Series-connected Cells Research Articles

Tandem small molecule organic photovoltaic cells with broad spectral response up to 1 μm and a high open-circuit voltage

We report a series-connected small molecule tandem photovoltaic cell utilizing two donors with complementary photovoltaic characteristics, lead phthalocyanine (PbPc) in the front subcell and boron subphthalocyanine chloride (SubPc) in the back subcell, to achieve both near infrared (NIR) response up to 1μm and high open-circuit voltage (VOC) of more than 1.5V in the same device. We find that the C60 layer thickness in the front subcell has a critical impact on the overall optical structure and photovoltaic performance of the tandem device. By combining transfer matrix calculations with subcell-selective spectral measurements, we are able to tune the optical field distribution inside the active layers and increase the photocurrent outputs from both subcells, leading to EQE>30% over the wavelength range 400nm<λ<900nm. This optimized tandem cell exhibits JSC=(5.5±0.1)mA/cm2, fill factor=0.54, VOC=1.53V, and a power conversion efficiency of (4.5±0.2)%.

An Optimized Topology for Next-to-Next Balancing of Series-Connected Lithium-ion Cells

Voltage balancing between series-connected cells of rechargeable lithium-ion batteries is important for battery life, autonomy, and safety. Active balancing is the designated choice for applications that are sensitive to energy losses and maximized autonomy. First, a well-known next-to-next balancing technique is presented and its design and operating limitations are analyzed. Then, the paper focuses on an evolution of the converter's architecture offering better performances while being more compact, requiring fewer and smaller filtering needs and still, being simple to implement. The experimental results of the balancing operation of a pack of eight cells connected in series show demonstrative and satisfactory results.

Modeling and Parameter Identification of Lithium-Ion Capacitor Modules

Lithium-ion capacitors (LiCs) are novel storage devices with a high power density and high energy density compared to conventional supercapacitors (SCs). This paper proposes a method to validate the previously developed characterization and modeling methods, which are the same as those used for a conventional SC with double-layer-activated carbon technology. This paper presents two relevant contributions. First, a full frequency range model and the experimental parameter identification of two kinds of LiC cells are presented. In order to extend the LiC cell parameter identification to a module composed of several series-connected cells, an aggregate model of the LiC module was investigated and validated. The results of experiments and numerical simulations demonstrate the value and effectiveness of the proposed model when the cells operate at room temperature.

Design and Safety Verification of a Distributed Charge Equalizer for Modular Li-Ion Batteries

This paper presents a novel charge equalization technique seamlessly integrated into a modular Battery Management System (BMS) for lithium-ion (Li-ion) batteries. The charge equalizer is a crucial element for an effective use of a Li-ion battery consisting of many series-connected cells. We describe a fully distributed charge equalizer based on a circular balancing bus, which outperforms other recently published approaches. Its safety requirements have formally been verified using a model checker, showing that formal methods and, in particular, the Symbolic Analysis Laboratory environment, can be effective to verify the safety requirements of a BMS.

Battery Management using Secondary Loads: A Novel Integrated Approach

Large lithium-ion battery systems for electric vehicle and stationary storage applications use from tens to hundreds of series-connected cells. In this paper we present an advanced integrated management structure and method that uses intelligent switching to balance series-connected cells. Our analysis shows that using this approach can increase the useful system capacity by upwards of 7% compared to conventional active or passive balancing systems by fully utilising cells with different capacities. The battery system efficiency in an electric vehicle has also been shown to increase by up to 3% compared to conventional systems. Further advantages of integrated balancing include simultaneous integration of both primary and secondary power loads or sources, as well as robustness to individual cell failure. This indicates significant potential for implementation in future lithium-ion battery packs.

Performance and Reliability Implications of Two-Dimensional Shading in Monolithic Thin-Film Photovoltaic Modules

We analyze the problem of partial shading in monolithically integrated thin-film photovoltaic (TFPV) modules, and explore how the shape and size of the shadows dictate their performance and reliability. We focus on the aspects of the shading problem unique to monolithic TFPV, arising from thin long rectangular series-connected cells, with partial shadows covering only a fraction of the cell area. Using calibrated 2-D circuit simulations, we show that due to the cell shape, the unshaded portion of partially shaded cell experiences higher heat dissipation due to redistribution of voltages and currents across the cells. We then use thermal imaging techniques to compare our results with module behavior under shade in realistic situations. We also analyze the effect of shadow size and orientation by considering several possible shading scenarios. We find that thin edge shadows can cause potentially catastrophic reverse bias damage, depending on their orientation. Finally, we show that external bypass diodes cannot protect the individual cells from shadow-induced reverse stress, but can limit the string output power loss for larger shadows.

Single-Switch Multioutput Charger Using Voltage Multiplier for Series-Connected Lithium-Ion Battery/Supercapacitor Equalization

Voltage equalization is essential for series-connected energy storage cells, such as lithium-ion cells and supercapacitors (SCs), because it ensures years of service life and maximizes the available energies of cells. Conventional equalizers and multioutput chargers require many switches and/or a multiwinding transformer, which are not desirable in terms of circuit simplicity and extendibility. In this paper, single-switch multioutput chargers based on a combination of an input cell (which generates a square voltage wave) and a voltage multiplier (which produces multiple uniform output voltages) are proposed. A fundamental operation analysis is made, and a dc equivalent circuit is derived to describe their charging and equalization behaviors. Experimental charge tests were performed separately for series-connected SC modules and lithium-ion batteries from voltage-unbalanced conditions. The least charged module/battery was charged preferentially; as a result, all of the modules/batteries were charged to a desired uniform voltage level while eliminating the voltage imbalance.

The effect of irradiance mismatch on a semi-dense array of triple-junction concentrator cells

A method to determine the effect of cell-to-cell non-uniformity on CPV module performance is applied to a high-concentration photovoltaic system, comprising a linear semi-dense array of 5 series-connected 1cm2 triple-junction concentrator cells (lattice-matched GaInP/GaAs/Ge). Characteristic irradiance distributions were obtained by Monte Carlo ray-tracing of the concentrator optical system. Spatial apodizing filters were then produced such that, when illuminated with a spatially uniform radiation source, the desired irradiance distribution is obtained behind the filter. An equivalent circuit model of the array was developed and fitted against reference uniform irradiance measurements at concentrations ranging from 10x to 1678x. The fitted model was then validated vis-à-vis measurements under non-uniform irradiance imposed by the filters at concentrations from 99x to 494x. Both experimental and simulated results show a characteristic stepped I–V behavior associated with irradiance mismatch in series-connected arrays. Under the worst-case conditions of irradiance mismatch over the array, the efficiency drops to 28.5% as compared to 39% for the case of perfect uniformity. Due to the translational symmetry of the primary concentrator, a redesign of the module with parallel cell interconnects enables to mitigate the effects of irradiance mismatch. The methods provide an experimentally verifiable approach to determine the effect of irradiance mismatch on concentrator solar cell arrays without the need for prototyping of the concentrator optics. They may further be applied to determine the underlying characteristics and properties of cells and arrays subjected to nonuniform irradiation.

High-Efficiency Digitally Controlled Charge Equalizer for Series-Connected Cells Based on Switching Converter and Super-Capacitor

The charge stored in series-connected lithium batteries needs to be well equalized between the elements of the series. We present here an innovative lithium-battery cell-to-cell active equalizer capable of moving charge between series-connected cells using a super-capacitor as an energy tank. The system temporarily stores the charge drawn from a cell in the super-capacitor, then the charge is moved into another cell without wasting energy as it happens in passive equalization. The architecture of the system which employs a digitally-controlled switching converter is compared with the state of the art, then fully investigated, together with the methodology used in its design. The performance of the system is described by presenting and discussing the experimental results of laboratory tests. The most innovative and attractive aspect of the proposed system is its very high efficiency, which is over 90%.

Phase-Disposition PWM Implementation for a Hybrid Multicell Converter

Many different topologies based on the multilevel converter approach have been presented in the last decades. In order to obtain improved output waveforms, several modulation techniques have been developed for such converters. Some of those modulation schemes can be directly applied to almost any multilevel converter, while others require significant adaptation in order to be implemented in certain specific converters. This paper presents the implementation of a phase-disposition level-shifted pulsewidth modulation to the recently introduced hybrid multicell converter (HMC). The HMC uses a flying capacitor module on each cell to generate a positive variable dc-link voltage. The variable dc link is inverted at fundamental switching frequency by an H-bridge to generate the cell output voltage. It is desirable to use PD-PWM for its well-known high-performance waveforms. In this paper, a state-machine decoder is developed to balance the floating capacitor voltages and to equalize the supplied power for the series-connected cells of the HMC when using a PD-PWM pattern. Experimental results obtained from a 10-kW prototype are presented, confirming the good-quality waveforms of the HMC modulated with PD-PWM and the adequate balance of all floating capacitors.

Measuring IV Curves and Subcell Photocurrents in the Presence of Luminescent Coupling

High-quality, direct-bandgap solar cells emit significant luminescence at their band edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III-V multijunction solar cell. We describe a technique that uses a set of light-emitting diodes (LEDs) and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.

Novel Hybrid Amorphous/Organic Tandem Junction Solar Cell

We report on a novel hybrid amorphous Si-organic series-connected tandem junction solar cell. The solar cell is fabricated on indium tin oxide (ITO)-coated glass and uses an a-(Si,C):H as the first cell and a P3HT/PCBM organic cell as the second cell. An intermediate ITO layer is used as an ohmic layer which provides an excellent contact to both the first and the second cells. By adjusting the bandgap and thickness of the first a-(Si,C):H cell, we achieve an almost complete matching of currents produced by the first and the second cells. The first cell produces ∼0.95–1.0-V open-circuit voltage, and the second cell produces ∼0.6-V open-circuit voltage. The combined cell produces 1.5-V open-circuit voltage and had a fill factor of 77%, showing the effectiveness of the intermediate ITO layer to act as an excellent connecting layer between the two cells.When such an ITO layer is not used, the fill factor is very poor. The solar conversion efficiency of the organic cell was 4.3%, whereas the efficiency of the tandem cell was 5.7%. We also measured the stability of the organic cell with and without an inorganic cell acting as a filter in front. It is shown that the degradation of the organic cell is much higher when it is subjected to a full solar spectrum, as compared with when it is subjected to light passing through an inorganic cell first, which filters out ultraviolet (UV) and blue photons. Thus,we showthat this new cell combination has the potential to significantly increase the efficiency of organic cells while also decreasing the instability. We also discuss the potential of achieving much higher efficiencies, that is approaching 20%, by using an appropriate combination of amorphous and organic cells. An example is shown next.

A High-Efficiency Active Battery-Balancing Circuit Using Multiwinding Transformer

This paper presents a simple circuit for balancing series-connected battery cells. The circuit is composed of one low-voltage metal-oxide-semiconductor field-effect transistor (MOSFET) for each cell and a symmetrical multiwinding transformer for a group of cells. Only one control signal is needed for all MOSFETs, and energy can be directly transferred from higher voltage cells to lower voltage cells. A small capacitor is added to form a resonant circuit with the transformer magnetizing inductance so that soft switching can be achieved in some operation range to obtain high efficiency. The circuit balance function and soft-switching condition are analyzed and verified by simulation and experiments. A circuit for balancing a 4- and 12-cell battery group is tested. The experimental results showed the effectiveness of the circuit. The energy transfer efficiency between cells can reach up to 93%. The circuit can be easily scaled to battery strings containing up to 12 battery cells, directly applicable to consumer electronics application where the number of cells is typically less than 12. A battery pack containing a large number of battery cells, such as the ones used in electric vehicles, can be balanced in groups, for example, in modules containing 12 cells each and with one additional circuit to balance among the modules.

Surface treatment patterning of organic photovoltaic films for low-cost modules

This work describes a patterning technique for the photoactive layer of organic photovoltaic modules. We demonstrate the fabrication of efficient poly[3-(hexyl)thiophene-2,5-diyl]:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based organic photovoltaic modules through a specific surface treatment, based on the deposition of a fluorinated self assembled monolayer (SAM) on top of the bottom electric contact. Direct self-patterning of the photoactive layer is achieved by the high contact angle between the SAM and the polymer solution, while a smooth topography is created by combining two solvents with different surface tensions and boiling points in the polymer:PCBM solution. The resolution of the patterning is approximately 400μm for modules based on a conventional cell architecture and 120μm for an inverted architecture. As a result, we show 25cm2 P3HT:PCBM based organic photovoltaic modules with 10 series-connected cells, fabricated via roll-to-roll compatible deposition and patterning techniques.

Fabrication of Monolithic Integrated Series-Connected GaAs Photovoltaic Cells for Concentrator Applications

Aiming at reducting in Joule energy loss of a photovoltaic cell under sunlight concentration, monolithic integration of GaAs cells has been realized, in which five subcells were connected in series and the total surface area of the cells occupied over 80% of the whole chip area. Using plasma etching with Cl2, a sufficiently sharp mesa for device isolation was obtained. Insulation between etched mesa sidewalls and interconnect electrodes proved to be the most significant issues for the purpose of eliminating shunt resistance and securing a reasonable fill factor; the SiO2 layer deposited by sputtering was much superior to polyimide as an insulator. The fabricated test device showed a short circuit current density of 20.7 mA/cm2 and an open circuit voltage of 4.79 V, which were consistent with the values for a single subcell.

Using Phase Effects to Understand Measurements of the Quantum Efficiency and Related Luminescent Coupling in a Multijunction Solar Cell

We analyze the quantum efficiency measurement of a series-connected multijunction solar cell by modeling the cell as an ac resistive-capacitive circuit and studying the complex current as a function of the light biasing. The photocurrent induced by directly absorbed photons is modeled as an independent current source, whereas the luminescent coupling current from higher bandgap to lower bandgap subcells is modeled as a dependent current source in the bottom subcell. We derive expressions for the equivalent impedance and the complex current in measurements of the top and bottom subcells of a two-junction device. High light biasing of the nonlimiting cell drives the magnitude and phase shift of the current toward well-defined limits, but insufficient light biasing yields a composite response that is not fully characteristic of either subcell. We recommend that the experimenter always monitor the phase shift of the signal for signs of insufficient light biasing, and to help identify genuine luminescent coupling effects.

Fabrication of Monolithic Integrated Series-Connected GaAs Photovoltaic Cells for Concentrator Applications

Aiming at reducting in Joule energy loss of a photovoltaic cell under sunlight concentration, monolithic integration of GaAs cells has been realized, in which five subcells were connected in series and the total surface area of the cells occupied over 80% of the whole chip area. Using plasma etching with Cl2, a sufficiently sharp mesa for device isolation was obtained. Insulation between etched mesa sidewalls and interconnect electrodes proved to be the most significant issues for the purpose of eliminating shunt resistance and securing a reasonable fill factor; the SiO2 layer deposited by sputtering was much superior to polyimide as an insulator. The fabricated test device showed a short circuit current density of 20.7 mA/cm2 and an open circuit voltage of 4.79 V, which were consistent with the values for a single subcell.

Class- E- based charge-equalisation circuit for battery cells

The aim of this study is to carry out a charge-equalisation circuit for ten series-connected lithium battery cells. This circuit is composed of a class-E inverter and a multi-winding transformer, which can constantly perform the equalisation task. In addition, the zero-voltage-switching feature of the class-E inverter helps reduce the switching loss and boosts the operation efficiency of the equalisation circuit. With the application of the class-E inverter, only one active switch is present in this circuit. Since adaptation of the duty ratio is not required, the circuit control task is very simple, and can be easily realised by a low-cost pulse-width-modulation (PWM) control chip.

Non-linear luminescent coupling in series-connected multijunction solar cells

The assumption of superposition or linearity of photocurrent with solar flux is widespread for calculations and measurements of solar cells. The well-known effect of luminescent coupling in multijunction solar cells has also been assumed to be linear with excess current. Here we show significant non-linearities in luminescent coupling in III-V multijunction solar cells and propose a simple model based on competition between radiative and nonradiative processes in the luminescent junction to explain these non-linearities. We demonstrate a technique for accurately measuring the junction photocurrents under a specified reference spectrum, that accounts for and quantifies luminescent coupling effects.

A Transformerless D-STATCOM Based on a Multivoltage Cascade Converter Requiring No DC Sources

This paper deals with a cascaded multilevel converter which has multiple dc voltage values (multivoltage cascade converter or hybrid multilevel converter) for a 6.6-kV transformerless distribution static synchronous compensator (D-STATCOM). A control method is proposed to realize dc voltage regulation of series-connected multiple cells in the STATCOM operation, making it possible to remove dc sources from all H-bridge cells. The simplified configuration without the dc sources makes the STATCOM small and lightweight. A downscaled STATCOM model rated at 220 V and 10 kVA is built and a series of verification tests is executed. Theoretical analysis and experimental results prove the stable operating performance of the proposed method in steady states and transient states.