Haacke Figure Of Merit Research Articles

Effect of thermal shock on ZnO/Ag/ZnO nano-multilayers: Photoelectric property degradation

We investigated the degeneration at photoelectric properties of typical multilayer transparent conductive electrodes (TCEs) of ZnO/Ag/ZnO under thermal shock by experiments and simulation. The experimental data indicated a reduction in the visible region transparency and an increase in sheet resistance for ZnO/Ag/ZnO after undergoing thermal cyclic shock. This resulted in a significant decrease in the Haacke figure of merit (FOM) from 30.7 × 10-3 Ω−1 to 7.8 × 10-3 Ω−1. We identified oxygen diffusion into Ag at the ZnO/Ag interface using molecular dynamics (MD) simulations. Concurrently, density functional theory (DFT) simulations showed that interstitial oxygen creates impurity charge centers within Ag, indirectly causing the deterioration of the photoelectric properties of ZnO/Ag/ZnO. This finding highlights the challenge of maintaining optoelectronic stability in TCO/metal/TCO (OMO) thin films for practical applications and provides both experimental and theoretical support for designing highly reliable optoelectronic devices.

Simultaneous Enhancement in Visible Transparency and Electrical Conductivity via the Physicochemical Alterations of Ultrathin-Silver-Film-Based Transparent Electrodes.

A methodology for the simultaneous modulation of the chemical and physical states of an amorphous TiOx layer surface and its impact on the subsequent deposition of a polycrystalline Ag layer are presented. The smoothened TiOx layer surface comprising chemically altered, oxygen-deficient states serves as a nucleating platform for Ag deposition, facilitating a marked increase (∼75%) in the nucleation number density, which strongly enhances the wettability of ultrathin Ag layers. The physically smoothened TiOx/Ag interface further reduces the optical and electrical losses. When the proposed methodology is applied to TiOx/Ag/ZnO transparent conductive electrodes (TCEs), exceptional TCE properties are yielded owing to the simultaneous improvement in visible transparency and electrical conductivity; specifically, a record-high 0.22 Ω-1 Haacke figure of merit is realized. TCEs are prepared on flexible substrates to verify their applicability as stand-alone flexible transparent heaters and as integrated heaters within electrochromic devices to enhance color-switching reactions.

Performance assessment of TCO/metal/TCO multilayer transparent electrodes: from design concept to optimization

Versatile multilayer designs based on a transparent conductive oxide (TCO)/metal/TCO structure are proposed to overcome the trade-off between their electrical and optical properties. The overall performance of the investigated multilayer designs based on ZnO/metal/ZnO and indium tin oxide (ITO)/metal/ITO structures is compared based on their Haacke figure of merit (FoM) at λ = 550 nm. The influence of both the thickness and position of the inserted silver and gold ultrathin metallic layer (ML) on the electrode FoM is studied. To address the trade-off between transparency and conductivity, a new hybrid approach combining the proposed multilayer designs and particle swarm optimization is conducted. The optimized multilayer design with the ITO/Ag/ITO structure is found to open a new avenue towards the achievement of ultrahigh FoM values of 135 × 10−3 Ω−1, superior to those found to date, with a high transmittance above 95% and a reduced sheet resistance of 4.7 Ω × sq−1. This enhancement can be attributed to the dual effects of the enhanced light management induced by effectively modulating the ML geometry and the reduced sheet resistance. The proposed design methodology therefore bridges the gap between high transparency and low sheet resistance, becoming suitable for use in high-performance optoelectronic applications.

Role of the Interface between Ag and ZnO in the Electric Conductivity of Ag Nanoparticle-Embedded ZnO.

The addition of Ag nanoparticles (Ag NPs) with an average size of 30 nm into ZnO increases the electric conductivity up to 1000 times. While a similar increase in the conductivity is observed in a mixture of Ag nanoparticles and Al-doped ZnO (AZO) films, a physical mechanism underlying the change in electric conductivity is not the same for Ag NP-added ZnO and Ag NP-added AZO. In Ag NP-added ZnO, an ohmic junction is formed at the ZnO-Ag interface, and electrons are accumulated in ZnO near the ZnO-Ag interface until electron-rich islands are connected. However, in Ag NP-added AZO, electrons in Ag NPs move to the AZO matrix via thermionic emission and travel through the AZO matrix. This change in electron transport at ZnO-Ag and AZO-Ag interfaces is due to the fact that the work function of ZnO (4.62 eV) is larger than those of Ag (4.24 eV) and AZO (4.15 eV). An increase in Ag NP content in the ZnO matrix leads to the overlap of the electron accumulation regions and forms a percolation path for the electron transport without deteriorating the electron mobility. Hence, the electron concentration increases to 2.4 × 1020/cm3 in the 1.4 vol % Ag NP-added ZnO film. In addition, Ag NPs have a negligible effect on the transmittance, and the best Haacke figure of merit (ΦH) values are 2.86 and 5.18 for ZnO:Ag NP and AZO:Ag NP, respectively.

Electrical percolation threshold evaluation of silver thin films for multilayer WO3/Ag/WO3 transparent conductive oxide

In this work, we developed an algorithm capable of identifying the electrical percolation threshold from AFM images of silver (Ag) thin films with different thickness. The percolation threshold was first determined experimentally using electrical measurements. Subsequently, the AFM images, film thicknesses, and conductivity values were fed to the algorithm to correlate the morphology from the AFM images and determine the existence of electrical conductivity. Then, the algorithm was calibrated using the threshold values found experimentally. Finally, the minimum thickness necessary for electrical percolation of silver thin films was determined to be around 7 nm. This methodology eliminates the urgency to carry out electrical measurements once the morphological ones are available, providing a criterion that allows to decide whether a film is suitable for its use as an intermediate layer in multilayer TCOs. To validate these results, indium free room temperature TCOs multilayer WO3/Ag /WO3 were grown and the Haacke figure of merit (FOM) were calculated. A good quality WO3-based TCO was obtained with a FOM value of 1.11 × 10−2 Ω−1 for WO3(10 nm)/Ag(7 nm)/WO3(20 nm).

NTO/Ag/NTO multilayer transparent conducting electrodes for photovoltaic applications tuned by low energy ion implantation

Multilayer structures with optimized Nb (3.7 at.%) doped TiO2 (NTO) having the layers as NTO/Ag/NTO (NAN) were fabricated to obtain high optical transmittance and low electrical resistivity which could be a suitable replacement of the conventional transparent conducting electrodes used in the energy conversion and optoelectronics devices. These optimized pristine films of NAN layers were deposited by sputtering and implanted with 40 keV N+ ions with fluences ranging from 1 × 1014 to 1 × 1016 ions/cm2. The N+ ion implantation leads to the improvement in the electrical conductivity of NAN films, confirmed by the Hall measurement. The resistivity of pristine film is 9.6 × 10−5 Ω cm which decreases to 5.5 × 10−5 Ω cm after ion implantation for the N+ ion fluence of 1 × 1016 ions/cm2. Electrical transport properties were studied in the temperature range of 80–340 K, and the results show stable behavior of films. This substitutional doping causes narrowing of band gap and improvement in the electrical conductivity. The optimized NAN multilayer films show a low sheet resistance of 6.9 Ω/□ and a high transmittance of ∼81% for the 1 × 1016 ions/cm2 fluence. The Haacke figure of merit (FOM) of 18 × 10−3 Ω−1 was obtained for the highest fluence (1 × 1016 ions/cm2). The X-ray photoemission spectroscopy study of the implanted samples revealed substitution of Ti by Nb in NTO film and appearance of Ti3+ state. The work function of pristine NAN films, measured using ultravoilet photoemission spectroscopy (UPS) was found to 4.63 eV, which matches with the work function of active layer of photovoltaic cell. On implantation, the O ions are replaced by N+ ions. These results indicate that the NAN films with N+ ion implantation are suitable for potential transparent conducting electrode (TCE) applications in photovoltaics due to their high transmittance, low electrical resistivity and compatibility for growth of further layers.

Measurement-based optimization and analysis of α-IGZO/Ag/α-IGZO transparent conducting electrodes fabricated using DC magnetron sputter deposition

Transparent conducting electrodes have been systematically developed for optoelectronic devices and applications in industry through materials exploration and manufacturing process optimization. The nanostructural, optical and electrical properties were correlated with the embedded Ag interlayer thickness for the first time, for α-IGZO/Ag/α-IGZO trilayer electrodes fabricated using DC magnetron sputter deposition. Optimizing the Ag interlayer thickness at 14 nm achieves a maximum Haacke figure of merit (FOM) of 7.3 × 10−3 Ω−1, at least two orders of magnitude greater than that of the single IGZO film, with X-ray diffraction patterns indicating a crystalline phase that is also retained for thicker layers. Electrical and surface topography measurements confirm a uniform, continuous Ag interlayer film with bulk metallic behavior and resistivity as low as 6 × 10−5 Ω cm. Experimentally-calibrated finite-element modeling and ray tracing simulations unravel in the multilayer films, backscattering and transmission effects from random surface roughness, as well as phonon- or ionized impurity-scattering mediated free carrier absorption plus surface/interface scattering and interference effects. Moreover, film thickness-sensitive tuning of the optical bandgap between 2.98 and 3.14 eV was demonstrated, yielding a maximum bandgap and Haacke FOM at the optimized Ag interlayer thickness, and with many-body interactions outcompeting the Burstein-Moss effect above this thickness.

AZO/Ag/AZO transparent flexible electrodes on mica substrates for high temperature application

AZO/Ag/AZO transparent flexible electrodes were fabricated on mica substrates by magnetron sputtering at room temperature. Effect of Ag inserting layer thickness on the optical and electrical properties of AZO/Ag/AZO was investigated. The Haacke figure of merit (FOM) exhibited a maximum of 30.2 × 10−3 Ω−1 with 8 nm's Ag inner layer. It had low resistivity of 3.5 × 10−5 Ω cm, average transmittance values of 83.1% in the visible wavelength region. Moreover, thermal durability of AZO/Ag(8 nm)/AZO was evaluated by annealing in vacuum for 0.5 h at different temperature. The FOM can be enhanced while the sample was annealed below 350 ℃. The resistivity and transmittance are respectively 3.69 × 10−4 Ω cm and 87.0% even after high temperature annealing up to 750 ℃. In addition, there was no appreciable change in the sheet resistance of multilayers after a severe bending test. The results indicate that the flexible and withstanding high temperature AZO/Ag/AZO multilayer transparent electrodes grown on mica have potential application for the next-generation flexible optoelectronics.

Control of refractive index by annealing to achieve high figure of merit for TiO2/Ag/TiO2 multilayer films

We modified the refractive index (n) of TiO2 by annealing at various temperatures to obtain a high figure of merit (FOM) for TiO2/Ag/TiO2 (45nm/17nm/45nm) multilayer films deposited on glass substrates. Unlike the as-deposited and 300°C-annealed TiO2 films, the 600°C-annealed sample was crystallized in the anatase phase. The as-deposited TiO2/Ag/as-deposited TiO2 multilayer film exhibited a transmittance of 94.6% at 550nm, whereas that of the as-deposited TiO2/Ag/600°C-annealed TiO2 (lower) multilayer film was 96.6%. At 550nm, n increased from 2.293 to 2.336 with increasing temperature. The carrier concentration, mobility, and sheet resistance varied with increasing annealing temperature. The samples exhibited smooth surfaces with a root-mean-square roughness of 0.37–1.09nm. The 600°C-annealed multilayer yielded the highest Haacke's FOM of 193.9×10−3Ω−1.

Highly flexible Al-doped ZnO/Ag/Al-doped ZnO multilayer films deposited on PET substrates at room temperature

We investigated the effects of the Ag layer thickness on the electrical and optical properties of AZO (36nm)/Ag/AZO (36nm) multilayer films that were deposited on polyethylene terephthalate (PET) substrates using a radio frequency magnetron sputtering method. The AZO/Ag/AZO films had transmittances over 74–89% at 550nm. The relationship between the transmittance and the Ag layer thickness was investigated with three-dimensional finite-difference time-domain (3D FDTD) simulations to understand high transmittance. As the Ag layer thickness increased from 15 to 23nm, the carrier concentration increased from 5.84×1021 to 9.66×1021cm−3, while the sheet resistance decreased from 10.15 to 3.47Ωsq−1. The Haacke figure of merit (FOM) was calculated for the samples with various Ag layer thicknesses; it was a maximum at 19nm (43.9×10−3Ω−1). The resistance change for the 100nm-thick ITO only films was unstable even after 5 cycles, while that of the AZO (36nm)/Ag (19nm)/AZO (36nm) film remained constant up to 1000 cycles.

Al-doped ZnO/Ag/Al-doped ZnO multilayer films with a high figure of merit

In this study, we investigated the effects of the Al-doped ZnO (AZO)-layer thickness on the optical and electrical properties of AZO/Ag/AZO multilayer films deposited on glass substrates at room temperature. The optimal AZO/Ag/AZO (36nm/19nm/36nm) multilayer sample exhibited a transmittance of approximately 93% at 550nm. As the AZO-layer thickness increased from 9 to 45nm, the carrier concentration gradually decreased from 1.87×1022 to 6.36×1021cm−3, while the sheet resistance slightly increased from 3.86 to 4.47Ωsq−1 and the charge mobility increased from 24.15 to 25.42cm2V−1s−1. The samples had smooth surfaces with a root mean square (RMS) roughness ranging from 0.40 to 1.23nm. The Haacke figure of merit (FOM) was calculated for the samples as a function of the AZO-layer thickness. The optimal AZO/Ag/AZO multilayer film had the highest FOM of 99.9×10−3Ω−1.

Effect of Gold Thickness and Annealing on Optical and Electrical Properties of TiO2/Au/TiO2 Multilayers as Transparent Composite Electrode on Flexible Substrate

Multilayer structures of TiO2/Au/TiO2 have been deposited onto flexible substrates by room temperature sputtering to develop indium-free transparent composite electrodes (TCEs). The effect of Au thicknesses on optical, electrical properties and the mechanism of conduction have been discussed. The electrical conductivity of the TCEs is solely contributed by the middle metal layer. The critical thickness (t c) of the Au mid-layer to form a continuous conducting layer is 10 nm, and the multilayer has been optimized to obtain a sheet resistance of 12.2 Ω/sq and an average optical transmittance of 86% at 590 nm. The Haacke figure of merit (FOM) for t c has one of the highest FOM with 18 × 10−3 Ω−1. The samples were annealed in various environments for 24 h up to 150°C.

Optimization of TiO2/Cu/TiO2 Multilayer as Transparent Composite Electrode (TCE) Deposited on Flexible Substrate at Room Temperature

Highly transparent conductive composite electrodes made of multilayer of TiO2 and Cu (TiO2/Cu/ TiO2) thin films (30/3‐9/30nm thick) are deposited onto flexible substrates at room temperature. The micro-structural, optical and electrical properties of the multilayer are studied with the use of atomic force microscopy, uv-visible spectrophotometry and Hall measurement tool. The Haacke figure of merit (FOM) has been calculated to evaluate the performance of the multilayer films. The highest FOM achieved is 6.4 ×10 −3 � −1 for a Cu thickness of 6 nm with a sheet resistance of 19 � /sq and an average transmittance of 81%.

Effect of Ag layer thickness on the electrical transport and optical properties of ZnO/Ag/MoOx transparent composite electrodes and their use in P3HT:PC61BM-based organic solar cells

In this paper, the optoelectronic properties and morphological features of indium-free ZnO/Ag/MoOx electrodes deposited by in situ combination of RF and DC sputtering at room temperature for low-cost organic photovoltaics has been discussed. The transparent composite electrode (ZnO/Ag/MoOx) was optimized by inspecting the formation of a continuous Ag thin film on top of the ZnO layer. It was found that a 9nm thick Ag film had the best Haacke figure of merit. Organic solar cells on these composite electrodes revealed good optical and electrical properties, making them a promising alternative indium-free and PEDOT:PSS-free organic solar cells.

Transparent conductive Sb-doped SnO2/Ag multilayer films fabricated by magnetron sputtering for flexible electronics

The effects of an embedded silver layer and substrate temperature on the electrical and optical properties of Sb-doped SnO2 (ATO)/silver (Ag) layered composite structures on polyethylene naphthalate substrates have been investigated. The highest conductivity of ATO/Ag multilayer films was obtained with a carrier concentration of 1.5×1022cm−3 and a resistivity of 2.4×10−5Ωcm at the optimum Ag layer thickness and substrate temperature. The photopic averaged transmittance and Haacke figure of merit are 81.7%, and 21.7×10−3Ω−1, respectively. In addition, a conduction mechanism is proposed to elucidate the mobility variation with increased Ag thickness. We also describe the influence of substrate temperature on the structural, electrical and optical properties of the ATO/Ag multilayer films, and propose a mechanism for the changes in electrical and optical properties at different substrate temperatures.

High quality transparent TiO2/Ag/TiO2 composite electrode films deposited on flexible substrate at room temperature by sputtering

Multilayer structures of TiO2/Ag/TiO2 have been deposited onto flexible substrates by room temperature sputtering to develop indium-free transparent composite electrodes. The effect of Ag thicknesses on optical and electrical properties and the mechanism of conduction have been discussed. The critical thickness (tc) of Ag mid-layer to form a continuous conducting layer is 9.5 nm and the multilayer has been optimized to obtain a sheet resistance of 5.7 Ω/sq and an average optical transmittance of 90% at 590 nm. The Haacke figure of merit (FOM) for tc has one of the highest FOMs with 61.4 × 10−3 Ω−1/sq.

Optimization of IGZO/Cu/IGZO Multilayers as Transparent Composite Electrode on Flexible Substrate by Room-temperature Sputtering and Post-Deposition Anneals

ABSTRACTHighly transparent composite electrodes made of multilayers of In- and Ga-doped ZnO and Cu (IGZO/Cu/IGZO) thin films (30/3-9/30 nm thick) are deposited onto flexible substrates at room temperature and by using radio frequency magnetron sputtering. The effect of Cu thickness on the electrical and optical properties of the multilayer stack has been studied in accordance with the Cu morphology. The optical and electrical properties of the multilayers are studied with the UV–Vis spectrophotometry, Hall measurement and four point probe analyses. Results are compared with those from a single IGZO layered thin film. The average optical transmittance and sheet resistance both decreases with increase of copper thickness and has been optimized at 6 nm Cu middle layer thickness. The Haacke figure of merit (FOM) has been calculated to evaluate the performance of the films. The highest FOM achieved is 6 x 10-3 Ω-1 for a Cu thickness of 6 nm with a sheet resistance of 12.2 Ω/sq and an average transmittance of 86%. The multilayered thin films are annealed upto 150 °C in vacuum, forming gas and O2 environments and the optical and electrical properties are studied and compared against the as-deposited samples. Thus IGZO/Cu/IGZO multilayer is a promising flexible electrode material for the next-generation flexible optoelectronics.

Effect of Silver thickness and Annealing on Optical and Electrical Properties of Nb2O5/Ag/Nb2O5 Multilayers as Transparent Composite Electrode on Flexible Substrate

ABSTRACTMultilayer structures of Nb2O5/Ag/Nb2O5 have been deposited onto flexible substrates by sputtering at room temperature to develop indium free composite transparent conductive electrodes. The optical and electrical properties of the multilayers are measured by UV–Visible spectroscopy, Hall measurement and four point probe and the effect of Ag thickness has been studied. The critical thickness of Ag to form a continuous conducting layer is found to be 9.5 nm and the multilayer stack has been optimized to obtain a sheet resistance of 7.2 Ω/sq and an average optical transmittance of 86 % at 550 nm. The Haacke figure of merit (FOM) has been calculated for the films, and the multilayer with 9.5 nm thick Ag layer has the highest FOM with 31.5 x 10-3 Ω/sq, which is one of the best FOM reported till date for room temperature deposition on flexible substrates. The multilayered samples are annealed in vacuum, forming gas, air and O2 environments and the optical and electrical properties are compared against the as-deposited samples.

Optimization of Nb2O5/Ag/Nb2O5 multilayers as transparent composite electrode on flexible substrate with high figure of merit

Different multilayer structures of Nb2O5/Ag/Nb2O5 have been deposited onto flexible substrates by sputtering at room temperature to develop an indium free transparent composite electrode. The effect of Ag thickness on the electrical and optical properties of the multilayer stack has been studied in accordance with the Ag morphology. The critical thickness of Ag to form a continuous conducting layer is found to be 9.5 nm. A new conduction mechanism has been proposed to describe the conduction before and after the critical thickness. The effective Hall resistivity of the optimized films is as low as 6.44 × 10−5 Ω-cm with a carrier concentration and mobility of 7.4 × 1021 cm−3 and 13.1 cm2 /V-s, respectively, at the critical Ag layer thickness. The multilayer stack has been optimized to obtain a sheet resistance of 7.2 Ω/sq and an average optical transmittance of 86% at 550 nm without any substrate heating or post-annealing process. The Haacke figure of merit (FOM) has been calculated for the films, and the multilayer with a 9.5 nm thick Ag layer has the highest FOM at 31.5 × 10-3 Ω−1, which is one of the highest FOM values reported for TCE deposited at room temperature on a flexible substrate.

Unexpected improved conductivity and systematic low-temperature anomalies of a new germanium zinc indium oxide system

The structure, electrical conduction behaviour, optical transmittance and Haacke figure of merit (FOM) of the Ge-Zn-In-O compound are investigated for the first time. Certain Ge-Zn-In-O compounds with Ge/(Ge + Zn + In) atomic ratio even greater than 10%, denoted as a-GeZnInO, can have a surprisingly 2 times higher conductivity than indium zinc oxide (IZO) with industrial standard indium/zinc atomic ratio of 83/17 (IZO83/17). With varying Ge concentration, unexpected appearance and disappearance of the crystalline In2O3 phase and systematic low-temperature resistivity anomalies (semiconductor-metal transition) were observed and interpreted.