High Sheet Resistance Research Articles

Mitigating Light and Elevated Temperature Induced Degradation in Multicrystalline Silicon Wafers and PERC Solar Cells Using Phosphorus Diffusion Gettering

Reducing light and elevated temperature induced degradation (LeTID) is important for the industrial success of PERC (passivated emitter and rear cell) solar cells fabricated on high‐performance multicrystalline silicon (multi‐Si) wafers. Although there has been a lot of progress in understanding the degradation kinetics, the defect(s) responsible for LeTID in multi‐Si wafers at elevated temperatures (≈80 °C, 1 Sun) has yet to be identified. In this study, the authors look at the possibility of using phosphorus diffusion gettering (PDG) for reducing LeTID in multi‐Si wafers and solar cells. By measuring light induced defect concentrations in multi‐Si wafers before and after LeTID, the authors observe that PDG can substantially reduce the average defect concentration. Trace element analysis using inductively coupled plasma mass spectrometry reveals that multi‐Si wafers from the edge of the ingot contain a high concentration of Cu, Ni and Ti in grains that degrade more than neighboring grains. To explore PDG for reducing LeTID in multi‐Si PERC solar cells, the authors fabricate cells with two different emitter profiles. Etching back a heavily diffused emitter to obtain a high sheet resistance is observed to improve the LeTID performance of the solar cells, an effect that is very likely related to a reduced impurity concentration within the wafer.

Electrolyte and Electrode Design for Dynamic Windows Based on Reversible Metal Electrodeposition

Dynamic windows, which allow electronic control over light and heat flux, enable responsive environments free of glare and unwanted heat. Electronic control over traditionally static performance characteristics like visible light transmittance and solar heat gain coefficients will allow dynamic windows to be integrated with algorithms to optimize for energy efficiency and comfort. Dynamic windows based on reversible metal electrodeposition (RME) represent an exciting, new class of electrochromic devices. Metals are ideal light-modulators for dynamic windows because they can be color neutral, inert, photostable, and opaque at 20-50 nanometer thicknesses. Our team recently demonstrated metal-based dynamic windows that boast cheap processing, fast switching, and stable performance over thousands of cycles on the 25 cm2 scale. Several technical challenges hinder further scalability of the technology. Indium tin oxide (ITO), the most ubiquitous transparent conductor, has a relatively high sheet resistance. By Ohm’s Law, maintaining a uniform current density across ITO electrodes necessitates a voltage drop proportional to the electrode area. Large-scale RME windows must be engineered with a tolerance to the voltage drop across the transparent electrodes in order to maintain optical uniformity. Additionally, undesired side reactions like the hydrogen evolution reaction (HER) set strict limits on the potentials that may be applied in aqueous electrolytes. The ideal system will enable uniform nucleation and growth of metal films over the potential range set by side reactions inherent to aqueous electrolytes, and the voltage drop inherent to transparent conductors. Prudent design of electrode structure and electrolyte composition enable uniform and reversible electrodeposition over large areas. First, we demonstrate a scalable, self-assembly process to fabricate transparent electrodes functionalized with Pt nanoparticles. The nanoparticles enable site-selective control over nucleation of metal deposits. Second, we establish selection criteria for the use of polymeric levelling agents in reversible metal electrodeposition systems. By understanding the binding affinity between levelling agents and metal ions in our system, we access a range of electrodeposit morphologies that offer tunable control over optical properties. Finally, we demonstrate 100 cm2 RME-based dynamic windows that switch uniformly from a clear state (60% transmission) to an opaque state (<5% transmission) in less than one minute. Figure 1

Transparent-flexible-multimodal triboelectric nanogenerators for mechanical energy harvesting and self-powered sensor applications

Triboelectric nanogenerators (TENGs) harvest and convert mechanical energy to electrical energy. TENGs that are transparent and flexible can be applied to various (opto-)electronic devices supporting finger- or pen-based touchscreen inputs. This paper presents a transparent, flexible TENG that harvests mechanical tapping energy (typically discarded) by simple placement on touchscreen devices. The developed TENG consists of flexible and transparent conducting electrodes (FTCE) with high transmittance (> 93%) and low sheet resistance (18.5 Ω/sq), and transparent 3D-hierarchical polydimethylsiloxane (PDMS) with porous pyramid-patterns. In this study, the developed TENG directly powered eight light-emitting diodes (LEDs) by harvesting the mechanical energy produced by tapping with a touch pen while playing a smartphone game. We also used the transparent TENG as a transparent single-electrode-based, self-powered raindrop detection sensor on a window for a smart home. Our results indicate that the proposed TENG can be used not only as an effective mechanical energy harvester for transparent, flexible, and next-generation optoelectronics devices but also as a self-powered sensor for future Internet-of-Things applications.

Investigation of Tellurite Based Fire-Through Dielectric (FTD) Ag Paste Contacts for Si Solar Cell with High Sheet Resistance Emitter

This paper reports the importance of PbO/TeO2 ratio in the formation of Ag2Te, which is responsible for increasing the reformed glass conductivity after sintering. The presence of Ag2Te, a degenerate semiconductor, seen in the Raman Spectra, after HNO3 etching of the gridline and its absence in the HF etched counterpart, supports this assertion. Thus, the increase glass conductivity after sintering has the potential of enabling formation of high quality contacts to emitter with high sheet resistance.

Study on the measurement accuracy of circular transmission line model for low-resistance Ohmic contacts on III-V wide band-gap semiconductors

The accuracy and error propagation for determining the low specific contact resistance of Ohmic contacts on III-V wide band-gap semiconductors based on the circular transmission line model have been analyzed and the validity of this method is discussed in detail. The accuracy is more susceptible to the factors including data fitting method, electrical measurement technique and contact area correction. By using the equations of the original circular transmission line model to extract the fitting parameters, the calculation accuracy is much improved and the inapplicability of the linear least-square fitting is prevented. To further improve the accuracy, a four-probe current-voltage measurement technique was adopted to reduce the parasitic series resistances and the uncertainty bound, especially for the Ohmic contact with low sheet resistance of the semiconductor. Moreover, we have studied the size effect of contact pads of patterns and demonstrated that contact area correction is necessary for the semiconductor with high sheet resistance. A comprehensive error analysis is also performed to fully understand all the impact factors on this advanced method of specific contact resistance measurement, which is benefit for device performance evaluation and failure analysis.

Thermally Stable, High Performance Transfer Doping of Diamond using Transition Metal Oxides

We report on optimisation of the environmental stability and high temperature operation of surface transfer doping in hydrogen-terminated diamond using MoO3 and V2O5 surface acceptor layers. In-situ annealing of the hydrogenated diamond surface at 400 °C was found to be crucial to enhance long-term doping stability. High temperature sheet resistance measurements up to 300 °C were performed to examine doping thermal stability. Exposure of MoO3 and V2O5 transfer-doped hydrogen-terminated diamond samples up to a temperature of 300 °C in ambient air showed significant and irreversible loss in surface conductivity. Thermal stability was found to improve dramatically however when similar thermal treatment was performed in vacuum or in ambient air when the oxide layers were encapsulated with a protective layer of hydrogen silsesquioxane (HSQ). Inspection of the films by X-ray diffraction revealed greater crystallisation of the MoO3 layers following thermal treatment in ambient air compared to the V2O5 films which appeared to remain amorphous. These results suggest that proper encapsulation and passivation of these oxide materials as surface acceptor layers on hydrogen-terminated diamond is essential to maximise their environmental and thermal stability.

Mechanical and electrical characterization of CVD-grown graphene transferred on chalcogenide Ge2Sb2Te5 layers

In the present work, we investigate some mechanical and electrical properties of a Chemical Vapor Deposition (CVD)-grown graphene layer transferred on 50 nm-thick Ge2Sb2Te5 chalcogenide and discuss the possible application of graphene as a scaled contact replacing metal lines in phase-change memory devices. At first, the graphene-chalcogenide interface was extensively investigated. The expected chemical composition was confirmed by means of Electron Energy Loss Spectrometry (EELS), and the absence of bond distortions in the chalcogenide layer after graphene transfer was proved through Raman spectroscopy. The latter evidenced also the presence of defects, further confirmed by Transmission Electron Microscopy (TEM) investigations. The quality of the contact stack was evaluated by means of the adhesion among layers and by the sheet resistance of the layers themselves. Scratch tests and numerical simulations revealed a stress distribution compatible with a failure of the interface where the graphene layer features a 2 %-void. Next, the contact resistance proved an effective good Ohmic quality with a high graphene sheet resistance, about 1400 Ohm/square, which suggests that a better optimized transfer process should be applied to reduce it more than one order of magnitude.

Graphene transparent conductive electrodes doped with graphene quantum dots-mixed silver nanowires for highly-flexible organic solar cells

Recent active studies on flexible photovoltaic cells strongly call for matchable flexible transparent electrodes. Graphene (GR) is one of the promising candidates as transparent conductive electrodes (TCEs) for flexible photovoltaic cells, but high sheet resistance of GR limits the efficiency of the cells. Here, we first fabricate GR TCEs doped with graphene quantum dots (GQDs)-mixed silver nanowires (Ag NWs) on polyethylene terephthalate substrates for highly-flexible organic solar cells (OSCs). With increasing doping concentration of GQDs to 0.03 g/L, the sheet resistance of the Ag NWs/GR TCE decreases to ∼92 Ω/sq whilst its work function increases to ∼4.53 eV, resulting in 3.66% power-conversion efficiency (PCE). In addition, the GQDs enhance the bending flexibility of the Ag NWs/GR TCEs, thereby maintaining the initial PCE of the OSCs over 90% even after 1000 bending cycles at a curvature radius of 4 mm.

Switching Vertical to Horizontal Graphene Growth Using Faraday Cage-Assisted PECVD Approach for High-Performance Transparent Heating Device.

Plasma-enhanced chemical vapor deposition (PECVD) is an applicable route to achieve low-temperature growth of graphene, typically shaped like vertical nanowalls. However, for transparent electronic applications, the rich exposed edges and high specific surface area of vertical graphene (VG) nanowalls can enhance the carrier scattering and light absorption, resulting in high sheet resistance and low transmittance. Thus, the synthesis of laid-down graphene (LG) is imperative. Here, a Faraday cage is designed to switch graphene growth in PECVD from the vertical to the horizontal direction by weakening ion bombardment and shielding electric field. Consequently, laid-down graphene is synthesized on low-softening-point soda-lime glass (6 cm × 10 cm) at ≈580 °C. This is hardly realized through the conventional PECVD or the thermal chemical vapor deposition methods with the necessity of high growth temperature (1000 °C-1600 °C). Laid-down graphene glass has higher transparency, lower sheet resistance, and much improved macroscopic uniformity when compare to its vertical graphene counterpart and it performs better in transparent heating devices. This will inspire the next-generation applications in low-cost transparent electronics.

Effects of diffusion process on potential induced degradation of silicon solar cells.

Potential induced degradation (PID) has recently been identified as one of the most important degradation mechanisms for silicon solar cells. It is widely considered that PID is closely related with the manufacture and application period of solar modules. In this study, the effects of diffusion sheet resistance on PID were verified and explained by testing the emitter doping profile, the minority carrier lifetime, the emitter saturation current, the electrical performance of different cells, and the PID process. With increasing sheet resistance of cells, the depth and saturation current density of the emitter both decreased, and the cell efficiency increased, whereas the PID phenomenon became serious. It was found that higher sheet resistance or thinner P-N junction could lead to higher PID sensitivity. Therefore, more attention should be paid to PID phenomenon as the photovoltaic industry develops in the direction of high sheet resistance.

Promising ITO-free perovskite solar cells with WO3–Ag–SnO2as transparent conductive oxide

Substituting indium-tin-oxide, one-pot deposition of WO3/Ag/SnO2films with high transmittance and low sheet resistance enables the formulation of high-performance perovskite solar cells.

Printing an ITO-free flexible poly (4-vinylphenol) resistive switching device

Resistive switching in a sandwich structure of silver (Ag)/Polyvinyl phenol (PVP)/carbon nanotube (CNTs)-silver nanowires (AgNWs) coated on a flexible PET substrate is reported in this work. Densely populated networks of one dimensional nano materials (1DNM), CNTs-AgNWs have been used as the conductive bottom electrode with the prominent features of high flexibility and low sheet resistance of 90 Ω/sq. Thin, yet uniform active layer of PVP was deposited on top of the spin coated 1DNM thin film through state of the art printing technique of electrohydrodynamic atomization (EHDA) with an average thickness of 170 ± 28 nm. Ag dots with an active area of ∼0.1 mm2 were deposited through roll to plate printing system as the top electrodes to complete the device fabrication of flexible memory device. Our memory device exhibited suitable electrical characteristics with OFF/ON ratio of 100:1, retention time of 60 min and electrical endurance for 100 voltage sweeps without any noticeable decay in performance. The resistive switching characteristics at a low current compliance of 3 nA were also evaluated for the application of low power consumption. This memory device is flexible and can sustain more than 100 bending cycles at a bending diameter of 2 cm with stable HRS and LRS values. Our proposed device shows promise to be used as a future potential nonvolatile memory device in flexible electronics.

An Implantable Transparent Conductive Film with Water Resistance and Ultrabendability for Electronic Devices.

Recently, instead of indium tin oxide, the random mesh pattern of metallic nanowires for flexible transparent conducting electrodes (FTCEs) has received a great amount of interest due to its flexibility, low resistance, reasonable price, and compliant processes. Mostly, nanowires for FTCEs are fabricated by spray or mayer coating methods. However, the metallic nanowire layer of FTCEs, which is fabricated by these methods, has a spiked surface roughness and low junction contact between the nanowires that lead to their high sheet resistance value. Also, the nanowires on FTCEs are easy to peel-off through exterior forces such as bending, twisting, or contact. To solve these problems, we demonstrate novel methods through which silver nanowires (AgNWs) are deposited onto a nanosize porous nitrocellulose (NC) substrate by electrophoretic deposition (EPD) and an opaque and porous substrate. Respectively, through dimethyl sulfoxide treatment, AgNWs on NC (AgNW/NC) is changed to the transparent and nonporous FTCEs. This enhances the junction contact of the AgNWs by EPD and also allows a permanent attachment of AgNWs onto the substrate. To show the mechanical strength of the AgNWs on the transparent nitrocellulose (AgNW/TNC), it is tested by applying diverse mechanical stress, such as a binding test (3M peel-off), compressing, bending, twisting, and folding. Next, we demonstrate that AgNW/TNC can be effectively implanted onto normal newspapers and papers. As paper electronics, light-emitting diodes, which are laminated onto paper, are successfully operated through a basic AgNW/TNC strip circuit. Finally, it is demonstrated that AgNW/TNC and AgNW/TNC on paper are water resistant for 15 min due to the insulation properties of the nonporous substrate.

Cu Microbelt Network Embedded in Colorless Polyimide Substrate: Flexible Heater Platform with High Optical Transparency and Superior Mechanical Stability.

Metal nanowires have been considered as essential components for flexible transparent conducting electrodes (TCEs) with high transparency and low sheet resistance. However, large surface roughness and high interwire junction resistance limit the practical use of metal wires as TCEs. Here, we report Cu microbelt network (Cu MBN) with coalescence junction and low surface roughness for next-generation flexible TCEs. In particular, the unique embedded structure of Cu MBN in colorless polyimide (cPI) film was achieved to reduce the surface roughness as well as enhance mechanical stability. The TCEs using junction-free Cu MBN embedded in cPI exhibited excellent mechanical stability up to 100 000 bending cycles, high transparency of 95.18%, and a low sheet resistance of 6.25 Ω sq-1. Highly robust Cu MBN-embedded cPI-based TCE showed outstanding flexible heater performance, i.e., high saturation temperature (120 °C) at very low voltage (2.3 V), owing to the high thermal stability of cPI and excellent thermal conductivity of the Cu MBN.

High aspect ratio silver grid transparent electrodes using UV embossing process

This study presents a UV embossing process to fabricate high aspect ratio silver grid transparent electrodes on a polymer film. Transparent electrodes with a high optical transmittance (93 %) and low sheet resistance (4.6 Ω/sq) were fabricated without any high temperature or vacuum processes. The strong adhesion force between the UV resin and the silver ink enables the fabrication of silver microstructures with an aspect ratio higher than 3. The high aspect ratio results in a low sheet resistance while maintaining a high optical transmittance. Multi-layer transparent electrodes were fabricated by repeating the proposed UV process. Additionally, a large-area of 8-inch touch panel was fabricated with the proposed UV process. The proposed UV process is a relatively simple and low cost process making it suitable for large-area production as well as mass production.

Progress in the development of industrial nPERT cells

This article addresses several issues and challenges that have been faced, when developing an industrially feasible n-type solar cell (n-PERT) process employing homogeneous gas-phase diffusions of front and back side. As a result of the presented developments and optimizations solar cell front side efficiencies close to 21% could be obtained.The first issue that has been investigated is the impact of rear side treatments, i.e. etching and diffusion, on the rear side contact formation. We have seen process instabilities in the rear side contacting step when moving to a back surface field with high sheet resistance, especially with current diamond sawed wafers becoming increasingly flat. The second challenge was to develop an edge isolation process that is superior to conventional laser edge isolation. Wet chemical edge isolation is used for the first time to obtain well defined reverse bias behavior. Less than -1.5 A were recorded at -12 V reverse bias. Furthermore, a major increase in efficiency was achieved by using Al-free Ag-paste for front- and back-side metallization. A significantly improved Voc demonstrates a reduced recombination current J0met. Finally we present the determination bifacial characteristics according to the Equivalent Irradiance method showing an effective efficiency ηeffective_BiFi20 of 24.6 % in applications with 20 % rear side illumination.

Transport properties of ultrathin YBa2Cu3O7−δ nanowires: A route to single-photon detection

We report on the growth and characterization of ultrathin YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have been also successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films, and critical current densities, which are only limited by vortex dynamics. The 10 nm thick YBCO nanowires without the Au capping present hysteretic current voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. Such bistability is associated in NbN nanowires to the presence of localized normal domains within the superconductor. The presence of the voltage switch, in ultrathin nanostructures characterized by high sheet resistance values, though preserving high quality superconducting properties, make our nanowires very attractive devices to engineer single photon detectors.

Si heterojunction solar cells employing graphene transparent conductive electrodes co-doped with gold chlorides and silver nanowires

Graphene/Si Schottky junction is highly attractive for its application in solar cells due to the excellence of graphene as a transparent conducting electrode. However, the high sheet resistance of the chemical vapor deposition-grown graphene films, mostly used for the solar cells, is still one of the principal factors limiting the power conversion efficiency of graphene/Si solar cells. Here, we report an approach of co-doping with gold (III) chloride (AuCl3) and silver nanowires (Ag NWs) to reduce the sheet resistance (Rs) of graphene as well as to control its work function (WG) for well-matched graphene/Si wafer junction. The molar concentration (nD) of AuCl3 is varied from 1 to 10 mM while the fraction (nA) of Ag NWs is fixed at 0.1 wt% for the fabrication of the solar cells, based on nA-dependent behaviors of Rs and WG. The co-doped graphene/n-Si Schottky solar cells show a maximum power-conversion efficiency of 7.01% at nD = 7.5 mM, resulting from the increased barrier height at the graphene/Si junction and the reduced sheet resistance/transmittance of graphene by the co-doping, as proved from the nD-dependent behaviors of open-circuit voltage, DC conductivity/optical conductivity ratio, series resistance, and external quantum efficiency.

One-Pot Synthesis and Purification of Ultralong Silver Nanowires for Flexible Transparent Conductive Electrodes.

Metal nanowires (NWs) have become the most promising candidates for the next generation of flexible transparent conductive electrodes (FTCEs), with high transmittance and low sheet resistance. In this work, ultralong silver NWs (Ag NWs), ∼220 μm (even larger than 400 μm) in length and ∼55 nm in diameter (aspect ratio: ∼4000), were synthesized via a one-pot polyol process using high molecular weight poly(vinylpyrrolidone) (Mw = 1 300 000) and an appropriate concentration of FeCl3 (12.5 μM) through hydrothermal reaction. The prepared Ag NWs were purified by a filter cloth (pore size: about 30 × 50 μm2) to remove the Ag nanoparticles and short-length Ag NWs. The FTCE based on the ultralong Ag NWs without any post-treatments exhibits low sheet resistance of 155.0 Ω sq-1 and transmittance of 97.70% at 550 nm. The outstanding performance of FTECs demonstrated that the ultralong Ag NWs are ideal materials for applications in flexible transparent optical devices.

Understanding the influence of tellurium oxide in front Ag paste for contacting silicon solar cells with homogeneous high sheet resistance emitter

This paper investigates TeO2, one of the front Ag paste additives, to understand its role in low contact and gridline resistances for screen-printed Si solar cell. It is concluded that TeO2 aids the reduction of molten glass frit viscosity during contact co-firing. This in turn, leads to uniform flow of molten glass frit, both in the gridline bulk and interface of gridline and SiNx. Therefore, the uniform wetting and etching of SiNx and consequently larger contact area of metal to Si compared to its counterpart without TeO2. Hence, the current transport mechanism from Si to gridline can be said to be both direct and tunneling. The Raman spectra showed a blue shift in the phase of the TeO2 after contact co-firing in the gridline bulk confirming a crystalline γ-TeO2.