Article pubs.acs.org/JPCC
Silica Nanoparticles for Enhanced Carrier Transport in Polymer-Based Short Channel Transistors Ali Veysel Tunc,† Andrea N. Giordano,‡ Bernhard Ecker,§,∥ Enrico Da Como,⊥ Benjamin J. Lear,‡ and Elizabeth von Hauff§,∥,* §
Organic Photovoltaics and Electronics, Institute of Physics, Albert-Ludwigs University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany ∥ Fraunhofer Institute for Solar Enery Systems (ISE), Heidenhofstr. 2, 79110 Freiburg, Germany † ̇ AK Ulusal Metroloji Enstitüsü (UME), Photonic and Electronic Sensors Laboratory, TÜ BIT ̇ AK Gebze Yerleşkesi Barış Mah. TÜ BIT Dr. Zeki Acar Cad. No:1, 41470 Gebze / Kocaeli, Turkey ‡ Department of Chemistry, Pennsylvania State University, 126 Davey Lab, University Park, Pennsylvania 16802, United States ⊥ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom S Supporting Information *
ABSTRACT: Electronic disorder in conducting polymers represents a fundamental limit for developing high performance polymer-based transistors (TFTs). Nanoscaled manipulation of polymer morphology with electrically inert nanostructures is an interesting and flexible strategy to enhance ordering in polymer films. We show that blending poly[2methoxy,5-(3′,7′-dimethyloctyloxy)]-1,4-phenylene vinylene (MDMOPPV) with silica nanoparticles leads to an increase in TFT performance, including an increase in hole mobility by over 10 times. By means of Raman spectroscopy we correlate variations in polymer structure induced by the silica to improvements in the electrical properties. We compare these results to MDMO-PPV blended with the fullerene derivative [6,6]phenyl-C61-butyric acid methyl ester (PCBM). Although PCBM leads to similar results in longer channel TFTs, no improvement in short channel behavior is observed. These results demonstrate a simple way to realize short channel polymer TFTs with enhanced performance. films prepared from the same materials. However, such structures are challenging to electrically contact due to parasitic contact resistances.14−18 Additionally, grain boundaries have to be carefully controlled within the transistor channel, particularly in short channel devices. Recent works have taken a new approach and focused on improving transistor performance by manipulating organic semiconductors microscopically, either with low concentration molecular doping,19,20 blending organic layers with insulators to induce localized lattice strain,21 or controlling molecular ordering.22−24 To date, the use of insulators to enhance transistor performance has been primarily applied to organic materials with a tendency to crystallize. In this study we demonstrate how blending amorphous polymers with nanostructures is a simple way to manipulate intramolecular ordering, and as a result, we increase carrier mobility by over 1 order of magnitude. The increase in transistor performance is comparable to the use of selfassembled monolayers to enhance molecular ordering at the
P
olymeric semiconductors have great potential for electronic applications due to low cost and flexible processing. For thin film transistor (TFT) applications, however, the characteristically low mobility values inherent to disordered polymer films represent a fundamental limit for device performance.1,2 As transistor speed τ is related to mobility (μ) and channel length (L) as τ ∼ L2/μ, shortening the channel length can increase device speed.3−5 Short channel behavior, that is, the lack of gate voltage modulation and/or saturation of the channel current with increasing source-drain voltage, is an undesirable effect commonly observed in organic TFTs6−10 leading to unreliable device output and poorly defined transistor parameters. In contrast to inorganic transistors, short channel behavior in organic TFTs is not a consequence of device geometry alone; we recently demonstrated that short channel behavior is highly correlated with morphological and, thus, energetic disorder in the semiconductor.11 This means that strategies to reduce disorder in organic films could both increase carrier mobility and allow for shorter channel devices with enhanced switching speed. Polymeric structures with macroscopic long-range order, such as crystals12 or fibers,13 have improved transport properties compared to amorphous © 2013 American Chemical Society
Received: September 3, 2013 Revised: September 26, 2013 Published: October 1, 2013 22613
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appropriate for observing the distribution of nanoparticles, it does not reveal information concerning how these dopants impact polymer structure, which is a critical aspect for understanding the effects of dopants on material properties. The structural-specific molecular effects of dopants upon conducting polymers is still widely debated, even for very wellstudied systems such as PPV doped with PCBM. They have been attributed to electrical interactions between polymer and fullerene, leading to a ground state charge-transfer state,25 or hole transport by PCBM molecules.33 Other researchers have attributed the effects to purely morphological variations in the polymer induced by the fullerene, such as enhanced planarization and ordering.26,34,35 McGehee and co-workers investigated fullerene intercalation in PPV side groups and correlated the intercalation effect occurring at low PCBM concentrations to an increased hole mobility in the polymer.36 Grey and co-workers demonstrated with the help of Raman spectroscopy37 that fullerene intercalation in PPV side groups decreases polymer chain ordering and suggested the interaction leads to better mobility is electronic. We chose Raman spectroscopy to investigate variations in polymer conformation between the blends, as it is very sensitive to local changes in conjugation length and planarization by probing the vibrational fingerprint of PPV-based polymers.38,39 The technique is well established to unravel the structural characteristics of different conjugated polymers at the molecular level.40,41 We chose to investigate MDMO-PPV blended with 20% PCBM, as previous studies demonstrated that fullerene intercalation in polymer side groups occurs at these low concentrations, making this blend the ideal candidate to determine the mechanism influencing the polymer properties. We compare these results to MDMO-PPV blended with silica. In Figure 2 the Raman spectra of MDMO-PPV (black); MDMO-PPV/silica (40%) (red); and MDMO-PPV/PCBM (20%) (blue) are shown. These concentrations were chosen taking into account that the density of PCBM (1.5 g/cm3)42 is
gate insulator or to increase polymer molecular weight. The addition of insulating silica nanoparticles is highly effective in enhancing TFT performance even in demanding configurations such as short channel devices. Additionally, a significant reduction in the use of the active polymer is achieved. These results demonstrate a simple, solution-based, and cost-effective method to manipulate the electrical properties of amorphous organic materials for the flexible design of electronics. The polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)]1,4-phenylene vinylene (MDMO-PPV) was blended with insulating silica nanoparticles. Silica was added to the polymer solution at concentrations of 20%, 40%, 60%, and 80% with respect to MDMO-PPV weight. In parallel we prepared blends of MDMO-PPV and 20%, 40%, 60%, and 80% of the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to compare the improvement in electrical performance on the same device architectures. PCBM has already been widely reported to increase hole mobility in amorphous PPV derivatives.25−28 In contrast to PCBM, the insulating silica nanoparticles employed here are 10−20 nm in diameter and are not expected to intercalate within polymer side groups. Figure 1 shows scanning electron microscopy (SEM) images of MDMO-PPV films containing (a) 20%, (b) 40%, (c) 60%,
Figure 1. SEM images of MDMO-PPV/silica blends with silica concentrations of (a) 20%, (b) 40%, (c) 60%, and (d) 80%.
and (d) 80% silica. The morphology of PPV/PCBM blends has been investigated extensively in one of our previous publications and by other groups.29,30 Phase segregation at the nanoscale between polymer and fullerene is observed to occur at PCBM concentrations between 40% and 50%,31,32 depending on solvent and polymer molecular weight. We compare this behavior to the distribution of silica nanoparticles in the MDMO-PPV matrix. The silica particles are not functionalized for solubility and, even at low concentrations, exhibit aggregation. Aggregates several hundred nanometers in size are observed within the polymer film, and the size and distribution of the aggregates increases with doping concentration. At higher concentrations (Figure 1c) and d)), the distribution of silica aggregates in the PPV film appears uniform and percolated by polymer dense regions. Though SEM is
Figure 2. Raman spectra of MDMO-PPV (black), MDMO-PPV:SiO2 (red) and MDMO-PPV:PCBM (blue). Both blends contain 60% dopant. The chemical structure of MDMO-PPV is depicted in the top left. The cartoons demonstrate the out-of-plane bend of the vinylene CH groups (968 cm−1 for pure MDMO-PPV) and the symmetric stretch of the phenyl ring. 22614
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roughly half of that of silica (2.68 g/cm3),43 so that a blend with 20% PCBM has a comparable volume fraction of dopant as the blend containing 40% silica. The spectra were measured with an excitation wavelength of 647 nm. This corresponds to a photon energy of incident light below MDMO-PPV absorption, allowing us to probe changes in the conjugated backbone of the polymer in the ground state without complications from resonance Raman enhancement or fluorescence. Interestingly, we observe that PCBM and silica influence the Raman bands very similarly. At 20% PCBM and 40% silica, significant shifts are apparent. Increasing the concentration of silica or PCBM does not further influence the spectra. We start by examining the band at 968 cm−1 (MDMO-PPV), which is assigned as an out-of-plane bend of the vinylene CH groups (inset in Figure 2). Due to the nature of this motion, changes to this band have been ascribed to changes in the planarity of the polymer backbone.37 In particular, red-shifts and increasing intensity (relative to the band at 1588 cm−1) are associated with increasing planarity. For both MDMO-PPV/ PCBM and MDMO-PPV/silica blends, we observe a shift in this band, accompanied by a slight decrease in intensity relative to the 1588 cm−1 band. This indicates a change in planarity, but taken alone, it remains unclear whether the planarity increases or decreases. It should be noted that comparison of Raman band intensities between different samples is nontrivial. A change in the relative intensities of the bands at 1588 cm−1 and 1625 cm−1, on the other hand, reflects a variation in conjugation length. From the literature, I1588/I1625 has been used as a marker for changes in conjugation length, as this value increases with increasing conjugation length.44 We note that, in terms of integrated intensity ratio, this value increases in blends (1.7 and 5.0 for SiO2 and PCBM, respectively) compared to pristine MDMO-PPV films (1.3). Taken together, the changes to the bands at 968, 1588, and 1625 cm−1 suggest an increase in planarity accompanied by an increase in conjugation length within the polymer, suggesting improved intramolecular order. This is confirmed by changes in the bandwidth of the Raman bands consistent with enhanced ordering. Both PCBM and silica lead to a narrowing of the full-width at half-maximum (fwhm) of the Raman bands; for example, the fwhm of the 1588 cm−1 band decreases from 27.6 (±1.0) cm−1 for the pure MDMO-PPV to 19.8 (±0.8) cm−1 and 20.6 (±0.5) cm−1 for the films blended with PCBM and silica, respectively. It is worth noting that the narrowing of this band that accompanies blending is the same, within error, for both dopants. The breadth of the bands originates from both inhomogeneous and homogeneous broadening, which result in Gaussian and Lorrenzian line shapes, respectively. Fitting of the bands to a Voigt profile shows that, upon addition of silica/PCBM, not only does the band become narrower, but the Gaussian contribution decreases. This is an indication that the inhomogeneity in the film also decreases upon doping. An additional interesting aspect here is the behavior of the Raman band at 1588 cm−1 of MDMO-PPV. This band is assigned to a symmetric stretching vibration of the phenyl ring (inset of Figure 2), and shifts in this band have been ascribed to charge transfer interaction between PPV derivatives and electron accepting molecules. In particular, for PPV doped with 2,4,7-trinitrofluorenone (TNF),44 a shift to lower energy was attributed to electron transfer from polymer to TNF. Similar band shifts observed for PPV:PCBM blends37 were also attributed to electronic interactions. In the MDMO-PPV/silica blends we observe identical behavior. As the silica particles are
both insulating and too large to intercalate the polymer side group, this strongly indicates that shifts in this band are related to morphological changes in the polymer and cannot be unequivocally assigned to charge transfer. As the next step, we investigated the influence of silica and PCBM on carrier transport and TFT output. We contrast these results with other well-studied techniques from the literature to induce molecular ordering in the transistor channel. The field effect mobility (μFE) for TFTs with channel lengths L = 20 μm was determined for MDMO-PPV blended at concentrations of 0, 20%, 40%, 60%, and 80% with silica and with PCBM. The TFTs were initially prepared on highly doped silicon substrates with a thermally grown layer of SiO2 as the gate oxide, with bottom contact Au source/drain electrodes. We first compare these results with substrates treated with a self-assembled monolayer of octadecyltrichlorosilane (OTS) which passivates the SiO2 surface and induces preferential molecular ordering at the oxide/semiconductor interface.45,46 Second we investigate the role of polymer molecular weight. High molecular weight (HMW) polymers have fewer chain ends and demonstrate considerably higher mobility values than low molecular weight samples of the same polymer.11 We investigate the influence of silica and PCBM on mobility in a HMW MDMO-PPV (Mn = 450 000 g/mol; Mw = 1 900 000 g/mol) and compare it to the results from the lower weight MDMO-PPV primarily used in this study (Mn = 25 000 g/mol; Mw = 88 000 g/mol). Figure 3 shows μFE vs doping concentration for MDMOPPV blended with (a) silica and (b) PCBM. The squares depict
Figure 3. μFE vs doping concentration for MDMO-PPV blended with (a) silica and (b) PCBM. The squares represent μFE values for standard devices with no OTS treatment, the circles for blends prepared OTS treated substrates, and the stars for blends prepared with HMW MDMO-PPV. The lines between data points are a guide for the eye, and the dashed lines represent μFE for 0% doping for each system.
mobility values for TFTs prepared on untreated SiO2 gate oxide surfaces, the circles for OTS treated devices, and the stars for blends prepared with HWM MDMO-PPV. The lines between the data points are intended as guides for the eye, and the dashed lines indicate the μFE values for the undoped systems. It can be seen that the hole mobility increases with both silica and PCBM concentrations in the blends, in the case of low molecular weight MDMO-PPV on untreated substrates, μFE increases from 6.3 × 10−7 cm2/(V s) (pure MDMO-PPV) to 1.3 × 10−5 cm2/(V s) (80% PCBM) and 8.3 × 10−6 cm2/(V s) (80% silica). We note here that the improvement in hole mobility is over 1 order of magnitude, an unprecedented result for blending organic semiconductors with inorganic insulators such as silica.22 The increase in mobility correlates with an overall increase in transistor performance, including a decrease in both contact resistance and threshold voltage, and an 22615
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Figure 4. Output characteristics for MDMO-PPV (lines), MDMO-PPV/silica (spheres), and MDMO-PPV/PCBM (stars) TFTs with L = 2.5 μm prepared on (a) untreated substrates and (b) OTS treated substrates.
Table 1. Field Effect Mobility μFE and ON/OFF Ratio for TFTs with Channel Lengths L = 20 μm and L = 2.5 μm active layer MDMO-PPV MDMO-PPV/PCBM (60%) MDMO-PPV/silica (60%)
L = 20 μm μFE (cm2/(V s)) −6
4.9 × 10 7.9 × 10−6 6.1 × 10−6
ON/OFF
L = 2.5 μm μFE (cm2/(V s))
ON/OFF
1.9 × 10 3.3 × 103 1.4 × 103
5.4 × 10−6 1.0 × 10−5 6.8 × 10−6
9.6 × 103 6.3 × 104 4.8 × 104
2
(solid lines), MDMO-PPV/PCBM (60%, stars), and MDMOPPV/silica (60%, spheres) for gate voltages of Vgs = 0−60 V, at steps of 20 V for (a) untreated and (b) OTS treated TFT substrates. As observed above for longer channel devices, OTS treatment generally improves TFT output. MDMO-PPV TFTs demonstrate ideal output characteristics without OTS; however, μFE is low (1 × 10−7 cm2/(V s)) and the ratio between Ids in the ON state and OFF state (ON/OFF = 1 × 102) is relatively poor, as a result of the small Ids. Applying OTS increases Ids but results in short channel behavior at higher source-drain (Vds) values. Interestingly, silica leads to an increase in Ids and ideal output behavior in the case of both OTS and non-OTS treated TFTs. Blends with PCBM, on the other hand, show a significant increase in Ids combined with highly nonideal output characteristics. The OTS treatment decreases Ids and slightly improves the output characteristics in these devices too, but the lack of saturation behavior is still dominant. We now discuss the influence of channel length on TFT parameters. Generally decreasing the channel length of the TFT increases the mobility values and the ON/OFF ratio. As mobility is field dependent, reducing L results in higher μFE values due to the increased electric field values in short channel devices.4 Additionally, the ON/OFF ratio increases, as carriers reach saturation velocities at lower applied voltages.7 Table 1 compares μFE and ON/OFF from TFTs prepared with OTS for L = 20 μm and L = 2.5 μm for MDMO-PPV, MDMO-PPV/ silica (60%), and MDMO-PPV/PCBM (60%). We observe that μFE increases by 10% in MDMO-PPV and MDMO-PPV/silica devices and by 300% in MDMO-PPV/PCBM devices. Due to the nonideality of the output characteristics of the MDMOPPV/PCBM TFTs, however, the extracted mobility is certainly overestimated. The ON/OFF ratio increases 10× for each blend when the channel is reduced. In summary, we have shown that silica and PCBM can be used to tune the carrier transport in MDMO-PPV as well as the output properties of MDMO-PPV TFTs. Raman spectroscopy was used to correlate the improvement in hole mobility with dopant-induced changes in intramolecular ordering, namely an increase in polymer planarity and conjugation length. In TFTs,
increase in the quality of the output characteristics (see Supporting Information). The OTS treatment leads to a significant increase in μFE in pristine MDMO-PPV (3.9 × 10−6 cm2/(V s)) compared to MDMO-PPV prepared on untreated SiO2. Doping at higher concentrations of silica/PCBM leads to further increases in the mobility, 6.1 × 10−6 cm2/(V s) (80% silica) and 1.3 × 10−5 cm2/(V s) (80% PCBM). Interestingly, the TFTs with and without the OTS treatment demonstrate the same mobility values at doping concentrations of 80% (silica and PCBM). The hole mobility is generally higher in the HWM MDMOPPV (2.6 × 10−5 cm2/(V s)) compared to the lower molecular weight polymer. Doping with both silica and PCBM increases μFE initially at low concentrations (up to 40%) to values approaching 10−5 cm2/(V s), after which the mobility decreases slightly with higher doping concentration. These results indicate the HMW MDMO-PPV exhibits better charge transport properties, which can be attributed to a lower density of chain end groups but also better intramolecular order, since the structure and the resulting electrical characteristics are weakly dependent on silica concentration. These results strongly suggest that the increase in hole mobility in MDMO-PPV induced by silica and PCBM is due to changes in molecular ordering. This indicates that doping can be used to achieve enhancements in TFT performance comparable to functionalization of the gate insulator, or employing highly ordered organic materials and structures. In short, doping with nanostructures represents a comparatively facile and flexible method for increasing device performance compared to techniques which require multiple processing steps and/or specific material requirements. We turn now to devices with more strict requirements in terms of electrical properties of the semiconductor, namely short-channel TFTs. To realize shorter channel devices with high performance, optimized molecular ordering in the channel is a prerequisite. We examine the influence of the dopants on the undesirable short-channel behavior in TFTs output, i.e. lack of saturation of the channel current Ids at higher drain-source voltages Vds. We investigate TFTs prepared with low molecular weight MDMO-PPV with channel lengths of L = 2.5 μm. Figure 4 shows the output characteristics of MDMO-PPV 22616
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Raman spectra were acquired using a Renishaw inVia Raman microscope equipped with an integral microscope (Leica DM2500 M). The excitation source was a 647 nm CrystaLaser CL-2000 diode laser (70 mW, model DL647-070). The integrated monochromator has a focal length of 250 mm. We used a 1200 l/mm grating, yielding a resolution of 1.9 cm−1.
blending with silica or PCBM results in a greater than 10-fold increase in μFE and improved ON/OFF behavior. Silica, however, is far more effective for improving the parameters of short channel devices, even suppressing nonideal output. This may be an effect of differences in the molecular ordering at the dielectric interface due to silica compared to PCBM. While silica appears to be distributed as uniform aggregates in the polymer matrix, fullerene intercalation in the polymer side groups36,37 or accumulation of fullerene at the substrate interface, and polymer at the top in the blend47 can disturb carrier transport along the channel. These effects are expected to increase with decreasing L, as preferential percolation paths for charge transport along the channel are reduced. These results are significant for the development of novel TFT architectures, including short channel and top gated devices, particularly in combination with gate insulators for which molecular ordering cannot practically be achieved with additional processing steps. In addition, the significant improvements of up to 1 order of magnitude in carrier transport in low molecular weight polymers are interesting for the processing of production-relevant systems such as soluble small molecules. As a result, systems with interesting chemical and optical properties can be tuned electrically with nanoscaled additives during film processing, decreasing the amount of active material necessary for device fabrication.
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ASSOCIATED CONTENT
* Supporting Information S
SEM images of MDMO-PPV blended with silica at 80%. Output characteristics for MDMO-PPV, MDMO-PPV/silica, and MDMO-PPV/PCBM TFTs with L = 20 μm and L = 2.5 μm. Transfer characteristics for MDMO-PPV/silica and MDMO-PPV/PCBM TFTs. Threshold voltage and contact resistance values for different concentrations of PCBM and silica. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: elizabeth.von.hauff@physik.uni-freiburg.de. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.V.T, E.D.C., and E.v.H. thank the DFG for support through the SPP1355 “fundamental processes in organic photovoltaics”. Additionally, E.v.H. and B.J.L. thank the bilateral funding initiatives from ALU Freiburg and Penn State.
EXPERIMENTAL METHODS Poly[2-methoxy, 5-(3′,7′-dimethyloctyloxy)]-p-phenylene vinylene (MDMO-PPV) used in this study was purchased from Sigma Aldrich and Merck. SiO2 nanoparticles (15−20 nm diameters) were purchased from Sigma Aldrich. [6,6]-PhenylC61-butyric acid methyl ester (PCBM) was purchased from Solenne BV. All chemicals were used as received. All organic layers were processed in an N2 filled glovebox. The concentration of MDMO-PPV was kept constant in dichlorobenzene (10 mg mL−1 for low molecular weight MDMO-PPV and 6 mg mL−1 for HMW MDMO-PPV). The amount of silica or PCBM was varied to achieve blends with 0%, 20%, 60%, and 80% of the dopant with respect to polymer weight. The solutions were left stirring overnight on a magnetic stirrer. TFTs were fabricated on highly n-doped silicon substrates with a thermally grown silicon oxide (SiO2) insulating layer (thickness 230 ± 10 nm) with 16 transistors per substrate. Source and drain contacts are interdigitated structures (10 nm ITO, 60 nm Au) with channel lengths L = 2.5, 5, 10, and 20 μm and channel width W = 1 cm. The substrates were purchased from Fraunhofer IPMS (Dresden). The substrates were cleaned in acetone and isopropyl alcohol in an ultrasonic bath for 10 min and then etched with O 2 plasma for 2 min. Octadecyltrichlorosilane (OTS) was applied to the SiO2 surface by immersion in a solution of 10 mM OTS in toluene for 20 min maintained at 60 °C, and the resulting surface was highly hydrophobic with a water contact angle between 94° and 96°. The semiconducting layers were spin coated onto precleaned substrates. The samples were then dried and annealed to evaporate residual solution. The electrical characterization of the TFTs was carried out in a cryostat at 10−6 mbar and room temperature in the dark. A Keithley 236 was used to regulate the source-drain voltage, and the gate voltage was regulated with a Keithley 2400 source measurement unit. A LabView program was used for automated measurements.
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REFERENCES
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The Journal of Physical Chemistry C
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