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Advances in Transparent Conducting Materials

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Advanced Composites".

Deadline for manuscript submissions: closed (31 May 2017) | Viewed by 74366

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Prof. Dr. Andrea Li Bassi

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Department of Energy, Politecnico di Milano, via Ponzio 34/3, 20133 Milano, Italy
Interests: growth of thin films and low-dimensional materials; optical and electronic properties of nanostructured oxides; nanomaterials for photoconversion and plasmonics; carbon nanostructures; vibrational spectroscopies; Scanning Tunneling Microscopy
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Prof. Dr. Carlo S. Casari

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Department of Energy, Politecnico di Milano, via Ponzio 34/3, 20133 Milano, Italy
Interests: nanostructured material growth; carbon nanostructures; structure, vibrational and electronic properties; nanostructured materials for energy applications
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Dear Colleagues,

Transparent conducting materials (TCM) are being investigated for the development of transparent electrodes in a wide variety of applications, ranging from photovoltaics and photocatalysis, photoelectrochemistry, transparent electronics, optoelectronics and light emitting diodes, to smart windows, flat panel displays, and touch screens.

Typically, conductivity of transparent materials is obtained by strong (degenerate) doping of wide-bandgap oxides. Traditionally, the most studied TCM are, thus, transparent conducting oxides (TCO) and some of them are commercially available and widely employed. However, current research involving different fields (from physics and chemistry to materials science and nanotechnology) still devotes extensive and renewed attention to this class of materials for a number of reasons:

the improvement of the material properties and performance often requires a compromise between electrical conductivity and transparency in different regions of the electromagnetic spectrum, so that there is still room for material optimization depending on the desired applications;
new TCM are today investigated or searched for, e.g., for cost reduction (In-free TCO; synthesis by solution processing instead of vacuum vapor deposition techniques); to address material stability in aggressive environments (e.g., TiO₂-based TCO for photoelectrochemical applications); to realize non-oxide TCM, based for instance on metal nanowire (NW) networks or 2D materials such as graphene; to develop p-type TCM (which are still far from applications but would be necessary, e.g., to realize transparent p-n junctions);
achievement of novel, additional functional properties beyond electrical conduction and transparency is often desirable: e.g. compatibility with plastic substrates or flexibility (amorphous TCOs, metal NWs, graphene), which requires low processing/synthesis temperatures; light scattering/trapping capability, that can be obtained by morphology modulation at the nano/mesoscale (e.g., using hierarchical nanostructures); large surface area/interface for the realization of diffuse heterojunctions, e.g., in organic/hybrid solar or photocatalysis devices; implementation of IR plasmonic effect; etc.;
a better understanding of the TCM physics, e.g., in terms of comprehension of the relationship between the complex and non-trivial defect chemistry and the electronic/optical functional properties, which of course would open the way to the possibility of a better engineering/tailoring of the properties.

This Special Issue is open to original and relevant contributions in this growing and strongly interdisciplinary field, addressing different aspects from synthesis, comprehension, material development, demonstration of novel functionalities, novel applications.

Prof. Dr. Andrea Li Bassi
Prof. Dr. Carlo Spartaco Casari
Guest Editors

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Keywords

multifunctional TCM
p-type TCO
non-oxide TCM
nanostructure-based approaches to TCM
defect chemistry of TCO
hierarchical nanostructures/mesoporous TCM
photovoltaics/photocatalysis applications of TCM
organic/hybrid devices
transparent electronics
TCM dielectrics
growth techniques
characterization techniques

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Published Papers (11 papers)

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Research

447 KiB

Open AccessArticle

Quantifying the Performance of P-Type Transparent Conducting Oxides by Experimental Methods

by Karsten Fleischer, Emma Norton, Daragh Mullarkey, David Caffrey and Igor V. Shvets

Materials 2017, 10(9), 1019; https://doi.org/10.3390/ma10091019 - 1 Sep 2017

Cited by 58 | Viewed by 8654

Abstract

Screening for potential new materials with experimental and theoretical methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early experimental stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estimated band gap. We discuss this using copper-deficient CuCrO

_{2}

as an example, as it was already shown to be a good p-type TCO grown at low temperatures. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment experimental methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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3742 KiB

Open AccessArticle

Characteristics of Carrier Transport and Crystallographic Orientation Distribution of Transparent Conductive Al-Doped ZnO Polycrystalline Films Deposited by Radio-Frequency, Direct-Current, and Radio-Frequency-Superimposed Direct-Current Magnetron Sputtering

by Junichi Nomoto, Katsuhiko Inaba, Shintaro Kobayashi, Takeshi Watanabe, Hisao Makino and Tetsuya Yamamoto

Materials 2017, 10(8), 916; https://doi.org/10.3390/ma10080916 - 9 Aug 2017

Cited by 16 | Viewed by 11545

Abstract

We investigated the characteristics of carrier transport and crystallographic orientation distribution in 500-nm-thick Al-doped ZnO (AZO) polycrystalline films to achieve high-Hall-mobility AZO films. The AZO films were deposited on glass substrates at 200 °C by direct-current, radio-frequency, or radio-frequency-superimposed direct-current magnetron sputtering at various power ratios. We used sintered AZO targets with an Al₂O₃ content of 2.0 wt. %. The analysis of the data obtained by X-ray diffraction, Hall-effect, and optical measurements of AZO films at various power ratios showed that the complex orientation texture depending on the growth process enhanced the contribution of grain boundary scattering to carrier transport and of carrier sinks on net carrier concentration, resulting in the reduction in the Hall mobility of polycrystalline AZO films. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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1602 KiB

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Deposition of Nanostructured CdS Thin Films by Thermal Evaporation Method: Effect of Substrate Temperature

by Nafiseh Memarian, Seyeed Mohammad Rozati, Isabella Concina and Alberto Vomiero

Materials 2017, 10(7), 773; https://doi.org/10.3390/ma10070773 - 9 Jul 2017

Cited by 61 | Viewed by 7088

Abstract

Nanocrystalline CdS thin films were grown on glass substrates by a thermal evaporation method in a vacuum of about 2 × 10⁻⁵ Torr at substrate temperatures ranging between 25 °C and 250 °C. The physical properties of the layers were analyzed by transmittance spectra, XRD, SEM, and four-point probe measurements, and exhibited strong dependence on substrate temperature. The XRD patterns of the films indicated the presence of single-phase hexagonal CdS with (002) orientation. The structural parameters of CdS thin films (namely crystallite size, number of grains per unit area, dislocation density and the strain of the deposited films) were also calculated. The resistivity of the as-deposited films were found to vary in the range 3.11–2.2 × 10⁴ Ω·cm, depending on the substrate temperature. The low resistivity with reasonable transmittance suggest that this is a reliable way to fine-tune the functional properties of CdS films according to the specific application. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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3927 KiB

Open AccessArticle

Comparative Study on ZnO Monolayer Doped with Al, Ga and In Atoms as Transparent Electrodes

by Dan Sun, Changlong Tan, Xiaohua Tian and Yuewu Huang

Materials 2017, 10(7), 703; https://doi.org/10.3390/ma10070703 - 26 Jun 2017

Cited by 18 | Viewed by 5022

Abstract

Transparent anodes are indispensable components for optoelectronic devices. Two-dimensional (2D) materials are attracting increasing research interest due to their unique properties and promising applications. In order to design novel transparent anodes, we investigated the electronic, optical, and electrical properties of 2D ZnO monolayers doped with Al, Ga, and In using the first-principles calculation in combination with the Boltzmann transport theory. When the doping concentration of Al, Ga, and In is less than 12.5 wt %, we find that the average transmittance reaches up to 99% in the visible and UV regions. Moreover, the electrical conductivity is enhanced for the Al, Ga, and In doped systems compared to that of the pristine ZnO monolayer. In particular, a good electrical conductivity with a significant improvement for the In doped ZnO monolayer is achieved compared to Al and Ga doping at the 6.25 wt % level. These results suggest that the ZnO monolayer based materials, and in particular the In doped ZnO monolayer, are promising transparent anodes for nanoscale electronic and optoelectronic applications. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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3326 KiB

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Tunable Multiple Plasmon-Induced Transparencies Based on Asymmetrical Grapheme Nanoribbon Structures

by Chunyu Lu, Jicheng Wang, Shubin Yan, Zheng-Da Hu, Gaige Zheng and Liu Yang

Materials 2017, 10(7), 699; https://doi.org/10.3390/ma10070699 - 26 Jun 2017

Cited by 6 | Viewed by 4075

Abstract

We present plasmonic devices, consisting of periodic arrays of graphene nanoribbons (GNRs) and a graphene sheet waveguide, to achieve controllable plasmon-induced transparency (PIT) by numerical simulation. We analyze the bright and dark elements of the GNRs and graphene-sheet waveguide structure. Results show that applying the gate voltage can electrically tune the PIT spectrum. Adjusting the coupling distance and widths of GNRs directly results in a shift of transmission dips. In addition, increased angle of incidence causes the transmission to split into multiple PIT peaks. We also demonstrate that PIT devices based on graphene plasmonics may have promising applications as plasmonic sensors in nanophotonics. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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1913 KiB

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Transparent Electrodes Based on Silver Nanowire Networks: From Physical Considerations towards Device Integration

by Daniel Bellet, Mélanie Lagrange, Thomas Sannicolo, Sara Aghazadehchors, Viet Huong Nguyen, Daniel P. Langley, David Muñoz-Rojas, Carmen Jiménez, Yves Bréchet and Ngoc Duy Nguyen

Materials 2017, 10(6), 570; https://doi.org/10.3390/ma10060570 - 24 May 2017

Cited by 63 | Viewed by 8832

Abstract

The past few years have seen a considerable amount of research devoted to nanostructured transparent conducting materials (TCM), which play a pivotal role in many modern devices such as solar cells, flexible light-emitting devices, touch screens, electromagnetic devices, and flexible transparent thin film heaters. Currently, the most commonly used TCM for such applications (ITO: Indium Tin oxide) suffers from two major drawbacks: brittleness and indium scarcity. Among emerging transparent electrodes, silver nanowire (AgNW) networks appear to be a promising substitute to ITO since such electrically percolating networks exhibit excellent properties with sheet resistance lower than 10 Ω/sq and optical transparency of 90%, fulfilling the requirements of most applications. In addition, AgNW networks also exhibit very good mechanical flexibility. The fabrication of these electrodes involves low-temperature processing steps and scalable methods, thus making them appropriate for future use as low-cost transparent electrodes in flexible electronic devices. This contribution aims to briefly present the main properties of AgNW based transparent electrodes as well as some considerations relating to their efficient integration in devices. The influence of network density, nanowire sizes, and post treatments on the properties of AgNW networks will also be evaluated. In addition to a general overview of AgNW networks, we focus on two important aspects: (i) network instabilities as well as an efficient Atomic Layer Deposition (ALD) coating which clearly enhances AgNW network stability and (ii) modelling to better understand the physical properties of these networks. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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5852 KiB

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Optical Design of Textured Thin-Film CIGS Solar Cells with Nearly-Invisible Nanowire Assisted Front Contacts

by Joop Van Deelen, Ahmed Omar and Marco Barink

Materials 2017, 10(4), 392; https://doi.org/10.3390/ma10040392 - 7 Apr 2017

Cited by 12 | Viewed by 4915

Abstract

The conductivity of transparent front contacts can be improved by patterned metallic nanowires, albeit at the cost of optical loss. The associated optical penalty can be strongly reduced by texturization of the cell stack. Remarkably, the nanowires themselves are not textured and not covered in our design. This was shown by optical modeling where the width of the nanowire, the texture height and the texture period were varied in order to obtain a good insight into the general trends. The optical performance can be improved dramatically as the reflection, which is the largest optical loss, can be reduced by 95% of the original value. The spectra reveal absorption in the Cu(In,Ga)Se₂ (CIGS) layer of 95% and reflection below 2% over a large part of the spectrum. In essence, a virtually black CIGS cell stack can be achieved for textured cells with a metal nanogrid. Moreover, it turned out that the ratio between the width of the nanowire and the height of the texture is a critical parameter for optical losses. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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2402 KiB

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Al-Doped ZnO Monolayer as a Promising Transparent Electrode Material: A First-Principles Study

by Mingyang Wu, Dan Sun, Changlong Tan, Xiaohua Tian and Yuewu Huang

Materials 2017, 10(4), 359; https://doi.org/10.3390/ma10040359 - 29 Mar 2017

Cited by 44 | Viewed by 8209

Abstract

Al-doped ZnO has attracted much attention as a transparent electrode. The graphene-like ZnO monolayer as a two-dimensional nanostructure material shows exceptional properties compared to bulk ZnO. Here, through first-principle calculations, we found that the transparency in the visible light region of Al-doped ZnO monolayer is significantly enhanced compared to the bulk counterpart. In particular, the 12.5 at% Al-doped ZnO monolayer exhibits the highest visible transmittance of above 99%. Further, the electrical conductivity of the ZnO monolayer is enhanced as a result of Al doping, which also occurred in the bulk system. Our results suggest that Al-doped ZnO monolayer is a promising transparent conducting electrode for nanoscale optoelectronic device applications. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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1001 KiB

Open AccessFeature PaperArticle

Codoping and Interstitial Deactivation in the Control of Amphoteric Li Dopant in ZnO for the Realization of p-Type TCOs

by Alessandra Catellani and Arrigo Calzolari

Materials 2017, 10(4), 332; https://doi.org/10.3390/ma10040332 - 23 Mar 2017

Cited by 14 | Viewed by 4636

Abstract

We report on first principle investigations about the electrical character of Li-X codoped ZnO transparent conductive oxides (TCOs). We studied a set of possible X codopants including either unintentional dopants typically present in the system (e.g., H, O) or monovalent acceptor groups, based on nitrogen and halogens (F, Cl, I). The interplay between dopants and structural point defects in the host (such as vacancies) is also taken explicitly into account, demonstrating the crucial effect that zinc and oxygen vacancies have on the final properties of TCOs. Our results show that Li-ZnO has a p-type character, when Li is included as Zn substitutional dopant, but it turns into an n-type when Li is in interstitial sites. The inclusion of X-codopants is considered to deactivate the n-type character of interstitial Li atoms: the total Li-X compensation effect and the corresponding electrical character of the doped compounds selectively depend on the presence of vacancies in the host. We prove that LiF-doped ZnO is the only codoped system that exhibits a p-type character in the presence of Zn vacancies. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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7273 KiB

Open AccessArticle

Carrier Compensation Induced by Thermal Annealing in Al-Doped ZnO Films

by Takashi Koida, Tetsuya Kaneko and Hajime Shibata

Materials 2017, 10(2), 141; https://doi.org/10.3390/ma10020141 - 8 Feb 2017

Cited by 22 | Viewed by 4789

Abstract

This study investigated carrier compensation induced by thermal annealing in sputtered ZnO:Al (Al₂O₃: 0.25, 0.5, 1.0, and 2.0 wt %) films. The films were post-annealed in a N₂ atmosphere at low (1 × 10⁻²³ atm) and high (1 × 10⁻⁴ atm) oxygen partial pressures (P_O2). In ZnO:Al films with low Al contents (i.e., 0.25 wt %), the carrier density (n) began to decrease at annealing temperatures (T_a) of 600 °C at low P_O2. At higher P_O2 and/or Al contents, n values began to decrease significantly at lower T_a (ca. 400 °C). In addition, Zn became desorbed from the films during heating in a high vacuum (i.e., <1 × 10⁻⁷ Pa). These results suggest the following: (i) Zn interstitials and Zn vacancies are created in the ZnO lattice during post-annealing treatments, thereby leading to carrier compensation by acceptor-type Zn vacancies; (ii) The compensation behavior is significantly enhanced for ZnO:Al films with high Al contents. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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2910 KiB

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Optoelectronic Properties and the Electrical Stability of Ga-Doped ZnO Thin Films Prepared via Radio Frequency Sputtering

by Shien-Uang Jen, Hui Sun, Hai-Pang Chiang, Sheng-Chi Chen, Jian-Yu Chen and Xin Wang

Materials 2016, 9(12), 987; https://doi.org/10.3390/ma9120987 - 6 Dec 2016

Cited by 16 | Viewed by 5197

Abstract

In this work, Ga-doped ZnO (GZO) thin films were deposited via radio frequency sputtering at room temperature. The influence of the Ga content on the film’s optoelectronic properties as well as the film’s electrical stability were investigated. The results showed that the film’s crystallinity degraded with increasing Ga content. The film’s conductivity was first enhanced due to the replacement of Zn²⁺ by Ga³⁺ before decreasing due to the separation of neutralized gallium atoms from the ZnO lattice. When the Ga content increased to 15.52 at %, the film’s conductivity improved again. Furthermore, all films presented an average transmittance exceeding 80% in the visible region. Regarding the film’s electrical stability, GZO thermally treated below 200 °C exhibited no significant deterioration in electrical properties, but such treatment over 200 °C greatly reduced the film’s conductivity. In normal atmospheric conditions, the conductivity of GZO films remained very stable at ambient temperature for more than 240 days. Full article

(This article belongs to the Special Issue Advances in Transparent Conducting Materials)

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