title: Beyond fullerenes: design of nonfullerene acceptors for efficient organic photovoltaics.
abstract: New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C60]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC60BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm(2)) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60-65% in the 380-620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC60BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials.
authors.affiliation: Department of Chemical Engineering and Department of Chemistry, University of Washington , 36 Bagley Hall, Seattle, Washington 98195-1750, United States.
authors.forename: Haiyan
authors.initials: H
authors.lastname: Li
authors.forename: Taeshik
authors.initials: T
authors.lastname: Earmme
authors.forename: Guoqiang
authors.initials: G
authors.lastname: Ren
authors.forename: Akinori
authors.initials: A
authors.lastname: Saeki
authors.forename: Saya
authors.initials: S
authors.lastname: Yoshikawa
authors.forename: Nishit M
authors.initials: NM
authors.lastname: Murari
authors.forename: Selvam
authors.initials: S
authors.lastname: Subramaniyan
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Crane
authors.forename: Shu
authors.initials: S
authors.lastname: Seki
authors.forename: Samson A
authors.initials: SA
authors.lastname: Jenekhe
mesh_terms: D011211:Electric Power Supplies; D037741:Fullerenes / Q000737:chemistry; D007094:Imides / Q000737:chemistry*; D015394:Molecular Structure; D055668:Photochemical Processes; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry*; D011789:Quantum Theory; D012993:Solar Energy
chemical_list: D037741:Fullerenes; D007094:Imides; D011084:Polycyclic Aromatic Hydrocarbons