Decrease In Series Resistance Research Articles

Efficiency increase in multijunction monochromatic photovoltaic devices due to luminescent coupling

We present a multijunction detailed-balance model that includes the effects of luminescent coupling, light trapping, and nonradiative recombination, suitable for the treatment of multijunction solar cells and photonic power converters—photovoltaic devices designed to convert the narrow-band light. The model includes both specular and Lambertian reflections using a ray-optic formalism and treats nonradiative processes using an internal radiative efficiency. Using this model, we calculate and optimize the efficiency of multijunction photonic power converters for a range of material qualities and light-trapping schemes. Multijunction devices allow increased voltage with lower current, decreasing series resistance losses. We show that efficiency increases significantly with an increased number of junctions, even without series resistance, when the device has an absorbing substrate. Such an increase does not occur when the device has a back reflector. We explain this effect using a simplified model, which illustrates the origin of the decreased radiative losses in multijunction devices on substrates.

Analysis of performance degradation during steady-state and load-thermal cycles of proton exchange membrane water electrolysis cells

A membrane-electrode assembly based on a 90 μm short-side-chain Aquivion® proton exchange membrane and containing low catalyst loadings, 0.4 mg IrRuOx cm−2 and 0.1 mg Pt cm−2 at anode and cathode, respectively, is investigated for combined thermal and load cycling at high current density (3 A cm−2) in water electrolysis cell. Durability tests under steady-state and load-thermal cycles are compared to evaluate the efficiency losses under specific operating conditions. Ac-impedance spectra and post-operation analyses are carried out to investigate the degradation mechanism. Catalyst degradation occurs more rapidly under cycled operation whereas mass transfer issues are relevant especially under steady-state mode. Membrane thinning appears to be affected by the uptime hours at high current density. The overall cell voltage increase is slightly larger for the cycled operation compared to the steady-state mode. However, this is essentially related to a compensation effect associated to a larger decrease of series resistance during the steady-state durability test. The dynamic electrolysis mode at high current density does not exacerbate significantly the degradation issues of low catalyst loading MEAs compared to a steady-state operation. This confirms the proper dynamic characteristics of the polymer electrolyte membrane electrolyser.

Efficient Cathode Interfacial Materials Based on Triazine/Phosphine Oxide for Conventional and Inverted Organic Solar Cells

Cathode interfacial layers (CIL) have been applied in organic solar cells (OSCs) for the enhancement of photovoltaic characteristics. Most of them are employed in either conventional organic solar cells (COSCs) or inverted organic solar cells (IOSCs) only. Herein, we have designed and synthesized two cathode interfacial materials, namely, 3-(4,6-bis(4-bromophenoxy)-1,3,5-triazin-2-yl)-2,6-difluorophenyl)diphenylphosphine oxide (Br-PO-TAZ) and 4,4′-((6-(3-(diphenylphosphoryl)-2,4-difluorophenyl)-1,3,5-triazine-2,4-diyl)bis(oxy))dibenzonitrile (CN-PO-TAZ), and utilized them as CILs for both COSCs and IOSCs. The incorporation of our new CIL layers significantly enhanced the photovoltaic performance compared to COSCs and IOSCs without the CILs. The CN-PO-TAZ exhibited a power conversion efficiency (PCE) of 8.19% for COSCs and 8.33% for IOSCs, whereas Br-PO-TAZ yielded a PCE of 8.15% for COSCs and 8.23% for IOSCs, respectively. The improved performance was attributed to the multiple favorable factors: significantly reducing leakage current, decreasing series resistance, suppressing recombination, efficient charge transport and collection. Moreover, the CIL layers helped for sustaining device stability because they served as an internal shield against humidity.

Effect of γ-ray irradiation on Schottky and ohmic contacts on AlGaN/GaN hetero-structures

The effect of γ-ray irradiation on the electrical transport characteristics of ohmic contacts and Schottky contacts having different dimensions on AlGaN/GaN hetero-structures has been reported. The contacts have been irradiated with three incremental doses of gamma irradiation up to the order of 104 Gray (Gy). The contacts are electrically characterized using current-voltage (I-V) analysis and the transfer length method (TLM). After each γ-ray dose, both the forward and reverse I-V characteristics show a gradual change associated with the variation in the dimensions of the Schottky contacts. A shift in the turn-on voltage (VF) with a tendency of improvement in the ideality factor (η) and decrease in series resistance(R) of the diode was observed whereas no change in Schottky barrier height (фB) was calculated. The reverse leakage current (IR) was reduced from 27 μA to 10 μA and the contact resistance (RC) value was reduced to 0.35 Ω-mm from 0.7 Ω-mm following the γ-ray irradiation. The Schottky contacts with larger dimensions and lower perimeter-to-area ratios (P/A) showed greater changes in the device parameters (VF, η, IR) due to high induced local heating. X-ray diffraction (XRD) analysis also shows reduction in dislocation density due to partial annealing effect. Finally, capacitance voltage (CV) characterization and Hall measurement shows no change in carrier concentration whereas improvement in the mobility has been observed due to traps/defects redistribution.

Humidity effect on electrical properties of graphene oxide back-to-back Schottky diode

A Schottky diode-based sensor is a promising structure for high sensitive and low power sensor. This paper investigates a device called back-to-back Schottky diode (BBSD) for humidity sensing operation. The BBSD provides simpler device configuration that can be fabricated using less complicated process. The current-voltage characteristic of the fabricated BBSD was measured at different relative humidity. From the obtained characteristics, series resistance, barrier height and ideality factor was analyzed. The device current increased at higher humidity level. The current increase could be associated to the decrease in series resistance, barrier height and ideality factor. When humidity decreased from 11 % to 97%, the barrier height showed reduction of 0.1 eV. The barrier height reduction was explained by considering electric field-induced reduction of graphene oxide. The observed result confirmed the device feasibility as promising simple and low cost humidity sensor.

Influence of NaErF4@NaYF4 nanoparticles doping on performances of Cu2ZnSn(S,Se)4 solar cell

We report the improvement of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells via doping NaErF4@NaYF4 (NEYF) nanoparticles. The NEYF nanoparticles were doped into the CZTSSe layer as an absorber in a solar cell with the conventional structure of SLG/Mo/absorber/CdS/i-ZnO/ITO/Al. It is found that NEYF doping could increase open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF) of the CZTSSe solar cell, which enables the best power conversion efficiency (PCE) increase from 4.03% of CZTSSe solar cell up to 7.10%. For the solar cell with a NEYF-doped CZTSSe (CZTSSe:NEYF) absorber, the improved VOC and JSC are mainly due to the decrease in reverse saturation current density (J0) and the improvement in photocurrent density (JL) brought by NEYF doping, while the enhancement in fill factor should be ascribed to the decreased series resistance (Rs). The mechanisms of change in the J0, JL, Rs and band gap (Eg) from NEYF doping are discussed in detail by comparative study of composition, structure and electrical properties of the CZTSSe:NEYF and CZTSSe films as well as interfacial structures of CZTSSe:NEYF and CZTSSe solar cells in the present work.

Effect of annealing temperature on structural and electrical properties of Al/Nb-doped TiO2 Schottky diodes on Pt–Si substrates

The Nb-doped TiO2 (NTO) thin films were deposited on Pt-coated Si substrates for fabrication of Al/NTO Schottky diodes using RF magnetron sputtering technique. The effects of annealing temperature on the microstructure, surface morphological and electrical properties are systematically investigated. X-ray diffraction pattern shows that the crystallinity of NTO thin films can be improved by adjusting the annealing temperature. The surface morphological analysis shows that the grain size and roughness can be affected by the annealing temperature. According to I–V characteristics, the ideal factor, barrier height and series resistance of Al/NTO Schottky diodes calculated by various methods are consistent well. The ideal factor and series resistance decrease with annealing temperature rising from 200 to 600 °C. When the NTO thin films are annealed at 600 °C, the Al/NTO Schottky diodes have good rectifying behaviors with an ideal factor of 4.97, a Schottky barrier height of 1.04 eV and a relatively low series resistance of 235 Ω. These results indicate that the prepared Al/NTO Schottky diodes have potential application prospects in rectifying devices.

Significantly Enhancing Response Speed of Self-Powered Cu2ZnSn(S,Se)4 Thin Film Photodetectors by Atomic Layer Deposition of Simultaneous Electron Blocking and Electrode Protective Al2O3 Layers.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin film is a promising material for optoelectronic devices. In this work, we fabricate Mo/CZTSSe/CdS/ZnO/ITO (ITO, indium tin oxide) heterojunction photodetectors with favorable self-powered characteristics. The photodetector exhibits exceptional high-frequency photoresponse performance whose -3 dB bandwidth and rise/decay time have reached 1 MHz and 240/340 ns, respectively. For further improvement, ultrathin Al2O3 layer prepared via atomic layer deposition (ALD) process is introduced at the Mo/CZTSSe interface. The influence of ALD-Al2O3 layer thickness and its role on the photoresponse performance are investigated in detail. The interfacial layer proved to serve as a protective layer preventing selenization of Mo electrode, resulting in the reduction of MoSe2 transition layer and the decrease of series resistance of the device. Accordingly, the -3 dB bandwidth is remarkably extended to 3.5 MHz while the rise/decay time is dramatically improved to 60/77 ns with 16 cycles of ALD-Al2O3 layer, which is 4-5 orders of magnitude faster than the other reported CZTSSe photodetectors. Simultaneously, it is revealed that the ALD-Al2O3 interfacial layer acts as an electron blocking layer which leads to the effective suppression of carrier recombination at the rear surface. Consequently, the responsivity and detectivity are enhanced in the entire range while the maximum values are up to 0.39 AW-1 and 2.04 × 1011 Jones with 8 cycles of ALD-Al2O3, respectively. Finally, the CZTSSe photodetector is successfully integrated into a visible light communication system and obtains a satisfying transfer rate of 2 Mbps. These results indicate the satisfying performance of CZTSSe-based thin film photodetectors with great potential applications for communication.

Self-Organized Back Surface Field to Improve the Performance of Cu2ZnSn(S,Se)4 Solar Cells by Applying P-Type MoSe2:Nb to the Back Electrode Interface.

Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells have been encountering a bottleneck period since the champion power conversion efficiency (PCE) of 12.7% was achieved by Kim et al. in 2014. One of the critical factors that impede its further development is the relatively low open-circuit voltage (VOC) caused by serious interface carrier recombination. In this regard, back surface field (BSF) employment is a feasible strategy to address the VOC issue of CZTSSe solar cells to some extent. Here, we demonstrated a self-organized BSF introduced by prompting interfacial MoSe2 layer transition from inherent n-type to desirable p-type with Nb doping (p-MoSe2:Nb). The BSF application can significantly reduce the carrier recombination at the back electrode interface (BEI) and lower down the back contact barrier height. The PCE of the corresponding cell was improved from 4.72 to 7.15% because of the enhancement of VOC and fill factor, primarily stemming from the doubling aspects of increased shunt resistance (RSh), decreased series resistance (RS), and alleviative recombination velocity of the BEI induced by the BSF. Our results suggest that introducing a BSF fulfilled with p-MoSe2:Nb is a facile and promising route to improve the performance of CZTSSe thin-film solar cells.

The effect of Rb doping on CZTSSe solar cells

Alkali metal doping is a promising method to enhance the performance of Cu2ZnSn(Se,S)4 (CZTSSe) solar cells due to its excellent effect on Cu(In,Ga)Se2 (CIGS) solar cells. In this work, rubidium (Rb) was doped in CZTSSe absorbers by sputtering Rb doped Cu2ZnSnS4 (CZTS) target with subsequent H2Se contained annealing. By this method, Na concentration in absorber was not decreased due to Rb doping. On the contrary, Na concentration was enhanced by 2.5 times in the absorber by Rb doping. Rb doping can also suppress the aggregation of Zn at CZTSSe/Mo interface and the formation of ZnSe, which result in the decrease of series resistance of solar cells. Eventually, the conversion efficiency of CZTSSe solar cells can be increased by Rb doping to 8.41%, which was accompanied by the increase of VOC, JSC and FF as well.

Influence of Temperature on the Output Parameters of a Photovoltaic Module Based on Amorphous Hydrogenated Silicon

The light load current-voltage characteristics of a solar photovoltaic module based on amorphous hydrogenated silicon have been studied at different temperatures under conditions of natural solar illumination (Рrad = 870 ± 10 W/m2). It has been found that the temperature dependence of the photocurrent has two slopes due to a change in the generation–recombination mechanism. The increase in the value of the short-circuit current with increasing temperature of the photovoltaic module is explained by a rise in the drift lengths of minority charge carriers due to an increase in the lifetime of minority carriers. In this case, the quasi Fermi level shifts to the conduction band, and the concentration of recombination centers decreases due to recharging of defective levels (D0 → D–). The decrease in the value of the open-circuit voltage with increasing temperature is explained by the exponential increase in the reverse saturation current and decrease in the band gap of the semiconductor. It has been found that the fill factor (FF) of the current–voltage characteristics decreases with increasing temperature, most likely due to a decrease in the shunt resistance (Rsh), which connects parallel to the p–n junction, consists of parasitic resistances, and leads to an increase in leakage currents. The temperature coefficient of the maximum output power has a positive value in the range of 320–332 K, i.e., increases with temperature. It has been revealed that the values of shunt and series resistance decrease with increasing temperature. A large loss of power output (up to 19%) has been observed on the series resistance of the solar photovoltaic module in the temperature range of 320–332 K. With increasing temperature, the loss of generated power on the shunt resistance grows sublinearly. The efficiency of the solar photovoltaic module decreases from 7.95 to 7.65% and has a coefficient of temperature dependence of efficiency, which decreases from $${{K}_{{{\text{Ef}}{{{\text{f}}}_{1}}}}}$$ ≈ –0.046 to $${{K}_{{{\text{Ef}}{{{\text{f}}}_{2}}}}}$$ ≈ –0.029%/K.

Copper–Nickel Alloy Plating to Improve the Contact Resistivity of Metal Grid on Silicon Heterojunction Solar Cells

As a dominant metallization technique of crystalline silicon solar cells, screen printing with silver paste has been generally used in photovoltaic industries. In case of the silicon heterojunction solar cells (SHJ) structure, a metal contact with silver paste has lower electrical conductivity than pure silver due to the other compositions of the paste. For the reason, copper plating is attractive substitute for the silver paste since the plated-copper contacts have high conductivity and easily reduce line width which is beneficial to light absorption. In this experiment, we studied copper–nickel (Cu–Ni) alloy plating to form a seed layer of the copper plating on an indium tin oxide (ITO) layer which is used for the transparent conductive oxide of the SHJ solar cells. As a requirement of suitable seed layer, contact resistivity (ρc) between the seed and the ITO is important to obtain high fill factor by decreasing series resistance of solar cells. Contact resistivity values of the samples with varied nickel contents in the Cu–Ni films were extracted by using transfer length method. Also, the composition ratio of the alloy layer was analyzed by energy dispersive spectrometer. Moreover, X-ray diffraction was used to compare lattice parameter and crystallite size of the film.

Improved Hole Injection in Bulk Heterojunction (BHJ) Hybrid Solar Cells by Applying a Thermally Reduced Graphene Oxide Buffer Layer

In this work, the utilization of graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotube (CNT) thin films as hole transport and electron-blocking layers in polymer/nanocrystal hybrid solar cells is demonstrated. A simple method has been used to modify the anode of hybrid solar cells by depositing these two solution-processable nanocarbon materials between poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and transparent indium tin oxide (ITO) layers. Upon the use of an rGO interlayer, we found a substantial improvement in power conversion efficiency (PCE) from 2.5% to 3.2% due to a decrease in series resistance (Rs). This decrease has been obtained by a careful tuning of the reduction degree of rGO, inducing optimization of the energy band alignment at the solar cell anode. In addition, charge extraction by linearly increasing voltage (CELIV) measurements show an increase in light-induced charge extraction of ca. 50%. Finally, the utilization of rGO as replacement for PEDOT:PSS is also presented. The findings reported in this work demonstrate the excellent potential of rGO as an efficient hole transport material in hybrid solar cells.

CH3NH3I Post-treatment of Organometal Halide Perovskite Crystals for Photovoltaic Performance Enhancement

Immersing organometal halide perovskite crystals in CH3NH3I solution converts PbI2 impurities to CH3NH3PbI3 perovskite, yielding excellent carrier transport. Hence, the power conversion efficiency of perovskite solar cells is improved from 17.7% to 19.3% based on increased light absorption and decreased series resistance.

Fabrication and performance evaluation of ultraviolet photodetector based on organic /inorganic heterojunction

Organic/inorganic ultraviolet photodetector was fabricated using thermal evaporation technique. Organic/inorganic heterojunction based on thermally evaporated copper (II) acetylacetonate thin film of thickness 200 nm deposited on an n-type silicon substrate is introduced. I–V characteristics of the fabricated heterojunction were investigated under UV illumination of intensity 65 mW/cm2. The diode parameters such as ideality factor, n, barrier height, ΦB, and reverse saturation current, Is, were determined using thermionic emission theory. The series resistance of the fabricated diode was determined using modified Nord's method. The estimated values of series resistance and barrier height of the diode were about 0.33 KΩ and 0.72 eV, respectively. The fabricated photodetector exhibited a responsivity and specific detectivity about 9 mA/W and 4.6 × 109 Jones, respectively. The response behavior of the fabricated photodetector was analyzed through ON-OFF switching behavior. The estimated values of rise and fall time of the present architecture under UV illumination were about 199 ms and 154 ms, respectively. Finally, enhancing the photoresponsivity of the fabricated photodetector, post-deposition plasma treatment process was employed. A remarkable modification of the device performance was noticed as a result of plasma treatment. These modifications are representative in a decrease of series resistance and an increase of photoresponsivity and specific detectivity. The process of plasma treatment achieved an increment of external quantum efficiency from 5.53% to 8.34% at −3.5 V under UV illumination.

Integrating a Charge Trapping Layer in Passivated Emitter Rear Contact Cell to Enhance Efficiency

In this letter, we propose a charge trapping technique to enhance passivated emitter rear contact solar cell efficiency without changing the fabrication process. We trap the charge by applying voltage to the rear contact between the SiNx passivation layer and Si substrate with special metal screen printing. Furthermore, different charge voltages were applied for IV curve, fill factor, efficiency, and series resistance comparisons. The IV curve was measured with white light, with results showing that the fill factor and efficiency increase and series resistance decrease with an increase in charge voltage. Finally, a model was proposed to explain the charge trapping effect in the passivation layer.

TiO2 nanofiber photoelectrochemical cells loaded with sub-12 nm AuNPs: Size dependent performance evaluation

Incorporation of gold nanoparticles (AuNPs) into titanium dioxide (TiO2) photoelectrodes has been used traditionally to increase the performance of photoelectrochemical cells (PECs) through their tailored optical properties. In contrast to larger AuNPs, previous studies have suggested that smaller AuNPs are the most catalytic or effective at increasing the photovoltaic (PV) performance of TiO2 photoelectrodes based on PECs. Despite this, AuNPs are often only compared between sizes of 12–300 nm in diameter due to the most common synthesis, the Turkevich method, being best controlled in this region. However, the optimum radius for citrate-capped AuNPs sized between 5 and 12 nm, and their influence on the PV performances has not yet been investigated. In addition to using AuNPs in the photoelectrodes, replacing traditional TiO2 NPs with one-dimensional nanofibers (NFs) is a promising strategy to enhance the PV efficiency of the PECs due their capability to provide a direct pathway for charge transport. Herein, we exploit the advantages of two different nanostructured materials, TiO2 NFs and sub-12 nm AuNPs (5, 8, 10, and 12 nm), and fabricate composite based photoelectrodes to conduct a size dependent performance evaluation. The PECs assembled with 8 nm AuNPs showed ∼20% improvement in the average power conversion efficiency compared to the control PECs without AuNPs. The highest performing PEC achieved a power conversion efficiency of 8%, which to the best of our knowledge, is among the highest reported for scattering layers based on pure anatase TiO2 NFs. On the basis of our comprehensive investigations, we attribute this enhanced device performance using 8 nm AuNPs in the TiO2 NF photoelectrodes to the improved spectral absorption, decreased series resistance, and an increase in electron transport and injection rate leading to an increase in current density and fill factor.

Moderately reduced graphene oxide via UV-ozone treatment as hole transport layer for high efficiency organic solar cells

Abstract This work demonstrates a new hole transport layer (HTL) via UV-ozone assisted moderate reduction of graphene oxide (GO) in organic solar cells (OSCs). The XPS and Raman measurements confirmed the successful reduction/removal of oxygen functional groups after UV-ozone treatment. Remarkably enhanced performance of the OSCs was demonstrated by the 15 min treated device with power conversion efficiency (PCE) of 4.07%, followed by the 10 min device with almost similar PCE of 4.03%, as compared to the device with untreated GO HTL exhibiting a much lower PCE of 2.80%. The significant enhancement in the device performance is attributed to the moderate reduction of oxygen functional groups, leaving behind highly conductive graphene as well as the decreased series resistance, increased shunt resistance and comparatively smooth HTL morphology. Further, it is also ascribed to enhanced short circuit current density, open circuit voltage and fill factor of UV-ozone treated GO which directly contributed to the improved PCE. Moreover, the OSCs with the UV-ozone treated GO HTL showed very good stability over the 250 h of exposure to ambient atmosphere.

High open circuit voltage c-Si/a-Si:H heterojunction solar cells: Influence of hydrogen plasma treatment studied by spectroscopic ellipsometry

One sided c-Si/a-Si:H heterojunction solar cells (Ag/Al/c-Si(n)/a-Si:H(i)/a-Si:H(p)/ITO/Ag) with open circuit voltage (Voc) as high as 711 mV were fabricated on double side polished n- type c-Si wafer (thickness 275–325 µm, resistivity of 5–10 Ω cm) by RF-PECVD technique in multi-chamber system. The cells were fabricated with hydrogen plasma treatment of a-Si:H(i) layer of different thicknesses prior to the deposition of top a-Si:H(p) layer. The thickness, optical band gap and microstructure of different a-Si:H layers in the cells were estimated using Spectroscopic ellipsometry. It was observed that hydrogen plasma treatment of a-Si:H (i) layer not only etched out the significant portion of the layer, but also improved the microstructure of the films and passivated the interface defects without deteriorating the electronic transport properties. The Voc of 711 mV is higher than the values reported in literature so far for single sided solar cells. This improvement in Voc is due to reduction of interface recombination sites, passivation of surface dangling bonds and decrease of series resistance with 2 min H2 plasma treatment on a-Si:H(i) layer before deposition of a-Si:H(p) layer, which also improved the short circuit current density (Jsc) and fill factor (FF). The results suggest hydrogen plasma treatment of a-Si:H(i) layer as an alternative approach to achieve surface passivation and improve open circuit voltage of c-Si/a-Si:H heterojunction cells.

Significantly improved efficiency of organic solar cells incorporating Co3O4 NPs in the active layer

Effect of various concentrations of fabricated cobalt oxide (Co3O4) nanoparticles (NPs) in the active layer of different donors and acceptors based hybrid organic bulk heterojunction-BHJ devices were investigated using inverted architecture. The organic active layer comprising different donors P3HT (poly(3-hexylthiophene-2,5-diyl) and PTB7 (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]][6,6]) and acceptor (phenyl-C-butyric acid methyl ester (PC60BM and PC70BM) materials. The incorporation of NPs in the binary blends of P3HT:PC60BM and PTB7:PC70BM enhanced the power conversion efficiency (PCE) of resulting ternary devices. This increase in PCE is mostly due to decrease in series resistance (Rs) with an optimum amount of Co3O4 in the organic photoactive layer. It was found that the ternary blend has higher absorbance relative to a binary blend of P3HT:PCBM. Addition of NPs in the active materials blend increased film roughness and form interpenetrating network to facilitate charges transportation in the active layer. The higher amount of NPs enhanced agglomerations which caused to break down the BHJ led to decrease the performance of the ternary devices.