Hierarchical structures and multiscale optical coupling for improved photodetectors

Hierarchical structures comprise of patterns which are themselves structured at lower length scales. Such structures can outperform the performance limits posed by continuum and non-hierarchical structures. However, the possible role of hierarchical structuring in photonics and optoelectronics are not well understood. Here we report on the implications of using materials structured at multiple length scales in the design of spectrally uniform photodetectors. We present our results on the design and fabrication of hierarchical structures comprising of alternating planar and nanostructured microscale domains interspersed with nanoscale objects. These structures show the possibility of obtaining unusual control over the flow of light compared to conventional non-hierarchical structures through the coupling of light across micro-domains. These multiscale structures are fabricated using self-assembly and a novel mechanical strain-augmented template based nano-molding process. We fabricate multi-periodic nanostructures embedded in a random distribution of micro-domains, using an initial template which has only monoperiodic structures. Organic photodetectors fabricated on these molded multiscale platforms show considerable improvements in spectral uniformity. This design advantage arises through the multiscale optical processes which preferentially filter light entering the absorber, which occur only in a hierarchically structured device. In summary, this paper explores unusual ways to control the flow of light using hierarchical structures. On the optoelectronic design front, hierarchical structures are seen to improve the spectral uniformity of photodetectors beyond the limits of continuum and non-hierarchical material design. Further, this effort pushes the limits of monodisperse self-assembly to fabricate static multiperiodic structures, using facile mechanical strain augmented nanofabrication, which was hitherto a challenge.