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Structural effects on the characteristics of organic field effect transistors based on new oligothiophene derivatives
Synthetic Metals 146 (2004) 365–371
Structural effects on the characteristics of organic field effect transistors based on new oligothiophene derivatives A.-L. Demana,∗ , J. Tardya , Y. Nicolasb , P. Blanchardb , J. Roncalib a
Ecole Centrale de Lyon et CNRS, Laboratoire d’Electronique, Opto´electronique et Microsyst`emes (LEOM), UMR CNRS 5512, 36 Avenue Guy de Collongue, 69134 Ecully Cedex, France b Groupe Syst` emes Conjugu´es Lin´eaires, CIMMA, UMR CNRS 6200, Universit´e d’Angers, 2 Boulevard Lavoisier, 49045 Angers, France Available online 13 September 2004
Abstract We report on a comparative investigation of a new series of oligothiophene derivatives as active semiconductor in organic field effect transistors (OFET). Quater- and sexithiophenes end-capped with linear hexyl chains or fused phenyl rings have been synthesized. The oligomers have been deposited by vacuum evaporation onto Si-p++ substrates. The gate dielectric was a bilayer PMMA/Ta2 O5 which ensures both good field effect mobility and rather low operating voltage. The thickness of the oligothiophene films was 80 nm. OFET performances were analyzed as a function of the nature of molecules. The analysis of the characteristics of the various devices shows that the nature and the number of the end group exert a considerable influence on the field effect mobility of the resulting devices. Adding hexyl end group improves the mobility up to 0.1 cm2 V−1 s−1 . Reference di-hexyl-sexithiophene OFETs exhibit a mobility of 0.13 cm2 V−1 s−1 . Fused phenyl ring end terminations have also been studied. Poorer performances are obtained with this termination. Our results are discussed in terms of steric interactions and also on the basis of the influence of the end group in the oligothiophene on the molecular arrangement on the surface of the substrate. © 2004 Elsevier B.V. All rights reserved. Keywords: Organic field effect transistors; Oligomers; Thiophene
1. Introduction Organic field effect transistors (OFETs) have been the subject of intensive investigations for more than 15 years. We nowadays benefit from a much better understanding of the fundamental transport mechanisms in these devices as well as from major technology improvements. Both have led to significant improvement of the performances of OFETs and field effect mobility comparable or even better than those obtained with amorphous silicon can now be reached with organic semiconductors (OSC) [1]. This enables us to foresee blooming of applications in coming years. These include driving circuits for future all organic OLEDs flat panel displays [2], plastic RF-ID circuits [3], gas sensors [4], chemical ∗
Corresponding author. Tel.: +33 4 7218 6065; fax: +33 4 7843 3593. E-mail address: [email protected] (A.-L. Deman).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.08.015
species sensors [5]. High quality OFETs are characterised by some major parameters such as: a high field effect mobility µFE , a high drain current in the on state, a as low as possible drain current in the off state, a low contact resistance for better charge injection, a low gate leakage current. Highest electrical performances are achieved through the improvement of mainly four interdependent technological steps: the molecular structure of the organic semiconductors, their crystallographic arrangement on the substrate (orientation, order, grain size), the gate dielectric (oxide or polymer, roughness, electronic surface states), the physicochemical and electrical properties of the organic semiconductor/dielectric and organic semiconductor/source-drain electrodes interfaces. Future technological applications will require specifically designed OSC combining high performances with adequate chemical and physical properties such as stability and processability. Such goals implies in turn a detailed understand-
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ing of the relationships between the chemical structure of the basic molecular unit and the electrical properties of the devices made from these OSC. Thiophene based polymers and oligomers are without doubt among the mostly studied organic semiconductors (OSC) for OFETs. The performances of oligothiophene OFETs, only surpassed by those in pentacene, are strongly dependent on the chemical structure of the molecules and, consequently, on the structural arrangement of the semiconductor layers on the gate insulator. The most common way to adjust the structure of the thiophene molecule was to modify the length of the chromophore, i.e. the number of thiophene units from 4 to 8 as a rule. Structural investigations showed a quasi-vertical orientation of 4T, 6T and 8T evaporated onto Si [6]. A comparison of OFETs built on these highly purified materials evidenced the superiority of 6T over 4T and 8T [7]. An increased grain size and a more uniform orientation was reported for 4T films deposited at increasing
substrate temperature up to 120 ◦ C [8]. In depth theoretical and experimental investigations of the influence of grain size and of grain boundaries on the transport mechanisms in 6T OFETs were also reported [9,10]. It is well-known that introduction of alkyl chain at the terminal ␣-positions of quater- and sexithiophenes 6T has led to a significant increase of hole mobility as a result of stronger intermolecular interactions between the -conjugated systems induced by the occurrence of van der Waals interactions between the lipophilic alkyl chains [11–13]. Alkyl substituted oligothiophene films also show a preferential vertical orientation of the molecule on the surface of the substrate and a layer-bylayer growth mode [14,15]. High mobility all organic oligothiophene OFETs were recently reported [16]. In view to evaluate both the intra-chain properties (density of electrons, ionization potential, etc.) as well as the packing ability of molecules in thin film state many other substituted oligothiophenes were proposed: thiophene-phenylene co-oligomer
Fig. 1. Chemical structure of the compounds synthesized for this study.
A.-L. Deman et al. / Synthetic Metals 146 (2004) 365–371
[17,18], oligo-thienylenevinylenes [19], acrylate derivative [20]. Interesting OFETs performance were recently reported with fused phenyl systems [21]. We have recently described the first example of oligothiophenes containing a benzothiophene system as terminal group [22]. In this context, we have prepared new OFETs based on various oligothiophenes as organic semiconductor and PMMA/Ta2 O5 bilayer as gate dielectric. The chemical structures of studied oligothiophenes are shown in Fig. 1. The first series of oligothiophenes involves unsubstituted and ␣,␣ -di-hexylquaterthiophenes 1 and 3, respectively, along with the unsymmetrical 5-hexyl2,2 :5 ,2 :5 ,2 -quaterthiophene 2. The well-known ␣,␣ di-hexyl-sexithiophene 4 has also been used as active material. The effect of the number of hexyl chain as well as the length of the oligothiophene on the performance of the corresponding transistor has been analyzed. Finally a new series of oligothiophenes (2, 5 and 6) has been synthesized by progressive replacement of one or two 5-hexyl-2-thienyl moieties of compound 2 by a benzo[b]thienyl group. The introduction of benzo[b]thiophene was expected to modify the electronic properties of the molecule, to improve intermolecular interactions via -stacking and to enhance the thermal stability of the molecule as well. The influence of the benzo[b]thiophene ring on the performance of the OFETs is also discussed.
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2. Experimental The synthesis and characterization of all new compounds will be reported in a forthcoming paper. Organic field effect transistors were processed by vacuum evaporation of oligomer at a pressure of 10−6 mbar. The Si-p++ substrate also acted as the gate electrode. The gate dielectric was made of a PMMA/Ta2 O5 bilayer. Ta2 O5 was deposited by e-beam evaporation onto Si kept at near room temperature. The de˚ and position rate and the thickness were respectively 5 A/s 80 nm. The relative dielectric constant of Ta2 O5 is εR = 21–22. Poly(methyl methacrylate) (PMMA) was spin coated from an anisole solution and then allowed to dry for 2 min at 120 ◦ C. The final PMMA thickness was 37.5 nm. This double layer dielectric was shown to provide both improved field effect mobility and lower operating voltage compared to SiO2 gate oxide on pentacene OFET [23]. OFETs with 200 nm thick thermal SiO2 gate oxide were also processed for comparison. Oligothiophene films were deposited by vacuum evaporation onto the substrate held at 70 ◦ C at a rate ˚ of 0.7 A/s. The vacuum during the evaporation process is 10−6 mbar. The final thickness is 85 nm for all the films. Following the organic semiconductor deposition, the device was completed by the evaporation of Au interdigited source
Fig. 2. Output ID –VD characteristics of three quaterthiophene based devices: (a) unsubstituted quaterthiophene (1), (b) mono-hexylquaterthiophene (2), (c) di-hexylquaterthiophene (3). A reference di-hexyl-sexithiophene (4) is also shown (d). Transfer characteristics are recorded for VD = −12 V.
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Table 1 Mobility, threshold voltage, on/off ratio, sub-threshold voltage swing and saturation current for hexyl substituted oligothiophene device processed on PMMA/Ta2 O5 gate dielectric (also reported is an unsubstituted 4T device processed on SiO2 for comparison) Compound 1 1 2 3 4
Gate dielectric SiO2 PMMA/Ta2 O5 PMMA/Ta2 O5 PMMA/Ta2 O5 PMMA/Ta2 O5
a
Mobility (cm2 V−1 s−1 ) 10−5
6× 2 × 10−3 2 × 10−2 6 × 10−2 1.25 × 10−1
VT (V) −50 −2 −0.4 0 −2.6
On/off ratio
S (V/dec.)
ID,sat a (A)
1.3 × 20 1.3 × 102 4 × 102 5.2 × 102
6.5 7 2.1 1.8 2.4
0.09 0.4 9 26 60
103
ID,sat was measured at VD = VG = −15 V. For the 4T device processed on SiO2 , ID,sat was measured at VD = VG = −80 V.
and drain. The channel width and length were respectively 19 mm and 115 m. Devices were characterised in air immediately after the process by the use of two Keithley model 2400 source-meter driven under LabviewTM environment. Although device ageing is not the subject of this paper, we could say that devices can be measured over a few days without noticeable change. In this study, the device configuration may be not fully optimised to provide the highest possible mobility and on/off ratio. For example, the area of the oligothiophene is not carefully delimited which could cause some lateral current on the edge of the device. The influence of other parameters such the surface roughness of Ta2 O5 and PMMA as well as the crystalline structure of the oligothiophene will be reported in a forthcoming paper. Nevertheless care has been taken to keep the process steps rigorously identical so
as to ensure that measured characteristics can be attributed unambiguously to the structure of the oligothiophene.
3. Results and discussion In Fig. 2 are shown typical drain current, ID , versus drain voltage, VD , characteristics at various gate voltages, VG, for the three quaterthiophene devices (1, 2, 3) and the reference di-hexyl-sexithiophene (4). All devices revealed to be of ptype as expected. We observe that, for given VD and VG , the drain current ID increases by a factor of 100 from compound 1 to 3 and increases again for 4 (50%). The characteristics for compound 1 (a) exhibit a high leakage current at low VD and high VG (above −9 V) attributed to drain-to-gate cur-
Fig. 3. Transfer characteristics of the same devices as in Fig. 2. The log(ID ) vs. VG plots in insert put in light the on/of ratio and the sub-threshold voltage swing.
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Fig. 4. Output and transfer characteristics of OFETs on devices 5 (a and b) and 6 (c and d). Transfer characteristics are recorded for VD = −12 V.
rent. On the other hand, plots (b)–(d) do show high quality device characteristics with a strong decrease of the current leakage relative to the drain current. We also observe that the drain current in the saturation regime may be not entirely flat as expected but shows a somewhat “wavy” shape or a slight decrease at high VD . Although no conclusive explanation can be given so far, we strongly suspect the occurrence of some trapping–detrapping mechanisms at the interface with PMMA. On an other hand, the devices show stress induced instabilities and a decrease of ID following several voltage scans should not be ruled out either. It should be noticed that all devices are operating at a rather low voltage (below 15 V) thanks to the use of Ta2 O5 which is a high dielectric constant oxide (εR ∼ 22 in our case). The use of Ta2 O5 alone would have given even smaller operating voltage. However pentacene OFETs made on bare Ta2 O5 exhibit some gate leakage which is almost entirely removed when a thin layer of polymer PMMA is deposited on top. The low dielectric constant of PMMA however lowers the overall gate capacitance which leads to an increase of operating voltage. Another advantage of high-k dielectric is the increase of carrier mobility in the accumulation layer [24]. A comparison of measured mobility with PMMA/Ta2 O5 and SiO2 (200 nm) will be given (see Table 1). The continuous improvement of quaterthiophene based devices upon end substitution with hexyl chains is clearly
observed. This is confirmed in Fig. 3 where the characteristics ID,sat as a function of VG are plotted for devices working in the saturation regime (drain voltage VD = 12 V). The plots in insert give log(ID,sat ) versus VG to evidence the on/off ratio and the sub-threshold voltage swing S. The threshold voltage VT remains about the same for all devices, say between −2 and 0 V. The mobility has been calculated in the saturation regime using Eq. (1): ID,sat =
W µCi (VG − VT )2 2L
(1)
where W and L are the channel width and length, respectively, Ci the gate dielectric capacitance per surface unit, µ the field effect mobility and VT the threshold voltage. In our devices, the W/L ratio and the gate capacitance are respectively 165/1 and 45.7 nF/cm2 . Table 1 summarises the data derived from characteristics reported in Figs. 2 and 3, as well as those obtained for compound 1 devices processed with a 200 nm thick SiO2 gate oxide in place of PMMA/Ta2 O5 . Data reported in Table 1 are corresponding to an average over several measurements on different devices and runs. We first observe the drastic improvement of the mobility – nearly two orders of magnitude – with PMMA/Ta2 O5 compared to SiO2 gate dielectric as well as the increase of ID,sat . Comparison of the three quaterthiophenes (1, 2, 3) clearly evidences the beneficial influence of
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Table 2 Mobility, threshold voltage, on/off ratio, sub-threshold voltage swing and saturation current for phenyl substituted oligothiophene devices processed on PMMA/Ta2 O5 gate dielectric Compound 1 5 6
Gate dielectric PMMA/Ta2 O5 PMMA/Ta2 O5 PMMA/Ta2 O5
a
Mobility (cm2 V−1 s−1 ) 10−3
2× 1.1 × 10−2 1.1 × 10−2
VT (V) −2 −0.5 −5
On/off ratio 20 3 × 102 1 × 102
S (V/dec.)
ID,sat a (A)
7 8.4 6.7
0.4 8 5
ID,sat was measured at VD = −20 V and VG = −16 V.
the hexyl end termination. Actually, the mobility increases from 2 × 10−3 to 6 × 10−2 cm2 V−1 s−1 in average. The on/off ratio also increases although it remains very low. ID,sat increases by a factor of nearly 7. The S factor also shows an improvement and reach a pretty good value of 1.8 V/dec. Dihexyl-sexithiophene (4) device shows a mobility and a saturation current both twice as high (µ = 1.25 × 10−2 cm2 V−1 s−1 and 60 A, respectively). Obtained mobility is quite similar to that previously published [12]. The influence of benzothiophene end termination (compound 5 and 6) on output and transfer characteristics of OFETs is shown in Fig. 4. We observe that devices based on compounds 5 and 6 exhibit a mobility of about one order of magnitude larger than compound 1. The maximum drain current and the on/off ratio are also much higher. However the two compounds 5 and 6 do not show much difference except a somewhat lower drain current in the saturation regime and a slightly smaller on/off ratio in the case of 6. On the other hand, comparison of compound 2 with only one hexyl end-chain and compound 5 with one hexyl and one benzothiophene end-group shows that compound 2 leads to a twice larger mobility and a much lower sub-threshold voltage swing (1.8 instead of 8.4 for compound 5). This would indicate a better molecular arrangement for compound 2 on the substrate. OFETs on hexyl substituted oligomers with alternating phenylene and thiophene on the backbone were reported [18]. Depending on the T/P ratio the mobility ranges between 2 and 9 × 10−2 cm2 V−1 s−1 in the case of vacuum deposited films. Single crystal Bi-phenyl-tri-thiophene cooligomer (BP3T) OFETs [17] were shown to have quite a good mobility (0.15 cm2 V−1 s−1 ). Table 2 summarizes our results on phenyl substituted oligithiophenes (compound 1, 5 and 6).
4. Conclusion In this paper we investigated OFETs based on diversely substituted quaterthiophenes and sexithiophenes. The OFETs were based on PMMA/Ta2 O5 bilayer gate dielectrics. This dielectric was shown to lead to improved mobility and lower operating voltages. We evidence the strong influence of the end substituent on FET performances. Unsubstituted and alkyl oligothiophenes with 1 or 2 hexyl end-groups were compared. Our results confirm that adding long alkyl chains lead to a significant
increase of mobility and this increase is continuous upon the number of alkyl substituents. Fused benzenic rings end-groups also improve the mobility compared to unsubstituted 4T but to a lesser extent than alkyl chains. A promising mobility of 10−2 cm2 V-1 s−1 is reach with these benzenic rings end groups. Work is currently in progress to improve this mobility through more adapted deposition conditions and to ascertain the origin of this improvement, namely enhanced stacking, preferential orientation of molecules or a layer-by-layer growth mode.
References [1] C.D. Dimitrakopoulos, P.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] H. Sirringhaus, N. Tessler, R.H. Friend, Synth. Met. 102 (1999) 857. [3] P.F. Baude, D.A. Ender, M.A. Haase, T.W. Kelly, D.V. Muyres, S.D. Theiss, App. Phys. Lett. 82 (2003) 3964. [4] B. Crone, A. Dodabalapur, A. Gelperin, L. Torsi, H.E. Katz, A.J. Lovinger, Z. Bao, Appl. Phys. Lett. 78 (2001) 2229. [5] C. Bartic, A. Campitelli, S. Borghs, Appl. Phys. Lett. 82 (2003) 475. [6] A.J. Lovinger, D.D. Davis, A. Dodabalapur, H.E. Katz, Chem. Mater. 8 (1996) 2836. [7] H.E. Katz, L. Torsi, A. Dodabalapur, Chem. Mater. 7 (1995) 2235. [8] W.A. Schoonveld, R.W. Stok, J.W. Veijtmans, J. Vrijmoeth, J. Wildeman, T.M. Klapwijk, Synth. Met. 84 (1997) 583. [9] G. Horowitz, M. Hajlaoui, R. Hajlaoui, J. Appl. Phys. 87 (2000) 4456. [10] G. Horowitz, M. Hajlaoui, Synth. Met. 122 (2001) 185. [11] H.E. Katz, A. Dodabalapur, L. Torsi, D. Elder, Chem. Mater. 7 (1995) 2238. [12] C.D. Dimitrakopoulos, B.K. Furman, T. Graham, S. Hedge, S. Purushothaman, Synth. Met. 92 (1998) 47. [13] R. Bourguiga, F. Garnier, G. Horowitz, R. Hajlaoui, P. Delannoy, M. Hajlaoui, H. Bouchriha, Eur. Phys. J. AP 14 (2001) 121. [14] A.J. Lovinger, H.E. Katz, A. Dodabalapur, Chem. Mater. 10 (1998) 3275. [15] J. Ackermann, C. Videlot, P. Raynal, A. El Kassmi, P. Dumas, Appl. Surf. Sci. 212–213 (2003) 26. [16] M. Halik, H. Mlauk, U. Zschieschang, G. Schimd, S. Ponomarenko, S. Kirchmeyer, W. Weber, Adv. Mater. 15 (2003) 917; M. Halik, H. Mlauk, U. Zschieschang, G. Schimd, W. Radlik, S. Ponomarenko, S. Kirchmeyer, W. Weber, J. Appl. Phys. 93 (2003) 2977. [17] K. Nakamura, M. Ichikawa, R. Fushiki, T. Kamikawa, M. Inoue, T. Koyama, Y. Taniguchi, Jpn. J. Appl. Phys. 43 (2004) 100. [18] M. Mushrush, A. Fachetti, M. Ledenfeld, H.E. Katz, T.J. Marks, J. Am. Chem. Soc. 125 (2003) 9414. [19] C. Videlot, J. Ackemann, P. Blanchard, J.M. Raimundo, P. Fr`ere, M. Allain, R. de Bettignies, E. Levillain, J. Roncali, Adv. Mater. 15 (2003) 306.
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Structural effects on the characteristics of organic field effect transistors based on new oligothiophene derivatives A.-L. Demana,∗ , J. Tardya , Y. Nicolasb , P. Blanchardb , J. Roncalib a
Ecole Centrale de Lyon et CNRS, Laboratoire d’Electronique, Opto´electronique et Microsyst`emes (LEOM), UMR CNRS 5512, 36 Avenue Guy de Collongue, 69134 Ecully Cedex, France b Groupe Syst` emes Conjugu´es Lin´eaires, CIMMA, UMR CNRS 6200, Universit´e d’Angers, 2 Boulevard Lavoisier, 49045 Angers, France Available online 13 September 2004
Abstract We report on a comparative investigation of a new series of oligothiophene derivatives as active semiconductor in organic field effect transistors (OFET). Quater- and sexithiophenes end-capped with linear hexyl chains or fused phenyl rings have been synthesized. The oligomers have been deposited by vacuum evaporation onto Si-p++ substrates. The gate dielectric was a bilayer PMMA/Ta2 O5 which ensures both good field effect mobility and rather low operating voltage. The thickness of the oligothiophene films was 80 nm. OFET performances were analyzed as a function of the nature of molecules. The analysis of the characteristics of the various devices shows that the nature and the number of the end group exert a considerable influence on the field effect mobility of the resulting devices. Adding hexyl end group improves the mobility up to 0.1 cm2 V−1 s−1 . Reference di-hexyl-sexithiophene OFETs exhibit a mobility of 0.13 cm2 V−1 s−1 . Fused phenyl ring end terminations have also been studied. Poorer performances are obtained with this termination. Our results are discussed in terms of steric interactions and also on the basis of the influence of the end group in the oligothiophene on the molecular arrangement on the surface of the substrate. © 2004 Elsevier B.V. All rights reserved. Keywords: Organic field effect transistors; Oligomers; Thiophene
1. Introduction Organic field effect transistors (OFETs) have been the subject of intensive investigations for more than 15 years. We nowadays benefit from a much better understanding of the fundamental transport mechanisms in these devices as well as from major technology improvements. Both have led to significant improvement of the performances of OFETs and field effect mobility comparable or even better than those obtained with amorphous silicon can now be reached with organic semiconductors (OSC) [1]. This enables us to foresee blooming of applications in coming years. These include driving circuits for future all organic OLEDs flat panel displays [2], plastic RF-ID circuits [3], gas sensors [4], chemical ∗
Corresponding author. Tel.: +33 4 7218 6065; fax: +33 4 7843 3593. E-mail address: [email protected] (A.-L. Deman).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.08.015
species sensors [5]. High quality OFETs are characterised by some major parameters such as: a high field effect mobility µFE , a high drain current in the on state, a as low as possible drain current in the off state, a low contact resistance for better charge injection, a low gate leakage current. Highest electrical performances are achieved through the improvement of mainly four interdependent technological steps: the molecular structure of the organic semiconductors, their crystallographic arrangement on the substrate (orientation, order, grain size), the gate dielectric (oxide or polymer, roughness, electronic surface states), the physicochemical and electrical properties of the organic semiconductor/dielectric and organic semiconductor/source-drain electrodes interfaces. Future technological applications will require specifically designed OSC combining high performances with adequate chemical and physical properties such as stability and processability. Such goals implies in turn a detailed understand-
366
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ing of the relationships between the chemical structure of the basic molecular unit and the electrical properties of the devices made from these OSC. Thiophene based polymers and oligomers are without doubt among the mostly studied organic semiconductors (OSC) for OFETs. The performances of oligothiophene OFETs, only surpassed by those in pentacene, are strongly dependent on the chemical structure of the molecules and, consequently, on the structural arrangement of the semiconductor layers on the gate insulator. The most common way to adjust the structure of the thiophene molecule was to modify the length of the chromophore, i.e. the number of thiophene units from 4 to 8 as a rule. Structural investigations showed a quasi-vertical orientation of 4T, 6T and 8T evaporated onto Si [6]. A comparison of OFETs built on these highly purified materials evidenced the superiority of 6T over 4T and 8T [7]. An increased grain size and a more uniform orientation was reported for 4T films deposited at increasing
substrate temperature up to 120 ◦ C [8]. In depth theoretical and experimental investigations of the influence of grain size and of grain boundaries on the transport mechanisms in 6T OFETs were also reported [9,10]. It is well-known that introduction of alkyl chain at the terminal ␣-positions of quater- and sexithiophenes 6T has led to a significant increase of hole mobility as a result of stronger intermolecular interactions between the -conjugated systems induced by the occurrence of van der Waals interactions between the lipophilic alkyl chains [11–13]. Alkyl substituted oligothiophene films also show a preferential vertical orientation of the molecule on the surface of the substrate and a layer-bylayer growth mode [14,15]. High mobility all organic oligothiophene OFETs were recently reported [16]. In view to evaluate both the intra-chain properties (density of electrons, ionization potential, etc.) as well as the packing ability of molecules in thin film state many other substituted oligothiophenes were proposed: thiophene-phenylene co-oligomer
Fig. 1. Chemical structure of the compounds synthesized for this study.
A.-L. Deman et al. / Synthetic Metals 146 (2004) 365–371
[17,18], oligo-thienylenevinylenes [19], acrylate derivative [20]. Interesting OFETs performance were recently reported with fused phenyl systems [21]. We have recently described the first example of oligothiophenes containing a benzothiophene system as terminal group [22]. In this context, we have prepared new OFETs based on various oligothiophenes as organic semiconductor and PMMA/Ta2 O5 bilayer as gate dielectric. The chemical structures of studied oligothiophenes are shown in Fig. 1. The first series of oligothiophenes involves unsubstituted and ␣,␣ -di-hexylquaterthiophenes 1 and 3, respectively, along with the unsymmetrical 5-hexyl2,2 :5 ,2 :5 ,2 -quaterthiophene 2. The well-known ␣,␣ di-hexyl-sexithiophene 4 has also been used as active material. The effect of the number of hexyl chain as well as the length of the oligothiophene on the performance of the corresponding transistor has been analyzed. Finally a new series of oligothiophenes (2, 5 and 6) has been synthesized by progressive replacement of one or two 5-hexyl-2-thienyl moieties of compound 2 by a benzo[b]thienyl group. The introduction of benzo[b]thiophene was expected to modify the electronic properties of the molecule, to improve intermolecular interactions via -stacking and to enhance the thermal stability of the molecule as well. The influence of the benzo[b]thiophene ring on the performance of the OFETs is also discussed.
367
2. Experimental The synthesis and characterization of all new compounds will be reported in a forthcoming paper. Organic field effect transistors were processed by vacuum evaporation of oligomer at a pressure of 10−6 mbar. The Si-p++ substrate also acted as the gate electrode. The gate dielectric was made of a PMMA/Ta2 O5 bilayer. Ta2 O5 was deposited by e-beam evaporation onto Si kept at near room temperature. The de˚ and position rate and the thickness were respectively 5 A/s 80 nm. The relative dielectric constant of Ta2 O5 is εR = 21–22. Poly(methyl methacrylate) (PMMA) was spin coated from an anisole solution and then allowed to dry for 2 min at 120 ◦ C. The final PMMA thickness was 37.5 nm. This double layer dielectric was shown to provide both improved field effect mobility and lower operating voltage compared to SiO2 gate oxide on pentacene OFET [23]. OFETs with 200 nm thick thermal SiO2 gate oxide were also processed for comparison. Oligothiophene films were deposited by vacuum evaporation onto the substrate held at 70 ◦ C at a rate ˚ of 0.7 A/s. The vacuum during the evaporation process is 10−6 mbar. The final thickness is 85 nm for all the films. Following the organic semiconductor deposition, the device was completed by the evaporation of Au interdigited source
Fig. 2. Output ID –VD characteristics of three quaterthiophene based devices: (a) unsubstituted quaterthiophene (1), (b) mono-hexylquaterthiophene (2), (c) di-hexylquaterthiophene (3). A reference di-hexyl-sexithiophene (4) is also shown (d). Transfer characteristics are recorded for VD = −12 V.
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Table 1 Mobility, threshold voltage, on/off ratio, sub-threshold voltage swing and saturation current for hexyl substituted oligothiophene device processed on PMMA/Ta2 O5 gate dielectric (also reported is an unsubstituted 4T device processed on SiO2 for comparison) Compound 1 1 2 3 4
Gate dielectric SiO2 PMMA/Ta2 O5 PMMA/Ta2 O5 PMMA/Ta2 O5 PMMA/Ta2 O5
a
Mobility (cm2 V−1 s−1 ) 10−5
6× 2 × 10−3 2 × 10−2 6 × 10−2 1.25 × 10−1
VT (V) −50 −2 −0.4 0 −2.6
On/off ratio
S (V/dec.)
ID,sat a (A)
1.3 × 20 1.3 × 102 4 × 102 5.2 × 102
6.5 7 2.1 1.8 2.4
0.09 0.4 9 26 60
103
ID,sat was measured at VD = VG = −15 V. For the 4T device processed on SiO2 , ID,sat was measured at VD = VG = −80 V.
and drain. The channel width and length were respectively 19 mm and 115 m. Devices were characterised in air immediately after the process by the use of two Keithley model 2400 source-meter driven under LabviewTM environment. Although device ageing is not the subject of this paper, we could say that devices can be measured over a few days without noticeable change. In this study, the device configuration may be not fully optimised to provide the highest possible mobility and on/off ratio. For example, the area of the oligothiophene is not carefully delimited which could cause some lateral current on the edge of the device. The influence of other parameters such the surface roughness of Ta2 O5 and PMMA as well as the crystalline structure of the oligothiophene will be reported in a forthcoming paper. Nevertheless care has been taken to keep the process steps rigorously identical so
as to ensure that measured characteristics can be attributed unambiguously to the structure of the oligothiophene.
3. Results and discussion In Fig. 2 are shown typical drain current, ID , versus drain voltage, VD , characteristics at various gate voltages, VG, for the three quaterthiophene devices (1, 2, 3) and the reference di-hexyl-sexithiophene (4). All devices revealed to be of ptype as expected. We observe that, for given VD and VG , the drain current ID increases by a factor of 100 from compound 1 to 3 and increases again for 4 (50%). The characteristics for compound 1 (a) exhibit a high leakage current at low VD and high VG (above −9 V) attributed to drain-to-gate cur-
Fig. 3. Transfer characteristics of the same devices as in Fig. 2. The log(ID ) vs. VG plots in insert put in light the on/of ratio and the sub-threshold voltage swing.
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Fig. 4. Output and transfer characteristics of OFETs on devices 5 (a and b) and 6 (c and d). Transfer characteristics are recorded for VD = −12 V.
rent. On the other hand, plots (b)–(d) do show high quality device characteristics with a strong decrease of the current leakage relative to the drain current. We also observe that the drain current in the saturation regime may be not entirely flat as expected but shows a somewhat “wavy” shape or a slight decrease at high VD . Although no conclusive explanation can be given so far, we strongly suspect the occurrence of some trapping–detrapping mechanisms at the interface with PMMA. On an other hand, the devices show stress induced instabilities and a decrease of ID following several voltage scans should not be ruled out either. It should be noticed that all devices are operating at a rather low voltage (below 15 V) thanks to the use of Ta2 O5 which is a high dielectric constant oxide (εR ∼ 22 in our case). The use of Ta2 O5 alone would have given even smaller operating voltage. However pentacene OFETs made on bare Ta2 O5 exhibit some gate leakage which is almost entirely removed when a thin layer of polymer PMMA is deposited on top. The low dielectric constant of PMMA however lowers the overall gate capacitance which leads to an increase of operating voltage. Another advantage of high-k dielectric is the increase of carrier mobility in the accumulation layer [24]. A comparison of measured mobility with PMMA/Ta2 O5 and SiO2 (200 nm) will be given (see Table 1). The continuous improvement of quaterthiophene based devices upon end substitution with hexyl chains is clearly
observed. This is confirmed in Fig. 3 where the characteristics ID,sat as a function of VG are plotted for devices working in the saturation regime (drain voltage VD = 12 V). The plots in insert give log(ID,sat ) versus VG to evidence the on/off ratio and the sub-threshold voltage swing S. The threshold voltage VT remains about the same for all devices, say between −2 and 0 V. The mobility has been calculated in the saturation regime using Eq. (1): ID,sat =
W µCi (VG − VT )2 2L
(1)
where W and L are the channel width and length, respectively, Ci the gate dielectric capacitance per surface unit, µ the field effect mobility and VT the threshold voltage. In our devices, the W/L ratio and the gate capacitance are respectively 165/1 and 45.7 nF/cm2 . Table 1 summarises the data derived from characteristics reported in Figs. 2 and 3, as well as those obtained for compound 1 devices processed with a 200 nm thick SiO2 gate oxide in place of PMMA/Ta2 O5 . Data reported in Table 1 are corresponding to an average over several measurements on different devices and runs. We first observe the drastic improvement of the mobility – nearly two orders of magnitude – with PMMA/Ta2 O5 compared to SiO2 gate dielectric as well as the increase of ID,sat . Comparison of the three quaterthiophenes (1, 2, 3) clearly evidences the beneficial influence of
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Table 2 Mobility, threshold voltage, on/off ratio, sub-threshold voltage swing and saturation current for phenyl substituted oligothiophene devices processed on PMMA/Ta2 O5 gate dielectric Compound 1 5 6
Gate dielectric PMMA/Ta2 O5 PMMA/Ta2 O5 PMMA/Ta2 O5
a
Mobility (cm2 V−1 s−1 ) 10−3
2× 1.1 × 10−2 1.1 × 10−2
VT (V) −2 −0.5 −5
On/off ratio 20 3 × 102 1 × 102
S (V/dec.)
ID,sat a (A)
7 8.4 6.7
0.4 8 5
ID,sat was measured at VD = −20 V and VG = −16 V.
the hexyl end termination. Actually, the mobility increases from 2 × 10−3 to 6 × 10−2 cm2 V−1 s−1 in average. The on/off ratio also increases although it remains very low. ID,sat increases by a factor of nearly 7. The S factor also shows an improvement and reach a pretty good value of 1.8 V/dec. Dihexyl-sexithiophene (4) device shows a mobility and a saturation current both twice as high (µ = 1.25 × 10−2 cm2 V−1 s−1 and 60 A, respectively). Obtained mobility is quite similar to that previously published [12]. The influence of benzothiophene end termination (compound 5 and 6) on output and transfer characteristics of OFETs is shown in Fig. 4. We observe that devices based on compounds 5 and 6 exhibit a mobility of about one order of magnitude larger than compound 1. The maximum drain current and the on/off ratio are also much higher. However the two compounds 5 and 6 do not show much difference except a somewhat lower drain current in the saturation regime and a slightly smaller on/off ratio in the case of 6. On the other hand, comparison of compound 2 with only one hexyl end-chain and compound 5 with one hexyl and one benzothiophene end-group shows that compound 2 leads to a twice larger mobility and a much lower sub-threshold voltage swing (1.8 instead of 8.4 for compound 5). This would indicate a better molecular arrangement for compound 2 on the substrate. OFETs on hexyl substituted oligomers with alternating phenylene and thiophene on the backbone were reported [18]. Depending on the T/P ratio the mobility ranges between 2 and 9 × 10−2 cm2 V−1 s−1 in the case of vacuum deposited films. Single crystal Bi-phenyl-tri-thiophene cooligomer (BP3T) OFETs [17] were shown to have quite a good mobility (0.15 cm2 V−1 s−1 ). Table 2 summarizes our results on phenyl substituted oligithiophenes (compound 1, 5 and 6).
4. Conclusion In this paper we investigated OFETs based on diversely substituted quaterthiophenes and sexithiophenes. The OFETs were based on PMMA/Ta2 O5 bilayer gate dielectrics. This dielectric was shown to lead to improved mobility and lower operating voltages. We evidence the strong influence of the end substituent on FET performances. Unsubstituted and alkyl oligothiophenes with 1 or 2 hexyl end-groups were compared. Our results confirm that adding long alkyl chains lead to a significant
increase of mobility and this increase is continuous upon the number of alkyl substituents. Fused benzenic rings end-groups also improve the mobility compared to unsubstituted 4T but to a lesser extent than alkyl chains. A promising mobility of 10−2 cm2 V-1 s−1 is reach with these benzenic rings end groups. Work is currently in progress to improve this mobility through more adapted deposition conditions and to ascertain the origin of this improvement, namely enhanced stacking, preferential orientation of molecules or a layer-by-layer growth mode.
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