Highly Stable Ion‐Exchange Doping of Organic Semiconductor Single Crystals for Reliable Flexible Sensors

Thin‐film single crystals of organic semiconductors represent a emerging class of materials for flexible printed electronic devices, including organic field‐effect transistors and strain sensors. Their potential arises from the demonstration of high‐mobility band transport, which significantly enhances both device performance and operational reliability. However, achieving stable and precise control over carrier concentration through chemical doping remains a fundamental challenge, restricting device architecture and broader application. In this study, the challenge is addressed by improving the environmental and thermal stability of chemically doped organic single crystals through the strategic selection of dopant anions. Specifically, ion‐exchange doping using the bulky tetrakis(3,5‐bis(trifluoromethyl)phenyl)borate (BArF) anion resulted in high electrical conductivity and exceptional stability under ambient conditions at 80 °C. The doped single crystals further exhibited excellent strain sensitivity, maintaining consistent strain sensing performance over 100 000 bending cycles, with conductivity drift suppressed to the order of ppm per hour under ambient conditions. These results highlight the importance of dopant ion design for stabilizing the doped state of organic semiconductors under environmental stress, without compromising mechanical flexibility or electronic functionality. This approach paves the way for robust and reliable flexible organic electronic devices.