Film-Depth-Dependent Charge Mobilities in Organic Semiconductor Films

Carrier mobility is a crucial parameter characterizing the performance of organic semiconductors in thin film optoelectronic devices. The organic semiconductor film is usually not uniform along the film-perpendicular direction. This non-uniformity affects the carrier mobility at various depths within the film, thereby influencing device performance. Conventional methods for measuring mobility are basically not available to determine film-depth-dependent charge density or mobility. In this study, an easily accessed approach was proposed to investigate the film-depth-dependent electronic properties. Soft plasma etching was introduced to achieve incremental etching of the film, without causing damage to the underlying sublayers. For doped films with intrinsic charge, film-depth-dependent charge density, and film-depth-dependent mobility were extracted from the transfer characteristics evolution during the etching. For materials without intrinsic charge, film-depth-dependences of mobility could also be estimated under light illumination. The proposed method achieves a film-depth resolution of approximately 10 nm. It is revealed that for some model conjugated polymers, the mobility at the top surface is one order higher than that at the buried region of the transistor. This feature leads to unfavorable field-effect mobility. This work provides insights into the film-depth-dependent electronic properties and a strategy to optimize the electronic device performance.