Film-Depth-Dependent Light Absorption Spectroscopy of Organic Thin Films

Abstract

Organic thin films, with thickness ranging from tens to hundreds of nanometers, are foundational to organic electronic devices. Yet, vertical phase separation during film deposition and postprocessing, along with the corresponding variations in crystallinity and photoelectric properties along the film-depth direction, critically influences device performance. Traditional characterization methods, such as cross-sectional electron microscopy, neutron reflectivity, or incremental etching coupled with mass spectrometry, suffer from key limitations: sample-destructive analysis, high-cost or inaccessible instrument, or an inability to directly probe depth-dependent optoelectronic behaviors inside the film. These shortcomings hinder the optimal design of high-performance devices. To overcome these challenges, we developed film-depth-dependent light absorption spectroscopy (FLAS), an innovative and accessible technique combining soft plasma etching with UV–vis spectroscopy. FLAS achieves ∼1 nm depth resolution of vertical phase separation, enables seminondestructive profiling of vertical properties, and directly correlates structural insights with device optimization. Over the past decade, FLAS has been widely applied for the depth-dependent analysis of organic films, and its effectiveness in elucidating the relationship between favorable film structure and high device performance has been validated. In this Account, we focus on the mechanisms and device applications of FLAS, aiming to enhance the understanding of vertical variations in organic films and to advance the fabrication of high-performance organic electronic devices. We begin by discussing the challenges associated with characterizing vertical phase separation and other depth-dependent variations in organic films, followed by the introduction of FLAS as an effective solution. We briefly highlight its advantages over conventional analytical techniques. Subsequently, we outline the principles of FLAS, including the Beer–Lambert law, which relates film absorbance to the content of different components, as well as the qualitative and quantitative relationships between depth-dependent absorption spectra and other depth-dependent variables including composition, crystallinity, optical and electrical variations. Elementary mathematical physics approaches, such as the least-squares method and the transfer matrix method (TMM), are employed to simulate film-depth-dependent composition and photoelectric field distribution, respectively. This is followed by a brief introduction to the supporting technique of surface-selective etching using low-pressure oxygen plasma, which enables the removal of surface layers without damage to the underlying sublayers. We then summarize several alternative depth-resolved analytical methods derived in the development of FLAS, such as film-depth-dependent light reflection and infrared spectroscopy. Next, the applications of FLAS in organic electronics, particularly in field-effect transistors, solar cells, photodetectors and thermoelectric devices, are explored. Finally, we evaluate FLAS and outline future prospects for depth-resolved characterizations in further investigations. This Account would provide a comprehensive understanding of how FLAS spectra correlate with other film-depth-dependent variables and offer a methodological framework for optimizing the performance of organic electronic devices.