Graphene-Induced Enhancement of n-Type Mobility in

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Graphene-induced Enhancement of Ntype Mobility in Perylenediimide Thin Films Srinivasa Rao Pathipati, Egon Pavlica, Andrea Schlierf, Mirella El Gemayel, Paolo Samori, Vincenzo Palermo, and Gvido Bratina J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp505463v • Publication Date (Web): 29 Sep 2014 Downloaded from http://pubs.acs.org on October 9, 2014

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The Journal of Physical Chemistry

Graphene-Induced Enhancement of n-type Mobility in Perylenediimide Thin Films Srinivasa Rao Pathipati1, Egon Pavlica1, Andrea Schlierf2, Mirella El Gemayel3, Paolo Samorì3, Vincenzo Palermo2, and Gvido Bratina1,* 1

Laboratory of Organic Matter Physics, University of Nova Gorica, Vipavska 13, SI-5000 Nova Gorica, Slovenia.

2

Istituto per la Sintesi Organica e la Fotoreattività-Consiglio Nazionale delle Ricerche (ISOFCNR), via Gobetti 101, 40129 Bologna, Italy.

3

Nanochemistry Laboratory, ISIS & icFRC, Université de Strasbourg & CNRS, 8 allée Gaspard Monge, 67000 Strasbourg, France

*

Corresponding author. E-mail: [email protected]. Tel. +386 5 365 35 00. Fax. +386 5 365

35 27

ACS Paragon Plus 1 Environment


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Abstract Organic thin-film transistor transfer characteristics and time-of-flight (TOF) photoconductivity measurements were used to investigate the effect of the addition of liquid-phase exfoliated graphene nanoflakes (GNs) on the electron mobility in thin films of N,N'-1H,1H-perfluorobutyl dicyanoperylene-carboxydi-imide (PDIF-CN2). Transfer characteristics measurements reveal that the charge carrier mobility of PDIF-CN2 increases by almost three orders of magnitude via blending with GNs. TOF photocurrent measurements confirm that the GNs improve the charge carrier transport in PDIF-CN2. We have found a strong dependence of the TOF-determined electron mobility on the excitation wavelength and obtained a maximum mobility of 0.17 cm2/Vs for charge carriers produced in GN:PDIF-CN2 blends, using photon energy of 5.9 eV. This value is in good agreement with the field-effect mobility of 0.2 cm2/Vs determined from transfer characteristics.

Keywords: grapheme nanoflakes, charge carrier transport, time-of-flight, organic field-effect transistors


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Introduction Functionalized perylenes have recently attracted considerable attention, since they combine high n-type mobility, ambient stability and solution processability

1-7

. Solution processing of these

materials in organic media makes them suitable for blending with graphene nanoflakes, aiming at exploiting the exceptional charge transport characteristics of this novel 2D material. Such a bicomponent material may be a good candidate for high-speed organic electronics as well as for light

harvesting

applications.

Among

perylene

perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN2)

1, 8

derivatives,

N,N'-1H,1H-

has gained increased interest

mostly because of reported band-like electron transport in PDIF-CN2 fibers

9

and single

crystals1-2, in the latter case exhibiting electron mobilities as high as 6 cm2/Vs1-2. Also, relatively high electron mobility and high Ion/Ioff ratios were reported in PDIF-CN2 thin-film-based organic thin film transistors (OTFTs)2, 10-13. OTFTs fabricated from solution processed perylenediimides suffer from disorder-induced trap states that effectively reduce charge carrier mobility2. One approach to overcome the adverse effects of disorder includes the introduction of a highly ordered and electrically performing material into the matrix of the organic semiconductor in order to decrease the number and size of the defects by generating percolation pathways for charge transport in the disordered regions. Device prepares by solution processing requires that both components are soluble or dispersable in compatible solvents, resulting in homogeneous blends that prospectively allow for upscalable deposition methods such as coating and printing. To ensure device performance, the additional material must also exhibit favorable alignment of electronic energy levels. Such alignment is a prerequisite for lossless transport between the organic semiconductor and the metallic contacts. Graphene and related systems can be obtained by liquid phase exfoliation in organic solvents. 1-2,



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Among various graphene forms, graphene nanoflakes (GNs) has been chosen in combination

with PDIF-CN2 because the latter has the advantages that perylene based n-type semiconductors are suitable for exfoliating graphite and stabilize graphene sheets in solution

15-17

. Moreover,

neglecting the size effects18, the position of Fermi level in GN’s19 is energetically well aligned with the lowest occupied molecular orbital (LUMO) in PDIF-CN220. We have therefore investigated the role of GNs on the charge carrier mobility in GN:PDIF-CN2 blends fabricated from solutions. This approach is fast, simple and technologically interesting, given that the molecules have the dual role of exfoliating graphite and contributing to charge transport and transistor switching. We studied charge transport on different length scales, by comparing results obtained by current-voltage (I-V) measurements of OTFTs and by time-offlight (TOF) photocurrent measurements. The results obtained indicate that the presence of GNs enhances the mobility of electrons from the 10-4 cm2/Vs observed in the neat PDIF-CN2 films up to 0.2 cm2/Vs in GN:PDIF-CN2 measured in the blends. Hole transport is also enhanced upon the addition of GNs. Their mobility increases from 6 x 10-5 cm2/Vs to 2 x 10-4 cm2/Vs. Experimental PDIF-CN2 (commercial name: ActivInk™ N1100) has been purchased from Polyera and used as provided. Graphene nanoflakes (GN) were prepared with liquid phase exfoliation of 3 mg/mL natural graphite flakes (Aldrich) in 0.1 mg/mL PDIF-CN2 solutions in chloroform (288306 SIGMA-ALDRICH, anhydrous, ≥99%, contains 0.5-1.0% ethanol as stabilizer). The mixtures were bath sonicated for 4.5 hours (Elmasonic P bath sonicator) and then centrifuged at 2200 rpm for 30 min to remove larger graphitic particles. Similar mixtures were also exfoliated in pure chloroform solutions, without the PDIF-CN2, for comparison sake and for the optical


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spectroscopy experiments. Absorption spectra were recorded using a Perkin-Elmer Lambda 950UV/VIS/NIR spectrophotometer. To measure the I-V characteristics of the OTFTs, we have fabricated bottom-gate bottom-contact transistors on heavily doped n-type silicon wafers serving as gate electrode with 230 nm-thick thermally grown SiO2 as the dielectric layer (Ci=15 nF/cm2) and pre-patterned interdigitated Au source-drain electrodes with channel length of 2.5 µm and channel width of 10 mm. SiO2 dielectric interface was treated with hexamethyldisilazane (HMDS), followed by thermal annealing at 80°C for 30 min. Au electrodes were rendered hydrophobic by chemisorption of a self-assembled monolayer (SAM) of decanethiol, obtained by immersion of the electrodes in a 3 mM solution in ethanol overnight. PDIF-CN2 was dissolved in chloroform at a concentration of 0.1 mg/ml and deposited by spin coating at a speed of 1500 rpm. This resulted in a film of thickness having 1.5±0.25 nm. Samples for TOF photocurrent measurements were fabricated on glass substrates, which were previously treated by HMDS. Layers of PDIF-CN2 or GN:PDIFCN2 were deposited by spin-coating using speeds of 500 rpm. This resulted in continuous, ≈20 nm thick layers, onto which two coplanar Al electrodes were deposited by vacuum evaporation (evaporation rate = 1 nm/s) using a shadow mask. The resulting effective inter-electrode separation (L) was 100 µm and 150 µm for samples comprising PDIF-CN2 and GN:PDIF-CN2, respectively. The reason for using longer channel length in the case of GN:PDIF-CN2 is in its superior charge carrier mobility. Faster charge carriers span the channel in shorter time, and their arrival to the collecting electrode can not be detected by the pick-up electronics. The problem is solved by increasing the channel length, i.e. the distance that the charge carriers must travel before arriving to the collecting electrode. In addition, increasing the channel resulted in a decrease of DC current. Sample fabrication for TOF and all electrical measurements were



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performed in a N2-filled glove box, at room temperature. A DC voltage ranging from ±300 V to ±500 V was applied between the electrodes. Biased samples were illuminated near the biased electrode by pulsed, variable-wavelength laser with pulse duration of 3 ns and the pulse repetition rate of 3 Hz. The opposite electrode was grounded via a 100 kΩ resistor, and the resulting time-dependent photocurrent (I(t)) was measured as a voltage drop across the resistor by a 2.5 GHz oscilloscope. We have measured I(t) at two different excitation wavelengths (λ): 530 nm and 210 nm. Results and discussion Figure 1 shows the AFM topography image of a film prepared by spin-coating on SiO2 a suspension obtained by liquid phase exfoliation of 3 mg/mL natural graphite flakes in 0.1 mg/mL PDIF-CN2 solutions in chloroform. It displays isolated graphene nanoflakes adsorbed on a flat substrate as evidenced in both the height (Figure 1a) and phase (Figure 1b) images. Their lateral size spans from 200 nm to several µm

21

, and their thickness varies from monolayers to

multilayers of nanoflakes. For the sake of comparison, pure PDIF-CN2 films were prepared on SiO2 substrates by spin-coating from solution in CHCl3 (in absence of GN). The AFM images in Fig. S1 in the supporting information reveal a very different morphology, characterized by flat patches with variable lateral dimensions (from ca. 50 nm to some µm) and thicknesses of 1.50±0.25 nm. The AFM phase image of GN:PDIF-CN2 blends (Fig. 1b) shows that each GN is surrounded by patches of molecules, which also partially covering their surfaces, yielding an irregular coating on some of the larger nanoflakes (Fig. S2). AFM image analysis (Fig. S3) gives an average grain density of ca. 1 µm-2, an average nanoflake length of ≈200 nm and an average thickness of ≈12


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nm. It is worth to point out that these numbers are just indicative, given the high polydispersity of the flakes size and the highly skewed distribution of their lateral and vertical size, which is not Gaussian, but follows instead a lognormal distribution (see Ref. 22). The transfer characteristics of OTFTs based on pure PDIF-CN2 and GN:PDIF-CN2 blend (0.1mg/ml concentration of PDIF-CN2 for both) as electroactive layer are shown in Fig. 2a and 2b, respectively. Typical saturation electron mobility obtained on a channel length (L) of 2.5 µm filled with a PDIF-CN2 layer amounted to 10-4 cm2/Vs while for GN:PDIF-CN2 it resulted 0.2 cm2/Vs. Ion/Ioff ratio of PDIF-CN2 films is 105. The transfer characteristic of GN:PDIF-CN2 based OTFTs exhibits Ion/Ioff ratio of 102. Ion/Ioff ratio decreases due to the percolation networks formed by GNs. Since graphene has no bandgap it will increase both Ion and Ioff currents. We note that by increasing the GN concentration, OTFTs exhibited increasingly graphene-like behavior, i.e. high electron mobility but low Ion/Ioff ratio. Below the reported concentration the increase of mobility was negligible. Further insight into the charge transport mechanism in these devices was obtained from TOF photocurrent measurements that were performed by using two different wavelengths. At a wavelength of λ=530 nm PDIF-CN2 has its absorption maximum while the absorption of GN is lower. Conversely, at λ=210 nm significant absorption of both PDIF-CN2 and graphene is observed. Fig. 3, shows (a) UV/VIS absorption of pure PDIF-CN2, pure GN and GN:PDIF-CN2 in CHCl3 solutions (see also Fig. S4 in SI showing both absorbance and photoluminescence spectra a) and b) the solid-state absorption spectrum of PDIF-CN2 on quartz glass). Absorption spectra of graphene show a typical, slowly decaying characteristic of suspensions. In the case of exfoliation in PDIF-CN2, this typical graphene characteristic is superimposed to the PDIF-CN2 bands that feature a broadened absorption maximum at 270 nm and the typical electronic S0-S1



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transition characterized by vibronic progression (resulting in absorption bands at 456 nm, 486 nm and 522 nm in CHCl3). The enhancement of charge transport properties observed in OTFTs by blending GNs with PDIF-CN2 was also evident in TOF photocurrent measurements. Figures 4a and 4b display the I(t) characteristics measured on PDIF-CN2 layers and GN:PDIF-CN2 layers, respectively. Solid symbols in both panels correspond to negative bias, whereas hollow symbols correspond to positive bias. The I(t) curves were obtained by illuminating the samples with a monochromatic light at λ=530 nm. Details of the TOF measurements can be found in Refs.

23-24

. Briefly, I(t)

curve reflects a photogenerated electric charge current that stems from charge carriers drifting in the inter-electrode electric field. From the polarity of the bias it is possible to measure charge transport of both electrons and holes, independently. For positive bias, I(t) reflects the hole current (hollow symbols) and for negative bias, I(t) reflects electron current (solid symbols). The carriers that are generated near the biased electrode by a laser pulse, become charge-separated by the applied electric field. For positively (negatively) charged electrode, the electrons (holes) drift to the illuminated electrode and contribute only a fraction of current in the I(t) curve. The opposite charged carriers instead drift away from the biased electrode and their I(t) is recorded. As we have shown previously24 by numerically solving the Poisson equation for the biased coplanar-electrode structures, the variation of the electric potential inside the channel exhibits a strong dependence on the distance from the electrodes. For example, if a biased electrode is at positive potential, the potential drops near each electrode (in the range of few µm). Further into the channel, it changes slowly with the distance. The carriers that are generated near the biased electrode initially sense a high acceleration due to the high electric field. This is manifested in the lineshape of I(t) as an initial surge. As the carriers enter the low-field region deeper in the


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channel, their effective drift velocity starts decreasing. Consequently, I(t) exhibits a relatively monotonic decrease. Upon arrival of the fastest carriers to the collecting electrode, a region of high electric field is again experienced. This may be manifested in the I(t) curve as a plateau or in the case of highly ordered materials by a cusp, followed by a rapid decrease in current due to the migration of the carriers into the electrode. The details of how the precise position of the cusp is reported in S7. In Fig. 4 we observe a higher photocurrent of electrons (solid symbols) comparing to the hole photocurrent (open symbols) at a constant magnitude of bias voltage. In addition to a monotonic decrease, electron I(t)s exhibit a change of slope over a relatively wide range of times (indicated by the arrow) followed by a noise signal, whose level was established in a separate measurement (see Supporting information page 8). From these curves we determine the transit time (ttr) of the fastest carriers (indicated by the arrow). We see that ttr is consistently shifted to lower values as the accelerating voltage is increased, as expected for drift transport of the electric charge carriers. Several methods have been proposed to assess ttr from I(t)24-29. Here we elected to compare measured I(t)s with the calculated I(t)s, which were obtained by the kinetic Monte Carlo simulations, based on the hopping transport theory and Gaussian disorder model25. Details of the simulation method that closely follows the method introduced by Bässler can be found in Ref. [6]. Briefly, the charge carrier hopping rate was simulated using Miller-Abrahams equation, and the disorder was simulated with a Gaussian distribution of site energies = ⁄ , where is “effective” width of site energies and temperature T is fixed to 300 K. The inverse decay radius of the localized electronic wave functions (γ), which determines the dependence of the Miller Abrahams hopping probability on the distance was defined with the relationship 2γa=10, in order to account hopping to the nearest neighbors, with the inter-site hopping distance a set to 30



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nm. This value corresponds to the grain size of spin-coated PDIF-CN2 layers, as determined by AFM of 1.5 nm-thick layer spin-coated on SiO2 (not shown). This effective hopping distance of 30 nm is an order of magnitude larger than the typical conjugated segment in disordered materials. In our simulation the hopping within conjugated segments was neglected and the hopping was assumed to occur only between grains. This assumption stems from the fact that charge mobility in polycrystalline layers strongly increases when the grain size increases and the density of grain boundaries is reduced. This suggests that the charge carrier transit time within a single grain is negligible relative to the transit time across grain boundary. Strictly speaking, a is the correlation length or the characteristic period of the percolation cluster, within which the charge transport occurs in negligible time frame. The presented simulation considers the hopping between such clusters, which occurs on a longer time frame. We considered PDIF-CN2 grains as localized states in the framework of hopping sites. The grain boundaries were treated as the energetic disorder imposed on the distribution of hopping states. The organic semiconductor (OS) layer was described as a three-dimensional array of 3333 sites in the direction of charge transport (x). In the two perpendicular directions (y and z) the number of sites was 3 and 105, respectively. In the simulation we assumed that the electric field varied along the x-direction, with the in-plane component described as ′ =

⁄

0 < ′ < ,

(1)

where ′ is the distance from the illuminated electrode and L is the distance between electrodes. The exemplified results of the I(t)s calculation are presented in Fig. 5, where we show, in double logarithmic scale, measured I(t) (symbols) and calculated I(t) (lines) for PDIF-CN2 layers at Vbias of ±400 V. In order to reproduce the measured I(t)s, the calculation comprised both electrons and


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holes. Their initial position was in the range of L/10, corresponding to the width of the laser beam in focus. Holes and electrons were subsequently allowed to hop through a manifold of transport states. While both types of carriers move through the same material system, and therefore experience the same degree of variation of intermolecular distances, i.e. structural (offdiagonal) disorder. On the other hand, charge transport in disordered organic semiconductors includes temporary polarization of molecules onto which charge carrier reside during hopping process. Fuchs et al. have shown with the framework of Marcus theory that spatial confinement of holes in such polarized environment is larger than the confinement of electrons. Consequently, the interaction of holes with neighboring molecules is stronger, giving rise to larger spread in site energies, so that ! > # .30 Indeed, in our case we find the best agreement between the simulated and measured I(t), if we use ! = 156 meV and # = 150 meV as simulation parameters. The calculated I(t)s are presented in Fig. 5 for -400V (solid line) and +400V (dashed line). For each curve, we simulated 105 holes and electrons and obtained ttr for each of them. The respective ttr distributions are presented in histograms in the lower part of Fig. 5. The solid-filled histogram corresponds to electrons, while the hashed histogram corresponds to holes. In order to obtain an equation for charge carrier mobility in the position dependent electric field (Eq(1)), we calculate ttr of a particle drifting with velocity $ ∙ , assuming constant µ. Next we average µ over the ensemble of ttr of simulated particles. The resulting formula for mobility is: 〈$〉 =

〈 〉

( bias -tr

(2)

where 〈- 〉 represents an arithmetic average of the inverse of transit times. On the samples tr

comprising pristine PDIF-CN2 (Fig. 4a) we have determined the electron mobility to be 1.3 x 104

cm2/Vs and hole mobility of 6 x 10-5 cm2/Vs.



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Fig. 6a presents in double logarithmic scale, the measured I(t) (symbols) and calculated I(t) (lines) for GN:PDIF-CN2 layers at Vbias = ±500V, represented in double logarithmic scale,. In contrast to the simulation of PDIF-CN2 layers, it was needed to set the inter-site hopping distance to 300 nm in order to reproduce monotonic slope of I(t) and broader cusp compared to pure PDIF-CN2 layers. Fig. 6b compares the measured I(t) at -500 V (solid squares) and the simulated I(t)s for the inter-site distance of 300 nm (solid line) and 30 nm (dashed line). The latter I(t) exhibits a broad cusp that reproduces measured values. According to Ref 24, a cusp is a signature of coherent charge transport. Coherent transport is established after the majority of charge carriers reaches hopping sites with similar energy. When these favorable sites are reached, the carriers drift with similar velocity. As a result, these carriers approach the opposite electrode collectively, and hence generate a cups in the I(t). However, the measured I(t) in Fig. 6b exhibits a broader cusp than simulated I(t) at a=30 nm, which indicates that the coherent transport is not reached throughout the GN:PDIF-CN2 layer. When we increase the inter-site distance to 300 nm, the number of hopping sites is reduced to 500 in the x direction. Accordingly, the number of hops is reduced and the hopping carriers have less probability to reach energetically favorable sites and coherent transport. Therefore, at the inter-site distance of 300 nm, the simulated I(t) exhibits a broader cusp, which better reproduces the measured I(t). In fact, the inter-site distance of 300 nm is of the same size as the typical width of GNs observed by AFM (Fig.1). Therefore, since the optimal inter-site distance I(t) simulations at a=300 nm better matches the measured I(t), we suggest that the GNs act as highly conductive segments, which interconnect PDIF-CN2 grains, where hopping sites that capture and trap carriers corresponds to GN sheet boundaries, at average distances on a scale of few hundreds nm, as compared with the 30 nm hopping sites present in the neat PDIF-CN2 grains. Consequently, the number of hopping


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sites that capture and trap carrier during the transport through the blend layer is reduced. For each of the calculated I(t)s in Fig. 6a we simulated 105 holes and electrons. The respective ttr distributions are presented as histograms in the lower part of Fig. 6a. The solid-filled histogram corresponds to electrons, hashed histogram corresponds to holes using ttr distributions and Eq. 2 we have obtained electron mobility of 9 x 10-4 cm2/Vs and hole mobility of 2 x 10-4 cm2/Vs. Such increase in mobility coupled to the observed changes in transfer curves (Fig. 2b) upon blending GNs to PDIF-CN2, suggests that GNs act as regions of high-mobility within the OS matrix. We note, however that the increase of the hole mobility resulted in a decrease of Ion/Ioff ratio. This is because, when the OFET is in “off” state for electrons, it is in “on” state for holes. Consequently, Ioff increases with the hole mobility, and the Ion/Ioff ratio decreases. The measured I(t)s (Figs. 4-6) were obtained by photons of λ=530 nm, which corresponds to the position of the maximum in absorption spectra for PDIF-CN2. The primary photon-molecule interaction is through creation of excitons. The resulting charge carriers are therefore the result of excitonic decay, and since GN:PDIF-CN2 comprises two materials with distinctively different absorption spectra, we expect that I(t) lineshape to vary with the excitation wavelength. Indeed, in Fig. 7 we compare electron I(t) obtained on GN:PDIF-CN2 on glass using a bias of -400 V, and two different photon wavelengths of 530 nm and 210 nm, shown as circles and squares, respectively. We see that I(t) obtained by high-energy, 210 nm photons decreases rapidly, but exhibits a distinct cusp before vanishing into the noise level. The I(t) lineshape indicates that the electrons have traveled across the channel and reached the collecting electrode. The transit time of the fastest carriers is ttr = 4×10-6 s, resulting in mobility of µ=0.17 cm2/Vs. This is one order of magnitude higher than the mobility of electrons obtained with low-energy photons.



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In order to explain this behavior, we consider the alignment of relevant electronic energy levels in GN:PDIF-CN2 and their relations relative to the energy of the incoming photons. Graphene work function is 4.56 eV

31

, while the position for LUMO in PDIF-CN2 is ELUMO = -4.5 eV8.

When low-energy photons (2.34 eV) are used for charge excitation, the primary absorption occurs in PDIF-CN2, where excitons are created. Absorption of these photons in graphene is almost three times smaller than the peak absorption in PDIF-CN2. After the excitonic decay, the electrons experience favorable alignment of LUMO in PDIF-CN2 and the Dirac point in graphene for their transport from PDIF-CN2 to graphene, and further to Al electrode (See Fig. S6). Based on the Anderson model, Al/PDIF-CN2 and Al/GN:PDIF-CN2 interfaces exhibit energy level offset of 0.2 eV and 0.3 eV, respectively. On the other hand, photons with energy of 5.9 eV (λ=210 nm) are significantly absorbed also in graphene by interband direct excitation of electrons from occupied into the unoccupied states near the saddle-point singularity at the M point of the Brillouin zone32. It has been shown that the correct treatment of optical response of graphene at these energies must include excitonic effect 33-35

. Theoretical modeling of the band transitions indicate excitonic lifetimes of only 0.5 fs34, so

that we may consider the carriers excited by ultraviolet light essentially as free on the timescale of our experiment. Comparison of the two I(t)s shows that electrons created by high energy photons exhibit almost two orders of magnitude shorter transit time (ttr =4×10-6 s) than the electrons created by 530 nm-photons. We argue that the observed dependence of ttr on the excitation wavelength reflects different electron transporting channels within the blend. Considering first electrons originating from decay of the excitons, which are the product of the interaction of photons with molecules, we note that in the high electric-field region near the biased (and illuminated) electrode, electrons


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drift towards the collecting electrode through the GN:PDIF-CN2 network, by sequentially occupying relaxed states (negative ions) on PDIF-CN2 molecules or quasi extended states residing on GNs. Their energy in the relaxed state is determined by the lattice deformation potential, and by vibronic and electronic polarizations. This implies that the residence time in the relaxed state is determined by the ratio between the electron kinetic energy and the sum of the three polarizations36. As kinetic energy the electron is increased the vibronic and lattice polarization times become too long to be effective, and with further increase of kinetic energy the electronic polarization is also lost. These hot electrons start behaving almost as free electrons whose transport is limited by elastic/inelastic scattering events within the layer. Excitation of GN:PDIF-CN2 by photons with λ=210 nm (energy = 5.9 eV) results (on the nanoseconds timescale of our excitation source) in charge-transfer (CT) excitons residing on PDIF-CN2 molecules, and hot electrons that reside on GNs. As has been shown by Tielrooij et al.

37

, the interaction of these hot electrons with the electrons on the Fermi surface, increases

their residence time in the conduction band of graphene. Consequently, due to applied electric field these hot electrons move through the GN:PDIF-CN2 layer, essentially absent of lattice, vibronic and electronic polarization. Their only significant recombination events are likely to occur at the GN/PDIF-CN2 interfaces. As shown above, the I(t) lineshape reflects predominantly the transport of the fastest carriers. Although the contribution of charge carriers resulting from the decay of CT excitons to the I(t) is not negligible, the lineshape in the region signaling the arrival of the fastest charge carriers is dominated by the fast electrons resulting from the excitation of GNs. A marked plateau at the end of I(t) therefore signals the arrival of hot electrons from GNs to the collecting electrode. From the markedly reduced ttr of these electrons we conclude that the interruptions of the high-energy transporting manifold are relatively short,



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and do not influence appreciably the transport properties of the blend. This again argues for GNs forming a percolating network within the blend. In summary, we have used combined time-of-flight photocurrent measurements and organic thin film transistor transfer characteristics measurement to assess the role of addition of graphene nanoflakes via blending on the electron mobility of PDIF-CN2. Both types of transport measurements indicate that graphene nanoflakes increase electron mobility. Transfer characteristic of transistors comprising GN:PDIF-CN2 layers exhibit an electron mobility of 0.2 cm2/Vs. These values are to be compared to the electron mobility of 10-4 cm2/Vs, obtained on transistors comprising pristine PDIF-CN2 OTFTs. Time-of-flight photocurrent measurements on thin layers of PDIF-CN2 blended with graphene nanoflakes exhibited strong dependence on the wavelength of the excitation light. Measurements using 530 nm resulted in an increase of mobility from 1.3 x 10-4 cm2/Vs on pristine PDIF-CN2 layers, to 9.0 x 10-4 cm2/Vs on blended PDIF-CN2 layers. Measurements using λ=210 nm on blended layers, yielded electron mobility of µ=0.17 cm2/Vs, which agrees well with the transfer characteristics measurements. We associate the observed wavelength dependence with different processes that are underway during the interaction of photons with blended layers. Low energy photons create mostly charge-transfer excitons on PDIF-CN2 matrix, whereas high-energy photons are capable of exciting hotelectrons in graphene nanoflakes. Transport of these hot electrons across the blended layer is almost free of influence of electronic, vibronic and lattice polarization. This results in considerable shorter electronic arrival times in time-of-flight experiment, and consequently higher electron mobilities.


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Acknowledgment This work was funded in part by the Slovenian Research Agency, program P1-0055, the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship, the EC Marie-Curie ITN-GENIUS (PITN-GA-2010-264694) as well as the International Center for Frontier Research in Chemistry (icFRC). S. R. P. acknowledges support from the ESF Project GOSPEL (Ref Nr: 09 Euro GRAPHENE- FP-001). Supporting Information Available Supporting information is available as a supplement to this paper in the file names “SI.docx”. This information is available free of charge via the Internet at http://pubs.acs.org



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Figures

a)

b)

Figure 1. Tapping Mode AFM topography images of graphene flakes exfoliated in 0.1 mg/mL solution of PDIF-CN2 in chloroform, supported on SiO2. a) Topography, Z=50nm. b) phase image, arbitrary units.


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a

b



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Figure 2. Typical transfer characteristics of thin-film transistor based on (a) neat PDIFCN2 layer, and (b) GN:PDIF-CN2 blend layer. Channel length = 2.5 µm. The arrows indicate the direction of voltage sweep.

Figure 3. UV/VIS absorption of pure PDIF-CN2, pure GN and GN:PDIF-CN2 in CHCl3 solutions, diluted 1:10 for optical measurement.


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Figure 4. Double logarithmic plot of time-of-flight photocurrent (I(t)) measured on spincoated layers of PDIF-CN2 and GN:PDIF-CN2 on glass. The layer under investigation was contacted with two coplanar Al electrodes. I(t) was measured by illuminating the organic layer at the biased electrode using photons of energy of 2.34 eV (λ=530 nm). The charge carriers were collected at the opposite electrode. Positive bias resulted in hole I(t) and negative bias resulted in electron I(t). Arrows indicate the position of the cusps, which approximately correspond to the transit time of the fastest carriers, obtained as a 10-percentile time of the transit time distributions, which are presented in Figs. 5 and 6. Electrode separation was 100 µm and 150 µm for PDI-CN2 and for blend, respectively.



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Figure 5. Comparison between experimental time-of-flight photocurrent (I(t)) measurements and simulated I(t) in PDIF-CN2. Empty and filled circles correspond to experimental data measured for electrons and holes, using bias voltage of -400 V and +400 V, respectively. Solid and dashed lines correspond to simulated curves for holes and electrons, respectively. The distance between electrodes L is 100 µm and the laser wavelength is 530 nm. The arrows (from left to right) represent the time of the cusp, the transit time of 10% of the fastest carriers (t10%) and the average transit time of electrons (red) and holes (blue). The latter was used to calculate mobility in Eq. 2. Transit time is obtained from Monte Carlo simulation of I(t) (solid and dashed lines) using a level of energetic disorder of 150 meV for electrons and 156 meV for holes. The histograms represent the transit time distributions of simulated electrons (solid-filled) and holes (hashed).


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Figure 6. Comparison between experimental time-of-flight photocurrent (I(t)) measurements and simulated

I(t) in GN:PDIF-CN2. a) Empty and filled squares

correspond to experimental data measured for holes and electrons, using bias voltage of +400 V and -400V, respectively. Solid and dashed lines correspond to simulated curves for holes and electrons, respectively. The distance between electrodes L is 100 µm and the laser wavelength is 530 nm. The arrows (from left to right) represent the time of the cusp, the transit time of 10% of the fastest carriers (t10%) and the average transit time of electrons (red) and holes (blue). The latter was used to calculate mobility in Eq. 2. The



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transit time is obtained from Monte Carlo I(t) simulation (solid and dashed lines) using a level of energetic disorder of 150 meV for electrons and 160 meV for holes. The histograms represent the transit time distribution of simulated electrons (solid-filled) and holes (hashed). b) Close view of measured and simulated I(t)s for two different hopping distances. Filled squares correspond to experimental data measured for electrons at 400V. Solid and dotted lines correspond to simulated curves obtained from Monte Carlo simulations of I(t) using an intersite hopping distance of 300 nm and 30 nm, respectively.

Figure 7. Double logarithmic plot of time-of-flight photocurrent (I(t)) measured on a layer of GN:PDIF-CN2, spin-coated on a glass substrate. The layer was contacted by two coplanar Al electrodes separated by 150 µm. One of the electrodes was at bias of -400V, and the opposite electrode was connected to zero potential with a testing resistance, so


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that electron current was measured.. I(t) was measured by illuminating the organic layer at the biased electrode using photons of λ= 210 nm (squares) and λ=530 nm (circles). The testing resistance, used to measure photocurrent was 1 kΩ and 100 kΩ at 210 nm and 530 nm, respectively. The arrows indicate the transit time of the fastest electrons that drifted from the biased electrode towards the opposite electrode.



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TOC graphics


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Graphene-Induced Enhancement of n-Type Mobility in

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