Variable-force tapping atomic force microscopy as a tool in the characterization of organic devices

A common method for characterizing the phase separation of materials in mixtures is tapping mode atomic force microscopy (AFM). However, AFM results are influenced by surface-energy effects and the employed tapping force, and it might therefore be difficult to attain correct information regarding the bulk with such a surface-imaging technique. In this work, we present a way of imaging material phase separation in an improved manner by recording a series of AFM images at different tapping force. More specifically, we have employed the variable-force AFM method on organic mixtures, comprising a conjugated polymer (MEH-PPV) and an ion-conducting polymer electrolyte (PEO-XCF(3)SO(3), X=Li, K, Rb), and we demonstrate that it is capable of reversibly sampling such materials not only on the surface, but also (indirectly) in the topmost part of the bulk. The analysis of the evolution of AFM phase images allows us to (indirectly) gain information about the bulk-phase separation of materials. We find that the variable-force AFM results correlate well with the device performance of light-emitting electrochemical cells employing such organic mixtures as the active material.