title: Slip-stacked perylenediimides as an alternative strategy for high efficiency nonfullerene acceptors in organic photovoltaics.
abstract: Perylenediimide (PDI)-based acceptors offer a potential replacement for fullerenes in bulk-heterojunction (BHJ) organic photovoltaic cells (OPVs). The most promising efforts have focused on creating twisted PDI dimers to disrupt aggregation and thereby suppress excimer formation. Here, we present an alternative strategy for developing high-performance OPVs based on PDI acceptors that promote slip-stacking in the solid state, thus preventing the coupling necessary for rapid excimer formation. This packing structure is accomplished by substitution at the PDI 2,5,8,11-positions ("headland positions"). Using this design principle, three PDI acceptors, N,N-bis(n-octyl)-2,5,8,11-tetra(n-hexyl)-PDI (Hexyl-PDI), N,N-bis(n-octyl)-2,5,8,11-tetraphenethyl-PDI (Phenethyl-PDI), and N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (Phenyl-PDI), were synthesized, and their molecular and electronic structures were characterized. They were then blended with the donor polymer PBTI3T, and inverted OPVs of the structure ITO/ZnO/Active Layer/MoO3/Ag were fabricated and characterized. Of these, 1:1 PBTI3T:Phenyl-PDI proved to have the best performance with Jsc = 6.56 mA/cm(2), Voc = 1.024 V, FF = 54.59%, and power conversion efficiency (PCE) = 3.67%. Devices fabricated with Phenethyl-PDI and Hexyl-PDI have significantly lower performance. The thin film morphology and the electronic and photophysical properties of the three materials are examined, and although all three materials undergo efficient charge separation, PBTI3T:Phenyl-PDI is found to have the deepest LUMO, intermediate crystallinity, and the most well-mixed domains. This minimizes geminate recombination in Phenyl-PDI OPVs and affords the highest PCE. Thus, slip-stacked PDI strategies represent a promising approach to fullerene replacements in BHJ OPVs.
authors.affiliation: Department of Chemistry and the Materials Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, The Argonne-Northwestern Solar Energy Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Patrick E
authors.initials: PE
authors.lastname: Hartnett
authors.forename: Amod
authors.initials: A
authors.lastname: Timalsina
authors.forename: H S S Ramakrishna
authors.initials: HS
authors.lastname: Matte
authors.forename: Nanjia
authors.initials: N
authors.lastname: Zhou
authors.forename: Xugang
authors.initials: X
authors.lastname: Guo
authors.forename: Wei
authors.initials: W
authors.lastname: Zhao
authors.forename: Antonio
authors.initials: A
authors.lastname: Facchetti
authors.forename: Robert P H
authors.initials: RP
authors.lastname: Chang
authors.forename: Mark C
authors.initials: MC
authors.lastname: Hersam
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks