Improvements in the Stability and Photocurrent of Organic-Semiconductor-Based Light-Addressable Potentiometric Sensors via ALD Stacked Sensing Membranes and Electron Transport Layers

This study presents the combination of the electron transport layer (ETL) and insulating layers in the organic semiconductor-based light-addressable potentiometric sensor (LAPS) for consideration for stability and improvement over a lifetime. PFN-Br was chosen as the ETL to replace ZnO as a more uniform organic semiconductor coating with fewer defects and higher photoelectrical transfer efficiency. The mixture of PTB7-Th and PC<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">71</sub>BM was spin-coated for the organic semiconductor layer. A stacked sensing membrane comprised of Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and HfO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was fabricated using atomic layer deposition (ALD) to improve the drift and hysteresis of a conventional single dielectric layer (e.g., pure Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>). Employing PFN-Br as ETL yielded improvements for maximum photocurrent of 24.35%, pH sensitivity of 2.35%, and hysteresis width of 9.24% compared to conventional ZnO. With the stacked sensing membrane, the drift coefficient was reduced by 22.4% and lasted for a 720-min measurement. The optimized organic semiconductor-based LAPS shows its feasibility with a sensitivity, linearity, drift coefficient, and hysteresis of 52.5 mV/pH, 99.9%, -2.53 mV/h, and -10.5 mV, respectively. These clear improvements mark a significant breakthrough for organic semiconductor-based LAPS into real pH sensing applications.