Abstract
Finding the optimal morphology of novel organic photovoltaic (OPV) polymer blends is a major obstacle slowing the development of more efficient OPV devices. With a focus on accelerating the systematic morphology optimisation process, we demonstrate a technique offering rapid high-resolution, 3-dimensional blend morphology analysis in the scanning electron microscope. This backscattered electron imaging technique is used to investigate the morphological features and length-scales defining the promising PffBT4T-2OD:PC70BM blend system and show how its photovoltaic performance is related to the nature of its phase separation. Low-voltage backscattered electron imaging can be used to probe for structure and domain stacking through the thickness of the film, as well as imaging surface morphology with highly competitive spatial resolution. For reference, we compare our results with equivalent images of the widely studied P3HT:PC60BM blend system. Our results also demonstrate that backscattered electron imaging offers significant advantages over conventional cross-sectional imaging techniques, and show that it enables a fast, systematic approach to control 3-dimensional active layer morphology in polymer:fullerene blends.
Highlights
Understanding the nature of phase separation in polymer blends is of great importance for obtaining the optimal performance from various blend systems [1]
Features are visible in all images that resemble phase separation in a polymer blend morphology
We have measured the image contrast between pure film samples of PffBT4T2OD, P3HT, PC70BM and PC60BM, and use this to assign the brighter regions in the images to polymer phases, with the darker regions being assigned to the fullerene
Summary
Understanding the nature of phase separation in polymer blends is of great importance for obtaining the optimal performance from various blend systems [1]. Polymer blends have found a wide range of applications in the current energy landscape, having been recently used in novel electrolyte layers in batteries [2] or dye-sensitised solar cells [3,4], for example. They are prevalent in the field of organic photovoltaics (OPV), where control over the phaseseparated morphology of the blend is a critical factor determining the photovoltaic power-conversion efficiency (PCE) [5,6,7,8,9]. In spite of its potential, this blend remains somewhat unexplored with no detailed model of its 3-dimensional morphology yet reported
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