Designing Intrinsically Stretchable Electrodes, Luminescent Films, and Light-Emitting Devices

Abstract

Soft materials and processing have enabled the development of a wide variety of wearable electronics including on-skin electrical, physical, and chemical sensors. Continuous monitoring of typical physiological data such as respiration rate, heart rate, contraction/expansion of muscles, and ground reaction forces through the sensors is particularly important for the understanding of human physiology and phenotypes that lead from health to diseases. One of the fundamental units of wearable electronics is a stretchable display that simultaneously visualizes signals from the bodyNet and provides feedback to the system. The ultimate goal in production of stretchable displays is to fabricate them directly on stretchable substrates that can be placed in conformal contact with human skin. However, the development of intrinsically stretchable displays is still underdeveloped due to the limitations of stretchable encapsulations and electrodes. Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of an air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. We propose an air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs). PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. We demonstrate a display that can be stretched up to 180%; it is composed of an air-stable SCCL that down-converts the SELDs blue emission and reemits it as green. Our work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays that have high color purity. To lower the operating voltage of the stretchable display, direct-current (DC) driven stretchable displays have been developed. Embedded silver nanowires (AgNWs) have been widely used as stretchable electrodes. However, the contact area is limited at the 1D AgNW/organic layer interface, so the embedded AgNWs have poor charge-injection properties. Energy-level misalignment at stretchable electrode/organic interfaces of ISOLEDs is another problem that must be solved. Hence, two-dimensional materials have been used as the electrode to form a complete two-dimensional contact at the electrode/organic interfaces. MXenes are a rapidly growing family of two-dimensional materials that are promising for optoelectronic applications because of numerous attractive properties, including high electrical conductivity. However, the poor environmental stability of MXene thin films and low work function (WF) of MXene electrodes are the biggest challenges to practical applications of solution-processed optoelectronics that require long environmental stability, high WF, and low sheet resistance (Rs). Herein, we present a Ti3C2Tx MXene with a compact structure and a perfluorosulfonic acid (PFSA) barrier layer as a promising electrode for organic light-emitting diodes (OLEDs). The electrode simultaneously exhibits excellent environmental stability, high WF at 5.84 eV and low Rs at 134.4 Ω/sq. The compact Ti3C2Tx structure after thermal annealing resists the intercalation of moisture and environmental contaminants. In addition, the PFSA surface-modification passivates interflake defects and modulates the WF. Thus, negligible changes in the Rs and WF (>5.60 eV) were observed even after 22 days of exposure to ambient air. Lastly, Ti3C2Tx MXene was applied for large-area and ten-by-ten passive matrix flexible OLEDs on six-by-six cm substrates for the first time. To further enhance the stability of the stretchable electrode, graphene has been introduced to overcome the unstable nature of MXene. We demonstrate highly-efficient ISOLEDs that use graphene-based two-dimensional-contact stretchable electrodes (TCSEs) that incorporate a graphene layer on top of embedded metallic nanowires. The graphene layer modifies the work function, promotes charge spreading, and impedes inward diffusion of oxygen and moisture. The work function (WF) of 3.63 eV is achieved by forming a strong interfacial dipole after deposition of a newly-designed conjugated polyelectrolyte with crown ether and anionic sulfonate groups on TCSE; this is the lowest value ever reported among ISOLEDs, which overcomes the existing problem of very poor electron injection in ISOLEDs. Subsequent pressure-controlled lamination yielded a highly efficient fluorescent ISOLED with an unprecedently high current efficiency of 20.3 cd/A, which even exceeds that of an otherwise-identical rigid counterpart. Lastly, a three-inch five-by-five passive matrix ISOLED was demonstrated using convex stretching. This dissertation can provide effective approaches for designing air-stable stretchable displays without using encapsulations and intrinsically stretchable high-efficiency optoelectronic devices with the introduction of two-dimensional materials.