Plastic-Syringe Induced Silicone Contamination in Organic Photovoltaic Fabrication: Implications for Small-Volume Additives

Abstract

Herein, the implications of silicone contamination found in solution-processed conjugated polymer solar cells are explored. Similar to a previous work based on molecular cells, we find this contamination as a result of the use of plastic syringes during fabrication. However, in contrast to the molecular case, we find that glass-syringe fabricated devices give superior performance than plastic-syringe fabricated devices in poly(3-hexylthiophene)-based cells. We find that the unintentional silicone addition alters the solution's wettability, which translates to a thinner, less absorbent film on spinning. With many groups studying the effects of small-volume additives, this work should be closely considered as many of these additives may also directly alter the solutions' wettability, or the amount of silicone dissolved off the plastic syringes, or both. Thereby, film thickness, which generally is not reported in detail, can vary significantly from device to device. KEYWORDS: organic, photovoltaic, solar, polymer, silicone, plastic, PDMS, P3HT ■ INTRODUCTIONIn the global search for clean and sustainable energy sources, organic photovoltaics (OPVs) have recently gained much attention. Facets such as solution processability, light weight, low cost, and the potential for roll-to-roll production make OPVs an advantageous option for the realization of greenpower generation. Popular modern-day organic solar cells are fabricated in what is known as the dispersed bulk-heterojunction (BHJ) architecture, formed from blends of conjugated polymers or small molecules with fullerene derivatives. 1 Since its introduction in the mid-1990s, many groups have suggested numerous methods for increasing BHJ power conversion efficiency (PCE). Among these are small-volume additives, in which a macromolecule or solvent are mixed with the BHJ blend at percentages typically less than 10%. 2−10 Most of these reports cite an improved morphology as the reason behind performance improvement, whether it is increased mobility from larger grain sizes, or increased exciton dissociation from smaller grain sizes. What is generally overlooked is how an additive changes the wetting of the BHJ blend solution on the substrate surface. As we show in this work, additive-induced differential wetting can lead to a notable change in film thickness, which alone can significantly affect light absorption and photocurrent generation. Moreover, films of different thicknesses dry at different rates, which in turn can affect the internal BHJ morphology and other device parameters. Thus, an additive-induced differential wetting can significantly affect device performance, without playing a direct role in altering morphology of the bulk BHJ film. We showcase the above using silicone as a small-volume additive, which is unintentionally introduced by plastic-syringes widely used in OPV fabrication.Recently, a work by Graham et al. showed that silicone contamination, induced during fabrication by the use of plastic syringes, can act as a small-volume additive. 2...

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