J. Phys. Chem. 1983, 87,868-872
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Flash Photolysis-Electron Paramagnetic Resonance Study of Light-Generated Paramagnetic Charge Carriers in Metal-Free a-,,8-, and x-Phthalocyanines Roger L. Sassevllle, Alan R. McIntosh, James R. Bolton, Department of Chemistty, University of Western Ontario, London, Ontario, Canada N6A 587
and John R. Harbour’ Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5L IJ9 (Received: April 19, 1982; In Final Form: October 19, 1982)
This flash photolysis-electron paramagnetic resonance (FPEPR) study provides spectroscopic evidence for the transient fate (?microseconds) of holes and electrons produced when light is absorbed by the organic semiconductor, metal-free phthalocyanine (H,Pc). Three stages in the transient response are identified: (1) a very rapid (-1 ps) depletion of trapped charges followed by (2) a repopulation of these sites and (3)-in pand cu-HzPconly-an additional population of sites creating trapped carriers within the bulk crystal and outside of the space charge region; these carriers subsequently recombine on the millisecond time scale. The magnitude of the absorption in stage 3 was demonstrated to be a function of the surface area of the pigment particles. These results are discussed in relation to the relative photoefficiency of these polymorphs. This technique therefore offers the potential for a mechanistic understanding of those events in semiconductorswhich ultimately control the limits for solar energy utilization and electrophotography.
Introduction When a semiconductor absorbs light with a photon energy greater than the band-gap energy, E,, charge separation may take place creating hole-electron (h+-e-) pairs’+ resulting in an injection of excess electrons in the conduction band and excess holes in the valence band. The application of this photoeffect has been of considerable interest of late.’-14 Experiments have been directed along two main fronts: (i) investigations of photoinduced charge-transfer processes occurring at semiconductor-solution interfaces’-” and (ii) the generation of useful electrical work from these charge separation^.'^-^^ For the most part little attention has been directed to the study of primary processes associated with the charge-separation step. Little is known about such processes as recombination and bhe trapping of holes or electrons at the surface and/or within the bulk of the semiconductor. Transient (1) N. F. Mott and E. A. Davis, “Electronic Processes in Non-Crystalline Materials”, Claredon Press, Oxford, 1979. (2) G . D. Watkins in “Point Defects in Solids”,J. H. Crawford,Jr., and L. M. Slifkin, Eds., Plenum Press, New York, 1975, Col. 2; G. D. Watkins in “Lattice Defects in Semiconductors”,F. A. Huntley, Ed., Institute of Phvsics. London. 1975. i3) M. H. Brodsky, Ed., “Topics in Applied Physics”, Vol. 36, Springer-Verlag, West Berlin, 1979. (4) N. B. Vidi and J. W. Corbett, Eds., ‘Radiation Effects in Semiconductors”. Conference Series No. 31. The Institute of Phvsics. Bristol, 1976. (5) L. Pal, G. Gauner, A. Janosay, and J. Solyom, Eds., “Organic Conductors and Semiconductors, Proceedings of the International Conference, Siofok, Hungary, 1976”, Springer-Verlag, West Berlin, 1977. (6) R. H. Wilson, CRC Crit. Rev. Solid State Mater. Sci., 12, 1 (1980), and references therein. (7) R. C. Ahyja and K. Ahuffe, Ber. Bensenges. Phys. Chem., 84, 68, 129, 138 (1980). (8) G. C. Hartmann and F. W. Schmidlin, J . Appl. Phys., 46, 266
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(9) G. C. Hartmann and J. Noolandi, J. Chem. Phys., 66, 3498 (1977). (10) Z. D. Popovic and J. H. Sharp, J. Chem. Phys., 66, 5076 (1977). (11) C. D. Jaeger, F.-R. F. Fan, and A. J. Bard, J. Am. Chem. Soc., 102, 2592 (1980). (12) R. 0. Loutfy and C.-K. Hsiao, Photogr. Sci. Eng., 24,155 (1980). (13) R. 0. Loutfy and E. R. Menzel, J . Am. Chem. Soc., 102, 4967 (1980). (14) A. K. Ghosh, D. L. Morel, T. Feng, R. F. Shaw, and C. A. Rowe, Jr., J. Appl. Phys., 45, 230 (1974).
0022-3654/83/2087-0868$0 1.50/0
photoinjection of charge has been used to probe the transport properties of semiconductors in which carrier transport may be viewed as a succession of carrier hops from one localized site to another.15J6 Although some work has been carried out on these processes occurring in inorganic semic~nducton,’-~ very little is known about such processes in organic semiconductors. Popovic and Sharp,lo using pulsed photoconductivity, measured transient action spectra for sublimed P-HzPc thin films (5.8 pm). When biased negatively (such that electrons are the dominant free carriers), the transient photoconductive response was observed to be a step function with a rise time of less than 40 ns, and, when biased positively (such that holes are the dominant free carriers), a similar sharp rise was observed followed by a slower rise in conductivity. These results were interpreted as a direct consequence of the photogeneration of free carriers, holes, and electrons, arising from a common singlet excited electronic state. In the positively biased case some of the photogenerated holes move very rapidly through the semiconductor lattice to the counterelectrode (fast transient), whereas others are trapped in the bulk but undergo electric field stimulated hole detrapping which results in the slower rising transient. This studylodemonstrates that phthalocyanines can in fact trap charge carriers. Pulsed photoconductivity can also be utilized to determine free-carrier lifetime^.'^ Usov and Benderskii,ls using this technique, have measured the lifetime of an electron in single crystals of P-HzPc to be T , = 1.9 p s and the lifetime of a hole to be q,= 1.4 ps at room temperature. Similar s t u d i e ~on l ~phthalocyanine ~~~ (15) F. W. Schmidlin, Phys. Rev. B, 16, 2362 (1977). (16) F. W. Schmidlin, PMOS.Mag., [Part]B, 41, 535 (1980). (17) K. Graff and H. Fisher in “Topics of Applied Physics”, Vol. 35, Springer-Verlag, West Berlin, 1978. (18) N. N. Usov and V. A. Benderskii, Phys. Status Solidi, 37, 535 (1970). (19) M. I. Federov and Va. A. Benderskii, Sou. Phys.-Semicond. (Engl. Transl.) 4, 1198, 1720 (1971). (20) J. McVie, R. S. Sinclair, and T. G. Truscott, J . Chem. SOC.Faraday Trans. 2, 74, 1870 (1978). (21) J. R. Harbour and R. 0. Loutfy, J . Phys. Chem. Solids, 43, 513 (1982).
0 1983 American Chemical Society
Charge Carriers in a-, @-, and x-Phthalocyanines
suggest that, upon absorption of light, excitons are formed which then ionize (dissociate) due to interactions with impurity centerslg or with lattice phonons.18 In a preceding paper,22we have fully described the EPR dark and light-induced characteristics of dispersed particles of @-H2Pcand x-H2Pc. We have shown that broad (AHpp= 5 G) EPR signals originally present in the solid state and in particle dispersions of @-H2Pcand x-H2Pc gradually disappear as the particles equilibrate. However, upon light irradiation, narrow (AHpp 1G) g = 2.00 EPR signals are generated. These light-induced EPR signals are inhomogeneously broadened and saturate at very low light intensities. It was concluded that these light-induced EPR signals probably arise from holes and electrons which are trapped at defect sites (trap sites) and that free carriers in transport states are not observable by EPR. This suggests, vide infra, that electrons and holes which escape geminate recombination may, in addition to random recombination, undergo respective trapping at defect sites. It is vitally important to understand the mechanism and the rates of processes prior to interfacial reactions or charge transport through a semiconductor,as it is these processes which will ultimately determine the efficiency of any useful device based on charge-carrier production and utilization. One device of considerable interest and importance is a photovoltaic cell based on the organic semiconductor p h t h a l o ~ y a n i n e ~in~ which t ~ ~ the pigment particles are dispersed in a polymer binder. Hence, with these views in mind we have chosen, in this preliminary study, to examine the kinetics and mechanisms of formation and decay of photogenerated excess free carriers in polycrystalline preparations of the organic semiconductors, metal-free @-phthalocyanine(@-H2Pc), x-HZPc,and a-HZPc, in dispersed media using the technique of flash photolysis-electron paramagnetic resonance (FPEPR).
Experimental Section Samples of @-HzPc,x-H2Pc, and a-H2Pcwere prepared and purified as previously described.21 Surface areas were determined by conventionalBET (Brunauer, Emmett, and Teller) adsorption technique^.^^ Surface areas of 7 and 70 m2/g were determined for two samples of @-H2Pcand x-H2Pc, respectively, whereas a-H2Pc had a surface area of 30 m2/g. The polydispersity, particle size, and phase of the samples were checked by scanning electron microscopy, X-ray, and infrared procedures.26 The surface charge for the phthalocyanine particles in methanol/glycero1 (1:l; v/v) dispersions, determined by microelectrophoretic techniques, was found to be negative although no absolute values of mobilities were determined. For studies involving the change in surface area of a particular polymorph, the following procedure was used. Approximately 150 g of /3-H2Pcwas milled in a 2-L polystyrene bottle containing 450 mL of 2-propanol and 3/8-in. diameter stainless-steel ball bearings. Samples (15 mL) were extracted twice a day for a period of 2 weeks. These aliquots were purified by three successive extractions with (22) R. L. Sasseville,J. R. Bolton, and J. R. Harbour, J. Phys. Chem., preceding article in this issue. (23) R. 0. Loutfy and J. H. Sharp, J . Chem. Phys., 71, 1211 (1979); R. 0. Loutfy, J. H. Sharp, and L. F. McIntyre, U S . Patent 4175982 (1979). (24) R. 0. Loutfy, J. H. Sharp, C. K. Hsiao, and R. Ho, J. Appl. Phys., 52, 5218 (1981). (25) Surface BET adsorption measurements were performed by B. Nash at Webster Xerox Center, New York, and S. Issler at Xerox Research Center, Canada. The independent determinations confirm cited values. (26) J. H. Sharp and M. Lardon, J . Phys. Chem., 72,3230 (1968); see also ref 16.
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(i) 2-proponol, (ii) acetone, and (iii) methanol and subsequently air dried. The EPR detection system has previously been well characteri~ed.~’ Using direct detection (output of preamplifier in microwave bridge in the absence of magnetic-field modulation) and a slight bandwidth modification to the preamplifier within the bridge, we obtain a l / e rise time of 0.4 ps in transient flash photolysis experiments. The output of the bridge was then digitized directly by a Nicolet 2090-111 digital oscilloscope transient recorder with 12 bits of vertical resolution. The transient recorder was interfaced with a Nicolet 1180 instrument computer for signal averaging and data analysis. Most of the experiments used a PRA Model 610C pulsed light source (30 Hz) with a flash half-life of 1.5 p s which was filtered with a Corning CS3-70 filter (A I500 nm). However, in one experiment a PRA Inc. Nitromite laser (Model LN 100) with a light pulse of 300 ps and an energy output of 50 pJ (A = 337 nm) was employed. In this configuration, a Nicolet Model 204 transient insert (50 ns per point) with 8 bits of vertical resolution was coupled to a HewlettPackard 461A amplifier operated at a gain of 40 db. The number of acquisitions in transient experiments was usually 32 768 and in order to ensure the authenticity of the kinetic traces we processed all on-resonance profiles by subtracting an off-resonance signal 1000 G upfield.
Results and Discussion The Model. In order to have a framework within which to present and discuss our results, we will first outline a model which we believe best describes the behavior of the phthalocyanine particles while also clarifying the experiment. Previous work1*13 has demonstrated that phthalocyanine behaves as a p-type semiconductor, and thus we base our model on a semiconductor in contact with a liquid or solid dispersing phase in which holes are the majority carriers. We propose the presence of defect sites, some of which are able to trap electrons and other holes, with the former associated more with bulk and the latter with the surface.11~22~28 In the dark-adapted dispersion, none of these defect sites are actually populated consistent with the fact that no EPR signal is observed. However, upon illumination, which creates holes and electrons, these sites become populated and EPR observable. It was shown by microelectrophoresis that the surface charge of these particles is negative, which suggests that trapped holes will be preferentially stabilized at the surface. The excess electrons would then be trapped at bulk defect sites with a population equal to the surface trapped holes and with a distribution in the bulk decreasing in number going away from the surface. This is in fact the definition of a space charge region which has the effect in this case of bending the bands downward (Figure la). Superimposed on this is an attraction of free holes (nonobservable by EPR) which can mirror the negative surface charge to a certain degree depending upon the population of the trapped sites. The implication is that, under repetitive flash photolysis conditions, a certain number of these trap sites will be occupied since the lifetime of these “steady-state’’ centers is long compared to the repetition frequency. Therefore, in the FPEPR experiment, the absorption of light from a single flash occurs in the presence of this steady-state concentration of trapped sites. The light which is absorbed (27) A. R. McIntosh, H. Manikowski, and J. R. Bolton, J.Phys. Chem., 83, 3309 (1979). (28) Y. C. Cheng and R. 0. Loutfy, J. Chem. Phys., 73, 2911 (1980). (29) R. 0. Loutfy and Y. C. Cheng, J. Chem. Phys., 73, 2902 (1980).
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