TY - JOUR
T1 - Crystallography, Morphology, Electronic Structure, and Transport in Non-Fullerene/Non-Indacenodithienothiophene Polymer:Y6 Solar Cells
AU - Zhu, Weigang
AU - Spencer, Austin P.
AU - Mukherjee, Subhrangsu
AU - Alzola, Joaquin M.
AU - Sangwan, Vinod K.
AU - Amsterdam, Samuel H.
AU - Swick, Steven M.
AU - Jones, Leighton O.
AU - Heiber, Michael C.
AU - Herzing, Andrew A.
AU - Li, Guoping
AU - Stern, Charlotte L.
AU - DeLongchamp, Dean M.
AU - Kohlstedt, Kevin L.
AU - Hersam, Mark C.
AU - Schatz, George C.
AU - Wasielewski, Michael R.
AU - Chen, Lin X.
AU - Facchetti, Antonio
AU - Marks, Tobin J.
N1 - Funding Information:
This research was mainly supported by the Center for Light Energy Activated Redox Processes (LEAP), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award DE-SC0001059 (T.J.M., project direction and advising; W.Z., devices; S.M.S., crystal growth; J.M.A., M.R.W., fs/ns optical spectroscopy; V.K.S., S.H.A., M. C. Heiber, and M. C. Hersam, IPDA; L.O.J., K.L.K., and G.C.S., theory), and the Office of Naval Research (ONR), under grant N00014012116 (G.L., GIWAXS). S.M. and D.M.D. acknowledge Award 70NANB14H012 from the U.S. Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design. Polymer synthesis and characterization efforts were funded by AFOSR grant FA9550-18-1-0320 (W.Z. and A.F., devices, microscopy, X-ray, and DSC). The 2DES study (A.P.S.) was supported by NSF grant CHE-1665021 to L.X.C. L.X.C. was partially supported by Solar Energy Photochemistry program of the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This work made use of the EPIC, BioCryo, Keck-II, and/or SPID facilities of Northwestern’s NUANCE Center, which received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205). This work (IPDA) made use of the MatCI Facility which receives support from the MRSEC Program (NSF DMR-1720139) of the Materials Research Center at Northwestern University. We thank the Integrated Molecular Structure Education and Research Center (IMSERC) for characterization facilities supported by Northwestern University, National Science Foundation (NSF) under NSF CHE-1048773, Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205), the State of Illinois, and the International Institute for Nanotechnology (IIN). Portions of this research (S.M., D.M.D., NEXAFS, R-SoXS, A.A.H., STEM) were carried out at the 7-ID Spectroscopy Soft and Tender (SST-1) Beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704, and at Beamline 11.0.1.2 of the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. D. Fischer, C. Jaye, E. Gann (NSLS-II) and C. Wang (ALS) are acknowledged for assisting with the experiments. J.M.A., S.H.A., S.M.S., and G.L. thank the NSF for graduate research fellowships. We sincerely thank D. Zhao for drawing and advising on figure design. W. Zhu thanks his grandmother for support.
Publisher Copyright:
© 2020 American Chemical Society.
PY - 2020/8/26
Y1 - 2020/8/26
N2 - Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3′':4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-πdistance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μe of 10-4 cm2 V-1 s-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.
AB - Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3′':4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-πdistance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μe of 10-4 cm2 V-1 s-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.
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U2 - 10.1021/jacs.0c05560
DO - 10.1021/jacs.0c05560
M3 - Article
C2 - 32698577
AN - SCOPUS:85090072774
SN - 0002-7863
VL - 142
SP - 14532
EP - 14547
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 34
ER -