Smart Passivation Materials with a Microencapsulated Liquid Metal for Self-Healing Conductors in Sustainable Electronic Devices Open Access

Research on flexible electronics began almost 20 years ago with the demand for flexible displays [1], [2]. However, planar and rigid wafer-based electronic devices are intrinsically incompatible with deformable organic systems. Thus, organic semiconductors and conducting polymers constitute appealing materials for large-area electronics, owing to their intrinsic flexibility, light weight, and low cost, especially when combined with roll-to-roll processes [3], [4]. The electrical failure of conductive pathways in highly integrated circuits results in the loss of function, which is often impossible to repair. This remains a long-standing problem hindering advanced electronic packaging [5]–[7]. Previous approaches to restore conductivity relied on external interventions in the form of heating [8] or manual delivery of relatively low conductivity materials [9], [10].

There have been several studies on stretchable conductors that can simultaneously self-heal, both mechanically and electrically. In particular, the use of liquid metals such as pure gallium or eutectic gallium alloy represents a promising approach to creating conductive pathways with metallic conductivity (~3 × 10⁴ S·cm⁻¹), benefitting from the low melting point (15.7 °C) and the viscosity (2 × 10⁻³ Pa·s) of the metal [11]. The ability to form conducting paths via their injection into micro-channels has proven useful for antennas, [12], [13] interconnects, [14], [15] soft electronics, [16], [17] reconfigurable optics, and electronics [18], [19]. The rapid restorative mechanism relies on the triggered release and transport of the liquid metal to the broken conductive pathway. For a relatively small volume fraction of microcapsules encapsulated in the liquid metal located on top of the conductors, the damaged sites could be healed with high efficiency. This healing system shows promise for more sustainable electronic devices with increased defect-tolerance, improved circuit reliability, and extended lifetime.

We propose a concept of a stretchable solar-powered smart watch embedded with a self-healing conductor, as shown schematically in Fig. 1, to autonomously restore conductivity in the mechanically damaged conductor.

Fundamentally, self-healing in this system is accomplished by the release and transport of a microencapsulated liquid metal to the damaged site. The liquid metal may react with the conductor and form an electrically conductive intermetallic compound. The liquid metal alloy was encapsulated via an in situ reaction of urea and formaldehyde as depicted in Fig. 2. The size of the microcapsule was controlled by varying the processing conditions. Microcapsules under 3 μm in diameter could be produced via sonication.

In order to demonstrate the feasibility of the self-healing conductor, we examined the electrical characteristics in a test specimen, before and after the mechanical damage. A Cu conductor was patterned on a rigid glass substrate and a passivation layer with the microcapsules was formed on top of it as shown in Fig. 3. Fig. 4(a)–(c) display the self-healing passivation films with different volumes of the microcapsules. During the cutting process (see Fig. 4(d)), the microcapsules in the passivation layer are ruptured and the liquid metal is released to the damaged sites of the conductor. The specimens were imaged by scanning electron microscopy (SEM) and optical microscopy, to reveal the localized release and transport of the liquid metal to the damaged site. The electrical performance of the conductor was monitored by measuring the normalized resistance, R_norm = R_sh/R_o, where R_o is the initial resistance before damage and R_sh is the resistance of the self-healed conductor. The value of R_norm ranges from zero for a specimen with no electrical conductance to one for a highly conductive specimen.

A representative specimen for evaluating the self-healing electrical performance is shown in Fig. 3. When the cutting load increases, both cracks and fracture occur successively. The simultaneous cutting damage of these lines triggered a loss of conductivity followed by restoration of the conductivity, owing to the repair of the primary conductive pathway. To investigate the time scale of the conductivity recovery, we monitored the normalized resistance (R_norm) of the specimen. R_norm increases sharply to infinity, corresponding to the breakage of the conductor. Although the restoration of the conductivity continued over a longer time, we monitored the resistance within 10 min; the resistance saturated after 5 min. For a representative specimen, an average resistance of 0.102 Ω was measured after 5 min, which is almost twice compared to the original resistance (0.054 Ω). Presumably, the continued healing process is due to the continued flow of the liquid metal, resulting in the restoration of the conductivity. In contrast, specimens without the microcapsules showed no evidence of conductivity restoration. In the self-healing specimen with the microcapsules (inset of Fig. 5(a)), R_norm rapidly recovered to over 89% compared to that of the undamaged specimen. Additional specimens fabricated with a passivation layer without the microcapsules showed no healing. Moreover, we also monitored the resistance between adjacent conductors and observed no short circuiting. For a subset of healed samples, the conductivity was monitored up to 1 h following the healing event and no loss of conductivity was observed. For the self-healing specimen, R_sh increased gradually to 2.8 after 2 min and then reverted to 1.8 after 5 min.

Further, the conductivity of the damaged conductor can be restored regardless of several repeated cuttings (Fig. 5(b)). It may be possible to achieve greater success in sample healing by optimizing the capsule size and the volume of the liquid metal released to the damaged area, as well as decreasing the distance between the capsule and the conductor.

One of the feasible applications of an electrically self-healing conductor is to improve the durability of flexible solar cells. In particular, metal electrode and an indium tin oxide (ITO) layer on a plastic substrate can be easily damaged by mechanical strain. Recent development of flexible PSCs has focused on device integration and interface engineering for low-temperature fabricating processes [21]–[23] but studies on the surface passivation and restoration of conductive layers from mechanical damage are rare. Fig. 6(a) exhibits a schematic of a flexible PSC incorporating self-healing passivation layers on an Au electrode. The role of the Au electrode is to connect the active areas and maintain device operation.

Fig. 6(b) shows the recovery test of the Au electrode on the flexible device and the OM images indicate that outflowing liquid metal fully filled the damaged site after cutting the Au electrode. Therefore, the electrical pathways in the Au electrode were almost perfectly recovered. Before cutting the Au electrode, the resistance between the active area and the Au electrode was found to be 0.82 Ω. In the absence of the passivation layer, the value increased significantly to 1.73 × 10¹⁰ Ω after cutting the Au electrode, accompanied by the breakdown of the device. However, the conductivity was fully recovered within 1 min when the passivation layer was employed. After healing, the resistance was found to be 1.47 Ω. Therefore, this recovery effect on the damaged Au electrode was reflected in the device performance. Fig. 6(c) exhibits the photocurrent J–V curves of the flexible device measured before cutting and after healing of the passivation Au electrode for backward (BW) sweep and forward (FW) sweep at a scan delay time of 200 ms. The pristine device was broken (blue lines in Fig. 6(c)), whereas the passivation device was recovered after cutting the Au electrode. Two-way voltage sweeps of the device clearly indicate the almost perfect recovery of the J–V curves, with retention of all photovoltaic parameters. This is the first example of a self-healing solar cell with a self-repairing conductor. We believe that the self-healing conductor will lead to enhanced longevity and device reliability in adverse mechanical environments, enabling new applications in microelectronics and electrical systems.

In this study, autonomic restoration of the electrical conductivity in a mechanically damaged metal (e.g. Cu and Au) conductor was demonstrated. Also self-healing of the conductor in a solar cell was demonstrated successfully. This concept of a self-healing conductor can also be used for other materials that suffer from mechanical issues during electrochemical reactions, including, electrode materials in fuel cells, water splitting, and catalysis. Beyond self-repairing devices, we envision that our concept can enable microelectronics that generates new circuits along stress-activated pathways, allowing for adaptive circuit architectures and improved circuit redundancy.

The authors would like thank Prof. Minwoo Park from Sookmyung Women's Univ. for his assistance in design and fabrication of flexible solar cell. Also the authors would like to thank Prof. Chan-Moon Chung from Yonsei Univ. for valuable suggestions in the development of liquid-metal capsules.