Organic Interface Properties and Key

Jul 26, 2008 - It is found that the presence of OH groups at the dielectric surface, already .... High Electron Mobility in Vacuum and Ambient for PDI...
0 downloads 0 Views 1MB Size
Download PDF
10130

J. Phys. Chem. B 2008, 112, 10130–10136

Correlation between Dielectric/Organic Interface Properties and Key Electrical Parameters in PPV-based OFETs Francesco Todescato,*,† Raffaella Capelli,† Franco Dinelli,‡ Mauro Murgia,† Nadia Camaioni,§ Mujie Yang,| Renato Bozio,⊥ and Michele Muccini† Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Via P. Gobetti 101, I-40129 Bologna, Italy, Consiglio Nazionale delle Ricerche (CNR), Istituto per i Processi Chimico Fisici, Via Moruzzi, 1 I-56124 Pisa, Italy, Consiglio Nazionale delle Ricerche (CNR), Istituto per la sintesi organica e la FotoreattiVita` (ISOF), Via P. Gobetti 101, I-40129 Bologna, Italy, Department of Polymer Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, China, and Dipartimento di Scienze Chimiche, UniVersita` degli Studi di PadoVa, Via Marzolo, 1, I-35131 PadoVa, Italy ReceiVed: February 11, 2008; ReVised Manuscript ReceiVed: June 9, 2008

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on polyphenylenevinylene derivatives. Through a systematic investigation of the most common dielectric surface treatments, a direct correlation of their effect on the field-effect electrical parameters, such as charge carrier mobility, On/Off current ratio, threshold voltage, and current hysteresis, has been established. It is found that the presence of OH groups at the dielectric surface, already known to act as carrier traps for electrons, decreases the hole mobility whereas it does not substantially affect the other electrical characteristics. The treatment of silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane and hexamethyldisilazane leads to an improvement in hole mobility as well as to a decrease in current hysteresis. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related not only to the OH groups density but also to the interaction between the dielectric and the semiconductor molecules. Specifically, the elimination of the OH groups produces the same effect observed with hexamethyldisilazane. The hole mobility values obtained with hexamethyldisilazane and polymer dielectrics are the highest reported to date for PPV-based field-effect transistors. Introduction In the recent past, the interest in organic electronics has grown quite rapidly. The main reason for this stems from the appealing and unique properties of organic semiconducting materials with respect to their inorganic counterparts.1 In particular, organic electronics offers low cost production and low temperature processing on a large and flexible active area. Organic fieldeffect transistors (OFETs) are the principal building blocks of a large number of electronic devices such as RFIDs, sensors, and active matrix displays.2 Furthermore, the promising results obtained with easy-processable conjugated polymers in organic light-emitting diodes (OLEDs) and OFETs encouraged their integration to realize organic light-emitting transistors (OLETs)3–7 and to pursue the production of low cost and large area allplastic optoelectronic devices.8,9 The key parameters to evaluate the OFET performances are the charge carrier mobility (µ), the On/Off current ratio, and the threshold voltage (Vt). For an accurate electrical characterization, it is also important to evaluate the current hysteresis and the variability in overall performances from device to device. For a device behavior compatible with application standards, these two latter parameters should be reduced to a minimum. At present, the characteristics of polymer-based * Corresponding author. E-mail: [email protected]. † Istituto per lo Studio dei Materiali Nanostrutturati. ‡ Istituto per i Processi Chimico Fisici. § Istituto per la sintesi organica e la Fotoreattivita `. | Zhejiang University. ⊥ Universita ` degli Studi di Padova.

OFETs are not as good as those of their inorganic counterparts. This has prevented so far a massive penetration of polymer electronic devices in the market. Nowadays it is common knowledge that the field-effect conduction occurs in a narrow region of the active material at the interface with the dielectric layer.10,11 Therefore, a careful control of the physical and chemical characteristics of this interface is crucial to improve the performances of OFET devices. Low µ values and noisy characteristics are commonly attributed to the presence of trap states for the charge carriers at the dielectric surface12–15 (i.e., silanol groups) or in the active materials (for instance, structural defects).16 The importance of dielectric surface treatments to reduce the trap density has already been underlined by a number of papers.17 This is valid for the case of oligomers18–21 and also for polymers and blends.22–24 However, none of these works can provide general correlations between the dielectric/organic interface properties following the different dielectric surface treatments and the OFET electrical parameters. In fact, different dielectric surface treatments have been used in combination with different active materials and device structures making it impossible to clearly describe the effects of each surface treatment on the final device parameters. It is therefore crucial, in order to gain new understanding of the device physics, to analyze the OFET behavior keeping fixed the active material and the device structure while systematically changing the dielectric layer or the dielectric surface chemical composition (Figures 1 and 2). We have thus decided to carry out a systematic investigation of two p-type polymers MEH-

10.1021/jp8012255 CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

Correlation in PPV-based OFETs

Figure 1. (a) Schematic drawing of an OFET device in top electrode configuration. (b) Drain and source layout employed in our studies.

PPV, (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) and OC1C10-PPV (poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene]). These two polymers are widely used in organic electronics5,25–29 and can be considered as representatives of a wide class of polymer semiconductors. They have been employed as active materials throughout this study whereas the dielectric/semiconductor interface has been changed to correlate its characteristics with the aforementioned OFET key parameters. We started by studying how different protocols for the cleaning of thermally grown silicon dioxide surfaces, the most common dielectric used for OFET, can affect the device properties. Then, in order to control the trap density at the dielectric interface, we have treated the thermally grown silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS). We additionally investigated, as another way to reducing the trap presence, an extra polymeric layer. For the purpose, polyvinyl alcohol (PVA), poly(methyl methacrylate) (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB) have been used. Different dielectric polymer layers, further to affect the charge carrier trap densities at the dielectric/organic interface, also involve distinct chemical and physical interactions with the organic semiconductor, which have direct influence on the OFET electrical characteristics.30 We pointed our attention to the effects of each dielectric surface treatment on the different OFET parameters and to the role of each dielectric chemical composition and of its interaction with the semiconducting material. Experimental Section The devices were fabricated in a top-electrode configuration (Figure 1) with the following channel length (L) and width (W) values: L ) 150, 300, or 600 µm and W ) 10 000 µm. The electrodes were made of gold sublimated at a base pressure of 1 × 10-4 mbar with a growth rate of 1 Å/s. During gold deposition, the samples were held at room temperature. The electrical characterization was carried out in a glovebox. The mobility and threshold voltage values were evaluated in the saturation regime via the acquisition of locus curves (Supporting Information Information Figures 1s and 5s) where they are not limited by the contact resistance at the metal/ organic interfaces. They were also measured in the linear regime via the acquisition of transfer curves. The results thus obtained were substantially similar. The substrates consisted of heavily doped n+2 silicon with a layer of thermally grown silicon dioxide (SiO2). The thickness of the oxide was 240 nm (Cox ) 1.5 × 10-8 F cm-1). The substrates were cleaned either with an oxygen plasma for 15 min at a power of 100 W or using the following protocol: sonication for 10 min in deionized water, 10 min in acetone, 10 min in chloroform, 10 min in acetone, and finally 10 min in deionized water again. We also tried HF etching by dipping

J. Phys. Chem. B, Vol. 112, No. 33, 2008 10131 the substrates in a 2% water solution for a few minutes and then rinsing in deionized water. This provides the removal of about 5 nm of the SiO2 layer. The gas phase treatments were carried out via exposing the SiO2 substrates to octadecyltrichlorosilane (OTS) or hexamethyldisilazane (HMDS) vapors in argon controlled atmosphere. In this case, the samples were previously cleaned with the sonication procedure described above. Then, the samples were rinsed in chloroform for the HMDS case and in heptane for the OTS case. The wettability of the treated surfaces was measured with a commercial contact angle instrument (Digidrop GBX, contact angle meter model DS). The rms roughness values of the various surfaces were measured by atomic force microscopy (NT-MDT, Solver PRO SPM): 0.3 nm for bare and 0.5 nm for HMDS or OTS treated SiO2 surfaces. The polymer films used as dielectrics were chosen to have a thickness between 50 and 200 nm. The films were obtained via spin-coating with the relevant parameters reported below. For PVA, we spun 200 µL of a 10 g/L water solution at 1500 rpm, for PMMA 150 µL of a 3 wt % ethyl-lactate solution at 6000 rpm, and for BCB, we spun 90 µL of a 10 wt % mesitylene solution at 6000 rpm. The thickness and electrical capacitance (per unit area) values were respectively 50 (PVA), 130 (PMMA), and 121 nm (BCB) and 1.2 × 10-7, 2.45 × 10-8, and 1.9 × 10-8 F cm-1. The dielectric constant used for the calculation of the electrical capacitance due to the polymeric buffer layer are respectively  ) 6.9 at 100 KHz for PVA,  ) 3.6 at 100 Hz for the PMMA, and  ) 2.65 at 1 GHz for the BCB. For PMMA and PVA, we used the dielectric constant found in the literature31 and measured at the lowest possible frequency as our devices are operated in a quasi-steady state. BCB is not a widely diffuse commercial polymer; thus, we use the dielectric constant supplied by the producer and reported in the literature.32 It is also reported that the dielectric constants do not drastically vary with the frequency. Thus, the possible variations in mobility values (some percent at most) do not substantially affect our results as these uncertainties fall within the typical variability from device to device (around 10%). Finally when the polymer buffer layers were employed, we calculated the total capacitance of the dielectric, SiO2 electrical capacitance plus the polymeric one, as the total capacitance of two capacitors in series.33 PVA was thermally annealed at 80 °C for 12 h, and PMMA was thermally annealed at 120 °C for about 12 h (both around their glass temperature). For an optimal BCB cross linking, the films underwent the following thermal curing process: a ramp from 30 to 150 °C at 4 °C/min, holding for 30 min at 150 °C, a ramp from 150 to 250 °C in 1 h, holding at 250° for 1 h, then finally quenched down to room temperature. The rms roughness values of the annealed films typically were 0.6 nm for PMMA and BCB and 0.4 nm for PVA Concerning the active materials, MEH-PPV was purified by chromatography (MW ) 40 000) while OC1C10-PPV was used as received from ADS dyes (MW ) 712 000). The glass transition temperature (Tg) values, measured by DSC, were 78.3 °C for MEH-PPV and 17.8 °C for OC1C10-PPV. The two polymers were spun onto the substrates from a chlorobenzene solution to obtain 100 nm thick films. We employed concentrations of 10 g/L for MEH-PPV and 5 g/L for OC1C10-PPV. The devices were operated after annealing for 15 min at 100 °C with the exception of devices with PMMA dielectric. In the case of MEH-PPV on PMMA, we analyzed samples as

10132 J. Phys. Chem. B, Vol. 112, No. 33, 2008

Todescato et al.

Figure 2. Chemical structures of the active polymers and of the dielectric layers. We started by studying how different protocols for the cleaning of thermally grown silicon dioxide surfaces, the most common dielectric used for OFET, can affect the device properties.

deposited and annealed at 75 and 100 °C. For OC1C10-PPV on PMMA, we studied samples as deposited and annealed at 50 and 100 °C. Results and Discussion SiO2 Cleaning Protocols. The first step of our work was dedicated to cleaning our substrates without any further treatments for the identification of the most effective cleaning procedure for the thermally grown silicon dioxide (SiO2) surfaces in order to obtain the best electrical performances. For this purpose, we chose three standard cleaning protocols, and we studied their effects onto the MEH-PPV device electrical features. The first one involved sequential sonication in different solvents (“wet” cleaning); the second one was the exposure to oxygen plasma, and the last one was etching in HF (see Experimental Section for details). The “wet” cleaning is the mildest process as it does not alter the surface but it only removes the impurities attached to it. In order to evaluate the effect of the cleaning protocol on the OFET response, we produced test devices by spin-coating the active polymer layer directly on SiO2 immediately after cleaning. All samples (with the exception of those with the PMMA dielectric, see details below) were annealed at 100 °C for 15 min. The electrical data are reported in Table 1. In the case of oxygen plasma cleaning, we expected to increase the amount of silanol (Si-OH) groups with a decrease of the Si-O-Si groups. With respect to the wet cleaning, we obtained similar Vt but slightly lower µ values and On/Off ratios

TABLE 1: Electrical Data of OFETs Based on MEH-PPV: Hole Mobility (µ), Threshold Voltages (Vt) and On/Off Ratiosa treatment

µ (cm2/Vs)

Vt (V)

On/Off ratio

plasma HF wet wet + OTS wet + HMDS wet + PVA wet + PMMA plasma + BCB

4.2 × 10 2.8 × 10-5 6.0 × 10-5 1.2 × 10-4 1.3 × 10-3 3.5 × 10-4 1.3 × 10-3 3.3 × 10-3

-8.5 -6.7 -10.9 -11.1 -9.1 -15.7 -8.7 -18.7

1.3 × 102 1 6.6 × 102 7.5 × 102 4.9 × 102 2.6 × 103 3.9 × 103 2.6 × 103

-5

a The samples with PMMA dielectric were annealed at 75 °C for 15 min; all other samples were annealed at 100 °C for 15 min.

(calculated as the ratio between the current in the On state divided by the current in the Off state). The I-V curves showed comparable hysteresis. In the case of etching in HF, a few nanometers of SiO2 were removed leaving more Si-O-Si groups on the surface with respect to the pristine SiO2 one. This treatment yielded by large the worst electrical behavior although the etched surface had the same rms roughness of about 0.5 nm. These data suggest that, in order to build OFETs using asgrown SiO2 as dielectric layer without any further treatment, the “wet” cleaning protocol is the best option. In particular, it yielded the highest µ and On/Off ratio, whereas Vt did not vary noticeably. Furthermore, this cleaning procedure allows one to obtain a good film adhesion without dewetting as can occur in the case of other surface treatments.

Correlation in PPV-based OFETs

J. Phys. Chem. B, Vol. 112, No. 33, 2008 10133

TABLE 2: Electrical Data of OFETs Based on OC1C10-PPV: Hole Mobility (µ), Threshold Voltages (Vt), and On/Off Ratiosa treatment

µ (cm2/Vs)

Vt (V)

On/Off ratio

wet wet + OTS wet + HMDS wet + PVA wet + PMMA plasma + BCB

2.5 × 10 1.6 × 10-4 1.3 × 10-3 3.9 × 10-4 1.1 × 10-3 7.1 × 10-4

-8.7 -14.4 -22.1 -15.2 -18.5 -20.9

1.3 × 102 1.7 × 102 1.7 × 102 1.1 × 103 1.9 × 103 6.9 × 102

-5

a The samples with PMMA dielectric were not annealed; all other samples were annealed at 100 °C for 15 minutes.

SiO2 Gas Phase Treatment. After this preliminary study, we treated the SiO2 surfaces in different ways (Figure 2) in order to change the chemical properties and the physical adhesion characteristics of the dielectric/organic interface. For this purpose, we used two molecular species: octadecyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS; Figure 2). In both cases, we employed the “wet” cleaning protocol for the SiO2 surface before any treatment. The results obtained are reported in Table 1 for MEH-PPV and Table 2 for OC1C10PPV. After the OTS treatment, the µ values increased in a reproducible manner. The Vt values were higher than in the untreated devices, and the On/Off ratios remained similar. Instead, the HMDS treatment improved substantially the OFET performances: much higher µ and On/Off ratio comparable with those obtained with the “wet” cleaning procedure. Regarding Vt, in the MEH-PPV case, it remains substantially similar to the wet cleaning treatment whereas a notable increase is observed for the OC1C10-PPV case. The I-V curves presented good saturation and low hysterisis in both cases, though slightly better for HMDS. In Figure 3, we report data for the case of MEH-PPV. OC1C10-PPV exhibits the same trend (Supporting Information Figure 4s). Additionally, for the HMDS case, we observed that the quality of the treatment is very important. A direct correlation between the contact angle of the HMDS treated SiO2 surface and the device performances can be established: the higher the contact angle, the higher the mobility and the On/Off ratio (Table 3). Higher values of contact angle are evidence of a more hydrophobic surface and therefore they correspond to lower silanol densities (Supporting Information Table 1s). This correlation was previously observed for polymers and oligomers but limited to the electron transport.12,14 For the hole transport, this trend was unknown also because no experimental evidence of the influence of the -OH groups on these charge carriers was available. Dielectric Polymer Layers. Finally, we also covered the SiO2 surfaces with a polymeric buffer thin film using three different dielectric polymers: PVA, PMMA, and BCB (Figure 2). Before the spin-coating of PVA and PMMA, we employed the “wet” cleaning protocol for the SiO2 surfaces, while only in the BCB case, the oxygen plasma cleaning was used in order to provide a better film adhesion. The electrical data are reported in Table 1 for MEH-PPV and in Table 2 for OC1C10-PPV. Starting with PVA, for both MEH-PPV and OC1C10-PPV, we observed that the current hysteresis in the I-V curves was higher (Figure 3d), and the µ values were lower with respect to HMDS. The curves do not reach a well-defined plateau at high drain-source voltages. On the contrary, when employing PMMA, we found nearly no hysteresis and a good saturation of the curves at high drain-source voltages (Figure 3e). The µ values were always higher than those for HMDS and PVA, with better On/Off ratio and similar or lower Vt. These electrical charac-

teristics were the best observed for the polymeric treatments and the best observed overall. High µ values (see Tables 1 and 2) with good electrical characteristics were also obtained when using BCB as the dielectric layer. The output curves showed fairly good saturation and rather low hysteresis (Figure 3f), however, not as good as the PMMA treatment. The On/Off ratios were similar with respect to PMMA. The only drawback was given by a slight increase in the Vt values. Finally, we would like to stress that the µ values obtained with these active polymers on PMMA, HMDS, and BCB are the highest reported in the literature to the best of our knowledge. In Figure 4, topographical images of MEH-PPV films on various dielectric layers are reported along with the rms roughness values. It can be observed that the morphology does not noticeably change from one to the other. The rms roughness values of the films after annealing typically were 0.6 nm for OC1C10-PPV and 0.7 nm for MEH-PPV. The annealing of the active films is extremely important to optimize the OFET behavior as we found out in MEH-PPV based devices. In fact, when we used PMMA as buffer layer, the best electrical characteristics were obtained after annealing for 15 min at 75 °C (glass transition temperature ) 78.3 °C). At higher annealing temperatures, we noticed the occurrence of dewetting of the active materials associated with a large degradation of the electrical characteristics. The same behavior was observed for the OC1C10-PPV. In the latter case, the best OFET performances were obtained with no annealing (glass transition temperature ) 17.8 °C). Otherwise, no dewetting phenomenon was observed on PVA, BCB, and HMDS, even when extending the annealing time from 15 min up to 1 h and the annealing temperature to 100 °C. This is evidence that MEH-PPV and OC1C10-PPV have less affinity with PMMA. Therefore, the molecules of the active polymer have less probability to interact with the dielectric layer at dielectric/semiconductor interface. Consequently, the trap formation is also expected to be reduced. Further Discussion The field effect conduction has been proven to occur in a very narrow region of the active material next to the interface with the dielectric layer.11 In this volume, the gate bias decays in a few nanometers. The presence of charge carrier traps on the dielectric surface has been recognized as the main cause of degradation of the device performances. In particular, the silanol groups present on the SiO2 surfaces have been demonstrated to act as traps for the electrons in polymer14 and oligomer12 based OFETs. In general, their effects can be recognized through a reduced µ, an increased current hysteresis, and a progressive shift of Vt toward higher values.15 The PPVs derivatives we have employed in this study have both p-type characteristics. They have also been shown to support ambipolar transport when using metals with work function values suitable for electron injection,5 which is not however the topic of this paper. From our data, it is clear that the -OH groups present on the SiO2 surfaces can affect the hole transport (Supporting Information Figure 3s). Additionally it is evident that the chemical environment is highly relevant. Field-Effect Mobility. Starting with the bare silicon dioxide surfaces, we immediately notice that the cleaning or treatment procedures that alter the silanol density affect the OFET performance. The “wet” cleaning protocol simply removes any organic contamination due to air exposure; the silanol density thus remains unvaried. This density depends on the modality of thermally driven oxide growth and is not easy to control.

10134 J. Phys. Chem. B, Vol. 112, No. 33, 2008

Todescato et al.

Figure 3. Output curves: Ids is normalized with respect to the channel width (W) and length (L) for OFETs based on MEH-PPV with different dielectrics: (a) SiO2 wet cleaned; (b) SiO2 treated with OTS; (c) SiO2 treated with HMDS; (d) SiO2 covered with a PVA buffer layer; (e) SiO2 covered with a PMMA buffer layer; (f) SiO2 covered with a BCB buffer layer.

TABLE 3: Electrical Data of OFETs Based on MEH-PPV for SiO2 Treated with HMDSa θc

µ (cm2/Vs)

Vt (V)

On/Off ratio

87° ( 2° 90° ( 2° 95° ( 2°

3.8 × 10 7.5 × 10-4 1.3 × 10-3

-6.0 -5.6 -9.1

1.2 × 101 2.6 × 102 4.9 × 102

-4

a A higher value of the contact angle (θc) is related to a lower density of the silanol (Si-OH) groups. All samples were annealed at 100 °C for 15 min. The different contact angles are obtained when nominally the same HMDS treatment is applied.

All of the output curves present low saturation: at high drainsource voltages, the curves do not reach a platea, and have high hysteresis (Figure 3). On the other hand, the oxygen plasma procedure is supposed not only to remove the organic contamination but also to increase the silanol density.34 Accordingly, we obtained a µ reduction that was reproducible in all of the devices analyzed. Vice versa, the HF cleaning protocol is intended to etch a certain amount of SiO2 depending on the concentration and time of exposure. From the pristine SiO2

surface, the silanol groups are removed leaving Si-O-Si terminal groups. Against our expectations, the electrical parameters are the worst of all. It is known that a reaction with the air humidity reintroduces the silanol groups. We suppose that in our procedure this reaction can occur almost immediately as the active material is spin-cast only after a few minutes. This results in a deterioration of the dielectric layer and of the interface with the semiconducting polymers leading to lower performance OFET devices (Supporting Information Figure 6s.a). The chemical treatment through gas phase is intended to reduce the silanol groups at the interface (see Figure 2). We have employed HMDS, which selectively attaches to silanol terminals, and OTS, which forms a self-assembled monolayer (SAM). The HMDS treatment, as expected, improves the device performances: Vt and the current hysteresis are reduced with respect to pristine substrates. Remarkably, we also observe that µ increases with increasing the contact angle (Table 3). The contact angle is a direct measurement of the silanol density (Supporting Information Figure 6s.b). This means that with

Correlation in PPV-based OFETs

J. Phys. Chem. B, Vol. 112, No. 33, 2008 10135

Figure 4. Atomic force microscopy (AFM) images of MEH-PPV films spin-cast on different dielectric substrates with the corresponding surface rms roughness. The lateral size is 5 × 5 µm2. The film deposited on PMMA was annealed at 75 °C for 15 min; all the others were annealed at 100 °C for 15 min.

better HMDS treatments the silanol density decreases and the free superficial traps are reduced. This result shows that the silanol groups can trap not only electron carriers but also hole carriers. Unexpectedly, the OTS treatment does not improve the electrical properties; with respect to the wet cleaning, only a slight increase in µ can be observed. OTS forms a SAM anchored to some silanol groups. When completely formed, the silanol groups should be fully screened. We therefore assume that either the SAM formed using the gas phase technique is not continuous or it does not fully screen the silanol groups. It has recently been shown that in order to improve the quality of the OTS gas phase treatment a second step to endcap residual -OH groups with HMDS is needed.35 It is therefore likely that this is the cause of the poor response we observe. The deposition of a thick polymeric dielectric film is sufficient to fully screen the influence of silanol groups that remain deep down in the surface. However, PVA still presents OH groups in the polymer chain though in a different chemical environment and with different acidity, as they are linked to aliphatic groups. In addition, the polymer film has an amorphous structure with the chains bundled together. On average, the OH groups are randomly oriented, and the total dipolar moment at the dielectric interface should be zero. The electrical performances are worse compared with HMDS but still better with respect to pristine SiO2 surfaces. This is evidence that the OH groups of the PVA polymer still act as traps. The lower trapping efficiency of these groups on the PVA, with respect to pristine SiO2 surface, can be explained considering either that the density of dipoles properly aligned at the free surface is lower in PVA than in SiO2 or it is presumable that the charge trapping strength of the silanol groups is larger than that one of the OH groups.

PMMA and BCB do not contain any OH species. Therefore, the large improvements in the electrical characteristics are well in accordance with this scenario. The µ values are the highest ever obtained with these PPVs, and the current hysteresis reduced to a minimum (Supporting Information Figure 2s). Threshold Voltage and Current Hysteresis. The second key parameter to be analyzed is the threshold voltage Vt. Whereas a common trend can be drawn for µ, the same does not apply for Vt. These variations could be related both to charge injection barriers and to the presence of deep traps in the semiconducting polymer films.36 Given that we always operate in a top electrode configuration with gold electrodes (the best suited Fermi level for hole injection), the metal/organic interface is expected to stay constant. We therefore suppose that different interfacial dielectric layers induced different deep traps in the active polymer layers. The different Vt values are probably related to the different chemical species in contact with the active materials as well as to the different adhesion of the semiconductor materials on the dielectric layer; that could influence the trap formation.37 The behavior of current hysteresis also presents some anomalies. After a certain number of measurements, the device performances decay, especially the current hysteresis, highly increases without any recovery even if left unbiased for a long time.38 We assign it to a damage of the interfacial layers of active material in contact with the dielectric (the zone responsible of conduction) probably due to the current flowing that involve a local heating. Again, different chemical interfaces in contact with the active conduction zone could give different damage levels. PMMA gives lower hysteresis, probably as a consequence of the weak interaction between the semiconducting and dielectric polymers. This weak interaction was con-

10136 J. Phys. Chem. B, Vol. 112, No. 33, 2008 firmed, as already mentioned before, by the dewetting of the active polymer, associated to a large increase in the rms roughness of the film, if the annealing temperature is increased above 75 °C. Conclusions In this paper, we have reported a systematic investigation on the role of the dielectric/organic interface in determining the key electrical parameters of polymer-based OFETs. For this purpose, we have employed two conjugated polymers, MEHPPV and OC1C10-PPV, whose properties have been already studied and described in the literature. As dielectric layers, we have employed thermally grown silicon dioxide layers in their pristine state, chemically treated with OTS and HMDS or covered with polymeric layers such as PVA, BCB, and PMMA. We have found that the presence of silanol groups strongly affects the hole transport: the higher their density, the worse the electrical performances. We have also established that the chemical environment matters as well. In particular, the OH groups present in the PVA molecules are less deleterious to hole transport than the silanol groups. The best performances have been found on PMMA, which does not have OH groups in the chemical structure and which shows the lowest affinity with the active polymeric layers. We have provided evidence that, with a reduced dielectric/organic interaction, the output curves present a reduced current hysteresis. The hole mobility values on PMMA, HMDS, and BCB are the highest values reported in the literature to date. This high mobility is also associated with other important electrical features such as low threshold voltage and current hysteresis, high On/Off ratio, and good saturation characteristics. Acknowledgment. This work was supported at Bologna by EU under Project No. FP6-IST-015034 (OLAS) and by Italian Ministry MIUR under Project Nos. FIRB - RBNE033KMA and FIRB - RBIP06JWBH (NODIS), and by Regione EmiliaRomagna under Project MISTER. Authors thank R. Zamboni for stimulating discussions. Supporting Information Available: Locus curves, mobility values versus the dielectric surfaces, output curves, and contact angle values for the various dielectrics. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Forrest, S. R. Nature 2004, 428, 911. (2) Zaumseil, J.; Sirringhaus, H. Chem. ReV. 2007, 107, 1296. (3) Muccini, M. Nat. Mater. 2006, 5 (8), 605. (4) Hepp, A.; Heil, H.; Weise, W.; Ahles, M.; Schmechel, R.; Von Seggern, H. Phys. ReV. Lett. 2003, 91 (15), 157406. (5) Zaumseil, J.; Friend, R. H.; Sirringhaus, H. Nat. Mater. 2006, 5, 69. (6) Dinelli, F.; Capelli, R.; Loi, M. A.; Murgia, M.; Muccini, M.; Facchetti, A.; Marks, T. J. AdV. Mater. 2006, 18, 1413.

Todescato et al. (7) Swensen, J. S.; Soci, C.; Heeger, A. J. Appl. Phys. Lett. 2005, 87, 253511. (8) Dimitrakopoulos, C. D.; Malefant, P. R. L. AdV. Mater. 2002, 14, 99. (9) Malliaras, G.; Friend, R. Phys. Today 2005, 58, 53. (10) Horowitz, G.; Peng, X.; Fichou, D.; Garnier, F. J. Appl. Phys. 1990, 67, 528. (11) Dinelli, F.; Murgia, M.; Levy, P.; Cavallini, M.; de Leeuw, D.; Biscarini, F. Phys. ReV. Lett. 2004, 92, 6802. (12) Facchetti, A.; Yoon, M.; Marks, T. J. AdV. Mater. 2005, 17, 1705. (13) Veres, J.; Ogier, S.; Lloyd, G. Chem. Mater. 2004, 16, 4543. (14) Chua, L. L.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2004, 16, 1609. (15) Smith, J. W. H.; Hill, I. G. J. Appl. Phys. 2007, 101, 044503-1. (16) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; De Leeuw, D. M. Nature 1999, 401, 685. (17) Yasuda, T.; Fujita, K.; Nakashima, H.; Tsutsui, T. Jpn. J. Appl. Phys. 2003, 42, 6614. (18) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dhem, C.; Schutz, M.; Malsch, S.; Effenberg, F.; Brunnbauer, M.; Stellaci, F. Nature 2004, 431, 963. (19) Jin, S. H.; Yu, J. S.; Lee, A. L.; Kim, J. W.; Park, B. G.; Lee, J. D. J. Koeran Phys. Soc. 2004, 44, 181. (20) Gorjanc, T. C.; Levesque, I.; D’Iorio, M. J. Vac. Sci. Technol. 2004, 22, 760. (21) Puigdolers, J.; Voz, C.; Orpella, A.; Quidant, R.; Matrin, I.; Vetter, M.; Alcubilla, R. Organic Electronics 2004, 5, 67. (22) Sirringhaus, H. AdV. Mater. 2005, 17, 2411. (23) Babel, A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656. (24) Goffri, S.; Muller, C.; Stingelin-Stutzmann, N.; Breiby, D. W.; Radano, C. P.; Andreasen, J. W.; Thompson, R.; Janssen, R. A. J.; Nielsen, M. M.; Smith, P.; Sirringhaus, H. Nat. Mater. 2006, 5, 950. (25) Tanase, C.; Wildeman, J.; Blom, P. W. M. AdV. Funct. Mater. 2005, 15, 2011. (26) Tanase, C.; Wildeman, J.; Blom, P. W. M.; Mena Benito, M. E.; De Leeuw, D. M.; Van Breemen, A. J. J. M.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen, J.; Schoo, H. F. M. J. Appl. Phys. 2005, 97, 123703-1. (27) Van Breemen, A. J. J. M.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen, J.; Schoo, H. F. M.; Mena Benito, M. E.; De Leeuw, D. M.; Tanase, C.; Wildeman, J.; Blom, P. W. M. AdV. Funct. Mater. 2005, 15, 872. (28) Jaiswal, M.; Menon, R. Polym. Int. 2006, 55 (12), 1371. (29) Chua, L. L.; Zaumseil1, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K.H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194. (30) Veres, J.; Ogier, S. D.; Leeming, S. W.; Cupertino, C.; Khaffaf, S. M. AdV. Funct. Mater. 2003, 13, 199. (31) Jin, S. H.; Yu, J. S.; Lee, C. A.; Wook, J.; Park, B. G.; Lee, J. D. J. Korean Phys. Soc. 2004, 44, 181. (32) Chen, H. L.; Chu, T. C.; Li, M. Y.; Ko, F. H.; Cheng, H. C.; Huang, T. Y. J. Vac. Sci. Technol. B 2001, 16, 2381. (33) Yoon, M.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 12851. (34) Lim, S. C.; Kim, S. H.; Lee, J. H.; Kim, M. K.; Kim, D. J.; Zyung, T. Synth. Met. 2005, 148, 75. (35) Ho, P. K. H.; Chua, L. L.; Dipnakar, M.; Gao, X.; Qi, D.; Wee, A. T. S.; Chang, J. F.; Friend, R. H. AdV. Mater. 2007, 19, 215. (36) Newmann, C. R.; Frisbie, C. D.; Da Silva Filho, D. A.; Bredas, J. L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (37) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 11–6431. (38) Kawakami, D.; Yasutake, Y.; Nishizawa, H.; Majima, Y. Jpn. J. Appl. Phys. (Part 2) 2006, 45, L1127.

JP8012255