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polymeric thin films using modified time-of-flight apparatus
ELSEVIER
Synthetic Metala 91 ( 1997) 169-171
Measurement of electron/hole mobility in organic/polymeric using modified time-of-flight apparatus
thin films
Abstract We report, using modified time-of-flight (TOF) apparatus. the measurement of the drift mobility of electrons/holes in thin films of vapordeposited tris( 8-hydruxyquinolinolato)aluminum (Alq,) and spin-cast poly(N-vinylcarbazole)(PVK) based on silicon. Drift mobilities of both carriers are strongly electric field and temperature dependent. At room temperature and an electric field of 2 X 10” V cm-‘. the effective mobilitiesofelectronandholearc IX 10-5and7.11X 10-hcm’V-‘s-‘. respectively, in a 200 nm thick samples corresponding to the two materials. 0 1997 Elsevier Science S.A. iY+w~~r~l.\: Mobility: Time of Bight: Silicon: Thin filrn~
1. Introduction Over the past a few years, organic and polymeric lightdiodes ( LEDs) or electroluminescent devices (ELs) have attracted much attention becauseof their potential application in the full-colour large-areaflat-paneldisplays operating at low drive voltages. Since the first demonstration of organic and polymer-based display devices by Tang and VanSlyke [ 11 and Burroughes et al. [ 21. respectively, the field hasadvancedrapidly toward the developmentof devices emitting in the red. green and blue wavelengths. A typical organic or polymeric light-emitting diode (LED) consistsof a hole-injecting contact. a hole transport layer. a light-emitting layer. an electron transport layer and electron-injecting contact in order. The various layers are typically less than 100nm thick and frequently somefunctions are combinedin a single layer. Electrons and holes recombine in the lightemitting layer. creating an excited state which decays to the ground state by emitting a photon. It has now been shown that devices can be made from a wide variety of organicsor polymers. Tris( S-hydroxyquinolinolato) aluminum ( Alq,) is frequently the material of choice for the electron transport/ light emissionlayer and is one of the materialsusedby Tang emitting
* Correspondin author. Tel.: + 86 013 I 8923 189: fax: + 86 013 I 8961 939; e-mail: ky @mai~.j\u.edu.cn 0379.6779/97/$17.00 0 1997 Elhevier Science S.A. Ail rights reserved PIZSO379-6779(97)01005-8
et al. [ 1,3] in the organic light-emitting diodes(OLEDs) and Kido et al. [4] in the polymeric light-emitting diodes ( PLEDs) Poly( N-vinylcarbazole) (PVK) is the materialof choice for the hole transport/light emissionlayer [5]. One of the important factors determiningthe performanceof these organic/polymeric devices is the behavior of the transport carriers in theseorganic/polymeric layers. But the organic/ polymeric thin films are highly resistive, low mobility solid, and the drift mobilities of the materialsare very difficult to measureusing genera1methods. The only report of a measurementof chargecarrier mobility in Alq, using the time-of-flight (TOF) technique is that made recently by Kepler et al. [ 61. In this paper. we report using modified time-of-flight (TOF) apparatusthe measurement of the drift mobility of electrons/holes in thin films of vapor-deposited tris( %hydroxyquinolinolato) aluminum ( Alq,) and spin-castpoly( N-vinylcarbazole) (PVK) based on silicon. The silicon substratealso acts asa carrier-generating layer. There are two advantagesby using silicon as substrates:(i) since there is strong intrinsic optical absorption in silicon over the visible range, the light sourcecan be choseneasily: (ii) the carriers are generatedin the silicon, so the interface betweenthe carrier-generatinglayer andcarrier-transporting layer is abrupt. The TOF techniqueinvolves generation of carriers with a short pulse of light and observation of the current displacedin the external circuit by the
motion ofthe carrier through the sample. In our study, carriers were generated in the silicon substrate and drifted in the organic/polymeric layer.
2. Experimental
2.4
results and discussion
The molecular structures of Alq, and PVK are shown in Fig. 1. Alq, films 200 nm thick were vapor deposited at a rate of 0.2-0.4 nm s- ’ onto the siIicon substrate; PVK films 200 nm thick were formed by spin-casting on silicon with chloroform solvent. There was a 12 nm thick, semitransparent gold layer on the side of the substrate prior to fabrication of the organic/polymeric film. The second electrode was a 100 nm gold film, vapor deposited on top of the organic/polymeric film. The pressure of the system was 6 X IO-” Torr during deposition. Nd:YAG laser pulses at 532 nm and 10 ns long were used to create carriers in the silicon substrate. The measurements were made at room temperature and at five electric fields: 2X lo”, 4 X lo’, 6X IO”, 8 X 10” and 1 X lo6 V cm- ‘. The current through the sample as a function of time was determined by using a storage oscilloscope to measure the voltage across a resistor placed in series with the sample. Five resistors varying in resistance from 50 Q to 10 kQ and several sweep speeds were used to cover several decades in time and current. All data for times shorter than ten RC time constants, where R is the series resistance and C is the sample capacitance, were discarded as invalid. The experimental results obtained for electrons in the Alq, layer and holes in the PVK layer, the illuminated electrode being negative or positive corresponding to electron or hole mobility measurement, at 2 X 10’ V cm- ’ are shown in Figs. 2 and 3. Figs. 2 and 3 show the TOF photocurrent signals from a structure where photocarriers generated in the silicon substrate are injected into and drift through the organic/polymeric layer under the influence of an external electric field. The process was initiated by a laser flash of 532 nm transmitted through the semitransparent gold electrode and absorbed in the silicon layer immediately next to the interface between silicon and organic/polymeric layer. Since the carrier mobility in silicon is much larger than that in organic/ polymeric materials, the time of carrier transport in silicon can be negligible. The arrival time of the photogenerated carriers at the counterelectrode is very clearly indicated in
0.8 -
0
10
20
Time
30
Ah3
0
[ P.S]
10
20
Time
30
of Alq,
and PVK.
40
50
[PS]
Fig. 3. Storage oscilloscope traces oftheTOFsigna1 ofholesphotogenerated in silicon and drift in PVK layer. The thickness of PVK was 200 nm and the electric fieldused was 2X 1o’V cm-‘.
Figs. 2 and3; the drift mobilitiesestimatedat 2 X lO:‘Vcm- ’ are close to 1X IO-” and 7.14X 10m6cm’ V-’ s-* correspondingto electronsin Alq, and holesin PVK which should obey the following Eq. ( 1) :
(1) where T, is the transit time of electronsin the organic/polymeric layer, D is the thicknessof theorganic/polymericlayer, E is the applied electric field, and p is the drift mobility of the carriers in organic/polymeric layer. It is frequently found that the drift mobility ,LLvaries with temperatureand electric field asGill reported {7] :
PVK structures
50
Fig. 2. Storage oscilloscope traces of the TOF signal of electrons photogenerated in silicon and drift in Alq, . layer. The thickness of Alq, was 300 nm and the electric field used was 2 X 16' V cm-l.
b4EJ)=h w( -2) ev(E) Fig. 1. The molecular
40
and
(2)
layer at room temperature; this may be due to adopting a silicon substrate, so that the signal is very strong, and the trap state for carriers in an organic thinner film is limited. Fig. 4 shows a plot of log ,u versus El” for carriers using our five data points. These results show that, to a good approximation, the field dependence of the drift mobility can be described by the relationship of Eq. (2). l.OE-5
t.
A
I
3. Conclusions
400
Fig. 1, The drift mobilities root of the electric field.
1 -=--T,,,
1 T
1 i””
600
of electrons
800
1000
1200
in Alq, and holes in PVK vs. square
( 3)
where E(, is the activation energy at zero electric field, Tis the temperature and k is Boltzmann’s constant. Such behavior closely approximated by this empirical equation has now been observed in many materials. In some materials the transport is so highly dispersive that evidence for a transit time can only be observed in a log-log plot of current versus time and, in that case, the value ofthe mobility obtained also varies with sample thickness. Some believe that, in the highly dispersive transport mode. the carrier thermal equilibration time within the manifold of transport states is considerably longer than the transit time and that the average velocity of the carriers decreases with time throughout the transit time. Such behavior has been observed in amorphous inorganic semiconductors and in the early work on disordered organic solids. In general, a highly dispersive behavior is not now observed in the disordered organic samples except at temperatures well below room temperature. Our results indicate that there is no dispersive behavior represented by the carrier mobility in the organic/polymeric
We have measured the mobility of electrons in vacuumdeposited films ofAlq, and holes in spin-cast films ofpoly(Nvinylcarbazole) (PVK) based on silicon using modified time-of-flight (TOF) apparatus. The drift mobilities of electrons and holes are strongly electric field and temperature dependent.
Acknowledgements This work was supported by National ‘863’ Project of China and the National Natural Science Foundation of China. Discussions with Dr X.J. Tian and Professor M.B. Yi are gratefully acknowledged.
References [ I I C.W. Tang and S.A. VanSlyke. Appl. Phys. Lett., 5 1 ( 1987) 913. [?I J.H. Burroughes,D.D.C. Bradley. A.R. Brown, R.N.Marks, K. Mackay, R.H. Friend, P.L. Burn and A.B. Holmes, Nature, 317 ( 1990) 539. [31 C.W. Tang, S.A. VanSlyke and C.H. Chrn, J. Appl. Phys., 65 (1989) 3610. [-I] J. Kido. K. Hougawa, K. Okuyama and K. Nagai, Appl. Phys. Lett.. 63 i 1993) 2627. [5] B.J. Chen, S.H. Xue, J.Y. Hou, H.Y. An, J.S. Huang, Y. Yang, S.Y. Liu, Y.G. Ma and J.C. Shen, Chin. Sci. Bull., 41 (1996) 1793. 161 R.G. Kepler, P.M. Beeson, S.J. Jacobh, R.A. Anderson, M.B. Sinclair, V.S. Valencia and P.A. Cahill, Appl, Phys. Lett., 66 ( 1995) 3618. 171 W.D. Gill. J. Appl. Phys.. 43 (1972) 5033.
Synthetic Metala 91 ( 1997) 169-171
Measurement of electron/hole mobility in organic/polymeric using modified time-of-flight apparatus
thin films
Abstract We report, using modified time-of-flight (TOF) apparatus. the measurement of the drift mobility of electrons/holes in thin films of vapordeposited tris( 8-hydruxyquinolinolato)aluminum (Alq,) and spin-cast poly(N-vinylcarbazole)(PVK) based on silicon. Drift mobilities of both carriers are strongly electric field and temperature dependent. At room temperature and an electric field of 2 X 10” V cm-‘. the effective mobilitiesofelectronandholearc IX 10-5and7.11X 10-hcm’V-‘s-‘. respectively, in a 200 nm thick samples corresponding to the two materials. 0 1997 Elsevier Science S.A. iY+w~~r~l.\: Mobility: Time of Bight: Silicon: Thin filrn~
1. Introduction Over the past a few years, organic and polymeric lightdiodes ( LEDs) or electroluminescent devices (ELs) have attracted much attention becauseof their potential application in the full-colour large-areaflat-paneldisplays operating at low drive voltages. Since the first demonstration of organic and polymer-based display devices by Tang and VanSlyke [ 11 and Burroughes et al. [ 21. respectively, the field hasadvancedrapidly toward the developmentof devices emitting in the red. green and blue wavelengths. A typical organic or polymeric light-emitting diode (LED) consistsof a hole-injecting contact. a hole transport layer. a light-emitting layer. an electron transport layer and electron-injecting contact in order. The various layers are typically less than 100nm thick and frequently somefunctions are combinedin a single layer. Electrons and holes recombine in the lightemitting layer. creating an excited state which decays to the ground state by emitting a photon. It has now been shown that devices can be made from a wide variety of organicsor polymers. Tris( S-hydroxyquinolinolato) aluminum ( Alq,) is frequently the material of choice for the electron transport/ light emissionlayer and is one of the materialsusedby Tang emitting
* Correspondin author. Tel.: + 86 013 I 8923 189: fax: + 86 013 I 8961 939; e-mail: ky @mai~.j\u.edu.cn 0379.6779/97/$17.00 0 1997 Elhevier Science S.A. Ail rights reserved PIZSO379-6779(97)01005-8
et al. [ 1,3] in the organic light-emitting diodes(OLEDs) and Kido et al. [4] in the polymeric light-emitting diodes ( PLEDs) Poly( N-vinylcarbazole) (PVK) is the materialof choice for the hole transport/light emissionlayer [5]. One of the important factors determiningthe performanceof these organic/polymeric devices is the behavior of the transport carriers in theseorganic/polymeric layers. But the organic/ polymeric thin films are highly resistive, low mobility solid, and the drift mobilities of the materialsare very difficult to measureusing genera1methods. The only report of a measurementof chargecarrier mobility in Alq, using the time-of-flight (TOF) technique is that made recently by Kepler et al. [ 61. In this paper. we report using modified time-of-flight (TOF) apparatusthe measurement of the drift mobility of electrons/holes in thin films of vapor-deposited tris( %hydroxyquinolinolato) aluminum ( Alq,) and spin-castpoly( N-vinylcarbazole) (PVK) based on silicon. The silicon substratealso acts asa carrier-generating layer. There are two advantagesby using silicon as substrates:(i) since there is strong intrinsic optical absorption in silicon over the visible range, the light sourcecan be choseneasily: (ii) the carriers are generatedin the silicon, so the interface betweenthe carrier-generatinglayer andcarrier-transporting layer is abrupt. The TOF techniqueinvolves generation of carriers with a short pulse of light and observation of the current displacedin the external circuit by the
motion ofthe carrier through the sample. In our study, carriers were generated in the silicon substrate and drifted in the organic/polymeric layer.
2. Experimental
2.4
results and discussion
The molecular structures of Alq, and PVK are shown in Fig. 1. Alq, films 200 nm thick were vapor deposited at a rate of 0.2-0.4 nm s- ’ onto the siIicon substrate; PVK films 200 nm thick were formed by spin-casting on silicon with chloroform solvent. There was a 12 nm thick, semitransparent gold layer on the side of the substrate prior to fabrication of the organic/polymeric film. The second electrode was a 100 nm gold film, vapor deposited on top of the organic/polymeric film. The pressure of the system was 6 X IO-” Torr during deposition. Nd:YAG laser pulses at 532 nm and 10 ns long were used to create carriers in the silicon substrate. The measurements were made at room temperature and at five electric fields: 2X lo”, 4 X lo’, 6X IO”, 8 X 10” and 1 X lo6 V cm- ‘. The current through the sample as a function of time was determined by using a storage oscilloscope to measure the voltage across a resistor placed in series with the sample. Five resistors varying in resistance from 50 Q to 10 kQ and several sweep speeds were used to cover several decades in time and current. All data for times shorter than ten RC time constants, where R is the series resistance and C is the sample capacitance, were discarded as invalid. The experimental results obtained for electrons in the Alq, layer and holes in the PVK layer, the illuminated electrode being negative or positive corresponding to electron or hole mobility measurement, at 2 X 10’ V cm- ’ are shown in Figs. 2 and 3. Figs. 2 and 3 show the TOF photocurrent signals from a structure where photocarriers generated in the silicon substrate are injected into and drift through the organic/polymeric layer under the influence of an external electric field. The process was initiated by a laser flash of 532 nm transmitted through the semitransparent gold electrode and absorbed in the silicon layer immediately next to the interface between silicon and organic/polymeric layer. Since the carrier mobility in silicon is much larger than that in organic/ polymeric materials, the time of carrier transport in silicon can be negligible. The arrival time of the photogenerated carriers at the counterelectrode is very clearly indicated in
0.8 -
0
10
20
Time
30
Ah3
0
[ P.S]
10
20
Time
30
of Alq,
and PVK.
40
50
[PS]
Fig. 3. Storage oscilloscope traces oftheTOFsigna1 ofholesphotogenerated in silicon and drift in PVK layer. The thickness of PVK was 200 nm and the electric fieldused was 2X 1o’V cm-‘.
Figs. 2 and3; the drift mobilitiesestimatedat 2 X lO:‘Vcm- ’ are close to 1X IO-” and 7.14X 10m6cm’ V-’ s-* correspondingto electronsin Alq, and holesin PVK which should obey the following Eq. ( 1) :
(1) where T, is the transit time of electronsin the organic/polymeric layer, D is the thicknessof theorganic/polymericlayer, E is the applied electric field, and p is the drift mobility of the carriers in organic/polymeric layer. It is frequently found that the drift mobility ,LLvaries with temperatureand electric field asGill reported {7] :
PVK structures
50
Fig. 2. Storage oscilloscope traces of the TOF signal of electrons photogenerated in silicon and drift in Alq, . layer. The thickness of Alq, was 300 nm and the electric field used was 2 X 16' V cm-l.
b4EJ)=h w( -2) ev(E) Fig. 1. The molecular
40
and
(2)
layer at room temperature; this may be due to adopting a silicon substrate, so that the signal is very strong, and the trap state for carriers in an organic thinner film is limited. Fig. 4 shows a plot of log ,u versus El” for carriers using our five data points. These results show that, to a good approximation, the field dependence of the drift mobility can be described by the relationship of Eq. (2). l.OE-5
t.
A
I
3. Conclusions
400
Fig. 1, The drift mobilities root of the electric field.
1 -=--T,,,
1 T
1 i””
600
of electrons
800
1000
1200
in Alq, and holes in PVK vs. square
( 3)
where E(, is the activation energy at zero electric field, Tis the temperature and k is Boltzmann’s constant. Such behavior closely approximated by this empirical equation has now been observed in many materials. In some materials the transport is so highly dispersive that evidence for a transit time can only be observed in a log-log plot of current versus time and, in that case, the value ofthe mobility obtained also varies with sample thickness. Some believe that, in the highly dispersive transport mode. the carrier thermal equilibration time within the manifold of transport states is considerably longer than the transit time and that the average velocity of the carriers decreases with time throughout the transit time. Such behavior has been observed in amorphous inorganic semiconductors and in the early work on disordered organic solids. In general, a highly dispersive behavior is not now observed in the disordered organic samples except at temperatures well below room temperature. Our results indicate that there is no dispersive behavior represented by the carrier mobility in the organic/polymeric
We have measured the mobility of electrons in vacuumdeposited films ofAlq, and holes in spin-cast films ofpoly(Nvinylcarbazole) (PVK) based on silicon using modified time-of-flight (TOF) apparatus. The drift mobilities of electrons and holes are strongly electric field and temperature dependent.
Acknowledgements This work was supported by National ‘863’ Project of China and the National Natural Science Foundation of China. Discussions with Dr X.J. Tian and Professor M.B. Yi are gratefully acknowledged.
References [ I I C.W. Tang and S.A. VanSlyke. Appl. Phys. Lett., 5 1 ( 1987) 913. [?I J.H. Burroughes,D.D.C. Bradley. A.R. Brown, R.N.Marks, K. Mackay, R.H. Friend, P.L. Burn and A.B. Holmes, Nature, 317 ( 1990) 539. [31 C.W. Tang, S.A. VanSlyke and C.H. Chrn, J. Appl. Phys., 65 (1989) 3610. [-I] J. Kido. K. Hougawa, K. Okuyama and K. Nagai, Appl. Phys. Lett.. 63 i 1993) 2627. [5] B.J. Chen, S.H. Xue, J.Y. Hou, H.Y. An, J.S. Huang, Y. Yang, S.Y. Liu, Y.G. Ma and J.C. Shen, Chin. Sci. Bull., 41 (1996) 1793. 161 R.G. Kepler, P.M. Beeson, S.J. Jacobh, R.A. Anderson, M.B. Sinclair, V.S. Valencia and P.A. Cahill, Appl, Phys. Lett., 66 ( 1995) 3618. 171 W.D. Gill. J. Appl. Phys.. 43 (1972) 5033.