Article pubs.acs.org/JPCC
Molecular Ordering in PCBM−Au(111) Interface Formation L. Tskipuri, Q. Shao, and J. Reutt-Robey* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2111, United States ABSTRACT: Structure evolution in monolayer films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) deposited on Au(111) was determined via ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Molecular organization was monitored from a glassy phase, produced via a pulsed microaersol molecular beam deposition source, through ordered arrangements, following thermal annealing. At lower PCBM densities, two double-row structures arise, involving distinct PCBM hydrogen-bonding motifs, reminiscent of patterns produced by solvent-free deposition. At higher PCBM monolayer densities, hexagonal close-packed islands of PCBM form, with a 0.98 nm nearestneighbor spacing in good agreement with structure predictions [Nápoles-Duarte et al. Phys. Rev. B 2008, 78, 035425]. Under UHV conditions, solvent molecules are retained in the organic layer to temperatures up to 200 °C, inhibiting PCBM ordering. Following complete solvent removal, nanosized hexagonal close-packed PCBM islands show arrested development, indicating a kinetic barrier to island growth, attributed to reorientational energy costs.
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INTRODUCTION Since their 1985 discovery, fullerenes have found increasing commercial usage as an electron-accepting n-type organic semiconductor material in solar cells. The chemical morphology of blended mixtures of the C60 derivative, [6,6]-phenyl-C61butyric acid methyl ester (PCBM), and electron-donating components such as copper phthalocyanine (CuPc) impact solar cell efficiency. Researchers are actively developing processing and chemical strategies to direct the nanophase segregation of active layers to achieve devices with better performance and energy conversion yields.1−3 Solution deposition methods in current manufacturing use, such as roll-to-roll printing and spin-casting, have limited control over film growth. Moreover, the significant waste in these deposition methods raises the cost of production and environmental concerns. To address some of these limitations, liquid deposition by directed flow of aerosols has recently been reported. In organic films grown by liquid aerosol deposition, submonolayer growth precision has been realized through controlled solvent droplet size, composition, and flux.4−9 Aerosol spray-deposited organic thin film transistors (OTFTs) and organic photovoltaic cells have been fabricated with demonstrated efficiencies comparable to those of spin-cast cells.10−16 While a large number of papers have focused on the optoelectronic properties of such devices, molecular organization in films produced by such far-from-equilibrium growth conditions is poorly understood. Device performance is known to depend strongly on the molecular arrangement in the active layer, which sets the ability to carry charge.1,17 Greater knowledge of molecular ordering is important for the advancement of organic electronic technologies and is needed to provide fundamental insight on far-from-equilibrium crystallization processes. In the present work, we examine molecular ordering in films of PCBM grown by the microaerosol deposition method. PCBM is a derivative of C60, made soluble in organic solvents © XXXX American Chemical Society
through the presence of the phenyl and methyl ester addends, allowing for its solution processability. This functionalization introduces highly anisotropic PCBM−PCBM interactions and leads to the solvated crystals that have been reported.17 A layered crystal structure of fullerene moieties, separated by sheets of the addends and an equivalent number of solvent molecules, was crystallized from dichlorobenzene. A monoclinic crystal structure with two disordered solvent molecules for each PCBM was grown from the solvent chlorobenzene. More recently, a novel dip coating procedure yielded single crystals of PCBM with hexagonal symmetry from chloroform solvents.18 Investigations of vapor-deposited PCBM monolayers on Au(111)19−21 report dramatically different PCBM packing arrangements than observed for the parent fullerene. Neat C60 adopts a hexagonal close-packed (hcp) monolayer structure on gold and other transition metal surfaces,22−24 with a nearestneighbor spacing of 1.02 nm, nearly identical to that in the bulk crystal. Reported PCBM arrangements on Au(111), in contrast, involve double rows of paired fullerene moieties spaced by the butyric acid methyl ester addends, oriented for hydrogen-bond formation. DFT theoretical computations demonstrated the stability of PCBM hydrogen-bonding motifs and predicted additional PCBM chain phases of comparable energy. In PVDgenerated films, PCBM arrangements are of much lower density than known fullerene monolayer phases and PCBM bulk crystal planes. These studies, and the anisotropy of PCBM, suggest that further structural tuning of the PCBM crystalline phases should be possible. Hexagonal close-packed monolayers of PCBM, free of molecular solvent, should be accessible through control of film growth parameters. Additional PCBM polymorphs, arising from the predicted PCBM−PCBM hydrogen-bonding interReceived: July 20, 2012 Revised: September 12, 2012
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dx.doi.org/10.1021/jp307201k | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
actions, should be accessible. Investigations of structure formation in PCBM films as a function of key growth parameters are needed to expand our knowledge of PCBM− PCBM and PCBM−surface interactions and provide general insight into structure evolution for such high-cohesivity molecular films. In this study we present investigations of the structure evolution in PCBM films, prepared by a novel microaerosol molecular-beam deposition method. The fine metering of this UHV-compatible deposition source bridges solution and physical vapor deposition methods. PCBM monolayer phases are formed from select solvents, and molecular-level packing arrangements are monitored in situ via UHV-STM. Newly observed packing arrangements include new chain-phase and hcp structures. Kinetic bottlenecks to long-range ordering are found and interpreted by DFT computation.
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EXPERIMENTAL SECTION
All experiments were conducted in a UHV-STM system with an integrated pulsed microaerosol molecular beam source, as previously described.7 Single crystal Au(111) substrates were first prepared with sputter−anneal cycles and then imaged, revealing growth substrates with the characteristic 23 × √3 herringbone reconstruction pattern25−27 and terrace widths in the 100−500 nm range. The pulsed microaerosol was generated from the pneumatic nebulization of 0.5 mmol/L solutions of PCBM (SES Research, 99%) in chloroform and 1,3-dichlorobenzene with a nitrogen carrier gas (10 psi). The microaerosol droplets (nominal dimension 10 μm) pass through a 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 long pulse trains (1000 pulses), pressure in the load-lock compartment reached 1 × 10−6 Torr, rapidly decreasing to
