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Xerographic dark discharge of PVK:TNF photoconductive material
ChemicalPhysics North-Holland
153 (1991) 305-312
Xerographic dark discharge of PVRTNF
photoconductive
material
Jean-Yves Moisan, Bernadette Andre and Roger Lever DPpartement OCM-TAC, Centre National d’Etudes des TMcommunications, B.P. 40, 22301 Lannion, France Received 22 October 1990
Xerographic dark discharge experiments on corona charged layers of poly N-vinylcarbazole (with different contents of trinitrofluorenone) have been carried out. The surface voltage decreasing against the time can be explained by the help of two mechanisms: the first, observed at the beginning of the dark discharge with sufficiently long corona charging times, is the detrapping effect of carriers injected by the corona current flowing through the sample during the charging sequence; the second is a thermal generation of carriers mechanism, previously proposed for capacitor structure-like materials. This thermal generation appearing electric-field-assisted according to a Poole-Frenkel effect.
1. Introduction There has been a considerable interest in the dynamics of charge photogeneration and transport in organic amorphous materials: poly N-vinylcarbazole and copolymers [ l-41, molecularly doped copolymers [ 1,2,5 ] and more recently polysilanes [ 6 1. Because of their high dark resistivity, these materials are of interest for electrostatic image storage and some have been used in xerographic reproduction and holographic optical switching systems [ 7 1. The high dark resistivity means low dark discharge kinetics, even if high electric fields, up to 1 MV/cm, are applied to the organic layers. Xerographic photodischarge is used to evaluate the photogeneration yield of the organic material, as it was previously used to study some amorphous chalcogenides (Se, As&,). So, with the same experimental set-up, two different properties of the same sample can be evaluated: the photogeneration yield [ 81 or quantum efficiency (which is the ratio of the number of charges swept from the sample surface and the number of absorbed photons in the bulk) and the dark discharge kinetics. It seems that this dark depletion discharge has mainly been studied for amorphous chalcogenide [9-l 11. Recently, some papers [ 12,131 discussed the dark discharge kinetics of some organic material and showed a reversible light-induced change, by xero0301-0104/91/$03.50
graphic measurements, on molecularly doped polymers. To our knowledge, no papers have been published concerning poly N-vinylcarbazole, with or without some acceptor molecule content, with attempt to explain the dark depletion behaviour. So we will present our results on dark discharge and attempt to explain the dark depletion kinetics by considering theoretical aspects and taking into account charge thermogeneration and surface injection.
2. Experimental PVK (from Janssen Chimica) was purified three times by dissolution in chloroform and reprecipitation in ethanol. TNF (2-4-7-trinitrofluorenone, from Janssen Chimica) is sublimated before use. These products (PVK and the indicated weight of TNF - in percentage of PVK) are dissolved in a chlorobenzene-toluene (50: 50) mixture and the layers are made by spin-coating on a 5 x 5 cm* IT0 glass substrate. Layers thicknesses (in the 2-5 urn range) are measured by spectrophotometric measurements in the IR range (with a Bruker 113V FTIR spectrometer). The sample (at 50 oC, unless otherwise specified ) , in a darkened experimental box, is first corona charged. A golden stainless-steel needle is connected
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
J.-Y. Molsan et al. / Xerographx dark discharge of PVK
306
to a high-voltage amplifier TREK Model 609A; the charging time can be monitored from 1 ms up to more than 1 s. In these experiments, a positive 6 kV voltage is used. At the end of the charging time, the sample is quickly moved (in about 0.1 s) under the probe of an electrometer (Monroe Model 244)) connected to a paper recorder. The surface voltage is recorded up to 4 hours. No changes were observed in the surface voltage decreasing kinetics, between a dark-rested sample ( 15 hours in the dark) and the same immediately tested sample (30 min. in the dark).
3. Theory When a charge is applied to the surface of a photoconductive layer, this charge immediately begins to decay. The rate at which the decay proceeds depends on the magnitude of the dark current through the layer (in a non-ohmic Z-V characteristic range), and this in turn depends on the carriers in the bulk. For these free carriers, three sources have to be considered: - thermally generated carriers in the film; - injection of carriers at the free surface of the layer: - injection of carriers at the interface of the layer with the IT0 conductive (and earth-connected) substrate. Generally the free surface of the layer behaves as a blocking layer and injection of carriers through this surface has not been taken into account so far. Schaffert [ 91 considered (for chalcogenide materials) the density of thermally generated carriers in the bulk to be negligible, but considered the interface layer to be an electric field sensitive electron barrier. During the corona charging time, a current flows through the bulk, depending on the density of free electrons and, at the end of the corona charging time (t =O), the discharge current depends in turn on the density of filled electron traps. So, he assumed that the dark decay is entirely due to carriers injected from the substrate and derived a mathematical expression for this decay: dV/I/=
- [A, +A1 ewat] dt
which gives: V= V0 exp[ -A,(
1-e-a’)/a-A2t]
with V the surface potential, V0 the surface potential at t = 0, the end of the charging time, (Ya probability factor related to the lifetime of electrons in the traps, A, and A2 are constants related respectively to the number of filled traps and to the number of free electrons in the bulk at t = 0. By adjusting the various parameters, a good agreement can be obtained between experimental results and the theoretical considerations [ 93. This mainly explains the heavy surface voltage decreasing at the beginning of the experiments (up to 250-300 s) for Se layers. This model is interesting because it takes into account the corona charging current flowing through the sample due to injection of charges through the interface layers and responsible for filled electron traps. Opposite to this “corona current model” from Schaffert, Abkowitz et al. [ 141 considered the capacitor-like structure sample, with a uniform electric field, after charging in the dark. They mainly considered the thermal generation of carriers in the bulk, assuming that only one sign of charge carriers is mobile on the time scale of the experiments. They wrote:
where E is the time dependent field, e the electronic charge, E the bulk dielectric constant, to the vacuum permittivity, GB the rate of thermal generation in the bulk, Js the rate of surimposed surface injection of carriers, and L the thickness of the layer. Assuming that Js is negligible, a depletion time td can be defined, such that the graph showing log (d I’/ dt) versus log(t) represents two linear segments, the inflexion point at td representing an abrupt change in the dark decay rate. And at td, we have V(f*) =$Vo. Before td the potential dV/dtcc - t-(‘-P)
decay is described by
,
and after td, by dV/dtcc -t-(‘+P).
The sum of the slopes of the two linear segments equals -2. The difference between V( t,,) and $ V. which could be observed, will be indicative of the rate of carriers injection from the interface. The parameter p is related to the width of the energy distribution
307
J.-Y. Moisan et al. /Xerographic dark discharge of PVK
of emission centers: p= 1 means a discrete emission center energy. An interesting point in this “electrostatic” model is that td is related to Ge, the rate of thermal carrier generation: the lower &, the higher G,. This depletion behaviour, with a definite td value, was observed for chalcogenide materials [ 9,11,14,15 ] and recently for molecular doped polymers [ 12 1. It is well known that the dark conductivity of amorphous solids is field dependent. For bulk electric properties, a Poole-Frenkel effect is expected: for example, the steady state photocurrents, measured in organic photoconductive materials, increase with the square root of the applied field [ 16- 18 1, but no definite conclusions were drawn about a Poole-Frenkel effect. If this Poole-Frenkel effect is assumed for thermal generation of carriers in the bulk [ 12,15 1, one can write (dVldt)ccexp(BJ--_)/kT,
where Q,is the barrier height for charge emission under zero electric field, and /3 the Poole-Frenkel coefficient. The theory of the Poole-Frenkel mechanism predicts: j?= (e3/7tte0)-L/Z.
TNF content of the sample, without changing the others testing parameters. 4.1. Charging time (tJ effect The xerographic dark discharge of a PVK-TNF 10% sample is shown in fig. 1. The charging time t, is increased from 70 up to 520 ms. The increase of V, with increasing t, is expected, but one can observe that f V. is never reached, even after more than 3 hours of measurement. Following the beginning of the xerographic dark discharge, two types of curves can be described. If t, 3 240 ms, the decreasing kinetics is obviously greater than when t, G 175 ms. In fig. 2, the depletion discharge versus time is shown. The two types of curves are now clearly observed: ( 1) For low t, values, or for high t times when tc < 175 ms, a set of parallel linear segments can be assumed to be the first segment expected in the “electrostatic model” from Abkowitz et al. The average
351
Assuming t = 3 for PVK with 10% by weight of TNF,
So, if one assumes a Poole-Frenkel mechanism, which means an electric field assisted thermal generation of carriers by lowering the height of the emission barrier, the slope of the graph [d V/dt=f( fi) ] equals /3. It indicates that the dark discharge of the sample is due to electric field assisted thermal generation of carriers, at least in the electric field range where this assumption is verified.
c
I
I
10
100
240ms
25t
4. Results and discussion
Successively the effect of three parameters will be analyzed. The first is the corona charging time t, of a sample, without changing the temperature. The second is the sample temperature during the charging time t, and the discharge time t. And the last is the
1
1000
1oc I00
Fig. 1. Decreasing surface voltage V versus time in the dark, following the indicated corona charging time. Thickness and temperature of the PVK-TNF 10% sample are given in the inset.
308
J.-Y. Moisan et al. /Xerographic dark dwharge of PVK
1oa
kv/.s,
-(dV/dt)
filled traps seems too low for an observable effect. So it appears that, in PVK-TNF lOoh, the xerographic dark discharge is due to two effects, depending on the charging conditions. For a “soft” charging sequence (here low t, times), few available traps are filled so the discharge is mainly due to bulk thermal generation. For “hard” charging conditions (here long t, times), two phenomena are superimposed: the first proposed bulk thermal generation of carriers, and the release of charges trapped in the bulk. As thermal generation is electric field assisted, it would be interesting to investigate if the Poole-Frenkel model is valid in some applied electric field range. One example (for t,= 240 ms) is given in fig. 3. The straight line drawn has a slope of 4.44 x 1Om4eV (cm/ V)-“2, which is quite close to the theoretical value. It appears, in the electric field range where bulk thermal generation is observed, this generation is electric field assisted, following a Poole-Frenkel mechanism.
1
ia
1
A
?Oms
l
1lOms
0
175ms
0
240ms
0
360ms
+
520ms
2.82/m 60°C
0.1
0.01
0001
l(
P
10
100
1
IO
1
0
Fig. 2. Xerographic depletion kinetics versus time for the same conditions as in fig. 1. Two different parallel segments are seen indicating two discharge processes.
slope for the drawn linear segments is 0.735 ( + 0.006), corresponding to a p value of 0.265: such a low value, meaning a large distribution of emission center energies, is reasonable, for an organic amorphous material. For dark-rested molecularly doped polymers a value of 0.05 was found [ 12 1. (2) For large t, values and for low discharging times t, another set of parallel linear segments is now assumed to be explained by the first “corona current model”. As for Se [ 9 1, this effect could be observed during t times lower than 1000 s. Since in this assumption the discharge current depends on the number of traps filled during the corona charging time, which is expressed by A, value, the depletion kinetics is higher for longer t,, meaning that the number of filled traps in the bulk is higher. On the other hand, because the probability factor cx does not depend on the number of tilled traps but on the local operating conditions, the second set of parallel segments shows the same slope. For too short t, times, the number of
PV Ch 0.1
0.01
0.001
.^^
6uu
-__ /vu
600
Fig. 3. Depletion kinetics versus square root of the remaining electric field. The slope of the straight line is 4.44~ 10e4 eV (cm/ V)-‘12 (theoretically 4.38x 1O-4 eV (cm/V)-1/2). Charging conditions are indicated.
J.-Y. Morsan et al. /Xerographic dark discharge ofPVK
But in the detrapping electric field range, this mechanism seems not to be valid. 4.2. Temperature effect In fig. 4 the dark depletion curves are presented for the same sample, corona charged in the same conditions but the temperature is increased from 20 up to 80 ’ C.The same assumptions, as in fig. 2, can be used: the beginning of the depletion kinetics follows the “corona current model” and the second time range of the depletion kinetics follows the “electrostatic model”. But some features will be emphasized: ( 1) Since the detrapping kinetics is probably thermally activated, a change is expected in the slope in the first time intervals of the depletion curves. This seems to be true in fig. 4. But more interesting is the time range where this “corona current model” appears to be valid (but superimposed to bulk thermal generation): 10
309
up to about 10 sat 80°C; up to about 50 s at 60°C; up to about 250 s at 40°C. At 20°C no evaluation can be made. So it clearly appears, because the sample temperature has no effect on charging conditions, that the detrapping kinetics is slower when the temperature is lowered. (2) On the other hand, the time range for the “electrostatic model” is lessened. Nevertheless, at 80°C a value of td could be proposed, about 4000 s. As the depletion is not only due to the thermally generated carriers, Vtd is expected to be less than 4 I’& which is true in the case that Vtd= 115 V and V03 260 V. But it is not possible to compute a significant value of p from the two parts of this graph. In the previous section, the “electrostatic model” depletion time range was associated with the remaining electric field range where the Poole-Frenkel mechanism seems to be obeyed. This is obviously observed in fig. 5, where this mechanism is valid in a remaining electric field less than about 0.7 MV/cm. But at 20°C after 4 hours of measurements, the remaining electric field is higher than this value, so no determination could be made. For the other temperatures, we obtained: at 80°C,B,,=3.83x 10e4eV (cm/V)‘/‘; at 60°C,&=4.44x 10e4eV (cm/V)‘/‘;
1
0.1
0 01
(E) “’ ( /cm)”
0.001
0 001 10
100
1000
11
)O
Fig. 4. Xerographic depletion kinetics versus time at different temperatures. The two previously described mechanisms (fig. 2) are again observed.
600
700
800
900
1000
Fig. 5. Depletion kinetics versus square root of the remaining electric field. The Poole-Frenkel mechanism seems to be obeyed, at each temperature, in the same electric field range.
310
J.-Y. Moisan et al. /Xerographic dark discharge of PVK
at 50°C,j?,.=4.42x 10e4eV (cm/V)‘/2. These three experimental values are in very good agreement with the theoretical value. So the temperature effects can be explained with the same assumptions as the charging conditions effect. During the first time range, the decreasing of the surface potential is explained by a detrapping mechanism; during the following time range, the decreasing of the potential is explained by a carrier thermal generation mechanism, obeying a Poole-Frenkel model. Changing the sample temperature will change the characteristic length of each process time range.
4.3. Dopant content effect
When a high content of doping molecules is added to PVK, the resistivity can be drastically lowered [ 191. This probably means that the conductivity mechanisms are changed or that their relative amplitudes are changed. The kinetics of decreasing surface voltage versus time are plotted in fig. 6: the depletion behaviours are identical for the samples with 2% and 1O”hTNF content. But the surface voltage for the 50°h TNF sample decreases more quickly. The differences between these samples are more
clearly seen in fig. I and an explanation can be proposed: ( 1) If each TNF molecule is assumed to be a possible trap (or to induce a trap in the bulk), the lifetime in each trap is expected to be the same for different TNF content. But the numbers of traps, filled during the charging time, should be greater with a higher TNF content, inducing a higher decreasing kinetics in the first depletion times. (2) No depletion time td is observed in the second time range. But because the slope is higher for a 50°h TNF content sample, this means that thep parameter is lower when TNF content is increased. If one remembers that p is related to the width of energy distribution of emission centers, it should be concluded that a higher dopant level increases the disorder in this amorphous material. According to the previous results, the Poole-Frenkel mechanism seems to be obeyed in this second time range, for a remaining electric field less than 0.55 MV/cm, as indicated in fig. 8. But, in this case, /3,,=5.50~10-4 eV (cm/V)-“2, which, is higher than the theoretical value. Generally a lower value than the theoretical one is obtained. This could be
300
x%
PVK - TbF x%
k-v 240 ms
250
200
150
+
2 %TNF
+
lO%TNF
)(
lO%TNF
0
50%
0
SO%TNF
/
100 0.001 1
Fig. 6. Decreasing surface voltage versus time for different contents of TNF in the sample. The corona charging conditions are indicated.
TNF
001
I
Fig. 7. Xerographic depletion kinetics samples with different TNF contents.
versus time for two PVK
J.-Y. Morsan et al. /Xerographic dark discharge of PVK
311
10
-(dV/di
PVK - - JF 10% 2.82,~ ,
60°C 12.5 mWh 1
- TNF ! 1%
na : 6 I I $0 ms 0.1
3JJm 60°C
0.01 0
/
L-L -I/2
0.001 L 500
600
700
800
-l/i ! ( /cm)
900
1000
Fig. 8. Xerographic depletion kinetics versus square root of the remainingelectric field. The slope ofthe straight line is 5.05~ 10m4 eV (cm/V)-“*.
explained by a different value of e with a 50% content.
IO0
Fig. 9. Surface voltage decreasing for a dark-rested sample ( 15 hours in the dark) and the same sample, highly illuminated just before the charging sequence.
5. Conclusion
4.4. Light illumination effect A large effect of a previous light illumination was observed on the xerographic dark discharge of molecularly doped polymers [ 121. This could be explained by photogenerated carriers, not recombined when the sample is submitted to a corona charging field. The dark discharge kinetics of a dark-rested sample ( 15 hours in the dark) is compared in fig. 9 to that of the same sample, “hardly” illuminated during 100 s and rested during 50 s before the corona sequence. For the illumination, we used a white light with energy density 3 orders of magnitude larger than light used in photodischarge experiments [ 9 1. As no effect was observed, it means that all the charges, photogenerated in the bulk, are recombined within 50 s. In these conditions, no light memory effect is observed in PVK-TNF materials.
Two mechanism are used to explain the xerographic dark discharge experiments on PVK-TNF material. One was said to be a “corona current model”, taking in account the corona current which flows through the sample during the charging sequence and whose effect is seen by the detrapping of carriers at the first time of the dark depletion. Obviously, this effect is increased by a longer charging time, as was demonstrated here. In the second “electrostatic model”, the dark discharge is only due to thermal generation of carriers. This mechanism is obviously superimposed on the first detrapping mechanism during the first times of each experiment. Since the thermal generation rate Gn is low, the decrease of the surface voltage is very low and experiments take hours. And the samples, in a capacitor-like structure, appear to be highly resistive. So, unfortunately, a more complete analysis of
312
J.-Y. Moisan et al. /Xerographic dark discharge of PVK
these two mechanisms from our experimental data seems to be hazardous. However, a more interesting point has to be emphasized. For lightly charged samples the surface voltage depletion is enterely described by considering the bulk thermal generation of carriers. On the other hand, for heavily charged samples, after a transition period depending on the sample temperature, the same mechanism is predominant. The length of this period is less than 200-300 s at room temperature, or even less at higher temperature. The thermal generation of carriers is the only mechanism involved for samples with a remaining electric field lower than about 0.5-0.7 MV/cm. Generally the photodischarge experiments (for determination of photogeneration quantum efficiency) are done in these conditions. The second aspect to be emphasized concerns the electric field assisted generation of carriers, which in our experiments, is in good agreement with a PooleFrenkel mechanism. It is well known that, in these same materials, the charge mobility is electric field sensitive, following a Poole-Frenkel mechanism. Therefore, in photoconductive organic materials (molecularly doped polymers or charge transfer complex sensitized PVK), electric conductivity proceeds through thermal or photogeneration of carriers which move in the bulk by hopping from site to site: this is in good agreement with a Poole-Frenkel description. Nevertheless, results showing a hole mobility decreasing with increasing electric field have been recently published for two very different materials, molecularly doped polymers [ 2 1 ] and Ge-backbone polymers [ 221. In contrast to non-photoconductive organic materials, in which the electric conductivity seems to proceed through dielectric relaxations [ 231, in this well known PVK-TNF materials electric conductivity seems to proceed through thermal or photogenera-
tion of carriers, moving plied electric field.
under the action of an ap-
References [ 1 ] J. Mart and G. Ptister, in: Electronic Properties of Polymers, Eds. J. Mot? and G. Ptister (Wiley, New York, 1982). [2] J. Mort and G. Ptister, Polymer-Plast. Technol. Eng. 12 (1979) 83. [ 31 C.-J. Hu, R. Oshima and M. Seno, J. Polymer Sci. Letters 26 (1988) 441. [4] P. Strohriegl, Mol. Cryst. Liquid Cryst. 183 (1990) 261. [ 51P.M. Borsenberger, E. Contois and D.C. Hoesterey, J. Chem. Phys. 68 (1978) 637. [ 61 M.A. Abkowitz, M.J. Rice and M. Stolka, Phil. Mag. B 6 I (1990) 25. [ 7 ] J.Y. Moisan, P. Gravey, R. Lever and L. Bonnel, Opt. Eng. 25 (1986) 151. [8] B. Andre, R. Lever and J.Y. Moisan, Chem. Phys. 137 (1989) 281. [ 9 ] R.M. Schaffer, ed., Electrophotography (Wiley, New York, 1981). [IO] M.A. Abkowitz and S. Maitra, J. Appl. Phys. 61 (1987) 1038. I1 ] C. Juhasz, M. Vaezi-Nejad and S.O. Kasap, J. Mater. Sci. 22 (1987) 2569. 121 Y. Kanemitsu and S. Imamura, Solid State Commun. 68 (1988) 701. 131 Y. Kanemitsu, D. Imanishi and S. Imamura, J. Appl. Phys. 66 ( 1989) 4526. 141 M. Abkowitz, F. Jansen and A.R. Melnyk, Phil. Mag. B 5 I (1985) 405. [ 151 SO. Kasap, M. Baxendale and C. Juhasz, J. Appl. Phys. 62 (1987) 171. [ 161 P.J. Reucroft and SK. Ghosh, Phys. Rev. B 8 (1973) 803. [ 171 K. Okamoto, S. Kusabayashi and H. Mikawa, Bull. Chem. Sot. Japan 46 (1973) 1953. [ 181 AI. Lakatos, J. Appl. Phys. 46 (1975) 1744. [ 191 A. Kuczkowski, Z. Greger, T. Stupkowski and B. Jachym, Polymer20 (1979) 1161. (201 W.D. Gtll, J. Appl. Phys. 43 (1972) 5033. [21] L.B. Schein, Mol. Cryst. Liquid Cryst. 183 (1990) 41. [22] M.A. Abkowitz, K.M. McGrane, F.E. Knier and M. Stolka, Mol. Cryst. Liquid Cryst. 183 ( 1990) 157. [ 231 J.Y. Moisan, B. Andre, R. Lever and C. Servens, to appear in Chem. Phys.
153 (1991) 305-312
Xerographic dark discharge of PVRTNF
photoconductive
material
Jean-Yves Moisan, Bernadette Andre and Roger Lever DPpartement OCM-TAC, Centre National d’Etudes des TMcommunications, B.P. 40, 22301 Lannion, France Received 22 October 1990
Xerographic dark discharge experiments on corona charged layers of poly N-vinylcarbazole (with different contents of trinitrofluorenone) have been carried out. The surface voltage decreasing against the time can be explained by the help of two mechanisms: the first, observed at the beginning of the dark discharge with sufficiently long corona charging times, is the detrapping effect of carriers injected by the corona current flowing through the sample during the charging sequence; the second is a thermal generation of carriers mechanism, previously proposed for capacitor structure-like materials. This thermal generation appearing electric-field-assisted according to a Poole-Frenkel effect.
1. Introduction There has been a considerable interest in the dynamics of charge photogeneration and transport in organic amorphous materials: poly N-vinylcarbazole and copolymers [ l-41, molecularly doped copolymers [ 1,2,5 ] and more recently polysilanes [ 6 1. Because of their high dark resistivity, these materials are of interest for electrostatic image storage and some have been used in xerographic reproduction and holographic optical switching systems [ 7 1. The high dark resistivity means low dark discharge kinetics, even if high electric fields, up to 1 MV/cm, are applied to the organic layers. Xerographic photodischarge is used to evaluate the photogeneration yield of the organic material, as it was previously used to study some amorphous chalcogenides (Se, As&,). So, with the same experimental set-up, two different properties of the same sample can be evaluated: the photogeneration yield [ 81 or quantum efficiency (which is the ratio of the number of charges swept from the sample surface and the number of absorbed photons in the bulk) and the dark discharge kinetics. It seems that this dark depletion discharge has mainly been studied for amorphous chalcogenide [9-l 11. Recently, some papers [ 12,131 discussed the dark discharge kinetics of some organic material and showed a reversible light-induced change, by xero0301-0104/91/$03.50
graphic measurements, on molecularly doped polymers. To our knowledge, no papers have been published concerning poly N-vinylcarbazole, with or without some acceptor molecule content, with attempt to explain the dark depletion behaviour. So we will present our results on dark discharge and attempt to explain the dark depletion kinetics by considering theoretical aspects and taking into account charge thermogeneration and surface injection.
2. Experimental PVK (from Janssen Chimica) was purified three times by dissolution in chloroform and reprecipitation in ethanol. TNF (2-4-7-trinitrofluorenone, from Janssen Chimica) is sublimated before use. These products (PVK and the indicated weight of TNF - in percentage of PVK) are dissolved in a chlorobenzene-toluene (50: 50) mixture and the layers are made by spin-coating on a 5 x 5 cm* IT0 glass substrate. Layers thicknesses (in the 2-5 urn range) are measured by spectrophotometric measurements in the IR range (with a Bruker 113V FTIR spectrometer). The sample (at 50 oC, unless otherwise specified ) , in a darkened experimental box, is first corona charged. A golden stainless-steel needle is connected
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
J.-Y. Molsan et al. / Xerographx dark discharge of PVK
306
to a high-voltage amplifier TREK Model 609A; the charging time can be monitored from 1 ms up to more than 1 s. In these experiments, a positive 6 kV voltage is used. At the end of the charging time, the sample is quickly moved (in about 0.1 s) under the probe of an electrometer (Monroe Model 244)) connected to a paper recorder. The surface voltage is recorded up to 4 hours. No changes were observed in the surface voltage decreasing kinetics, between a dark-rested sample ( 15 hours in the dark) and the same immediately tested sample (30 min. in the dark).
3. Theory When a charge is applied to the surface of a photoconductive layer, this charge immediately begins to decay. The rate at which the decay proceeds depends on the magnitude of the dark current through the layer (in a non-ohmic Z-V characteristic range), and this in turn depends on the carriers in the bulk. For these free carriers, three sources have to be considered: - thermally generated carriers in the film; - injection of carriers at the free surface of the layer: - injection of carriers at the interface of the layer with the IT0 conductive (and earth-connected) substrate. Generally the free surface of the layer behaves as a blocking layer and injection of carriers through this surface has not been taken into account so far. Schaffert [ 91 considered (for chalcogenide materials) the density of thermally generated carriers in the bulk to be negligible, but considered the interface layer to be an electric field sensitive electron barrier. During the corona charging time, a current flows through the bulk, depending on the density of free electrons and, at the end of the corona charging time (t =O), the discharge current depends in turn on the density of filled electron traps. So, he assumed that the dark decay is entirely due to carriers injected from the substrate and derived a mathematical expression for this decay: dV/I/=
- [A, +A1 ewat] dt
which gives: V= V0 exp[ -A,(
1-e-a’)/a-A2t]
with V the surface potential, V0 the surface potential at t = 0, the end of the charging time, (Ya probability factor related to the lifetime of electrons in the traps, A, and A2 are constants related respectively to the number of filled traps and to the number of free electrons in the bulk at t = 0. By adjusting the various parameters, a good agreement can be obtained between experimental results and the theoretical considerations [ 93. This mainly explains the heavy surface voltage decreasing at the beginning of the experiments (up to 250-300 s) for Se layers. This model is interesting because it takes into account the corona charging current flowing through the sample due to injection of charges through the interface layers and responsible for filled electron traps. Opposite to this “corona current model” from Schaffert, Abkowitz et al. [ 141 considered the capacitor-like structure sample, with a uniform electric field, after charging in the dark. They mainly considered the thermal generation of carriers in the bulk, assuming that only one sign of charge carriers is mobile on the time scale of the experiments. They wrote:
where E is the time dependent field, e the electronic charge, E the bulk dielectric constant, to the vacuum permittivity, GB the rate of thermal generation in the bulk, Js the rate of surimposed surface injection of carriers, and L the thickness of the layer. Assuming that Js is negligible, a depletion time td can be defined, such that the graph showing log (d I’/ dt) versus log(t) represents two linear segments, the inflexion point at td representing an abrupt change in the dark decay rate. And at td, we have V(f*) =$Vo. Before td the potential dV/dtcc - t-(‘-P)
decay is described by
,
and after td, by dV/dtcc -t-(‘+P).
The sum of the slopes of the two linear segments equals -2. The difference between V( t,,) and $ V. which could be observed, will be indicative of the rate of carriers injection from the interface. The parameter p is related to the width of the energy distribution
307
J.-Y. Moisan et al. /Xerographic dark discharge of PVK
of emission centers: p= 1 means a discrete emission center energy. An interesting point in this “electrostatic” model is that td is related to Ge, the rate of thermal carrier generation: the lower &, the higher G,. This depletion behaviour, with a definite td value, was observed for chalcogenide materials [ 9,11,14,15 ] and recently for molecular doped polymers [ 12 1. It is well known that the dark conductivity of amorphous solids is field dependent. For bulk electric properties, a Poole-Frenkel effect is expected: for example, the steady state photocurrents, measured in organic photoconductive materials, increase with the square root of the applied field [ 16- 18 1, but no definite conclusions were drawn about a Poole-Frenkel effect. If this Poole-Frenkel effect is assumed for thermal generation of carriers in the bulk [ 12,15 1, one can write (dVldt)ccexp(BJ--_)/kT,
where Q,is the barrier height for charge emission under zero electric field, and /3 the Poole-Frenkel coefficient. The theory of the Poole-Frenkel mechanism predicts: j?= (e3/7tte0)-L/Z.
TNF content of the sample, without changing the others testing parameters. 4.1. Charging time (tJ effect The xerographic dark discharge of a PVK-TNF 10% sample is shown in fig. 1. The charging time t, is increased from 70 up to 520 ms. The increase of V, with increasing t, is expected, but one can observe that f V. is never reached, even after more than 3 hours of measurement. Following the beginning of the xerographic dark discharge, two types of curves can be described. If t, 3 240 ms, the decreasing kinetics is obviously greater than when t, G 175 ms. In fig. 2, the depletion discharge versus time is shown. The two types of curves are now clearly observed: ( 1) For low t, values, or for high t times when tc < 175 ms, a set of parallel linear segments can be assumed to be the first segment expected in the “electrostatic model” from Abkowitz et al. The average
351
Assuming t = 3 for PVK with 10% by weight of TNF,
So, if one assumes a Poole-Frenkel mechanism, which means an electric field assisted thermal generation of carriers by lowering the height of the emission barrier, the slope of the graph [d V/dt=f( fi) ] equals /3. It indicates that the dark discharge of the sample is due to electric field assisted thermal generation of carriers, at least in the electric field range where this assumption is verified.
c
I
I
10
100
240ms
25t
4. Results and discussion
Successively the effect of three parameters will be analyzed. The first is the corona charging time t, of a sample, without changing the temperature. The second is the sample temperature during the charging time t, and the discharge time t. And the last is the
1
1000
1oc I00
Fig. 1. Decreasing surface voltage V versus time in the dark, following the indicated corona charging time. Thickness and temperature of the PVK-TNF 10% sample are given in the inset.
308
J.-Y. Moisan et al. /Xerographic dark dwharge of PVK
1oa
kv/.s,
-(dV/dt)
filled traps seems too low for an observable effect. So it appears that, in PVK-TNF lOoh, the xerographic dark discharge is due to two effects, depending on the charging conditions. For a “soft” charging sequence (here low t, times), few available traps are filled so the discharge is mainly due to bulk thermal generation. For “hard” charging conditions (here long t, times), two phenomena are superimposed: the first proposed bulk thermal generation of carriers, and the release of charges trapped in the bulk. As thermal generation is electric field assisted, it would be interesting to investigate if the Poole-Frenkel model is valid in some applied electric field range. One example (for t,= 240 ms) is given in fig. 3. The straight line drawn has a slope of 4.44 x 1Om4eV (cm/ V)-“2, which is quite close to the theoretical value. It appears, in the electric field range where bulk thermal generation is observed, this generation is electric field assisted, following a Poole-Frenkel mechanism.
1
ia
1
A
?Oms
l
1lOms
0
175ms
0
240ms
0
360ms
+
520ms
2.82/m 60°C
0.1
0.01
0001
l(
P
10
100
1
IO
1
0
Fig. 2. Xerographic depletion kinetics versus time for the same conditions as in fig. 1. Two different parallel segments are seen indicating two discharge processes.
slope for the drawn linear segments is 0.735 ( + 0.006), corresponding to a p value of 0.265: such a low value, meaning a large distribution of emission center energies, is reasonable, for an organic amorphous material. For dark-rested molecularly doped polymers a value of 0.05 was found [ 12 1. (2) For large t, values and for low discharging times t, another set of parallel linear segments is now assumed to be explained by the first “corona current model”. As for Se [ 9 1, this effect could be observed during t times lower than 1000 s. Since in this assumption the discharge current depends on the number of traps filled during the corona charging time, which is expressed by A, value, the depletion kinetics is higher for longer t,, meaning that the number of filled traps in the bulk is higher. On the other hand, because the probability factor cx does not depend on the number of tilled traps but on the local operating conditions, the second set of parallel segments shows the same slope. For too short t, times, the number of
PV Ch 0.1
0.01
0.001
.^^
6uu
-__ /vu
600
Fig. 3. Depletion kinetics versus square root of the remaining electric field. The slope of the straight line is 4.44~ 10e4 eV (cm/ V)-‘12 (theoretically 4.38x 1O-4 eV (cm/V)-1/2). Charging conditions are indicated.
J.-Y. Morsan et al. /Xerographic dark discharge ofPVK
But in the detrapping electric field range, this mechanism seems not to be valid. 4.2. Temperature effect In fig. 4 the dark depletion curves are presented for the same sample, corona charged in the same conditions but the temperature is increased from 20 up to 80 ’ C.The same assumptions, as in fig. 2, can be used: the beginning of the depletion kinetics follows the “corona current model” and the second time range of the depletion kinetics follows the “electrostatic model”. But some features will be emphasized: ( 1) Since the detrapping kinetics is probably thermally activated, a change is expected in the slope in the first time intervals of the depletion curves. This seems to be true in fig. 4. But more interesting is the time range where this “corona current model” appears to be valid (but superimposed to bulk thermal generation): 10
309
up to about 10 sat 80°C; up to about 50 s at 60°C; up to about 250 s at 40°C. At 20°C no evaluation can be made. So it clearly appears, because the sample temperature has no effect on charging conditions, that the detrapping kinetics is slower when the temperature is lowered. (2) On the other hand, the time range for the “electrostatic model” is lessened. Nevertheless, at 80°C a value of td could be proposed, about 4000 s. As the depletion is not only due to the thermally generated carriers, Vtd is expected to be less than 4 I’& which is true in the case that Vtd= 115 V and V03 260 V. But it is not possible to compute a significant value of p from the two parts of this graph. In the previous section, the “electrostatic model” depletion time range was associated with the remaining electric field range where the Poole-Frenkel mechanism seems to be obeyed. This is obviously observed in fig. 5, where this mechanism is valid in a remaining electric field less than about 0.7 MV/cm. But at 20°C after 4 hours of measurements, the remaining electric field is higher than this value, so no determination could be made. For the other temperatures, we obtained: at 80°C,B,,=3.83x 10e4eV (cm/V)‘/‘; at 60°C,&=4.44x 10e4eV (cm/V)‘/‘;
1
0.1
0 01
(E) “’ ( /cm)”
0.001
0 001 10
100
1000
11
)O
Fig. 4. Xerographic depletion kinetics versus time at different temperatures. The two previously described mechanisms (fig. 2) are again observed.
600
700
800
900
1000
Fig. 5. Depletion kinetics versus square root of the remaining electric field. The Poole-Frenkel mechanism seems to be obeyed, at each temperature, in the same electric field range.
310
J.-Y. Moisan et al. /Xerographic dark discharge of PVK
at 50°C,j?,.=4.42x 10e4eV (cm/V)‘/2. These three experimental values are in very good agreement with the theoretical value. So the temperature effects can be explained with the same assumptions as the charging conditions effect. During the first time range, the decreasing of the surface potential is explained by a detrapping mechanism; during the following time range, the decreasing of the potential is explained by a carrier thermal generation mechanism, obeying a Poole-Frenkel model. Changing the sample temperature will change the characteristic length of each process time range.
4.3. Dopant content effect
When a high content of doping molecules is added to PVK, the resistivity can be drastically lowered [ 191. This probably means that the conductivity mechanisms are changed or that their relative amplitudes are changed. The kinetics of decreasing surface voltage versus time are plotted in fig. 6: the depletion behaviours are identical for the samples with 2% and 1O”hTNF content. But the surface voltage for the 50°h TNF sample decreases more quickly. The differences between these samples are more
clearly seen in fig. I and an explanation can be proposed: ( 1) If each TNF molecule is assumed to be a possible trap (or to induce a trap in the bulk), the lifetime in each trap is expected to be the same for different TNF content. But the numbers of traps, filled during the charging time, should be greater with a higher TNF content, inducing a higher decreasing kinetics in the first depletion times. (2) No depletion time td is observed in the second time range. But because the slope is higher for a 50°h TNF content sample, this means that thep parameter is lower when TNF content is increased. If one remembers that p is related to the width of energy distribution of emission centers, it should be concluded that a higher dopant level increases the disorder in this amorphous material. According to the previous results, the Poole-Frenkel mechanism seems to be obeyed in this second time range, for a remaining electric field less than 0.55 MV/cm, as indicated in fig. 8. But, in this case, /3,,=5.50~10-4 eV (cm/V)-“2, which, is higher than the theoretical value. Generally a lower value than the theoretical one is obtained. This could be
300
x%
PVK - TbF x%
k-v 240 ms
250
200
150
+
2 %TNF
+
lO%TNF
)(
lO%TNF
0
50%
0
SO%TNF
/
100 0.001 1
Fig. 6. Decreasing surface voltage versus time for different contents of TNF in the sample. The corona charging conditions are indicated.
TNF
001
I
Fig. 7. Xerographic depletion kinetics samples with different TNF contents.
versus time for two PVK
J.-Y. Morsan et al. /Xerographic dark discharge of PVK
311
10
-(dV/di
PVK - - JF 10% 2.82,~ ,
60°C 12.5 mWh 1
- TNF ! 1%
na : 6 I I $0 ms 0.1
3JJm 60°C
0.01 0
/
L-L -I/2
0.001 L 500
600
700
800
-l/i ! ( /cm)
900
1000
Fig. 8. Xerographic depletion kinetics versus square root of the remainingelectric field. The slope ofthe straight line is 5.05~ 10m4 eV (cm/V)-“*.
explained by a different value of e with a 50% content.
IO0
Fig. 9. Surface voltage decreasing for a dark-rested sample ( 15 hours in the dark) and the same sample, highly illuminated just before the charging sequence.
5. Conclusion
4.4. Light illumination effect A large effect of a previous light illumination was observed on the xerographic dark discharge of molecularly doped polymers [ 121. This could be explained by photogenerated carriers, not recombined when the sample is submitted to a corona charging field. The dark discharge kinetics of a dark-rested sample ( 15 hours in the dark) is compared in fig. 9 to that of the same sample, “hardly” illuminated during 100 s and rested during 50 s before the corona sequence. For the illumination, we used a white light with energy density 3 orders of magnitude larger than light used in photodischarge experiments [ 9 1. As no effect was observed, it means that all the charges, photogenerated in the bulk, are recombined within 50 s. In these conditions, no light memory effect is observed in PVK-TNF materials.
Two mechanism are used to explain the xerographic dark discharge experiments on PVK-TNF material. One was said to be a “corona current model”, taking in account the corona current which flows through the sample during the charging sequence and whose effect is seen by the detrapping of carriers at the first time of the dark depletion. Obviously, this effect is increased by a longer charging time, as was demonstrated here. In the second “electrostatic model”, the dark discharge is only due to thermal generation of carriers. This mechanism is obviously superimposed on the first detrapping mechanism during the first times of each experiment. Since the thermal generation rate Gn is low, the decrease of the surface voltage is very low and experiments take hours. And the samples, in a capacitor-like structure, appear to be highly resistive. So, unfortunately, a more complete analysis of
312
J.-Y. Moisan et al. /Xerographic dark discharge of PVK
these two mechanisms from our experimental data seems to be hazardous. However, a more interesting point has to be emphasized. For lightly charged samples the surface voltage depletion is enterely described by considering the bulk thermal generation of carriers. On the other hand, for heavily charged samples, after a transition period depending on the sample temperature, the same mechanism is predominant. The length of this period is less than 200-300 s at room temperature, or even less at higher temperature. The thermal generation of carriers is the only mechanism involved for samples with a remaining electric field lower than about 0.5-0.7 MV/cm. Generally the photodischarge experiments (for determination of photogeneration quantum efficiency) are done in these conditions. The second aspect to be emphasized concerns the electric field assisted generation of carriers, which in our experiments, is in good agreement with a PooleFrenkel mechanism. It is well known that, in these same materials, the charge mobility is electric field sensitive, following a Poole-Frenkel mechanism. Therefore, in photoconductive organic materials (molecularly doped polymers or charge transfer complex sensitized PVK), electric conductivity proceeds through thermal or photogeneration of carriers which move in the bulk by hopping from site to site: this is in good agreement with a Poole-Frenkel description. Nevertheless, results showing a hole mobility decreasing with increasing electric field have been recently published for two very different materials, molecularly doped polymers [ 2 1 ] and Ge-backbone polymers [ 221. In contrast to non-photoconductive organic materials, in which the electric conductivity seems to proceed through dielectric relaxations [ 231, in this well known PVK-TNF materials electric conductivity seems to proceed through thermal or photogenera-
tion of carriers, moving plied electric field.
under the action of an ap-
References [ 1 ] J. Mart and G. Ptister, in: Electronic Properties of Polymers, Eds. J. Mot? and G. Ptister (Wiley, New York, 1982). [2] J. Mort and G. Ptister, Polymer-Plast. Technol. Eng. 12 (1979) 83. [ 31 C.-J. Hu, R. Oshima and M. Seno, J. Polymer Sci. Letters 26 (1988) 441. [4] P. Strohriegl, Mol. Cryst. Liquid Cryst. 183 (1990) 261. [ 51P.M. Borsenberger, E. Contois and D.C. Hoesterey, J. Chem. Phys. 68 (1978) 637. [ 61 M.A. Abkowitz, M.J. Rice and M. Stolka, Phil. Mag. B 6 I (1990) 25. [ 7 ] J.Y. Moisan, P. Gravey, R. Lever and L. Bonnel, Opt. Eng. 25 (1986) 151. [8] B. Andre, R. Lever and J.Y. Moisan, Chem. Phys. 137 (1989) 281. [ 9 ] R.M. Schaffer, ed., Electrophotography (Wiley, New York, 1981). [IO] M.A. Abkowitz and S. Maitra, J. Appl. Phys. 61 (1987) 1038. I1 ] C. Juhasz, M. Vaezi-Nejad and S.O. Kasap, J. Mater. Sci. 22 (1987) 2569. 121 Y. Kanemitsu and S. Imamura, Solid State Commun. 68 (1988) 701. 131 Y. Kanemitsu, D. Imanishi and S. Imamura, J. Appl. Phys. 66 ( 1989) 4526. 141 M. Abkowitz, F. Jansen and A.R. Melnyk, Phil. Mag. B 5 I (1985) 405. [ 151 SO. Kasap, M. Baxendale and C. Juhasz, J. Appl. Phys. 62 (1987) 171. [ 161 P.J. Reucroft and SK. Ghosh, Phys. Rev. B 8 (1973) 803. [ 171 K. Okamoto, S. Kusabayashi and H. Mikawa, Bull. Chem. Sot. Japan 46 (1973) 1953. [ 181 AI. Lakatos, J. Appl. Phys. 46 (1975) 1744. [ 191 A. Kuczkowski, Z. Greger, T. Stupkowski and B. Jachym, Polymer20 (1979) 1161. (201 W.D. Gtll, J. Appl. Phys. 43 (1972) 5033. [21] L.B. Schein, Mol. Cryst. Liquid Cryst. 183 (1990) 41. [22] M.A. Abkowitz, K.M. McGrane, F.E. Knier and M. Stolka, Mol. Cryst. Liquid Cryst. 183 ( 1990) 157. [ 231 J.Y. Moisan, B. Andre, R. Lever and C. Servens, to appear in Chem. Phys.