Green Inks for the Fabrication of Organic Solar Cells: A Case Study on PBDTTPD:PC 61 BM Bulk Heterojunctions

Out of a database of 1239 solvents, we found some nonhalogenated solvents within the solubility spheres that comply with a strict list of solvent requirements including evaporation and toxicity criteria

Christian Sprau; Ana Milena Cruz; Lorenzo Bautista; Laura Molina; Michael Wagner; Christos L. Chochos; Mónica Della Pirriera; Alexander Colsmann

2021

Scholarcy highlights

  • The absorbance spectrum of the light-harvesting layer processed from chlorobenzene in Figure 2d is very similar to previous reports with most pronounced contributions from PBDTTPD in the wavelength regime beyond 450 nm and phenyl C61-butyric acid methyl ester dominating the spectrum in the wavelength regime below 450 nm, with its characteristic feature at 330 nm. Solar cells cast from the solvent candidate o-xylene exhibited good film forming properties and similar absorbance characteristics, yielding an almost identical average PCE of 4.2%, whereas all other solvents produced only moderate results
  • To determine the Hansen solubility parameters of the semiconductor components, PBDTTPD and phenyl C61-butyric acid methyl ester were separately dissolved in mixtures of a good main solvent and a cosolvent
  • The HSP approach is a powerful tool to facilitate the first step of layer formation, i.e., the dissolution of the organic semiconductors
  • The situation becomes even more complex, if a binary solvent is used, where the initially optimal solvent ratio gained from the HSP analysis may change during stage two, which is the initial drying process, e.g., due to different boiling points of the binary solvent components
  • The investigations presented in this work clearly demonstrate some of the limitations of the HSP formalism that are important to consider when looking for other suitable solvents for the deposition of bulk heterojunction solar cells
  • Diminished by shunting, as became visible in the gradually increasing current under reverse bias both under illumination and in the dark. This was most drastically visible when the devices were fabricated from 80:20 mixtures of p-xylene:BA, where all key parameters, including the VOC, were significantly deteriorated, yielding PCEs of less than 1%
  • We found the solvent eucalyptol in our HSP study which is, as a natural product, fully ecofriendly and biodegradable, and which, in combination with the ecofriendly BA and AA produced organic solar cells with good performance

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