Front Electrode Research Articles

Investigation on Low Firing Copper for Front Electrode of Si-Based Solar Cell Applications.

Solar cell is one of the most popular alternative energies. The aim of this study is to construct an ohmic contact between front electrode and Si-based solar cell by a Newly-invented low-cost paste and low temperature sintering process. The core-shell of CucoreAgshell powders were prepared for making high solid content paste, then screen printing the fine line on laser-opening H-pattern silicon substrate and applying firing process. Because the silver coverage is more than 95% and silver nanoparticles start to melt at 200 °C. The shell of nanoparticles of silver not only is used to prevent copper from oxidized, but also connected core Cu particles for enhancing the conductivity of CucoreAgshell. TEM, EDS, SEM were used to examine the microstructure of CucoreAgshell. Fourpoint probe and transmission line model were employed to analyze the sheet resistance and the specific contact resistance. The lowest specific contact resistivity is 0.005 Ωcm2, sheet resistance is 0.0138 Ω/ and the lowest resistivity of front electrode measured is 2.65 × 10-5 Ωcm when CucoreAgshell paste with 94 wt% solid content was fired at 550 °C.

Roll-over behavior in current-voltage curve introduced by an energy barrier at the front contact in thin film CdTe solar cell

Roll-over phenomenon in the current–voltage (J–V) curve is often observed in a CdTe thin film solar cell. The roll-over phenomenon, which is occurred near the open-circuit voltage in a light J–V curve, is due to Schottky energy barrier formed at the CdTe/metal interface in a CdTe solar cell back contact. In this study we report a J–V roll-over phenomenon which is induced by an energy barrier at the front contact of a CdTe solar cell. Two kinds of oxides, namely, Al2O3 and SnO2, were deposited as high-resistance transparent (HRT) layer between the window layer CdS and the fluorine doped tin oxide (FTO) front electrode in CdTe solar cells. These two oxides present much different electronic band alignment with FTO and CdS. SnO2 formed almost no energy barrier with CdS, this allowed smooth transport for photo-generated electrons from CdTe to CdS and FTO. However, Al2O3 formed a high energy barrier with CdS. The rather high energy barrier with a value of 3.43 eV at the CdS/Al2O3 interface induced a J–V roll-over phenomenon in a CdTe thin film solar cell, which dramatically led to a quick decrease for the cell device efficiency. The electron transport at the FTO/Al2O3/CdS interface is governed by tunneling effect. The results presented in this study demonstrate that the band structure at the front electrode plays an important role for the performance of a CdTe thin film solar cell.

Work Function-Driven Hot Electron Extraction in a Bimetallic Plasmonic MIM Device

The Localized Surface Plasmon Resonance (LSPR) can cause hot electrons to be emitted from metal nanostructures. A key issue is how these hot electrons can be efficiently extracted and used for example in energy harvesting or sensing applications. One way is to create a plasmonic metal–insulator–metal device (MIM). In a plasmonic MIM device hot electrons (or holes) are injected from a nanostructured surface electrode across the insulator into the back electrode, and a current flows. Here we present a new, bimetallic plasmonic MIM device where the different work functions of the metal electrodes result in a built-in electrical field which facilitates hot electron extraction. This eliminates the need for an externally applied bias. The nanostructured front electrode, with 52 nm wide and 10 μm long gold nanorods, is separated from the back electrode of aluminum by an insulator layer of Al2O3 with thicknesses varying between 3 and 18 nm. A maximum device responsivity of about 2.2 μA/W is measured when the Al2O...

Multi-scale texturing for a-Si/mc-Si thin-film tandem solar cells

Glass texturing by sandblasting or cream etching methods was introduced for the light trapping purpose for a-Si/mc-Si thin-film tandem solar cells. The nm-scale features of transparent conductive oxide (TCO) front electrode and μm-scale features of textured glass form a multi-scale texturing scheme, which is potentially beneficial to the longer wavelength absorption in mc-Si cell and hence increase the total current of the tandem solar cell. Conformal growth model was applied successfully to analyze thin film growth on textured glass in this work. Power conversion efficiency improvements of ∼1.0% and ∼0.8% have been achieved with solar cells on sandblasting and cream-etching textured glass, respectively.

Advanced light management techniques for two-terminal hybrid tandem solar cells

Multi-junction solar cells are considered for various applications, as they tackle various loss mechanisms for single junction solar cells. These losses include thermalization and non-absorption below the band gap. In this work, a tandem configuration comprising copper-indium-gallium-di-selenide (CIGS) and hydrogenated amorphous silicon (a-Si:H) absorber layers is studied. Two main challenges are addressed in this work. Firstly, the natural roughness of CIGS is unfavorable for monolithically growing a high quality a-Si:H top cell. Some sharp textures in the CIGS induce shunts in the a-Si:H top junction, limiting the electrical performance of such a configuration. To smoothen this interface, the possibility of mechanically polishing the intermediate i-ZnO layer has been explored. The second challenge that is addressed, is the significant current mismatch in these tandem architectures. To enhance absorption in the current-limiting top cell, the ZnO:Al front electrode was textured by means of wet-etching the entire tandem stack. We demonstrated that one can manipulate the morphology of the random textures by varying the growth conditions of the ZnO:Al, leading to better light management in these devices.

4H-SiC Detector in High Photons and Ions Irradiation Regime

A Silicon carbide detector, based on a Schottky diode technology with an interdigitated front electrode design, was employed to monitor photons and low-energy ions. Photons were generated by a He-Cd laser with maximum optical power density of 64 mW/cm2, while a plasma generated by a pulsed laser at low intensity (1010 W/cm2), was used to produce low-energy Al and C ions. The measured detector yield in the case of the photon detection increases linearly with the optical power density, and shows saturation at high power densities, being the saturation value depending on the applied detector bias. A similar behavior is observed for ion detection, where the detector response saturates with increasing ion energy. Carrier generation rate was extracted from the experimental data obtaining almost equal values for photon and ion irradiations, allowing also to extract a maximum generation rate of about $8 \times 10^{19}$ carriers/cm $^{3}\cdot \text {s}$ as upper limit for the linear operation of the detector.

Enhanced efficiency of hybrid amorphous silicon solar cells based on single-walled carbon nanotubes and polymer composite thin film

We report a simple approach to fabricate hybrid solar cells (HSCs) based on a single-walled carbon nanotube (SWCNT) film and thin film hydrogenated amorphous silicon (a-Si:H). Randomly oriented high-quality SWCNTs with conductivity enhanced by means of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate are used as a window layer and a front electrode. A series of HSCs are fabricated in ambient conditions with varying SWCNT film thicknesses. The polymethylmethacrylate layer drop-casted on fabricated HSCs reduces the reflection fourfold and enhances the short-circuit Jsc, open-circuit Voc, and efficiency by nearly 10%. A state-of-the-art J–V performance is shown for SWCNT/a-Si HSC with an open-circuit voltage of 900 mV and an efficiency of 3.4% under simulated one-sun AM 1.5 G direct illumination.

Efficiency enhancement of silicon nanowire solar cells by using UV/Ozone treatments and micro-grid electrodes

Silicon nanowire solar cells were fabricated by metal catalyzed electroless etching (MCEE) followed by thermal chemical vapor deposition (CVD). In this study, we investigated two effects, a UV/ozone treatment and the use of a micro-grid electrodes, to enhance light absorption and reduce the optic losses in the solar cell device. The UV/ozone treatment successfully improved the conversion efficiency. The micro-grid electrodes were then applied in solar cell devices subjected to a back surface field (BSF) treatment and rapid thermal annealing (RTA). These effects improved the conversion efficiency from 9.4% to 10.9%. Moreover, to reduce surface recombination and improve the continuity of front electrodes, we optimized the etching time of the MCEE process, giving a high efficiency of 12.3%.

Fine line screen printed silver electrodes for copper electrodeposition

Purpose In this paper, results of the studies on the copper deposition on screen-printed fine-line front electrode of the solar cell were presented. The silver consumption is an important problem according to growing silver prices. The proposed solution of those problems might be printing of a very thin silver seed layer and subsequent copper plating. This process can be an alternative for typically used screen printing. The purpose of this study was the optimization of the finger path fabrication process to obtain required geometric of fingers for copper deposition. Design/methodology/approach In this paper, double-step metallization process was analyzed. The first step of an electrode formation is screen printing, the second one is copper electrodeposition. Presented investigations were focused on the optimization of the finger path fabrication process to obtain required geometric dimensions and sharp border of fingers. The morphology of the electrodes was characterized by scanning electron microscope before and after copper deposition. The X-ray analysis of elemental arrangement and cross-section profiles of fingers were made using energy-dispersive X-ray fluorescence spectrometer. Findings Presented investigations were focused on the optimization of the finger path fabrication process to obtain required geometric dimensions and sharp border of fingers without any silver particles. The main problem of non-uniform silver paste distribution close to the border of printed finger paths was eliminated by selection of appropriate paste and printing parameters. The obtained coatings were soft, ductile and bright. Originality/value The novelty of the presented approach is modification of the printing parameters, especially for copper deposition. In this paper, the reasons of the widening of electrodes during the copper deposition process is analyzed.

Anti-reductive properties of AZO/FTO bilayered transparent conducting films

ABSTRACTAluminium-doped zinc oxide (AZO)/fluorine-doped tin dioxide (FTO) double-layered transparent conducting films were manufactured on quartz glass substrates by spray pyrolysis. Like top films, AZO has marvellous reduction-resistant properties during a-Si:H deposition. However, as thin films with features of self-textured surfaces, SnO2:F (FTO) provides high light scattering for front electrode in photovoltaic devices. The distribution of elements in surface and interface has been analysed by glow discharge spectrometer. The anti-reductive behaviour in ambient hydrogen plasma condition was studied and the results showed that hydrogen ions are dispersed in the AZO layer but not diffused to the FTO layer, thus avoiding the reduction of SnO2 to SnO or Sn during hydrogen plasma bombardment. Besides, this phenomenon also indicated that the coating of the AZO layer on FTO electrode can increase the resistance of FTO to hydrogen. AZO film protects FTO from reduction, and the resistivity to hydrogen plasma of AZO/FTO bilayer films has been improved.

On the double peak structure of avalanche photodiode response to monoenergetic x-rays at various temperatures and bias voltages

The double response of a large area avalanche photodiode, a planar RMD model S1315, to 6-keV x-rays was investigated as a function of APD biasing voltage and for different operating temperatures. Our data are consistent with the interpretation that the dissimilar APD response is due to x-ray interactions in the different APD-layer structures; interactions in the APD entrance layer just below the front electrode, where the electric field intensity is very low lead to pulses with higher risetime and lower amplitudes, when compared with interactions in the deeper layers where the electric field is more intense. Average pulse risetime values of 14 and 7 ns have been measured in our setup, the slower pulses presenting average amplitudes which are around 20% lower than those of the faster pulses. While the fast risetime does not depend significantly on APD biasing voltage and on temperature, the slow risetime presents a slight decrease with increasing bias voltage and decreasing temperature, a behaviour that is consistent with the increase of the electric field as a result of the increase in the APD biasing voltage. The fraction of the slow pulses reduces from 60% to 40% as the APD biasing increases from about 1.58 to 1.64 kV, indicating a reduction in the thickness, from 25 to 15 μm, in the weak-electric-field entrance layer.

Ultrathin Resonant-Cavity-Enhanced Amorphous Germanium Solar Cells on ZnO Honeycomb Electrodes

Ultrathin resonant-cavity-enhanced solar cells based on less than 30 nm thick hydrogenated amorphous germanium (a-Ge:H) absorbers have the potential to replace standard micrometer-thick microcrystalline silicon absorbers in silicon thin-film solar cell technology, as they yield comparable infrared absorption. Combined with the concept of a folded solar cell on a three-dimensional structured ZnO honeycomb electrode, grown in a cost-effective scalable electrochemical process, the strong absorption of this ultrathin type of cell can be even further enhanced. Our results show an increase of 11% in the short-circuit current density of an a-Ge:H solar cell on a honeycomb electrode compared to a flat reference cell, with no detrimental effects in the electrical properties. The benefit in the optical performance is mainly related to a broadband increase in the quantum efficiency due to an increased absorber volume as well as suppressed interferences inside the front electrode. At the same time, the desired resonances inside the resonant-cavity-enhanced solar cell itself are maintained.

Bifacial color realization for a-Si:H solar cells using transparent multilayered electrodes

Transparent hydrogenated amorphous silicon (a-Si:H) thin-film solar cells, in which the colors of the front and rear faces can be adjusted individually, were developed for implementation in building-integrated photovoltaic (BIPV) windows. The proposed ultrathin transparent multilayered electrodes (TMEs) are highly conductive and transparent, consisting of a bottom layer of gallium-doped zinc oxide (GZO), a thin metal layer of Ag, and an optoelectronic controlling layer (OCL) of GZO. They can be used for both the front and rear electrodes of transparent a-Si:H solar cells, and the resulting solar cells show a 5.0% average power conversion efficiency and 18.3% average transmittance. The various dual colors on the front and rear of the cell can be individually adjusted by changing the thickness of the OCL on each face of the TME, without significantly changing the efficiency. The cell colors were quantitatively investigated using the color coordinates in a chromaticity diagram based on the International Commission on Illumination (CIE; 1931) standard to show that the simulated and observed colors match well under both LED and fluorescent light sources. We also show that the observable color of the BIPV windows is determined by indoor and outdoor light sources, and dynamically varies as a function of illuminance of the light source as the day gradually progresses into night. This study helps to elucidate the color behavior of the BIPV windows and to develop a-Si:H BIPV windows that show various colors with stable electrical power generation.

Gender Differences in Spontaneous and Evoked Activities of the Human Brain

The purpose of this study was to analyze sex-related differences in spontaneous and evoked brain activity. The study included 11 women and 10 men aged between 18 and 27 years; the subjects were adapted to the conditions of electrophysiological experiment. Differences in spontaneous activity were estimated by EEG spectral characteristics in frequency ranges corresponding to θ, α, β1, β2, γ1, and γ2 rhythms at rest with the eyes open and closed. The differences in evoked activity were estimated by the parameters of evoked potentials (EP) recorded upon the presentation of different types of stimuli (images and words) and categories (“fruits” and “vessels”). Behavioral differences were estimated by the efficiency of stimuli recognition. No gender differences were observed in the efficiency of stimuli recognition (number of errors, time of reaction). In the EEG of male subjects at rest, fast frequencies (β and γ) were less marked than in the female subjects, which may indicate a higher level of activation of CNS structures in women. Regardless of stimuli type and category, the amplitude of EP components in men is lower than that in women, which may be caused by lower level of CNS activation. However, the difference in the latency of EP components depended on the type of stimuli used. During the recognition of visual images, the latency of earlier EP components (N1, P2) in men was longer mostly on back and/or right electrodes. The latencies of the later EP components (P3, N3) in male subjects were longer on the back electrodes and shorter on the front electrodes than those in female ones. During the recognition of words, the latency of P1, N1, P2 and N2 components in men were shorter on temporal and temporoparietal electrodes of the left hemisphere. This may indicate that visual image in men are perceived slower but comprehended faster than in women. On the other hand, verbal stimuli in men were perceived faster but comprehended slower than in women.

Transparent and \u2018opaque\u2019 conducting electrodes for ultra-thin highly-efficient near-field thermophotovoltaic cells

Transparent conducting electrodes play a fundamental role in far-field PhotoVoltaic systems, but have never been thoroughly investigated for near-field applications. Here we show, in the context of near-field planar ultra-thin ThermoPhotoVoltaic cells using surface-plasmon-polariton thermal emitters, that the resonant nature of the nanophotonic system significantly alters the design criteria for the necessary conducting front electrode. The traditional ratio of optical-to-DC conductivities is alone not an adequate figure of merit, instead the desired impedance matching between the emitter and absorber modes along with their coupling to the free-carrier resonance of the front electrode are key for optimal device design and performance. Moreover, we demonstrate that conducting electrodes ‘opaque’ to incoming far-field radiation can, in fact, be used in the near field with decent performance by taking advantage of evanescent photon tunneling from the emitter to the absorber. Finally, we identify and compare appropriate tunable-by-doping materials for front electrodes in near-field ThermoPhotoVoltaics, specifically molybdenum-doped indium oxide, dysprosium-doped cadmium oxide, graphene and diffused semiconductors, but also for ‘opaque’ electrodes, tin-doped indium oxide and silver nano-films. Predicted estimated performances include output power density ~10 W/cm2 with >45% efficiency at 2100 °K emitter temperature and 60 Ω electrode square resistance, thus increasing the promise for high-performance practical devices.

Outdoor organic photovoltaic module characteristics: Benchmarking against other PV technologies for performance, calculation of Ross coefficient and outdoor stability monitoring

A comparison of performance parameters for first, second and third generation PV technologies has been conducted. Organic photovoltaic (OPV) modules displayed markedly different outdoor performance characteristics to other PV technologies owing to the positive temperature coefficient, lower thermal mass and response under low light conditions. The linear relationship between irradiance and module temperature rise above ambient is studied, leading to calculation of values for the Ross coefficient for OPV modules. OPVs are shown to possess a lower Ross coefficient than poly-Si, due to the lower absorption of infrared radiation. The effect of wind speed on the Ross coefficient is also investigated, showing the effect that module structure has upon outdoor PV performance, with the OPV module cooling quicker under windy conditions than the poly-Si due to a lower thermal mass. A long term stability study on OPV modules with a silver nanowire-zinc oxide (AgNW-ZnO) composite front electrode has showed two phases of degradation: a short initial burn-in with significant drops in performance; followed by stabilisation and degradation progressing at a much slower rate. During the burn-in period the modules showed diurnal reversible degradation in the short circuit current (ISC), whereas open circuit voltage (VOC) and fill factor (FF) show a steady decline. The reversible degradation is assumed to be related to the desorption of oxygen from the ZnO layer during the day due to UV excitation, leading to an increase in trap formation and a drop in current generation capacity, followed by re-adsorption of the oxygen overnight.

Impact of metal electrode work function of CH3NH3PbI3/p-Si planar heterojunction perovskite solar cells

The use of p-type silicon (p-Si) as hole transport materials (HTM) has been suggested to enhance the stability of perovskite-based solar cell (PSC). We design and simulate the CH3NH3PbI3/p-Si planar heterojunction perovskite solar cell and the power conversion efficiency exceeding 18%. In the solar cell, the metal electrode work function of perovskite solar cells is a decisive factor for built-in voltage (Vbi). The device simulation revealed that the Vbi and VOC increases when rear electrode work function (фM−rear) decreasing from −4.3eV to −4.7eV, the Vbi almost saturate to the plateau and the performance is optimal when фM−rear=−4.8 to −5.2eV. However, the work function of the front electrode (фM−front) also has a significant effect on the performance of p-Si perovskite solar cell. When фM−front decreasing from −3.4 to −4.0eV, a higher performance can be obtained and the Vbi almost saturate to the plateau. The results showed the importance of the фM matching to maintain Vbi, which is useful guideline for the design of the p-Si perovskite solar cell.

On the origin of the changes in the opto-electrical properties of boron-doped zinc oxide films after plasma surface treatment for thin-film silicon solar cell applications

The modification of the steep and sharp valleys on the surface of the boron-doped zinc oxide (BZO) front electrodes by plasma surface treatment is a critical process for avoiding a significant reduction in the electrical performance of thin-film silicon solar cells. In this work, we report the origin of the changes in the electrical and optical properties of the BZO films that occur after this process. On the basis of an analysis of the chemical states, we found an improvement of the carrier concentration along with the treatment time that was mainly due to an increase of the oxygen vacancy. This indicated a deficiency of the oxygen in the BZO films under argon-ion bombardment. The red-shift of the A1 longitudinal optical mode frequency in the Raman spectra that was attributed to the existence of vacancy point defects within the films also strengthened this argument. The significant reduction of the haze ratio as well as the appearance of interference peaks on the transmittance spectra as the treatment time was increased were mainly due to the smoothing of the film surface, which indicated a degradation of the light-scattering capability of the BZO films. We also observed a gain of the visible-region transmittance that was attributed to the decrease of the thickness of the BZO films after the plasma surface treatment, instead of the crystallinity improvement. On the basis of our findings, we have proposed a further design rule of the BZO front electrodes for thin-film silicon solar cell applications.

Effect of Different Front Metal Design on Efficiency Affected by Series Resistance and Short Circuit Current Density in Crystalline Silicon Solar Cell

In commercial solar cells, the pattern of the front electrode is critical to effectively assemble the photo generated current. The power loss in solar cells caused by the front electrode was categorized as four types. First, losses due to the metallic resistance of the electrode. Second, losses due to the contact resistance of the electrode and emitter. Third, losses due to the emitter resistance when current flows through the emitter. Fourth, losses due to the shading effect of the front metal electrode, which has a high reflectance. In this paper, optimizing the number of finger on a 4 × 4 solar cell is demonstrated with known theory. We compared the short circuit current density and fill factor to evaluate the power loss from the front metal contact calculation result. By experiment, the short circuit current density(Jsc), taken in each pattern as 37.61, 37.53, and 37.38 mA/cm2 decreased as the number of fingers increased. The fill factor(FF), measured in each pattern as 0.7745, 0.7782 and 0.7843 increased as number of fingers increased. The results suggested that the efficiency(Eff) was measured in each pattern as 17.51, 17.81, and 17.84 %. Throughout this study, the short-circuit current densities(Jsc) and fill factor(FF) varied according to the number of fingers in the front metal pattern. The effects on the efficiency of the two factors were also investigated.

Numerical simulation and experimental validation of inverted planar perovskite solar cells based on NiOx hole transport layer

Numerical simulation of inverted planar perovskite solar cells based on NiOx hole transport layer was performed with AMPS-1D program. The simulated device parameters were shown to agree well with our experimental work. The simulated results revealed that the device contained typical p-i-n junction configuration. The optimum thickness of the absorber, the effects of the absorber quality, the defect density of interfaces, the effects of VBO and CBO, the interface contact at front and back electrodes were analyzed. Open-circuit voltage mainly depended on the defect density in CH3NH3PbI3 layer, the recombination at HTL/CH3NH3PbI3 and ETL/CH3NH3PbI3 interface, the values of VBO and CBO, while short-circuit current mainly depended on the thickness of CH3NH3PbI3 layer. Fill factor was significantly influenced by the interface contact at front and back electrodes. Remarkably, a power conversion efficiency of 21.8% is obtained under optimised conditions. Real devices with PCE of up to 15% were obtained by initially optimizing the preparation of CH3NH3PbI3 absorber layer. Our work can provide some important guidance for device design and optimization from the considerations of both theory and experiment.