Reduced Series Resistance Research Articles

Performance Optimization of Au-Free Lateral AlGaN/GaN Schottky Barrier Diode With Gated Edge Termination on 200-mm Silicon Substrate

In this paper, a further leakage reduction of AlGaN/GaN Schottky barrier diodes with gated edge termination (GET-SBDs) has been achieved by optimizing the physical vapor deposited TiN as the anode metal without severe degradation of ON-state characteristics. The optimized GET-SBD multifinger power diodes with 10 mm anode width deliver ~4 A at 2 V and show a median leakage of 1.3 μA at 25 °C and 3.8 μA at 150 °C measured at a reverse voltage of -200 V. The temperature-dependent leakage of Si, SiC, and our GaN power diodes has been compared. The breakdown voltage (BV) of GET-SBDs was evaluated by the variation of anode-to-cathode spacing (LAC) and the length of field plate. We observed a saturated BV of ~600 V for the GET-SBDs with LAC larger than 5 μm. The GET-SBD breakdown mechanism is shown to be determined by the parasitic vertical leakage current through the 2.8 μm-thick buffer layers measured with a grounding substrate. Furthermore, we show that the forward voltage of GET-SBDs can be improved by shrinking the lateral dimension of the edge termination due to reduced series resistance. The leakage current shows no dependence on the layout dimension LG (from 2 to 0.75 μm) and remains at a value of ~10 nA/mm. The optimized Au-free GET-SBD with low leakage current and improved forward voltage competes with high-performance lateral AlGaN/GaN SBDs reported in the literature.

Enhanced Performance of Inverted Polymer Solar Cells by Combining ZnO Nanoparticles and Poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyfluorene)] as Electron Transport Layer

A highly efficient inverted polymer solar cell (PSC) has been successfully demonstrated by using a ZnO nanoparticle (NP) and poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyfluorene)] (PFN) bilayer structure as an effective electron collecting layer. This ZnO/PFN bilayer structure is designed to combine the advantages of both ZnO and PFN, based on the performance comparison of ZnO-only, PFN-only, and ZnO/PFN bilayer devices in our work. ZnO NPs can serve as an efficient electron transport and buffer layer for reduced series resistance, while the PFN interlayer can improve the energy level alignment of devices through the formation of an interfacial dipole. With the enhanced electron extraction induced by the ZnO/PFN bilayer structure and PTB7:ICBA:PC71BM ternary system, the corresponding inverted PSC device shows a high PCE of 9.3%, which is more than a 15% improvement compared to the ZnO- or PFN-only devices.

Binder-Free Graphene Organogels as Cost-Efficient Counter Electrodes for Dye-sensitized Solar Cells

Graphene organogels (GOGs) filled with organic electrolytes may function as high-activity, low-cost electrodes for energy conversion and storage devices such as Li ion batteries, supercapacitors, and dye-sensitized solar cells (DSSCs), because of their ideal electron-transport and ion-diffusion pathways through an interconnected 3D porous framework self-assembled from highly conductive and high-specific-area graphene sheets. Here, graphene hydrogels prepared by a modified hydrothermal method are converted into organogels with a specific surface area up to ∼1298m2g−1 by a simple solvent-exchange approach, and pressed onto titanium meshes to form GOG films as economical, wearable counter electrodes for DSSCs. Without optimizing TiO2 photoanodes, GOG-based DSSCs show a markedly enhanced short-circuit current density (16.34mAcm−2) and thus an impressive power conversion efficiency of 7.2%, higher than those using graphene aerogels (11.6mAcm−2, 5.9%) and commercial Pt films (10.2mAcm−2, 5.9%) as counter electrodes under otherwise identical conditions. The improved efficiency is ascribed to a substantial reduction in charge-transfer resistance and series resistance, which is correlated with the high conductivity and high specific area of GOGs.

High-power, high-efficiency 808 nm laser diode array

High-power, high-efficiency 808 nm laser diode arrays for pumping solid-state lasers have been widely used in industrial, scientific, medical and biological applications. The tendency of development of 808 nm laser diode pumping with high power, high efficiency and long lifetime is well-known. Diode-pumped solid-state system with high-efficiency laser diode array has many advantages such as compact volume, lower weight and energy saving. Currently, commercial 808 nm diode laser arrays with lower power conversion efficiency of about 50%-55%, due to the optical absorption losses for GaAs-based epitaxial materials, have been reported. In order to reduce series resistance and thermal resistance, heavily doped p-type waveguide and cladding layers are employed. However, the absorption loss on the free carriers in heavily doped p-type layers is dominant, leading to a lower power conversion efficiency. In order to achieve a high efficiency, the following requirements must be considered: improving the internal quantum efficiency by reducing the carrier leakage and increasing the electron injection efficiency; minimizing the voltage drop by optimizing the operating voltage; reducing the series resistance and thermal resistance of device; minimizing the internal loss including free-carrier absorption loss and scattering loss by designing optimized waveguide and cladding structure. In this paper, optimizing the epitaxial structure and fabricating technologies are demonstrated to achieve the high efficiency and high power. The asymmetric broad waveguide epitaxial structure with lower absorption loss in p-type waveguide and cladding layer is designed in order to achieve the above goals. The high-efficiency epitaxial structure is optimized including the thickness, doping and composition for each layer structure. The strained quantum well diode laser with lower transparency current and higher differential is of benefit to achieving the high power. A novel asymmetric broad waveguide structure is designed by optimizing the waveguide thickness and component of p-waveguide so as to reduce carrier absorption loss, the optical absorption loss in this epitaxial structure is achieved to be as low as 0.63 cm-1. The wafer is grown by metalorganic chemical vapor deposition on an n-GaAs substrate. The optimized growth conditions and substrates orientation are extensively studied to improve the crystal quality and reduce the internal loss and defects. The wafer is processed using standard procedures. For the fabricated 1-cm laser diode array mounted on P-side down on copper microchannel cooled heatsink, the device shows an output power of 150 W under an operating current of 135 A with an emitting wavelength of 809 nm, an operating voltage of 1.76 V, a slope efficiency of as high as 1.25 W/A, and maximum power conversion efficiency of as high as 65.5%, which is the highest level of 808 nm diode laser array with an output power of 150 W.

Improving the performance of perovskite solar cells with glycerol-doped PEDOT:PSS buffer layer**Project supported by the National Natural Science Foundation of China (Grant Nos. 61264002, 61166002, 91333206, and 51463011), the Natural Science Foundation of Gansu Province, China (Grant No. 1308RJZA159), the New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-13-0840), the Research Project of Graduate

In this paper, we investigate the effects of glycerol doping on transmittance, conductivity and surface morphology of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)) (PEDOT:PSS) and its influence on the performance of perovskite solar cells. . The conductivity of PEDOT:PSS is improved obviously by doping glycerol. The maximum of the conductivity is 0.89 S/cm when the doping concentration reaches 6 wt%, which increases about 127 times compared with undoped. The perovskite solar cells are fabricated with a configuration of indium tin oxide (ITO)/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al, where PEDOT:PSS and PC61BM are used as hole and electron transport layers, respectively. The results show an improvement of hole charge transport as well as an increase of short-circuit current density and a reduction of series resistance, owing to the higher conductivity of the doped PEDOT:PSS. Consequently, it improves the whole performance of perovskite solar cell. The power conversion efficiency (PCE) of the device is improved from 8.57% to 11.03% under AM 1.5 G (100 mW/cm2 illumination) after the buffer layer has been modified.

Efficiency enhancement of the MAPbI3−xClx-based perovskite solar cell by a two-step annealing procedure

The development of a novel two-step annealing method has contributed to the significantly improved performance of the MAPbI3−xClx based perovskite solar cell (PSC). By utilizing a two-step annealing method, we obtained a high power conversion efficiency (PCE) of 12.72% with a current density (Jsc) of 18.35 mA cm−2, an open circuit voltage (Voc) of 0.90 V and a fill factor (FF) of 0.71. Noticeably, the two-step annealed device shows no hysteresis and exhibits a PCE which is approximately 1.2 times greater than that of the one-step annealed device. The improvement in device efficiency is ascribed to the reduced series resistance, increased parallel resistance, better surface coverage, lower leakage current and stronger crystallization of the MAPbI3−xClx perovskite layer.

The impact of low Al-content waveguides on power and efficiency of 9xx nm diode lasers between 200 and 300 K

We present results on investigations of 9xx nm, GaAs-based diode lasers with 100 μm wide, 4 mm long stripes operating at temperatures between 200 and 300 K. As temperatures are reduced to 218 K, efficiency and power are improved. We analyze the characteristic parameters and subsequently mitigate the limiting factors by altering the vertical epitaxial design, seeking to further increase the optical output power and efficiency. The temperature dependence of internal parameters is obtained from length-dependent measurements, showing that the improved performance is due to an increased differential internal efficiency. Series resistance is confirmed as the main remaining limit to higher efficiency at high power. We show that the series resistance can be reduced by lowering the aluminum content in the AlxGa1−xAs waveguide which increases the carrier mobility. Although poor optical performance is seen at room temperature in low Al-content structures due to carrier leakage, at 218 K this is suppressed and is accompanied by significantly reduced series resistance. However, at high powers we observe further (non-thermal) power saturation in low Al-content structures even at 208 K, which limits the efficiency gain at high powers. Analysis of the series resistance of laser structures with different waveguide compositions at temperatures between 208 and 298 K reveals that series resistance is a function of the barrier height around the quantum well. This is taken as first evidence for carrier transport being a significant limit to electrical resistance. Finally, structures with an optimized Al-content are shown to maintain conversion efficiency >65% to output powers of 20 W, substantially higher than previously reported at 300 K (∼55% at 20 W).

Material and Device Improvement of GaAsP Top Solar Cells for GaAsP/SiGe Tandem Solar Cells Grown on Si Substrates

With its wide bandgap and good diode performance, GaAsP is an excellent candidate for the top cell in a silicon-based multijunction tandem device. Even though the material is not lattice matched to silicon, inclusion of a graded SiGe buffer between the GaAsP layer and the Si substrate has previously been demonstrated to enable lattice matching. The SiGe layer may then serve as a high-quality current-matched bottom cell to form a tandem dual-junction structure. This paper describes the design, fabrication, analysis, and improvement of the GaAsP top solar cell in a three-terminal GaAsP/SiGe tandem solar cell on a silicon substrate. Uncertified GaAsP top cell efficiencies have been improved from 8.4% to 18.4% with bandgap voltage offsets ( $W_{{\rm oc}}$ ) of 0.48 and 0.31 V under concentration factors of 1 and 20 ×, respectively. This progress is made by improved III–V material quality, reduced series resistance, and an addition of antireflection coating. Improving the optics, material quality, and fill factor (FF) should further improve the efficiency of the GaAsP top cell in this tandem structure grown on an Si substrate.

Cobalt/molybdenum ternary hybrid with hierarchical architecture used as high efficient counter electrode for dye-sensitized solar cells

Cobalt selenide/molybdenum selenide/molybdenum oxide (Co0.85Se/MoSe2/MoO3) ternary hybrid is synthesized by a facile hydrothermal method and used as efficient Pt-free counter electrode (CE) for dye-sensitized solar cells (DSSCs). Field emission scanning electron microscopy observes that Co0.85Se/MoSe2/MoO3 possesses hierarchical architecture with one-dimensional (1D) nanorods, two-dimensional (2D) nanosheet and three-dimensional (3D) nanoparticle. Cyclic voltammogram indicates that Co0.85Se/MoSe2/MoO3 hybrid have larger current density than sputtered Pt electrode. Electrochemical impedance spectroscopy shows that cooperation among Co0.85Se, MoSe2 and MoO3 reduces series resistance and charge transfer resistance. The DSSC based on Co0.85Se/MoSe2/MoO3 CE achieves a power conversion efficiency of 7.10%, which is higher than that of the DSSC based on sputtered Pt CE (6.03%).

Silver Nanowire Transparent Conductive Electrodes for High-Efficiency III-Nitride Light-Emitting Diodes.

Silver nanowires (AgNWs) have been successfully demonstrated to function as next-generation transparent conductive electrodes (TCEs) in organic semiconductor devices owing to their figures of merit, including high optical transmittance, low sheet resistance, flexibility, and low-cost processing. In this article, high-quality, solution-processed AgNWs with an excellent optical transmittance of 96.5% at 450 nm and a low sheet resistance of 11.7 Ω/sq were demonstrated as TCEs in inorganic III-nitride LEDs. The transmission line model applied to the AgNW contact to p-GaN showed that near ohmic contact with a specific contact resistance of ~10−3 Ωcm2 was obtained. The contact resistance had a strong bias-voltage (or current-density) dependence: namely, field-enhanced ohmic contact. LEDs fabricated with AgNW electrodes exhibited a 56% reduction in series resistance, 56.5% brighter output power, a 67.5% reduction in efficiency droop, and a approximately 30% longer current spreading length compared to LEDs fabricated with reference TCEs. In addition to the cost reduction, the observed improvements in device performance suggest that the AgNWs are promising for application as next-generation TCEs, to realise brighter, larger-area, cost-competitive inorganic III-nitride light emitters.

Inverted, semitransparent small molecule photovoltaic cells

We demonstrate semitransparent small molecule organic photovoltaic (OPV) cells based on inverted mixed and hybrid planar-mixed heterojunction (PM-HJ) structures comprised of a neat acceptor layer located beneath the donor/acceptor mixed region. The fill factor increases from 0.53 ± 0.01 for the mixed HJ to 0.58 ± 0.01 for the PM-HJ due to reduced series resistance, whereas the internal quantum efficiency increases from an average of 75% to 85% between the wavelengths of λ = 450 nm and 550 nm. The inverted, semitransparent PM-HJ cell achieves a power conversion efficiency of PCE = 3.9% ± 0.2% under simulated AM1.5G illumination at one sun intensity with an average optical transmission of T¯ = 51% ± 2% across the visible spectrum, corresponding to > 10% improvement compared with the mixed HJ cell. We also demonstrate an inverted semitransparent tandem cell incorporating two PM-HJ sub-cells with different absorption spectra. The tandem cell achieves a PCE = 5.3% ± 0.3% under simulated AM1.5G at one sun intensity with T¯ = 31% ± 1% across the visible. Almost identical efficiencies are obtained for tandem cells illuminated via either the cathode or anode surfaces.

Worldwide photovoltaic energy yield sensitivity from a variety of input losses

The energy yield of a system depends largely on how the photovoltaic (PV) panels perform at different weather conditions (particularly plane of array irradiance and module temperature) summed over the hourly meteorological data for a year at the given site. Some of the PV performance parameters such as ‘efficiency at low light/efficiency at standard conditions’ and the ‘P MAX thermal coefficient’ can vary between technologies and may be able to be improved by better production methods (for example, improving uniformity, reducing shunts, reducing series resistance or improving the thermal design). The sensitivity of energy yield (kilowatt hour/year) against PV input value will be site dependent, for example, high insolation sites will usually experience higher module temperatures so that improvements in temperature coefficients would be more beneficial at higher than at lower insolation. This study studies the effect of the sensitivity of energy yield at five different sites (from northern Europe to desert) of six different PV input values (including low-light efficiency, P MAX temperature coefficient and nominal operating cell temperature). It quantifies the benefits at each site of good temperature coefficients or improving the low-light efficiency and also the need for wide V MP inverter ranges at sites with wide seasonal temperature differences.

Carbon Nanotube-Based Supercapacitors with Excellent ac Line Filtering and Rate Capability via Improved Interfacial Impedance.

We report the fabrication of high-performance, self-standing composite sp(2)-carbon supercapacitor electrodes using single-walled carbon nanotubes (CNTs) as conductive binder. The 3-D mesoporous mesh architecture of CNT-based composite electrodes grants unimpaired ionic transport throughout relatively thick films and allows superior performance compared to graphene-based devices at an ac line frequency of 120 Hz. Metrics of 601 μF/cm(2) with a -81° phase angle and a rate capability (RC) time constant of 199 μs are obtained for thin carbon films. The free-standing carbon films were obtained from a chlorosulfonic acid dispersion and interfaced to stainless steel current collectors with various surface treatments. CNT electrodes were able to cycle at 200 V/s and beyond, still showing a characteristic parallelepipedic cyclic votammetry shape at 1 kV/s. Current densities are measured in excess of 6400 A/g, and the electrodes retain more than 98% capacity after 1 million cycles. These promising results are attributed to a reduction of series resistance in the film through the CNT conductive network and especially to the surface treatment of the stainless steel current collector.

Enhanced Performance of Polymeric Bulk Heterojunction Solar Cells via Molecular Doping with TFSA.

Organic solar cells based on bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]2NH) bulk doped poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT):C71-butyric acid methyl ester (PC71BM) were fabricated to study the effect of molecular doping. By adding TFSA (0.2-0.8 wt %, TFSA to PCDTBT) in the conventional PCDTBT:PC71BM blends, we found that the hole mobility was increased with the reduced series resistance in photovoltaic devices. The p-doping effect of TFSA was confirmed by photoemission spectroscopy that the Fermi level of doped PCDTBT shifts downward to the HOMO level and it results in a larger internal electrical field at the donor/acceptor interface for more efficient charge transfer. Moreover, the doping effect was also confirmed by charge modulated electroabsorption spectroscopy (CMEAS), showing that there are additional polaron signals in the sub-bandgap region in the doped thin films. With decreased series resistance, the open-circuit voltage (Voc) was increased from 0.85 to 0.91 V and the fill factor (FF) was improved from 60.7% to 67.3%, resulting in a largely enhanced power conversion efficiency (PCE) from 5.39% to 6.46%. Our finding suggests the molecular doping by TFSA can be a facile approach to improve the electrical properties of organic materials for future development of organic photovoltaic devices (OPVs).

Scalability of multi-junction organic solar cells for large area organic solar modules

We investigate the scalability of multi-junction organic photovoltaic cells (OPV) with device areas ranging from 1 mm2 to 1 cm2, as well as 25 cm2 active area solar modules. We find that the series resistance losses in 1 cm2 vs. 1 mm2 OPV cell efficiencies are significantly higher in single junction cells than tandem, triple, and four junction cells due to the lower operating voltage and higher current of the former. Using sub-electrodes to reduce series resistance, the power conversion efficiency (PCE) of multi-junction cells is almost independent of area from 1 mm2 to 1 cm2. Twenty-five, 1 cm2 multi-junction cell arrays are integrated in a module and connected in a series-parallel circuit configuration. A yield of 100% with a deviation of PCE from cell to cell of <10% is achieved. The module generates an output power of 162 ± 9 mW under simulated AM1.5G illumination at one sun intensity, corresponding to PCE = 6.5 ± 0.1%, slightly lower than PCE of discrete cells ranging from 6.7% to 7.2%.

Effects of Temperature on I-V Characteristics of InAs/GaAs Quantum-Dot Solar Cells

The current-voltage (I-V) characteristics of quantum-dot (QD) solar cells under illumination at various temperatures are presented. Stacked of high-density self-assembled InAs/GaAs QDs were incorporated into the Schottky-barrier-type solar cell structure. The I-V characteristics reveal that both short-circuit current and open-circuit voltage of the QD solar cell reduce when the measurement temperature increases. This result is unexpected and inconsistent with a basic solar cell theory where the temperature is believed to cause the enhancement of the short-circuit current. By considering the solar-cell circuit model, we can explain the obtained I-V curves by a high series resistance of the cell structure. Theoretical exclusion of the series resistance shows a substantial improvement of solar cell fill factor and efficiency. This work therefore suggests that reduction of series resistance by properly doping of the epitaxial layers can improve these devices.

Capacitive-Piezoelectric Transducers for High-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> Micromechanical AlN Resonators

A capacitive-piezoelectric (also known as, capacitive-piezo) transducer that combines the strengths of capacitive and piezoelectric mechanisms to achieve a combination of electromechanical coupling and Q higher than otherwise attainable by either mechanism separately, has allowed demonstration of a 1.2-GHz contour-mode aluminum nitride (AlN) ring resonator with Q > 3000 on par with the highest measured d 31-transduced AlN-only piezoelectric resonators past 1 GHz, and a 50-MHz disk array with an even higher Q > 12 000. Here, the key innovation is to separate the piezoelectric resonator from its metal electrodes by tiny gaps to eliminate metal material and metal-to-piezoelectric interface losses thought to limit thin-film piezoelectric resonator Q , while also maintaining high electric field strength to preserve a strong piezoelectric effect. While Q increases, electromechanical coupling decreases, but the k $^{2}_{\rm eff}{}\cdot {}$ Q product can still increase overall. More importantly, use of the capacitive-piezo transducer allows a designer to trade electromechanical coupling for Q , providing a very useful method to tailor Q and coupling for narrowband radio frequency (RF) channel-selecting filters for which Q trumps coupling. This capacitive-piezo transducer concept does not require dc-bias voltages and allows for much thicker electrodes that reduce series resistance without mass loading the resonant structure. The latter is especially important as resonators and their supports continue to scale toward even higher frequencies. [2013-0395]

Study the efficiency of single crystal CdTe/ZnCdS solar cell at various temperatures and illumination levels

Abstract CdTe is the best suited semiconductor for solar cells due to its band gap value 1.47 eV which is close to solar spectrum, low sublimation temperature and high absorption coefficient in the range of solar spectrum. To improve the photovoltaic performance of CdS/CdTe thin film solar cells, the CdS window layer is alloyed with different concentration of ZnS to reduce the resistivity and increase the band gap values. The single crystal CdTe based solar cell devices were prepared by vacuum evaporation method and have undergone for different temperature at various illumination levels to enhance the cell efficiency. We have achieved 14.37% efficiency and increased short circuit current density and open circuit voltage by reducing series resistance of the cell.

Studies on the Properties of Organic Photovoltaic Cells Using TiOx and DMDCNQI as Double Buffer Layers.

Various types of n-type buffer layers have been used in organic electronic devices. These buffer layers turned out to expedite carrier injection and reduce series resistance, leading to good performance of organic electronic devices. In our current work, we have fabricated organic photovoltaic (OPV) cells consisting of ITO/PEDOT:PSS/P3HT:PCBM/TiOx/DMDCNQI/AI which were fabricated in the presence of air. To incorporate the individual advantages of each n-type buffer layer, a DMDCNQI and TiOx layers were inserted to act as n-type double buffer layers. This leads to an increase of short-circuit current (JSC) and fill factor (FF) with good stability, in comparison to P3HT:PCBM based conventional cells. The results imply that the structures of double buffer layers can provide possible alternative to achieving high performance and air durability.

Novel dye-sensitized solar cell architecture using TiO2-coated Ag nanowires array as photoanode

With the aim of reducing series resistance and increasing dye loading, novel dye-sensitized solar cell architecture was designed with TiO2 nanoparticle-coated Ag nanowires array as the photoanode. Ag nanowire array was prepared by anodic aluminum oxide (AAO) template-assisted electrochemical deposition route. Then, Ag nanowires were coated by TiO2 nanoparticles in hydrothermal process. The structures of the photoanode were characterized by field emission scanning electron microscopy (FESEM). Ag nanowires are covered by a layer of very fine nanoparticles with a diameter of less than 5 nm. X-ray diffraction (XRD) and selected-area electron diffraction (SAED) show that Ag nanowires have a strong preferred orientation in (220) direction and the TiO2 coating layer is a polycrystalline structure. With this photoanode, 3.2 % conversion efficiency is achieved for the cell sensitized with N3 dye.