Understanding Morphology Evolution In Printed Organic Solar Cells

Polymer-based organic photovoltaics (OPVs) have emerged as a promising renewable energy candidate suitable for inexpensive and scalable production, being lightweight, flexible, and amenable to low-energy solution processing. However, despite having surpassed 10% power conversion efficiency (PCE) - widely held as the threshold for commercial viability - OPVs are still mostly constrained to lab-scale devices fabricated by spin coating. Efforts to translate to scalable roll-to-roll printing trail significantly in efficiency, commonly by an order of magnitude, highlighting the need to better understand the processing-morphology-performance relationship in the context of linear printing methods. The work presented will focus on two aspects of OPV development: 1) process control to translate from spin coating to printing in order to achieve scalable high-performance devices, and 2) application of improved tools for nanoscale morphological characterization. To the former, a thermodynamic model of phase separation is presented for a model polymer:fullerene system. Next we investigate a high-performance system which has demonstrated > 10% PCE via spincoating but only exhibits 1% PCE when roll-to-roll printed due to differences in drying dynamics and phase separation. OPV bulk heterojunctions are characterized using synchrotron X-ray scattering techniques, elucidating the impact of a critical residual chemical additive on the phase-separated morphology. It is discovered that excessive additive residence time within the semi-dry film gives rise to a hierarchal morphology that severely degrades device performance. Using the understanding gained in this study, we are able to achieve a printed OPV with 5.33% PCE, which is among the highest performing roll-to-roll OPVs to date. To the latter, we address the fact that commonly used microscopy techniques suffer from significant shortcomings for imaging OPVs. We demonstrate the first application of a technique known as Photo-induced Force Microscopy (PiFM) for imaging OPVs with nanoscale chemical specificity. Results from image processing are corroborated with established synchrotron methods and photovoltaic device performance, revealing excellent quantitative agreement. Further, we demonstrate that images from atomic force microscopy (AFM) and PiFM show poor correlation, highlighting the need to move beyond standard AFM for morphology characterization of bulk heterojunctions. We emphasize that PiFM is high-throughput, lab-scale, ambient, and requires no special sample preparation, filling an important underserved role in imaging of OPVs.