Ross A. Hatton
David Muñoz-Rojas
Seung Hwan Ko
L. Jay Guo- 1Department of Chemistry, University of Warwick, Coventry, United Kingdom
- 2Univ. Grenoble Alpes, CNRS, Grenoble INP, LMGP, Grenoble, France
- 3Department of Mechanical Engineering, Seoul National University, Seoul, South Korea
- 4Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, United States
Editorial on the Research Topic
Window Electrodes for Emerging Thin Film Photovoltaics
Photovoltaics (PVs) fabricated by printing at low temperature onto flexible substrates are attractive for a broad range of applications in buildings and transportation, where flexibility, color-tuneability, and light-weight are essential requirements. Two emerging PV technologies on the cusp of commercialization are organic PVs and perovskite PVs. CIGS, CdTe, and a-Si solar cells also have potential applications in flexible PVs. It is widely recognized that these classes of PV will only fulfill their full cost advantage and functional advantages over conventional thin film PVs if a suitable transparent, flexible electrode is forthcoming (Lu et al., 2018). Indium tin oxide (ITO) is the most popular transparent conductor material for opto-electronics including solar cells and displays. However, the fragile ceramic nature makes ITO unsuitable for future electronics such as flexible, stretchable, and wearable electronics because it will easily develop cracks under mechanical deformation. Instead, optically thin film or metallic nanowire networks (Sannicolo et al., 2016) of the most electrically conductive metals copper (Cu), silver (Ag), and gold (Au) have shown promising potential, in spite of the oxidation and parasitic absorption problem of Cu and the high material cost problem of Ag and Au. Whilst the chemical, thermal, and electrical stability of transparent electrodes based on these metals presents challenges, it has been shown that thin coating layers can be very efficient in preserving their integrity and properties (Celle et al., 2018). Additionally, low-temperature, high-throughput deposition techniques, such as spatial atomic layer deposition (SALD) (Muñoz-Rojas and MacManus-Driscoll, 2014; Khan et al., 2018), can be used to deposit these protective layers.
The four articles in this Research Topic relate to different aspects of the development of transparent electrodes based on optically thin films of Au (Lucarelli and Brown), Ag (Wang et al.), and Cu (Bellchambers et al.; Pereira and Hatton). In all cases the metal deposition method of choice is thermal evaporation. Given that large area roll-to-roll deposition of metals by vacuum evaporation is an established industrial process for the production of low cost food packaging and insulation foils, it is plausible that this method of metal deposition offers a path to the fabrication of low cost transparent electrodes for PVs.
For films of Cu, Ag, and Au to have high optical transparency and low sheet resistance they must have a thickness < ~10 nm and a uniform slab-like structure, which is notoriously difficult to achieve on glass and technologically relevant transparent plastic substrates such as polyethylene terephthalate due to the high surface energy of these metals. To address this challenge, various approaches have been developed, e.g., with the use of seed layer or introducing metal “doping” to suppress the 3D island growth, leading to smooth metal films (Zhang et al., 2014). In this special issue Wang et al. report a novel nucleation inducing seed layer based on a bilayer of ultra-thin (0.6 nm) Au and polyethyleneimine (PEI) for Ag films, which substantially outperforms either of these nucleation seed layers on their own, reducing the percolation threshold for evaporated Ag films from 6 to 3 nm. Using this approach, in conjunction with a high-refractive index anti-reflection layer, a sheet resistance of ~9 Ω/sq and a remarkable transmittance of ~93% is demonstrated together with outstanding long-term environmental and mechanical stability. Whilst to date studies reporting hybrid organic-inorganic metal nucleation layers are sparse (Bellchambers et al., 2019). Wang et al. have shown that the benefits can be substantial and in some cases may off-set the disadvantage associated with the extra complexity in fabrication. There is also a very large pallet of material combinations that could be explored and so this is a fertile area for future research.
Perovskite PVs present a particular challenge for metal substrate electrode design because of the chemical incompatibility of most metals toward the iodine-containing compounds used to form narrow bandgap perovskite semiconductors as well as the iodine-containing degradation products (Bastos et al., 2017). For this reason Lucarelli and Brown have used a 10 nm thick film of Au, which offers superior chemical resistance toward oxidation as compared to Ag and Cu. Whilst gold is 80–100 times more costly than Ag, in this context its very low thickness and high stability may justify its use in high performance perovskite PVs since Au is already widely used in the electronics industry. Lucarelli and Brown show that a solution processed SnO2 layer is particularly effective as a nucleation seed layer for evaporated Au films, and that by depositing the same oxide on top, to match the optical impedance across the metal film, the far-field transparency is greatly improved. SnO2 is also an inspired choice because it can be interfaced with TiO2, which is the most widely used hole-blocking charge extraction layer in perovskite PVs. On plastic substrates these triple layer electrodes offer remarkable stability toward bending through tiny radius of curvature (as small as 1.5 mm) and in PV devices a power conversion efficiencies of 7.6% is achieved.
Whilst Cu based transparent electrodes are very appealing due to the much lower cost of Cu (1% that of Ag), the susceptibility of Cu to oxidation in air imposes a challenge. To exploit the advantage of low material cost and good electrical conductivity without the oxidation problem (Han et al., 2014). Pereira and Hatton demonstrated a Cu –zinc oxide bilayer electrode supported on flexible polyethylene terephthalate (PET) with a sheet resistance of 11.3 Ω/sq and average transparency of 84.6% in the wavelength range of 400–800 nm. The Cu film is perforated with a dense array of sub-micron diameter apertures fabricated using polymer-blend lithography, which is found to suppress reflection, particularly for wavelengths >550 nm. Compared with ITO, the bilayer electrode showed superior stability toward bending deformation, and organic PVs using this electrode in place of ITO achieved a high power conversion efficiency of 8.7%. With the similar objective to realize high performance Cu based transparent electrodes less prone to oxidation, Bellchambers et al. passivated a 9 nm Cu film electrode with an 0.8 nm aluminum passivation layer, both deposited by simple thermal evaporation. The results showed superior oxidation stability with very little change (3.4%) in sheet resistance after 5,000 h in air because of the segregation of aluminum-copper-oxide to boundaries between the Cu crystallites upon exposure to air, which retards oxidation at those sites in the Cu film most vulnerable to oxidation. These two original studies on new Cu thin film based flexible transparent conductors are expected to accelerate research into low-cost transparent flexible electrodes in numerous emerging optoelectronic devices.
Author Contributions
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
RH gratefully acknowledges the United Kingdom Engineering and Physical Sciences Research Council (EPSRC) for funding (Grant number: EP/N009096/1). DM-R thanks funding from the Agence Nationale de la Recherche (ANR, France) via the Projects ANR-16-CE05-0021 (DESPATCH), ANR-17-CE05-0034 (OXYGENE), and the Investissements d'avenir program (ANR-15-IDEX-02) through the project Eco-SESA.
References
Bastos, J. P., Manghooli, S., Jaysankar, M., Tait, J. G., Qiu, W., Gehlhaar, R., et al. (2017). Low-cost electrodes for stable perovskite solar cells. Appl. Phys. Lett. 110:233902. doi: 10.1063/1.4984284
Bellchambers, P., Walker, M., Huband, S., Dirvanauskas, A., and Hatton, R. A. (2019). Enhanced oxidation stability of transparent copper films using a hybrid organic-inorganic nucleation layer. ChemNanoMat 5, 619–624. doi: 10.1002/cnma.201800667
Celle, C., Cabos, A., Fontecave, T., Laguitton, B., Benayad, A., Guettaz, L., et al. (2018). Oxidation of copper nanowire based transparent electrodes in ambient conditions, and their stabilization by encapsulation: application to transparent film heaters. Nanotechnology 29:085701. doi: 10.1088/1361-6528/aaa48e
Han, S., Hong, S., Ham, J., Yeo, J., Lee, B., Kang, B., et al. (2014). Fast plasmonic laser nanowelding for a Cu-nanowire percolation network for flexible transparent conductors and stretchable electronics. Adv. Mater. 26, 5808–5814. doi: 10.1002/adma.201400474
Khan, A., Nguyen, V. H., Muñoz-Rojas, D., Aghazadehchors, S., Jiménez, C., Nguyen, N. D., et al. (2018). Stability enhancement of silver nanowire networks with conformal ZnO coatings deposited by atmospheric pressure spatial atomic layer deposition. ACS Appl. Mater. Interfaces. 10, 19208–17. doi: 10.1021/acsami.8b03079
Lu, H., Ren, X., Ouyang, D., and Choy, W. C. H. (2018). Emerging novel metal electrodes for photovoltaic applications. Small 14:1703140. doi: 10.1002/smll.201703140
Muñoz-Rojas, D., and MacManus-Driscoll, J. (2014). Spatial atmospheric atomic layer deposition: a new laboratory and industrial tool for low-cost photovoltaics. Mater. Horizons 1:314. doi: 10.1039/C3MH00136A
Sannicolo, T., Lagrange, M., Cabos, A., Celle, C., Simonato, J., and Bellet, D. (2016). Metallic nanowire-based transparent electrodes for next generation flexible devices: a review. Small 12:6052. doi: 10.1002/smll.201602581
Keywords: transparent electrode, organic solar cell, organic photovoltaic, perovskite solar cell, solar cell, photovoltaic, metal film
Citation: Hatton RA, Muñoz-Rojas D, Ko SH and Guo LJ (2020) Editorial: Window Electrodes for Emerging Thin Film Photovoltaics. Front. Mater. 7:72. doi: 10.3389/fmats.2020.00072
Received: 10 January 2020; Accepted: 10 March 2020;
Published: 31 March 2020.
Edited by:
Cher Ming Tan, Chang Gung University, TaiwanReviewed by:
Karthik Ramasamy, Los Alamos National Laboratory (DOE), United StatesAndy T. Wu, Luvata Waterbury Inc., United States
Copyright © 2020 Hatton, Muñoz-Rojas, Ko and Guo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Ross A. Hatton, ross.hatton@warwick.ac.uk