Higher Short-circuit Current Research Articles

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

AbstractOrganic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short‐circuit currents (JSC) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open‐circuit voltages (VOC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl‐C61‐butyric acid methyl ester (PC61BM), that achieves a high VOC (0.9 V) with very low energy losses (Eloss = 0.52 eV) from the energy of absorbed photons, a respectable JSC (13 mA cm−2), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field‐dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge‐carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the JSC, VOC, and FF can all be improved, even with very low energetic offsets.

Technical and Economic Analysis of the R-Type SFCL for HVDC Grids Protection

Mainly used in ac grids, resistive-type superconducting fault current limiters may be still more interesting for dc systems due to their capability to reduce high short circuit currents appearing in case of dc cable fault. This limiter reduces the breaking capability, speed, and energy requirements of the required dc circuit breaker allowing the implementation of electromechanical breakers for fault current interruption. These breakers have lower breaking capability, on-load losses, and investment costs in comparison with hybrid circuit breakers based on power electronics. This paper presents a technical and economic analysis of a superconducting fault current limiter used in a radial three-terminal high voltage dc grid to protect a cable link. Based on simulation studies using an electromagnetic transient program, an effective system protection will be demonstrated with special attention to the continuity of power flow through healthy parts of the grid.

Designing 1,5‐Naphthyridine‐2,6‐dione‐Based Conjugated Polymers for Higher Crystallinity and Enhanced Light Absorption to Achieve 9.63% Efficiency Polymer Solar Cells

AbstractHighly crystalline conjugated polymers represent a key material for producing high‐performance thick‐active‐layer polymer solar cells (PSCs). However, despite their potential, a limited number of crystalline polymers are used in PSCs because of the lack of highly coplanar acceptor building blocks and insufficient light absorptivity (α < 105) of most donor (D)–acceptor (A)‐type polymers. This study reports a series of novel 3,7‐di(thiophen‐2‐yl)‐1,5‐naphthyridine‐2,6‐dione (NTDT) acceptor‐based conjugated polymers, PNTDT‐2T, PNTDT‐TT, and PNTDT‐2F2T, synthesized with 2,2′‐bithiophene (2T), thieno[3,2‐b]thiophene (TT), and 3,3′‐difluoro‐2,2′‐bithiophene (2F2T) donor units, respectively. PNTDT‐2F2T exhibits superior polymer crystallinity and a much higher absorption coefficient than those of PNTDT‐2T or PNTDT‐TT because of adequate matching between highly coplanar A (NTDT) and D (2F2T) building blocks. A bulk heterojunction solar cell based on PNTDT‐2F2T exhibits a power conversion efficiency of up to 9.63%, with a high short circuit current of 18.80 mA cm−2 and fill factor of 0.70, when a thick active layer (>200 nm) is used, without postfabrication hot processing. The findings demonstrate that the polymer crystallinity and absorption coefficient can be effectively controlled by selecting appropriate D and A building blocks, and that NTDT is a novel and versatile A building block for highly efficient thick‐active‐layer PSCs.

Novel Short-Circuit Detection in Li-Ion Battery Architectures

As energy storage devices become more important in the management of the world’s power supply, safeguarding large battery assemblies against catastrophic failure (CF) has become a top priority. Although lithium-ion batteries are found in a wide array of applications, from mobile phones to commercial airliners, the continued expansion of lithium-ion batteries is hindered by safety, durability, and reliability concerns. One of the reasons for CF is battery internal short-circuit (SC), which causes thermal runaway. Unfortunately, no fast industry-accepted method to forecast CF exists. Recently, we proposed a new electrochemical approach to detect battery SC far before CF can occur.1 The approach is based on the application of a constant voltage (VTEST) to a short-circuited cell (battery) at a slight discharge overvoltage from fractions of mV to several mV, depending on the battery system and desired speed of SC detection. Under this slight polarization, causing minimal battery disturbance, the response signal (measured current) transitions from an initial high negative value to a stabilized positive value through a current zero crossing point. A stabilized, positive current under this potentiostatic condition is a metric for SC detection and is SC current at absolute value in the case when the cell’s genuine self-discharge is negligible. For a short-circuited cell with notable genuine self-discharge, the stabilized positive current is a sum of two currents: genuine self-discharge current and SC. The current zero crossing point time is another useful and practical metric for SC detection. This method was validated on rested, single 18650 Li-ion cells with shorts of different severity and demonstrated high precision. Lower-resistance shorts result in shorter time to current zero crossing point and higher SC current. Diagnostics at field applications, such as electric drive vehicles, require fast determination of SCs in battery architectures consisting of numerous cells in serial-parallel connections. In order to determine the usefulness of the method, we tested a dozen SC scenarios for one-string, consisting of three 18650s cells, and dual-string, consisting of six cells. In order to mimic field scenarios, the study was conducted on resting architecture and when the architecture was connected to the external known resistive load. In the field, the load is battery-controlling circuitry, such as battery management system, with vehicle equipment connected to the battery. It is assumed that a vehicle is parked, not driven, during this type of diagnostic. The SC resistor was applied across a single cell or several single cells randomly positioned in battery architectures, while potentiostat was connected to short-circuited cell, not-short-circuited cell, and a string or double-string containing short-circuited and not-short-circuited cells. If a potentiostat is connected to a healthy not-short-circuited cell in the battery architecture consisting of other short-circuited cells, there is no SC current running in the diagnostic circuit. In contrast, short-circuited cells produce SC currents detectable on all levels of architecture. Using this methodology, it is feasible to first detect problems on the full battery architecture level, then problematic string(s) can be found and, in the string, problematic cells with high self-discharge (and potential shorts) can be identified. This method appears to be a universal diagnostic tool used for a battery over its life span. Initially, it can be used for the detection of faulty cells under production, and then it can be applied at the beginning of life during initial characterization (mapping) of battery performance. It can also be used for periodic state-of-health checks during the operational life of the battery in comparison to the beginning of life. When the battery reaches the end of life, which is 80% of state-of-charge for electric-drive vehicle applications, it may be repurposed for secondary use in residential or industrial storage. This is the time for checking for shorts to avoid problems in larger battery architectures. Lastly, the approach can be used for first responders on battery-related accidents. This patent-pending method is accurate, fast, non-invasive, and applicable to any battery chemistry or design with capability to detect nascent shorts. The method can be adapted for battery management systems for monitoring battery state-of-health at any time and in any state of charge. The technology based on this method can be used in electric-drive vehicles; stationary energy storage; military, aeronautic, and portable electronic devices; and many other applications.

Silicon Solar Cell Architecture with Front Selective and Rear Full Area Ion‐Implanted Passivating Contacts

In this work, the application of carrier‐selective passivating contacts based on tunneling silicon‐dioxide and ion‐implanted poly‐Si in front and rear contacted Si solar cells is presented. This paper addresses the need to minimize the contact recombination while still keeping high short circuit current. We aim to solve such trade‐off with a novel solar cell architecture called Passivated Rear and Front ConTacts (PeRFeCT). Such design employs a selective passivating contact combined with standard homojunction on the front side in order to minimize contact recombination, while achieving high optical transparency and a full area passivating contact on the rear side. The opto‐electrical modeling of this front/rear contacted architecture indicates a potential efficiency above 26%. As technology demonstration, we also report on the optimization of front surface field and processing of 2.8 × 2.8 cm2 wide solar cells leading to a 20.1% conversion efficiency.

Pixelated speckle image holography carrier fringes for efficient superimposed light harvesting in organic solar cells

An inverted organic solar cell (OSC) device structure by incorporating pixelated speckle image holography carrier fringes (SIHFs) for efficient superimposed light harvesting is demonstrated. The proposed SIHF based OSCs yield an 18.2% increase in power conversion efficiency (PCE) compared to that of the flat control devices. Moreover, compared to the common two-dimensional (2D) periodic grating patterned OSCs, SIHF based devices achieve 7.8% higher short-circuit current (JSC) and 10.0% higher PCE. This observable improvement in PCE of SIHF based OSCs is mainly ascribed to the geometric effect due to the unique chaotic carrier fringes of SIHFs.

Influence of allophane distribution on photocatalytic activity of allophane–titania composite films

Allophane–titania composite films were prepared from titanium alkoxide sols dispersing the natural clay mineral allophane as a nano-sized adsorbent. The composites of the titania dispersing the allophane particles in the whole film efficiently degraded trichloroethylene and acetaldehyde better than the normal titania and the titania dispersing the allophane only on the film surface. The titania dispersing the allophane particles in the whole film produced lower amounts of the intermediate products in certain stages than the titania. These results are due to the effective reactant transport from the allophane to titania. Photocatalytic degradation of glucose and photoelectrochemical measurements using an electrolyte solution containing glucose indicated that oxidative degradation of the glucose enhanced the generation of electricity during UV irradiation. A higher short circuit current was observed using the film dispersing the allophane on the surface layer with an ca. 50-nm thickness than the titania and those dispersing the allophane only on the film surface. Allophane effectively adsorbed the glucose molecules and transported them to the titania, then the products were desorbed into the liquid phase. Therefore, the holes were efficiently transported and consumed by the glucose oxidation.

Sequential Dip-spin Coating Method: Fully Infiltration of MAPbI3-xClx into Mesoporous TiO2 for Stable Hybrid Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have reached a power conversion efficiency of 22.1% in a short period (∼7 years), which has been obtainable in silicon-based solar cells for decades. The high power conversion efficiency and simple fabrication process render perovskite solar cells as potential future power generators, after overcoming the lack of long-term stability, for which the deposition of void-free and pore-filled perovskite films on mesoporous TiO2 layers is the key pursuit. In this research, we developed a sequential dip-spin coating method in which the perovskite solution can easily infiltrate the pores within the TiO2 nanoparticulate layer, and the resultant film has large crystalline grains without voids between them. As a result, a higher short circuit current is achieved owing to the large interfacial area of TiO2/perovskite, along with enhanced power conversion efficiency, compared to the conventional spin coating method. The as-made pore-filled and void-free perovskite film avoids intrinsic moisture and air and can effectively protect the diffusion of degradation factors into the perovskite film, which is advantageous for the long-term stability of PSCs.

Pulsed-laser-ablation based nanodecoration of multi-wall-carbon nanotubes by Co\u2013Ni nanoparticles for dye-sensitized solar cell counter electrode applications

We report here on the use of pulsed KrF-laser deposition technique (PLD) for the decoration of Multi-wall carbon nanotubes (MWCNTs) by Co–Ni nanoparticles (NPs) to form highly efficient counter electrodes (CEs) for use in Dye-sensitized solar cells (DSSC). By varying the number of laser ablation pulses (NLP = 500–60,000) of the KrF laser, we were able to control the average size of the Co–Ni NPs and the surface coverage of the MWCNTs by the nanoparticles. The PLD-based decoration of MWCNTs by Co–Ni NPs is shown to form novel counter electrodes, which significantly enhance the power conversion efficiency (PCE) of the DSSCs. Indeed, the DSSCs based on the PLD-decorated Co–Ni counter electrodes (obtained at the optimal NLP = 40,000) are shown to exhibit a PCE value as high as 6.68%, with high short circuit current (Jsc = 14.68 mA/cm2) and open circuit voltage (Voc = 0.63 V). This represents a PCE improvement of ~190% in comparison to the DSSCs with pristine MWCNTs (PCE = 2.3%) and ~7.4% PCE increase than that of the conventional DSSC made with a sputtered Platinum-based counter electrode. By systematically investigating the local nanostructure of the Co–Ni decorated CEs, we found that the Co–Ni NPs layer exhibits a porous cauliflower-like morphology, of which surface roughness (RMS) is NLP dependent. Interestingly, both PCE and roughness of the Co–Ni NPs layer are found to exhibit the same NLP dependence, with a maximum located around the optimal NLP value of 40,000. This enabled us to establish, for the first time, a linear correlation between the achieved PCE of DSSCs and the local roughness of their CEs decorated by Co–Ni NPs. Such a correlation highlights the importance of maximizing the surface area of the Co–Ni coated MWCNTs on the CEs to enhance the PCE of the DSSCs. Finally, Ultra-violet Photoelectron Spectroscopy (UPS) measurements revealed a significant decrease in the local work function (Φ) of Co–Ni NPs decorated MWCNTs based CEs (at NLP = 40,000, Φ = 3.9 eV) with respect to that of either pristine MWCNTs (Φ = 4.8 eV) or sputtered-Pt (Φ = 4.3 eV) counter-electrodes. This Φ lowering of the Co–Ni/MWCNTs based CEs is an additional advantage to enhance the catalytic reaction of the redox couple of the electrolyte solution, and improve thereby the PCE of the DSSCs.

Effect of Isomeric Structures on Photovoltaic Performance of D-A Copolymers.

Two donor-acceptor copolymers based on isomeric acceptor units, [7,7'-bithieno[2',3':4,5]thieno[2,3-d]thieno[3,2-b]pyridine]-5,5'(4H,4'H)-dione (BTTP) and [2,2'-bithieno[2',3':4,5]thieno[2,3-d]thieno[3,2-b]pyridine]-5,5'(4H,4'H)-dione (iBTTP), are developed to study the effect of isomeric structures on photovoltaic performance. Compared with PBDTBTTP, PBDTiBTTP possesses a smaller bandgap for good light harvesting and a better π-π stacking for higher hole mobility. PBDTiBTTP solar cells present balanced mobilities and good nanoscale phase separation, giving a power conversion efficiency (PCE) of 6.51%, with higher short-circuit current (Jsc ) and fill factor (FF).

Design and Application of Superconducting Fault Current Limiter in a Multiterminal HVDC System

Voltage source converter based HVdc (VSC-HVdc) systems are prone to high short-circuit current during transmission line faults. The situation for multiterminal HVdc (MTDC) systems is worse. The characteristics of superconducting material are ideal to limit the fault current in HVdc systems. This paper presents a novel use of the resistive type of superconducting fault current limiter (SFCL) in the MTdc network with the function of limiting the high current. The working principles of fault current limiter and a three-terminal HVdc system are modeled in detail using PSCAD/EMTDC software. The hybrid operation of the SFCL in the three-terminal HVdc system is tested in this paper for the fault response of the MTdc system. The performances of SFCL under different fault conditions are analyzed. The simulation results show that the fault current is effectively restrained and the SFCL can act as an efficient protective device for VSC-based multiterminal HVdc systems.

Influence of heating temperature of Se effusion cell on Cu(In, Ga)Se2 thin films and solar cells

In this study, copper indium gallium di-selenide (Cu(In, Ga)Se2, CIGS) thin films are prepared at different heating temperatures of Se effusion cell (Tsec) between 150 °C and 190 °C. The Tsec has a great influence on the deposition rate, chemical composition, morphology and grain structure of the CIGS absorbers. The CIGS thin film solar cell fabricated at 160 °C shows best efficiency of 11.6% in this series due to the relatively higher open circuit voltage (Voc) and fill factor (FF) as well as a high short circuit current (Jsc). By further optimizing device and material properties, the higher efficiency CIGS thin film solar cell of 17.2% is achieved.

Efficient Semitransparent Solar Cells with High NIR Responsiveness Enabled by a Small-Bandgap Electron Acceptor.

Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%-6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV-vis-near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an Egopt of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm-2 . Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.

Direct arylation polymerization approach for the synthesis of narrow band gap cyclopentadithiophene based conjugated polymer and its application in solar cell devices

Direct arylation polymerization is a simple, cost-effective and efficient method for synthesizing conjugated polymers. The synthesis of a narrow band gap conjugated polymer, PCPDTTBT, comprising of alternating cyclopentadithiophene (CPDT) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole (TBT) units was carefully optimized by direct arylation polycondensation. The synthesized polymer has a number average molecular weight of 36.3kgmol−1 with a low PDI of 1.25 as measured by GPC against polystyrene standards. PCPDTTBT exhibits an absorption maximum at 617nm in THF solution whereas in thin film, the absorption spectrum was broader and a maximum was observed at 650nm. The HOMO, LUMO and band gap calculations were estimated using cyclic voltammetry and were found to be −5.20eV, −3.57 and 1.63eV, respectively. Bulk heterojunction polymer solar cells were fabricated by blending PCPDTTBT and PC71BM in three different ratios with six cells for each ratio to optimize the efficiency. It was found that the blend ratio of 1:2 exhibited a high short-circuit current of 10.03mAcm−2, open-circuit voltage of 0.567V and a power conversion efficiency of 1.86%. The morphology of the active layer was investigated by employing atomic force microscopy (AFM) in tapping mode and the film of active layer with 1:2 ratio of PCPDTTBT: PC71BM showed most favorable morphology with less phase separation.

1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells.

A small molecule TBTT-1 with 5-(2-ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. TBTT-1 exhibits a broad absorption with a low optical band gap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm-2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π-π stacking and reduce the scales of phase separation, which led to improved exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance.

Photovoltaic performance and impedance spectroscopy of a purely organic dye and most common metallic dye based dye-sensitized solar cells

In this work, performances of an organic electron donor–π-bridge–electron acceptor (D–π–A) type RK1 dye and the most common metallic N719 dye based dye-sensitized solar cells (DSSCs) were compared. To gain insight into the behavior of these devices, current–voltage and impedance spectroscopic measurements were performed. From the current–voltage data, the parameters of cells including open-circuit voltage, short-circuit current, fill factor, solar energy-to-electricity conversion efficiency, series resistance, shunt resistance, and ideality factor were extracted. The RK1 based cell showed higher photovoltage, higher short-circuit current, solar energy-to-electricity conversion efficiency, higher shunt resistance and ideality factor. Using the Mott-Schottky plots, electrical properties of the cells were investigated and lower electron concentration was found for the RK1-DSSC. This high performance can be attributed to the higher absorption coefficient of the RK1 dye and relatively higher induced positive band-shift of the conduction band edge of the TiO2 semiconductor. The trends of the recombination resistance and the chemical capacitance as observed in the measured impedance spectra have also confirmed high photovoltaic performance of the RK1 based cell. Impedance spectra of the devices were observed both voltage and frequency dependent. At low frequencies, significant contribution of trap states was observed.

Separation of Transformers for Class 1E Systems in Nuclear Power Plants

In order to supply electric power to the safety related loads, safety and reliability of onsite power have to be ensured for the safety function performance in nuclear power plants. Even though the existing electric power system of APR1400 meets the requirements of codes regarding Class 1E system, there is a room for improvement in the design margin against the voltage drop and short circuit current. This paper discusses the amount that the voltage drop and short circuit current occur in the existing electric power system of APR1400. Additionally, this paper studies with regard to the improved model that has the extra margin against the high voltage drop and short circuit current by separation of unit auxiliary transformer (UAT) and standby auxiliary transformer (SAT) for the Class 1E loads. The improved model of the electric power system by separation of UAT and SAT has been suggested through this paper. Additionally, effects of reliability and cost caused by the electric power system modification are considered.

Advantages of using a DC power system on board ship

This article is structured to present an overview of a DC ship power system. The main DC grid configurations will be presented and a difference to the AC system configuration will be highlighted. Compared to the AC power system used on board ships, DC has some obvious benefits which will be explored in this paper. These benefits include: improvement of prime mover efficiency and reduction of fuel costs, weight and space savings, unity power factor operation of generators, lower transmission losses, faster and simpler parallel connection of generators and simpler implementation of energy storage. Finally, some of the challenges introduced with the DC technology will also be explored. These include: high short-circuit currents, DC protection concept and expensive and possibly non-profitable energy storage system solutions.

A Novel Bis-Lactam Acceptor with Outstanding Molar Extinction Coefficient and Structural Planarity for Donor–Acceptor Type Conjugated Polymer

A novel electron-accepting bis-lactam building block, 3,7-dithiophen-2-yl-1,5-dialkyl-1,5-naphthyridine-2,6-dione (NTDT), and a conjugated polymer P(NTDT-BDT) comprising NTDT as an electron acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) as an electron donor are designed and synthesized for producing efficient organic solar cells. The thermal, electronic, photophysical, electrochemical, and structural characteristics of NTDT and P(NTDT-BDT) are studied in detail and compared with those of the widely used bis-lactam acceptor 3,6-dithiophen-2-yl-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione (DPPT) and its polymer P(DPPT-BDT). Compared to DPPT derivatives, NTDT and P(NTDT-BDT) exhibit remarkably higher absorption coefficients, deeper highest occupied molecular orbital energy levels, and more planar conformations. A bulk heterojunction solar cells based on P(NTDT-BDT) exhibit power conversion efficiency of up to 8.16% with high short circuit current (Jsc) of 18.51 mA cm–2, one of the highest Jsc values yet obt...

Multiphysics analysis of busbars with various arrangements under short‐circuit condition

This study presents a coupled electric–magnetic–thermal–mechanical analysis of various busbar arrangements under short-circuit conditions. The Lorentz force, mechanical displacement, and temperature rise in three busbar arrangements are investigated. Busbar systems are studied considering the detailed two-way interactions among separate continuum physics. To this aim, the multiphysics modelling of busbar systems is presented where the coupled electric–magnetic–thermal–mechanical set of equations are solved numerically using finite-element method. Caused by electromagnetic force, the mechanical displacement of conductors in horizontal and vertical arrangements involving multiple conductors in each phase is illustrated. The effects of conductor cross-sectional area on the temperature rise caused by short-circuit current are also investigated. To verify the multiphysics modelling approach under high short-circuit currents, the simulation results are compared with those obtained by existing closed formulas. Moreover, after practical investigation of several busbar systems, the proposed multiphysics modelling approach is applied to the busbar system of a low-voltage switchgear installed in a cement plant.