Exciton Delocalization in H2OBPc1âxMOBPcx - ACS Publications

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Exciton Delocalization in HOBPc MOBPc (M = Co, Cu, Ni, Mn) Crystalline Thin Film Organic Alloys Lane Wright Manning, Naveen Rawat, Cody J. Lamarche, Rory Waterman, Randall L. Headrick, and Madalina Ioana Furis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02343 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

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The Journal of Physical Chemistry

Exciton Delocalization in H2OBPc1-xMOBPcx (M = Co, Cu, Ni, Mn) Crystalline Thin Film Organic Alloys Lane W. Manning§, Naveen Rawat§, Cody Lamarche#, Rory Waterman†, Randall L. Headrick§, Madalina Furis§* §

Materials Science Program and Department of Physics

University of Vermont, 82 University Place, Burlington, VT, 05405, U.S. E-mail: [email protected] Telephone: (802) 656 5177 †

Department of Chemistry

University of Vermont, 82 University Place, Burlington, VT, 05405, U.S. #

Department of Astrophysics

Cornell University, 108 Space Sciences Building, Ithaca, NY 14853, U.S.

ACS Paragon Plus Environment

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ABSTRACT:

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Novel solution-processing deposition techniques in tandem with chemical

synthesis design of small molecule soluble derivatives represent a viable avenue for exploring organic analogues of semiconductor alloyed systems, where excitonic properties are tunable through alloy concentration. incidence

x-ray

Here these properties are explored using absorption, grazing

diffraction

(GIXRD)

and

temperature-dependent/time-resolved

photoluminescence spectroscopy (TRPL) in a series of crystalline thin film alloys of metal-free (H2OBPc) and metal (MOBPc) octabutoxy-phthalocyanine, H2OBPc1-xMOBPcx (M = Co, Cu, Ni, or Mn) where 0.5 ≥ x ≥ 0.001. Films are fabricated using a solution-processed, novel hollow pen-writing technique that results in millimeter-sized crystalline grains with long-range macroscopic order for all concentrations. The spectroscopy experiments produce two important results that offer great insight into the fundamental quantum mechanics of delocalized excitons in small molecule semiconductors.

First, they indicate that the delocalization of bandgap

excitons previously observed in pure H2OBPc films extends over approximately ten molecules, and second they reveal that the presence of the MOBPc molecule inhibits the formation of this delocalized exciton for x > 0.09.

Furthermore, the MOBPc molecule introduces a highly

localized state with a strong photoluminescence signature.


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INTRODUCTION Organic small molecules are at the forefront of fundamental experimental and theoretical studies aimed at understanding the physics of electrons in organic electronic devices and the quantum mechanical interactions taking place at the molecular level in these systems.1-3 Recently, certain families of aromatic ringed molecules (such as the acenes, the coronenes, and phthalocyanines) have shown great promise for flexible organic electronics and photovoltaic devices due to their unique broad visible/IR absorption bands and large carrier mobilities.4-8 In addition to their cost-effectiveness and unique functionalities, organic small molecules are also viable due to stability and robustness, many showing little degradation after exposure to air even above room temperature.9 Recent commercial applications include, but are not limited to, organic photovoltaics, organic LEDs, flexible organic displays and organic field effect transistors.10-18 A determining factor for how these organic molecules will perform in device-related applications is their excitonic properties in the solid state, specifically in thin films.6,19-21 In organic π-conjugated molecules, photoexcitation leads to the formation of excitons (strongly bound electron-hole pairs), therefore generating free carriers is not a trivial process. This is because the binding energy of these electron-hole pairs are typically greater than the room temperature thermal energy,22 and dissociation will occur in the presence of large electric fields at heterojunctions.23-24 With this in mind, understanding the dynamics of exciton diffusion and localization (or delocalization) becomes a necessity for organic electronic applications.25 Furthermore, the processing method, the purity, and the crystalline quality of the films themselves can also greatly impact exciton diffusion.26-29


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The studies reported here focus on quantifying the spatial range of intermolecular interactions and exciton delocalization that can directly impact carrier mobilities and exciton diffusion in small molecule semiconductors. Our approach is inspired by a wealth of studies in inorganic semiconductor alloys (such as AlGaN, InGaN, SiGe etc.)30-40 where compositional disorder is known to result in exciton localization by alloy potential fluctuations, as well as recent reports of successful donor/acceptor organic alloys containing two molecular species with similar geometries.41 This pursuit benefits from a solution-processed deposition technique developed inhouse, that accommodates a wide range of molecules for rapid fabrication of alloyed organic thin films with macroscopic crystalline grain sizes. The molecules of choice belong to the phthalocyanine family, a well-known and well-studied small molecule π-conjugated organic semiconductor that offers the advantage of tuning the bandgap without changing the molecular structure of the carbon-nitrogen ring. Alloyed thin film samples of H2OBPc1-xMOBPcx (with 0.5 ≥ x ≥ 0.001) were fabricated from solution mixtures of 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (H2OBPc) combined with various transition-metal octabutoxy-phthalocyanines (MOBPc). The choice of transition metal species include cobalt(II) (CoOBPc), copper(II) (CuOBPc), nickel(II) (NiOBPc) and manganese(II) (MnOBPc) 1,4,8,11,15,18,22,25-octabutoxy-phthalocyanine. A series of temperature-dependent ultrafast photoluminescence techniques are employed to probe the excitonic states, and all films are characterized through Grazing Incidence X-Ray Diffraction (GIXRD) in order to correlate the results of the luminescence studies to the crystalline ordering. These experiments reveal the bulk delocalized bandgap exciton previously observed in the H2OBPc films42 is significantly changed by the introduction of the MOBPc molecules only for concentrations of MOBPc larger than x = 0.09. Based on this observation we estimate the range


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of exciton delocalization extends over at least 10 molecules in this system. This range is significantly smaller than the size of random alloy fluctuations, estimated from GIXRD experiments.

EXPERIMENTAL METHODS Materials. Phthalocyanines (or tetrabenzotetraazaporphyrins) are synthetic analogues of the porphyrin molecule that contain four isoindole subunits (a benzo-fused pyrrole) at each corner, connected together by a series of nitrogen atoms.43-45

Phthalocyanine serves as a parent

molecule for an entire family of derivatives, where one can attach varying alkylated side-chains to increase solubility, shift Q-band absorption, or adjust intermolecular distances along the π-π stacking direction in the crystalline phase.46-48 They have very intense absorption maxima and high extinction coefficients in the visible and near IR spectral domain,44,47,49 with the Q-band extending into the 600-800 nm wavelength range, making them ideal candidates for photovoltaic devices, as this overlaps exactly with the maximum solar photon flux spectral window.12,50-51 This is in contrast to many other organic semiconductors whose visible absorption spectra are blue-shifted with respect to the phthalocyanines. The central hydrogen atoms of the metal free phthalocyanine molecule can also be replaced with over 70 different metals (the focus here will be on the d-shell transition metals), allowing for significant variation in electronic, magnetic and structural properties.47,52-55 Significant changes occur in the excitonic properties of OBPcs (and other similar organic small molecules) in the solid state, including broadening of spectroscopic features56-57 and the presence of a delocalized exciton, as a result of π-π stacking and intermolecular interactions along the stacking axis in the crystalline phase.42, 58-60 In these thin films the OBPc molecules will order in


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pseudo one-dimensional chains, stacking in a co-facial arrangement along a crystalline axis as a result of π-π and van-der-Waals interactions between molecules that are strongest in a face-toface orientation.45,52 The orientation of the π-π stacking axis in reference to the unit cell axes changes drastically from one phthalocyanine derivative to another,42,61 making it impossible to directly infer information on the structural and electronic properties from the better known insoluble parent molecules. Chemistry and Purification Techniques. Impure 1,4,8,11,15,18,22,25- Octabutoxy29H,31H-Phthalocyanine (H2OBPc) powder was purchased from Sigma-Aldrich and was chromatographed over silica gel using toluene and ethyl acetate as eluents. After two individual columns, the product was collected and recrystallized by dissolving it in acetone. Bright green crystals were collected and dried under vacuum. The purity was confirmed by NMR and solution absorption, showing good agreement with reported literature.47,62 1,4,8,11,15,18,22,25-copper-octabutoxy-phthalocyanine (CuOBPc) was purchased from Sigma-Aldrich and further purified by column chromatography using dichloromethane and diethyl ether (1:10 ratio) and a toluene and ethyl acetate combination as eluents. This was followed by a recrystallization in THF and ethanol. Crystals were then collected and dried for thin film fabrication. 1,4,8,11,1,5,18,22,25-nickel-octabutoxy-phthalocyanine

(NiOBPc)

was

purchased

from

Sigma-Aldrich and purified using column chromatography in toluene. This was followed by a recrystallization in THF and ethanol. Crystals were then collected and dried for thin film fabrication. 1,4,8,11,15,18,22,25-cobalt-octabutoxy-phthalocyanine (CoOBPc) was synthesized by starting with cobalt (II) acetate (1.1750 mg, 3.7 mmol, purchased from Sigma Aldrich) and added to a


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refluxing solution of purified H2OBPc (200mg, .102 mmol) in butanol (15 ml). The solution was maintained at reflux for 2 hours under nitrogen after adding the metal and cooled and column chromatographed twice (in silica gel, using toluene and ethyl acetate as eluents). The green blue fraction was dried under reduced pressure, and further purified by column chromatography, and subsequently was set to recrystallize in THF and ethanol (~85% yield). The crystals obtained were then dried under vacuum. 1,4,8,11,15,18,22,25-manganese-octabutoxy-phthalocyanine (MnOBPc) was synthesized by starting with manganese (II) acetate (630mg, 3.7 mmol, purchased from Sigma-Aldrich) and adding it to a refluxing solution of purified H2OBPc (110mg, .102 mmol) in butanol (15 mL). The solution was maintained at reflux for 2 hours under nitrogen after adding the metal salt, and subsequently cooled and column chromatographed twice (in silica gel, with chloroform and methanol as eluents). The red wine fraction was then dried under atmospheric pressure (~60% yield). Substrate Surface Treatment.

C-plane crystalline sapphire slides were treated with a

triethoxyphenylsilane solution. The deposition grade triethoxyphenylsilane was purchased from Sigma-Aldrich. In order to treat the substrates, they were submerged in a solution of 3% triethoxyphenylsilane/97% toluene and kept under nitrogen for 15 hours at a temperature of 90o C. After the substrate is pulled from the PTS solution, a quick methanol rinse is performed before thin film deposition. The sapphire substrates were purchased from Meller Optics. Alloyed Film Deposition Technique.

For a typical alloyed phthalocyanine film, the

individual H2OBPc and MOBPc were each dissolved in a .5% by weight solution (for ex. 0.0005g/100µl) in identical solvents (typically toluene).

After sonicating each individual

solution for 15 minutes, they were combined by volume into another solution (using the


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appropriate ratio desired), and then this combined solution was sonicated for an additional 20 minutes to assure complete mixing. The mixed solution was then immediately deposited at room temperature onto the PTS treated c-plane cut sapphire substrate using the hollow capillary penwriting technique. This hollow pen-writing technique involves loading the mixed solution into a hollow borosilicate glass capillary pen, while the solution is held in the pen by capillary forces. Film deposition is then achieved by letting droplets of solution at the end of the pen to make contact with the substrate and then laterally translating the substrate at finely controlled speeds between 8-10µm/s. Ultrafast Spectroscopy Experiments.

The excitation source for the time-resolved

photoluminescence (PL) experiments was a PicoQuant PDL 800-D pulsed diode laser driver and a LDH-D-C-730 laser head (wavelength = 734 nm, pulse duration = 70 ps).

The

photoluminescence spectra were recorded using a Princeton Instruments CCD camera, coupled to a Princeton Instruments 0.5 m spectrometer. The time-resolved data was recorded using a time-correlated single-photon counting system (PicoHarp 300 from PicoQuant) and a PDM series avalanche photodiode (Full Width at Half Maximum (FWHM) response time of 130ps) mounted to the side exit port of the same spectrometer. For temperature-dependent experiments, sample temperatures were varied continuously using a variable-temperature continuous-flow Oxford Microstat He cryostat (5 K to 300 K), and a long working distance (15 mm) objective lens was used to focus the incoming excitation beam. The absorption spectra were measured at room temperature in the range of 400-1100 nm using a quasi-monochromatic (1 nm bandwidth) tunable, incoherent, tungsten halogen light source, and an achromatic 5 cm focal length lens that created an incoherent beam diameter size of approximately 50 µm. A long working distance telescope was used to monitor the beam position on the sample surface, while detection was


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accomplished using Thorlabs Si diode amplified detector. The same telescope set-up was used for the photoluminescence experiments to monitor the excitation beam position on the sample. All PL spectra were corrected for the CCD response and grating reflectivity across the spectral range of interest. A correction factor was also introduced to account for the s and p reflectivity and transmittance of the beam splitter used in the experiments. Grazing Incidence X-Ray Diffraction (GIXRD) Experiments. GIXRD measurements were performed at the Cornell High Energy Synchrotron Source (CHESS), on the G2 line, with x-rays at 10.05 ± .01 keV (λ = 1.2337 Å), and the scattering data collected using a 640-element onedimensional diode-array. A set of Soller slits were used on the detector arm to provide an inplane resolution of 0.2°. GIXRD patterns were collected on the same thin films previously characterized with the spectroscopy experiments mentioned above. AFM Measurements. Atomic force microscopy was performed at The University of Vermont, with an Asylum Research MFP-3D-BIO (Asylum Research, an Oxford Instruments company; Santa Barbara, CA). Images were obtained in the ‘‘contact’’ mode in air, using a 225 µm pyramidal silicon probe containing a silicon tip with tip radius 0.09, feature (3) becomes sharper and loses intensity until it completely disappears in the x = 0.5 thin film. In contrast, the localized exciton, feature (2), that was always absent at low temperatures from the spectrum of the pure metal-free species dominates the low temperature spectrum for the x = 0.5 film. This is reminiscent of inorganic semiconductors, where the


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excitonic behavior in the lightly doped systems retain the properties of the pure material, while alloyed systems can have distinct properties that in many cases are due to a new crystalline phase. However, in stark contrast to inorganic alloys where the excitonic line-width broadens, and the overall intensity decreases due to alloy fluctuations,32-33,37 the narrowing of the exciton line-width and its blue shift at large MOBPc concentrations indicate that the formation of the delocalized exciton is inhibited altogether. This observation correlates with the previous GIXRD spectra (Figure 7), and the absorption spectra (Figure 6), that infer a significant change in the structural ordering for the films with MOBPc concentration larger than x = 0.09. We can then conclude in our phthalocyanine alloys, the critical concentration of MOBPc at which the delocalized exciton disappears is approximately 9%. Since the presence of a delocalized exciton is exclusively due to the π-π stacking in the ordered H2OBPc chain, as well as the coupling to the lattice vibrations, we can infer that the average distance between two MOBPc molecules becomes roughly equal to the spatial extent of the exciton delocalization around approximately x = 0.09 (10:1 H2OBPc:CoOBPc). The correlation between the delocalized exciton state and the compositional fluctuations cannot simply be understood through an analogy to inorganic alloy semiconductors. This is because in phthalocyanines, the coupling of π-states to lattice vibrations is comparable in strength to the Coulomb long-range intermolecular interactions and cannot be treated as a perturbation to a hydrogen-like exciton model. The presence of compositional fluctuations in the system does not merely create an alloy potential fluctuation that could trap the bandgap exciton (as is the case for inorganic systems),30-32,34-35,80,84 but more importantly disturbs the long-range vibrational coupling, inhibiting the formation of the delocalized exciton altogether.


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For MOBPc concentrations larger than x = 0.09, the π-π stacking along the molecular chain will be significantly different, since the probability of having a nearest neighbor of the opposite species becomes larger. The π-π overlap along the stacking axis is now perturbed on a length scale comparable to the exciton delocalization in the pure species. Therefore, in these alloys, the compositional fluctuations will gradually diminish the spatial extent of the delocalized exciton at low temperatures, for concentration x > 0.09. If we assume a perfectly random distribution of these molecules, our result implies the spatial extent of the delocalized exciton is on average at least 10 lattice spacings (or roughly 40 Å) along the stacking axis in the pure H2OBPc crystalline films. This interpretation is supported by the spectrum of the H2OBPc0.5CoOBPc0.5 film, where the probability of having a nearest neighbor of the opposite species is at least 50%. In that case, the π-π stacking and vibrational coupling that lead to the observation of the delocalized exciton in the pure H2OBPc is completely inhibited, and we observe only the contribution from the localized exciton polarized in the plane of the molecule. This is a universal behavior observed for all of the d-shell transition-metal ions studied (Co, Cu, Ni, Mn), as all the four alloyed systems exhibited a similar trend with increasing MOBPc concentrations (See Figure 8b for Cu, and Supporting Information for Ni and Mn in Figure S8 and S10 respectively).

Our

observations also indicated the concentration that characterizes the switch from localized to delocalized excitons is slightly different for the various transition metals studied in these alloyed thin films, most likely as a result of the somewhat different packing of molecules in the various MOBPc crystalline structures. The other remarkable trait of all alloyed films is the presence of a sharp luminescence emission feature located at 850 nm (feature (1) in Figure 8), never before reported in pure H2OBPc thin


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films.

The feature is strongly temperature-dependent, with a full-width at half-maximum

(FWHM) rapidly increasing, and an energy rapidly decreasing, with temperature. In some of these samples (such as x = 0.5 and x = 0.09 CoOBPc alloy temperature dependent luminescence plots presented in Figure S6 and Figure S7 respectively), the 5 K spectrum even exhibits multiple vibrational replicas of the main luminescence peak. The sharpness of this feature and its temperature dependence is reminiscent of emission from localized states (or bound excitons) r

in bulk semiconductors, where the ∆k = 0 selection rule for optical recombination results in the presence of strong phonon replicas for these transitions. In contrast, for delocalized (or free) excitons, the selection rules are relaxed due to strong phonon coupling already present in the system. Since excitonic emission is significantly quenched in the solid state for MOBPc molecules as mentioned previously, this sharp emission cannot simply be associated with luminescence from the CoOBPc dopants.

Instead, we believe this feature is likely associated with an optical

transition involving one of the delocalized π-orbitals of the H2OBPc chain and an electronic state localized on the CoOBPc molecule. It can be viewed as the impurity state to conduction band recombination often observed in inorganic semiconductors. It could also be regarded similarly to an intermolecular charge transfer exciton where either the hole or electron is delocalized along the H2OBPc chain, while the other carrier is largely localized on the MOBPc molecule. We want to emphasize that unlike our previous studies of disordered pure H2OBPc thin film systems,66 the 850 nm feature is not a result of disorder at the grain boundary or some other type of defect in the sample. Our focused beam luminescence experiments confirm the presence of this feature everywhere inside the grains in the crystalline films. While there is no consistent trend for the evolution of this feature’s intensity with the H2OBPc/MOBPc mixing concentration,


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its relative intensity with respect to the excitonic recombination, is much larger in the higher concentration thin films (x = 0.09 and above), confirming it is related to the presence of MOBPc molecules.

The presence of this feature in all the alloyed films studied, regardless of

concentration, as well as its presence in all the grains of any given thin film provides further evidence that we are not simply making H2OBPc thin films with inclusions of pure MOBPc aggregates.

Figure 9. a) Time-resolved delocalized exciton photoluminescence decay for seven different ratios of H2OBPc1-xMOBPcx at 5 K. The corresponding mixing ratios of metal-free to metal are listed next to each spectrum. b) High injection lifetime dependence on CoOBPc concentration. The lifetimes were extracted from the fitted decay slopes as illustrated in the inset.


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This sharp feature is very much present even in the lower concentrations (x = 0.02 and below), dispelling the idea that we might simply be looking at emission at a grain boundary between nanometer size MOBPc inclusions and the H2OBPc matrix. If the metal-free and metal OBPc were completely phase separated, we would either record luminescence of pure H2OBPc or no luminescence at all, which is clearly not the case here. Instead, as mentioned in detail above, a gradual extinction of the delocalized exciton, in favor of an enhancement in the localized exciton is observed with increasing MOBPc concentration at low temperatures. Low Temperature Time-Resolved Photoluminescence. The alloy formation is supported by the low-temperature evolution of radiative lifetimes in the H2OBPc1-xCoOBPcx alloy films shown in Figure 9. The most notable finding in this data is the evolution of the bandgap exciton recombination lifetimes as a function of concentration at 5 K shown in Figure 9b. For most of the alloyed films, the delocalized exciton lifetimes are similar to the one measured in pure H2OBPc films, decreasing slightly with increasing MOBPc concentration. The only remarkable difference is seen for the x = 0.5 alloy film bandgap exciton, whose lifetime is four times longer than the one measured at low temperature in the pure H2OBPc film. We interpret these longer lifetimes as a confirmation that the formation of the delocalized exciton is completely inhibited when the nearest neighbors are molecules of a different type. We also investigated the photoluminescence decay for the 850 nm and found it to be very similar to the bandgap exciton (see Figure S11), supporting the hypothesis that the delocalized states of metal –free Pc are involved in this transition. CONCLUSIONS To summarize our findings, we were able to fabricate alloyed crystalline thin films of the organic molecule phthalocyanine using varying concentrations of the MOBPc species mixed


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with H2OBPc while still retaining crystalline ordering.

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We performed photoluminescence

studies as a function of MOBPc concentration that provided an estimation of the spatial extent of the exciton delocalization. Varying the concentration from x = 0.001 to x = 0.5 introduces changes in the crystalline phase structure that result in a drastic change in excitonic states for x > 0.09. At the x = 0.5 (1:1) concentration, the delocalized exciton formation is entirely inhibited due to the lack of long-range vibrational coupling between H2OBPc molecules. This is a universal behavior observed for all alloyed films created from transition-metal species of octabutoxy-phthalocyanines. From a broader perspective, the ease of fabricating crystalline organic alloys for a wide range of molecular concentrations opens up incredible avenues for engineering emergent electronic properties on a length-scale relevant to device applications. Organic alloys are fundamentally different from mixtures that result in bulk heterojunctions, where poor miscibility and phase separation are encouraged. Instead they represent a paradigm shift to systems where large miscibility is highly desirable and results in crystalline films with long-range order. This allows for new electronic properties such as the existence of delocalized excitons, and more importantly, provides a tuning mechanism for this delocalization. Our study also reveals the robustness of this deposition technique that is essentially molecule independent, where the process is governed by solubility, organic solvent and deposition speed, thus accommodating a large variety of small organic molecules.

This could then lend itself to designing and

engineering properties of organic semiconductors in the solid state, such as carrier mobilities, diffusion lengths and other properties. Our group is already pursuing the characterization of low-temperature magnetism properties in these alloys, and preliminary results hold great promise with regards to the possibility of tuning the spin-exchange interactions between the d-shell metal


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ions as a function of MOBPc concentration. However, these results are beyond the scope of this paper and will be reported elsewhere. ASSOCIATED CONTENT Absorbance and photoluminescence spectra of Ni and MnOBPc-based alloys. Cross sections of X-ray scattering maps and domain size estimations. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email:[email protected] Telephone: (802) 656 5177 Author Contributions L.W.M. and R. L. H. fabricated all thin film samples mentioned herein, while N. R. and R.W. synthesized and purified the soluble Pc molecules used in this study. L.W.M., N.R., C.L. and M.F performed ultrafast spectroscopy and GIXRD experiments. All authors contributed to the writing of the paper. ACKNOWLEDGMENTS L.W.M. would like to sincerely thank Dr. Zhenwen Pan, Kim Hua, Victoria Ainsworth, Lauren Paladino, and Michael Arnold (Physics, University of Vermont) for their assistance, as well as Dr. Tony Wetherby (Chemistry, University of Vermont) for advising on the synthetic chemistry routes for soluble Pc derivatives.


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In addition, the authors would like to thank John M. Hughes and Nico Pedrial (Geology, University of Vermont) and Jeffery Ulbrandt (Physics, University of Vermont) for great conversations and advice about the X-Ray experiments. A special thank you to Dr. Arthur Woll at the CHESS facility for imparting his expertise on the GIXRD experiments on the G2 beamline. This material is based upon work supported by the National Science Foundation under DMR CAREER Grant No. 1056589, REU Grant No. 1062966, and MRI Grant No. 0821268. This work is also based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. The AFM portion of this project was performed in the Microscopy Imaging Center and supported by award number S10RR025498 from the National Center for Research Resources for purchase of the Atomic Force Microscope. REFERENCES (1) Cicoira, F.; Santato, C., Organic electronics: emerging concepts and technologies. John Wiley & Sons: Weinheim, Germany, 2013. (2) Forrest, S. R., The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 2004, 428, 911-918. (3) Podzorov, V., Organic single crystals: Addressing the fundamentals of organic electronics. MRS Bull. 2013, 38, 15-24.


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Figure 1. a) Chemical structure of non-peripheral octabutoxy-substituted metal-free phthalocyanine (H2OBPc). b) Chemical structure of non-peripheral octabutoxy-substituted cobalt (II) phthalocyanine (CoOBPc). c) Polarized optical microscope images of a typical solution deposited thin film of H2OBPc. d) Polarized optical microscope images of a typical solution deposited thin film of CoOBPc. 82x105mm (300 x 300 DPI)



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Figure 2. Polarized optical microscope images of H2OBPc0.75CoOBPc0.25 at various deposition speeds. a) 6.25 µm/s b) 7.59 µm/s c) 8.48 µm/s d) 9.82 µm/s. Slower speeds a) and b) are prone to producing aggregates, while faster speeds, d) give rise to uneven and overlapping grain boundaries. In this case the optimum speed of deposition was determined to be c) 8.48 µm/s. 82x56mm (300 x 300 DPI)


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Figure 3. Polarized optical microscope images of alloyed thin films with various concentrations. a) H2OBPc0.67CoOBPc0.33 b) H2OBPc0.98CoOBPc0.02 c) H2OBPc0.91NiOBPc0.09 d) H2OBPc0.99CoOBPc0.01. Long-range ordering with large grain size is achieved even at high concentrations of MOBPc. 82x56mm (300 x 300 DPI)



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Figure 4. Room temperature absorption spectra of H2OBPc, CoOBPc and H2OBPc0.83 CoOBPc0.17 thin films. The alloyed system spectrum bears the signature of an enhanced bandgap exciton feature (marked with an arrow), previously encountered in the metal-free species. 82x61mm (300 x 300 DPI)


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Figure 5. a) Room temperature absorption spectra of H2OBPc0.83CoOBPc0.17, recorded for eight different crystalline grains within a single sample. b) Room temperature absorption spectra of H2OBPc0.83CuOBPc0.17, recorded for eight different crystalline grains within a single sample. 63x102mm (300 x 300 DPI)



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Figure 7. Powder mode grazing incidence x-ray scattering map of 6 thin film samples. a) H2OBPc b) H2OBPc0.98 CoOBPc0.02 c) H2OBPc0.91CoOBPc0.09 d) H2OBPc0.75CoOBPc0.25 e) H2OBPc0.5CoOBPc0.5 f) CoOBPc. Peaks assigned to H2OBPc are marked with circles, and those assigned to CoOBPc with diamonds. 127x90mm (300 x 300 DPI)


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Figure 6. a) Absorption spectra of eight different concentrations of H2OBPc1-xCoOBPcx thin films at room temperature. b) Absorption spectra of five different concentrations of H2OBPc1-xCuOBPcx thin films at room temperature. In addition, absorption spectra from each pure thin film is shown for comparison. 65x158mm (300 x 300 DPI)



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Figure 8. a) Luminescence spectra of ten different ratios of H2OBPc1-xCoOBPcx at 5 K and at room temperature. b) Luminescence spectra of five different ratios of H2OBPc1-xCuOBPcx at 5 K and at room temperature. The corresponding mixing ratios of metal-free to metal are listed next to each spectrum. Excitation wavelength is equal to 734 nm. 129x121mm (300 x 300 DPI)


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Figure 9. a) Time-resolved delocalized excitonic photoluminescence decay for seven different ratios of H2OBPc1-xMOBPcx at 5 K. The corresponding mixing ratios of metal-free to metal are listed next to each spectrum. b) High injection lifetime dependence on CoOBPc concentration. The lifetimes were extracted from the fitted decay slopes as illustrated in the inset. 65x108mm (300 x 300 DPI)



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Exciton Delocalization in H2OBPc1âxMOBPcx - ACS Publications

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Exciton Delocalization in H2OBPc1âxMOBPcx - ACS Publications