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Determination of the trap distribution in polycrystalline coronene layers
Mat. R e s . B u l l . , Vol. 19, p p . 531-534, 1984. P r i n t e d in t h e USA. 0025-5408/84 $3.00 + .00 C o p y r i g h t (c) 1984 Perg~amon P r e s s L t d .
DETER~INATION OF TI~ TRAP DISTRIBUTION IN POLYCRYSTALLINE CORONENE LAYERS
W. Mycielski and B. Zi6~kowska-Pawlak Technical University of ~6d~, Institute of Physics, ul. W61cza~ska 219, 93-005 &6d~, Poland ( R e c e i v e d J a n u a r y 4, 1984; Communicated by A. R a b e n a u )
ABSTRACT The d.c. electrical conductivity of polycrystalline coronene layers was investigated. The shape of the current-voltage curves indicates that, in the layers investigated, the space-charge-limlted currents are observed. From these characteristics three discrete trap levels in the coronene layers were found: 0.67, 0.72 and 0.81 eV. The total density of localized states is of the order of 1046 cm-'.
Introduction The field of organic molecular solids has attracted considerable interest in recent years. It is connected with the prospect of their practical application, for instance as photoconductors or substitutes for semiconductors in electronic devices. On the other hand relatively simple organic materials may be treated as model substances for very complicated organic systems, e.g. polymers and biologically important compounds. The most extensively studied of the low molecular weight organic materials are the llnear-condensed aromatic compounds such as polyacenes /anthracene, tetracene/ and polyphenyls /p-terphenyl, p-quaterphenyl/. Relatively few investigations relate to the ring-condensed compounds, for example coronene, C~Ha, whose molecule is shown in Fig. I. In the present paper we report experimental results relating to d.c. conductivity o~ ~olycTystalline coronene in the spacecharge-limited currents (SCLC) regime. The SCLC technique is a well-known method of the trap distribution investigation in high resistivity materials (I). The main aim o~ these investigations has been to determine depths and densities of trapping centres, but attempts have been made to derive other information as well, for example the shape of the energy distribution of these traps. Many models have been 531
532
W. MYCIELSKI, et al.
Vol. 19, No. 4
worked out by assuming a discrete (2), uniform (3), exponential 4), double-exponential (5) or Gaussian (6) distribution 0f the rapping levels. A very useful method of analysis of SCLC characteristics, which leads directl~ to the trap distribution, was proposed by ManXredottl et al. ( 7 1. In this original method the parameters or traps energy and density of localized states may be obtained without introducing an a-priori hypothesis regarding the nature of the trap distribution. This analysis is clearly valid only in the regime of strong trapping, i.e. when n , ~ n ~ (n~ is the density of trapped carriers, n~ is the freecarrier density). In this case, the maximum in the plot of dnJdEv versus E, ( where E, is the energy of the quasi-Permi level) may be identified to a good approximation as a discrete or a diffuse trapping level. This m e t h o d w a s examined with prosperity in our group both 1or organic {8,9~ and inorganic materials (10) and in this paper we present its application to the analysis of SCLC curves ~or polycrystalline coronene in the form or thin evaporated layers.
~
Experimental Thin films of coronene were prepared by thermal vacuum evaporation at a pressure of the order of 1~ 5 Torr. For investigation of electrical properties, these ~ilms were sand-8 o wiched between gold bottom 0 and alu~inium top electroo des. The thicknesses of the 0 o samples ( 0.5 - 2.0 ~m ) were ---_9 determined using an inter0 o o rerometer microscope or by o capacitance measurements 0 o using an effective dieleca o bo° Co tric permittlvity z~o of o ~ o -10 o g 2.6 x 10- ~ Fm -~. The area of 0 0 o the samples was about 9 m m ~ . 0 o 0 0 o Isothermal current0 0 voltage characteristics o 0 0 -11 were studied using a conventional circuit with the 0 0 0 current source (d.c. supply o 0 0 o 0 power O - 200 V and connec0 o ° oo ted in series resistance o 0 -12 o 0 1 0 7 - 1046E'~) and Vakutronic 0 0 VAJ-51 electrometer.Because o 0 0 0 0 of strong polarization o 0 effects the steady-state 0o° 0 value of the current was -13 registrated at the time 1 2 10 - 15 minutes a/'ter every /ogU(V) change o5 the applied voltage. FIG. 1 The current-voltage dependences The same samples were also used to the carrier 1or thin coronene films of diStransport investigations, Zerent thickness: a- 0.5 @m, particulary to the electron b - 1.1 ~m, c - 1.7 ~m. The insert shows a structure mobility measurements. The o5 the coronene molecule. experimental procedure and
@o :!
/
/
°
Vol. 19, No. 4
POLYCRYSTALLINE CORONENE LAYERS
533
results, interpreted in the terms o% so-called "dispersive transport", are described elsewhere (11). Results and Discussion Figure I shows typical current-voltage characteristics ~or coronene layers on a log-log scale. The shape of these curves indicates that, in the layers iDvestigated, space-charge-limited currents are observed. Plots of n versus E ~curve a) and dn~/dE~ versus E (curve b} are shown in Pig. 2. The density of trapped carriers n and free-carrler density n were extracted numerically from the I-U characteristics using Man%redotti's model (7,9) and experimental values o~ the current density and the applied voltage. Also, the data on the electron mobility in polycrystalline coronene (~ ~ i~* cm:V -~ s-l), reported by us elsewhere (111, were used for the calculations. The energy o% the quasi-Fermi level was calculated ~rom the usual expression Nb E F = kT i n ~ , ~8
1.6 t
where N~= 3 x 10~ cm -~ is the band density of states (12) and kT = 0.025 eV (all measurements were made at room temperature). F~om Fig. 2, using an approximation EFt,, ~ E,, the energy o~ the traps 0.69, 0.74 and 0.83 eV one can obtain. In accordance with the theoretical predictions (7), the precise relation between E~ and EFt.. is given by formula
!
~6
__b "b
2
0
0.4
f
!
0Y
0.8
'~1
EFmax = E t + kT in g,
0 ).9
where g is the degeneracy lactor (g = 2). Taking into account FIGo 2 this relation, the mean Plots of n~ (curve a) and dnt/dE~ depths of the trapping (curve b) versus energy E~ of the levels for several quasi-Ferml level. This results samples are: 0.67, were extracted from Fig. I, curve b. 0.72 and 0.81 eV with an accuracy ~ 0.04 eV. The relatlvelysmall values of the full width at half ~aximum ( F W H M < 3.5 kT) indicate the existence of the discrete levels in the material investigated. Total densities of states N~ , calculated from formula
Er(eV)-.-
Nt = 4 k T [ ~
]
,
are 1.1 x 104~ , 8.5 x 10 ~ and 2.0 x 1046 cm -3, for each trap level, respectively. Notice that these results were determined from the first measurement for each sample. The next measurements
534
W. MYCIELSKI, et al.
Vol. 19, No. 4
give the same values of the energy depths of the traps but smaller densities of states (probably because of partial ~illlng of the trapping centres). Our results may be comparised with recent data relating to the trap distribution in coronene layers which were obtained using the thermally-stimulated currents (TSC) technique ( 13 ). From the TSC spectra analysis also three trapping levels were found: 0.69, 0.78 and 0.81 eV. One can see that these results are, in a limit of experimental error, identical to that ones obtained by us from the SCLC curves analysis. Acknowledgements This work was carried out under Research Project MR.I-5. The ~uthors are indebted to Mrs. E. Staryga for help in preparation of the samples and very usefull discussions. The authors are also indebted to Dr. A. Lipi~ski for help and encouragement during this work.
I. 2. 3. 4. 5. 6. 7.
8.
References M.A. Lampert and P. Mark, Current Injection in Solids, Academic Press, New York - London 1970 R.S. M~ller, Solid State Electronics, 6, 25(1963) J.L. Hartke, Phys. Rev., 125, 1177(1962) P. Mark and W. Helfrich, J. Appl. Phys., 33, 205(1962) J. Sworakowski and K. Pigo~, J. Phys. Chem. Solids, 30, 491 ( 1969 ) E.A. Silinsh, Phys. Status Solidi A, 3, 817 (1970) C. Manfredotti, C. De Blasi, S. Galassini, G. Micocci, . Ruggiero and A. Tepore, Phys. Status Solidi A, 36, 569 1976) W. Mycielski and A. Lipi~ski, Phys. Status Solidi A, 49, K41
~
(1978)
9. H. Kasica, A. Lipi~ski and J. ~wi~tek, Mat. Res. Bull., 16, 461 (1981) 10. W. Mycielski, A. Lath, A. Lipi~ski, M. Mitkova and Z. Boncheva-Mladenova, Thin Solid Films, 84, L177 (1981) 11. W. Mycielskl, B. Zi61kowska-Pawlak and A. Lipixlski, Thin Solid Films (in the press) 12. H. B~ssler, G. Herrmann, N. Riehl and G. Vaubel, J. Phys. Chem. Solids, 30, 1579 (1969) 13. E. Staryga, B. Zi6lkowska-Pawlak, W. MyclelsKi and A. Lipidski, Proc. Conf. "Molecular Crystals 85", Krak6w (Poland;, Sept. 5 - 8, 1985, ed. Institute of Nuclear Physics, Krak6w, Report No 1225/PS, p. 114
DETER~INATION OF TI~ TRAP DISTRIBUTION IN POLYCRYSTALLINE CORONENE LAYERS
W. Mycielski and B. Zi6~kowska-Pawlak Technical University of ~6d~, Institute of Physics, ul. W61cza~ska 219, 93-005 &6d~, Poland ( R e c e i v e d J a n u a r y 4, 1984; Communicated by A. R a b e n a u )
ABSTRACT The d.c. electrical conductivity of polycrystalline coronene layers was investigated. The shape of the current-voltage curves indicates that, in the layers investigated, the space-charge-limlted currents are observed. From these characteristics three discrete trap levels in the coronene layers were found: 0.67, 0.72 and 0.81 eV. The total density of localized states is of the order of 1046 cm-'.
Introduction The field of organic molecular solids has attracted considerable interest in recent years. It is connected with the prospect of their practical application, for instance as photoconductors or substitutes for semiconductors in electronic devices. On the other hand relatively simple organic materials may be treated as model substances for very complicated organic systems, e.g. polymers and biologically important compounds. The most extensively studied of the low molecular weight organic materials are the llnear-condensed aromatic compounds such as polyacenes /anthracene, tetracene/ and polyphenyls /p-terphenyl, p-quaterphenyl/. Relatively few investigations relate to the ring-condensed compounds, for example coronene, C~Ha, whose molecule is shown in Fig. I. In the present paper we report experimental results relating to d.c. conductivity o~ ~olycTystalline coronene in the spacecharge-limited currents (SCLC) regime. The SCLC technique is a well-known method of the trap distribution investigation in high resistivity materials (I). The main aim o~ these investigations has been to determine depths and densities of trapping centres, but attempts have been made to derive other information as well, for example the shape of the energy distribution of these traps. Many models have been 531
532
W. MYCIELSKI, et al.
Vol. 19, No. 4
worked out by assuming a discrete (2), uniform (3), exponential 4), double-exponential (5) or Gaussian (6) distribution 0f the rapping levels. A very useful method of analysis of SCLC characteristics, which leads directl~ to the trap distribution, was proposed by ManXredottl et al. ( 7 1. In this original method the parameters or traps energy and density of localized states may be obtained without introducing an a-priori hypothesis regarding the nature of the trap distribution. This analysis is clearly valid only in the regime of strong trapping, i.e. when n , ~ n ~ (n~ is the density of trapped carriers, n~ is the freecarrier density). In this case, the maximum in the plot of dnJdEv versus E, ( where E, is the energy of the quasi-Permi level) may be identified to a good approximation as a discrete or a diffuse trapping level. This m e t h o d w a s examined with prosperity in our group both 1or organic {8,9~ and inorganic materials (10) and in this paper we present its application to the analysis of SCLC curves ~or polycrystalline coronene in the form or thin evaporated layers.
~
Experimental Thin films of coronene were prepared by thermal vacuum evaporation at a pressure of the order of 1~ 5 Torr. For investigation of electrical properties, these ~ilms were sand-8 o wiched between gold bottom 0 and alu~inium top electroo des. The thicknesses of the 0 o samples ( 0.5 - 2.0 ~m ) were ---_9 determined using an inter0 o o rerometer microscope or by o capacitance measurements 0 o using an effective dieleca o bo° Co tric permittlvity z~o of o ~ o -10 o g 2.6 x 10- ~ Fm -~. The area of 0 0 o the samples was about 9 m m ~ . 0 o 0 0 o Isothermal current0 0 voltage characteristics o 0 0 -11 were studied using a conventional circuit with the 0 0 0 current source (d.c. supply o 0 0 o 0 power O - 200 V and connec0 o ° oo ted in series resistance o 0 -12 o 0 1 0 7 - 1046E'~) and Vakutronic 0 0 VAJ-51 electrometer.Because o 0 0 0 0 of strong polarization o 0 effects the steady-state 0o° 0 value of the current was -13 registrated at the time 1 2 10 - 15 minutes a/'ter every /ogU(V) change o5 the applied voltage. FIG. 1 The current-voltage dependences The same samples were also used to the carrier 1or thin coronene films of diStransport investigations, Zerent thickness: a- 0.5 @m, particulary to the electron b - 1.1 ~m, c - 1.7 ~m. The insert shows a structure mobility measurements. The o5 the coronene molecule. experimental procedure and
@o :!
/
/
°
Vol. 19, No. 4
POLYCRYSTALLINE CORONENE LAYERS
533
results, interpreted in the terms o% so-called "dispersive transport", are described elsewhere (11). Results and Discussion Figure I shows typical current-voltage characteristics ~or coronene layers on a log-log scale. The shape of these curves indicates that, in the layers iDvestigated, space-charge-limited currents are observed. Plots of n versus E ~curve a) and dn~/dE~ versus E (curve b} are shown in Pig. 2. The density of trapped carriers n and free-carrler density n were extracted numerically from the I-U characteristics using Man%redotti's model (7,9) and experimental values o~ the current density and the applied voltage. Also, the data on the electron mobility in polycrystalline coronene (~ ~ i~* cm:V -~ s-l), reported by us elsewhere (111, were used for the calculations. The energy o% the quasi-Fermi level was calculated ~rom the usual expression Nb E F = kT i n ~ , ~8
1.6 t
where N~= 3 x 10~ cm -~ is the band density of states (12) and kT = 0.025 eV (all measurements were made at room temperature). F~om Fig. 2, using an approximation EFt,, ~ E,, the energy o~ the traps 0.69, 0.74 and 0.83 eV one can obtain. In accordance with the theoretical predictions (7), the precise relation between E~ and EFt.. is given by formula
!
~6
__b "b
2
0
0.4
f
!
0Y
0.8
'~1
EFmax = E t + kT in g,
0 ).9
where g is the degeneracy lactor (g = 2). Taking into account FIGo 2 this relation, the mean Plots of n~ (curve a) and dnt/dE~ depths of the trapping (curve b) versus energy E~ of the levels for several quasi-Ferml level. This results samples are: 0.67, were extracted from Fig. I, curve b. 0.72 and 0.81 eV with an accuracy ~ 0.04 eV. The relatlvelysmall values of the full width at half ~aximum ( F W H M < 3.5 kT) indicate the existence of the discrete levels in the material investigated. Total densities of states N~ , calculated from formula
Er(eV)-.-
Nt = 4 k T [ ~
]
,
are 1.1 x 104~ , 8.5 x 10 ~ and 2.0 x 1046 cm -3, for each trap level, respectively. Notice that these results were determined from the first measurement for each sample. The next measurements
534
W. MYCIELSKI, et al.
Vol. 19, No. 4
give the same values of the energy depths of the traps but smaller densities of states (probably because of partial ~illlng of the trapping centres). Our results may be comparised with recent data relating to the trap distribution in coronene layers which were obtained using the thermally-stimulated currents (TSC) technique ( 13 ). From the TSC spectra analysis also three trapping levels were found: 0.69, 0.78 and 0.81 eV. One can see that these results are, in a limit of experimental error, identical to that ones obtained by us from the SCLC curves analysis. Acknowledgements This work was carried out under Research Project MR.I-5. The ~uthors are indebted to Mrs. E. Staryga for help in preparation of the samples and very usefull discussions. The authors are also indebted to Dr. A. Lipi~ski for help and encouragement during this work.
I. 2. 3. 4. 5. 6. 7.
8.
References M.A. Lampert and P. Mark, Current Injection in Solids, Academic Press, New York - London 1970 R.S. M~ller, Solid State Electronics, 6, 25(1963) J.L. Hartke, Phys. Rev., 125, 1177(1962) P. Mark and W. Helfrich, J. Appl. Phys., 33, 205(1962) J. Sworakowski and K. Pigo~, J. Phys. Chem. Solids, 30, 491 ( 1969 ) E.A. Silinsh, Phys. Status Solidi A, 3, 817 (1970) C. Manfredotti, C. De Blasi, S. Galassini, G. Micocci, . Ruggiero and A. Tepore, Phys. Status Solidi A, 36, 569 1976) W. Mycielski and A. Lipi~ski, Phys. Status Solidi A, 49, K41
~
(1978)
9. H. Kasica, A. Lipi~ski and J. ~wi~tek, Mat. Res. Bull., 16, 461 (1981) 10. W. Mycielski, A. Lath, A. Lipi~ski, M. Mitkova and Z. Boncheva-Mladenova, Thin Solid Films, 84, L177 (1981) 11. W. Mycielskl, B. Zi61kowska-Pawlak and A. Lipixlski, Thin Solid Films (in the press) 12. H. B~ssler, G. Herrmann, N. Riehl and G. Vaubel, J. Phys. Chem. Solids, 30, 1579 (1969) 13. E. Staryga, B. Zi6lkowska-Pawlak, W. MyclelsKi and A. Lipidski, Proc. Conf. "Molecular Crystals 85", Krak6w (Poland;, Sept. 5 - 8, 1985, ed. Institute of Nuclear Physics, Krak6w, Report No 1225/PS, p. 114