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Electrical properties of phenylene vinylene oligomer thin films
ELSEVIER
SyntheticMetals76 (1996) 187-190
Electrical properties of phenylene vinylene oligomer thin films P. Le Rendu a, T.P. Nguyen b, 0. Gaudin b, V.H. Tran ’ a hboratoire de Chimie des Sofides, Institut des Mate’riallx de Nantes, 2 rue de la Houssinike, 44072 Nantes Cedex 03, France b Laboratoire de Physique Cristalline, Institut des Mate’riaux de Nantes, 2 rue de la HoussiniBre, 44072 Nantes Cedex 03, France c Luboratoires des Mate’riarcx Organiques aux Proprie’th Sptkijqnes, CNRS, BP 24, 69390 Vernaison, France
Abstract In this work, we report the results of electrical investigations on phenylene vinylene oligomer thin films with 5-phenyl, 4-vinyl compound (4PV). Synthesis of the oligomer powder was performed using a Wittig reaction of phosphonium salt with carbonyl group and thin films weredeposited by conventional thermal evaporation under vacuum of the obtained powder.The conduction mechanisms of oligomer-based diodes werestudied by measuring the current-voltage-temperature characteristicsand the thermally stimulated current. In the high-temperature range, Poole-Frenkel emission is observedwhile, at lower temperatures,a hopping processprobably occurs.Data from the thermally stimulated current measurements involve shallow traps (0.42 eV) which might originate from the 4PV/metal interface region. Keywords:
Phenylenevinylene;Poole-Frenkeleffect; Thermallystimulatedcurrent;Conductionmechanism;Films
1. Introduction The recent discovery of the electroluminescent properties of conjugated polymers has encountered the research of new synthesis techniques and new materials which can be used in future electronic display devices [ 11. Among these polymers, poly (para-phenylene vinylene) (PPV) is intensively studied [ 2-41 because of its interesting physical properties (stability, electrical and optical properties, etc.). Phenylene vinylene oligomers have not yet been well studied, although preliminary optical measurements 1.51 in these materials reveal a very promising possibility of use in optoelectronic devices. The main advantage of these oligomers with regard to PPV is that they can be obtained by thermal evaporation in vacuum yielding thin films with uniform and controlled thickness. Therefore, devices can be easily fabricated without breaking the vacuum and thus can avoid exposure of the samples in air which may involve undesirable effects. The work we present in this paper is part of a programme aiming at characterizing the phenylene vinylene oligomers for their future applications in electronic industrial use. We have investigated the 5pheny1, 4-vinyl compound (abbreviated as 4PV) and we compare its electrical properties to those of PPV. 2. Experimental Synthesis of polymer from precursor solution gives long polymer chains. For instance, the main reaction in the PPV 0379-6779/96/$15.000 1996ElsevierScienceS.A.All rightsreserved
synthesis [ 61 uses a bi-functional compound so that the reaction can continue after condensation it the first chain end. In this case, polymerization can be initiated from short length molecules, but this method of preparation hardly allows the control of the polymer chain length. To obtain well-controlled chain length from PPV precursor, one can either modify the parameters such as temperature or duration during the reaction or select the interesting part of the obtained molecules by a separation process. These methods give polymers with molecular weight in a relatively narrow range. In order to synthesize oligomers of defined formulae, we use classical organic chemistry methods. In the case of 4PV, we have to create a chain with five phenyl units bounded to four vinyl ones. This synthesis was done by associating one phenyl unit with two elements, each of them containing two phenyls bound to one vinyl. It consists of creating two double bonds between constituent units. This can be done with the Wittig reaction in which a czrbonyl group reacts with a phosphonium salt, giving the desired double bond. All the chemical products were purchased from Aldrich. Dichloro-para-xylene and triphenyl phosphine were used as reagents for the phosphonium salt. This salt is obtained by the following reaction using N,N-dimethylformamide (DMF) as solvent at 60 “C for 2 h with an excess of triphenyl phosphine: ClCH2-Q-CH&l + 2P& + 2Cl- + @P + CH+Z’-CH2P
+ @s
188
P. Le Rendu et al. /Synthetic
The salt was then washed with ether and dried at 80 “C under vacuum. The base used for the Wittig reaction was lithium hydride and the reaction is conducted using pure alcohol as solvent: base
Q,P=CH-@-CH=P@,
+ 2RCH0 --) RCH=CH-@CH=CHR
+ 2PcP30
When R is a-CH=CH-@, the 4PV is obtained after 2 h. The powder is filtered, washed with pure alcohol and dried. The obtained product is pulverulent and of bright yellow colour. The devices were obtained by thermal evaporation at room temperature under high vacuum conditions (about 10v6 Torr) of a bottom electrode on a cleaned glass substrate, followed by the oligomer powder evaporation with a rate of about 1 nm s-l. The top electrodes were then deposited without breaking the vacuum. Chromium was used for both top and bottom electrodes. Four samples of 2 X 2 mm* active area each were obtained on the same substrate. The thicknesses of the polymer films were measured by an Alphastep unit and they varied from 0.1 to 1 km. Optical measurements including UV-Vis, IR and Raman spectroscopies were performed on bare polymer films to control their quality. Electrical experiments were carried out with a set-up already described in our previous paper [7]. All measurements were performed under vacuum.
Metals
76 (1996)
187-190
diode. From results obtained in PPV-based diodes [ 71, it is suggested that injection of electrons has occurred from the top electrode into the oligomer film. Although the sample has identical electrodes at both sides of the polymer film, the asymmetric characteristics may indicate that the nature of the contacts is different following the method of formation. It would mean that, when the polymer is deposited on the metal electrode, the junction formed may be purely ohmic. In contrast, the metal layer on the polymer film would form apotential barrier leading to rectifying behaviour of the structure. This seems to be a suitable way to explain the experimental characteristic in the oligomer diode. In PPV devices [8], using the surface investigation method, we have pointed out that the rectifying behaviour of symmetrical diodes (for instance Cr-PPV-Cr structures) can be related to the different nature of the inter-facial layers. In addition, in asymmetric configurations, the measured current in geometrically identical samples does not show a clear dependence on the work function of the metal used for electrodes and this suggests that the real potential barrier at the metal-polymer contact may be governed by factors such as surface states or intermediate layers that could exist at the interface. However, the contact formation between the polymer and the metallic films needs to be investigated by further spectroscopic methods, such as photoelectron spectroscopy (XPS) or optical spectroscopies (IR, Raman), before we arrive at a definitive understanding of the charge injection mechanism. Assuming the thermionic emission at the potential barrier, the current density can be expressed as a function of the voltage by [ 91 J=J, exp(eV/@T)
3. Results and discussion 3.1. Current-voltage (I-V) characteristics
Fig. 1 shows the plot of the current density versus the applied field at ambient temperature. The forward current is obtained with negative bias of the top electrode. The curve is asymmetric and non-linear. The threshold field in the forward direction is about 1.5 X lo7 V m-l for the Cr top electrode
J Wcm2) 6 lo8:
-2 lo‘* / -3.0 10’
I -1.5 10’
I
I 1.5 10’
E C~h-4 Fig. 1. Current density vs. applied field for a Cr-4PV-0 K. The thickness of the polymer film is 0.6 km.
I 3.0 10’ diode at T=293
(1) where J is the current density, Jo the reverse leakage current density of the junction, e the electronic charge, V the forward voltage, 77the ideality factor, k the Boltzmann constant and T the absolute temperature. Jo is given by JO=A*p
exp( - e&,lkT)
(2)
where A* is the Richardson constant ( 120 A Km2 cm-* for the free electron model) and & the barrier height. The value of 77determined from the experimental curve is very high (about 66) ; therefore, true thermionic emission would not be considered here [ 91. It is known that an ideality factor superior to unity usually reflects the presence of an insulating layer between the metal and the semiconductor layers in a Schottky barrier [ lo]. The appearance of such a layer on the oligomer film is not obvious since the device was obtained in a vacuum chamber. It supports, however, the explanation of the asymmetry of the Z-V characteristics observed in symmetric oligomer diodes. The value of +, determined from the experimental curve is about 0.63 eV. Notice that this value was obtained assuming that the free electron model is applicable. In fact, the experimental Richardson constant is found in many cases to be inferior to the theoretical one [9], since the effective mass of the carriers
P. Le Rendu et al. /Synthetic
0
I 1
I 2
I 3
4
v lr2 (Volt’“) Fig. 2. Plot of the current vs. square root of the applied voltage for a Cr4PV-Cr diode at T= 293 K. The thickness of the polymer film is 0.6 km.
may differ from that of free electrons. Therefore, the barrier height could be smaller than the calculated value determined above. Fig. 2 shows the plot of the current versus the square root of the applied voltage at room temperature. For V> 1.5 V, a straight line is obtained. The linear variation of the current can correspond either to the Poole-Frenkel mechanism or to the Schottky emission [ 111. The Schottky-Richardson emission involves charge carrier injection from the electrode to the semiconductor via the field-assisted lowering of the potential barrier at the contact, The Z-V relationship is given by Z=A,T’
76 (1996)
189
187-190
3.2. Temperaturedependence
I(A) lo’*.~
10’14 0
Metals
exp{ [ &( V/d) 1’2- &,I /kT}
The dependence of the current on the temperature is shown in Fig. 3. The evolution of the current clearly shows two regions: below 275 K, the current depends slightly on the temperature; above 275 K, a sharp increase of the current occurs with increasing temperature. This observation shows that two different mechanisms are acting in the two temperature regions. At high temperatures, the process has a high activation energy which is typical for electronic conduction. The plot of the activation energy 4 versus the applied voltage is shown in Fig. 4. A straight line is obtained which intercepts the C$axis at +. N 1.9 eV. As the mechanism in this region is identified as the Poole-Frenkel emission, the value found represents the energy of the localized states with regard to the conduction band edge at the applied field. At temperatures below 275 K, the activation energy is very weak (about 0.01 eV) indicating that hopping of charge carriers probably occurs in this region [ 121. The results obtained here are in good agreement with those already reported in PPV films [ 71. We should notice however that the mean energy level of the localized states in the band gap of PPV is about 0.4 eV from the bottom of the conduction band. In the case of 4PV film, this value determined from Eq.
1(A)
(3)
1’2, A,, a constant, e the electronic where & = ( e314r& charge, V the applied voltage, d the thickness of the film, E the relative dielectric constant, co the dielectric constant of free space, k the Boltzmann constant and +s the contact potential barrier. For the Poole-Frenkel mechanism, the carriers are emitted by the lowering of the potential barrier from the localized levels in the semiconductor bulk into the conduction band. The current is given by
~1000.00000 d I..... . .. . . . c .*a*.................I.... ...... k
IO-”
1o”2 3
I
/
3.5
4
1 0.5
I 1
1
/
1
I
I
4.5 5 5.5 6 6.5 7 1000/r (K-g -.Fig. 3. Variation of the current as a function of the inverse of temperature for a Cr-PPV-Cr diode: (a) 3 V; (b) 6 V; (c) 9 V; (d) 12 V. The thickness of the polymer film is 0.6 pm.
(4) where B. is a constant and PPF = 2( e3/4rrrEEo) if2 = Zp,. Distinction between the two processes can be done by comparing the theoretical value of p with the experimental one obtained by calculating the slope of the curve Z= f( V”‘). The relative dielectric constant determined from capacitance measurements of the sample is about 2.5, which yields the following values of the constant /3: ,f3% = 4 X 1O-24 J V”2 m1’2 and PPF= 8.6 X 1O-24 J V”2 m*‘2. The slope of the straight line portion of the curve in Fig. 2 gives a value of 7.2 X 1O-24 J V”2 m1’2 for p, closer to &than ps. Therefore, it seems plausible to assume that carriers are transported through the polymer film by the field-assisted Poole-Frenkel mechanism.
0.4 0
I I I I / 1.5 2 2.5 3 3.5 4 vlD (VOP) Fig. 4. Plot of the activation energy vs. square root of the applied voltage for a Cr-4PV-0 diode. 0
P. Le Rendu et al. /Synthelic Metals 76 (1996) 187-190
190
I@) I
1 lo-lo
I
I
I
1
I
I
O-1 lo-IO-
conduction mechanism. An alternative process explaining the TSC current could be the release of charges from the interfacial layer [ 151, as already observed in some semiconductor devices. Further experiments are needed to elucidate the origin of the charge transport in this process.
4. Conclusions -4 lo-‘O-5 1O”O -6 do
I 170
I
I
I
I
/
I
190 210 230 250 270 290 310 TOO Fig. 5. Thermally stimulated current spectra for a Cr-4PV-Cr diode with different biases V,: (a) 3 V; (b) 6 V; (c) 9 V. The thickness of the polymer tilmis0.6~m;TH=293K;TL=100K;R=1.85Kmin-’. 150
(4) is much higher (about 1.9 eV) , suggesting that a deep energetic level is involved. A band-to-band emission process with a high activation energy could also be considered as already observed in other types of polymers [ 131. 3.3. Thermally stimulated current (TX)
In TSC measurements, the following procedure was adopted. At high temperature TH,the sample was biased at a given voltage V,. While maintaining the bias on, the sample was cooled down to low temperature TL.When thermal equilibrium was reached, the sample was short circuited, then the current was monitored upon heating the device at a constant rate R. Fig. 5 shows the TSC spectra of a 4PV sample at various V,. We can see that a current peak appears at 277 K for V, = 3 V (curve a) which shifts to higher temperature for higher bias (curves b and c) I On the other hand, the area delimited by the TSC curve, which represents the released charge, increases with increasing bias. The trap depth E, of the released charge can be evaluated using the varying heatingrate technique. It consists in recording the TSC spectra with the same applied bias and with different heating rates. The treatment proposed by Booth [ 141 gives a simple way to determine Ep We obtain via this technique a value of 0.42 eV for the energy level of the trapped charges. This value is close to that obtained by the same technique for PPV film but we should notice that, in this case, the involved energy corresponds to the localized states in the band gap of the polymer. For 4PV thin films, the energy found from the TSC technique differs significantly from that derived from the Z(r) characteristic, implying that the charge carriers are not the same in the two processes. The energy of the released charges from the TSC experiment shows that the traps are shallow and the contribution of the trapped charges is almost negligible in the
We have investigated the electrical properties of 4PV oligomer films by current-voltage-temperature characteristics and thermally stimulated current studies. The conduction mechanism is identified as field-assistedPoole-Frenkel emission for temperatures higher than 275 K, while, in the lowtemperature range, hopping of charge carriers between localized states with weak energy activation probably occurs. TSC results show that the trapped charge carriers with mean energy of 0.42 eV are different from those involved in the above processes. In addition to their remarkable optical properties [ 51, the possibility of obtaining thin films by vacuum deposition makes the studied oligomers very attractive for future electronic devices such as light-emitting diodes (LEDs) or fieldeffect transistors (FETs) on which we are now working.
References [l] R. Friend, D. Bradley and A. Holmes, Phyys,World, (Nov.) (1992) 42.
A.R. Brown, D.D.C. Bradley, J.H. Burroughes, R.H. Friend, NC. Greenham, P.L. Bum, A.B. Holmes and A. Kraft, Appl. Phys. Lert., 61 (1992) 2793. [3] S. Karg, W. Riess, V. Dyakonov and M. Schwoerer, Synd. Met., 54 (1993) 427. [4] C. Zhang, H. von Seggem, K. Pakbaz, B. Kraabel, H.W. Schmidt and A.J. Heeger, Synrh.Met., 62 (1994) 35. [5] J.M. Nunzi, F. Charra, N. Pfeffer, T.P. Nguyen and V.H. Tran, Nonlinear Opt., in press. [6] R.A. Wessling and R.G. Zimmermann, US Pafew No. 3401152 (1968). [7] T.P. Nguyen, V.H. Tran and V. Massardier, J. Phys.: Condens. Matter, 5(1993) 1. [S] V.H. Tran, V. Massardier, A. Guyot and T.P. Nguyen, Polymer, 34 (1993) 3179. [9] E.H. Rhoderick, Metal-Semiconductor Contact, Clarendon Press, Oxford, 1980. [lo] B.L. Sharma, MetalSemicondllctor Schottky Barriers and Their Applications, Plenum, New York, 1984. [ 111 J.G. Simmons,in L.I. MaisselandR. Glang (eds.), Handbook ofThin Film Techology, McGraw-Hill, New York, 1970, Ch. 14. [ 121 S. Roth, in M. Pollak and B.I. Shklovskii (eds.), Hopping Transport in Soli& North-Holland, Amsterdam, 1991, Ch. 11. [ 131 See, for example: E.M. Cavalcante and M. Carnpos, Synrh.Met., 5557 (1993) 4918. [ 141 A.H. Booth, Can. J. Chem., 32 (1954) 214. [ 153 T.P. Nguyen and S. Minn, Solid State Commun., 31 (1979) 233. [2]
SyntheticMetals76 (1996) 187-190
Electrical properties of phenylene vinylene oligomer thin films P. Le Rendu a, T.P. Nguyen b, 0. Gaudin b, V.H. Tran ’ a hboratoire de Chimie des Sofides, Institut des Mate’riallx de Nantes, 2 rue de la Houssinike, 44072 Nantes Cedex 03, France b Laboratoire de Physique Cristalline, Institut des Mate’riaux de Nantes, 2 rue de la HoussiniBre, 44072 Nantes Cedex 03, France c Luboratoires des Mate’riarcx Organiques aux Proprie’th Sptkijqnes, CNRS, BP 24, 69390 Vernaison, France
Abstract In this work, we report the results of electrical investigations on phenylene vinylene oligomer thin films with 5-phenyl, 4-vinyl compound (4PV). Synthesis of the oligomer powder was performed using a Wittig reaction of phosphonium salt with carbonyl group and thin films weredeposited by conventional thermal evaporation under vacuum of the obtained powder.The conduction mechanisms of oligomer-based diodes werestudied by measuring the current-voltage-temperature characteristicsand the thermally stimulated current. In the high-temperature range, Poole-Frenkel emission is observedwhile, at lower temperatures,a hopping processprobably occurs.Data from the thermally stimulated current measurements involve shallow traps (0.42 eV) which might originate from the 4PV/metal interface region. Keywords:
Phenylenevinylene;Poole-Frenkeleffect; Thermallystimulatedcurrent;Conductionmechanism;Films
1. Introduction The recent discovery of the electroluminescent properties of conjugated polymers has encountered the research of new synthesis techniques and new materials which can be used in future electronic display devices [ 11. Among these polymers, poly (para-phenylene vinylene) (PPV) is intensively studied [ 2-41 because of its interesting physical properties (stability, electrical and optical properties, etc.). Phenylene vinylene oligomers have not yet been well studied, although preliminary optical measurements 1.51 in these materials reveal a very promising possibility of use in optoelectronic devices. The main advantage of these oligomers with regard to PPV is that they can be obtained by thermal evaporation in vacuum yielding thin films with uniform and controlled thickness. Therefore, devices can be easily fabricated without breaking the vacuum and thus can avoid exposure of the samples in air which may involve undesirable effects. The work we present in this paper is part of a programme aiming at characterizing the phenylene vinylene oligomers for their future applications in electronic industrial use. We have investigated the 5pheny1, 4-vinyl compound (abbreviated as 4PV) and we compare its electrical properties to those of PPV. 2. Experimental Synthesis of polymer from precursor solution gives long polymer chains. For instance, the main reaction in the PPV 0379-6779/96/$15.000 1996ElsevierScienceS.A.All rightsreserved
synthesis [ 61 uses a bi-functional compound so that the reaction can continue after condensation it the first chain end. In this case, polymerization can be initiated from short length molecules, but this method of preparation hardly allows the control of the polymer chain length. To obtain well-controlled chain length from PPV precursor, one can either modify the parameters such as temperature or duration during the reaction or select the interesting part of the obtained molecules by a separation process. These methods give polymers with molecular weight in a relatively narrow range. In order to synthesize oligomers of defined formulae, we use classical organic chemistry methods. In the case of 4PV, we have to create a chain with five phenyl units bounded to four vinyl ones. This synthesis was done by associating one phenyl unit with two elements, each of them containing two phenyls bound to one vinyl. It consists of creating two double bonds between constituent units. This can be done with the Wittig reaction in which a czrbonyl group reacts with a phosphonium salt, giving the desired double bond. All the chemical products were purchased from Aldrich. Dichloro-para-xylene and triphenyl phosphine were used as reagents for the phosphonium salt. This salt is obtained by the following reaction using N,N-dimethylformamide (DMF) as solvent at 60 “C for 2 h with an excess of triphenyl phosphine: ClCH2-Q-CH&l + 2P& + 2Cl- + @P + CH+Z’-CH2P
+ @s
188
P. Le Rendu et al. /Synthetic
The salt was then washed with ether and dried at 80 “C under vacuum. The base used for the Wittig reaction was lithium hydride and the reaction is conducted using pure alcohol as solvent: base
Q,P=CH-@-CH=P@,
+ 2RCH0 --) RCH=CH-@CH=CHR
+ 2PcP30
When R is a-CH=CH-@, the 4PV is obtained after 2 h. The powder is filtered, washed with pure alcohol and dried. The obtained product is pulverulent and of bright yellow colour. The devices were obtained by thermal evaporation at room temperature under high vacuum conditions (about 10v6 Torr) of a bottom electrode on a cleaned glass substrate, followed by the oligomer powder evaporation with a rate of about 1 nm s-l. The top electrodes were then deposited without breaking the vacuum. Chromium was used for both top and bottom electrodes. Four samples of 2 X 2 mm* active area each were obtained on the same substrate. The thicknesses of the polymer films were measured by an Alphastep unit and they varied from 0.1 to 1 km. Optical measurements including UV-Vis, IR and Raman spectroscopies were performed on bare polymer films to control their quality. Electrical experiments were carried out with a set-up already described in our previous paper [7]. All measurements were performed under vacuum.
Metals
76 (1996)
187-190
diode. From results obtained in PPV-based diodes [ 71, it is suggested that injection of electrons has occurred from the top electrode into the oligomer film. Although the sample has identical electrodes at both sides of the polymer film, the asymmetric characteristics may indicate that the nature of the contacts is different following the method of formation. It would mean that, when the polymer is deposited on the metal electrode, the junction formed may be purely ohmic. In contrast, the metal layer on the polymer film would form apotential barrier leading to rectifying behaviour of the structure. This seems to be a suitable way to explain the experimental characteristic in the oligomer diode. In PPV devices [8], using the surface investigation method, we have pointed out that the rectifying behaviour of symmetrical diodes (for instance Cr-PPV-Cr structures) can be related to the different nature of the inter-facial layers. In addition, in asymmetric configurations, the measured current in geometrically identical samples does not show a clear dependence on the work function of the metal used for electrodes and this suggests that the real potential barrier at the metal-polymer contact may be governed by factors such as surface states or intermediate layers that could exist at the interface. However, the contact formation between the polymer and the metallic films needs to be investigated by further spectroscopic methods, such as photoelectron spectroscopy (XPS) or optical spectroscopies (IR, Raman), before we arrive at a definitive understanding of the charge injection mechanism. Assuming the thermionic emission at the potential barrier, the current density can be expressed as a function of the voltage by [ 91 J=J, exp(eV/@T)
3. Results and discussion 3.1. Current-voltage (I-V) characteristics
Fig. 1 shows the plot of the current density versus the applied field at ambient temperature. The forward current is obtained with negative bias of the top electrode. The curve is asymmetric and non-linear. The threshold field in the forward direction is about 1.5 X lo7 V m-l for the Cr top electrode
J Wcm2) 6 lo8:
-2 lo‘* / -3.0 10’
I -1.5 10’
I
I 1.5 10’
E C~h-4 Fig. 1. Current density vs. applied field for a Cr-4PV-0 K. The thickness of the polymer film is 0.6 km.
I 3.0 10’ diode at T=293
(1) where J is the current density, Jo the reverse leakage current density of the junction, e the electronic charge, V the forward voltage, 77the ideality factor, k the Boltzmann constant and T the absolute temperature. Jo is given by JO=A*p
exp( - e&,lkT)
(2)
where A* is the Richardson constant ( 120 A Km2 cm-* for the free electron model) and & the barrier height. The value of 77determined from the experimental curve is very high (about 66) ; therefore, true thermionic emission would not be considered here [ 91. It is known that an ideality factor superior to unity usually reflects the presence of an insulating layer between the metal and the semiconductor layers in a Schottky barrier [ lo]. The appearance of such a layer on the oligomer film is not obvious since the device was obtained in a vacuum chamber. It supports, however, the explanation of the asymmetry of the Z-V characteristics observed in symmetric oligomer diodes. The value of +, determined from the experimental curve is about 0.63 eV. Notice that this value was obtained assuming that the free electron model is applicable. In fact, the experimental Richardson constant is found in many cases to be inferior to the theoretical one [9], since the effective mass of the carriers
P. Le Rendu et al. /Synthetic
0
I 1
I 2
I 3
4
v lr2 (Volt’“) Fig. 2. Plot of the current vs. square root of the applied voltage for a Cr4PV-Cr diode at T= 293 K. The thickness of the polymer film is 0.6 km.
may differ from that of free electrons. Therefore, the barrier height could be smaller than the calculated value determined above. Fig. 2 shows the plot of the current versus the square root of the applied voltage at room temperature. For V> 1.5 V, a straight line is obtained. The linear variation of the current can correspond either to the Poole-Frenkel mechanism or to the Schottky emission [ 111. The Schottky-Richardson emission involves charge carrier injection from the electrode to the semiconductor via the field-assisted lowering of the potential barrier at the contact, The Z-V relationship is given by Z=A,T’
76 (1996)
189
187-190
3.2. Temperaturedependence
I(A) lo’*.~
10’14 0
Metals
exp{ [ &( V/d) 1’2- &,I /kT}
The dependence of the current on the temperature is shown in Fig. 3. The evolution of the current clearly shows two regions: below 275 K, the current depends slightly on the temperature; above 275 K, a sharp increase of the current occurs with increasing temperature. This observation shows that two different mechanisms are acting in the two temperature regions. At high temperatures, the process has a high activation energy which is typical for electronic conduction. The plot of the activation energy 4 versus the applied voltage is shown in Fig. 4. A straight line is obtained which intercepts the C$axis at +. N 1.9 eV. As the mechanism in this region is identified as the Poole-Frenkel emission, the value found represents the energy of the localized states with regard to the conduction band edge at the applied field. At temperatures below 275 K, the activation energy is very weak (about 0.01 eV) indicating that hopping of charge carriers probably occurs in this region [ 121. The results obtained here are in good agreement with those already reported in PPV films [ 71. We should notice however that the mean energy level of the localized states in the band gap of PPV is about 0.4 eV from the bottom of the conduction band. In the case of 4PV film, this value determined from Eq.
1(A)
(3)
1’2, A,, a constant, e the electronic where & = ( e314r& charge, V the applied voltage, d the thickness of the film, E the relative dielectric constant, co the dielectric constant of free space, k the Boltzmann constant and +s the contact potential barrier. For the Poole-Frenkel mechanism, the carriers are emitted by the lowering of the potential barrier from the localized levels in the semiconductor bulk into the conduction band. The current is given by
~1000.00000 d I..... . .. . . . c .*a*.................I.... ...... k
IO-”
1o”2 3
I
/
3.5
4
1 0.5
I 1
1
/
1
I
I
4.5 5 5.5 6 6.5 7 1000/r (K-g -.Fig. 3. Variation of the current as a function of the inverse of temperature for a Cr-PPV-Cr diode: (a) 3 V; (b) 6 V; (c) 9 V; (d) 12 V. The thickness of the polymer film is 0.6 pm.
(4) where B. is a constant and PPF = 2( e3/4rrrEEo) if2 = Zp,. Distinction between the two processes can be done by comparing the theoretical value of p with the experimental one obtained by calculating the slope of the curve Z= f( V”‘). The relative dielectric constant determined from capacitance measurements of the sample is about 2.5, which yields the following values of the constant /3: ,f3% = 4 X 1O-24 J V”2 m1’2 and PPF= 8.6 X 1O-24 J V”2 m*‘2. The slope of the straight line portion of the curve in Fig. 2 gives a value of 7.2 X 1O-24 J V”2 m1’2 for p, closer to &than ps. Therefore, it seems plausible to assume that carriers are transported through the polymer film by the field-assisted Poole-Frenkel mechanism.
0.4 0
I I I I / 1.5 2 2.5 3 3.5 4 vlD (VOP) Fig. 4. Plot of the activation energy vs. square root of the applied voltage for a Cr-4PV-0 diode. 0
P. Le Rendu et al. /Synthelic Metals 76 (1996) 187-190
190
I@) I
1 lo-lo
I
I
I
1
I
I
O-1 lo-IO-
conduction mechanism. An alternative process explaining the TSC current could be the release of charges from the interfacial layer [ 151, as already observed in some semiconductor devices. Further experiments are needed to elucidate the origin of the charge transport in this process.
4. Conclusions -4 lo-‘O-5 1O”O -6 do
I 170
I
I
I
I
/
I
190 210 230 250 270 290 310 TOO Fig. 5. Thermally stimulated current spectra for a Cr-4PV-Cr diode with different biases V,: (a) 3 V; (b) 6 V; (c) 9 V. The thickness of the polymer tilmis0.6~m;TH=293K;TL=100K;R=1.85Kmin-’. 150
(4) is much higher (about 1.9 eV) , suggesting that a deep energetic level is involved. A band-to-band emission process with a high activation energy could also be considered as already observed in other types of polymers [ 131. 3.3. Thermally stimulated current (TX)
In TSC measurements, the following procedure was adopted. At high temperature TH,the sample was biased at a given voltage V,. While maintaining the bias on, the sample was cooled down to low temperature TL.When thermal equilibrium was reached, the sample was short circuited, then the current was monitored upon heating the device at a constant rate R. Fig. 5 shows the TSC spectra of a 4PV sample at various V,. We can see that a current peak appears at 277 K for V, = 3 V (curve a) which shifts to higher temperature for higher bias (curves b and c) I On the other hand, the area delimited by the TSC curve, which represents the released charge, increases with increasing bias. The trap depth E, of the released charge can be evaluated using the varying heatingrate technique. It consists in recording the TSC spectra with the same applied bias and with different heating rates. The treatment proposed by Booth [ 141 gives a simple way to determine Ep We obtain via this technique a value of 0.42 eV for the energy level of the trapped charges. This value is close to that obtained by the same technique for PPV film but we should notice that, in this case, the involved energy corresponds to the localized states in the band gap of the polymer. For 4PV thin films, the energy found from the TSC technique differs significantly from that derived from the Z(r) characteristic, implying that the charge carriers are not the same in the two processes. The energy of the released charges from the TSC experiment shows that the traps are shallow and the contribution of the trapped charges is almost negligible in the
We have investigated the electrical properties of 4PV oligomer films by current-voltage-temperature characteristics and thermally stimulated current studies. The conduction mechanism is identified as field-assistedPoole-Frenkel emission for temperatures higher than 275 K, while, in the lowtemperature range, hopping of charge carriers between localized states with weak energy activation probably occurs. TSC results show that the trapped charge carriers with mean energy of 0.42 eV are different from those involved in the above processes. In addition to their remarkable optical properties [ 51, the possibility of obtaining thin films by vacuum deposition makes the studied oligomers very attractive for future electronic devices such as light-emitting diodes (LEDs) or fieldeffect transistors (FETs) on which we are now working.
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