Microgravity as a Tool for a Molecular Understanding of the Active Layer Formation in Organic Solar Cells

Solution–processed organic photovoltaic (OPV) devices have gained serious attention during the last decade.¹ The active layer of an OPV solar cell consists of a thin solid film of an electron donor blended with an electron acceptor. One important factor for the performance is the nanostructured morphology of the active layer. It is known that molecular interactions govern the morphology formation process, whether it is a polymer–polymer system or a polymer-small molecule system.² Understanding the factors determining this structure formation on a molecular level enables not only morphology control, but also the prediction of suitable and more environmentally friendly solvents for a greener processing of OPV.<br/>One way to enable a detailed study of the initial stage of the phase separation and hence the influence of molecular interactions, is to slow down the phase-separation process. Minimizing the influence from gravity on the film formation is known to slow down this process during the film formation.^3,4<br/>In this work we have prepared active layers under microgravity conditions at parabolic flights.⁵ For this purpose, we have designed a custom–built experiment setup for dip–coating from volatile solutions under microgravity conditions, meeting the security measures for parabolic flights.⁶ The resulting thin blend films are characterized using AFM and AFM-IR, the latter combining the high resolution of AFM with the chemical fingerprint of infrared spectroscopy. It is shown that the morphology is similar to films prepared at 1g conditions, but with differences that can be related to the absence of a gravitational field during the drying of the applied liquid coating. Film thickness as well as the size of structures due to phase–separation are shown to depend on the level of gravity during the drying of the films.<br/>In parabolic flights, the microgravity phase lasts for 20–25 seconds, which is too short to ensure complete evaporation of the solvents. To guarantee complete evaporation under microgravity conditions, sounding rocket experiments with 6 minutes of microgravity will be performed in the autumn 2023. The construction of the equipment for sounding rocket flights is ongoing and the design concept will be described.<br/><br/>References:<br/>1. Brabec, C. J. et al. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 22, 3839–3856 (2010).<br/>2. Ye, L. et al. Quantitative relations between interaction parameter, miscibility and function in organic solar cells. Nat. Mater. 17, 253–260 (2018).<br/>3. Bamberger, S. et al. in Separations Using Aqueous Phase Systems, edited by D. Fisher and I. A. Sutherland (Springer, Boston, MA, 1989), pp. 281–286.<br/>4. Bailey, A.E. et al. Spinodal Decomposition in a Model Colloid-Polymer Mixture in Microgravity, Phys. Rev. Lett. 99(20), 205701 (2007).<br/>5. 70^th ESA Parabolic Flight Campaign (2018) and 78^th ESA Parabolic Flight Campaign (2022).<br/>6. Ericsson, L.K.E. et al. An experimental setup for dip-coating of thin films for organic solar cells under microgravity conditions, Rev. Sci. Instrum. 92, 015108 (2021).