activation energy, lower trap density and higher ratio of free to trapped carriers (θ) were observed in BTO/LNO in contrast to BTO/LCMO. Furthermore, the lower activation energy indicates the presence of shallow trap level.

CZTSe solar cells prepared by co-evaporation of multilayer Cu–Sn/Cu,Zn,Sn,Se/ZnSe/Cu,Zn,Sn,Se stacks

In this work, thin-film kesterite Cu2ZnSnSe4 (CZTSe) solar cells were prepared using a novel precursor configuration employing co-evaporated layer stacks of Mo/Cu–Sn/Cu,Zn,Sn,Se/ZnSe/Cu,Zn,Sn,Se. It is found that this sequential deposition of the constituants leads to the formation of large CZTSe grains on the surface and fine grains at the Mo interface of the absorber, respectively. Prototype CZTSe solar cells using this stacked approach achieve power conversion efficiencies of up to 7.9% at an open-circuit voltage of 430 mV and a fill-factor of 62%. The analysis of temperature-dependent current density–voltage characteristics indicates that bulk Schottky–Read–Hall recombination is the dominant recombination mechanism for the devices fabricated from the proposed stack. In addition, the influence of pre-annealing of each stacked layer on the absorber growth and device performance is examined and discussed.

Temperature and dopant dependence of hole transport in a green light emitting polyspirobifluorene polymer

In the present work, hole transport in a green light emitting polyspirobiflorene polymer has been studied. We have realized better hole injection by using tetra-fluoro-tetracyano-quinodimethane hole injection layer in the polymer. The temperature dependent current density–voltage characteristics of polyspirobifluorene thin films have been investigated. Hole mobility has been calculated by direct current space charge limited conduction method. Further, the charge transport of the interface modified polymer has been considered as a function of doping concentrations. The current density has been found to increase with the doping concentrations. The conductivity of the polyspirobifluorene has also been calculated at room temperature and found to be increased with the dopant concentrations. The present study opens a new way to improve the optoelectronic devices.

Giant Voc Boost of Low‐Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers

Large‐scale industrial fabrication of Cu(In,Ga)Se2 (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium‐free dry vacuum process suitable for in‐line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum‐based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module‐size glass/Mo/CIGS stack. The buffer‐window stack consisting of Zn(O0.75S0.25)/i‐ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open‐circuit voltage (Voc). Temperature‐dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed Voc boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing.

Thermionic emission current in graphene-based electronic devices

A new current equation for graphene/semiconductor or graphene/metal junctions in graphene-based electronic devices is proposed based on the thermionic emission theory. Temperature-dependent current density predicted by the proposed model agrees well with those experimental data reported in the literature. It can also explain the electric field and temperature-dependent effective Schottky barrier height observed in experiments. This is because a high drift velocity in graphene and its dependence on temperature can lead to a change in the effective Schottky barrier height. Due to the nonlinearity between current and temperature, the Richardson’s law will be broken down. The proposed model will benefit to better understand the current transport mechanism in graphene-like materials and graphene-based electronic devices.

Improved charge transport in PANI:PSS by the uniform dispersion of silver nanoparticles

In the present work, polyaniline:poly (4-styrenesulfonic acid) (PANI:PSS) and silver (Ag) nanoparticle–incorporated PANI:PSS (4-styrenesulfonic acid) are synthesized through a chemical route. The synthesized materials are characterized via X-ray diffractogram (XRD), ultraviolet–visible (UV–Vis) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). The thermal stability of the samples is realized via thermogravimetric analysis (TGA) study. To gain detailed information about the electrical charge transport properties of the synthesized samples, the temperature-dependent current density (J) and voltage (V) data of the samples within a vast temperature range (143 K–303 K) are recorded. The deep analysis of those data reveals that the electrical charge transport mechanism within PANI:PSS and its composites with Ag nanoparticles is governed by a trap-dependent space-charge-limited current (SCLC) mechanism. The evaluated field-dependent mobility data at different temperatures show that silver nanoparticle incorporation within the PANI:PSS matrices below the percolation threshold enhances charge carrier mobility by reducing the trap density. From the analysis of the electrical data, various parameters associated with the charge transport mechanism are estimated, including disorder parameters, the density of states, the optimal hopping distance, and localization lengths. All the parameters estimated from the detailed analysis indicate that a small amount of silver nanoparticle dispersion within a PANI:PSS matrix enhances the overall semiconducting nature of PANI:PSS.

Analysis of Recombination Mechanisms in RbF-Treated CIGS Solar Cells

In this paper, we studied the effect of rubidium fluoride (RbF) post-deposition treatment (PDT) on the properties of Cu(In,Ga)Se2 (CIGS) solar cells. Specifically, the recombination mechanisms were analyzed by a series of characterizations including thermal and optical defect spectroscopies, temperature dependent current density–voltage measurements, and time resolved photoluminescence. It was found that the main effect of RbF PDT on the solar cell was an increase of the open circuit-voltage, $V_{{\text{oc}}}$ , by 30 mV due to a decrease of the values of the diode quality factor and reverse saturation current. Recombination mechanisms were identified as being in the CIGS space charge region, likely at the grain boundaries and near the CIGS surface. Breakdown of contributions to the $V_{{\text{oc}}}$ increase showed that part of it is due to an increase of the majority carrier concentration (16 mV) and another to the increase in the minority carrier lifetime (1 mV). The latest is mostly due to a reduction in the EV+0.99 eV deep-level trap density. An additional CIGS surface modification (contributing 13 mV), observed by the secondary ion mass spectrometry, is essential to explain the full change in $V_{{\text{oc}}}$ .

Charge carrier transport in poly(p-phenylene vinylene):methanofullerene photovoltaic blends

The charge carrier transport and energetic disorder in OC1C10-PPV:PCBM blends with different fullerene concentrations are investigated. We demonstrate that the temperature-dependent current density versus voltage (J–V) characteristics of PCBM electron-only device and OC1C10-PPV hole-only device can be accurately described using our recently introduced improved mobility model. Furthermore, the $$J-V$$ characteristics of OC1C10-PPV:PCBM blends that were measured in electron-only and hole-only devices for different wt% of PCBM can also be accurately described by the improved mobility model. Additionally, we find that the width of the Gaussian density of states $$\sigma$$ and zero-field mobility of electron and hole in OC1C10-PPV:PCBM blends are the function of wt% PCBM. For electron-only devices based on OC1C10-PPV:PCBM blends, the electron mobility gradually increases with increase in the PCBM weight ratio, while the width of the Gaussian density of states $$\sigma$$ gradually decreases with increase in the PCBM weight ratio. Surprisingly, the hole mobility and the width of the Gaussian density of states $$\sigma$$ in hole-only devices based on OC1C10-PPV:PCBM blends show an identical behavior. These results provide information about the energetic disorder and a simplified modeling of the charge transport in disordered organic semiconductors.

Electrical and optical characterization of CdTe solar cells with CdS and CdSe buffers—A comparative study

A study is reported comparing the electrical and optical properties of CdTe solar cells, prepared using CdS and CdSe buffer layers, to investigate defects in the bulk and interface, carrier transport, and recombination. Temperature dependent capacitance–voltage measurement and admittance spectroscopy were used to extract carrier concentration, resistivity, charge carrier mobility, and their temperature dependence. The authors identify the presence of two defect signatures corresponding to carrier freeze-out and the formation of a Schottky back-contact barrier. The back-contact barrier height (≈300 meV) extracted from the temperature dependent current density–voltage (JVT) experiment was confirmed by conventional admittance spectroscopy. The activation energies of mobility (resistivity) are 101.2 ± 2.5 meV (92.6 ± 2.3 meV) and 84.7 ± 2.7 meV (77.6 ± 4.5 meV) for CdS and CdSe buffer layers, respectively. Intensity dependent photoluminescence analysis demonstrates that the CdSe/CdTe device exhibits lower radiative efficiency than the CdS/CdTe device. This confirms the presence of higher defects in the CdSe/CdTe device corroborated by temperature dependent VOC analysis. The comparative electrical and optical analysis provides insight into improving the performance of CdTe solar cell device by selenization.

An Investigation of the Role of Radiative and Nonradiative Recombination Processes in InAs/GaAs $_{1-x}$ Sb $_{x}$ Quantum Dot Solar Cells

An InAs/GaAs $_{0.86}$ Sb $_{0.14}$ quantum dot solar cell and a GaAsSb control cell were investigated using temperature-dependent current density–voltage ( J–V ), external quantum efficiency, photoluminescence (PL), and electroluminescence (EL) measurements. Thermally activated defect states associated with the GaAsSb matrix material are found to account for the reduction of the performance of the solar cell. The rapid quenching of the PL and EL intensity, along with the shift (above 150 K) of the dominant recombination process during spontaneous emission (EL), further indicates the prevalence of nonradiative processes at elevated temperatures in these systems. These findings are also supported by a reduction in the open-circuit voltage at elevated temperatures in these devices.

Low work function intermetallic thin film as a back surface field material for hybrid solar cells

In this study, MgNd intermetallic is deposited as a thin film using magnetron sputtering. Ultraviolet photoelectron spectroscopy (UPS) is employed to determine the work function of the MgNd thin film. It is found that the MgNd thin film has a low work function of 3.5 eV, leading to its application in hybrid poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/n-Si solar cells as a back surface field (BSF) material. By introducing a MgNd thin film into the back Si/electrode interface of a hybrid solar cell, obvious improvements in the open circuit voltage (Voc) and fill factor (FF) of the device are observed, leading to an increase by more than 26% in power conversion efficiency (PCE). Temperature-dependent current density–voltage (J-V) and Voc-light intensity measurements demonstrate that the introduction of a MgNd thin layer increases the built-in voltage at the rear interface and decreases the recombination current in the device. These improvements are attributed to the efficacy of the MgNd thin film as a BSF by reducing minority carrier recombination and enhancing majority carrier transport.

Mechanism of Carrier Transport in Hybrid GaN/AlN/Si Solar Cells

The particularities of the carrier transport in p–n-GaN/n-AlN/p–n-Si and n-GaN/n-AlN/p–n-Si structures were investigated through temperature-dependent current density and forward voltage (J–V) measurements, carrier distribution, and transport modeling. Despite the insulating properties of AlN, reasonably high current densities were achieved under forward bias. The experimental relationship between the current density and forward voltage was accurately approximated by an expression accounting for space-charge-limited current in the AlN layer and non-linear characteristics of the p–n junction formed in silicon. We suggest that extended defects throughout the AlN volume are responsible for the conduction, although the limited data available do not allow the accurate identification of the type of these defects.

Improvement of kesterite solar cell performance by solution synthesized MoO3 interfacial layer

In this study, an ultra‐thin MoO3 layer synthesized by a solution‐based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open‐circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.

Dynamic thermoelectric model of a light-emitting structure with a current spreading layer

The non-stationary thermoelectric model of the axisymmetric heterostructure of a light-emitting device is considered taking into account positive feedback mechanisms and the effect of the current-spreading-layer resistance. Taking into account the current localization effect, the nonuniform distribution of the heterojunction current density over the heterostructure area is determined. The non-stationary thermal conductivity equation with temperature-dependent current density flowing into the heterojunction is solved by the numerical–analytical iterative method. Based on the developed model, the current density, temperature, and thermomechanical stress distributions for the heterojunction plane are determined.