Article pubs.acs.org/JPCC
Vertical Phase Separation in Bilayer [6,6]-Phenyl‑C61-butyric Acid Methyl Ester:Zinc Phthalocyanine Films Qian Shao, Levan Tskipuri, and Janice E. Reutt-Robey* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20740, United States S Supporting Information *
ABSTRACT: We report an ultrahigh vacuum scanning tunneling microscopy study of thermally driven interface rearrangement in binary films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and zinc phthalocyanine (ZnPc), a model electron acceptor−electron donor system for organic solar cells. Neat PCBM films have been previously shown to undergo a transition from a disordered (glassy) phase to a crystalline hexagonal close-packed (hcp) arrangement above a critical packing density of 0.9 molecules/nm2. We now show how local PCBM density has a critical impact on binary film structure evolution. Bilayer films of PCBM and ZnPc undergo a spontaneous vertical phase separation to PCBM/ZnPc/Au(111) stacking at lower (0.9 molecules/nm2) PCBM density and after annealing to 177 °C, crystalline hexagonal closepacked (hcp) monolayer islands of PCBM form. Stable monolayers of ZnPc on Au(111), displaying a nearly square lattice phase and density of 0.55 molecules/nm2, are produced following annealing to 127 °C. Within this monolayer, ZnPc adopt flat-lying orientations with respect to Au(111). We now build upon these previous works, investigating nanophase separation in binary mixtures and molecular orientation at the resulting donor−acceptor interfaces. We show how bilayer PCBM−ZnPc films prepared with two distinct initial film structures transform to the layered structure motif in this present work. We show how this vertically phaseseparated arrangement is thermodynamically favorable but requires thermal activation. We further identify configurations from which this stacked structure is inaccessible. The results provide guidance for tailoring interface structures in molecular semiconductor mixtures.
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RESULTS AND DISCUSSION Measurements were conducted on films of bilayer thickness in order to explore interface formation and the possible vertical separation of solutes at the substrate. Codeposited Bilayers. A bilayer film with equal parts PCBM and ZnPc was deposited from the aerosol mixture on the room temperature Au(111) surface, as described above. An STM image of the as-grown bilayer film is shown in Figure 1a.
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EXPERIMENTAL SECTION All experiments were conducted in a UHV-STM system with a pulsed microaerosol molecular beam source as previously described.5 Single crystal Au(111) substrates were first prepared with cycles of 25 min sputtering followed by a 20 min annealing at 490 °C. Subsequent STM images revealed growth substrates with the characteristic 23√3-herringbone reconstruction pattern16 and terrace widths in the 100−500 nm range. Such substrates were then subjected to aerosol spray deposition. The pulsed microaerosol mist, consisting of 1−50 μm size droplets, was generated from the pneumatic nebulization of 0.5 mmol/L each solution of PCBM and ZnPc in chloroform with a nitrogen carrier gas (970 Torr). Droplet size was reduced to the 1−10 μm diameter range via an in-line cyclonic spray chamber and then fed to a solenoidactuated pulsed nozzle valve (1 mm diameter), generating a directed aerosol beam that is then twice differentially pumped and skimmed before impacting the room-temperature Au(111) surface. For prolonged pulse trains (1000 pulses), pressure reached 1 × 10−6 Torr, decreasing to 90% of the surface area. A large-scale example is shown in Figure 3c. In patches where this stacked structure is not observed, the PCBM monolayer is generally found. This PCBM monolayer is generally a low-density 0.9 molecule/nm2, yields PCBM monolayers with coexisting hcp regions (0.9 molecules/nm2) and disordered regions.4 Covering such a two-phase PCBM monolayer with a ZnPc layer, followed by annealing, reveals two variations in the displacement of PCBM. Figure 6a shows isolated PCBM
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Figure 6. Impact of local PCBM density (ρ) on PCBM displacement to the second layer: (a) ρlow, where PCBM is displaced to the second layer. (b) ρhigh, PCBM nucleates to form a hcp island (inset) in contact with Au(111), which cannot be displaced to the second layer.
ASSOCIATED CONTENT
S Supporting Information *
A discussion of the electrostatic energy calculation and the height histogram analysis of annealed films. This material is available free of charge via the Internet at http://pubs.acs.org.
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molecules displaced to the second layer by ZnPc. Figure 6b shows undisplaced PCBM arranged as crystalline PCBM islands. In addition, displaced PCBM, both isolated and loosely packed, are observed. The inset of Figure 6b reveals the crystalline PCBM islands to be hcp, with a nearest-neighbor 0.97 nm distance, in good agreement with our previous study of hcp PCBM on Au(111). Such hcp PCBM are not displaced by ZnPc. In contrast, the isolated and loosely packed PCBM molecules occupy the second layer and are evidently displaced by ZnPc. Second-layer and first-layer PCBM molecules are distinguished by a small apparent height difference of ∼0.1 nm. These results indicate that ZnPc molecules preferentially displace PCBM molecules from Au(111) in regions of lower PCBM density to form the stacked arrangement (Figure 5b). In regions of hcp PCBM, the PCBM−PCBM interactions are strongly stabilizing and PCBM molecules are not displaced. Further annealing time yields no significant additional changes in film structure. Bilayer PCBM−ZnPc film structures are shown to depend explicitly on film processing conditions. The stacked PCBM/ ZnPc/Au structure is favored and kinetically accessible from initial film structures with lower PCBM densities. The multiphase structure with coexisting hcp PCBM domains and PCBM/ZnPc/Au(111) stacking is kinetically accessible from films with higher local PCBM densities. Once formed, the multiphase structure is thermodynamically preferred, as the PCBM−PCBM interactions are much stronger than the electrostatic interactions that lead to PCBM/ZnPc/Au(111) stacking.
AUTHOR INFORMATION
Corresponding Author
*Tel: 301-405-1807. Fax: 301-314-2779. E-mail: rrobey@umd. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The National Science Foundation (through the CHE-MSN award CHE1310380) is gratefully acknowledged for financial support.
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