Concept: Transparent electrodes
For the realization of high-efficiency flexible optoelectronic devices, transparent electrodes should be fabricated through a low-temperature process and have the crucial feature of low surface roughness. In this paper, we demonstrated a two-step spray-coating method for producing large-scale, smooth and flexible silver nanowire (AgNW)-poly3,4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) composite electrodes. Without the high-temperature annealing process, the conductivity of the composite film was improved via the lamination of highly conductive PEDOT:PSS modified by dimethyl sulfoxide (DMSO). Under the room temperature process condition, we fabricated the AgNW-PEDOT:PSS composite film showing an 84.3% mean optical transmittance with a 10.76 Ω sq(-1) sheet resistance. The figure of merit Φ(TC) was higher than that obtained from the indium tin oxide (ITO) films. The sheet resistance of the composite film slightly increased less than 5.3% during 200 cycles of tensile and compression folding, displaying good electromechanical flexibility for use in flexible optoelectronic applications.
We report the fabrication of a highly flexible indium tin oxide (ITO) electrode that is completely transparent to light in the visible spectrum. The electrode was fabricated via the formation of a novel ITO nanoarray structure, consisting of discrete globular ITO nanoparticles superimposed on an agglomerated ITO layer, on a heat-sensitive polymer substrate. The ITO nanoarray spontaneously assembled on the surface of the polymer substrate by a simple sputter coating at room temperature, without nanolithographic or solution-based assembly processes being required. The ITO nanoarray exhibited a resistivity of approximately 2.3 × 10(-3) Ω cm and a specular transmission of about 99% at 550 nm, surpassing all previously reported values of these parameters in the case of transparent porous ITO electrodes synthesized via solution-based processes at elevated temperatures. This novel nanoarray structure and its fabrication methodology can be used for coating large-area transparent electrodes on heat-sensitive polymer substrates, a goal unrealizable through currently available solution-based fabrication methods.
- Proceedings of the National Academy of Sciences of the United States of America
- Published about 4 years ago
Next-generation flexible electronics require highly stretchable and transparent electrodes. Few electronic conductors are both transparent and stretchable, and even fewer can be cyclically stretched to a large strain without causing fatigue. Fatigue, which is often an issue of strained materials causing failure at low strain levels of cyclic loading, is detrimental to materials under repeated loads in practical applications. Here we show that optimizing topology and/or tuning adhesion of metal nanomeshes can significantly improve stretchability and eliminate strain fatigue. The ligaments in an Au nanomesh on a slippery substrate can locally shift to relax stress upon stretching and return to the original configuration when stress is removed. The Au nanomesh keeps a low sheet resistance and high transparency, comparable to those of strain-free indium tin oxide films, when the nanomesh is stretched to a strain of 300%, or shows no fatigue after 50,000 stretches to a strain up to 150%. Moreover, the Au nanomesh is biocompatible and penetrable to biomacromolecules in fluid. The superstretchable transparent conductors are highly desirable for stretchable photoelectronics, electronic skins, and implantable electronics.
Transparent carbon electrodes composed of single-walled carbon nanotubes or graphene were used as the bottom electrode in flexible inverted perovskite solar cells. Their photovoltaic performance and mechanical resilience were compared and analyzed using various techniques. Whereas a conventional inverted perovskite solar cells using indium tin oxide showed a power conversion efficiency of 17.8%, the carbon nanotube- and graphene-based cells showed efficiencies of 12.8% and 14.2%, respectively. A thin layer of MoO3 was used for its stable doping effect. The difference in photovoltaic performance between the carbon nanotube- and graphene-based cells was due to the inferior morphology and lower transmittance of the carbon nanotube films compared with the graphene films. Raman spectroscopy and four-probe measurements after strain application revealed that the graphene-based cells were more susceptible to strain than the carbon nanotube-based cells for a given magnitude of strain. Cyclic flexural testing also showed higher resilience of the carbon nanotube-based cells compared with graphene-based cells, though the difference was marginal. Overall, graphene is a better option than carbon nanotubes as the bottom transparent electrode in perovskite solar cells. However, the transfer step for graphene has lower reproducibility, and thus the development of better graphene transfer methods would help maximize the current capacity of graphene-based cells.
Solution processed poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) transparent electrodes (TEs) offer great potential as a low cost alternative to expensive indium tin oxide (ITO). However, strong acids are typically used for enhancing the conductivity of PEDOT:PSS TEs, which produces processing complexity and environmental issues. This work presents an environmentally friendly acid free approach to enhance the conductivity of PEDOT:PSS using a light oxygen plasma treatment in addition to solvent blend additives and post treatments. The plasma treatment was found to significantly reduce the sheet resistance of PEDOT:PSS TEs from 85 to as low as 15 Ω sq-1, which translates to the highest reported conductivity of 5012 S/cm for PEDOT:PSS TE. The plasma treated PEDOT:PSS transparent electrode resulted in an ITO-free perovskite solar cell efficiency of 10.5%, which is the highest reported efficiency for ITO-free perovskite solar cells with a PEDOT:PSS electrode that excludes the use of acid treatments. This is the first demonstration of this technology. Moreover, the PEDOT:PSS TEs enabled better charge extraction from the perovskite solar cells and reduced the hysteresis in the current density-voltage (J-V) curves.
Silver nanowires (Ag NWs) are the promising materials to fabricate flexible transparent electrodes, aiming to replace indium tin oxide (ITO) in the next generation of flexible electronics. Herein, a feasible polyvinylpyrrolidone (PVP)-mediated polyol synthesis of Ag NWs with different aspect ratios is demonstrated and high-quality Ag NWs transparent electrodes (NTEs) are fabricated without high-temperature thermal sintering. When employing the mixture of PVP with different average molecular weight as the capping agent, the diameters of Ag NWs can be tailored and Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are obtained. Using these as-synthesized Ag NWs, the uniform Ag NWs films are fabricated by repeated spin coating. When the aspect ratios exceed 500, the optoelectronic performance of Ag NWs films improve remarkably and match up to those of ITO films. Moreover, an optimal Ag NTEs with low sheet resistance of 11.4 Ω/sq and a high parallel transmittance of 91.6% at 550 nm are achieved when the aspect ratios reach almost 1000. In addition, the sheet resistance of Ag NWs films does not show great variation after 400 cycles of bending test, suggesting an excellent flexibility. The proposed approach to fabricate highly flexible and high-performance Ag NTEs would be useful to the development of flexible devices.
In this study, low-voltage, printed, ion gel-based electrochromic devices (ECDs) were successfully fabricated. While conventional dispensing printing provides irregularly printed electrochromic (EC) gels, we improved the adhesion between the printed gel and the substrate by applying an external voltage. This is called electrostatic-force-assisted dispensing printing. As a result, we obtained well defined, printed, EC gels on substrates such as indium tin oxide (ITO)-coated glass. We fabricated a gel-based ECD by simply sandwiching the printed EC gel between two transparent electrodes. The resulting ECD, which required a low coloration voltage (~0.6 V), exhibited a high coloration efficiency (η) of 161 cm(2)/C and a large transmittance contrast (~82%) between the bleached and colored states at -0.7 V. In addition, electrostatic-force-assisted dispensing printing was utilized to fabricate directly patterned ECDs.
Transparent conductive film (TCF) has found wide applications. Indium tin oxide (ITO) is currently the most widely used transparent electrode. However, major problem of ITO is the lacking of flexibility, which totally limits its applications. Here, we report a highly flexible transparent electrode consisting of freestanding ITO nanofiber network fabricated by blow spinning, the advantage of which is its high-efficiency, low cost and safety. When the bending radius decreased to 0.5 mm, the resistance of the transparent electrodes only increased by 18.4%. Furthermore, the resistance was almost unchanged after thousands of bending cycles at 3.5 mm bending radius.
Transparent electrodes (TEs) have attracted significant scientific, technological, and commercial interest in recent years due to the broad and growing use of such devices in electro-optics, consumer products (touch-screens for example), solar cells, and others. Currently, almost all commercial TEs are fabricated through “top-down” approaches (primarily lithography-based techniques), with indium tin oxide (ITO) as the most common material employed. Several problems are encountered, however, in this field, including the cost and complexity of TE production using top-down technologies, the limited structural flexibility, high-cost of indium, and brittle nature and low transparency in the far-IR spectral region of ITO. Alternative routes based upon bottom-up processes, have recently emerged as viable alternatives for production of TEs. Bottom up technologies are based upon self-assembly of building blocks - atoms, molecules, or nanoparticles - generating thin patterned films that exhibit both electrical conductivity and optical transparency. In this Feature Article we discuss the recent progress in this active and exciting field, including bottom-up TE systems produced from carbon materials (carbon nanotubes, graphene, graphene-oxide), silver, gold, and other metals. The current hurdles encountered for broader use of bottom-up strategies along with their significant potential are analyzed.
In this paper, we demonstrated a highly-flexible all-fiber based transparent piezoelectric harvester (ATPH) by using the direct-write, near-field electrospinning (NFES) technique and polyvinylidene fluoride (PVDF) micro/nano fibers (MNFs) as source materials. Here, we comprehensively show that transferred high performance transparent electrodes with Au-coated nanowire (NW) electrodes can be obtained using a facile and scalable combined fabrication route of both electrospinning and sputtering processes. Au-coated MNFs of a.c. 110 nm thick can significantly reduce junction resistance, which results in high transmittance (90%) at low sheet resistance (175 Ω sq(-1)). The Au-coated MNFs electrodes also show great flexibility and stretchability, which easily surpass the brittleness of indium tin oxide (ITO) films. Further improvement in ATPH performance was realized by rolling the device into a cylindrical shape, resulting in an increase in power output due to the cooperatively enhanced effect. The rolled ATPH with 0.34 cm diameter produces a high output voltage of ∼4.1 V, current ∼295 nA at a strain of 0.5% and 5 hz. This can efficiently run commercially available electronic components in a self-powered mode without any external electrical supply.