May/June_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 0 1 9 actions from charged functional groups have been utilized as a natural self- healable polymeric binder for Si electro- des [18] . Produced by adding gelation agent (ammoniumpersulfate) into com- posite solution by complexing PEDOT: PSS with poly(vinyl alcohol), this poly- mer hydrogel binder is able to maintain both the electric contact and integrity of Si electrodes [19] . In essence, the devel- opment of efficient self-healing binders is one of the challenges to maintaining good adhesion and electrodes integrity. Dynamically covalent crosslinked networks can be also fabricated using poly(ethylene oxide)-based networks with various amounts of lithium bis-(tri- fluoromethanesulfony)imide. These so- lid polyelectrolytes not only self-heal to recover conductivity, but also dis- solve back to monomers to make Li- ion batteries cleaner and sustainable [20] . Due to the increasing need of wearable electronic devices, flexible solid-state Li-ion batteries with self-healing elec- trolytes are also promising candidates. Highly stretchable and self-healable polymer electrolytes composites were fabricated using poly(4-vinylpyrindine) (propyl-trimethylammonium) as elec- trolyte backbonewith 1-ethyl-3-methyl- imidazolium bis(trifluoro-methanesul- fonyl)imide ionic liquid and lithium salt filled into the interspace between chain segments [21] . Polyethylene glycol (PEG) was uti- lized as an active layer in the develop- ment of self-healable perovskite solar cells, where hygroscopic PEG scaffold stabilized perovskite films, render- ing the solar cell resistant to moisture (Fig. 3) [22] . Quantum dot (QD) solar cells is perhaps one of the areas of great interest because the conversion ef- ficiency can be very high, but the de- velopment of the miniband transport and collection is the challenge [23] . One can envision that various QD configu- rations can be developed by control- lable spacing between the dots using precisely designed copolymers. These copolymers could serve two functions, control of the efficiency and self-heal- ing of defects. Also, intensive research activities have been and are being pur- sued toward the development of low- cost organic photovoltaic (OPV) devices based on organic semiconductors with backbones comprised mainly of alter- nating C–C and C=C bonds [24] . FUTURE OUTLOOK The rate at which self-healable polymers are being developed, as mani- fested by new projects and many pub- lications, indicate that these materi- als will revolutionize all energy sectors and will become a standard for future applications. Aside from devices, there are endless opportunities to devel- op sustainable and cost-effective en- ergy facilities as well as new materials. For example, the use of self-healable polymers for maintaining functions of nuclear plants or extending the life- span of wind turbines is one aspect that will lead to energy savings. On the oth- er hand, utilizing radioactive elements for the development of new self-heal- able materials or exploring damage-re- pair cycles for harvesting energy are untapped territories. In terms of ma- terials design, phase-separated and multi-layered structures are highly promising because morphology control will enable tailoring electrical proper- ties and self-healing. Novel fabrication processes will also facilitate a vehicle for the development of omni-self-heal- able devices, and there are opportu- nities to use electric and/or magnetic fields to control damage-repair cycles. Furthermore, thermally or mechani- cally induced self-healing will be high- ly attractive for semi-conductive and/ or conductive components of energy harvesting and storage devices. Com- bined with the ability to sense envi- ronmental changes, self-healing will be advantageous in the development of in- terfaces between man-made electron- ic devices and low current biological systems. One can envision that energy harvesting can be achieved using ener- gy from growing plants or physical mo- tion. The inner circle in the illustration on the first page shows existing devices, and a number of opportunities for new developments in sensing and monitor- ing self-healable flexible electronics, fibrous devices, energy utilities (for ex- ample, H 2 refill hoses), nanogenerators, integrated multi-functional circuits, or biological and quantum computing are Fig. 3 — Steps involved in self-healing of PPSCs: (1) Water absorption by perovskite; (2) perovskite hydrolysis into PbI 2 and MAIH 2 O; and (3) restrained MAI by PEG reactions with nearby PbI 2 forming perovskites. PEG exhibits strong interactions with MAI, thus preventing evaporation. In-situ reactions of MAI and PbI 2 to form perovskites upon film removal from the vapor source. Figure reproduced with permission from Ref. 22.