Atom probe tomography of small-molecule organic semiconducting materials

Abstract

For organic electronics, film morphology is crucial to device performance, requiring techniques with both high spatial resolution and chemical sensitivity that are suitable for these materials. This work demonstrates that atom probe tomography (APT) is well-suited to this purpose. It can provide sub-dalton mass resolution, detection thresholds of less than 100 ppm, and spatial distribution of molecules with better than nanometer precision. These capabilities mean that APT can successfully analyze systems of interest to the organic electronics community, revealing new morphological information that can enable better devices through improved understanding of structure-property relationships. To demonstrate the power of APT to uncover structure-property relationships in organic systems that have proven extremely difficult to probe using existing techniques, three examples are discussed: (1) a model organic photovoltaic system in which a chemical reaction occurs at the heterointerface, explaining a change in open circuit voltage; (2) a model organic light-emitting diode (OLED) system in which molecular segregation occurs in the emissive layer bulk, which has ramifications for efficiency; and (3) controlled ultraviolet exposure of an OLED emitter in which photodegradation occurs, quantifying degradation product hierarchies. These examples illustrate the power of APT to enable new insights into organic molecular materials. Additionally, a new tomographic reconstruction method is presented that corrects for near field trajectory aberrations. It does so by correcting for detector density fluctuations in an unbiased way that generates an ensemble of solutions. This is demonstrated with a simulated sample of amorphous Si with small B clusters, a system in which there is a large field difference that can completely obscure clustering signal. Comparing this new method with the standard commercial protocol, the new method improves the accuracy of reconstruction and allows for better spatial signal recovery. This enables analysis of more challenging materials systems with APT. APT creates numerous opportunities for studying organic electronic systems. As a result of its spatially resolved chemical information, APT allows for quantitative understanding of composition, morphology, phase behavior, device physics, and device degradation. APT is invaluable for furthering our understanding of organic electronic systems and enables us to collect information that was previously inaccessible.