Flexible Distributed Bragg Reflectors from Nanocolumnar Templates.

Technological evolution of flexible optoelectronic devices, such as solar cells or light emitting diodes,[1–6] must be accompanied by that of flexible optical and electronic materials in order to fulfill the increasing requirements of higher performance and functionality. In that regard, last years have seen a boost of the research in this field, a wide variety of approaches having been taken to prepare flexible components.[7–11] Integration into devices will require stability of their properties upon bending and stretching, adaptability to different types of substrates, durability, etc.[12] Among optical materials employed in optoelectronics, distributed Bragg reflectors (DBRs) play a central role as either photon frequency filters, optical cavities to enhance spontaneous or stimulated emission[13] or simply as mirrors to increase the time of residence of photons in absorbing electrodes and thus improve their photovoltaic performance.[14–16] In order to attain the flexible version of the classical stratified multilayer structure characteristic of DBRs, a common strategy is to stack thin polymer layers of alternate refractive index.[17] Enhanced spontaneous emission has been observed from flexible optical cavities prepared based on this approach.[18] Also, by rolling two polymers around a cylindrical core, tunable elastic optical multilayered fibers have been attained.[19] However, elastomers usually possess low refractive indexes, typically comprised in a narrow range, so a small dielectric contrast is typically achieved. This means that a large number of layers must be stuck in order to reach intense reflections and only in narrow spectral ranges. One way to increase the refractive index contrast between layers relies on the deposition of hybrid precursors made of a mixture of polymers and higher refractive index inorganic particles.[20] In this way, the refractive index contrast is increased, although its value is limited by the amount of particles that can be dispersed in the polymers. In addition, this approach requires a high degree of physicochemical compatibility of the particle surface with the polymer, or the strict control of the particle formation in a polymeric medium.[21]