Mapping nanoscale effects of localized noise-source activities on photoconductive charge transports in polymer-blend films

摘要

We develolped a method to directly image the nanoscale effects of localized noise-source activities on photoconducting charge transports in domain structures of phase-separated polymer-blend films of Poly(9,9-di-n-octylfluorenyl-2,7-diyl) and Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole). For the imaging, current and noise maps of the polymer-blend were recorded using a conducting nanoprobe in contact with the surface, enabling the conductivity (sigma) and noise-source density (N-T) mappings under an external stimulus. The blend-films exhibited the phase-separation between the constituent polymers at domains level. Within a domain, high sigma (low N-T) and low sigma (high N-T) regions were observed, which could be associated with the ordered and disordered regions of a domain. In the N-T maps, we observed that noise-sources strongly affected the conduction mechanism, resulting in a scaling behavior of sigma alpha N-T(-0.5) in both ordered and disordered regions. When a blend film was under an influence of an external stimulus such as a high bias or an illumination, an increase in the sigma was observed, but that also resulted in increases in the N-T as a trade-off. Interestingly, the Delta sigma versus Delta N-T plot exhibited an unusual scaling behavior of Delta sigma alpha Delta N-T(0.5), which is attributed to the de-trapping of carriers from deep traps by the external stimuli. In addition, we found that an external stimulus increased the conductivity at the interfaces without significantly increasing their N-T, which can be the origin of the superior performances of polymer-blend based devices. These results provide valuable insight about the effects of noise-sources on nanoscale optoelectronic properties in polymerblend films, which can be an important guideline for improving devices based on polymer-blend.