Variation Of Open Circuit Voltage Research Articles

Improved electrical properties of silicon quantum dot layers for photovoltaic applications

The low density of silicon quantum dots (Si QDs) in a dielectric matrix is one of the plausible reasons for the low power conversion efficiency (PCE) of Si QD solar cells due to large spacing between QD and substrate. In this study, the electrical properties of the Si QD layers were improved by increasing the density of Si QDs and reducing the barrier width. In the previous studies, Si QD layers were fabricated by alternating deposition of SiO2/SiOx layers and subsequent high temperature (1100°C) annealing, where the diffusion of Si species in the SiOx layer into Si substrate was observed. In order to prevent the diffusion, we propose the substitution of the SiOx layer with an amorphous Si (a-Si) layer. In [SiO2 (2nm)/SiOx (2nm)]×34 multilayer structures, the first few SiOx layers were substituted with a-Si layers. As a result, Si QDs could be well formed in the a-Si layer with high density and survived near the interfaces between the first Si QD layer and Si substrate. The distance of the nearest Si QDs from the Si substrates was also reduced from 7.2nm to 3.6nm, which results in the effective carrier transfer by tunneling between the Si QDs. The PCE of heterojunction B-doped Si QDs/crystalline Si solar cells was enhanced by using the Si QD structures with the improved electrical properties. Since the variations of open-circuit voltage and short-circuit current are not so large, the enhancement of PCE is mainly related with the improvement of fill factor due to the decrease of series resistance.

Li Pre-Doping of Amorphous Si Thin Flakes in Li-Aromatic Complex Solutions

Li-Si alloy negative-electrodes have attracted much attention for use in lithium ion batteries with high energy density because of their high theoretical capacity (ca. 4200 mAh g-1). However, several problems, such as poor cycleability and low Coulombic efficiency particularly in the initial cycles, remain to be solved for practical use. The poor cycleability is attributed to a large volumetric change during charge/discharge cycling. Hence, we designed nano-flaked Si called Si Leaf-Powder® (Si-LP), which can relieve the physical stress caused by expansion and shrinkage of Si during alloying/dealloying reactions with lithium. We have reported that Si-LP with a thickness of 100 nm showed good cycleability.1) Another serious problem for Si negative-electrode is a large irreversible capacity (Qirr ) in the first cycle. Li pre-doping is a useful technique to cancel the large Qirr . We also reported that the large Qirr (1000~1500 mAh g−1) can be drastically reduced by Li pre-doping using a direct contact method with Li foil.2) Unfortunately, however, several issues, such as control of Li-doping level and homogenous Li-doping, remained to be solved for practical use. In this study, we developed a Li pre-doping technique for the amorphous Si-LP electrode using Li-naphthalene and Li-anthracene complex solutions.3) The Si-LP composite electrode consisted of 83.3 wt.% Si-LP, 5.6 wt.% Ketjen black, and 11.1 wt.% carboxymethyl cellulose sodium salt. The Si-LP electrode was pre-doped using a coin-type two-electrode cell and Li foil as a lithium source and a reference electrode. The solutions were tetrahydrofuran (THF) containing naphthalene (NTL) or anthracene (ATC) of which the concentration ranged from 0.1 to 1.0 M. The pre-doping was carried out by leaving the cell for a given time under the open-circuit condition. The variation of open circuit voltage (OCV) with time was monitored. After pre-doping, the cell was disassembled in an Ar-filled glove box and then the Si-LP electrode was rinsed with pure THF to remove the residual solution. Charge and discharge properties of the pre-doped Si-LP electrodes were investigated using a coin-type two-electrode half-cell at a C/6 rate (1 C = 4200 mAh g-1) at 30oC. The working and counter electrodes were the pre-doped Si-LP electrode and a Li foil, respectively. The electrolyte used was 1.0 M LiPF6/EC+DEC (1:1 by vol.). Figure 1 shows the variation of OCV of pre-doping cells with time. The voltages descended to about 60 and 40 mV within 5 h after the start of pre-doping in 0.1 and 0.5 M Li-NTL solution, respectively. On the other hands, the voltage almost unchanged at around 240 mV during the first 10 h for Li-ATC, and then began to decrease gradually. Table 1 shows charge capacity (Qcha ), discharge capacity (Qdis ) and pre-doped capacity (Qpre ) of the Si-LP electrodes in the 1st cycle in 1.0 M LiPF6 / EC + DEC after pre-doping in Li-NTL or -ATC complex solution. If the irreversible capacity is about 1200 mAh g-1, which corresponds to the initial irreversible capacity of pristine Si-LP electrodes in 1.0 M LiPF6/EC+DEC, Qpre is calculated from the following equation; Qpre = Qdis – Qcha + 1200 mAh g-1 As a result, Qpre was 2345 and 2429 mAh g-1 for 1 h pre-doping in 0.5 M Li-NTL and 10 h pre-doping in 0.1 M Li-NTL solution, respectively. These values were close to typical reversible capacity of Si-LP (about 2300 mAh g-1). These results suggest that the Si-LP electrodes were fully pre-doped in these conditions. On the other hands, the Qdis was almost the same as Qcha , for 1 h pre-doping in 0.1 M Li-NTL solution, that is, the Qpre compensated for Qirr . The OCV at the end of pre-doping was 110 mV, as shown in Fig. 1. Very similar results were obtained for 22 h pre-doping in Li-ATC solution; Qdis was close to Qcha and the OCV reached 140 mV at the end of pre-doping. To clarify the meaning of the OCV, a pristine Si-LP electrode was charged at a constant voltage of 120 mV for 3 h, and then discharged. The discharge capacity was evaluated to be 1500 mAh g-1, which was almost equivalent to Qirr of 1200 mAh g-1. These results indicate that Li-doping level can be controlled by OCV. References 1) M. Saito et al., Solid State Ionics, 225, 506 (2012). 2) T. Okubo et al., Solid State Ionics, 262, 39 (2014). 3) T. Tabuchi et al., J. Power Sources, 146, 507(2005). Figure 1

Hexagonal BC3: A Robust Electrode Material for Li, Na, and K Ion Batteries.

We have investigated the stability, maximum intercalation capacity, and voltage profile of alkali metal intercalated hexagonal BC3 (MxBC3), for 0 < x ≤ 2 and M = Li, Na, and K. Our calculations, based on dispersion-corrected density functional theory, show that these intercalation compounds are stable with respect to BC3 and their bulk metal counterparts. Moreover, we found that among all MxBC3 considered, the maximum stable capacity corresponds to an x value of 1.5, 1, and 1.5 for Li, Na, and K, respectively. These values are associated with large gravimetric capacities of 572 mA h/g for Na and 858 mA h/g for Li and K. Importantly, we show that metal intercalated hexagonal BC3 has the advantage of a small open-circuit voltage variation of approximately 0.49, 0.12, and 0.16 V for Li, Na, and K, respectively. Our results suggest that BC3 can become a robust alternative to graphitic electrodes in metal ion batteries, thus encouraging further experimental work.

Understanding device-structure-induced variations in open-circuit voltage for organic photovoltaics.

We investigate the structural influences on the device performance, especially on open-circuit voltage (V(OC)) in squaraine (SQ)/fullerene (C60) bilayer cells. Simply changing the SQ thickness could lead to 40% variation in V(OC) from 0.62 to 0.86 V. The ionization potential (IP) of SQ films and recombination at the anode surface as well as donor/acceptor (D/A) interface sensitively vary with film thicknesses, which account for the shifts in V(OC). The anode recombination can be effectively suppressed by preventing direct contact between C60 and the anode with a buffer layer, delivering an elevated V(OC). Through polarized infrared-multiple-angle incidence resolution spectroscopy measurement, the molecular structure of SQ films is found to gradually evolve from lying-down on indium-tin oxide substrates with noncentrosymmetric orientation at low thicknesses to random structure at high thicknesses. The different molecular orientation may yield different strengths of electronic coupling, which affects the charge-carrier recombination and thus V(OC). Moreover, the oriented SQ films would spontaneously compose aligned dipole moments at the D/A interface because of the strong dipolar effects in SQ molecules identified by density functional theory calculations, whereas no aligned interfacial dipole moment exists in the random structure. The resulting interfacial dipole moments would form an electric field at the D/A interface, leading to variations in the IP and thus impacting V(OC). Our findings demonstrate that V(OC) in organic photovoltaic cells is critically associated with the molecular orientation that affects the charge-carrier recombination and interfacial dipole alignment, which should be seriously taken into consideration for the design of organic molecules and optimization of the cell efficiency.

Trap‐Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular‐poly(3‐hexylthiophene):fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor–acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap‐assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Dye-sensitized solar cells using nanomaterial/PEDOT–PSS composite counter electrodes: Effect of the electronic and structural properties of nanomaterials

Nanomaterials with different electronic and structural properties are employed to construct nanomaterial/PEDOT–PSS composite counter electrodes (CEs) for dye-sensitized solar cells (DSCs). It is found that the photovoltaic performance of the DSC is strongly affected by the addition of nanomaterials. The increase in the fill factor with the incorporation of nanomaterial with PEDOT–PSS is due to the reduced charge transfer resistance (RCT) at electrolyte/CE interface and porous diffusion impedance (Zdiff) of redox couple in porous CE, as evidenced by electrochemical impedance spectrum and Tafel polarization measurements. RCT and Zdiff are determined by the structural property of the nanomaterial, and are low in condition of nanomaterials with large specific surface area. The variations in open circuit voltage (VOC) and short circuit current density (JSC) depend on the electronic property of the nanomaterial, i.e., higher VOC and JSC are achieved in the condition of n-type nanomaterial. This work provides a new perspective for the design of nanomaterial/PEDOT–PSS composite CE.

Developing 3-D Model of Intermediate-Temperature SOFC with GDC Electrolyte

SOFCs with GDC electrolytes present challenges for stack modeling due to the variation in conductivity and chemical activity of GDC as a function of effective oxygen partial pressure P O2, which varies significantly down the channel in SOFC anodes with increasing fuel utilization. This paper presents a 3-D model for an intermediate-temperature SOFCs with GDC electrolytes, Ni/GDC anodes, and LSCF/GDC cathodes. The model uses fitted kinetics to capture, internal reforming and water-gas-shift in the anode as well as variations in open circuit voltage and activation overpotentials as a function of flow composition and temperature along the length of the channel. The model is validated against button cell data. The results show that for 600 and 650°C inlets, super-equilibrium concentrations of CH4 remain after 5 cm of channel. For the same fuel stoichiometry and cell voltage, humid H2 provides over 30% more power than reformate with more than 5% CH4.

A study of donor/acceptor interfaces in a blend of P3HT/PCBM solar cell: Effects of annealing and PCBM loading on optical and electrical properties

Optical, structural and electrical properties based on blend of P3HT: PCBM thin films have been investigated for several PCBM loading whilst exposed to air. Experimental evidence indicates that the peak absorption for PCBM with highest loading concentration show greater reduction in intensity upon annealing temperature of 175°C. This provides useful information regarding PCBM segregation associated with annealing process. Comparison of GIXRD patterns indicates an increase in intraplanar d spacing and polymer grain size upon thermal annealing. Variation in open circuit voltage and short circuit current with annealing temperatures, suggests that the latter remains almost constant up to around 100°C and improves gradually with temperature thereafter, whilst the former improves with temperature and the onset of deterioration in current density is linked to percentage PCBM loading. These results are further explained or supported by optical absorption spectroscopy to account for PCBM clustering. A power conversion efficiency of 3.38% was achieved at 175°C for devices fabricated and tested in air.

Theoretical analysis of the characteristics of the solid oxide fuel cells with a bi-layer electrolyte

From the analytical model derived earlier [17], analytical expressions for the relative thickness ratio rs of a bi-layer electrolyte and the maximum power density of a fuel cell are developed. Using these expressions, together with the other relationships from the analytical model, the characteristics of solid oxide fuel cells (SOFCs) with a bi-layer electrolyte are analyzed and theoretical analysis of the effect of the configuration of a bi-layer electrolyte on the SOFC performance is performed. The results show that the effectiveness of the bi-layer electrolyte depends strongly on its configuration. In the analyses, the variations of open circuit voltage and the maximum power density with the thickness ratio at different total electrolyte thicknesses and different operating temperatures are obtained. Furthermore, by taking into considerations of the oxygen partial pressure at the interface between the two layers of electrolytes, an analytical expression for the critical relative thickness ratio, above which, the electrolyte is stable, is obtained.

Performance analysis of solid oxide fuel cell using reformed fuel

Fuelling SOFC with reformed fuel can be beneficial due to it being cheaper compared to pure hydrogen. A biomass fuel can be easily modeled as a reformed fuel, as it can be converted into H2 and CO using gasification or biodegradation, the main composition of product from a reformer. Hence in this study it is assumed that feed to the fuel cell contains only H2 and CO. A closed parametric model is formulated. Performance is analyzed with changes in temperature, pressure and fuel ratio; considering the possible voltage losses, like ohmic, activation, mass transfer and fuel crossover. Performance curves consisting of operating voltage, fuel utilization, efficiency, power density and current density are developed for both pure hydrogen and mixture of CO and H2. Variations of open circuit voltage with temperature, power density with current density, operating voltage with current density and maximum power density with fuel utilization are also evaluated.

A study of the factors influencing the performance of ternary MEH-PPV:porphyrin:PCBM heterojunction devices: A steric approach to controlling charge recombination

A series of three porphyrins varying only in the steric bulk of their peripheral groups have been synthesised and integrated into the active layer of bulk heterojunction solar cells. The porphyrins broaden the spectral response of the device and contribute to the total photocurrent generated. More importantly, the device characteristics change systematically with increasing steric bulk on the peripheral meso phenyl groups of the tetra-phenyl porphyrin. Optical, scanning probe and scanning transmission X-ray microscopy are used to demonstrate that the observed changes do not arise from morphological differences in film structure. Cyclic voltammetry, UV–vis spectroscopy and DFT calculations are used to establish that the porphyrin LUMO, HOMO and bandgap are independent of side group. We conclude that the variations in open-circuit voltage with side group are the result of the porphyrin acting as a bimolecular recombination centre, with an efficiency that is dependent on the side group type. The possibility of designing optimised macromolecules for OPV devices based on an understanding the effect of porphyrin steric bulk upon device performance is discussed.

The Effect of Ion Exchange Membrane on the Electrical Conduction in Metal Fuel Cell

In this study, The cation exchange membrane and the anion exchange membrane affect in electrical conduction of metal fuel cell was investigated. Magnesium material as anode electrode and the NaCl solution dissolved with 5~15wt% as electrolyte were used for the metal fuel cell. It was found that magnesium slag where flows toward the air electrode was suppressed by using ion exchange membrane. The open circuit voltage variation during discharge has very flat pattern by using ion exchange membrane, but the case which is not the exchange membrane, the open circuit voltage increased according to time. When using the anion exchange membrane, the electric current was higher case of the cation exchange membrane, as a result of higher equivalent conductivity in anion Cl-. The cation exchange membrane was observed with the fact that the output power is excellent in compared with anion exchange membrane.

Effect of O2 concentration on performance of solid oxide fuel cells with V2O5 or Cu added (LaSr)(CoFe)O3–(Ce,Gd)O2−x cathode with and without NO

Abstract A solid oxide fuel cell unit is constructed with Ni-(Ce,Gd)O2−x (GDC) as the anode, yttria-stabilized zirconia as the electrolyte, and V2O5 or Cu added (LaSr)(CoFe)O3–GDC as the cathode. The effect of the O2 concentration on the open circuit voltage (OCV) is studied and a mass-transfer limited OCV is observed. The power density with Cu addition can be much higher than that with V2O5 addition but the effect of the O2 concentration with Cu addition is larger than that with V2O5 addition. Without the presence of NO, both the power density and the OCV decrease with decreasing O2 concentration. The OCV variation can be substantial with the variation of the flow rate, the O2 concentration and the NO concentration. The presence of CO2 can increase the OCV while that of NO can decrease the OCV; however, a synergistic effect can occur on the OCV when NO is present at a very low O2 concentration which results in a sudden drop of the OCV.

Steady and Unsteady Modeling of Single PEMFC with Detailed Thermoelectrochemical Model

A steady and unsteady water and thermal model was developed to consider the effects of local pressure on the cell performance, pressure drop, open-circuit voltage variation with stack temperature, and water-vapor effects on membrane conductivity. These considerations made the model physically more reasonable as well as more suitable for various operating conditions. Additionally, this model combined the along-flow-channel model and catalyst layer model, which represent a significant improvement to proton exchange membrane fuel cell (PEMFC) modeling. The model could predict the distributions of a series of important parameters along the flow channel and in the catalyst layer. Furthermore, the transient performance of the fuel cell can be simulated with this model. The modeling results agreed reasonably with the available experimental results from the literature. The results show that the humidification of both anode and cathode is very important for the performance of PEMFC which could also be improved by increasing the flow inlet temperatures within a reasonable range. Pressure loss is one of the important parameters that affect total system efficiency and optimization. This model could be used as part of a PEMFC stack or entire system modeling.

Along-channel mathematical modelling for proton exchange membrane fuel cells

Proper water and thermal management is essential for obtaining high performance of proton exchange membrane fuel cells (PEMFCs). A steady, two-dimensional water and thermal management model was developed, aiming at considering pressure effects (i.e. the effects of local pressure on the cell performance), pressure drop, open circuit voltage variation with stack temperature, water vapour effects on membrane conductivity, which made the model physically more reasonable and more suitable for various operating conditions. The model could predict the distributions of a series of important parameters along the flow channel, and thus the effects of various operating and design parameters on the fuel cell performance could be investigated easily by numerical trial-and-error method. The modelling results compared well with the available experimental results from the literatures. The results also showed that the humidification of both anode and cathode is crucial for the performance of PEMFCs. The model could be a very useful engineering tool for the optimization of PEMFCs. Copyright © 2005 John Wiley & Sons, Ltd.

Modeling of the impedance response of porous metal hydride electrodes

Porous metal hydride electrodes of the alloy MmNi 3.5–3.7Co 0.7–0.8 Mn 0.3–0.4Al 0.3–0.4 have been characterized by means of impedance spectroscopy. A mathematical model for the impedance response, including effects of diffusion of hydrogen, surface kinetics, conductivity in the metal phase and the solution phase, as well as a continuous, lognormal, particle size distribution, was implemented and fitted to the experimental results by application of a least square fitting routine. The model is based on physical parameters, thus avoiding problems related to the conventional interpretation of impedance spectra in terms of equivalent circuits. Very good agreement between experimental results and model results was obtained for a wide range of frequencies, indicating that physical parameters to a great extent can be determined under realistic operating conditions. The latter was confirmed by independent measurements of the variation in open circuit voltage with respect to the state of charge of the electrode. The model provides an improved methodology for the determination of diffusion coefficients based on electrochemical impedance data. Furthermore, the model can be applied for parametric studies of metal hydride electrodes.

Influence of buffer layers on the performance of polymer solar cells

The influence of different anode buffer layers on the performance of solar cells based on blends of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (DiO-PFDTBT), acting as electron donor, and [6,6]-phenyl-C61-butyric acid methylester (PCBM), acting as electron acceptor was studied. The buffer layers were constructed from different forms of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT-PSS). Variations in open-circuit voltage, short-circuit current, and fill factor were observed. These were attributed to the slight rearrangement of the insulating layer PSS on top of PEDOT, which altered the charge injection at the interface between anode and photoactive layer.

Open-Circuit Voltage Study of Graphite-Coated Copper Foil Electrodes in Lithium-Ion Battery Electrolytes

The open-circuit voltage (OCV) of graphite-coated copper foil electrodes in Li-ion battery electrolytes was found to vary over time. A detailed study showed that the OCV first rapidly decreased until reaching a minimum, and then gradually increased until reaching a steady state. These results were compared with OCV studies of graphite-coated aluminum foil and copper foil without graphite coating. The influence of hydrofluoric acid and thermal treatment of the graphite coating was also studied. Combined with copper dissolution studies using atomic absorption spectroscopy, it was found that the interaction of the graphite coating with electrolyte solution was the main causative factor that resulted in the OCV variation. © 2002 The Electrochemical Society. All rights reserved.

Electron Affinity Difference in CdS/Si Solar Cells

The electron affinity difference ΔEc=χ1-χ2, in CdS/Si solar cells fabricated by four different processes is measured from observations of the variations of open circuit voltage with temperature. For CdS/Si cells, the values of ΔEc lie between 0.43 and 0.48 eV and are found to be independent of the process of cell fabrication. The use of CdZnS in place of CdS decreases the value of ΔEc to 0.3 eV. The method used for the measurement of ΔEc is very simple. The values of the saturation current I0 for different types of cells are estimated from the slopes of qVoc versus kT characteristics and compared with those obtained from ln I versus V curves. The values of I0 found for each cell by the two methods are in good agreement.

Comparison of the cubic and layered polymorphs of titanium disulphide

Lithium intercalation into the three hosts, layered stoichiometric TiS 2, layered non-stoichiometric Ti 1.09S 2 and cubic TiS 2C are compared. It is shown that the variation of open circuit voltage with lithium content for TiS 2C may be described by an equation first derived for concentrated electrolyte solutions. Data on the diffusion of lithium in each host is extracted from ac measurements. The composition dependence of the pre-exponential factor and activation energy for the self diffusion of lithium ions is presented. Broad minima are noted in each of the plots and discussed in terms of interactions between the guest charges. It is shown that the data are consistent with significant changes in the concentration of mobile lithium ions rather than changes in their mobility alone.