(354f) High Performance Roll-to-Roll Printed PTB7-Th/PCBM Organic Solar Cells

Despite having surpassed 10% power conversion efficiency (PCE), widely held as the threshold for commercial viability, high performance organic photovoltaics (OPVs) are still mostly constrained to lab-scale devices fabricated by spin coating. Efforts to produce scalable printed OPVs trail significantly in efficiency, highlighting the need to better understand the processing-morphology-performance relationship in the context of linear printing processes. Here we investigate the promising OPV system PTB7-Th/PC71BM, which has demonstrated >10% PCE via spincoating but only exhibits ~1% PCE when roll-to-roll printed due to differences in drying dynamics. Of particular interest is the ubiquitous alcohol wash post-treatment applied to the dried active layer, which induces a significant improvement in device performance, and its crucial role for printed films. While it has been speculated that the primary utility of the alcohol post-treatment is to remove the additive 1,8-diiodooctane (DIO) residue in the dried film, we find here that the wash process itself dramatically impacts morphology in printed films regardless of the presence of DIO. Here we employ various x-ray characterization techniques to probe phase separation, crystallinity, and molecular orientation, as well as in-situ grazing-incidence x-ray diffraction (in-situ GIXD) to monitor morphological evolution during the isopropanol post-treatment process. It is discovered that isopropanol induces significant donor polymer alignment and enhanced π-π degree of crystallinity. Through the understanding gained in this study, we are able to achieve a roll-to-roll printed OPV with 5% PCE, which is to our knowledge the highest reported performance for a roll-to-roll printed single junction photoactive layer on a flexible substrate.