1,8-diiodooctane acts as a photo-acid in organic solar cells

Abstract

The last decade saw myriad new donor polymers, among which benzodithiophene-co-thienothiophene polymers are attractive due to their relatively high power conversion efficiency in bulk heterojunction solar cells. We examine the effect of UV-light on the stability of these polymers. The relationship between the polymer chemical structure and the UV-stability of the cells is explored on the one hand, and on the other hand, the effect of additives on their UV-stability: 1,8-diiodooctane against 1-chloronaphthalene in the cells and 1,8-octanedithiol in solution. For example, PBDTTT-E with 18% efficiency loss is more stable than PBDTTT-ET with 36% loss throughout the exposure. While 1,8-diiodooctane acts as photo-acid and leads to accelerated degradation of the solar cells, 1-chloronaphthalene does not. Acidity is known to be detrimental to the efficiency and stability of organic solar cells. The degradation is initiated upon UV-irradiation by the cleavage of the side chains, resulting in more electron traps and by the formation of iodine, dissolved HI and carbon-centered radicals from 1,8-diiodooctane as revealed by 1H NMR spectrum. The 1,8-octanedithiol spectra do not show such species. Finally, the mechanisms behind the effect of 1,8-diiodooctane are explained, paving the way for the design of new, efficient as well as stable materials and additives.

Structure dependent photostability of ITIC and ITIC-4F

“…Nevertheless, as a lesson learned from the fullerene-based OPV world, it is now well known that DIO leads to an accelerated device degradation. 6,7 Therefore, an in-depth understanding of the basic morphological properties related to such novel highly efficient acceptors, and how these can ultimately impact the device efficiency and stability, seems essential.…”

Effects of the Reduction and/or Fluorination of the TT‐Units in BDT‐TT Polymers on the Photostability of Polymer:Fullerene Solar Cells

“…These figures show the comparative PCE decays for the best performing devices while Figure S3, Supporting Information displays the comparative average trend in PCE decays, for all seven PFSCs. The trends are similar in both 1D and 2D PFSCs, however, it is more pronounced in the 2D PFSCs, certainly due to the effect of the alkylthienyl substituent on the BDT, which were previously shown [33,37] through 1 HNMR of the monomers to make them much less stable.…”

“…Details of device counts with the efficiency spread of the 1D and 2D PFSCs are displayed in Figure S1C–E, Supporting Information. Note, the solar cells are not processed with additive, such as diiodooctane (DIO) in order to avoid the degradation effects from the additive. Nonetheless, in general, the reported PCEs are close enough to the literature values, especially for devices without DIO.…”

“…Thus, leading to their faster degradation compared to the other solar cells of the same family. The alkoxy groups are known to be easily cleavable in films and solutions under continuous exposure to light, heat, oxygen or the combination of any of the above. This supports the notion that the increase in trap density is the reason behind the observed trends in degradation.…”

Water Compatible Direct (Hetero)arylation Polymerization of PPDT2FBT: A Pathway Towards Large‐Scale Production of Organic Solar Cells

“…As demonstrated by Sprau, [51] AA is less toxic than most solvent additives, such as 1,8-diiodooctane (DIO), 1-chloronaphtalene (CN), and even DPE, and is also very compatible with o-xylene as the main solvent. Furthermore, DIO decreases the photostability of OSCs, due to its photo-acidity, [52] in contrast to AA, which showed the opposite effect on the polymer:fullerene bulk heterojunction (BHJ). [53] The solvent composition was optimized as well as the polymer:fullerene ratio.…”