Formation of Porous Conjugated Polymer Films via Spontaneous Phase Separation and Their Gas Sensor Applications—Theoretical Examination of Pore Structures and Sensing Kinetics

Controlling the miscibility between mixture components induces spontaneous phase separation into distinct domain sizes. This process results in porous conjugated polymer (CP) films with varying pore sizes after the selective removal of auxiliary components. In this study, we propose a phase separation method for fabricating meso/macroporous CP films by mixing CP with auxiliary components that induce phase separation during film formation. By adjusting the content of PCBM, a well-known material for creating heterojunction structures, a porous structure was successfully fabricated. Additionally, we designed several model compounds to mix with CP and calculated the solubility distance using the Hansen solubility parameter, providing insights into the solubility of organic materials. As the difference in solubility parameters between the matrix CP and the auxiliary components increases, the pore size also increases. The pore size was effectively observed through atomic force microscopy, revealing increased root mean square and surface area, which allows precise control over the degree of phase separation.<br/>Moreover, we explored the application of porous CP films as field-effect transistors (FETs) type gas sensor platforms. The porous structure enhances detection sensitivity and improves detection speed when used in FET-based gas sensors for NO₂ detection. The electrical properties of the CP are largely maintained even after pore formation. However, excessive pore formation can cause pores to extend near the dielectric layer of CP-based FETs, resulting in partial degradation of the carrier-transporting active channel in the FET. The performance of the sensor can be enhanced by employing a FET-based gas sensor with porous structure to facilitate the adsorption and desorption of NO₂. The porous structure-based gas sensor exhibited remarkable sensitivity of 3,800 %/ppm and selectivity for NO_2,with an exceptional limit of detection of 10 ppb. The initial adsorption of the analyte occurs rapidly through the pores, generating a charge influenced by the electrical properties of the employed CP. Therefore, the quantitative analysis of the response-recovery trend of the FET sensor using the Langmuir isotherm suggests that the response speed can be improved by more than 2.5 times with a 50-fold increase in NO₂ sensitivity compared with pristine CP, which has no pores.<br/>These findings highlight the potential of utilizing blend films and porous structures for various applications, showcasing their effectiveness in controlling solubility parameters, promoting phase separation, and enhancing the performance of electronic devices and gas sensors.