Metal grid technologies for flexible transparent conductors in large-area optoelectronics

Highlights

• Basic concepts in order to design and evaluate flexible metal grids.

• Established methods for processing metal grids for transparent conductors.

• Emerging soft lithography and printing methods for processing metal grids.

• Metal grids application in tandem perovskite solar cells, organic light emitting diodes, and electrochromic deviced.

Abstract

Mechanically flexible optoelectronic devices such as flexible displays, touch-screens, wearable electronics and solar cells are attracting significant commercial interest. In these devices, a transparent conductor is an essential element that delivers or collects the electrical current to the active material while it allows light to enter or exit from the device. The transparent conductor is composed of a transparent conductive film and a metallic grid providing electrical conduction over the large area. In this article, we review the established processes used by the industry as well as emerging solution-based methods for processing metal grids. Furthermore, we review the issues and potentials of these emerging processes facing for large-area deployment. In the final section, we evaluate three applications of flexible transparent conductors in: perovskite-based solar cells, organic light emitting diodes and electrochromic windows.

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.