Continuous-Flow Direct Writing of Hybrid TiO₂ Flexible Photo-Electrodes: Processing, Microstructure and Functionality Interrelations

Abstract

Flexible electronics manufacturing from functional inks is a versatile approach gaining interest from both industry and academia at an accelerated pace; towards its full development, research studies establishing connections between the inks processing conditions and final materials functionalities become necessary. In this work, we report on the relations between synthesis, continuous - flow direct writing parameters, and low energy intensity post-processing of functional TiO₂ hybrid ink patterns. Such inks are printed on heat sensitive polymer substrates with typical application in dye solar cell photoelectrodes; nevertheless, their versatility spans a wide range of other applications from sensors to photocatalysts. For the ink formulation, we use an initial crystalline nanoparticle TiO₂ phase that provides the main functionality of the printed films. We also add a Ti-precursor that, when post-treated, provides connecting paths for the initial phase thus forming continuous porous structures. We find that the ink’s formulation plays a pivotal role by providing the means for tuning its rheological properties (necessary for successful direct writing), the ink-substrate interactions, and the printed microstructures. We further discuss the implications of such compositional variations, introduced when adding polymeric agents, such as polyacrylic acid, on the crystallization of the Ti-organic precursor into TiO₂ bridges between the nanoparticles. We finally report on the electrical properties of the printed TiO₂ photoelectrodes as compared to conventionally fabricated counterparts. The design, continuous – flow direct writing, and the subsequent mild thermal-energy treatments of hybrid sol-gel based TiO₂ inks may hold the key for large-scale and sustainable additive manufacturing of flexible functional components for a range of applications.