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

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(3-hexylthiophene) (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 degrees, 45 degrees-61 degrees, and 8 degrees 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.