Designing 2D-based Electrodes for Intrinsically Stretchable Organic Light-Emitting Diodes with High Efficiency

Abstract

The emergence of wearable electronics, as an alternative to conventional stationery and hand-held instruments, requires integrated devices to be stretchable to form intimate contact with human skin. Such systems can use on-skin sensors to monitor physiological signals by continuously and in real time, and can directly visualize of signals by seamless incorporation of stretchable displays. However, compared with the realm of on-skin sensors, the development of intrinsically stretchable displays is stagnated. The state-of-the-art stretchable displays consisting of fully stretchable layers normally exhibit the low performance due to the limitations of stretchable electrode characteristics and the absence of a protocol of the lamination process. The development of intrinsically-stretchable organic light-emitting didoes (ISOLEDs) has stagnated due to the lack of ideal stretchable electrode materials to overcome the existing issue of poor charge injection at the electrode interface, and consequent high turn-on voltage Von > 7 V, although the stretchability has been improved significantly. Herein, we present a two-dimensional-contact stretchable electrode (TCSE) that consists of AgNWs as the conductive filler, two-dimensional graphene as the work function (WF) modifier on the surface, GO binder to enhance conductivity and stretchability, and dopants to control the WF. The WF of the TCSE can be modulated to 5.69 eV and 4.04 eV by p- and n-type doping, respectively. TCSE can fulfill the required energy-level alignment with reduced the charge-injection barrier and form a two-dimensional electrical contact with the adjacent organic layer upon lamination for efficient charge injection. An ISOLED using the laminated doped TCSE exhibited low Von of 5.5 V, maximum luminance (Lmax) of 1130 cd/m2, current efficiency (CE) of 9.3 cd/A, and a stable light-emission with LT50 = 7.07h (the lifetime at 50% initial luminance under continuous operation). Furthermore, we demonstrated a three-inch five-by-five passive matrix ISOLED using the convex stretching method. To further develop the practical applications of intrinsically stretchable optoelectronic devices with enhanced performances, a new lamination method has been introduced to obtain a uniform lamination contact and strong adhesion at the interface. A minimum operating voltage of intrinsically stretchable organic light-emitting diodes (ISOLEDs) is always required for practical applications. However, the lack of protocols for the lamination makes it extremely challenging to attain a reliable ISOLED without inducing any degradations. Herein, we present a solvent-vapor-assisted lamination (SVAL) method to reinforce the cathode interface, which lowers the operation voltage and increases the stretchability of ISOLEDs. Achieving a uniform contact and strong adhesion at the interface is the key to attaining reliable lamination. A cold-pressing (CP) treatment was first introduced to reduce the surface roughness of silver nanowires before the embedding process. Subsequent solvent vapor treatment before the lamination partially solvates the surface of the active layer with an increase in the segmental motion of polymer chains, which substantially increases the interfacial adhesion. These benefits from a combination of CP and SVAL considerably reduced threshold voltage Vth (i.e., indicates the voltage where current shows an abrupt increase for light-emission) from 6.7 to 2.7 V. The ISOLED also exhibits excellent mechanical stretchability, with no significant change in luminance under 30% strain. CVD-grown graphene suffers from its poor solution processability and long process time for transferring process, making it less compatible with solution-processed devices. To simplify the process of the two-dimensional contact stretchable electrode, we have introduced MXene as the two-dimensional contact layer for the stretchable AgNW electrode instead of graphene. MXene is a fast-growing material having two-dimensional structure for optoelectronic devices owing to its attractive properties such as WF modulating capabilities, metallic conductivity, and solution processability with hydrophilic groups on the surface. Herein we demonstrate MXene-based 2D contact stretchable electrodes for intrinsically stretchable phosphorescent organic light-emitting diodes (ISPhOLEDs). MXenes serve as the 2D interlayer to increase the electrical contact area without sacrificing the stretchability. The WF of MCSE can be successfully tuned from 3.79 to 5.71 eV for cathode and anode applications. Besides, we have also proposed a stretchable gradient hole injection layer to facilitate the hole injection with the addition of PFSA. In terms of the light-emitting layer, stretchability can be achieved with the addition of elastomer as the plasticizer. Lastly, the intrinsically stretchable phosphorescent light-emitting device with the high efficiency has been fabricated for the first time. This dissertation can provide effective guidelines for designing two-dimensional contactable stretchable electrodes with 2D materials and intrinsically stretchable organic light-emitting diodes with high efficiency.