Model Supported In-Line Characterizaton of Roll-to-Roll Coated Solar Cells

The application of scalable processing methods like roll-to-roll coating is being adopted with increasing intensity in the field of organic and hybrid perovskite solar cells. This development calls for characterization methods matching the speed and volume of such deposition techniques. One such method is <i>in situ</i> X-ray scattering which has followed step, allowing high-throughput analysis of the effect of fine-grained variation of processing parameters and inline analysis of the kinetics of structure formation.<br/>Analyses of non-crystalline nanostructure in thin films coated on flexible substrates are still few and low in detail. The analysis is complicated by the overlapping contributions of scattering from substrate and other nanostructured layers, but there are also considerable advantages by working with coatings on a flexible substrate. Besides studying the coated thin films under the actual processing conditions that allow scaling to mass production, it also facilitates large scale analysis of processing parameters with high resolution. Böttiger <i>et al</i>.¹ demonstrated this as a structural analysis extension to an earlier large scale solar cell device analysis experiment, where constituent concentration ratio and film thickness was varied over tens of meters of roll-coated devices². In this manner, “recording” thousands of experiment variations along the substrate foil, high-resolution analysis of parameter space is allowed by “playing back” the coating, either on a fast testing device to record solar cell device parameters (IV-curves), or through a high-flux X-ray beam to record the small- and wide-angle scattering from the coated film. Acquisition times down to a few hundred milliseconds can be realized corresponding to a spatial resolution along the coated foil of a millimeter to a centimetre for foil speeds between 0.5 to 5 m/min³. For an experiment where a component concentration is varied from 0% to 100%, this corresponds to a resolution in concentration parameter space of better than 0.01%, or better than 0.001% for the case of additive concentration, varied from 0% to 5%¹. This is well below what can be expected in mixing accuracy from syringe pumps and uncertainties in the flow dynamics of the microchannel solution mixing structure. With fewer measurements along the coated foil and thus a lower resolution of parameter space similar experiments can be completed in a few days with an optimized laboratory instrument where acceptable counting statistics can be achieved in 100 s exposures^4-6.<br/>We have focused on developing an in-line methodology that is supported by molecular dynamics simulation⁷ in order to disentangle the scattering fingerprints that may eventually be used to steer mesoscale structure formation to achieve the optimal photovoltaic performance of a bulk heterojunction. As will be shown, the methodology may be applied both on laboratory scale setups⁸ as well as on synchrotron beam lines, and may even be used in conjunction with coating conditions far removed from ambient, i.e. with fully heated solution, coating head and substrate to handle materials that otherwise quickly aggregate and gelate. In combination with optical probes and machine learning techniques, the methodology is expected to close the lab-to-fab gap that continues to hold back the commercial breakthrough of organic solar cells.<br/>1. A.P.L. Böttiger <i>et al.</i> <i>J. Mater. Chem.</i>, <b>22</b>(<b>42</b>) 22501–22509 (2012).<br/>2. J. Alstrup <i>et al.</i>, <i>ACS Appl. Mater. Interfaces</i>, <b>2</b>(<b>10</b>) 2819–27 (2010).<br/>3. L.H. Rossander, <i>et al.</i>, <i>AIP Adv.</i>, <b>4</b>(<b>8</b>) 087105, 8 p. (2014).<br/>4. L. H. Rossander <i>et al.</i>, <i>Energy Environ. Sci.</i>, <b>10</b>, 2411-2419 (2017).<br/>5. N.K. Zawacka, <i>et al.</i>, <i>J. Mater. Chem. A</i>, <b>2</b>(<b>43</b>) 18644–18654 (2014).<br/>6. T.R. Andersen, <i>et al.</i>, <i>Energy Environ. Sci.</i>, <b>7</b>(<b>9</b>) 2925–2933 (2014).<br/>7. A. S. Gertsen <i>et al.</i>, <i>Phys. Rev. Materials</i> <b>4</b>, 075405 (2020).<br/>8. M. K. Sørensen <i>et al.,</i> <i>J. Vis. Exp.</i> (169), e61374, doi:10.3791/61374 (2021).