J. Phys. Chem. 1996, 100, 20035-20042
20035
Charge Carrier Dynamics in Pulse-Irradiated Polyphenylenevinylenes: Effects of Broken Conjugation, Temperature, and Accumulated Dose Gerwin H. Gelinck and John M. Warman* Radiation Chemistry Department, IRI, Delft UniVersity of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands ReceiVed: July 10, 1996; In Final Form: September 26, 1996X
The transient conductivity resulting from nanosecond pulsed ionization of alkoxy-substituted phenylenevinylene/ethylidene copolymers, “dMOM-PPV(n)”, with n the fractional vinylene content, has been studied using the pulse radiolysis time-resolved microwave conductivity (PR-TRMC) technique. Minimum values of the sum of the charge carrier mobilities within the bulk solids, ∑µmin, have been estimated from the endof-pulse conductivity. For the freshly precipitated materials at room temperature, ∑µmin decreases gradually with decreasing n from 1.8 × 10-7 m2/(V s) for n ) 1 (full conjugation) to 0.4 × 10-7 m2/(V s) for n ) 0.57. After annealing dMOM-PPV(1) at 100 and 150 °C, ∑µmin at room temperature increased to 3.2 × 10-7 and 8.0 × 10-7 m2/(V s), respectively. No significant effect of high-temperature annealing was found for n e 0.87. On cooling dMOM-PPV(1) from 150 to -50 °C, ∑µmin decreased initially with an activation energy of approximately 0.07 eV but approached a plateau at the lowest temperatures. The after-pulse decay of the conductivity was disperse in all cases. First half-lives of several microseconds were found for n ) 1. The decay kinetics were independent of the dose in the pulse. Large accumulated radiation doses (up to 1.2 MJ/kg) did not effect the end-of-pulse conductivity but did increase the decay rate. This effect could be reversed by high-temperature annealing.
Introduction Opfermann1
first reported that polyIn 1970 Ho¨rhold and phenylenevinylene (PPV) was a moderately good organic photoconductor. Since then many reports have appeared in which the photoconductive properties of PPV and its derivatives have been investigated. The experimental methods applied can be divided into three categories: (1) continuous illumination with detection of the steady-state photocurrent,2-6 (2) laser flash photolysis with time-resolved detection of transient conductivity changes down to picosecond time scales,5-8 and (3) time-offlight (TOF) studies of charge carrier drift in an electric field.3,9-11 All of these methods have been applied to thin solid films. Recently, a modification of approach 2 was reported in which microwaves were used to detect conductivity changes on flash photolysis of dilute PPV solutions.12 This allowed measurements to be made of the photoconductive properties of isolated PPV chains. Using the “steady-state” approach, attention has focused mainly on the dependence of the photoconductivity on the wavelength, intensity, and polarization of the incident light as well as the influence of the electric field strength. Information on charge carrier dynamics, such as charge carrier mobilities and relaxation kinetics, is not readily obtained using this approach. Time-resolved conductivity measurements are capable of providing direct information on the relaxation kinetics of initially formed mobile charge carriers. A common finding in PPV derivatives, which has been substantiated by time-resolved optical absorption studies,13,14 is that the decay kinetics of charge carriers are invariably disperse, i.e., nonmonoexponential.4-6,12 In addition to kinetic information, estimates can be made of the product of the quantum yield for free charge carrier pair formation and the sum of the charge carrier mobilities, φp∑µ, * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)02051-5 CCC: $12.00
from the absolute magnitude of the conductivity transients. The values reported for PPV films using dc conductivity methods range from 2 × 10-6,8 via 6 × 10-7,5 to 8 × 10-8 m2/(V s).7 Values of φp∑µ ) 2 × 10-8 m2/(V s) for a gel suspension and 2 × 10-9 m2/(V s) for isolated polymer chains in solution have been obtained using the microwave technique.12 Since φp must be less than unity, φp∑µ represents the minimum possible value of the mobility. It has been suggested based on results for transpolyacetylene5,7,8 that the actual mobilities in the solid films may be as much as a factor of 100 higher; i.e., φp may be as low as 0.01. The TOF technique is in theory capable of providing a direct measure of charge carrier mobilities. Its application to mixed morphology materials such as solid PPVs is, however, fraught with difficulties. This is caused by the very disperse nature of the TOF transients observed.9,15 This is ascribed to charge carrier trapping during the course of interelectrode drift and to barriers to charge transport posed by domain boundaries. The mobility values, if obtainable at all, may therefore be considered to also be lower limits to the values which pertain at short times within well-organized monodomains of the material. Accordingly, the maximum TOF mobilities found for PPV or its derivatives are on the order of 10-7 m2/(V s) or less,3,10,11 i.e., considerably lower than the estimates from time-resolved conductivity measurements at short times based on φp ) 0.01. In the majority of TOF studies the hole has been identified as the major charge carrier for times on the order of a microsecond or longer after photoionization. In the work reported here, the dynamics of charge carriers in bulk solid samples of PPV derivatives have been studied using the pulse radiolysis time-resolved microwave conductivity (PRTRMC) technique. This method offers certain advantages over photo dc conductivity methods for studying bulk polymorphic materials. A (sub)nanosecond pulse of 3 MeV electrons produces a known, low concentration (100 µM or less) of ionization events which are close to uniformly distributed © 1996 American Chemical Society
20036 J. Phys. Chem., Vol. 100, No. 51, 1996
Gelinck and Warman the sample was monitored as a change in microwave absorption by the sample over the frequency range 27-38 GHz. For small changes the change in the reflected microwave power, ∆P, is directly proportional to the radiation-induced conductivity change, ∆σ,
∆P/P ) -A∆σ
Figure 1. Molecular structure of the polymers studied with n the fraction of vinylidene residues. For the “fully conjugated” polymer, dMOM-PPV(1), n g 0.98.
throughout the sample (“radiation doping”). Any resulting change in conductivity of the material is monitored as an increase in the attenuation of 30 GHz microwaves traversing the sample. The transient conductivity can be monitored from nanoseconds to milliseconds following a single ionizing pulse. The method requires no ohmic contacts and is relatively free from grain boundary and dielectric polarization effects. It is applied here to a study of charge carrier dynamics in bulk solid PPV materials in which the conjugation has been deliberately broken by a known fraction of saturated ethylidene residues. Experimental Section Materials. The molecular structures of the dMOM-PPV(n) compounds are shown in Figure 1. Their synthesis and characterization have been described previously.16 The molecular weights were determined by gel-permeation chromatography, which was calibrated using polystyrene standards in THF. The weight-average molecular weight, MW, of all polymers studied were in the range of 200 000-500 000 with a typical dispersion of 2-3. The average fraction of vinylidene residues, n, was determined from the 1H NMR spectral contribution of ethylidene group protons. For the “fully conjugated” polymer, denoted dMOM-PPV(1), NMR signals from the methoxyethylidene residues were not detectable indicating n g 0.98. Pulse Radiolysis. Approximately 30 mg of the materials was compressed by hand into a cavity with dimensions of 6 × 3 × 2 mm3 in a PMMA block using a close-fitting Teflon rod. The PMMA block was placed inside the microwave cell which consisted of a 14 mm length of gold-plated copper waveguide with internal dimensions 7.1 mm × 3.55 mm closed at one end with a metal plate, as described previously.17 For dMOM-PPV(1) and dMOM-PPV(0.87) more material was available and two cells were filled completely to a length of ca. 10 mm with ca. 150 mg of polymer. The cell was contained in a cryostat which was capable of covering the temperature range from -100 to 200 °C. The upper limit was reduced to 120 °C when using the PMMA sample holder. The materials were irradiated by single 5-50 ns pulses of 3 MeV electrons from a van de Graaff accelerator. The penetration depth of the primary 3 MeV electrons is ca. 15 mm in materials with a density of ca. 1 g/cm3, such as dMOM-PPV(n). This is much larger than the sample thickness of 3.5 mm which ensures close to uniform energy deposition within the material.17 The energy deposited in the pulse, D, was accurately known and equal to 0.54Q Gy, i.e., 540FQ J/m3 with F the density of the material in g/cm3 and Q the beam charge in nC which was routinely measured. The maximum initial concentration of e-h pairs produced was approximately 100 µM, i.e., approximately 3 × 10-5 ionization events per phenylenevinylene unit. Microwave Detection. After the generation of charge carriers by pulse radiolysis, any change in the conductivity of
(1)
The sensitivity factor A can be calculated as described previously and used to derive the absolute magnitude of the radiationinduced conductivity.17-19 Transient changes in the output of the microwave detector diode on a nanosecond time scale were detected using a Tektronix SCD 1000 digital oscilloscope (time resolution ca. 1 ns) to accurately determine the conductivity at the end of the pulse. The decay kinetics of the conductivity were studied by monitoring the output of the microwave detector using a tandem combination of a Tektronix 2205 oscilloscope with a 7A13 plugin and a Sony/Tektronix RTD 710 digitizer capable of monitoring data points from 10 ns to 1 ms on a pseudologarithmic time base following a single pulse. The radiation-induced conductivity changes were on the order of 10-3 S/m or less. The data are discussed in terms of the conductivity change normalised by the dose in the pulse, ∆σ/D in units of S m2/J. The PR-TRMC technique is more fully described elsewhere.17,19,20 Radiation Damage. The effect of large accumulated radiation doses on the conductivity transients was studied in the following way: Transients were measured for the fresh sample using as few single pulses as possible. The sample was then subjected to repetitive pulsing at 10 Hz depositing approximately 10 J/kg per pulse for a given length of time. Single-shot conductivity measurements were then repeated before applying a further train of repetitive pulses, and so on. After clear signs of radiation damage were observed in the form of a change in decay rate of the transient conductivity, the effect of annealing was studied by heating to 150 °C and recooling to room temperature. The maximum accumulated dose used was 1.2 MGy ()1.2 × 109 J/m3 for a 1 g/cm3 material) corresponding to an energy deposition of 3.6 eV per PV unit. The Conductivity/Mobility Relationship. The end-of-pulse conductivity per unit dose absorbed in the sample, ∆σeop/D, is related to the sum of the mobilities of the charge carriers present, ∑µ, by17,19,21
∆σeop/D ) Feop∑µ/Ep
(2)
In (2), Ep (the “pair formation energy”) is the average energy absorbed per initial ionization event for high energy electrons and Feop is the fraction of initially formed charge carrier pairs that survives geminate recombination and is still present at the end of the pulse. Since Feop has a maximum value of 1, minimum values of ∑µ can be determined if Ep is known from
∑µmin ) Ep[∆σeop/D]
(3)
Ep can be estimated using the relationship to the band-gap energy, Eg, derived by Alig et al.,22
Ep ≈ 2.73Eg + 0.5
(4)
Relationship 4 is found to be applicable over the range from inorganic semiconductors with band gaps of only approximately 1 eV to insulating, saturated hydrocarbon liquids with band gaps larger than 8 eV. If a medium consists of several molecular components the fraction of energy deposited initially in a particular component
Charge Carrier Dynamics in Polyphenylenevinylenes
J. Phys. Chem., Vol. 100, No. 51, 1996 20037 TABLE 1: Effect of Annealing on the Radiation-Induced Conductivity Characteristics of dMOM-PPV(n) Derivativesa n
∆σeop/D annealing ∑µminc τj,maxd temp (°C)b (×10-8 S m2/J) (×10-7 m2/(V s)) (ps)
1e 0.96 ( 0.03 0.87 ( 0.03 0.63 ( 0.07 0.57 ( 0.07
100 150 100 f 100 f 100 f 100 f
Figure 2. Dose-normalized radiation-induced conductivity transients obtained at room temperature on freshly precipitated polymers: dMOMPPV(1) (open circles), dMOM-PPV(0.87) (filled triangles), and dMOMPPV(0.57) (squares) using a 20 ns pulse.
n is proportional to its relative electron density Z(n). If the components have different Ep values, the overall average value of Ep will be given by
〈Ep〉 ) {∑[Z(n)/Ep(n)]}-1
(5)
For example, in the present materials we can differentiate between the alkoxy side chains with an Ep value of ca. 24 eV (Eg ≈ 8.5 eV23) and Z(n) ) 87/139, and the polymer backbone with an Ep of ca. 6 eV (Eg ≈ 2.1 eV for a dialkoxy-substituted PPV chain24) and Z(n) ) 52/139 per PV unit. An 〈Ep〉 value of 12 eV is therefore derived. Use of this overall value to calculate ∑µmin values which refer just to charge carriers on the polymer chains is considered to be justified since charge carriers formed initially within the side chain regions would be expected to migrate rapidly to the backbone in view of its much lower ionization potential (4.7 eV24 compared with 8.5 eV23) and much larger electron affinity (2.6 eV24 compared with 0 eV23). The value of the parameter Feop, which determines the relationship between the actual mobility sum and ∑µmin, i.e., ∑µmin/Feop will be discussed in the next section. Results and Discussion All of the materials studied showed a transient increase in conductivity on pulsed irradiation, indicating the formation of mobile charge carriers. This is illustrated by the traces in Figure 2. The transients are characterized by a “fast”, in-pulse component and a “slow” component which decays on a time scale much longer than the pulse length. The fast component is in general small and is thought to be mainly spurious since similar transients are observed in the absence of the polymer sample. We cannot, however, completely rule out the possibility that part at least of the fast component arises from initiallyformed, mobile charge carriers which undergo subnanosecond localization or geminate recombination. In what follows we will first present and discuss the values of the charge carrier mobilities that can be estimated from the absolute magnitude of the end-of-pulse conductivity. We then turn to the after-pulse decay kinetics of the conductivity before finally presenting data on the effects on the conductivity transients of very large accumulated doses of radiation. Mobility. In Table 1 are listed the values of the dosenormalized end-of-pulse conductivity, ∆σeop/D, found at room temperature for the different samples studied together with the corresponding minimum mobility values, ∑µmin, using a pair formation energy of 12 eV derived as described in the previous
1.5 2.7 6.7 1.1 1.5 0.9 0.9 0.3 0.4 0.3 0.3
1.8 3.2 8.0 1.3 1.8 1.1 1.1 0.4 0.5 0.4 0.4
18 10 4 24 18 29 29 80 64 80 80
a All experimental values were measured at room temperature. b The samples were kept at the annealing temperature for approximately 45 min. c Calculated from ∆σeop/D using eq 3 and Ep ) 12 eV. d Calculated from ∑µmin using equation 6 and a jump distance of 7 Å. e Nominal “fully conjugated polymer”; could contain up to 2% ethylidene residues and/or 2% branches. f Limited by PMMA holder.
section. The values of ∆σeop/D refer to the conductivity level immediately after the pulse, i.e., the end-of-pulse height of the slowly decaying component. There are reasons to believe that ∑µmin is in fact probably close to the “true” value of the mobility sum, ∑µ. In other words, the end-of-pulse survival probability, Feop, is expected to be close to unity in the present materials. This is based on the following considerations: Most of the ionization events induced by high-energy radiation lead to secondary electrons with considerable excess kinetic energy. As a result, the average separation between an electron and its sibling hole after thermalization in condensed organic materials is 30-40 Å.25 This is much farther than the distance between neighboring polymer chains. The hole and its associated electron will therefore have a high probability of becoming localized on different, well-separated polymer backbones. In a recent flash photoconductivity study of a dMOM-PPV(1) gel, it was found that charge carrier pairs which are formed by interchain charge transfer have lifetimes of hundreds of nanoseconds.12 Most pairs formed by high-energy radiation would therefore be expected to have lifetimes well in excess of the nanosecond pulse lengths used. The end-of-pulse survival probability, Feop, should therefore be close to unity. The room temperature value of ∑µmin for a freshly precipitated sample of the fully conjugated polymer was found to be 1.8 × 10-7 m2/(V s). This increased considerably, however, when the sample was annealed at an elevated temperature. After heating to 150 °C for example, ∑µmin increased to 8 × 10-7 m2/(V s). A substantial increase in ∑µmin at room temperature was found even after a fresh sample had been annealed at only 100 °C. No indication was found of a specific temperature, corresponding for example to a solid to liquid crystalline phase transition, above which annealing suddenly became operative. The substantial increase in ∑µmin on annealing at high temperatures is a clear indication of the importance of the morphology of the material in charge transport. An X-ray diffraction study of a similar PPV derivative has shown that annealing at high temperatures increases the degree of crystallinity of the sample and enhances chain alignment.26 It cannot be decided, however, whether the annealing effect on the mobility observed in the present experiments results from better intramolecular alignment of the conjugated backbones or better π-π coupling between neighboring chains. Probably it is a combination of both. The value of ∑µmin ) 8 × 10-7 m2/(V s) for dMOM-PPV(1) is considerably larger than the value of 1.3 × 10-7 m2/(V s)
20038 J. Phys. Chem., Vol. 100, No. 51, 1996 found using the PR-TRMC technique for an analogous annealed poly(p-phenylene) (PPP) derivative27 but compares well with an average value of ∑µmin of 7 × 10-7 m2/(V s) found for poly(3-alkylthiophenes).28 It is, however, still approximately 3 orders of magnitude less than the ∑µmin values, in the range 2 to 20 × 10-4 m2/(V s), which have been determined using the present technique for polycrystalline samples of polydiacetylene derivatives.29,30 The mobility for dMOM-PPV(1) is also considerably lower than the ∑µmin values on the order of 10-5 m2/(V s) found for the crystalline solid phase of self-organized columnar aggregates of discotic aromatic molecules.19,31 A comparison of the present results with previous photoconductivity studies can best be made with those which also involved the use of fast, time-resolved techniques. The parameter determined in those experiments was φp∑µ with φp the overall quantum yield of charge carrier pairs. The term φp∑µ in photoconductivity studies is therefore analogous to ∑µmin in the present work. In a picosecond study of unsubstituted PPV a value as high as φp∑µ ) 6 × 10-7 m2/(V s) has been found.5 An alkoxy-substituted derivative, MEH-PPV, however, displayed a conductivity transient which was reported to be at least 1 to 2 orders of magnitude smaller,4,5 i.e., in the range of 10-8-10-7 m2/(V s). In nanosecond time-resolved flash photoconductivity studies of aligned, unsubstituted PPV values of φp∑µ ) 8 × 10-8 7 and 2 × 10-6 m2/(V s)8 parallel to the direction of alignment of the polymer backbones have been reported. Our present result of ∑µmin ) 8 × 10-7 m2/(V s) for an annealed bulk sample of dMOM-PPV(1) is seen to lie within the range of φp∑µ values determined in the flash photoconductivity studies. In the latter, however, charge carriers are formed in the first instance on a single chain as either local excitations or electron-hole pairs. The quantum yield for heterogeneous exciton dissociation and the probability of escape from rapid geminate recombination, both of which contribute jointly to φp, could be considerably less than unity in contrast to the value of close to unit thought to be appropriate for the analogous parameter Feop in the present work. In the absence of a measured value of φp for PPV derivatives, it has frequently been assumed that this parameter is similar to the value of 0.01 estimated for polyacetylene. If this value is applied to the φp∑µ values given in the previous paragraph, then mobilities are derived which are all substantially larger than the PR-TRMC value of ∑µmin. Possible reasons for the difference are discussed below. The most obvious sources of error in the derivation of mobilities from the conductivity data are the yield parameters φp and Feop. We have put forward arguments above for concluding that Feop probably does not differ significantly from unity and hence that ∑µmin is probably close to ∑µ. We consider that an absolute minimum value of Feop would be 0.3 and hence that ∑µ could not be larger than ∑µmin by more than a factor of 3. The parameter φp, as mentioned above, is taken from experiments on a different material and it is possible that the value for PPVs is in fact significantly larger. This would reduce the values of ∑µ estimated from the photoconductivity results. In addition to the uncertainties in the yield parameters there could also be morphological differences between the materials investigated which could underly differences in the charge transport properties. For example, the present dMOM-PPV(1) material was a precipitated bulk solid whose properties could be changed by annealing and we cannot be certain that the maximum degree of order possible was achieved by annealing at 150 °C. The photoconductivity experiments, on the other hand, were carried out on thin layers prepared in situ from a
Gelinck and Warman
Figure 3. Dose-normalized radiation-induced conductivity transients obtained at room temperature for freshly precipitated dMOM-PPV(1) before and after annealing for 45 min at 150 °C.
prepolymer. Also, the absence of alkoxy groups for the unsubstituted PPV materials could allow better chain ordering and more rapid chain-to-chain diffusion. The measurement of the conductivity in the direction of the chains for aligned samples might also result in larger mobility values if intrachain diffusion is the dominant transport mechanism. Direct determinations of the individual charge carrier mobilities in PPV derivatives by the time-of-flight (TOF) technique have been attempted but without general success.3,9-11 The TOF measurements invariably display very disperse displacement currents with only slight evidence, if any, of Gaussian transport with a well-defined drift time in any of the samples studied. This can be explained by the complex morphology of the materials and the lack of monodomain samples with dimensions of tens of microns. The TOF mobility estimates which have been made are invariably lower than the value of 8 × 10-7 m2/(V s) found in the present work. This is to be expected since the bulk drift of charge carriers necessary to traverse the interelectrode gap in a TOF cell will be mainly controlled by barriers to charge transport presented by domain boundaries within the material, even if spurious effects due to electrode contact problems and medium polarization have been eliminated. Using the TRMC method, electrode contact problems are absent as are effects due to dielectric polarization and fieldinduced drift to domain boundaries. The maximum field strengths of only a few volts per centimeter and the ultrafast reversal time of tens of picoseconds ensure that what is monitored is only a small perturbation of the otherwise random diffusional motion of the charges. The nanosecond time scale of the measurements ensures that the charge carriers are usually probed prior to their diffusional drift to intrinsic chemical or physical trapping sites. Processes such as thermalization and polymer backbone relaxation will, however, occur on a much shorter time scale so that the carriers observed should be fully relaxed polarons whose diffusional motion is controlled by the transport of both charge and the associated lattice distortion. The very low average concentration of charge carriers formed in the pulse ensures that secondary interactions to form bipolarons cannot occur on a time scale of nanoseconds. The temperature dependence of the mobility for dMOM-PPV(1) is complicated by the pronounced annealing effects mentioned above. The Arrhenius-type plot shown in Figure 4 is therefore for a cooling trajectory from 150 to -50 °C. The temperature dependence resembles that found for the “slow” component of φp∑µ in the picosecond photoconductivity study of unsubstituted PPV,5 i.e., a close to temperature-independent behavior at the lowest temperatures but an increase at higher temperatures. The activation energy corresponding to the
Charge Carrier Dynamics in Polyphenylenevinylenes
J. Phys. Chem., Vol. 100, No. 51, 1996 20039
Figure 4. Temperature dependence of the end-of-pulse conductivity of dMOM-PPV(1) obtained on cooling from 150 °C. The dotted line corresponds to an activation energy of 0.07 eV.
dashed line in Figure 4 is 0.07 eV which is slightly lower than the value of ca. 0.1 eV found in the photoconductivity work. The general, qualitative agreement between the two techniques suggests that this temperature behavior is a fundamental characteristic of the motion of polaronic charge carriers in PPV and its derivatives. The pronounced negative influence of broken conjugation on the radiation-induced conductivity is shown by the ∑µmin values in Table 1 and is illustrated by the actual conductivity transients for n ) 1, 0.87, and 0.57 in Figure 2. The difference between the fully conjugated polymer and the n < 1 materials is accentuated after annealing since the compounds with the smaller n values are much less affected by this procedure. Because of this, the mobilities for the n ≈ 0.6 compounds are a factor of 20 lower than that for the 150 °C annealed dMOMPPV(1). A strong negative influence of the conjugation length on the photoconductivity was also observed by Shen et al.8 The much lower mobility for the low n materials was as expected and can readily be ascribed simply to a considerably reduced intrachain diffusional motion caused by the barrier presented by the saturated ethylidene units. The chain bends induced by the ethylidene segments could in addition result in a less well-organized morphology which could also play a role in reducing the overall rate of diffusional charge migration via interchain transfer. The TRMC technique is incapable of differentiating the individual contributions of the negative and positive charge carriers. Most TOF investigations have, however, identified the hole as the major carrier in PPV compounds. While the positive charge will be delocalized over several phenylenevinylene units, it is nevertheless of interest to apply a simple site-to-site hopping model to charge motion which results in an effective jump time, τj. This is related to the mobility by
τj ) edj2/6kBTµ (s)
(6)
where dj is the jump distance. The values of ∑µmin in Table 1 have been used to derive the values of τj in Table 1 based on the assumption that only intrachain motion with a jump distance of 7 Å, i.e., the intrachain distance between PV units, is operative. As can be seen, for the 150 °C annealed dMOM-PPV(1) sample, the jump time is a few picoseconds and for the lowest n value compounds it extends to several tens of picoseconds. Interchain hopping could also contribute to the overall diffusional motion of charge, particularly in zones where aggregation results in strong π-π interaction between neighboring backbones. For columnarly stacked discotic molecules, charge migration occurs only via such π-π interactions between neighboring aromatics and intermolecular jump times of a few
picoseconds or less have been found to be operative.32 Future experiments on aligned materials should help to elucidate the relative magnitudes of intra- and interchain diffusion in the present polymeric compounds. Conductivity Decay Kinetics. The transient radiationinduced changes in the conductivity of freshly precipitated and 150 °C annealed dMOM-PPV(1) are shown in Figure 3 for times up to 1 ms after the pulse. As discussed in the previous section, the increase in conductivity on annealing is attributed to an increase in the degree of crystallinity and enhanced chain alignment within the sample as evidenced by an X-ray diffraction study of a related compound.26 In addition to the lower end-of-pulse value found for the fresh sample, the decay over the first ca. 100 ns is seen to be more rapid than after annealing. For longer times, however, the forms of the decays closely resemble each other. The results can be explained if only a small part of the freshly precipitated material is in the form of well-organized domains with the remainder consisting of disorganized, amorphous regions which apparently can reorganize on heating. In the latter regions, defects and trapping sites would be expected to be more abundant. This would explain both the lower end-of-pulse conductivity and the faster initial decay over the first 100 ns found for the fresh sample. Changing the dose by a factor of 6 for the annealed dMOMPPV(1) sample was found to have no appreciable effect on either ∆σeop/D or the decay kinetics. If the decay had been due to second-order interactions between freely diffusing carriers, as for example in homogeneous recombination between oppositelycharged carriers or the condensation of like-charged polarons to form lower mobility bipolarons, then a more rapid decay for the higher dose (higher charge carrier concentration) would have been observed. The lack of a dose dependence is of some importance since bipolaron formation has been suggested to occur in PPV materials subsequent to photoionization.4 We conclude that the decrease in conductivity in dMOMPPV(1) is most probably due to localization of the mobile charge carrier(s) at intrinsically present trapping sites. Whether the defects responsible for trapping in the annealed sample are of a chemical or a physical nature, however, cannot be definitely decided. An estimate of the average distance that a charge carrier diffuses prior to trapping, RT, can be obtained from RT ≈ (6t1/2µkBT/e)1/2. For a half-life of 5 µs and a mobility of 8 × 10-7 m2/(V s) RT is found to be ca. 800 nm. This is somewhat larger than the few hundred nanometer thickness of the active polymer layer in molecular electronic devices. As indicated above, the fact that the initial more rapid decay for the fresh sample can be removed by annealing indicates that structural defects are important trapping sites in the unannealed material. The first half-life of the conductivity is found to decrease substantially, e.g., from 5 to 0.4 to 0.2 µs for n ) 1, 0.87, and 0.57. This may indicate that the role of structural defects as trapping sites becomes more important as the degree of conjugation decreases since we have no reason to believe that chemical defects would be directly related to n. In connection with possible second-order effects mentioned in a previous paragraph, it should be pointed out that the concentrations of charge carriers formed in the present pulseradiolysis experiments are orders of magnitude lower than those prevailing in laser photoconductivity studies of thin layer samples. Because of the high penetrating power of the 3 MeV electrons used (15 mm in 1 g/cm3 density materials), the initial charge carrier concentration is only approximately 6 × 1022 m-3 (10-4 M) even for the largest doses used, 105 J/m3. This corresponds, on the average, to approximately 3 × 10-5
20040 J. Phys. Chem., Vol. 100, No. 51, 1996
Gelinck and Warman
ionizations per PV unit throughout the bulk of the sample. In flash-photolysis experiments similar total amounts of energy to that in an electron pulse, i.e., on the order of millijoules, are absorbed within a layer only a few microns thick. This results in energy densities on the order of 107 J/m3 and correspondingly orders of magnitude higher concentrations of excited and ionic species with a concomitant increase in the rates of second-order interactions. In a recent publication we reported microwave conductivity transients obtained on flash photolysis of dMOM-PPV(1) as a dilute solution of isolated polymer chains and as a partially aggregated gel suspension. A much larger and more slowly decaying transient photoconductivity was found for the gel, t1/2 ≈ 200 ns compared with 30 ns for the solution. The t1/2 value of 5 µs found for the bulk solid in the present work is in turn considerably longer than that for the gel. This can be attributed to the larger average distance between the ionization events when using high-energy radiation. Thus, in the flash-photolysis experiments ca. 1% of the PV units were excited compared to only 3 per 105 PV units in the pulse-irradiated solid. As a result of the much higher concentration on flash photolysis, the decay of the photoconductivity for the dMOM-PPV gel suspension was in fact found to be controlled mainly by second-order combination reactions between the charge carriers.12 The decay of the radiation-induced conductivity is very disperse, i.e., nonmonoexponential, for all samples and under all conditions. In Figure 5 are shown transients taken at different temperatures for the 150 °C annealed sample of dMOM-PPV(1). While the first half-life of the conductivity remains within the range of 2-5 µs, the form of the decay is seen to change considerably. In particular, the initial rate of decay appears to decrease as the temperature is raised. This would be in accord with a mechanism involving a distribution of trap depths with localization occurring at increasingly deep traps as time goes on as has been proposed to explain disperse conductivity decays found in dc experiments on PPV3,9 and many other polymeric matrices.33-36 At elevated temperatures localization in shallow traps, responsible for the initial decay at low temperatures, becomes inoperative because of the rapid thermal detrapping of carriers. At the lowest temperature of -50 °C the close to linear dependence of the log-log representation would suggest an inverse power law dependence to apply while at 150 °C the form resembles more that of a stretched exponential. We are at present investigating empirical forms which can incorporate both types of decay. One of these involves the use of a timedependent rate coefficient for the disappearance of charge carriers given by (7)
dNp/dt ) -ANp/[1 + Bt]n
(7)
which for n ) 1 gives
Np(t) ) Np(0)/[1 + Bt]A/B
(8)
and for n * 1,
Np(t) ) Np(0) exp{-A[(1 + Bt)(1-n) - 1]/(1 - n)B} (9) Equation 9 was used to calculate the empirical fits to the data shown in Figure 5. This aspect of the work is ongoing and will receive more attention in a subsequent publication. Effect of Accumulated Dose. Experimental studies of the transient radiation-induced conductivity of samples are carried out in the single-pulse mode with only 4-16 pulses usually being averaged to improve the signal to noise ratio. In the
Figure 5. Decay kinetics of dose-normalized radiation-induced conductivity transients obtained at different temperatures for 150 °C annealed dMOM-PPV(1). Also shown are the fits using eq 9.
present work the maximum dose per pulse used was 100 Gy (ca. 105 J/m3) for a 50 ns, 4 A pulse and the maximum accumulated dose during a series of measurements on a single sample was on the order of 20 kGy or less. For these total dose conditions no appreciable deterioration of the magnitude or decay kinetics of the conductivity transients over the course of the measurements could be detected. Because of the great importance attached to the degradation of polymeric materials in device applications, it was decided to carry out a study of the effect of much larger accumulated radiation doses on the conductivity transients. Accordingly, a sample was irradiated up to a total dose of 1.2 MGy ()1.2 × 109 J/m3) with single-shot transients being taken at intermediate doses. The maximum dose corresponded to an average energy deposited per PV unit of 3.6 eV. The results are shown in Figure 6. As can be seen the value of ∆σeop/D was unaffected by accumulated dose even up to the maximum. The decay of the conductivity was, however, much more sensitive with the first half-life decreasing by a factor of approximately 2 for a dose of 50 kGy. The first and second half-lives are plotted against accumulated dose in Figure 7. For the largest dose the first half-life was reduced by close to an order of magnitude and the second half-life was reduced by even more.
Charge Carrier Dynamics in Polyphenylenevinylenes
Figure 6. Radiation-induced conductivity transients for 150 °C annealed dMOM-PPV(1) as a function of accumulated dose.
J. Phys. Chem., Vol. 100, No. 51, 1996 20041 to a factor of 20 for 40% ethylidene residues. This is attributed mainly to a decrease in the rate of intrachain charge transport caused by the σ-bonded linkages. A detrimental effect of sp3 hybridization on chain alignment and interchain π-π interactions may, however, also play a role. The first half-life of the transient conductivity for the fully conjugated polymer is 2-5 µs. In all cases the decay of the conductivity is disperse and is attributed to increasingly deep trapping of mobile charge carriers at intrinsic structural and/or chemical defects. The conductivity decays more rapidly for the less well-conjugated materials probably due to greater structural disorder. No evidence is found for second-order processes such as homogeneous recombination or bipolaron formation being responsible for the decrease in conductivity. A generalized decay equation which incorporates both inverse power and stretched exponential time dependences is shown to fit the data quite well. For very high accumulated doses, up to 1.2 MGy or 3.6 eV per phenylenevinylene unit, the end-of-pulse conductivity remains unchanged. The first half-life however, decreases by an order of magnitude. The latter effect is attributed to the build up of defect-trapped radical ion sites which function as recombination centers for subsequently formed mobile charge carriers. These can be removed by thermally-induced recombination at high temperatures. The charged defects can, however, be completely removed by thermally-induced detrapping leading to recombination at higher temperatures. Acknowledgment. Materials were kindly provided by Dr. Emiel G. J. Staring from Philips Research Laboratories, Eindhoven, The Netherlands.
Figure 7. First (circles) and second (squares) half-lives for dMOMPPV(1) as a function of accumulated dose. Also shown as filled symbols are the first and second half-lives of the sample after annealing at 150 °C subsequent to a total accumulated dose of 1.2 MGy.
These effects were found to be completely reversed if the sample was annealed at 150 °C as shown by the lifetime values in Figure 7. This result indicates that the defects produced do not involve permanent chemical change such as the formation of carbonyl groups as suggested to explain degradation of samples on photolysis. The most likely explanation is the gradual build up of trapped charge carriers at intrinsic defect sites which at sufficiently high concentration begin to act as recombination centers for the carriers produced in subsequent single pulses. These “electronic” defects can then be annealed out by inducing their own recombination at a high enough temperature in an analogous way to that occurring in thermoluminescence studies of irradiated solids. Conclusions The radiation-induced conductivity of freshly precipitated, fully conjugated dMOM-PPV can be increased by close to a factor of 5 by annealing at 150 °C. This is attributed to an improvement in chain alignment and an increase in organized domains. The (pseudoisotropic) mobility of charge carriers in an annealed sample at room temperature is estimated to be close to 8 × 10-7 m2/(V s). This is the minimum value determined from the end-of-pulse conductivity, ∑µmin, and corresponds to an intrachain jump time of ca. 5 ps. The mobility is only weakly thermally activated, increasing from a low temperature plateau value of 5 × 10-7 m2/(V s) at -50 °C to 15 × 10-7 m2/(V s) at 150 °C with an activation energy of 0.07 eV at the highest temperatures. The presence of saturated ethylidene units in the backbone decreases markedly the radiation-induced conductivity, by up
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