J. Phys. Chem. zyxwvuts 1984,88, zyxwvu 3139-3142 3139 Flash Photolysis Electron Paramagnetic Resonance Studies of Charge-Carrier Production in Sublimed Films of Phthalocyaninet Roger L. Sasseville, Alan R. McIntosh, James R. Bolton,* Photochemistry Unit, Department zyxwvut of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 and John R. Harbour* Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5K 2LI (Received: September zyx 6, 1983; In Final Form: January 15, 1984) Phthalocyaninefilms deposited on glass substrates by sublimation yield the a-phase which by heating in air may be converted to the P-phase. These films are diamagnetic in the dark; however, on exposure to light with photon energy above the band gap paramagnetism is observed by electron paramagnetic resonance (EPR) methods. Transient flash photolysis EPR techniques reveal that the triplet state is an important precursor to radical production in that the phenomenon of chemically induced dynamic electron polarization (CIDEP) is observed. The polarization pattern of the CIDEP signals is uniquely in microwave emission which could only arise from a “triplet” mechanism where electron transfer occurs from a phthalocyanine triplet state with a preferential population of one of the triplet sublevels. zyxwvu Our findings indicate that at least some of the light-induced charge carriers in this system arise from ionization of the phthalocyanine triplet state. Introduction Solid films and particles of phthalocyanine have proven to be of considerable interest as active elements in organic photovoltaic devices. Solid phthalocyanine is considered to be a ”p-type” semiconductor and, under a variety of circumstances, optical excitation from the valence band to the conduction band results in the creation of electron-hole The detailed examination of the creation and dissociation of these electron-hole pairs has been useful in the study of charge transfer across organic solid- liquid interface^.^,^ Equally important are the investigations of photoinduced electron-transfer reactions and electron donor-ac- ceptor quenching reactiom6 The above concepts have been exploited in the development of Schottky-barrier phthalocyanine-based photovoltaic cells.@ These devices can be classified into two basic designs with respect to the absorber or sensitizer: one where the phthalocyanine is sublimed onto a substratelo and the other where phthalocyanine particles of uniform size are dispersed in a polymer binder.” This latter design has been used in the most efficient phthalo- cyanine-based photovoltaic cell developed to date. The phthal- ocyanine particles in this cell are of the X phase (X-H,Pc) sandwiched between a “NESA” (Sb-doped SnOJ electrode (ohmic contact) and a semitransparent Al or In Schottky-barrier electrode’ (blocking contact). The mechanism of free-carrier production in the above systems is, however, as yet unclear, particularly as to the nature of the photoactive excited state. Proposals for an excited singlet state: triplet state,I2 or an exciton mechanism3 have been made, with most of the evidence leaning toward a singlet-state mechanism. Coupled spectroscopic and near-infrared photoelectrical ex- periments12 indicate that the carriers in metallophthalocyanines are created via the triplet state; however, in the case of metal-free H2Pc it has been argued that the triplet cannot be responsible as the S-T transition is strongly forbidden.I3 However, our results will show that at least some of the carriers must originate from the triplet state in this case. We have earlier investigated the ~teady-state’~ and transient15 electron paramagnetic resonance (EPR) spectra in dark and light-irradiated polycrystalline phthalocyanine particles. We have shown, as others,6 that the light-induced generation of charge carriers (holes and electrons) occurs in less than 1 zyxwvuts ps and that recombination times, which are temperature dependent, may be PuMication No. 3 18, Photochemistry Unit, Department of Chemistry, Unversity of Western Ontario. as long as seconds. These results have been useful in elucidating the operational mechanism in particle-based phthalocyanine photovoltaic cells. In this paper we extend our studies to sublimed films of metal-free phthalocyanine (both a-H2Pc and P-H,Pc) where we present strong evidence that a triplet state is a precursor for charge separation. Experimental Section Several standard glass slides (75 zyxwv X 25 mm, Fisher Scientific) were cut to a dimension of 4 X 25 mm, washed with a lukewarm mild soap solution, rinsed many times with glass-distilled water, and allowed to air-dry. The slides were then carefully mounted in an Edwards Model E306 vacuum evaporator in which the phthalocyanine (H,Pc) was sublimed to various thicknesses onto the surface of the slides by using standard procedures. The pressure in the evaporator was maintained between 0.75 X and 1.15 X lo4 torr, where it was possible to effect the sublimation of the phthalocyanine at a minimum temperature in the quartz (1) (a) Kearns, D. R.; Tollin, G.; Calvin, M. zyxwv J. Chem. Phys. 1960, 32, 1020. (b) Kearns, D. R.; Calvin, M. Ibid. 1961, 34, 2022. (2) (a) Heilmeier, G. H.; Warfield, G. J. Chem. Phys. 1963, 38, 897. (b) Westgate, C. R.; Warfield, G. Ibid. 1966, 46, 94. (3) (a) Usov, N. N.; Benderskii, V. A. Phys. Status Solidi 1970,37, 535. (b) Cox, G. A.; Knight, P. C. J. Phys. C 1974, 7, 146. (4) Ahuja, R. C.; Hauffe, K. Ber. Bunsenges. Phys. Chem. 1980,84,68, 129, 138. (5) (a) Harbour, J. R.; Hair, M. L. Photochem. Photobiol. 1978,28, 721. (b) Menzel, E. R.; Loutfy, R. 0. Chem. Phys. Lett. 1980, 72, 522. (c) Loutfy, R. 0.; Menzel, E. R. J. Am. Chem. SOC. 1980, 102, 4967. (6) (a) Popovic, Z. D.; Sharp, J. H. J. Chem. Phys. 1977,66, 5076. (b) Hartmann, G. C.; Noolandi, J. Ibid. 1977, 66, 3498. (7) (a) Loutfy, R. 0.; Sharp, J. H.; Hsiao, C.-K; H o , R. J. Appl. Phys. 1981, 52, 5218. (b) Loutfy, R. 0.; Sharp, J. H. J. Chem. Phys. 1979, 71, 1211. (8) Popovic, Z. D.; Loutfy, R. 0. J. Appl. Phys. 1981, 52, 6190. (9) Ghosh, A. K; Morel, D. L.; Feng, T.; Shaw R. F.; Rowe C. A. J. Appl. (10) Wagner, H. J.; Loutfy, R. 0. J. VUC. Sci. Technol. 1982, 20, 300. (1 1) Loutfy, R. 0; McIntyre, L. F. Can. J. Chem. 1983, 61, 72; Sol. (12) (a) Hackett, C. F. J. Chem. Phys. 1971,55, 3178. (b) Minami, N. (13) Harrison, S. E. J. Chem. Phys. 1969, 50, 4739. (14) Sasseville, R. L.; Bolton, J. R.; Harbour, J. R. J. Phys. Chem. 1983, Phys. 1974,45, 230. Energy Mater. 1982, 6, 467. Ibid. 1980, 72, 6317. 87. 862. (15) Sasseville, R. L.; McIntosh, A. R.; Bolton, J. R.; Harbour, J. R. J. Phys. Chem. 1983, 87, 868. 0022-3654/84/2088-3139.$01.50/0 0 1984 American Chemical Society