Increase In Short-circuit Current Research Articles

Orthogonal solubility in fully conjugated donor-acceptor block copolymers: Compatibilizers for polymer/fullerene bulk-heterojunction solar cells

Donor-acceptor (D-A) type fully conjugated block copolymer systems have been rarely reported due to the challenges in synthetic approaches to prepare well-defined low-polydispersity products. In this work, fully conjugated block copolymers are synthesized in a one-pot reaction through Stille coupling polycondensation, by utilizing the end-functional polymer copolymerization method. End-functional P3HT are copolymerized with AA (2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole) and BB (4,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole, TBT) type monomers, respectively. The orthogonal solubility between the very soluble P3HT donor and the insoluble PCDTBT acceptor block improves the purity of block copolymers as well as distinct nano-scale phase-separation compared with other reports on miscibility of donor and acceptor polymer block. Further purification via preparative GPC is carried out to remove the excess of unreacted P3HT and free PCDTBT as well as to achieve low polydispersity of block copolymers. The chemical structure of the P3HT-b-PCDTBT block copolymers are verified via 1H-NMR, and further confirmed by FTIR spectra. The block copolymer shows broad absorption and moderate optical band gap of 1.8 eV. Furthermore, the fully conjugated block copolymer films exhibit significant fine structures, much smoother film morphology compared to P3HT/PCDTBT polymer blends. By adding a small amount of block copolymer P3HT-b-PCDTBT as a compatibilizer into the bulk-heterojunction of P3HT:PC61BM blends, polymer solar cells with an 8% increase of short circuit current (J sc and 10% increase of power conversion efficiency (PCE) are achieved owing to the improvement of the active-layer film morphology. To the best of our knowledge, this is the first report on donor-acceptor type fully conjugated block copolymer as an effective ternary additive in polymer: fullerene bulk heterojunction solar cells.

Dark carrier dynamics and electrical characteristics of organic solar cells integrated with Ag-SiO2 core-shell nanoparticles

Short circuit current increase in organic solar cells due to increased light absorption, mediated by metal nanoparticles is widely reported, but the underlying influence of changes in electrical properties of dark carriers and trap states in organic solar cells is not well explored. In this work, Ag-SiO2 core-shell nanoparticles in an optimized blend with ZnO sol-gel was used to improve device performance of PCPDTBT:PC70BM based organic solar cells and allowed direct examination of the electrical properties while isolating optical effects. Impedance spectroscopy revealed dopant-induced dark carriers from impurities in the active layer or surface trap states. The modified ZnO devices containing dopant-induced dark carriers exhibited enhanced carrier mobility and reduced lifetime compared to reference devices. We attribute the reduced dark carrier lifetime to short-lived and shallow trap states that reduce monomolecular recombination under illuminated operating conditions, thereby improving charge extraction efficiency. A combination of changes in dark carrier dynamics and improvements in electrical properties was responsible for the improved fill factor and efficiency of devices with Ag-SiO2 core-shell nanoparticles.

Locally placed nanoscale gold islands film within a TiO2 photoanode for enhanced plasmon light absorption in dye sensitized solar cells

As metal nanostructures demonstrated extraordinary plasmon resonance, their optical characteristics have widely been investigated in photo-electronic applications. However, there has been no clear demonstration on the location effect of plasmonic metal layer within the photoanode on both optical characteristics and photovoltaic performances. In this research, the gold (Au) nano-islands (NIs) film was embedded at different positions within the TiO2 nanoparticulate photoanode in dye-sensitized solar cells (DSSC) to check the effect of plasmon resonance location on the device performance; at the top, in the middle, at the bottom of the TiO2 photoanode, and also at all the three positions. The Au NIs were fabricated by annealing a Au thin film at 550 °C. The DSSC having the Au NIs-embedded TiO2 photoanode exhibited an increase in short circuit currents (Jsc) and power conversion efficiency (PCE) owing to the plasmon resonance absorption. Thus, the PCE was increased from 5.92% (reference: only TiO2 photoanode) to 6.52% when the Au NIs film was solely positioned at the bottom, in the middle or at the top of TiO2 film. When the Au NIs films were placed at all the three positions, the Jsc was increased by 16% compared to the reference cell, and consequently the PCE was further increased to 7.01%.

Spark discharge in conductive liquid with microbubbles

Pulse electrical breakdown in 15% water solution of Isopropyl alcohol with air microbubbles from a pointed anode has been studied experimentally. It is shown, that the breakdown is always initiated from the bright region near the anode (anode “spot”). Detailed investigation into dynamic current-voltage characteristics and synchronized images reveals that it is thermal instability in the near anode region that causes spark channel initiation and development. The breakdown voltage, spark channel propagation speed and short-circuit current increase when the microbubbles are presented in the solution. The spark channel propagation speed is about 4-12 m/s and grows along with microbubbles concentration.

Theoretical framework for performance evaluation of silicon quantum dot solar cell under low concentration illumination

This article describes a silicon quantum dot (Si QD) solar cell with absorption enhancement due to quantum-confinement in the front-side emitter region, which helps in the improvement of the short-circuit current density. The Si QD solar cell is theoretically mimicked using an equivalent circuit to account the possible recombination losses under low concentration illumination. The electrical output is estimated using proposed theoretical model by considering the fundamental properties of intrinsic Si reported in the literature. An increase of ∼7.5% in generated current density is observed due to extra absorption of incident radiation through Si QD, which is in accordance with the experimental findings. The performance of Si QD/c-Si solar cell has been studied with respect to low concentration illumination, wherein with increasing CR from 1 sun to 5 suns, the short circuit current density increases from 29.4 to 147.2 mA/cm2 and the open circuit voltage increases from 0.565 to 0.609 V. This study provides a theoretical framework for design optimization of a future Si QD solar cells to achieve better performance of the device.

Soft imprinted Ag nanowire hybrid electrodes on silicon heterojunction solar cells

We demonstrate enhanced efficiencies in front-contacted silicon heterojunction (SHJ) solar cells using silver nanowire-based hybrid electrodes. SHJ cells typically suffer from shading losses due to reflection from macroscopic sun-facing metal fingers, which must be closely spaced to avoid resistive losses in the transparent conductive electrode (TCE). Using substrate conformal imprint lithography (SCIL) we fabricate silver nanowire electrodes on practical scale (4.0 cm2) planar SHJ cells. These electrodes exhibit anomalous transmission and a 7-fold improvement in sheet conductance relative to a standard ITO layer, enabling larger finger spacings and reducing reflection losses without compromising the cell fill factor. Over 70% of the ITO is replaced with transparent SiNx, reducing the use of indium while improving the anti-reflective performance. Combined, the reduced shading and reflection raises the short circuit current density increases by 2.1 mA cm−2, yielding an absolute increase in cell efficiency of 1.0%. These engineered hybrid electrodes provide a practical pathway towards front-contacted SHJ cells with a reduced dependence on rare metals and high efficiencies.

Nanostructured organosilicon luminophores in highly efficient luminescent down-shifting layers for thin film photovoltaics

We use a new type of solution processed luminescent molecules – nanostructured organosilicon luminophores (NOLs) – in printed luminescent down-shifting layers for photovoltaic light harvesting. We show that NOLs exhibit a high luminescent efficiency of 82–90% when added into two chemically different polymer matrices that are commonly used for photovoltaic encapsulation: polyvinyl butyral and ethylene-vinyl acetate. The coated layers are optimized to maximize both UV absorbance and visible light emission and transmittance. Attaching the luminescent layers onto a CIGS solar cell significantly improves the external quantum efficiency in the UV region: from 1% to 55%at 360nm. As a result, the short circuit current density and power conversion efficiency increase by up to 3.2–4.3%. After experimental verification of our optical simulation model, we employ it to determine the ideal molecular structure of NOLs for luminescent down-shifting applied to CIGS.

Application of nanostructured silver film in multilayer contact system of Ti/Mo/Ag silicon photoconverters

In this article we have synthesized multilayer contact systems Ti/Ag and Ti/Mo/Ag with nanostructured silver films using two vacuum methods, namely the method of electron-beam evaporation and HF magnetron sputtering. Using the data of atomic force microscopy, X-ray diffraction and mass spectroscopy of the secondary neutrals in the paper we have investigated the influence of the application method and of the annealing conditions on the grain formation processes in silver thin films and mass transfer between the metal layers of the contact system. The decrease of surface resistivity of the developed contacts and, respectively, the increase of short-circuit current of the heterojunction silicon solar cells have been determined.

Enhanced performance of thin-film amorphous silicon solar cells with a top film of 2.85 nm silicon nanoparticles

The performance of thin-film amorphous silicon-based n+-i-p+ solar cells in the presence of an exterior top thin film of mono-dispersed ultra-small 2.85nm diameter silicon nanoparticles has been examined. The silicon nanoparticles have a 530–700nm red luminescence band with a band peak at 628nm. Reflectance and the external/internal quantum efficiency (EQE/IQE) of the devices were measured over the range of 300–800nm. With the Si NPs there is a reduction in reflectivity, which improves the coupling of light to the solar cell. The enhancement in IQE is found to be over the spectral slice 360–610nm lying between the confinement and the direct band gaps. An enhancement in the IQE in the range of 390–530nm with a maximum of 31% at 445nm and a smaller one in the range of 530–610nm with a maximum of 11% at 590nm was observed. The increase in the spectral response corresponds to 11.42% increase in short circuit current in agreement with our J–V measurements. The IQE enhancement is explained in terms of two competing channels: excitonic radiative recombination down-conversion in the range 530–610nm and charge separation and collection in the range 360–530nm.

Improving the Efficiency of Poly-Crystalline Photovoltaic Module Using Special Transparent Glass Covers

Photovoltaic systems have become recently the most attractive and promising technology compared with other solar energy conversion devices. Solar energy reaches the earth’s surface in wavelengths between 0.300 and 2.50 μm. Silicon based photovoltaic systems convert only the wavelength between 0.35 to 0.82 μm of solar energy to electricity. The rest of incident solar radiation will be converted to heat, which will increase the operating temperature of the device and decrease the output power and efficiency. Maintaining the operating temperature of the PV systems at low and desired value was the main emphasis of different researches through the last decades. In this research, a special transparent glass cover (STGC) was used in order to cool down the photovoltaic module and, therefore, increase the output power and the efficiency of the module. The result showed that STGC led to 2.75% improvement in open circuit voltage, 9.6% increase in short circuit current, 26.4% increase in the output power, and 3% increase in the efficiency of the module.

Encapsulation of Tandem Organic Luminescence Solar Concentrator With Optically Transparent Triple Layers of SiO2/Epoxy/SiO2

Developing an organic luminescent solar concentrator (LSC), featuring ultralong lifetime and high transparency simultaneously, is crucial for building-integrated photovoltaic applications, such as solar energy harvesting clear windows. In this paper, a tandem organic LSC is encapsulated and connected with three optically transparent layers, namely an encapsulating epoxy layer and two insulating SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers that prevent dissolving the organic dyes into the epoxy layer. Experimental results demonstrate that the encapsulated organic LSC maintains the high average transmission of 60% in the visible range of 390-750 nm, and has an ultralong lifetime of ~ 6.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> h under illuminated test in laboratory environment, which is around five times longer than that of the organic LSC without any encapsulation. In addition, experiments confirm that most of the photoluminescence radiation generated in the organic dyes is trapped in the high-index SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /epoxy/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> structure, and guided between the glass substrate before emerging from the four edges of the organic LSC sample for conversion to electricity. A 30% increase in short-circuit current is attained, in comparison with a similar unencapsulated organic LSC structure.

Numerical simulation of silicon heterojunction solar cells with Si/Si1-xGex quantum wells

Heterojunction with intrinsic thin-layer (HIT) solar cells attract attention due to their high open circuit voltage and stable performance. However, short circuit current density is difficult to improve due to light losses of transparent conductive oxide and hydrogenated amorphous silicon passivation (a-Si:H) layer and low absorption coefficient of crystalline silicon (c-Si). Silicon germanium alloy (Si/Si1-xGex) quantum wells and quantum dots are capable of improving low light utilization by strong optical absorption in the infrared region. In this article, opto-MoS2of the HIT solar cells integrated with Si/Si1-xGex quantum wells (HIT-QW) as a surface absorber are investigated by numerical simulation with Technology Computer Aided Design (TCAD). The influences of germanium content on the MoS2of HIT solar cells with long carrier lifetimes of Si1-xGex layers (p*) and defect-free a-Si:H/c-Si interface are investigated at first. The simulation results indicate that optical utilization in the infrared region is enhanced with the increase of germanium fraction, while open circuit voltage degrades due to the decreasing of the energy band gap of Si1-xGex, radiative recombination and auger recombination mechanism in the Si/Si1-xGex quantum wells. And the conversion efficiency reaches a maximum value at a germanium fraction of 0.25 then drops distinctly. When the germanium fraction increases from 0 to 0.25, the short circuit current density increases from 34.3 mA/cm2 to 34.8 mA/cm2, while the open circuit voltage declines from 749 mV to 733 mV. Hence, the conversion efficiency increases from 21.5% to 21.7% due to the fact that the enhancement of short circuit current density compensates for the reduction of open circuit voltage. When the germanium content increases to more than 50%, a serious open circuit voltage loss of more than 130 mV associated with the energy band gap loss of Si1-xGex arises in the HIT-QW solar cells, which indicates that the dominating carrier transport mechanism changes from shockley diffusion to recombination in the Si/Si1-xGex quantum wells. Subsequently, the influences of interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si1-xGex quantum wells are discussed. Both interface holes at a-Si:H/c-Si interface and bulk holes in Si1-xGex quantum wells can be recombined through the interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si1-xGex quantum wells, respectively, which restricts the position of hole fermi level in the open circuit condition. When the germanium fraction increases, the influence of interface defects at a-Si:H/c-Si interface becomes weak on the degradation of open circuit voltage compared with the significant influence of the bulk trap centers. Moreover, p* of longer than 510-5 s is necessary for the retention of electrical performance of HIT-QW solar cells by the simulation. Based on this research, high-efficiency HIT solar cells can be achieved by incorporating high-quality Si/Si0.75Ge0.25 quantum wells, which also requires the impactful passivation of a-Si:H/c-Si interface.

Improvement of Jsc in a Cu2ZnSnS4 Solar Cell by Using a Thin Carbon Intermediate Layer at the Cu2ZnSnS4/Mo Interface

Back contact modification plays an important role in improving energy conversion efficiency of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, an ultrathin carbon layer is introduced on molybdenum (Mo)-coated soda lime glass (SLG) prior to the deposition of CZTS precursor to improve the back contact and therefore enhance CZTS solar cell efficiency. By introducing this layer, the short circuit current (Jsc) and device conversion efficiency increase for both nonvacuum (sol-gel) and vacuum (sputtering) methods. Specifically, for the sol-gel based process, Jsc increases from 13.60 to 16.96 mA/cm(2) and efficiency from 4.47% to 5.52%, while for the sputtering based process, Jsc increases from 17.50 to 20.50 mA/cm(2) and efficiency from 4.10% to 5.20%. Furthermore, introduction of this layer does not lead to any deterioration of either open circuit voltage (Voc) or fill factor (FF).

GaAs quantum dot solar cell under concentrated radiation

Effects of concentrated solar radiation on photovoltaic performance are investigated in well-developed GaAs quantum dot (QD) solar cells with 1-Sun efficiencies of 18%–19%. In these devices, the conversion processes are enhanced by nanoscale potential barriers and/or AlGaAs atomically thin barriers around QDs, which prevent photoelectron capture to QDs. Under concentrated radiation, the short circuit current increases proportionally to the concentration and the open circuit voltage shows the logarithmic increase. In the range up to hundred Suns, the contributions of QDs to the photocurrent are proportional to the light concentration. The ideality factors of 1.1–1.3 found from the VOC-Sun characteristics demonstrate effective suppression of recombination processes in barrier-separated QDs. The conversion efficiency shows the wide maximum in the range of 40–90 Suns and reaches 21.6%. Detailed analysis of I-V-Sun characteristics shows that at low intensities, the series resistance decreases inversely proportional to the concentration and, at ∼40 Suns, reaches the plateau determined mainly by the front contact resistance. Improvement of contact resistance would increase efficiency to above 24% at thousand Suns.

Increased Power Conversion Efficiency of Dye-Sensitized Solar Cells with Surface-Modified Photoelectrode

A nanoporous TiO2 film was modified with an aqueous potassium chloride (KCl), and the resulting film was applied to the photoelectrode of dye-sensitizes solar cell (DSSC). The DSSC with KCl-modified photoelctrode showed an increase in short circuit current (Jsc) and open circuit voltage (Voc), resulting in a power conversion efficiency (PCE) of 7.04%, compared to that (6.36%) of reference device with bare TiO2. The suppression of charge recombination between photo-injected electrons and triiodide (I3−) ions was found to increase both Jsc and Voc of the device with KCl-modified photoelctrode.

Nanocrystalline Silicon Oxide Emitters for Silicon Hetero Junction Solar Cells

In this study we developed a hydrogenated nanocrystalline silicon oxide (p)nc-SiOx:H as an emitter window layer for silicon heterojunction solar cells. We investigated the variation of refractive index and crystalline volume fraction at different growth conditions by Plasma Enhanced Chemical Vapor Deposition (PECVD) and we show that combining a low refractive index (n ∼ 2.65) and low parasitic absorption the (p)nc-SiOx:H emitter can replace the standard (p)a-Si:H, which leads to a short circuit current increase of up to 4%. We also show a method to reduce the incubation layer thickness in the initial stage of growth using a CO2 plasma treatment of the intrinsic amorphous layer surface prior to the emitter deposition. Lifetime measurements prove that the plasma treatment and the emitter layer deposition did not compromise the passivation layer quality. Moreover, in order to improve the p-emitter/n-type TCO contact, a highly doped nc-Si:H layer is implemented on top of the emitter, which leads to lower series resistance (Rs,light) and higher fill factor (FF) without affecting the open circuit voltage (Voc).

Neural control of submucosal gland and apical membrane secretions in airways.

The mechanisms that lay behind the low-level secretions from airway submucosal glands and the surface epithelium in the absence of external innervation have been investigated in small areas (1.0–1.5 cm2) of mucosa from sheep tracheas, freshly collected from a local abattoir. Glandular secretion was measured by an optical method while short circuit current was used as a measure of surface secretion. Activation of neurones in the intrinsic nerve net by veratrine alkaloids caused an immediate increase in both glandular secretion and short circuit current, both effects being blocked by the addition of tetrodotoxin. However, agents known to be acting directly on the glands, such as muscarinic agonists (e.g., carbachol) or adenylate cyclase activators (e.g., forskolin) were not influenced by tetrodotoxin. The toxin alone had no discernable effect on the low-level basal secretion shown by unstimulated glands. Calu-3 cell monolayers, generally agreed to be a surrogate for the secretory cells of submucosal glands, showed no sensitivity to veratrine alkaloids, strengthening the view that the veratrine-like drugs acted exclusively on the intrinsic nerve net. The data are discussed in relation way in which transplanted lungs can maintain mucociliary clearance and hence a sterile environment in the absence of external innervation, as in transplanted lungs.

Interplay Between Side Chain Pattern, Polymer Aggregation, and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk‐Heterojunction Solar Cells

Poly(benzo[1,2‐b:4,5‐b′]dithiophene–alt–thieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C71‐butyric acid methyl ester (PC71BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.

Antireflection coatings for solar panel power output enhancement

ABSTRACTThe impact of nanostructured broadband antireflection (AR) coatings on solar panel performance has been projected for a broad range of panel tilt angles at various locations. AR coated films have been integrated on test panels and the short-circuit current has been measured for the entire range of panel tilts. The integration of the AR coatings resulted in an increase in short-circuit current of the panels by eliminating front sheet reflection loss for a broad spectrum of light and wide angle of light incidence. The short-circuit current enhancement is 5% for normal light incidence and approximately 20% for off-angle light incidence. The National Renewable Energy Laboratory (NREL) System Advisor Model (SAM) predicts that this AR coating can yield at least 6.5% improvement in solar panel annual power output. The greatest enhancement, approximately 14%, is predicted for vertical panels. The AR coating’s contributions to vertical mount panels and building-integrated solar panels are significant. This nanostructured broadband AR coating thus has the potential to lower the cost per watt of photovoltaic solar energy.

Bulk heterojunction organic solar cells fabricated by oblique angle deposition.

Bulk heterojunction organic solar cells based on small molecules are often fabricated by the co-deposition of donor and accepter materials on substrates placed horizontally. An intimate blend of donor and acceptor molecules is the common product in as-prepared samples. Using metal phthalocyanine as the donor and fullerene as the acceptor, we tilt the substrate to deposit the active layer. Both short circuit current density and open circuit voltage increase obviously in bulk heterojunction organic solar cells fabricated by oblique angle deposition. Analysis of the active layer reveals that phase segregation occurs in obliquely deposited bulk heterojunction layers. The mechanism of phase segregation is discussed. A change in the stacking style of fullerene molecules on metal phthalocyanine grains is proposed on the tilted substrate. This result provides a simple method to form phase segregation in bulk heterojunction layers.