Elucidating the Impact of Alcohol Post-Processing in High Performance Roll-to-Roll Printed Organic Photovoltaics

Despite having surpassed 10% power conversion efficiency (PCE), which has long been held as the threshold for commercial viability, high performance organic photovoltaics (OPVs) are still mostly constrained to lab-scale devices fabricated on rigid substrates by spin coating. In order to fully realize the often-touted advantages of being lightweight, flexible, and scalable, it is necessary to translate this high efficiency to roll-to-roll printing methods. 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 continuously roll-to-roll printed. 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) in spin coated films, we find 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 wide-angle x-ray scattering (in-situ GIWAXS) to monitor morphological evolution during the alcohol post-treatment process. It is discovered that isopropanol induces significant polymer alignment and enhanced π-π relative 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.