Polarity dependent surface charge retention on CYTOP fluoropolymer for durable TENG applications

Polymer-based dielectric materials with inherent quasi permanent charge storage stand out as the predominant choice for the development of high-performance Triboelectric Nanogenerators (TENG) for different self-powered sensing applications. However, a key research avenue in further augmenting the performance of such devices is in the discovery of advanced triboelectric polymers as well as deciphering the surface charge retention mechanism and quantification of existing polymer or electret materials. This work reports the polarity dependent surface charge retention capability of Cyclic Transparent Optical Polymer (CYTOP), which is well-known for its electret-based applications due to high surface charge density, high breakdown voltage, optical transparency and CMOS compatibility. Here, a systematic investigation of the charge stability and retention of injected charge on CYTOP thin film is performed in two steps. Firstly, charge injection of both polarities is performed using the contact mode of Atomic Force Microscopy (AFM) by applying a fixed DC bias (±10 V) over a small area. Later, the surface potential is mapped using Kelvin Probe Force Microscopy (KPFM) at different time intervals to decipher the surface charge retention properties under different bias conditions. Within the first hour after charge writing, the surface potential decay rate of ∼20 µV/s and ∼0.14 mV/s are observed for the cases of negative and positive charge writing, respectively. The written charge saturates after almost an hour with only 10% decay from the initial amount for the negative charge injection whereas 85% decay is perceived for the positive counterpart. Such discrepancy is attributed to the chemical structure and the morphology of the deposited film and energy required to overcome the trapped site for charged of different polarities. The injected negative charge persists even after a week ensuring long-term applicability of CYTOP. After optimizing the surface charge retention properties vis-à-vis defining the corresponding position in the triboelectric series, high-performance pre-charged triboelectric nanogenerator (PC-TENG) based self-powered sensing applications are demonstrated employing large area negative charge injected CYTOP thin films as the active material.