ReviewMetal grid technologies for flexible transparent conductors in large-area optoelectronics
Graphical abstract
Introduction
Flexible optoelectronic devices are the integrated part of many existing and emerging consumer electronic products [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. Examples include flexible displays, flexible touch-screens, wearable electronics, radio-frequency identification (RFID) and etc. Moreover they constitute the basic building blocks of the devices used in emerging subjects such as artificial skin[12,13] and internet of things (IoT) [14]. The ultimate goal of artificial skin is to integrate different types of sensors and readouts such as light emitting diodes (LED) for instance, into a flexible and stretchable platform [15,16]. In the context of IoT, flexible optoelectronic devices play a key role for optical and electronic sensing of physical quantities on moving and flexible objects [[17], [18], [19]].
A flexible transparent conducting electrode (TCE) exists in every flexible optoelectronic device and it is responsible for delivering the electrical power to the distributed elements of the device; in addition it allows light to enter or exit from the device. Therefore, for flexible applications, TCEs need to be conductive, transparent, bendable and stretchable. The requirements for electrical conductivity scale with area of the devices, where larger devices requiring higher electrical conductivities. On the other hand, fundamentally, there is a trade-off between conductivity and transparency of a single material [20]. Improving the electrical conductivity requires either larger quantity of metal or larger density of free-carriers, both of which lead to the deterioration of the transparency. Therefore, hybrid structures are often pursued to decouple the optical properties of the TCE from its electronic requirements [7,[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]].
In applications such as displays, organic light emitting diodes (OLEDs) and photovoltaics, we generally deal with large-areas (>25 cm2) and therefore TCEs formed by the combination of a metallic grid and a transparent conductive film (TCF) are used. Essentially, the TCF provides the electrical conductivity within a unit-cell area of the metallic network and the metallic grid establishes the electrical conductivity over the entire TCE area. The TCF is either a transparent conductive oxide (TCO) or a network of metallic nanowires.
TCOs are wide band-gap degenerate semiconductors with In2O3:Sn (ITO) and ZnO being the prominent ones. ITO with annual sales of nearly $1.6 billion in 2013, accounts for 93% of the entire TCE market [2,34]. Good chemical and environmental stability along with high transmittance (>93%) at low sheet resistance (10 Ω.sq−1) comprise the primary reasons for the popularity of ITO [2,35]. Doped ZnO materials such as ZnO:Al [[36], [37], [38], [39]] and ZnO:B [40] have features similar to ITO. However, they suffer from low chemical stability in acids as well as degradation under applied electric potential.
Networks of metallic nanowires such as Ag, Au, Cu, and Ni are studied as another alternatives to ITO [2]. The main advantages of these conductors lie in their excellent mechanical stability under bending and stretching tests as well as the fabrication of these electrodes using low-cost solution-based processes; however, so far, their significant optical absorption hindered their application in major optoelectronic devices [[41], [42], [43]]. In this review we exclude TCFs because there are several recent reviews on this subject [[44], [45], [46]].
Metallic grids are integrated onto these TCFs using a large number of printing, lithographic and self-assembly processes. These processes can be classified in two main categories: i) processes that require addressing scientific challenges to be scaled up and ii) processes that already exist in large-scale; however, their integration onto flexible substrates and perspective of fully roll-to roll processing might require development. There have been recently several reviews on digital printing technologies [1] and self-assembly processes [47,48] which falls in the former category. Therefore, in this review we primarily focus on metal grid technologies for developing large-area transparent conducting electrodes.
Fig. 1 shows the schematic of the steps in processing flexible optoelectronic devices. The process starts with a polymer on which moisture barrier layers are deposited to prevent water and oxygen ingression to the device and planarize the polymer surface. TCF, either TCO or metallic nanowires, and the metal grid are deposited on these barrier layers providing the transparency and the electrical conduction needed to electrically drive the device. Subsequently, the device is fabricated, encapsulated and finally tested.
In this review, first the basic concepts in order to design and evaluate flexible metal grids are discussed. In the subsequent section, standard and emerging technologies for fabrication of metal grids are reviewed. We review achievements in large-area metal grid TCEs and evaluate their performance through a critical analysis of different technologies for flexible optoelectronic applications. In the last section, we show successful examples of integration of TCO-metal grids into semi-transparent perovskite solar cells for high-efficiency perovskite silicon tandem applications, flexible OLEDs for lighting, and electrochromic smart windows.
Section snippets
Basic concepts of flexible large-area TCEs
How do we estimate the resistance of a TCO-metal grid hybrid? How do we design the size and spacing of metallic lines in a TCO-metal grid hybrid electrode? To answer these questions, we first describe an actual large-area electrode in which three elements i.e. busbars, metallic lines and TCO are classified according to their function. Then, the procedures to evaluate the mechanical and optical properties of these electrodes are discussed.
Metal grid technologies
Metal grids are often placed on top of TCF as shown in Fig. 3a because i) such processing is simple and ii) the TCO layer provides an extra barrier layer for the device and the grid. However due to the large thickness of the metallic grids, often a few microns, the metallic lines need to be passivated by dielectric coatings to avoid short circuit in the device layers on top. The sheet resistance of the metal grid/TCO electrode should be in the range of 10–50 Ohm/sq. with 90% transparency for
Applications
In this section, we review a number of technologies that metal grid-TCO hybrid has been used in large-area devices. The applications we review are perovskite-based solar cells, organic light emitting diodes (OLEDs) for lighting applications and the emerging subject of electrochromic devices.
Conclusion
TCEs in optoelectronic devices are responsible for delivering the electrical power to the distributed elements of the device; in addition, they allow light to enter or exit from the device. In large-area (>25 cm2) optoelectronic devices such as large-area displays, OLED lamps, photovoltaics and etc., the TCEs are formed by the combination of a metallic grid and a TCF (either a TCO or a network of metallic nanowires). We explained the basic concepts in order to design and evaluate flexible metal
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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