Tuning the Organic Solar Cell Performance of Acceptor 2,6-Dialkylaminonaphthalene Diimides by Varying a Linker between the Imide Nitrogen and a Thiophene Group

Abstract

Core-substituted naphthalene diimides (NDI) are promising candidates as acceptors for organic solar cells. To study their structure−property relationships, a series of 2,6-dialkylamino-NDI compounds with various substituents were synthesized, characterized, and tested in bulk heterojunction solar cells by blending with regioregular poly(3hexylthiophene) (P3HT). The imide substituents consisted of a linker connected to a thiophene group, where the linker was phenyl, methyl, or ethyl. The core substituents were cyclohexylamino or 2-ethylhexylamino. While the various substituents had little effect on the optoelectronic properties in solution, they strongly affected device performance and blend morphology. Under the conditions studied, the best performance was obtained with the methyl linker combined with the cyclohexylamino core substituent, with a power conversion efficiency of 0.48% and a high open circuit voltage of 0.97 V. For blends of P3HT with modified NDI non-fullerene acceptors, the methyl linker promoted larger phase-separated domains than the ethyl or phenyl linkers. DFT calculations showed that the linker determines the orientation of the thiophene conjugated plane with respect to the NDI conjugated plane. That angle was 114°, 45°−61°, and 8°for the methyl, phenyl, and ethyl linkers, respectively. Using thiophene at the end of the imide substituent adds a unique dimension to tune morphology and influence the molecular heterojunction between donor and acceptor.

Processing additive suppresses phase separation in the active layer of organic photovoltaics based on naphthalene diimide

“…2 shows the UV/vis absorption spectra of RF1 in solution and as a film. As previously reported [39], RF1 has an absorption band in the visible range near 630 nm due to the amino/imide push/pull system of these core-substituted naphthalene diimide molecules. A slight broadening of the visible absorption band is apparent in the solid-state.…”

“…Solar cells had a configuration of glass/ITO/PEDOT:PSS (30 nm)/P3HT:RF1 (70-80 nm)/Ca (25 nm)/Al (100 nm) and devices were tested as-cast or annealed at 100°C for 1 h. These annealing conditions were chosen based on previous tests for comparable core-substituted NDI molecules [39]. All active layer thicknesses were similar, between 70 nm and 80 nm thick.…”

“…1 M n = 33.9 kg mol À1 , M w = 38.8 kg mol À1 , PDI = 1.15. The synthesis and characterization of RF1 was reported elsewhere [39]. PEDOT:PSS was purchased from Heraeus Materials Technology, LLC.…”

“…The acceptor that combined the methyl linker and the cyclohexylamino core substitu- Fig. 1), gave the best performance when blended with P3HT [39]. Interestingly, the blend morphology did not appear ideal, showing large crystalline domains.…”

Charge Generation Pathways in Organic Solar Cells: Assessing the Contribution from the Electron Acceptor

“…134 In a recent report by Sauvéet al, 2,6-dialkylaminonaphthalene diimides ( Figure 22) were studied as nonfullerene acceptors with P3HT as the donor. 135 The absorption spectra of these NDIs can be seen in Figure 23 (panels a and b). All of the materials had solution absorption spectra similar to that already reported for 2,6-dialkylamino NDIs, 136,137 with a strong absorption at 500−660 nm, which is a charge transfer band associated with the push−pull system between the donating alkylamino core substituents and the withdrawing imides, and a high energy band at 350−360 nm, which is due to the localized transitions in the naphthalene core.…”