Solution-processible nanomaterials for flexible electronic production
Flexible solar cells emerge as the need for clean energy resources arises in many complex scenarios, demanding device tolerance with irregular geometries. Bendable or even foldable devices with atypical form factors fit for a wide range of applications, such as integration with building designs, smart home “Internet-of-Things” systems, wearable electronics. Implementation of nanomaterials into flexible devices, owing to their solution processible and flexible nature, can further lower production costs while improving device flexibility; tunable properties of nanomaterials can also add in versatility of device functions. Ultrathin CuInSe₂ nanocrystal solar cells with a total thickness less than 20 μm, are enabled by both nanomaterial incorporation and photonic lift-off technique. Photonic lift-off provides ultrafast, clean, and high-throughput processing, to separate flexible devices from the underlying rigid support. CuInSe₂ nanocrystal solar cells can be produced in flexible format through the photonic lift-off process, after the device flexibility was greatly improved by the replacement of indium tin oxide in the top contact with spray-coated silver nanowires. In contrast to nanowire assembly produced from spray coating with a disordered morphology, regular assembly of nanowires which exhibits a higher level of uniformity, could lead to enhancement of the intrinsic nanowire properties. A simple toluene-water system, utilizing the interface formed between the two immiscible liquids, is able to drive nanowires to align side by side into monolayers, creating centimeter-wide coverage. Uniformity in nanowire alignment across hundreds of microns gives rise to anisotropic properties such as birefringence, revealing their potential to be used as building blocks of optoelectronics. Last but not the least, near-infrared active plasmonic CuInSe₂ nanocrystals were achieved through cation exchange. A post-synthetic heat treatment of CuInSe₂ nanocrystals with Cd²⁺ sources induced plasmon-enhanced absorption around 0.85 eV. Cd incorporation generates free carriers and expands the pristine chalcopyrite crystal lattice, as a result of the introduced substitutional and interstitial defects. These studies provided deep insights into the structure and properties of solution-processible nanomaterials, to advance their applications for flexible electronic production.