Supramolecular Organization-Electrical Properties Relation in

Article pubs.acs.org/JPCC

Supramolecular Organization-Electrical Properties Relation in Nanometric Organic Films Priscila Alessio,*,† Maria Luisa Braunger,† Ricardo Flavio Aroca,‡ Clarissa de Almeida Olivati,† and Carlos José Leopoldo Constantino† †

UNESP Univ Estadual Paulista, DFQB, Faculdade de Ciências e Tecnologia, Presidente Prudente, SP, 19060-900, Brazil University of Windsor, Windsor, Ontario N9B 3P4, Canada

‡

S Supporting Information *

ABSTRACT: The need to improve the performance of electronic devices based on organic materials has been at the center of structure−property relation research, where the main objective is to develop low cost and flexible electronic components in large-scale. Important contributions to the performance of such devices have been made based on studies of the supramolecular organization of small organic molecules, primarily those with π-conjugated systems. Here we examine the relationship of supramolecular organization and the electrical properties of a substituted perylene tetracarboxylic diimido in nanometric films fabricated by physical vapor deposition (PVD). The morphology of the thin solid films is probed with optical and electron microscopy and the supramolecular characterization includes vibrational and electronic spectroscopies. The electrical properties are studied using AC and DC measurements with interdigitated electrodes and diode-like structures. A higher conductivity is observed when measured with the diode-like structure. It seems to be associated with a perpendicular orientation of the electric field with respect to the π−π molecular stacking, favoring the charge transport through these π aggregates. The results enhance the understanding of organic electronics, helping surface engineering to harness supramolecular organization to improve performance of thin film devices.

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INTRODUCTION Research in nanometric thin solid films is of great interest due to its potential scientific and technological impact in several areas of materials science, surface science, and applied physics.1 In particular, films of organic semiconductors have potential for applications in displays, solar cells, sensors, and flexible electronics.2,3 Among the organic semiconductors, perylene derivatives have been extensively studied due to its excellent optoelectronic properties such as high electron affinity and large π-orbital overlap in the solid state.4,5 Wurthner and Stolte reported the importance of long-range order in the electronic properties of organic thin-film transistors, which facilitates the electronic coupling and percolation.6 They also indicated that the device performance is improved when using vacuum deposition methods. In particular, the physical vapor deposition (PVD) is known to promote supramolecular organization in thin solid films.7−10 Perylene derivatives have been widely investigated as n-type semiconductors in solar cells due to their high electron mobility, photochemical stability, and strong optical absorption.11 Another important point is its inexpensive production on the kiloton scale as industrial pigments. Using a perylene derivative as electron acceptor, it was possible to fabricate one of the most efficient organic fullerene-free solar cells, up to 6%.12 The main approach to improve the efficiency of fullerene © XXXX American Chemical Society

free organic solar cells is to achieve the optimum morphology of the donor−acceptor mixture. In this way, it is important to explore the relationship between supramolecular organization and the optoelectronic properties in thin films based on perylene derivatives.13−17 Here, we study the influence of the supramolecular organization, induced during PVD thin film fabrication, on the electrical properties of thin solid films of bis(2-carboxyethylimido perylene) (PTCD-COOH). DC and AC electrical measurements are complemented with spectroscopic characterization, including UV−vis absorption, FTIR, Raman, fluorescence, surface-enhanced resonance Raman scattering (SERRS), and X-ray diffraction (XRD), while morphology is probed using atomic force microscopy (AFM) and scanning electron microscopy (SEM).

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MATERIALS AND METHODS Organic semiconductor PVD films of PTCD-COOH were prepared in a vacuum system Boc Edwards model Auto 306. The PTCD-COOH (MW = 534.5 g/mol), which molecular structure is shown in the inset in Figure 1, was provided by Dr. J. Duff Xerox Research Center in Canada. The PVD film Received: March 31, 2015 Revised: May 5, 2015

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DOI: 10.1021/acs.jpcc.5b03093 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

obtained by film deposition onto gold interdigitated electrodes (IDE) and the perpendicular one was obtained by depositing the PVD film on a transparent ITO electrode and later evaporating aluminum on top of the PVD film, leading to ITO/ PTCD-COOH/Al structure (diode-like). The PVD film deposited onto IDE was 90 nm mass thick, and 50 and 90 nm thick films were prepared for the diode-like structure. Also, for the IDE device, current versus time measurements (I vs t) were performed under both dark and illumination with a halogen lamp (150 mW/cm2) in order to investigate the existence of photoconductivity in this material.

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RESULTS Film Growth−Quartz Crystal Microbalance and UV− Vis Absorption Spectroscopy. The growth of PTCDCOOH PVD films is monitored using the quartz crystal microbalance (in situ) and UV−vis absorption (ex situ) techniques, and the extinction spectra for 10, 52, 100, and 150 nm mass thicknesses are shown in Figure 1. The absorbance at 500 nm shows a linear dependence with the film thickness (inset in Figure 1), indicative of a good control of the amount of material deposited in every evaporation step within the range investigated here, characteristic of a controlled process of film growth.10,18−21 The monitoring process is important because, for different materials, the linear film growth appears to depend on the amount of material deposited in each evaporation step. For instance, in the work of Volpati et al.10 it was observed the evaporation process ought to be in steps up to 40 nm for a perylene derivative, and in Zanfolim et al.,21 the evaporation step was 10 nm for zinc phthalocyanine. The PTCD in solution exhibits characteristic features in the UV−vis absorption spectra associated with the π−π* transitions of aromatic hydrocarbons (chromophore), presenting a well-defined vibronic structure.22−24 This is also the case of PTCD-COOH solution spectrum shown in Figure 1, where absorption maxima (related to π−π* transition) observed at 528, 492, and 460 nm correspond to (ν0−0), (ν0−1), and (ν0−2) transitions from electronic ground state to different vibronic levels of the first electronic excited state, respectively. The energy relative to this vibronic structure is about 0.172 eV or 1386 cm−1. The UV−vis absorption spectra of the PTCDCOOH PVD films are also shown in Figure 1. There is a broadening of the absorption bands showing two maxima at 574 and 500 nm, quite different from the solution spectrum. These two maxima may be explained by Kasha’s theory,25 according to which J or H aggregates can be formed as a function of the angle (α) between electronic dipoles and molecular axis of the aggregate. J aggregates are characterized by red-shifted maximum compared (574 nm) to the monomer absorption band (528 nm), being induced by “head-to-tail” arrangement of the dipoles.25,26 In contrast, the absorption band of H aggregates is characterized by blue-shifted maximum (500 nm) due to “card pack” arrangement of the dipoles (faceto-face molecular stacking).25 Notably, as seen in Figure 1, the same UV−vis absorption spectra for PVD films with different thicknesses are observed. However, a deconvolution of the UV−vis absorption bands using Gaussian function (Figure S1 and Tables T1−T3 in Supporting Information) reveals the appearance of a new band for the thicker films at about 460 nm, as seen in Tables T1−T3. The latter implies an increase in the total FWHM (full width at half-maximum) with the increase in thickness, in both the blue and red direction. This feature is a result of the film growth

Figure 1. UV−vis absorption spectra for PTCD-COOH in solution (dashed line) and forming PVD films with mass thicknesses of 10, 52, 100, and 150 nm. Insets: molecular structure of PTCD-COOH and UV−vis absorbance at 500 nm against film thickness.

thicknesses were monitored with a quartz crystal microbalance using density of 1.1 g/cm3. For the manufacturing of PTCDCOOH PVD films, about 5.0 mg of its powder was placed in a tantalum boat, which was heated by an electric current under a vacuum atmosphere of 10−6 Torr. Electric current was slowly adjusted to approximately 1.5 A (10 V). The evaporation rate used was about 0.1−0.3 nm/s. Different substrates were used, depending on the characterization to be performed. For instance, for UV−vis analysis, PVD films were deposited on quartz substrate with thicknesses of 10, 52, 100, and 150 nm. PVD film with thickness of 150 nm was deposited on ZnSe substrate and onto a silver mirror for FTIR measurements in transmission and reflection modes, respectively. The UV−vis measurements were performed using a spectrophotometer Varian model Cary 50. Raman spectra were obtained with a Renishaw micro-Raman system model inVia equipped with a Leica microscope, which allows the acquisition of spectral mapping point-by-point with spatial resolution of about 1 μm2 using a 50× objective lens and CCD detector. The laser lines at 514.5 and 633 nm were used with 1800 lines/mm grating resulting in a spectral resolution of about 4 cm−1. Raman images could be obtained through an automated XYZ platform, collecting spectra over a line previously selected for mapping. The FTIR measurements were performed on a Bruker spectrometer model Vector 22 with spectral resolution of 4 cm−1 and 128 scans, using a DTGS detector. The spectra in reflection mode were obtained with an incident angle of radiation of 80° using the Bruker accessory A118. The AFM images were obtained using an atomic force microscope Nanosurf model EasyScan 2 in the contact mode with a resolution of 512 lines/area and analyzed in Gwiddeon software. The FEG-SEM images were obtained using a FEI microscope Quanta 200F under low vacuum (1.0 Torr) and without metallization of the film surface. Room temperature XRD measurements were carried out using Shimadzu diffractometer model XRD-6000 with the Cu Kα line at 0.1542 nm. The diffractogram of the PVD film (150 nm on glass) was scanned over the range of 3° to 60° (2θ), with a step rate of 2°/min (2θ). The PTCD-COOH electrical characterization was carried out for different structures and electrode types. For the DC measurements, a Keithley 238 source measurement unit was used, and for the AC characterization, a Solartron model 1260A impedance analyzer was employed. The parallel contact was B


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2b). The SERRS spectrum shows three main bands assigned to vibrational modes of the perylene chromophore,10,30 and the plasmon enhancement is seen in Figure 2b by plotting both RRS and SERRS spectra in the same scale. The enhancement factor estimated was about 103, which was calculated considering the intensity ratio SERRS/RRS of the band at 1294 cm−1 for the spectra obtained under the same experimental conditions. The correspondence between RRS and SERRS spectra reveals the Ag islands deposited onto the PTCD-COOH film is indicative of physical adsorption. Thus, the enhancement mechanism can be assigned to the electromagnetic effect, basically related to the amplification of the electromagnetic field surrounding the Ag nanoparticles supported by Ag localized surface plasmon resonances.31−33 Emission Spectra. When excited by radiation within the electronic resonances, PTCD molecules exhibit strong fluorescence, which can overpower the Raman scattering. This effect is observed when using the excitation 633 nm laser line to study the PTCD-COOH PVD films. Figure 3 shows the

with a brickwork arrangement outlining multilayers with formation of H and J aggregates, which means that aggregation increases with the film thickness.25,27 Raman Scattering (RS) and Surface-Enhanced Resonance Raman Scattering (SERRS). Vibrational Raman scattering technique is a versatile characterization tool sensitive to crystallinity while providing molecular fingerprint information. Figure 2 shows the resonance Raman spectra recorded

Figure 3. Fluorescence spectra for PTCD-COOH powder and PVD films with 10, 30, 80, and 125 nm and pre-SERRS spectrum for the 10 nm PTCD-COOH PVD film under 6 nm Ag island film. Laser line at 633 nm.

Figure 2. (a) RRS spectra obtained for PTCD-COOH powder and PVD films with 10 and 150 nm on glass, and SERRS for 10 nm film coated with 6 nm Ag; (b) RRS and SERRS of 10 nm PVD film plotted in the same scale. Laser line at 514.5 nm.

fluorescence spectra obtained for powder and PVD films with 10, 30, 80, and 125 nm and SERRS for the film with 10 nm coated with Ag islands. It is seen the PTCD-COOH films exhibit a large and intense fluorescence band with maximum at 742 nm. This fluorescence is characteristic of perylene, being attributed to excimer emission.34 It is known that the excimer emission becomes predominant in thin films of perylene derivatives thicker than two monolayers.35 The excimer emission is facilitated by the molecular orientation provided by the PVD film formation, in which the PTCD chromophores are arranged parallel to each other. The PTCD-COOH powder displays fluorescence with a blue shift of about 40 nm (maximum at 700 nm) in relation to the PTCD-COOH PVD film, which is explained by the formation of J and H aggregates in the film.8 The fluorescence intensity increases linearly with the thickness of the PTCD-COOH PVD film (inset in Figure 3). However, depositing 6 nm Ag onto the 10 nm PTCD-COOH PVD film, the film fluorescence is quenched by the presence of Ag islands and the surface-enhanced phenomenon (SERRS) is observed. It is known that perylene can exhibit the so-called surface-enhanced fluorescence (SEF).36,37 However, this requires that the perylene chromophore is not in contact

with the 514.5 nm laser line for PTCD-COOH powder and PVD films of 10 nm on glass with and without 6 nm Ag island film evaporated on top (cover layer) and 150 nm on glass. Since the excitation laser line at 514.5 nm is within the absorption band of PTCD-COOH, the resonance Raman scattering (RRS) is recorded. The RRS spectra of the powder and PVD films shown in Figure 2 are practically identical, supporting the claim on the subject of chemical integrity of the PTCD-COOH molecules when forming PVD films. In addition, it is observed the spectra are completely determined by the vibrational modes of the perylene chromophore, assigned respectively to C−H bending + ring stretching and perylene ring stretching for the peaks at 1294, 1372 cm−1, respectively, and CC stretching for the peaks at 1567 and 1587 cm−1.28,29 Very minor changes in relative intensities are due to an increase in the aggregation with film thickness, and between powder and PVD films, changes can be associated with differences in molecular organization. The surface-enhanced resonance Raman scattering (SERRS) is obtained after the deposition of 6 nm Ag film onto the PTCD-COOH PVD film with 10 nm mass thickness (Figure C


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Figure 4. (a) Optical microscopy image superimposed to the line Raman mapping (left) and Raman line mapping in 3D (right) for PTCD-COOH PVD film with 150 nm. (b) Topographic AFM image for 10 nm PTCD-COOH PVD film. (c) SEM image for 100 nm PTCD-COOH PVD film.

with the metallic surface to prevent the quenching effect.37 Therefore, the fluorescence suppression observed for PTCDCOOH suggests the perylene molecules are oriented face-on in relation to the substrate surface (PTCD chromophore plane parallel to the substrate surface) or edge-on touching the Ag surface by the PTCD chromophore longer axis. These conditions allow the deposition of the Ag island film onto the PTCD chromophore, leading to the quenching effect. Since no Raman signal is observed for PTCD-COOH using 633 laser line the enhancement factor could be estimated considering the ratio pre-SERRS/spectral noise. The enhancement factor was found to be about 105 for the band at 1293 cm−1 (Figure 3), which is around 102 higher than that estimated for SERRS using the laser line at 514.5 nm. The latter is in agreement with the model considering the electromagnetic mechanism, which predicts the enhancement factor follows SERS > SERRS > SEF.37 In the case of the 633 nm laser line, it is in preresonance (pre-SERRS) with the UV−vis absorption band of the PTCDCOOH PVD film (Figure 2), while 514.5 nm laser line is in full resonance (SERRS). Morphology at Micro- and Nanoscales: Micro-Raman and AFM. The morphology of the PTCD-COOH PVD films was investigated at the micrometer scale using optical microscopy in the micro-Raman system, which allows combination of morphological and chemical information, as shown in Figure 4a for a 150 nm PVD film. A line Raman mapping is superimposed to an optical image obtained with a 50× objective lens (500× magnification). The Raman mapping is obtained by collecting spectra with step of one micrometer along a predefined line with 100 μm length. The line in gray scale corresponds to the intensity of the Raman band at 1302 cm−1 assigned to C−H bending + ring stretching (brighter spot

corresponds to higher intensity). All the 101 Raman spectra collected along the 100 μm line are also shown in Figure 4a. The surface of the PVD films is homogeneous at micrometer scale since no large aggregates in the optical image and irregularities in the Raman mapping are observed. This microscopic homogeneity is characteristic of the PVD technique, being observed for PVD films of various materials such as perylenes, phthalocyanines, polymers, and other macromolecules.10,18−20 At nanometer scale, the morphological study was carried out through AFM and SEM, and the corresponding images are in Figure 4b,c for 10 and 100 nm PTCD-COOH PVD films. The AFM image in Figure 4b shows a few isolated clumps leading to a root-mean-square (RMS) roughness of 1.94 nm (scanned area: 5 μm × 5 μm), and the RMS value ranges between 2.16 nm in the aggregated area (area A in Figure 4b) and 1.22 nm in the smooth area (area B in Figure 4b). The SEM analysis also revealed a few small clumps at the film surface, as shown in Figure 4c. These large aggregates can be the origin of the higher Raman intensity observed at the beginning of the line mapping shown in Figure 4a. Molecular Organization: XDR and FTIR Absorption Spectroscopy. The XRD (Figure S2 in Supporting Information) measurements reveal a pattern with two diffraction peaks with different intensities indicating that the film is amorphous/nanocrystalline structure38,39 The peaks at 25° and 27.4° corresponding to a d space of 3.6 and 3.2 Å, respectively. It can be attributed to the π−π* stacking of the adjacent perylene since the distances of π−π* stacking between the perylene cores ranging approximately at 3.5 Å.40,41 In the work reported by Lovinger et al., similar results were found.42 The appearance of the peak in 27.4° (102 plane) reveals a D


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molecular orientation is also consistent with the differences in relative intensities of the band at 1686 cm−1, assigned to CO symmetric stretching (in-plane mode). The dynamic dipole moment of this vibration mode is along the shorter axis of the PTCD chromophore, and dominates the spectrum recorded in transmission mode. In addition, the PTCD-COOH molecules preferentially oriented face-on in PVD film is also consistent with the fluorescence and SERRS analysis through 633 nm laser line, that is, fluorescence quenching for the PVD film covered with 6 nm Ag and revealing SERRS effect. Electrical Characterization: DC and AC Measurements. The DC electrical measurements were performed applying voltage (V) and measuring the electrical current response (I). For the diode-like structured device, the electric field E = V/d46 was calculated in order to compare the results for samples with different PVD film thickness, d (50 and 90 nm). Figure 6a shows the energy diagram for the diode-like device (ITO/PTCD-COOH/Al), for which the energy gap (Eg) and the LUMO energy (Elumo) of the PCTD were obtained using the experimental data from UV−vis absorption and cyclic voltammetry measurements, respectively, and the calculation following the procedure reported47 (Figure S3 in Supporting Information). Figure 6b shows I versus E curve for the diode-like devices and it is seen a rectifying diode behavior, probably due to the semiconducting characteristics of perylene48 and the work functions of the ITO49 and Al50 electrodes, which promote different barriers for charge injection.51 The rectification factors were calculated (E ∼ 15 MV/m) and the value obtained was about 3 for both devices. For higher electric fields (E ∼ 20 MV/m), the curves show a slight variation of electric current for devices with films of different thickness (50 and 90 nm), which are associated with transport phenomena. These differences are probably related to the thin film aggregation, which agrees to the UV−vis results. This deviation can be explained by the Poole-Frenkel effect,52,53 which is the exponential dependence of mobility with the applied electric field. Related to the AC analysis, the real part of the conductivity (σ′) was obtained from impedance spectra for frequencies ranging from 1 Hz to 1 MHz. In Figure 6c, we show σ′ versus frequency for different DC bias up to 8 V for the diode-like device with 90 nm of thickness film. The curves shape suggest that the active layer of the device follows a typical behavior seen in disordered materials54,55 following a frequency power law dependence. The low frequency behavior of the σ′ vs frequency plots, herein called σdc, shows a tendency of the conductivity to reach a distinct constant value for each applied DC bias stimulus. As shown in Figure 6c, an increasing in the bias magnitude results in a plateau of the constant conductivity in the frequency range considered, reaching maximum at 8 V. This result was similar to that obtained for the 50 nm thickness film. The value obtained for σdc in the diode-like structured device moves up approximately 5 orders of magnitude (10−9 to 10−5 S/m) as the DC bias voltage is increased from 0 to 8 V, superimposed to the 100 mV AC stimulus. This change in conductivity is probably due to the reduction of the interfacial barriers56 (organic semiconductor/Al) and subsequent increase of the charge carrier injection in the device. In a previous work, we have demonstrated a σdc increase about 2 orders of magnitude for another organic macromolecule, the thin film of cobalt phtalocyanine, for DC bias from 0 to 5 V.57 For the IDE device, the increase of the current as a function of the DC voltage (I vs V) is linear, as seen in Figure 7a,

preferred orientation of perylenes cores parallel to the substrate. However, the molecular orientation is not sharply defined due to a proportion of (110) planes (at 25°) parallel to the substrate.42 The FTIR spectra recorded in transmission and reflection modes allow probing the molecular organization in thin solid films. In fact, infrared spectroscopy can provide information on the molecular structure, intermolecular interactions and molecular orientation at microscopic and molecular levels.43 The latter is a powerful tool in the study of organic films of small aromatic molecules such as perylene derivatives.8,43 External reflection−absorption IR probes the film formed above the metal surface by measuring the absorption of the reflected beam, and the spectral analysis is conducted according to the surface selection rules.44 Briefly, (i) the infrared absorption intensity (I) for a given fixed molecule in the film depends on the scalar product between the electric field of the incident radiation (E⃗ ) and the dynamic dipole moment (μ′)⃗ for each vibrational mode; (ii) the incident electric field is parallel to the substrate plane in the transmission mode; and (iii) the incident electric field is polarized preferentially perpendicular to the substrate plane in the reflection mode (grazing angle).44 Therefore, a given molecular orientation may be determined considering the bands assigned to vibrations with the dynamic dipole moment parallel or perpendicular metal surface. FTIR spectra obtained for the PTCD-COOH powder (ATR), for the PVD film in transmission (film on Zn Se plate) and reflection (film on Ag mirror) modes are shown in Figure 5. The main characteristic FTIR bands are seen for both

Figure 5. FTIR spectra for PTCD-COOH powder (ATR mode) and PVD film containing 150 nm in transmission (ZnSe substrate) and reflection (Ag mirror substrate) modes. Inset: preferential molecular organization of the PTCD-COOH in PVD films (face-on).

powder and PVD film, supporting the chemical integrity of the molecule after the thermal process of film fabrication, which agree with the Raman results. However, there are significant differences in relative intensities of absorption bands comparing the spectra of the PVD films in transmission and reflection modes, a clear sign of anisotropy in the film due to molecular organization. For instance, the band at 812 cm−1 dominates the reflection mode spectrum, but is practically vanished in the transmission mode spectrum. This band is assigned to the characteristic C−H wagging vibration, an out-of-plane mode with dynamic dipole moment perpendicular to the PTCD chromophore plane. The same pattern is observed for the bands at 737 and 859 cm−1, also assigned to out-of-plane modes.30,45 Considering the selection rules, it can be extracted that the molecule is oriented face-on (flat) with respect to the metal substrate. This E


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Figure 6. (a) Energy diagram for the diode-like device (ITO/PTCD-COOH/Al). (b) I vs E plots for the diode-like devices with thicknesses of 50 and 90 nm. (c) Plots of σ′ vs frequency for the diode-like device (90 nm film thickness) with different applied DC bias.

Figure 7. (a) I vs V plot for the PTCD-COOH film deposited onto the IDE. (b) Z′ vs frequency curves for PTCD-COOH film deposited onto IDE, under different applied DC biases. (c) Representation of the perylene molecule chromophore, laid over the substrate, and their interaction with the electric field vector (E⃗ ) in the diode-like device (ITO/PTCD-COOH/Al) and PTCD-COOH deposited onto IDE.

the cell constant was determined to be 5.1 m−1 and the IDE device conductivity (σdc) resulted in 2.9 × 10−10 S/m. The electrical conductivity reported in literature for perylenes nonsubstituted are about 10−14 to 10−11 S/m,60−62 which are low values compared to the result presented here. Tkachenko et al.63 also observed higher conductivity for PTCD derivative

characteristic of the ohmic behavior.58 The film resistance (R) can be obtained from this plot, calculated directly from y = a + bx equation, where the slope b is 1/R. Using a model from Olthuis et al.,59 it is possible to obtain the conductivity of the device from its resistance. In ref 59, the cell constant accounts for the digit height, spacing, number, and length. In our case, F


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as on the charge carrier mobility and on the photoinjection from the illuminated electrodes.9 When the light is turned off, the photogenerated excitons diffuse through the perylene organic layer until they dissociate at a defect site or at a metal/organic-layer interface.67 In Figure 8, it is possible to observe a fast reduction of the photocurrent (light off) followed by a slow decay approaching to the dark current initial value. The slow decay of the current features the persistent photoconductivity, a phenomenon where the conductivity remains high for a while even after light exposure, which has already been observed in organic semiconductors by several groups.68−70 Since the electrical conductivity in perylene derivatives is due to intrinsic hopping of small polarons,71,72 the persistent photoconductivity is probably related to the polarons trapped which are then released and dissociated at an interface.73 It has been reported that more ordered structures of perylene derivatives lead to higher photoconductivity.9,74 Draper et al. also demonstrated that the materials maintain their photoconductivity for several hours after light irradiation (persistent photoconductivity) even in air and in the presence of water.74 According to Mo et al., more ordered perylene films are formed with smaller alkyl chains, once longer chains may hinder the tightly molecular packing.75 The last examples showed the impact of the film organization in the photoconductivity of perylene derivatives, which is an important feature for a possible application of perylene films as devices in the organic electronic area. Perylene derivatives used to be widely applied in organic solar cells at the beginning of the organic photovoltaic age,76,77 and then they lost their popularity for the C60 derivatives.78 However, studies showing the potential for the photoconductivity enhancing in perylene films are improving their application potential and consequently bringing back the deserved prominence of these organic molecules.63

as compared to the PTCD nonsubstituted. The results of AC measurements for that same device are shown in Figure 7b; the real part of the impedance as a function of frequency (Z′ vs frequency). In contrast to the behavior of the diode-like structure, for the IDE device, R does not change as the DC bias is applied in the AC measurements. This is probably related to the ohmic contacts56 of the IDE device, in which there is no difference in the electrode work functions. The conductivity value obtained at low frequency and zero bias at AC measurements for the diode-like structure is related to the bulk of the material. It is possible to observe that this conductivity is about 1 order of magnitude higher than that obtained for parallel contact (IDE device) in DC measurements. It is important to remember that the chromophores are arranged parallel on the substrate, as observed by FTIR (faceon). Thus, the difference between the conductivities may be due to the electric field vector (E⃗ ) in relation to the PTCD chromophore orientation. In the IDE device, the chromophores are arranged parallel to E⃗ , whereas for the diode-like device they are perpendicular to E⃗ . The interaction between E⃗ and the chromophores in both devices are illustrated in Figure 7c. The results show that the diode-like device attains higher conductivity. A similar result was observed by Dey and Pal64 in layer-by-layer nickel-phthalocyanine films (NiPc). This fact is probably due to the perpendicular orientation between E⃗ and the perylene chromophores, improving the charge carrier transport along the π stacked aggregates. This is a common feature in materials assembled from planar aromatic molecules, in particular in those assembled with fibrillar morphology.65,66 In the IDE device, the charge transport along the E⃗ direction (parallel to the chromophores) is probably hampered by the lateral groups attached to the PTCD chromophores. Photoconductivity tests were also carried out for the PTCDCOOH thin film. Figure 8 shows the I versus t curves in dark

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CONCLUSIONS We show the influence of the supramolecular organization of the PTCD-COOH PVD film on its electrical properties using different device geometries (IDE and diode-like devices), which promote distinct orientation of the electrical field in relation to PTCD-COOH molecular organization. The PTCD-COOH molecular organization in PVD films is determined. The UV− vis absorption reveals a linear relationship between absorbance and film thickness for film growth by PVD. The Raman data (RRS and SERRS) disclose the chemical integrity of the PTCD-COOH molecules after the thermal evaporation process and corroborate the UV−vis and infrared absorption measurements. A plasmon enhancement factor of about 103 and 105 are estimated for SERRS and SERS respectively. Quenching of the emission when the organic film is covered with AgNPs indicates a preferential face-on PTCD-COOH molecular organization in the PVD films. The latter is confirmed by FTIR measurements performed in transmission and reflection modes. The morphology investigated by micro-Raman mapping, optical microscopy, AFM, and SEM is found to be homogeneous for both micro- and nanoscale. The results for DC electrical measurements for the diode-like device (ITO/PTCD-COOH/Al) indicate a Schottky diode behavior with a rectification factor of 3 for both thicknesses (50 and 90 nm) at least for high electric fields, probably due to the increasing aggregation with film thickness, consistent with UV− vis results. For the IDE device, the results for DC electrical measurements have a linear behavior (I vs V), with calculated

Figure 8. I vs t plot for the PTCD-COOH in IDE device under dark and illuminated conditions.

and under illumination, where a 5 V DC stimulus is applied in the IDE device. The electric current increases (photocurrent) around 17 nA when the device is exposed to light. The electric current enhancing due to the light exposure (photoconductivity effect), is basically related to the photogenerated excitons diffusing through the organic layer.67 However, the photocurrent measured in the I versus t curve depends not only on the photogeneration of charge carriers into the bulk, but as well G


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The Journal of Physical Chemistry C conductivity 2.9 × 10−10 S/m. The AC electrical measurements for the diode-like device show a conductivity increase of about 5 orders of magnitude (10−9 a 10−5 S/m) with DC bias voltage. In contrast, the IDE device does not present dependence on DC bias. Regarding the relationship between supramolecular organization and electrical properties in PTCD-COOH films, the electrical measurements show higher conductivity for the diode-like device when compared to the IDE one. The latter is related to the perpendicular orientation of the electric field in relation to the π−π stacking aggregation of the PTCD-COOH molecules in the diode-like device. Finally, as proof-of-principle, the IDE device presents photoconductivity effect, with current escalating around 17 nA when illuminated. Finally, the results discussed here are consistent with the recent approaches being proposed to reach more efficient fullerene free organic solar cells. Once an optimum molecular architecture with the best conductivity is known, the search for optimum morphology of the donor−acceptor mixture can be explored.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1, S2, and S3 showing the UV−vis spectra deconvolution, XRD spectrum, and cyclic voltammetry, respectively, and Tables T1−T3 showing UV−vis spectra deconvolution data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03093.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from FAPESP and CNPq. Special thanks to Prof. Souza, A.E. for XRD measurement.

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REFERENCES

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