Organic solar cells are receiving considerable attention as low-cost alternatives to silicon photovoltaics because their active layers can be manufactured through low temperature solution processes. They can also be produced as flexible or semi-transparent photovoltaic technologies which considerably facilitates their integration into a smart and sustainable society. The active layers based on conjugated polymer electron donors (EDs) blended with electron acceptors (EAs) are usually spin-coated from chlorinated solutions. However, during spin-coating, 90% of the chlorinated solution is ejected outside the substrate, which results in large amounts of costly material and hazardous solvent wastes (Figure 1).<br/>Here, we introduce a new conjugted polymer thin film fabrication method called push-coating which generates no material waste and reduces the amount of employed hazardous solvent by a factor of 20. A minimal amount of solution is placed between a substrate and a flat polydimethylsiloxane (PDMS) stamp, which spreads through capillary forces within the boundaries of the substrate or the PDMS stamp to produce uniform wet films. As the small amount of solvent diffuses into PDMS, uniform thin films can be easily manufactured.<br/>We successfully fabricated ED:fullerene push-coated organic solar cells with power conversion efficiencies up to 5.8%, a value similar to those of spin-coated ones. However, EA:non-fullerene acceptors push-coated active layers do not perform as well as spin-coated ones. Through a systematic study involving coefficients of solvent diffusion into PDMS, crystallite formation and crystallite orientation, we elucidate the morphological differences between the spin-coated and push-coated films and correlate those with their photovoltaic performances.<br/>Despite the slightly lower performances obtained in push-coated ED:non-fullerene acceptor active layers compared to spin-coated ones, push-coating demonstrates a great promise for the low-cost and environment-friendly fabrication of functional polymer thin films that could be applied in a variety of fields. In fact, the use of push-coating can be broadened to any field employing polymer films produced using solvents that diffuse into PDMS.

Low-cost, green & sustainable push-coated conjugated polymer-based thin films for environment-friendly organic photovoltaics

ACS Spring Meeting 2021

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