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Solution-phase deposition of oligomeric TFT semiconductors
ELSEVIER
Synthetic
Metals
102 (1999)
897-899
Solution-phase deposition of oligomeric TFT semiconductors H.E. Katz, W. Li, A.J. Lovinger. J. Laquindanum Bell Laboratories-Lucent
Technologies,
600 Mountain
Avenue. Murray
Hill, NJ 07974
Abstract Until now, most organic-based FETs have been fabricated by subliming the active films onto device substrates at high vacuum, whereby usehI orientational order and intergrain connectivity are obtained. For lower-cost devices, a major challenge is to duplicate this performance in semiconductors deposited from solution. We have prepared several thiophene-based compounds. both oligomers and thienoaromatics, that display high mobilities, in some cases equivalent to vacuum-deposited films, when cast Tom common solvents. The similarities between liquid- and vaporphase processed films suggest that oligomer solutions may be valuable for the production of all-organic electronic circuitry.
Organic semiconductors for applications in “a& organic*’ or “all-plastic” electronics fall into three main classes: perfectly rigid, planar molecules such as pentacene and phthalocyanines, semirigid linear oligomers such as thiophene tetramers, hexamers, and octamers, and semiflexible polythiophenes, especially regioregular poly(alkylthiophene)s. We anticipate that a primary advantage of using these compounds as active materials in thin film transistor (TFT)-based circuits will be more facile processing relative to “silicon” electronics technology. The most complete realization of this advantage would be if the semiconductors could be deposited for TFT fabrication directly from solution. Such deposition has been most convincingly demonstrated for regioregular poly(3-hexylthiophene)[ 1,2], where mobilities approaching 0.1 cm’/Vs and coverages of many cm’ have been obtained. As an alternative , it would be desirable to process molecular solids similarly, since some of them are more readily prepared in an undoped form leading to a higher on/off ratio. Furthermore, the very highest mobilities have been observed from crystalline films of oligomer-sized molecules such as pentacene[3], dihexyla-quaterthiophene[4,5], and dihexylanthradithiophene[6]. In this paper, we describe evidence that some members of this class of molecules can indeed be cast as semiconducting films from common solvents, in some cases displaying mobilities that rival those obtained 6om high-vacuum deposition.
dissolution was obtained. and casting this solution onto a heated conductive substrate that had been coated with a dielectric, either 3000 Angstroms of SiOz on doped silicon or a photocured polyimide on ITO-glass. The solvent was removed f?om the heated film under partial vacuum, ca 0.25 atmosphere. Source and drain electrodes were applied by evaporation of gold through a shadow mask. the socalled “top contact>’ geometry. Current-voltage characteristics were obtained in air with a HewlettPackard semiconductor analyzer, and capacitance was measured at a blank region of the dielectric using an Anders-Hagerling bridge at I kHz. Results Our initial data have been obtained From DHa6T TFTs. The mobilities are strongly dependent on film morphology, which is determined in turn by the nature of the solvent and the deposition temperature. The smoothest and best performing films, covering ca IO mm2. were deposited at moderately elevated “C) from chlorobenzene. temperatures (50-60 Solvents trichlorobenzene, and 3-methylthiophene. that are slightly less polarizable. such as anisole and toluene. were not as useful because the active compounds tended to precipitate Tom the solvent pool rather than nucleating on the dielectric surface. Concentrations below 0.1% were employed in order to avoid the deposition of rough, powdery films; even though such films are thicker, electrical contact is not well maintained among the grains and apparent mobilities for such samples are low. Where films are smooth. mobilities as high as 0.1 cm’/Vs were occasionally obtained. and a mobility of 0.03 cm’/Vs was routine. both on oxide and polymer dielectrics. On/off ratios were typically in the thousands (up to 3 MV/cm gate field. including depletion). Ordering in these films. assessed by x-ray difiaction. is similar in magnitude and orientation to that of vapor-deposited films. The on/off ratio and threshold voltage were extremely sensitive to solvent purity and leakage Figures I and 2 show through the dielectric. characteristics for a DHa6T TFT film cast from
Experimental Dihexyl-a-sexithiophene (DHa6T) was synthesized as previously described[7], by the Stille coupling of 5-hexyl-5’-tributylstannyl-2,2’bithiophene with 5.5’-dibromo-2,2’-bithiophene. The pentameric analogue DHaST was synthesized analogously, coupling to 2,5-dibromothiophene. The synthesis of dihexylanthradithiophene (DHADT) has been reported.[6] TFTs were fabricated by heating a mixture of semiconductor and an aromatic solvent until complete 0379.6779/99/$ - see front PII: SO379-6779(98)00380-4
matter 0 1999 Elsevier
Science
S.A.
All rights
reserved.
898
H.E. Katz et al. I Synthetic
chlorobenzene onto a silicon substrate. optimized for sharp turn-on behavior. Current is in amps. and voltage in volts. Further improvements could undoubtedly result from casting the films at lower pressure or under an inert atmosphere, better controlling the dielectric surface, and purifying the semiconductor at ultrahigh vacuum. A second set of experiments was performed on DHaST. a compound we expected to be more soluble and less easily doped. Since this compound had not been characterized previously, we first investigated its TFT mobility as a vacuum-deposited film. Figure 3 shows the temperature-dependent mobility of this compound sublimed onto SiOJSi. A sharp maximum is observed near 1.50 “C. At this temperature. x-ray difhaction and electron microscopy reveal an optimization of molecular ordering and intergrain connectivity, previously shown to be determining factors in achieving high mobility. The magnitudes of the measured mobilities compare favorably with DHa6T, even though the DHa5T is shorter and less symmetric. Two of the three points (indicated as square and diamond) plotted for the 50 ‘C temperature were derived t?om chlorobenzene-cast films, again demonstrating that mobilities can be obtained for these oligomers as solution-deposited films that are comparable to those of their evaporated films. Finally, it is possible to prepare semiconducting films of extremely rigid and high-melting compounds. ADT consists of two thiophene rings fused to the ends of an anthracene molecule. As such. it is a hybrid of pentacene, which displays the highest reported TFT mobility. and the thiophene oligomers, which are more amenable to solution processing, especially under ambient conditions. The dihexyl-terminated derivative, DHADT, despite melting at 395 “C, is sufficiently soluble in chlorobenzene to cast a film. When dried at 100 “C, the film had a TFT mobility of 0.01-0.02 cm*/Vs. On the other hand. a saturated solution of pentacene handled under similar conditions photobleached in minutes.[5]
Metals
102 (1999)
897-899
3.51cs6 -310" -2.5 106 I -2108 ‘D -1.5 la6 -1 IO6 -510' 0 510' 20
0
-20
,;a0
a0 -100-120 D
Figure 1
i;...__..........! o.ooo5 : _.......... : IJ.......:...............: -I TIDj ; o; ; SQR o ,001 L .,.,......; i (................!:a : 0.00, 5 :.
100
i.............”
50
i
0
‘o’...j
1
a
-
-50
-100 -150
Ys
Figure 2
Conclusion Alkylthiophene-terminated compounds can be induced to form thin films displaying transistor action by simple solution-based processes under ordinary laboratory conditions. For oligothiophene derivatives, mobilities are comparable to those that have been previously reported for similar compounds deposited under high vacuum. A major remaining challenge is to extend the coverage area of these films so that larger devices or device arrays can be prepared. Means of accomplishing this. as well as integrating these devices into more complex components. such as “smart pixels” and complementary circuits, are under investigation.
0.12m
3 0.1 r-F-
5 0.08
2 0.06 kg E 0.04 0.0
0
50
100 150 T, deg C
200
250
H.E. Katz
et al. I Synthetic
Metals
102 (1999)
dihexyl-a-6T
dihexyl-a-5T
dihexylanthradithiophene References I. 2. 3. 4. 5. 6. 7.
Z. Bao. A. Dodabalapur. A.S. Lovinger. Appt. Phys. Len. 69 (1996) 4108. H. Sininghaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. Y.Y. Lin, D.J. Gundlach, S.F. Nelson. T.N. Jackson, IEEE Trans. Elect. Dev. 44 (1997) 1325. H.E. Katz A.J. Lovinger. J. Laquindanum, Chem. Mater. 10 (1998) 457. F. Gamier, Chem. Phys. 227 (1998) 253. J. Laquindanum. H.E. Katz. A.J. Lovinger. J. Am. Chem. Sot. 120 (1998) 664. H.E. Katz, J. Laquindanum, A.J. Lovinger, Chem. Mater. 10 (1998) 633.
897-899
Synthetic
Metals
102 (1999)
897-899
Solution-phase deposition of oligomeric TFT semiconductors H.E. Katz, W. Li, A.J. Lovinger. J. Laquindanum Bell Laboratories-Lucent
Technologies,
600 Mountain
Avenue. Murray
Hill, NJ 07974
Abstract Until now, most organic-based FETs have been fabricated by subliming the active films onto device substrates at high vacuum, whereby usehI orientational order and intergrain connectivity are obtained. For lower-cost devices, a major challenge is to duplicate this performance in semiconductors deposited from solution. We have prepared several thiophene-based compounds. both oligomers and thienoaromatics, that display high mobilities, in some cases equivalent to vacuum-deposited films, when cast Tom common solvents. The similarities between liquid- and vaporphase processed films suggest that oligomer solutions may be valuable for the production of all-organic electronic circuitry.
Organic semiconductors for applications in “a& organic*’ or “all-plastic” electronics fall into three main classes: perfectly rigid, planar molecules such as pentacene and phthalocyanines, semirigid linear oligomers such as thiophene tetramers, hexamers, and octamers, and semiflexible polythiophenes, especially regioregular poly(alkylthiophene)s. We anticipate that a primary advantage of using these compounds as active materials in thin film transistor (TFT)-based circuits will be more facile processing relative to “silicon” electronics technology. The most complete realization of this advantage would be if the semiconductors could be deposited for TFT fabrication directly from solution. Such deposition has been most convincingly demonstrated for regioregular poly(3-hexylthiophene)[ 1,2], where mobilities approaching 0.1 cm’/Vs and coverages of many cm’ have been obtained. As an alternative , it would be desirable to process molecular solids similarly, since some of them are more readily prepared in an undoped form leading to a higher on/off ratio. Furthermore, the very highest mobilities have been observed from crystalline films of oligomer-sized molecules such as pentacene[3], dihexyla-quaterthiophene[4,5], and dihexylanthradithiophene[6]. In this paper, we describe evidence that some members of this class of molecules can indeed be cast as semiconducting films from common solvents, in some cases displaying mobilities that rival those obtained 6om high-vacuum deposition.
dissolution was obtained. and casting this solution onto a heated conductive substrate that had been coated with a dielectric, either 3000 Angstroms of SiOz on doped silicon or a photocured polyimide on ITO-glass. The solvent was removed f?om the heated film under partial vacuum, ca 0.25 atmosphere. Source and drain electrodes were applied by evaporation of gold through a shadow mask. the socalled “top contact>’ geometry. Current-voltage characteristics were obtained in air with a HewlettPackard semiconductor analyzer, and capacitance was measured at a blank region of the dielectric using an Anders-Hagerling bridge at I kHz. Results Our initial data have been obtained From DHa6T TFTs. The mobilities are strongly dependent on film morphology, which is determined in turn by the nature of the solvent and the deposition temperature. The smoothest and best performing films, covering ca IO mm2. were deposited at moderately elevated “C) from chlorobenzene. temperatures (50-60 Solvents trichlorobenzene, and 3-methylthiophene. that are slightly less polarizable. such as anisole and toluene. were not as useful because the active compounds tended to precipitate Tom the solvent pool rather than nucleating on the dielectric surface. Concentrations below 0.1% were employed in order to avoid the deposition of rough, powdery films; even though such films are thicker, electrical contact is not well maintained among the grains and apparent mobilities for such samples are low. Where films are smooth. mobilities as high as 0.1 cm’/Vs were occasionally obtained. and a mobility of 0.03 cm’/Vs was routine. both on oxide and polymer dielectrics. On/off ratios were typically in the thousands (up to 3 MV/cm gate field. including depletion). Ordering in these films. assessed by x-ray difiaction. is similar in magnitude and orientation to that of vapor-deposited films. The on/off ratio and threshold voltage were extremely sensitive to solvent purity and leakage Figures I and 2 show through the dielectric. characteristics for a DHa6T TFT film cast from
Experimental Dihexyl-a-sexithiophene (DHa6T) was synthesized as previously described[7], by the Stille coupling of 5-hexyl-5’-tributylstannyl-2,2’bithiophene with 5.5’-dibromo-2,2’-bithiophene. The pentameric analogue DHaST was synthesized analogously, coupling to 2,5-dibromothiophene. The synthesis of dihexylanthradithiophene (DHADT) has been reported.[6] TFTs were fabricated by heating a mixture of semiconductor and an aromatic solvent until complete 0379.6779/99/$ - see front PII: SO379-6779(98)00380-4
matter 0 1999 Elsevier
Science
S.A.
All rights
reserved.
898
H.E. Katz et al. I Synthetic
chlorobenzene onto a silicon substrate. optimized for sharp turn-on behavior. Current is in amps. and voltage in volts. Further improvements could undoubtedly result from casting the films at lower pressure or under an inert atmosphere, better controlling the dielectric surface, and purifying the semiconductor at ultrahigh vacuum. A second set of experiments was performed on DHaST. a compound we expected to be more soluble and less easily doped. Since this compound had not been characterized previously, we first investigated its TFT mobility as a vacuum-deposited film. Figure 3 shows the temperature-dependent mobility of this compound sublimed onto SiOJSi. A sharp maximum is observed near 1.50 “C. At this temperature. x-ray difhaction and electron microscopy reveal an optimization of molecular ordering and intergrain connectivity, previously shown to be determining factors in achieving high mobility. The magnitudes of the measured mobilities compare favorably with DHa6T, even though the DHa5T is shorter and less symmetric. Two of the three points (indicated as square and diamond) plotted for the 50 ‘C temperature were derived t?om chlorobenzene-cast films, again demonstrating that mobilities can be obtained for these oligomers as solution-deposited films that are comparable to those of their evaporated films. Finally, it is possible to prepare semiconducting films of extremely rigid and high-melting compounds. ADT consists of two thiophene rings fused to the ends of an anthracene molecule. As such. it is a hybrid of pentacene, which displays the highest reported TFT mobility. and the thiophene oligomers, which are more amenable to solution processing, especially under ambient conditions. The dihexyl-terminated derivative, DHADT, despite melting at 395 “C, is sufficiently soluble in chlorobenzene to cast a film. When dried at 100 “C, the film had a TFT mobility of 0.01-0.02 cm*/Vs. On the other hand. a saturated solution of pentacene handled under similar conditions photobleached in minutes.[5]
Metals
102 (1999)
897-899
3.51cs6 -310" -2.5 106 I -2108 ‘D -1.5 la6 -1 IO6 -510' 0 510' 20
0
-20
,;a0
a0 -100-120 D
Figure 1
i;...__..........! o.ooo5 : _.......... : IJ.......:...............: -I TIDj ; o; ; SQR o ,001 L .,.,......; i (................!:a : 0.00, 5 :.
100
i.............”
50
i
0
‘o’...j
1
a
-
-50
-100 -150
Ys
Figure 2
Conclusion Alkylthiophene-terminated compounds can be induced to form thin films displaying transistor action by simple solution-based processes under ordinary laboratory conditions. For oligothiophene derivatives, mobilities are comparable to those that have been previously reported for similar compounds deposited under high vacuum. A major remaining challenge is to extend the coverage area of these films so that larger devices or device arrays can be prepared. Means of accomplishing this. as well as integrating these devices into more complex components. such as “smart pixels” and complementary circuits, are under investigation.
0.12m
3 0.1 r-F-
5 0.08
2 0.06 kg E 0.04 0.0
0
50
100 150 T, deg C
200
250
H.E. Katz
et al. I Synthetic
Metals
102 (1999)
dihexyl-a-6T
dihexyl-a-5T
dihexylanthradithiophene References I. 2. 3. 4. 5. 6. 7.
Z. Bao. A. Dodabalapur. A.S. Lovinger. Appt. Phys. Len. 69 (1996) 4108. H. Sininghaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. Y.Y. Lin, D.J. Gundlach, S.F. Nelson. T.N. Jackson, IEEE Trans. Elect. Dev. 44 (1997) 1325. H.E. Katz A.J. Lovinger. J. Laquindanum, Chem. Mater. 10 (1998) 457. F. Gamier, Chem. Phys. 227 (1998) 253. J. Laquindanum. H.E. Katz. A.J. Lovinger. J. Am. Chem. Sot. 120 (1998) 664. H.E. Katz, J. Laquindanum, A.J. Lovinger, Chem. Mater. 10 (1998) 633.
897-899