Back Electrode Research Articles

Roll and roll-to-roll process scaling through development of a compact flexo unit for printing of back electrodes

We report manufacture of fully printed and coated polymer solar cells on a small scale roll-to-roll coater representing the intermediate scale between laboratory and pilot scale. We highlight the enormous span in scale between the laboratory scale and the intended industrial scale by a factor of >100.000 and detail how this enormous scale must be covered by equipment that follow the scale. Especially the intermediate scale between equipment that can fit inside a fume cupboard and the typical pilot equipment with a footprint having the size of a large room presents a challenge that comprises some of the most critical steps in the scaling process. We describe the development of such a machine that comprise web guiding, tension control and surface treatment in a compact desk size that is easily moved around and also detail the development of a small cassette based flexographic unit for back electrode printing that is parsimonious in terms of ink usage and more gentle than laboratory scale flexo units where the foil transport is either driven by the flexo unit or the flexo unit is driven by the foil transport. We demonstrate fully operational flexible polymer solar cell manufacture using this new roll and roll-to-roll (R3) approach and compare with the existing methods.

Toward Barrier Free Contact to Molybdenum Disulfide Using Graphene Electrodes

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm(2)/(V s) in MoS2 at low temperature.

Effect of spherical silver powders with different sizes in back electrode paste on the conversion efficiency of silicon solar cells

In the present paper, spherical silver powders of three different sizes (0.5–2.0 µm) and high dispersity were synthesized by a chemical reduction method using ascorbic acid as reducing agent and silver nitrate as oxidant. Then the properties of three back-side silver pastes made of spherical silver powders with three different sizes (D90 < 1.0, 1.5 and 2.0 µm) were investigated when these back-side silver pastes were printed and further formed to back-side electrodes on solar cells. The results showed that the surface consistency, weldability and welding tension of the back-side silver electrodes, as well as the photoelectric conversion efficiency of the solar cells increased with the decreasing of silver particle size. A series of comparative experiments confirmed that the spherical silver powder (a) with the size of D90 < 1.0 µm was optimal for making back-side silver paste. The average welding tension of formed back-side silver electrode and the photoelectric conversion efficiency of the solar cells have reached 7.6 N and 17.60 %, respectively, which meet the requirements of current commercialization for back-side silver pastes. In addition, the electrical performance of back-side silver pastes with different silver contents was also investigated. In order to reach the demands of the highest photoelectric conversion efficiency and the lowest cost for back-side silver pastes, the back-side silver pastes prepared by silver powder (D90 < 1.0 µm) with the content of 54.72 wt% was found to be optimal.

A novel method to prepare ZnO/Ni multilayer thin film used in back electrode for solar cell

In order to improve the conversion efficiency of the solar cell, the ZnO/Ni multilayer thin film used in back electrode was prepared through the electroless plating and sol–gel methods. The morphology, structure and resistivity of the electroless nickel coating were measured by scanning electron microscopy, X-ray diffraction, and four-point probe resistivity tester, respectively. It was found that the nickel coating presented crystalline structure with the lowest resistivity of 4.33×10−5Ωcm. The effect of the ZnO sol concentration, annealing temperature and the number of the spin-coating layers on the crystal structure, film morphology and photoelectric performance were also studied. The light trapping structure was found on the surface of ZnO layer when the sol concentration, annealing temperature and the number of the spin-coating layers were 0.75mol/L, 300°C and 4, respectively. The average transmittance in the visible light range (390–780nm) was about 1.06% by visible spectrophotometer. The ZnO/Ni multilayer film could be used in back electrode for solar cell and would improve the properties of the solar cell.

Plasmonic electrodes for bulk-heterojunction organic photovoltaics: a review

Here, we review recent progress on the integration of plasmonic electrodes into bulk-heterojunction organic photovoltaic devices. Plasmonic electrodes, consisting of thin films of metallic nanostructures, can exhibit a number of optical, electrical, and morphological effects that can be exploited to improve performance parameters of ultrathin photovoltaic active layers. We review the various types of plasmonic electrodes that have been incorporated into organic photovoltaics such as nanohole, nanowire, and nanoparticle arrays and grating electrodes and their impact on various device performance parameters. The use of plasmonic back electrodes can impact device performance in a number of ways because the mechanisms of performance improvements are often a complex combination of optical, electrical, and structural effects. Inverted bulk heterojunction device architectures have been shown to benefit from the multifunctionality of plasmonic back electrodes as they can minimize space-charge effects and reduce hole carrier collection lengths in addition to providing improved light localization in the active layer. The use of semi-transparent plasmonic electrodes can also be beneficial for organic photovoltaics as they can exhibit a variety of optical properties such as light scattering, light localization, extraordinary transmission of light, and absorption-induced transparency, in addition to providing an alternative to metal oxide–based transparent electrodes.

Fabrication of polycrystalline CdTe thin-film solar cells using carbon electrodes with carbon nanotubes

The effects of adding carbon nanotubes (CNTs) to carbon back electrodes in polycrystalline CdTe thin-film solar cells were investigated. The CNTs were prepared by arc discharge under atmospheric pressure. The conductivity of the obtained CNT film with a density of 1.65 g/cm3 was approximately 2.6 × 103 S/cm. In the CdTe solar cells using carbon back electrodes with CNTs, the fill factor (FF) was improved as a result of adding CNTs with a concentration of 1 to 5 wt %. The improvement of FF was mainly due to the decrease in the series resistance of the CdTe solar cell. Furthermore, the open-circuit voltage (VOC) was improved by the CNT addition. The improvement of VOC was probably due to the reduction of the back barrier at the back contact.

AgAl alloy electrode for efficient perovskite solar cells

We demonstrate an efficient mixed halide perovskite solar cell employing a thermally evaporated AgAl alloy as a back electrode.

Effects of bathocuproine/Ag composite anode on the performances of stability polymer photovoltaic devices

In this work, the composite anode of BCP/Ag replaces the composite anode of Ca/Al, and the PTB7:PC71BM acts an as active layer for polymer solar cells. Calcium (Ca) is not a desirable candidate as electron extraction layer (EEL) for long-term stability polymer solar cells (PSCs) on account of its nature of active metal. And then, due to the poor stability of Al, which is not a desirable candidate as electrode, the bathocuproine (BCP) layer acts as an exciton blocking layer in organic device such OLEDs and small molecule solar cells, which has a k value that is close to zero for a broad range of wavelengths. The Ag has the nature of better chemical stability and conductivity than Al. In the device architecture described below, we replace the typical back metal electrode composed of a thin Ca layer and a thicker Al electrode by a few nanometer thick bathocuproine (BCP) layer and a thick 150 nm Ag layer. We investigate the effects of BCP thickness on the power conversion efficiency (PCE) and stability. The results reveal that the photovoltaic performances are improved, and a PCE of 6.82% at the 5 nm of BCP thickness, higher than the PCE of Ca/Al acted composite anode, is achieved. The substitution of BCP for Ca, can largely enhance light harvesting and exhibits an optimal light absorption by the active layer. This enhanced reflectivity of the buffer layer/electrode back contact results in an increase of the short circuit current. Compared with the devices of Ca/Al composite anode, it increases Jsc and external quantum efficiency with BCP/Ag composite anode. At the same time, it has the better stability of BCP/Ag composite anode of device, and almost the same PCE decrease ratio as free BCP devices and significantly improves the stability compared with Ca/Al composite anode. The stability test shows the better stability of BCP/Ag as composite anode than that of Ca/Al composite anode. The PCE of the device with Ca/Al as composite anode rapidly decreases by about 70% after 50 hour servicing due to the poor stabilities of Ca and Al. The device with BCP/Ag as composite anode shows favorable stability, owing to the PCE moderate decrease by less than 30% after the same story time. Our results indicate that substitution of BCP/Ag for Ca/Al composite anode is an alternative candidate for high performance and longterm photo stability PSCs.

Modeling of Capacitive MEMS Microphone with Square Membrane or Plate Using Integral Method

An analytical approach describing the acoustic pressure field inside a thin layer of thermo-viscous fluid trapped between the square membrane or plate and planar backing electrode of the electrostatic receiver, which involves integral formulation, is presented. The use of integral formulation enables to choose between a model of membrane and plate easily and to avoid the limitations of the classical method whereby the acoustic field is expressed as a sum of eigenmodes coupled with modal expression of the diaphragm displacement. The analytically calculated pressure sensitivity of the receiver is compared to the numerically calculated one and the discrepancies are discussed.

Analysis on Bowing and Formation of Al Doped P+ Layer by Changes of Thickness of N-type Wafer and Amount of Al Paste

【In this study, in order to improve the efficiency of n-type monocrystalline solar cells with an Alu-cell structure, we investigate the effect of the amount of Al paste in thin n-type monocrystalline wafers with thicknesses of $120{\mu}m$ , $130{\mu}m$ , $140{\mu}m$ . Formation of the Al doped $p^+$ layer and wafer bowing occurred from the formation process of the Al back electrode was analyzed. Changing the amount of Al paste increased the thickness of the Al doped $p^+$ layer, and sheet resistivity decreased; however, wafer bowing increased due to the thermal expansion coefficient between the Al paste and the c-Si wafer. With the application of $5.34mg/cm^2$ of Al paste, wafer bowing in a thickness of $140{\mu}m$ reached a maximum of 2.9 mm and wafer bowing in a thickness of $120{\mu}m$ reached a maximum of 4 mm. The study's results suggest that when considering uniformity and thickness of an Al doped $p^+$ layer, sheet resistivity, and wafer bowing, the appropriate amount of Al paste for formation of the Al back electrode is $4.72mg/cm^2$ in a wafer with a thickness of $120{\mu}m$ .】

Band diagram construction of CdTe/Sb2Te3 interface using synchrotron radiation

The CdTe/Sb2Te3 interface was prepared by co-evaporation on an ultra-thin glass substrate and studied “in-situ” by photoelectron spectroscopy using a tunable synchrotron radiation photon source. The results indicated that the as-deposited antimony telluride was a mixture of Sb–Te phase and Sb2Te3 structure. The band diagram showed a conduction band offset (ΔECB) of 0.65eV at the interface. A thin layer of Sb2Te3 came into being on the surface of CdTe with a subsequent 300°C anneal. The sputter depth profile suggested that the annealed interface was actually constituted by the “substrate CdTe/interface Sb+Cd donors/inner Sb–Te mixture/surface Sb2Te3” structure. The band diagram gave ΔECB=1.35eV for the annealed interface. It was found that there was a thin barrier of 0.2eV induced by Sb+Cd donors for the hole transportation at the interface. To create a thin p+ layer between CdTe and the back electrode, a 50Å thick Sb2Te3 layer was used for the back contact to CdTe thin film solar cells. J–V characteristic showed that this thin p+ back contact greatly enhanced the cell performance.

Experimental and theoretical investigation of the superior contact properties of dielectrophoretically processed graphene and tantalum nitride electrodes

AbstractWe have explored the dielectrophoretic process (DEP) to reach large scale integration of field effect transistors based on multilayer graphene (GraFET) deposited between source and drain metal electrodes. In our devices graphene lays on top of a TaOx gate dielectric deposited on an n+ silicon substrate, which is used as the back gate electrode, while the metal electrodes are made of TaN. These GraFETs reached a maximum transconductance of −19 mS/mm, which is higher than in silicon MOSFETs. To explain the origin of such good performance at the atomic level, ab initio calculations were conducted assuming two different models of the graphene/TaN interface: Ta‐ and N‐terminated δ‐TaN[111] surfaces with one, two, three, and four layers of graphene on top. We have found that all graphene layers are considerably deformed due to strong interaction with both metal surfaces, and remain non‐planar up to the fourth layer. We have also found that, except for the first graphene layer on TaN, all other layers display the Dirac cones in their locally projected density of states (PDOS). The reason for the absence of the Dirac cones in the first layer, an indication of poor conductance for monolayer graphene on TaN, is the metal induced surface states and not graphene deformation. Finally, due to the large work function of N‐terminated TaN, the first few graphene layers closest to the interface are strongly p‐doped, while for the Ta‐terminated TaN which has low work function the first few graphene layers are slightly n‐doped. The better adherence of graphene on N‐terminated TaN and the strong p‐doping of the interfacial graphene layers may explain the measured low contact resistance.

Capacitance spectroscopy of Cu(In,Ga)Se2-based solar cells with a Pt back electrode

This work contributes to the discussion on the influence of the back electrode on the capacitance spectroscopy results of Cu(In,Ga)Se2-based solar cells. Devices with Mo and Pt back electrodes were investigated by means of admittance spectroscopy and carrier concentration profiling. We present arguments supporting the statement that admittance spectroscopy signals, which are commonly observed and reported, cannot be exclusively attributed to the non-ohmic character of the back electrode.

Photoelectrochemistry of photosystem I bound in nafion.

Developing a solid state Photosystem I (PSI) modified electrode is attractive for photoelectrochemical applications because of the quantum yield of PSI, which approaches unity in the visible spectrum. Electrodes are constructed using a Nafion film to encapsulate PSI as well as the hole-scavenging redox mediator Os(bpy)2Cl2. The photoactive electrodes generate photocurrents of 4 μA/cm(2) when illuminated with 1.4 mW/cm(2) of 676 nm band-pass filtered light. Methyl viologen (MV(2+)) is present in the electrolyte to scavenge photoelectrons from PSI in the Nafion film and transport charges to the counter electrode. Because MV(2+) is positively charged in both reduced and oxidized states, it is able to diffuse through the cation permeable channels of Nafion. Photocurrent is produced when the working electrode is set to voltages negative of the Os(3+)/Os(2+) redox potential. Charge transfer through the Nafion film and photohole scavenging at the PSI luminal surface by Os(bpy)2Cl2 depends on the reduction of Os redox centers to Os(2+) via hole scavenging from PSI. The optimal film densities of Nafion (10 μg/cm(2) Nafion) and PSI (100 μg/cm(2) PSI) are determined to provide the highest photocurrents. These optimal film densities force films to be thin to allow the majority of PSI to have productive electrical contact with the backing electrode.

Performance Improvement of Dye Sensitized Solar Cells by Using a Novel Operation Mode

A novel operation mode is proposed to enhance the photovoltaic performance of dye-sensitized solar cell (DSSC) by applying an external bias on the back electrode. The measured data shows that the photovoltaic conversion efficiency and open circuit voltage of DSSC are significantly improved by more than 38.8% and 5.8% under the stimulated AM 1.5 sunlight by using the biased operation mode compared to the conventional operation mode without an external bias applied. The improved photovoltaic performance could be attributed to the suppressed back reaction from the recombination between the photoelectrons produced from dyes and the I3- ions in electrolyte under the novel operation mode.

Creeping discharge developing on vegetable-based oil / pressboard interface under AC voltage

The behavior of creeping discharges developing on the pressboard surface was investigated experimentally in palm fatty acid ester oil, natural rapeseed oil and commercial mineral oil under variable ac voltages with 60 Hz, respectively. The oilimpregnated pressboard was used as the solid dielectric and it was immersed completely into the same oil. The tungsten needle electrode was installed in the one side of pressboard surface without the counter electrode as a high voltage electrode and a grounded fine copper rod was adhered tightly in the other side of pressboard as a back side electrode. The discharge shape, streamer extension, streamer velocity and tracking at the oil/pressboard interface were observed as a function of the time and voltage by using a still camera equipped with a night viewer. Simultaneously, the discharge current and dissipated energy were measured. The gas components dissolved in oils by the creeping discharge were also examined by the dissolved gas-in-oil analysis (DGA). In this paper, it is presented that there are distinctive events due to the occurrence of creeping discharges.

Growth of Mo Thin Films on Flexible Polymer and Metal Foil Substrates for Solar Cell Application

The Mo thin films have been used as the back electrodes of Cu (In,Ga)Se2and Cu2ZnSn (S,Se)4thin film solar cells. In this study, the Mo thin films were sputtered on flexible polyimide, polyethylene terephthalate, stainless steel foil, Ti foil, and Al foil substrates. The experimental results indicate that the Mo thin films with promising properties can grow on different flexible substrates. The favourable substrate material is stainless steel foil. By post-annealing, the average sheet resistance of Mo thin film on stainless steel foil substrate reduces to 2.05 Ω/sq.

Silver versus white sheet as a back reflector for microcrystalline silicon solar cells deposited on LPCVD‐ZnO electrodes of various textures

AbstractWe compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5‐µm‐thick low‐pressure chemical vapor deposition (LPCVD)‐ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD‐ZnO layer (with thickness below 200 nm). For this latter design, the optical loss in the nano‐rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar‐plasma treatment. Because of its superior lateral conductivity, the thin‐ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD‐ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin‐ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active‐layer thickness of only 1.1 µm. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Study on Si-Based LED Integrated Packaging with Vertical Structure

We study the influence on the luminous efficiency of Si-based LED with different chip spacing. It is found that the optimal chip spacing is 2mm. Furthermore, multi-chips LED with top and back electrodes, according to a certain permutations and combinations way, are packaged integrally on the independent silver reflective layers. We find the LED integrated package devices have higher driving voltage and lower driving current than traditional LED devices with top and back electrodes. In addition, our study shows that independent silver reflective layers can obtain a better luminous efficiency and thermal property.

Pseudomagnetic fields in a locally strained graphene drumhead

Recent experiments reveal that a scanning tunneling microscopy (STM) probe tip can generate a highly localized strain field in a graphene drumhead, which in turn leads to pseudomagnetic fields in the graphene that can spatially confine graphene charge carriers in a way similar to a lithographically defined quantum dot (QD). While these experimental findings are intriguing, their further implementation in nanoelectronic devices hinges upon the knowledge of key underpinning parameters, which still remain elusive. In this paper, we first summarize the experimental measurements of the deformation of graphene membranes due to interactions with the STM probe tip and a back gate electrode. We then carry out systematic coarse grained, (CG), simulations to offer a mechanistic interpretation of STM tip-induced straining of the graphene drumhead. Our findings reveal the effect of (i) the position of the STM probe tip relative to the graphene drumhead center, (ii) the sizes of both the STM probe tip and graphene drumhead, as well as (iii) the applied back-gate voltage, on the induced strain field and corresponding pseudomagnetic field. These results can offer quantitative guidance for future design and implementation of reversible and on-demand formation of graphene QDs in nanoelectronics.