Optimal Cell Performance Research Articles

Synthesis and application of TiO2 single-crystal nanorod arrays grown by multicycle hydrothermal for dye-sensitized solar cells

TiO2 is a wide band gap semiconductor with important applications in photovoltaic cells. Vertically aligned TiO2 nanorod arrays (NRs) are grown on the fluorine-doped tin oxide (FTO) substrates by a multicycle hydrothermal synthesis process. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED). It is found that dye-sensitized solar cells (DSSCs) assembled by the as-prepared TiO2 single-crystal NRs exhibit different trends under the condition of different nucleation and growth concentrations. Optimum cell performance is obtained with high nucleation concentration and low growth cycle concentration. The efficiency enhancement is mainly attributed to the improved specific surface area of the nanorod.

Modeling the Mechanical Behavior of P3HT/Fullerene Blends For Photovoltaic Applications

ABSTRACTOrganic solar cells, comprised of P3HT-fullerene blends, have the potential for photovoltaic energy applications. However, there is limited understanding of the mechanical behavior of these devices, and how this behavior can be tailored for optimal organic solar cell performance and device reliability. Therefore, a recently developed computational approach that is based on a constitutive representation of semi-crystalline polymers and fullerenes is used to identify the dominant morphological and microstructural characteristics that would affect the mechanical behavior of the active layer. The predictions indicate that stress and dislocation-density accumulation in interfacial regions and tie molecules play a significant role on the overall behavior.

Effects of metal oxide as an anode interlayer for organic photovoltaics

In this study, polymer:fullerene bulk-heterojunction hybrid solar cells with the structure indium tin oxide (ITO)/nickel oxide (NiO)/poly (3-hexylthiophene) (P3HT):[6, 6]-phenyl C61-butyric(PCBM):titania (TiO2):platinum (Pt) nanoparticles (NPs)/Ca/Al were fabricated. The effects of a p-type NiO thin layer deposited by thermal evaporation between the active layer P3HT:PCBM:TiO2:Pt and ITO on cell performance were examined. The results show that the NiO interfacial layer between the ITO and active layer can increase the efficiency and stability of the prepared hybrid solar cells. The optimum cell performance by ITO/NiO(5nm)/P3HT:PCBM:TiO2 (15wt.%):Pt (0.03wt.%)/Ca/Al (best cell structure) is an open-circuit voltage (Voc)=0.61V, short circuit current density (Jsc)=6.22mA/cm2, fill factor (FF)=54.8%, and η=2.1%.

Efficiency enhancement of flexible dye-sensitized solar cell with sol–gel formed Nb2O5 blocking layer

We investigated the effect of a Nb2O5 blocking layer formed through the sol–gel method introduced to a titanium metal foil electrode in a flexible dye sensitized solar cell. The blocking layer formed directly on the working electrode physically separates the working electrode from the electrolyte, and prevents back transfer of electrons from the electrode to the electrolyte. The gel processing conditions (sol reaction time) and heat treatment temperature used in formation of the Nb2O5 blocking layer have been shown to affect the performance of the dye sensitized solar cell and optimal values of these parameters have been determined. A sol reaction time of 45 min and heat treatment temperature of 550 °C has been observed to result in optimal cell performance (η = 6.185%, Jsc = 13.233 mA/cm2, Voc = 0.672 V, ff = 0.694). Introduction of an Nb2O5 blocking layer enhances solar cell efficiency by 39.7%, which is much greater than the increase of 24.6% observed in a similar cell containing a TiO2 blocking layer under standard illumination conditions. The results obtained via Nb2O5 have been observed to be superior to those obtained via a TiO2 blocking layer.

Effect of cathode microporous layer composition on proton exchange membrane fuel cell performance under different air inlet relative humidity

The effects of cathode microporous layer (MPL) composition including carbon black and polytetrafluoroethylene (PTFE) loadings on the performance of a proton exchange membrane fuel cell (PEMFC) are evaluated under different relative humidity (RH) at air inlet. Both acetylene black and composite carbon black which is made of acetylene black and black pearls 2000 (BP) are used in the experiments. The results show that the optimal cell performance at different air RH depends on the cathode MPL composition. When acetylene black is used, the optimal composition at higher RH is found to be 1.5 mg cm−2 carbon loading and 20 wt% PTFE content, while an increase in PTFE content could improve the cell performance at lower RH states. The cell performance also could be enhanced significantly at lower RH when composite carbon black is employed, while it fails to increase the cell performance at higher RH. The composite carbon black with 30 wt% BP is found to give the optimal cell performance at low RH conditions. The results benefit the understanding for the role of cathode MPL in running PEM fuel cells under insufficient air humidification.

Optimization of dye adsorption time and film thickness for efficient ZnO dye-sensitized solar cells with high at-rest stability

Photoelectrodes for dye-sensitized solar cells were fabricated using commercially available zinc oxide (ZnO) nanoparticles and sensitized with the dye N719. This study systematically investigates the effects of two fabrication factors: the ZnO film thickness and the dye adsorption time. Results show that these two fabrication factors must be optimized simultaneously to obtain efficient ZnO/N719-based cells. Different film thicknesses require different dye adsorption times for optimal cell performance. This is because a prolonged dye adsorption time leads to a significant deterioration in cell performance. This is contrary to what is normally observed for titanium dioxide-based cells. The highest overall power conversion efficiency obtained in this study was 5.61%, which was achieved by 26-μm-thick photoelectrodes sensitized in a dye solution for 2 h. In addition, the best-performing cell demonstrated remarkable at-rest stability despite the use of a liquid electrolyte. Approximately 70% of the initial efficiency remained after more than 1 year of room-temperature storage in the dark. To better understand how dye adsorption time affects electron transport properties, this study also investigated cells based on 26-μm-thick films using electrochemical impedance spectroscopy (EIS). The EIS results show good agreement with the measured device performance parameters.

Effect of crystallization in frits on the contact of AgSi under fast firing

The thermophysical properties of glass in Ag pastes must be controlled to enhance the optimal cell performance. In order to verify the effect of crystallization of glass on the formation of Ag recrystallites on n+ emitter, several glasses that crystallize with increase in the firing temperatures were fabricated. The glass transition temperature, crystallization peak temperature, and melting temperature were determined using a differential thermal analyzer. The crystallization phases of glasses were verified via X-ray diffraction. The microstructures between the Ag electrodes and Si emitter were characterized by using a scanning electron microscope. The experimental results indicate that crystallization of glass significantly influence the viscous flow with increasing temperatures, which in turn affects the interface structures between the Ag electrodes and Si emitter.

Influence of ionomer content on the proton conduction and oxygen transport in the carbon-supported catalyst layers in DMFC

Proton conduction and oxygen transport in the cathodes of direct methanol fuel cells (DMFCs) are affected by the loading and distribution of Nafion ionomer. These effects of ionomer in the catalyst layer with carbon-supported catalysts on the cathode performance were investigated by electrochemical techniques and single cell testing in order to improve the power output of DMFC. It is found that varying the Nafion content has minimal effect on the electrochemical surface area (ESA) and catalyst utilization, but has significant effect on pore structure, proton conduction, and oxygen transport in the catalyst layer. A higher content of Nafion ionomer is needed in DMFC cathodes to achieve the same specific conductivity as in proton exchange membrane fuel cells (PEMFC) due to the higher catalyst loading in DMFCs. Optimal cell performance is achieved with a balance among catalytic activity, proton conductivity, and oxygen transport. These findings could provide guidelines for electrode design, fabrication, and prediction of cell performance.

Thermal modelling of Li-ion polymer battery for electric vehicle drive cycles

Time-dependent, thermal behaviour of a lithium-ion (Li-ion) polymer cell has been modelled for electric vehicle (EV) drive cycles with a view to developing an effective battery thermal management system. The fully coupled, three-dimensional transient electro-thermal model has been implemented based on a finite volume method. To support the numerical study, a high energy density Li-ion polymer pouch cell was tested in a climatic chamber for electric load cycles consisting of various charge and discharge rates, and a good agreement was found between the model predictions and the experimental data. The cell-level thermal behaviour under stressful conditions such as high power draw and high ambient temperature was predicted with the model. A significant temperature increase was observed in the stressful condition, corresponding to a repeated acceleration and deceleration, indicating that an effective battery thermal management system would be required to maintain the optimal cell performance and also to achieve a full battery lifesapn.

All-Solid-State, Semiconductor-Sensitized Nanoporous Solar Cells

Despite the rapid increase in solar cell manufacturing capacity (~50 GW(p) in 2011), maintaining this continued expansion will require resolving some major fabrication issues. Crystalline Si, the most common type of cell, requires a large energy input in the manufacturing process, which results in an energy payback time of years. CdTe/CdS thin film cells, which have captured around 10% of the global market, may not be sustainable for very large-scale use because of limited Te availability. Thus, research in this field is emphasizing cells that are energy efficient and inexpensive and use readily available materials. The extremely thin absorber (ETA) cell, the subject of this Account, is one of these new generation cells. Since the active light absorber in an ETA cell is no more than tens of nanometers thick, the direct recombination of photogenerated electrons and holes in the absorber should not compete as much with charge removal in the form of photocurrent as in thicker absorber materials. As a result, researchers expect that poorer quality semiconductors can be used in an ETA cell, which would expand the choice of semiconductors over those currently in use. We first describe the ETA cell, comparing and contrasting it to the dye-sensitized cell (DSC) from which it developed and describing its potential advantages and disadvantages. We then explain the mechanism(s) of operation of the ETA cell, which remain controversial: different ETA cells most likely operate by different mechanisms, particularly in their photovoltage generation. We then present a general description of how we prepare ETA cells in our laboratory, emphasizing solution methods to form the various layers and solution treatments of these layers to minimize manufacturing costs. This is followed by a more specific discussion of the various layers and treatments used to make and complete a cell with emphasis on solution treatments that are important in optimizing cell performance and explaining the possible modes of action of each of these treatments. Finally, we show how ETA cells have improved over the years, their present efficiencies, our expectations for the future, and the challenges that we foresee to fulfill these expectations.

Effects of modified flow field on optimal parameters estimation and cell performance of a PEM fuel cell with the Taguchi method

The study applies a three-dimensional model simulating the transport phenomenon and electrochemical reactions of full scale serpentine channels to determine the best arrangement of cuboid rows at the axis in the anode and cathode channels. With the best arrangement of the cuboid rows in the channels, the Taguchi methodology is used in the experiment to obtain the optimal operating parameters for three objectives with the minimum pressure drops in anode and cathode channels, and maximum electrical power. The results show that the interactions of flow fields between each cuboid and the current collector surface generate less overall deflection effect and force more reactant gases into the catalyst layer to have more uniform current density distributions. The electrical power is 30% greater for the three objectives optimization than for minimum pressure drops optimization and the pressure drops 275% less for the three objectives optimization than for maximum electrical power optimization.

Large area solar cells with efficiency exceeding 19% in pilot series designed for conventional module assembling

Abstract Performance, reliability and cost are the key parameters in terms of wafer material, solar cell design and module fabrication to transfer a new solar cell concept into production. In this paper we present results based on pilot series of large area (156 mm x 156 mm wafer format) passivated emitter and rear solar cells on Czochralski grown boron doped wafers. Median efficiencies above 19% averaged over 50 wafer batches are received. These high-efficiency cells benefit from a passivated lowly doped industrial emitter on the front side, a seed and plate front side metallization and a dielectric passivated rear side using local contacts for rear side contacting. In order to optimize cell performance in a large-scale solar cell production a careful selection of the Cz silicon material is mandatory taking into account the electrical quality along the whole ingot. Solar cell results are presented before and after light-induced degradation using adjacent material along the complete ingot of different material supplier. Our passivated emitter and rear cells are conventionally solderable and can be integrated into a standard module assembling. The stability of our high-efficiency solar cells is shown by fabricating mini-modules that successfully pass standard climate chamber tests.

Thin-film CdTe cells: Reducing the CdTe

Polycrystalline thin-film CdTe is currently the dominant thin-film technology in world-wide PV manufacturing. With finite Te resources world-wide, it is appropriate to consider the limits to reducing the thickness of the CdTe layer in these devices. In our laboratory we have emphasized the use of magnetron sputtering for both CdS and CdTe achieving AM1.5 efficiency over 13% on 3 mm soda-lime glass with commercial TCO and 14% on 1 mm aluminosilicate glass. This deposition technique is well suited to good control of very thin layers and yields relatively small grain size which also facilitates high performance with ultra-thin layers. This paper describes our magnetron sputtering studies for fabrication of very thin CdTe cells. Our thinnest cells had CdTe thicknesses of 1 μm, 0.5 μm and 0.3 μm and yielded efficiencies of 12%, 9.7% and 6.8% respectively. With thinner cells Voc, FF and Jsc are reduced. Current–voltage (J–V), temperature dependent J–V (J–V–T) and apparent quantum efficiency (AQE) measurements provide valuable information for understanding and optimizing cell performance. We find that the stability under light soak appears not to depend on CdTe thickness from 2.5 to 0.5 μm. The use of semitransparent back contacts allows the study of bifacial response which is particularly useful in understanding carrier collection in the very thin devices.

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

AbstractInterdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (VOC) due to the surface passivation and heterojunction contacts, and high short circuit current density (JSC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence VOC and JSC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since JSC and VOC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.

Single-step fabrication of ABPBI-based GDE and study of its MEA characteristics for high-temperature PEM fuel cells

Single-step fabrication of a Poly(2,5-benzimidazole) (ABPBI)-based gas diffusion electrode (GDE) by directly adding a carbon-supported-catalyst to a homogeneous ABPBI solution prior to deposition and its membrane electrode assembly (MEA) were investigated for high-temperature proton exchange membrane (PEM) fuel cell applications. The ABPBI and LiCl dosages of the catalyst layer were varied. The characterizations of the resulting electrodes and/or MEA for the gas permeability, electrical resistance, specific electrochemical surface area, AC impedance, cyclic voltammetry and high-temperature PEM fuel cell performance were carried out. The high-temperature PEM fuel cell was successfully demonstrated at temperatures of up to 180 °C under ambient pressure operation. The fuel cell performance was evaluated by using dry hydrogen/oxygen gases, which added the advantage of eliminating the complicated humidification system of Nafion cells. The obtained results revealed that a catalyst layer with an ABPBI content of 15 wt.% and an ABPBI/LiCl ratio of 1:2 was sufficient to obtain the optimal cell performance with better electrochemical properties of low cell impedance, high electrochemical activity, low contact resistance and short activation time.

Microstructural Characterization of Composite Electrode Materials in Solid Oxide Fuel Cells via Image Processing Analysis

Among various fuel cells, solid oxide fuel cells (SOFCs) offer the highest energy efficiency, when taking into account the thermal recycling of waste heat at high temperature. However, the highest efficiency and lowest pollution for a SOFC can be achieved through the sophisticated control of its constituent components such as electrodes, electrolytes, interconnects and sealing materials. The electrochemical conversion efficiency of a SOFC is particularly dependent upon the performance of its electrode materials. The electrode materials should meet highly stringent requirements to optimize cell performance. In particular, both mass and charge transport should easily occur simultaneously through the electrode structure. Matter transport or charge transport is critically related to the configuration and spatial disposition of the three constituent phases of a composite electrode, which are the ionic conducting phase, electronic conducting phase, and the pores. The current work places special emphasis on the quantification of this complex microstructure of composite electrodes. Digitized images are exploited in order to obtain the quantitative microstructural information, i.e., the size distributions and interconnectivities of each constituent component. This work reports regarding zirconia-based composite electrodes.

Numerical Analysis of Water Transport Through the Membrane Electrolyte Assembly of a Polymer Exchange Membrane Fuel Cell

The effects of water transport through membrane electrolyte assembly of a polymer exchange membrane fuel cell on cell performance has been studied by a one-dimensional, nonisothermal, steady-state model. Three forms of water are considered in the model: dissolved water in the electrolyte or membrane, and liquid water and water vapor in the void space. Phase changes among these three forms of water are included based on the corresponding local equilibriums between the two involved forms. Water transport and its effect on cell performance have been discussed under different operating conditions by using the value and the sign of the net water transport coefficient, which is defined by the net flux of water transported from the anode side to the cathode side per proton flux. Optimal cell performance can be obtained by adjusting the liquid water saturation at the interface of the cathode gas diffusion layer and flow channels.

Numerical study on channel size effect for proton exchange membrane fuel cell with serpentine flow field

Abstract This work numerically investigates the effect of the channel size on the cell performance of proton exchange membrane (PEM) fuel cells with serpentine flow fields using a three-dimensional, two-phase model. The local current densities in the PEM, oxygen mass flow rates and liquid water concentrations at the interface of the cathode gas diffusion layer and catalyst layer were analyzed to understand the channel size effect. The predictions show that smaller channel sizes enhance liquid water removal and increase oxygen transport to the porous layers, which improve cell performance. Additionally, smaller channel sizes also provide more uniform current density distributions in the cell. However, as the channel size decreases, the total pressure drops across the cell increases, which leads to more pump work. With taking into account the pressure losses, the optimal cell performance occurs for a cell with a flow channel cross-sectional area of 0.535 × 0.535 mm 2 .

The formation of water droplets in an air-breathing PEMFC

Air-Breathing Proton Exchange Membrane Fuel Cells (AB-PEMFC) have the potential to supersede lithium-ion batteries in portable electronics. However, their water management issue has yet to be resolved to ensure optimum cell performance and safe system operation. In this paper, the formation of water droplets and their aggregation in the cathode flow channels of an operating AB-PEMFC is investigated by direct visualisation under various operating conditions. The developed optical set-up enables observation of droplet formation on the surface of the membrane from the top and side view of the channels simultaneously. The two orthogonal views reveal that during formation the receding and advancing droplet contact angles are almost identical with values that increase, in a similar trend to the droplet height, with increasing droplet diameter. Water films were able to develop and maintain direct contact with the side wall of the channels even under the effect of gravitational force. The aggregation of water droplets in the channels was strongly influenced by the change in the air and hydrogen stoichiometry conditions. However, these operating parameters appear to have no significant effect on the water extraction from the channels contrary to load and temperature, where temperature has proved to be the most effective water removal mechanism with minimum reduction in the current density of AB-PEMFC.

Effects of operating temperatures on performance and pressure drops for a 256 cm 2 proton exchange membrane fuel cell: An experimental study

Cell performance and pressure drop were experimentally investigated for two commercial size 16 cm × 16 cm serpentine flow field proton exchange membrane fuel cells with Core 5621 and Core 57 membrane electrode assemblies at various cell temperatures and humidification temperatures. At cell temperature lower than the humidification temperature, the cell performance improved as the cell temperature increased, while reversely at cell temperature higher than the humidification temperature. At a specified cell temperature, increasing the cathode and/or anode humidification temperature improved the cell performance, and their effects weakened as cell temperature decreased. The effects of the cell and the humidification temperature on the pressure drops were closely related to the reactant feed mode. For the constant stoichiometric flow rate mode, both cathode and anode pressure drops increased as humidification temperature and average current density increased. For the constant mass flow rate mode, both cathode and anode pressure drops increased as humidification temperature increased, while anode pressure drops decreased and cathode pressure drops increased as average current density increased. The optimal cell performance occurred at cell temperature of 65 °C and humidification temperature of 70 °C. The effects of these operating parameters on the cell performance and pressure drop were analyzed based on the catalytic activity, membrane hydration, and cathode flooding.