ion

J. Phys. Chem. 1993,97, 944-951

944

Photocurrent Generation and Cbarge Transport in SnOz/Ion-Exchange Polymer-ZnTPP/Au Cells M. F. Lawrence,' Z. Huang,t C. H.Langford, and I. Ordoned Department of Chemistry and Biochemistry, Concordia University, 1455 de Maisonneuue W., Montreal, Quebec, Canada H3G 1M8 Received: February 26, I992

A variety of experiments have been performed to investigate electrical conductivity in a system composed of zinc meso-tetraphenylporphyrin (ZnTPP) incorporated in an ion-exchange polymer. This polymer is a blend of poly(4-vinylpyridine-co-styrene) and a random ternary copolymer with cationic sites. Thin films of this mixture were cast from solution onto SnO2 optically transparent electrodes, and conductivity measurements were then made possible by the evaporation of gold contacts on top of the films. The variation of dark current as a function of applied voltage, observed using polymer films with or without ZnTPP, follows the behavior expected of an insulator containing traps. Analysis of the space-charge limited currents in these samples enables the determination of a total trap density of Nt = 3 X lOI4~ m and - ~a trap depth Et of -0.73 eV. The addition of dye molecules to the polymer blend is found to have little effect on the density of deep traps as well as on to 4 X the charge mobility which varies from 3 X cm2 V-' s-'. The temperature dependence of the dark conductivity, however, shows that the activation energy of the conduction process changes from a A E of -4 eV for the polymer alone, to a L!?of - 2 eV for the polymer4ye system, in the same temperature range. Photoconductivity measurements performed in the visible on the polymer4ye system clearly indicate that light absorption by ZnTPP is responsible for the observed photocurrents. The response times of the photoinduced currents, during a light-on/light-off sequence, are characteristic of systems in which conductivity is strongly influenced by the trapping and recombination of charge carriers. The isothermal time dependence of photocurrent decay indicates the existence of an isolated narrow distribution of traps with an Et of -0.78 eV. Arguments are brought forth which indicate that these trapping centers may correspond to pyridinium sites present in the ion-exchange polymer. From these observations, a discussion concerning the energetic relation between the individual components involved and the effect of traps on the photogenerated charge carriers is presented to account for the photoconductor's overall behavior.

Introduction It was approximately40 years ago that the first detailed studies of the electrical and photoelectrical behaviors of organic and inorganic molecular solids began.l-8 In large part these were aimed at getting a better understanding of important photochemical processes such as photosynthesis and vision and also to promote their use as photographic sensitizers. It is also not many years ago that interest in the electrical properties of polymers was effectively limited to their electrical insulating ability. However, this situation has changed remarkably, and the molecular engineering of chemical structures and molecular assemblies has since led to applicationssuchas photovoltaicenergy conversion, photocatalysis, electrocatalysis, and electrophotography. One of the major reasons for this lies in the great versatility of molecular structures which allow for modification of a system's architecture through chemical synthesis. Of particular interest here are developments concerning the use of an ion-exchange polymer enabling the modification of electrodes to produce surfaces with a high affinity for redox This polymer consists of a random ternary copolymer containing two types of hydrophilic cationic groups and one hydrophobic styrene group, as shown in Figure 1B, which is blended with poly(4-vinylpyridine-co-styrene), shown in Figure 1C. Studies made using this polymer, referred to as polyXI0, have led to interesting results with respect to the fabrication of synthetic organic systems capable of photoinduced charge separation. The main feature of this type of polymer blend is its spontaneous tendency to segregate into hydrophilic and hydro-

* Address correspondence to this author.

' Present address: INRS-Energie, C.P. 1020, Varennes, Quebec, Canada

J3X 1S2. Present address: Institute for Toxicology, University of Southern California, 1985 Zonal Ave., Los Angeles, CA 90033. f

C

0

'

&(Et)3 CI-

L&(EtOH)3 CI-

Figure 1. Molecular structures of (A) zinc meso-tetraphcnylporphyrin, (B) random ternary copolymer, and (C) poly(4-vinylpyridine-eo-styrene).

phobic domains following the casting of films from methanol solution onto substrates. These films provide the dual advantage of providing a transparent, highly adherent support for the immobilization of chromophores at the surface of semiconductor electrodes for the purpose of photosensitization (TiO2, Sn02, CdS, ...), while maintaining high diffusion coefficients for anionic redox specieswhen these electrodesare used in an electrochemical (solution) environment. The photoelectric behavior of Sn02 optically transparent electrodes (OTE) coated with polyXI0, to which soluble dye molecules have been added (porphyrins, phthalocyanines), was previously studied under electrochemical conditions (hexacyanoferrate[II/III] aqueous electrolyte, Pt counter electrode).I2 Upon illumination of these systems in the visible, anodic short-

0022-3654/58/2097-0944S04.00/0 Q 1993 American Chemical Society

The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 945

Photocurrent Generation and Charge Transport I

I

I

I

I

I I

+--I

POLYMER

I

Figure 2. Schematic representation of the photoinduced electron flow originating at the irradiateddye-polyXIO modified SnOz in contact with a F c ( C N ) ~ ~ -redox / ~ - couple. The insert shows the photocurrent-time reponse Observed with this system during a light-on/light-offsequence.

circuit photocurrents generated through light absorption by the chromophore, are believed to proceed via both “wet” (ionic) and “dry” (electronic) conduction mechanisms within the polymer (Figure 2). However, preliminary measurements have also shown that the dye-polyXI0 system’s photocurrent-time responses, in the solid state, were parallel to those obtained with the wet (photoelectrochemical) cells.I2 The time constants associated with the rise and decay of photocurrents for wet cells are not experimentallydistinguishablefrom those obtained with dry cells, which indicates that the kinetics of the overall photocurrent generation process are being controlled by the polymer itself rather than by the efficient ionic transport characteristic of the film. To identify and possibly eliminate the cause of this behavior, an investigation of charge transport in the absence of solution electrolyte within these films has been performed. In an attempt to better characterize the conduction of photogenerated charges in this type of system, this article presents a detailed investigation of the dark conductiveand photoconductive behavior of SnOz electrodes coated with polyX10 containing ZnTPP (Figure 1A). The following results provide more insight concerning the energetic relation between the principal components involved, and their role in charge carrier photogeneration, migration, and recombination. This investigationand the analysis presented in the followup paper dealing with recombination processes are also aimed at clarifying certain aspects of photocurrent generation and charge-transport behavior common to disordered organic systems in general. Finally, it is hoped that this paper demonstrates how the adoption of the concepts and methods of solid-state physics, and their application from a physical chemistry perspective, provide effective means to characterize the electrical and photoelectrical properties of complex molecular systems.

Experimental Dewls The preparation of the random ternary copolymer has been described previously. Elemental analysis performed on the final product used in this study gave the following weight percentages: C (69.37%), H (8.82%), N (4.38%), C1(10.37%), and 0 (7.06% by difference), which agree reasonably well with the theoretical percentages of C (69.20%), H (8.57%), N (4.24%), C1(10.76%), and 0 (7.23%) that one expects for a 1:l:l ratio of the three different pendant groups present in the ternary copolymer (Figure 1B). A 2% (w/v) solution of this polymer was prepared in methanol. The poly(4-vinylpyridmcco-styrene) (Aldrich Chemical Co.), having a styrene content of 10% (Figure lC), was used as received and dissolved in methanol to give a 2% (w/v) solution. Equal volumes of these two solutions were then mixed together to constitute the polyXI0 solution. To prepare the dye/polyXIO blend, 75 mg of ZnTPP (Strem Chemicals,Inc.), used as received and dissolved in 10 mL of methanol, was added to 20 mL of the polyXI0 solution.

Thin ZnTPP-polyXIO films were cast by depositing 0.3-mL amounts of this solution with a syringe onto clean conducting SnOz substrates (1.5 X 3.0 cmz, ps = 25-30 S l sq-I, Swift Glass Co.), which were placed on a carefully leveled plate to ensure uniform film thicknesses. They were then kept under an inverted crystallization dish in the presence of CaC12 during evaporation of the solvent. As typically observed when casting films by solvent evaporation, the dye/polymer in solution tends to concentrate at theedgesof theSnO2substratecausing the portion of filmcovering the central area to have a dye concentration lower than expected when considering that of the initial solution. All thickness, spectroscopic, and electrical measurements relate to that central portion of film which was reproducibly uniform in terms of thickness and dye concentration. Film thicknesses obtained in this way were of 5 pm. Some of the dye-polyXIO film was then removed with methanol to expose the underlying SnOz substrate. To act as an insulator between the SnOzand the gold electrode to be evaporated over the polymer film, nail varnish (-0.5 mm thick) was applied over half of the e x p e d SnO2. This was followed by evaporation of a 1 cm2gold electrode under vacuum ( 1 V Torr, Varian VK12B cryogenic pump) onto the selected surface of the modified Sn02. The sandwich cells were completed by using epoxy to attach copper leads to the gold and SnOz electrodes, and silver paint to establish a good electrical contact. Absorption spectra of the ZnTPP/polyXIO films were taken with a model HP8452 UV-vis diode array spectrophotometer. Film thicknesseswere measured with a Dek-Tak surface profilometer (Sloan). The cells were encased in a Pyrex glass container having an inlet and outlet for the purpose of purging with nitrogen gas, and two stainless steel rods enabling electrical contact with the cells from the outside. Flexible heating tape was wrapped around the glass container in order to change the temperature of the cell. The temperature was controlled with a variac, and a thermocouple placed just next to the sample was employed to monitor the temperature. This entire system was installed in a Faraday cage having a 2 cmz diameter window for sample illumination. A Keithley Model 6 17 programmable electrometer was used as both the voltage source and the current or resistance monitor. The current at various applied voltages and temperatures was also monitored by coupling the electrometer to a strip chart recorder. Monochromatic illumination of the ZnTPP-polyXIO films through the SnOz was obtained with a 150-W xenon lamp and a manual/motor driven monochromator (Bausch and Lomb). The illuminated cell area was 1 cmz in all cases and incident light intensities were measured with a radiometer (Optikon model 88 X LC, with a Model 400 sensor head). All conductivity measurementswere performed on cells under nitrogen atmosphere, after stabilization of the dark current.

-

ReSultN

Dark Conductivity. The typical dark current-voltage curves obtained with polyXI0 alone and with polyXIO-ZnTPP are shown in Figure 3. These measurements were performed with the SnOz contact acting as the cathode. The dark currents at field strengths of 103-105 V cm-1 vary with increasing voltage according to I, = aY

(1)

where Vis the applied voltage and a is a constant. At voltages lower than VSCL,the parameter s was found to have values of 1.0-1.3 and, therefore, Ohm’s law holds. In the region between VSCLand VTFL,s varied between 1.9 and 2.1 which is indicative of space charge limited (SCL) currents and, consequently, the current-voltage characteristic may be described byI3qi4

I, = 9Aee,,pdVz/8LJ where A is the electrode area, e the dielectric constant,

the

946

Lawrence et al.

The Journal of Physical Chemistry, Vol. 97, No.4, 1993 0.5

4

+ ?L! 3 V Y L

c5 =

O0.1 a2I

3.2

0.1

I

I

,

vTFL

1

I

1

I

,oo

V Figure 3. Dependence of dark current density on the applied voltage: (A) polyXI0 alone and (B) polyXISZnTPP.

3.3

3.4

IOOO/ T K-’ Figure 5. Dependence of dark current density on the temperature for a polyXIO-ZnTPP sample under an applied voltage of (A) 25 and (B) 35 V.

the absorbance at 566 nm and using c = 2.1 X 104 dm3 mol-’ cm-l),19 has only a minor effect on the mobility. It has been well established that the appearance of a region where the current is proportional to v.! (Figure 3), is characteristic of an insulator containing In this situation thecurrents are expressed as

Id = ~ A E E & P / B L ~ (4) where m is the microscopic electron mobility and Q is given by Ne, exp(-E,/kT) (5) Nt where Ncrr is the effective state density for the material, Nt is the trap density, Et is the trap energy, k is the Boltzmann constant, and Tis the absolute temperature. Equation 4, thus indicates that the slope of a In Id vs T plot, at an applied voltage within the SCL region, enables the determination of Et. These results are shown in Figure 5 for a polyXIO-ZnTPP sample, for two different applied voltages, which both yield the same value of Et = 0.73 eV. The values of Et for polyXI0 films with or without dye were found to be in the same range 0.72-0.75 eV. The currents at applied voltages greater than VTFL, the voltage at which the traps present in the film are filled, rise even more rapidly with the s parameter varying from about 2.7 to 2.9.VTFL is known to be directly proportional to the density of traps involved, N,, as given by14,15

4=

0

lo00

500

lS00

V2 Figure 4. Dependence of the dark current density on the applied voltage squared, in the SCL region: (A) polyXI0 alone and (B) polyXI0ZnTPP.

permittivity of free space, Pd the charge drift mobility, and L the distance between the electrodes. Equation 2 indicates that the drift mobility can be obtained from the slope of the Id vs v2 curve in the SCL region, as shown in Figure 4, by using the relation & = [8L3dId/d(v2)]/9t@

(3) Taking c = 3, which is common for most polymer materials,l4-I6 the drift mobilities for polyXI0 alone and for polyXI0-ZnTPP were calculated to be 3 x 10-9 cm2 V-l s-’ and 4 X cm2 V-I s-I, respectively. Such low mobilitiessuggestthat charge transport in these materials is controlled by a hopping mechanism operating between localized sites. It should be noted that a value of 3 for E of polyXI0 is consistent with taking an average value of E’S reported by Duke et al.,I7J*for styrene and pyridine pendant group polymers, if the respective mole fractions of styrene and pyridine groups present in this blend are the only ones being considered (Figure lB,C). Although these groups make up for 70% of the pendant sites in polyXI0, the actual E value is expected to be slightly higher in view of the two different ionic sites which account for 30% of the overall pendant group concentration. It is important to note also that the presence of ZnTPP in the polymer film, at a concentration of about 1 X lOI7 cm-3 (calculated from

Nt = 9 ~ ~ , ~ ’ ~ ~ / 8 q L ~ (6) where q is the electron charge. With a VTFLof approximately 40 V for both polyXI0 alone and polyXI0-ZnTPP, N, was found to be -3 X IOl4 cm-). The fact that Nt and Et are unchanged by the presence of the dye molecules would suggest that these trapping states are associated to the polymer. The influence of these traps on charge mobility in the polyXI0-ZnTPP system is determined by the following expression (7) Inserting the experimental values of Et = 0.73 eV and Nt = 3 X lOI4 into eq 5, and taking Ncn = 1 X loz1cm-3, 4 at room temperature is equal to 2 X lW, and eq 7 then yields a microscopic mobility of p,, = 1.5 X lo-’ cm2 V-1 S-I. Equation 7 signifiesthat, in theabsence of trapping, thedrift and microscopic mobilities are identical. Figure 6 shows the temperature dependence of the dark conductivity for polyXI0 alone, in the temperature range 23-42 OC. The activation energy of the conduction process, AE, can kj

4pO

Photocurrent Generation and Charge Transport

The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 947

30Tj 0

R

3I

-

Y C

n

'0 E

L

'E

0

4001

4.0

1

I

I\

I

3.0

200

2.0

2 V

.u

L

-

0

0

r

&.

(3

3

n

11.0

100

D

0v) 0

ki

a

W

.-b .->

0.0

0 400

500 600 700 Wavelength (nm) Figure 8. Absorption spectrum and action spectrum (under an applied field of 400 Vcm-I, at room temperature) of the polyXIO-ZnTPPsystem. The insert highlights the photocurrents from 500 to 650 nm.

U

0

3

U E

0"

I

I

I

I

I

OFF

I

t

3.2

3.3

I

1

3.5

3.4

IOOO/ T ( K-'

Figure 6. Temperature dependence of thedark conductivity for polyXI0 alone.

"r

2o -0

n

100

300

500

t (sec) Figure 9. Photocurrent-time response of the SnO&lyXIO-ZnTPP/ Au cell during a light-on/light-off sequence, a t room temperature under an applied field of 1200 V cm-1.

.-.-3> Y

U C 0

0

2x1f1.: 1%lO-y I

I

3.2

3.3

I

3.4

1000/ T ( K-' ) Figurel. Temperaturedependenceof thedarkconductivity

for polyXI0-

ZnTPP.

be derived from the Arrhenius plot for the dark conductivity by using20 u = uo exp(-AE/ZkT)

(8) where 00 is the preexponential conductivity. For polyXI0 alone, hE values were found to vary between 3.7 and 4.1 eV. In the case of the polyXIO-ZnTPP system, however, conductivities in the same temperature range show two distinct regions of different slope (Figure 7). The slopes obtained from values appearing in region A of Figure 7, gave AE's similar to those obtained with the polyXI0 alone. The slopescalculated from region B at lower temperatures, gave U s ranging from 1.9 to 2.2 eV, in good agreement with the energy difference between the ground and first excited singlet states of ZnTPP.21-22 Wotocollductivity. Figure 8 shows the absorption spectrum of the polyXIO-ZnTPP films on SnO2. The absorption maxima observed for these films in the red (566 and 606 nm) were found to be very similar to those found for ZnTPP in CHCI3 solution

containing a large excess of poly(4-vinylpyridine), which causa the bands to shift relative to those obtained in CHClp alone (562 and 602 nm).I9 This is an indication that some of the ZnTPP in these films is coordinated by the pyridine moiety in polyXI0. The photocurrents observed as a function of wavelength with the Sn02/PolyXIO-ZnTPP/Au cells, illuminated through the Sn02, correspond to the absorption spectrum of the dye. The polyXI0 alone shows no absorption in the visible and therefore, the photocurrent is obviously due to absorption of light by the ZnTPP. The action spectra were normalized at a constant photon flux of 3 X IOI4 photons s-1 cm2, based on the light intensity incident on the cell at 606 nm. The results shown in Figure 8 also tend to indicate that light absorption in the Soret band of ZnTPP, compared to that in the Q band, is more efficient at photogenerating charges. Figure 9 shows the complete photocurrent response profile obtained duringa light on/light offsequence, at room temperature, with a polyXIO-ZnTPP cell under an applied field of 1200 V cm-I and 606-nm illumination. The photocurrent responses observed following illumination at each of the two other maxima in the visible (566 and 430 nm) had a similar profile, differing only in the magnitude of the steady state photocurrent. These results are parallel to those reported previously in the study of wet cells12 and are characteristic of systems where photoconductivity is greatly influenced by the trapping of charge carriers. In the following experiments, 606 nm was specifically chosen as the excitation wavelength to provide a fairly uniformgeneration of charge carriers throughout the sample and, therefore, enable the attribution of observed behaviors to processes occurring in the bulk rather than at the illuminated surface contact. Photocurrent responses for the SnO*/PolyXIO-ZnTPP/Au cells were measured at different temperatures (23-40 "C) and at different appliedfields (1000-1500Vcm-1). Theexperimentswerecarried out on numerous cells prepared in the same way and the reproducibility was generally good.

Lawrence et al.

948 The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 8 n

4: 5 x 1 i 1 1 W

4

C

g

-11

5x10

-

0 0

4

0

r

a

,

-111

1x10

3.1

\ [ 5.5

3.5

1000/ T ( K-' ) Figure 10. Temperature dependence of the steady-state photocurrents for polyXIO-ZnTPP under applied fields of (A) lo00 and (B) 1500 V cm-1.

OC

d,

W d

-

29.4OC

100

200

-

Figure 12. Variation of Iph during the decay at longer times (>16 s), for polyXIO-ZnTPP under an applied field of 1200 V cm-l and at temperatures of (A) 23.5 and (B) 29.4 'C. The insert presents the variation of 1ph-l at shorter times for the same temperatures and applied field.

4

E,

20 '\',

0.70

. \

100

200

(4

0.75

1

0

300

t (sec)

-23.5 --- 25.5 O C -..-..

0

I

0.80

0.85

I

I

300

t (sec) Figure 11. Time dependence of photocurrent decay for polyXI&ZnTPP at three different temperatures and an applied field of 1200 V cm-I.

The steady-state photocurrents, ZO,were found to increase with temperature according to

\

1

where P is the preexponential photocurrent and u p h is the activation energy of photoconductivity. Figure 10 presents these results, which show a dependency of the slopes on the applied field. The slopes, u p h , were found to be -0.4 and -0.3 eV at fields of lo00 and 1500 V cm-I, respectively. The typical isothermal photocurrent decay vs time curves observed after attainment of the steady-state current under 606-nm light, and an applied field of 1200 V cm-I, are shown in Figure 1 1 for three differenttemperatures. In all cases, the decays were characterized by a relatively rapid rate of decrease at shorter times ( t I15 s), slowing down progressively at longer times. During the first 15 s, the photocurrent decay appears to result from a recombination process of a bimolecular nature (insert, Figure 12), which then changes to a process showing an exponential dependence (Figure 12). Photoconduction studies performed by Beck on a variety of semiconducting polymers23.24 have revealed a similar behavior. Based on the theory of isothermal decay currents developed by Simmons and Tam25v26a plot of rpht vs log t (where Iph is the decaying photocurrent and t is the time) is known to permit the direct determination of dominant trap distributions in semiconductors and insulators without even having an a priori knowledge of the trap parameter^.^^,^' The measurements of isothermal

100

10

1000

t (sec) Figure 13. Variation of I p h t with time for polyXIO-ZnTPP under an applied field of 1200 V cm-I, at temperatures of (A) 23.5 and (B) 25.5 OC.

decay currents made in this study were plotted in the Ipht vs log t form, and they appear as asymmetrical bell shaped CUNCS (Figure 13). The log t scale can be converted to an energy scale by using the following equati0n:~5

E, = k T In ( v t )

(10) where Et is the trap energy relative to conduction states enabling either electron or hole transport, and Y is the attempt-to-escape frequency of the trapped charge carriers. The conversion, however, requires knowledge of the attempt to escape frequency. This parameter can be obtained by measuring the Zpht vs log t characteristic at twodifferent temperatures, say, T Iand Tz, while maintaining the applied field at a fixed value (Figure 13). One then takes a time tl on the Iph(Tl)t-log t curve and a time 12 on the Iph(T2)t-log t curve at which a prominent part of the characteristic appears corresponding to an energy, say, E l . From

Photocurrent Generation and Charge Transport eq 10, it is then apparent that TI, T2, t l , and simultaneous equations

t2

satisfy the two

E , = &TIIn (vt,), E, = &T2In ( v f 2 )

(1 1) from which Y was calculated to be -4 X 10” s-I which is in good agreement with literat~revalues.’~ Taking this value, deep traps distributed in a narrow energy range of -0.1 eV and centered at an Et ranging from 0.77 to 0.80 eV, are found to be present in the polyXIO-ZnTPP films. These values for Et are in close agreement with the trap depths determined from the SCL current measurements performed in the dark.

Discussion

Nature of Traps in the PolyXIO-ZnTPP System. A photoconductor’s behavior is generally determined by the properties of its localized defect states; their density, their location in terms of energy (depth), and their cross section for capture of free carriers. The early studies dealing with this subject were based upon the theory of carrier injection in insulators first given by Mott and Gurney.28 Rose,l3329 and later Rose and Lampert30 developed the classical theory of SCL currents that was subsequently applied to the field of organic crystals by Helfrich and Mark.31 Since then SCL current measurements, as a diagnostic means to determine the characteristics of trapping sites, have been used extensively to study dark conductivity and photoconductivity in organic materials showing some aptitudes for photovoltaic energy conversion or for possible use in electronic d e ~ i c e s . ~The ~ - numerous ~ investigationshave solidly established that (a) the energy distribution of traps determines the form of the dark current-voltage curves and the magnitude of the SCL currents and (b) the energy distribution of traps also determines the form of the photocurrent-time curves and the response time in photoconductivity measurements. In organic solids, a discrete trapping level is generally related to the presence of a specific chemical species, whereas traps forming a quasi-continuous distribution of energy levels are associated to the statistical dispersion of structural imperfections. For a trap of chemical origin, the electronic affinity or the ionization energy will determine the type of charge carrier (hole or electron) it is capable of trapping. Structural defects, on the other hand, are the cause of local variations in the polarization energy of charge carriers and can act as traps for carriers of either t ~ p e . ~ I - ~ 3 Consideringthe treatment of isothermaldecay currents (Figure 13), the narrow distribution in energy and the depth of traps in polyXIO-ZnTPP films tend to support their attribution to the presence of a specific chemical species. It is also clear from the current-voltage measurements (Figures 3 and 5 ) , that these traps are associated to the polymer rather than to the dye. Of the different sites present in polyXI0, the pyridinium ions which are known to form in such films by protonation of the pyridine groups, appear to be likely candidates for this role. Indications to this effect have been provided in the previous report concerning the electrochemical response of polyXIO:’2 (i) Cyclic voltammetry experiments performed on SnO2/polyXIO electrodes in contact with aqueous 1 M KCl electrolyte at pH 6.7 (using platinum foil as counter electrode), have revealed a cathodic wave peaking at - 4 . 4 5 V vs NHE correspondingto the reduction of pyridinium ions to pyridine radicals. (ii) Under the same electrochemical conditions, none of the other pendant group sites present in polyXI0 (Figure 1) were found to be electroactivein the potential range of -1.2 to 1.O V vs NHE. The reduction potential determined electrochemically has indicated the existenceof electron traps that are located, following the usual assumption that 0 V vs NHE = -4.5 eV vs vacuum on the absolute electron energy ~ c a l e , 4at ~ --4.05 ~~ eV vs vacuum. Comparing this to the excited-state levels of thedye, the trapping level is found to lie at -0.7 eV below the first excited singlet state

+

The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 949

of ZnTPP which is located at -3.35 eV vs vacuum. The Et values estimated from the measurements of dark conductivity and photoconductivity, performed on the solid-state cells, agree reasonably well with that of the previous electrochemical experiments. This interpretation is also consistent with the AE for darkconductivity and the charge mobility in polyXIO-ZnTPP films (Figure 7 and 4), which reveal the existence of localized charge-carrying states lying at - 2 eV above the ground-state energy of ZnTPP. Under illumination, these states are expected to mediate the transport of photogenerated electrons, while those lying near the ground-state energy of ZnTPP should be active in the transport of holes. Additional information favoring the proposed identity of the deep traps is provided by the field dependence of h E p h (Figure 10). Since the pyridinium sites carry a single positive charge, the trapping and detrapping process can be described as T++e-+To (12) The detrapping process in this particular case is known as the Poole-Frenkel effect. If this model is operative, the relationship4749

G = Goexp{-(E,

- BE1/2)/&T)

where G is the current (or photoconductive) gain, GOis a constant, @ is the PooleFrenkel coefficient, and E is the applied field, can be taken as an approximate description of the field and temperature dependence of photoconductivity. Comparing eq 13 to eq 9 for the temperature dependence of steady-state photocurrents, the activation energy for photoconductivityis then expressed by u p h (E, which indicates that the barrier, Et, associated to the promotion of an electron from a trap state to a conduction state, can be lowered by the amount Taking Et to be 0.8 eV, the values of A E p h determined at applied fields of lo00 and 1500 V cm-I for the polyXIO-ZnTPP system follow the trend described by eq 14. A value of B = 1.3 X l e 2(cm/V)V2 is thus obtained, which is close to the room temperature value of 1.7 X (cm/V)lI2 predicted by the Poole-Frenkel theory for a material having e = 3. Photocooductor Behavior. The effect of traps on the performance of a photoconductor is characterized by the measurement of macroscopic observables such as (i) the ratio of the number of photogenerated electrons passing through the photoconductor to the number of photons it absorbs, per unit time, and (ii) the time required for the pbotocurrent to rise or decay to a steadystate value following a variation in the incident light intensity, at a fixed applied field. The first of these observables defines the current gain, G, of the photoconductor, and the second one the characteristic response time tph. The tph for the rise, or decay, of photocurrent is generally greater than the average lifetime of a free charge carrier, T , because of the filling, or emptying, of traps that are in thermal equilibrium with the free carriers. Considering electrons, this effect is expressed by the relation14ss0

t,,

= (n, + n h / n

where n is the total number of free electrons and n, is the total number of trapped electrons effectively in thermal contact with the free electrons. The average lifetime of the free photogenerated electrons at steady state is defined by To

= No/F

(16)

where NOcorresponds to the number of free electrons generated in the sample by the incident light, at steady state, and F is the number of photons absorbed by the sample per unit time (number of excitations per second). It should be noted that T and 70 represent different lifetimes; T corresponds to the lifetime of

Lawrence et al.

950 The Journal of Physical Chemistry,Vol. 97,No. 4, 1993

photogenerated free electrons during the rise or the decay of photocurrent, whereas TO corresponds to the lifetime of photogenerated free electrons at steady state. During the rise or decay of photocurrent, the densities of free and trapped electrons vary gradually and the lifetime changes accordingly. Under steadystate conditions, however, there is a constant supply of photogenerated charges and the densities of free and trapped electrons remain unchanged with time and, therefore, TOis a constant. For the sake of definiteness, the sample calculations that follow will focus only on photoconductive decay, and the result shown in Figure 1 1 for the decay at room temperature, will be taken as an example. The current gain of a photoconductor is expressed as27950 where T,is the transit time of the electrons from cathode to anode. Taking the value of Io = 48 X A obtained at room temperature for a polyXIO-ZnTPP sample illuminated at 606nm(Figure9),forwhichF= 1.8 X 1014photonss-l(calculated from the photon flux of 3 X 1014photons s-I incident on a surface area of 1 cm2, and the absorbance of 0.4, at 606 nm), G is found to be 1.7 X 10-6 electron/absorbed photon. The transit time in eq 17 is given by

T,= LIP$

(18) where E is the electric field across the specimen. Taking the values E = 1200 V cm-1, L = 5 pm (Figure 9), and pd = 4 X cm2 V-' s-I (Figure 5 ) , T,under these conditions is found to be 104 s. Introducing this value and that of G in eq 17 yields an average lifetime of TO = 1.8 X 10-4s. With eq 16 and the values for TOand F, we then find the steady-state density of free electrons generated by light to be n , ~= No/volume of sample = 6.5 X 1013 cm-3. Since each absorbed photon creates an electron-hole pair and given that each trapped electron forms a neutral site, it is reasonable to assume based on the condition of charge neutrality that a close balance exists between free charge carriers, i.e., n , ~ = PO,where po is the steady-state density of free holes. At this point the total density of free carriers is 1.3 X 1014cm-3, which is nearly equivalent to half of the density of traps, Nt, present in these samples. Upon removal of light we therefore have n p , and the decay might be expected to involve the recombination of oppositely charged carriers which may be affected by trapping. This situation is consistent with the bimolecular rate law observed during the first 15 s or so of photocurrent decay (Figure 12). At steady state, if one assumed that the annihilation of free carriers is strictly due to direct recombination, without trapping, the rate equation for the bimolecular process would be written as

-

value is given by which is in fact the same as that defined by q 16, TO = 1.8 X 10-4 s. The decay curves shown in Figure 1 1, however, totally disagree with this treatment. The photocurrents, which are taken to be proportional to the charge carrier densities, decay to half of their steady state values only after several seconds. This indicates that, even at short times, the recombination process is influenced by trapping events. When the decay law starts changing ( t = 16 s, Figures 1 1 and 12), the density of free electrons (and free holes) at room temperature has diminished by about one-fifth of its steady-state value. Compared to the situation which prevails immediately after the removal of light, the number of times that a free electron may be captured by and emitted froma trap before it is annihilated through recombination becomes substantially greater, Le., the free and trapped electrons maintain thermal contact. This suggests that during the decay period at longer times, the ratio of free to trapped electrons shouldremain approximatelyconstant. Under these conditions photocurrent decay is governed by a single differential equation: d(n

+ n,)/dt = - n / r

(24) where the density of electrons that now needs to be taken into account in the recombination process is not n, but (n + n,). Setting 6 = n,/n = constant, eq 24 reduces to (1

+ 6 ) dn/dt = -n/T

(25)

which has the solution

where (1

+6

) =~ t,, = (n,

+n)r/n

(27) Le., the characteristic response time is greater than the lifetime by the ratio of total (trapped + free carriers) to free carriers. This description is consistent with the exponential photocurrent decay observed in the polyXIO-ZnTPP films at longer times.

Conclusion

Dark conductivity measurements have demonstrated that the incorporationof ZnTPP in polyXI0 has little influence on charge transport through the system. The drift mobility and the total trap density remain essentially the same in polyXI0 films with or without dye. The low mobility suggests that charge transport occurs by hopping between localized statesand that themigration dno/dt = f - K n g , = f - Kn: = 0 (19) is effectively controlled by the polymer phase. wheref = F/volume of the sample = 3.6 X 101' excitations ~ m - ~ Incorporationof thedyeleads to photosensitizationofpolyXI0 s-I, K is the bimolecular rate constant in cm3 s-I carrier-I, and in the visible, with the ground and first excited singlet state of the steady-state concentration of free electrons is given by ZnTPP acting as the energy gap for the onset of photocurrent in the red. The fact that this photon energy equals the activation no = ( f / K ) ' / 2 energy of 2 eV obtained for dark conductivity in the polyXIOZnTPP films (M is -4 eV for polyXI0 alone), implies the Using the calculated values off and n,~,eq 20 gives a K of 1 existence of a "conductivity gap" which is essentially defined by X cm3s-I came+ for a polyXIO-ZnTPP sample at room the dye. The photoconductive behavior of the polyXIO-ZnTPP temperature, under an applied field of 1200 V cm-1. When light system as a function of time is dictated by the presence of a is removed, the rate equation becomes narrow distribution of deep traps which are believed to be pyridinium sites. The effect of these traps on photogenerated dnldt = -Kn2 (21) electrons results in a slow photocurrent-time response and also which has the solution a low photoconductive gain. The photocurrent decay at shorter times can be attributed to trap controlled recombination of a n = no/(l Knot) (22) bimolecular nature. However, beyond 15s or so, the photocurrent Equation 22 indicates that, in this case, the time required for is found to decrease exponentially with time, which could be the free electron density to decrease to one-half of its steady state interpreted as indicative of a unimolecular process. More insight

-

+

Photocurrent Generation and Charge Transport concerning the theoretical treatments applicable to this type of decay will be presented in a following arti~le.~l The similarity in the responses observed between the solidstate cells and the previously studied "wet" cells,I2 where the basic difference in cell configuration lies in the replacement of the gold electrode by a liquid electrolyte, would indicate that the machanisms of charge generation, migration, and recombination are basically the same in both systems. These results suggest that efforts to improve photocurrentsand response times of similar ion-exchangepolymer/dye systemswill need to be directed toward enhancingthe polymer's "dry" chargeconductingproperties, while maintaining its high affinity for redox reactants.

Acknowledgment. This research was supported by the Natural Scienca and Engineering Research Council of Canada and the MinistCre d'Enseignement Sup€rieureet de la Sciencedu Qu6bec.

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