Chem. Mater. 2009, 21, 5519–5526 5519 DOI:10.1021/cm902699s
Mechanism for Oxygen-Enhanced Photoconductivity in Rubrene: Electron Transfer Doping3 Ashok J. Maliakal,*,† Judy Y.-C. Chen,‡ Woo-Young So,§ Steffen Jockusch,‡ Bumjung Kim,‡ Maria Francesca Ottaviani,#,( Alberto Modelli,^,|| Nicholas J. Turro,‡ Colin Nuckolls,‡ and Arthur P. Ramirez† †
LGS Innovations, 15 Vreeland Road, Florham Park, New Jersey 07932, ‡Department of Chemistry and § Department of Applied Physics, Columbia University 3000 Broadway, New York, New York 10027, # Institute of Chemical Sciences, University of Urbino, Piazza Rinascimento 6, 61029 Urbino, Italy, ^ Department of Chemistry, University of Bologna, via Selmi 2, 40126 Bologna, Italy, and ||CIRSA, via S. Alberto 163, 48100 Ravenna, Italy. ( Current address: Department of Geological Sciences, Chemical and Environmental Technologies Scientific Campus Loc. Crocicchia 61029 Urbino, Italy. Received August 31, 2009
The oxygen-enhanced photoconductivity observed in crystalline rubrene is investigated using electron paramagnetic resonance (EPR) spectroscopy and steady-state and time dependent photoconductivity (PC) measurements. The EPR data indicate the presence of rubrene radical cation and oxygen radical anion pairs formed within the crystalline structure when rubrene is irradiated in the presence of oxygen. Radical lifetimes determined using EPR spectroscopy correlate well with transient PC data and provide strong evidence that the rubrene radical cation is the charge carrier responsible for enhanced conduction. This process is reversible, although photodegradation is also observed. The oxygen-enhanced PC of rubrene is thus explained by an electron transfer mechanism that generates radical cation “hole” carriers within the crystal via the oxygen acceptor. Introduction Rapid advances in the development of organic thin film transistors have given scientists and engineers the hope of achieving flexible electronic devices such as displays, radio frequency ID tags, and potentially large area organic photovoltaics.1-3 However, further understanding of the underlying issues relating to charge transport in these materials is critical to realize the full potential of devices based on molecular solids. In this regard, studies of single crystal devices have shed much light on the charge transport processes, energetics of trap states, and 3 This publication involves research sponsored by the U.S. Department of Energy under grant no. DE FG02-04ER 46118 and Columbia University. *Corresponding author.
(1) Dimitrakopoulos, C.; Malenfant, P. Adv. Mater. 2002, 14, 99–117. (2) Reichmanis, E.; Katz, H.; Kloc, C.; Maliakal, A. Bell Labs Tech. J. 2005, 10, 87–106. (3) Organic Field Effect Transistors; Bao, Z., Locklin, J., Eds.; CRC Press: Boca Raton, FL, 2007. (4) Goldmann, C.; Krellner, C.; Pernstich, K. P.; Haas, S.; Gundlach, D. J.; Batlogg, B. J. Appl. Phys. 2006, 99, 034507. (5) Lang, D.; Chi, X.; Siegrist, T.; Sergent, A.; Ramirez, A. Phys. Rev. Lett. 2004, 93, 086802-1–086802-4. (6) Krellner, C.; Haas, S.; Goldmann, C.; Pernstich, K. P.; Gundlach, D. J.; Batlogg, B. Phys. Rev. B 2007, 75, 245115. (7) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E. Phys. Rev. Lett. 2004, 93, 086602. (8) Calhoun, M.; Sanchez, J.; Olaya, D.; Gershenson, M.; Podzorov, V. Nat. Mater. 2008, 7, 84–89. (9) Gershenson, M.; Podzorov, V.; Morpurgo, A. Rev. Mod. Phys. 2006, 78, 973. (10) Sundar, V.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willet, R.; Someya, T.; Gershenson, M.; Rogers, J. Science 2004, 303, 1644–1646. (11) Menard, E.; Marchenko, A.; Podzorov, V.; Gershenson, M. E.; Fichou, D.; Rogers, J. A. Adv. Mater. 2006, 18, 1552–1556. r 2009 American Chemical Society
control of dopants in organics.4-8 Most notably, rubrene has been the subject of several investigations,7,9-11 in part due to its easy crystallization and low band gap (∼2 eV), which is well-suited for many electronics applications. Especially interesting for applications involving either light emission or conversion is rubrene’s photoconductivity, which is dramatically enhanced in the presence of oxygen.12,13 Despite several publications implicating oxygen’s role, there is as of yet no plausible explanation as to the microscopic origin of this photoconductivity enhancement.6,12,14 The rubrene endoperoxide species has been identified as a substantial impurity arising from the reaction of rubrene with oxygen in the presence of light.15 The endoperoxide has been observed as a surface layer on crystalline rubrene as well as an impurity (∼1%) within the bulk rubrene crystal.16,17 The role of the endoperoxide in the photoconduction mechanism has been questioned, however, because of its possession of a deeper HOMO level than rubrene, a shorter conjugation length, and a shape that would create severe local strain as a defect in (12) Mathews, N.; Fichou, D.; Menard, E.; Podzorov, V.; Mhaisalkar, S. Appl. Phys. Lett. 2007, 91, 212108. (13) So, W. Y.; Wikberg, J. M.; Lang, D. V.; Mitrofanov, O.; Kloc, C. L.; Siegrist, T.; Sergent, A. M.; Ramirez, A. P. Solid State Commun. 2007, 142, 483–486. (14) Nakayama, Y.; Machida, S.; Minari, T.; Tsukagishi, K.; Noguchi, Y.; Ishii, H. Appl. Phys. Lett. 2008, 93, 173305. (15) Harada, Y.; Takahashi, T.; Fujisawa, S.; Kajiwara, T. Chem. Phys. Lett. 1979, 62, 283. (16) Kafer, D.; Witte, G. Phys. Chem. Chem. Phys. 2005, 7, 2850–2853. (17) Mitrofanov, O.; Lang, D. V.; Kloc, C.; Wikberg, J. M.; Siegrist, T.; So, W. Y.; Sergent, M. A.; Ramirez, A. P. Phys. Rev. Lett. 2006, 97, 166601.
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the rubrene crystal structure.14 As such, a priori one would expect that the rubrene endoperoxide would have a deleterious effect on conductivity (either dark or photoinduced) within the rubrene crystal. An alternative pathway for the interaction of oxygen with excited state polyaromatic hydocarbons (PAHs) is via electron transfer,18-20 which has been used to explain photostability in substituted pentacenes21,22 and charge transfer in poly(3-hexylthiophene)23 and between the surface of an anthracene crystal and aqueous solution.24 In this pathway, electron transfer from the excited state PAH to oxygen (eq 1) results in the generation of the PAH radical cation and the superoxide radical anion. This reaction pathway competes with intersystem crossing (ISC) to the triplet, followed by singlet oxygen sensitization, or in the case of rubrene, singlet oxygen sensitization can occur simultaneously with the ISC.25 ISC is not competitive with fluorescence in the absence of oxygen.26 Both in solution as well as in thin films the electron transfer mechanism can result in further degradation reactions of PAHs.27 However, this electron transfer can also be reversible, and although favored in the excited state, back electron transfer is operative in the ground state (see eq 1 and Scheme 2).21
Maliakal et al. Scheme 1. Photoconductivity Setup
derivative (such as rubrene endoperoxide). By providing direct chemical evidence via EPR spectroscopy for both the donor and acceptor states involved in the photoconduction mechanism in rubrene, we provide strong evidence to associate the superoxide anion radical with the acceptor state responsible for the oxygen enhanced photoconductivity in rubrene.17 Experimental Section
On the basis of these observations, we hypothesize that the role of oxygen in enhancing rubrene photoconductivity is the result of electron transfer between oxygen and excited rubrene molecules. In the present study, we have successfully employed electron paramagnetic resonance (EPR) spectroscopy to observe directly both the superoxide radical anion acceptor and the rubrene radical cation donor within the rubrene crystal. Furthermore, we have correlated the lifetime of the rubrene radical cation EPR spectra with the transient photoconductivity data from single-crystal devices. These results are thus consistent with a charge transfer mechanism for photoconductivity enhancement that does not rely on a rubrene (18) Kavarnos, G. Fundamentals of Photoinduced Electron Transfer; VCH: New York, 1993. (19) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausalito, CA, 1991. (20) Mazur, M.; Blanchard, G. J. J. Phys. Chem. B 2004, 108, 1038–1045. (21) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem. Mater. 2004, 16, 4980–4986. (22) Northrup, B.; Houk, K.; Maliakal, A. Photochem. Photobiol. Sci. 2008, 7, 1463–1468. (23) Abdou, M. S. A.; Orfino, F. P.; Son, Y.; Holdcroft, S. J. Am. Chem. Soc. 1997, 119, 4518–4524. (24) Pope, M.; Swenberg, C. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999. (25) Wu, K. C.; Trozzolo, A. M. J. Phys. Chem. 1979, 83, 2823–2826. (26) Lohmannsroben, H.-G. Appl. Phys. B: Laser Opt. 1988, 47, 195– 199. (27) Clar, E. Polycyclic Aromatic Hydrocarbons; Academic Press: London, 1964; Vol. 1.
Rubrene Crystal Growth and Device Fabrication. Rubrene crystals were grown using the vapor transport method designed by Laudise et al.28 A temperature-gradient furnace was prepared and rubrene powder (Acros Organics, 99%) was placed in a hot zone (250 C). The pressure of the system was decreased to 1 10-1 Torr. After 2-3 days, crystals can be obtained in crystallization zone (∼200 C). Electrodes (Gold) were deposited through a shadow mask using an Edwards Auto 306 thermal evaporator at a pressure of 5 10-6 Torr. Photoconductivity Measurement. Photoconductivity measurement is divided into two sections: DC and transient measurement. To obtain constant input photon flux, 100 W tungsten halogen lamp is operated all the time at the constant input power, 55.3 W, controlled by AL924A(elc) power supply. The light source is monochromated by SPEX TRIAX180 (Jobin Yvon), and the monochromated light is focused on the species, mounted in a vacuum probe station (MMR), with a 1 mm diameter. While the samples are mounted, the pressure of the chamber and the temperature of the species are controlled depending on the process conditions. All the material processes are conducted in the dark; the shutter is closed. DC photocurrent is measured by Keithley 6517A every 20 s after a new bias is applied while the shutter is open. Transient photocurrent is measured by HP4155A semiconductor parameter analyzer with a constant electric field, 167 V/cm, applied; it is recorded at intervals of 100 ms while the light is modulated by a shutter. Experimental Details for EPR Measurement. For EPR measurements, a quartz tube (3 mm inner diameter) containing rubrene crystals was evacuated to remove air and backfilled with oxygen to a pressure of 1 atm. The sample tube was placed into the rectangular cavity of a Bruker EMX spectrometer operating at X-band (9.5 GHz). The sample was (28) Laudise, R. A.; Kloc, C. S., P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449–454.
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Scheme 2. Proposed Mechanism of Photoconductivity for Rubrene in the Presence of Oxygena
a (A) Light absorption generates excited state of rubrene within crystal. (B) Interstitial oxygen (present through annealing of rubrene crystal with oxygen) serves as electron acceptor to generate radical ion pair. Rubrene radical cation serves as charge carrier which can migrate in the presence of an electric field. However, ground state rubrene radical cation thermodynamically favors back electron transfer with superoxide radical anion to regenerate rubrene and interstitial oxygen. (C) Degradative reactions of the rubrene radical cation are also observed and these eventually degrade the conductivity of the device.
irradiated inside the EPR cavity using a 300 W Xe lamp (ILC Technology, Inc.) in conjunction with a water-cooled pyrex filter to remove UV-light below 320 nm and IR light to avoid heating of the sample. The temperature for experiments at 120 K was controlled with a variable-temperature controller (ER 4131 VT, Bruker) that uses evaporating liquid N2 as coolant. Computational Details and Simulation. Calculations were performed with the Gaussian 03 set of programs.29 The total energies of the neutral and ion states and the hyperfine coupling constants were obtained at the density functional theory (DFT) level using the B3LYP hybrid functional,30 with the standard 6-31G(d) basis set, using the unrestricted formalism for the open-shell species. Computation of the EPR signals was accomplished by means of the SimFonia program by Bruker, considering a powder system with the unpaired electron spin coupling with the proton spin of the hydrogen nuclei at I = 1/2.
Results and Discussion Oxygen has been identified by us as a dopant in singlecrystal rubrene devices by both two-point photoconductivity and also field effect transistor measurements.13,17 In the present work, rubrene single crystals were grown using the vapor transport method (in argon atmosphere). Gold electrodes were deposited onto the crystals using (29) Frisch, M. J. T., G.W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M. E.; Replogle, S.; Pople, J. A. Gaussian; Gaussian Inc.: Pittsburgh, PA, 2003. (30) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
thermal evaporation through a shadow mask, and wires were connected to the measurement pads outside the crystals; refer to the inset Figure 1B. The devices were initially subjected to annealing in the measurement chamber under vacuum for 24 h at 100 C in order to improve contact properties of the gold electrodes and to remove residual oxygen that might be attached on the surface of the rubrene crystals during the sample preparation. The photoconductivity measurement was performed at 27 C under either vacuum or in an oxygen atmosphere, and the input power of lamp was maintained at 55.3W in order to keep the photon flux constant. The vacuum-annealed devices showed the lowest levels of conductivity in the presence of light (see Figure 1) when measured under a vacuum. When the measurement chamber is backfilled with oxygen at 15 psi, the photocurrent rises by 4-5% depending on the individual crystal used (see Table 1). Variations between crystals are anticipated because of differences in individual crystal parameters (shape, surface, impurities, and strain effects). For each type of crystal morphology, however, observable increases in photoconductivity are measured in the presence of oxygen. When the crystals are exposed to oxygen for longer periods of time (1-19 h) at 27 C, further increases in photoconductivity are measured, and are believed to result from increased diffusion of oxygen into the crystal. Finally, rubrene single crystals are annealed in an oxygen environment for 24 h at 100 C to assist oxygen diffusion into the crystals. In this process condition, we observe the highest level of oxygen induced photoconductivity (about 14 times enhancement). An alternative explanation for this photoconductivity enhancement is a possible change in the contact resistance as a result of oxygen treatment at higher temperatures. Although a 4-probe measurement would definitively rule this alternative hypothesis out, we can still reasonably associate this data point as an extension of our room
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Figure 1. Photoconductivity at RT of rubrene single crystal after various oxygen treatment processes under 500 nm illumination. Keithley 6517A was used for the photoconductivity measurement. (A) Photocurrent density versus field. (inset, typical device) (B) Photoconductance squared versus time, used to calculate oxygen diffusivity within rubrene crystal. The conductance was measured at a constant electric field of 167 V/cm; each data point (open square) represents a separate I-V measurement. Table 1. Evaluation of Oxygen Enhanced Photoconductivity at 100 V/cm Electric Field oxygen treatment (time in hours)
device 1
current density (A/cm) relative enhancement
vacuum
0
1
19
4.16 10-9 1
4.37 10-9 1.05
4.96 10-9 1.19
6.37 10-9 1.53
oxygen treatment (time in hours) vacuum device 2
current density (A/cm) relative enhancement
-10
9.61 10 1
temperature observations, because all the devices in our study were annealed in vacuum at 100 C for 24 h prior to the initial measurements. Furthermore, we do not anticipate oxygen in the second annealing step affecting contact resistance because the interface between the gold contact and the rubrene surface is protected by the gold electrode. The observation of enhanced photoconductivity in oxygen treated samples, even when the photoconductivity measurements are performed in vacuum, suggests that oxygen incorporated into the bulk of the crystal, as opposed to surface oxygen, is critical. To support our hypothesis that oxygen diffusion into the bulk of the rubrene crystal is responsible for the observed photoconductivity enhancement, we have
0
2 -9
1.00 10 1.04
1.75 10 1.82
24 (373 K) -9
1.30 10-8 13.6
estimated the oxygen diffusivity within the rubrene crystal using the results from Figure 1B. Generally, conductance is given by eq 2. SðtÞ ¼
W eμQðtÞ L
ð2Þ
where W is the width of the device, L is the length of it, e is the elementary charge, μ is carrier mobility, and Q(t) is the areal concentration of carrier, i.e., rubrene radical cation. However, based on the stoichiometry of eq 1, Q(t) can be equivalently represented as the concentration of superoxide within the rubrene crystal QO2-(t). Here, QO2-(t) can be related to the dissolved oxygen by estimation of the
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yield of superoxide φO2- and the relation in eq 3 below. The dissolved oxygen concentration as a function of time can be calculated using the relation in eq 4:31
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Using the appropriate inputs for light intensity and rubrene photophysics, the superoxide yield is estimated to be on the order of 1 10-4 (see the Supporting Information for details). The mobility of the photogenerated carriers is taken to be ∼1 cm2/(V s), and W/L in this work is 1.25. With these values and the geometry of the device, the diffusivity can be estimated from the slope of the line in Figure 1B. Our estimate for the diffusivity of O2 (DO2)
within the rubrene crystal from our photoconductivity data is on the order of ∼1 10-14 cm2/s at 27 C. Unfortunately, the lack of reliable data for oxygen concentration in rubrene limits the accuracy of our estimate, but it suffices for the purposes of our current work, i.e., determining the plausibility of oxygen diffusion as a mechanism to understand the photoconductivity data in Figure 1b. It is noted that even if the oxygen concentration within rubrene is much lower than we have estimated, a higher diffusion coefficient would result for this calculation. For example, if CO2 were as low as 1 1014 molecules/cm3, it would result in an estimate for DO2 on the order of 1 10-9 cm2/s. This range of values from our estimate can be compared with literature values for oxygen diffusion in polymers such as P3HT (∼1 10-8 cm2/s)23,33 and for oxygen diffusion reported in organic crystals (titaniumoxo phthalocyanine) (1 10-14 to 1 10-16 cm2/s).34 From this comparison, we conclude that the time dependence of the photoconductivity data in Figure 1b is consistent with the hypothesis of oxygen diffusion into the bulk of the rubrene. The mechanism of oxygen enhanced photoconductivity in rubrene that we envision is illustrated in Scheme 2, in which a rubrene molecule is excited by absorption of light into its singlet excited state. The resulting singlet excited state is energetic enough to favor electron transfer to a nearby interstitial oxygen molecule. The resulting geminate radical cation-radical anion pair can separate via migration of the hole to other rubrene molecules in the crystal by hopping drift, or alternatively in the presence of an electric field, charge separation can occur. To support this hypothesis for oxygen doping of rubrene, we have measured the solid state EPR spectra of crystalline rubrene prepared under analogous conditions to the samples used in photoconductivity measurements. These samples are crushed to increase surface area prior to placement in a quartz tube (3 mm diameter) for EPR measurement. The samples were irradiated directly within the EPR cavity with white light filtered to remove UV and a water filter to prevent sample heating. Measurements are performed in both 1 atm oxygen, as well as under vacuum conditions. For samples under a vacuum (absence of O2) no EPR signal was observed in the absence of light and also under illumination (see the Supporting Information). However, in the presence of 1 atm O2, EPR signals were observed under illumination. Typical EPR spectra taken at room temperature are illustrated in Figure 2. Before irradiation, no EPR signal was observed (Figure 2a). Under light illumination, a broad unresolved signal is growing (line width about 8 G) centered at g = 2.0039 (based on DPPH reference at g = 2.0036). As an example, Figure 2b shows this broad signal after an irradiation time of 15 min. This broad signal was simulated (see Figure 2e) using the coupling constants obtained for the rubrene cation
(31) Sze, S. Semiconductor Devices: Physics and Technology, 2nd ed.; Wiley: New York, 2001. (32) Saunders, M.; Jininez-Vazquez, H.; Cross, R. J. J. Am. Chem. Soc. 1994, 116, 2193–2194.
(33) Guillet, J.; Andrews, M. Macromolecules 1992, 25, 2752–2756. (34) Luer, L.; Egelhart, H.-J.; Oelkrug, D.; Winter, G.; Hanack, M.; Weber, A.; Bertagnolli, H. Synth. Met. 2003, 138, 305–310.
QðtÞ ¼ QO2 - ðtÞ ¼ QO2 ðtÞφO2 QO2 ðtÞ = CO2
pffiffiffiffiffiffiffiffiffiffi DO2 t
ð3Þ ð4Þ
where CO2 is the volume concentration of the dissolved oxygen at the rubrene surface at equilibrium with the atmospheric oxygen and DO2 is the diffusivity of oxygen. There is very little published data for dissolved oxygen concentration CO2 within organic crystals. For our estimation of oxygen diffusivity, we have to use of data available from the literature for poly-3-hexyl thiophene, a semicrystalline polymer. CO2 can be calculated using the relationship in eq 5, SO2 is the solubility coefficient, pO2 is the oxygen pressure kB is the Boltzmann constant, and T is temperature (values for SO2 have been reported for poly-3-hexylthiophene (SO2 = 2.8 10-3).23 C O2 ¼
SO2 pO2 kB T
ð5Þ
At 27 C and 1.02 atm, the value of CO2 is calculated to be 7 1016 O2 molecules/cm3. It may be argued that CO2 for this semicrystalline polymer could be higher than that for a denser crystalline solid such as rubrene. Unfortunately, to the best of our knowledge, there is no reliable data for oxygen solubility in organic crystals that we can use to improve our estimation. We have found data in which Xe is incorporated into C60 under high pressure, and under these conditions 0.008% Xe is incorporated on mole fraction basis.32 Although the Xe@C60 is an endohedral complex, because of its size, its incorporation into the C60 requires steric distortions, and so may serve as a reasonable model for oxygen incorporation into rubrene. Based on the value of 0.008% incorporation, we can calculate a value for CO2 of ∼1 1017 which is very close to the value of CO2 in P3HT. The conductance-vs-time equation can be expressed in terms of the diffusivity by combining eqs 2, 3, and 4 yielding eq 6. SðtÞ =
pffiffiffiffiffiffi W eμφO2 - CO2 Dt L
ð6Þ
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Figure 2. EPR spectra of rubrene crystals in the presence of 1 atm O2 (a) before and (b, c) after irradiation with light (>320 nm) at room temperature for (b) 15 and (c) 30 min. Microwave power: 20 mW. Spectrum d was generated by subtraction of spectrum b from c. The red spectra e-g are simulated spectra for experimental spectra b-d, respectively.
from B3LYP/6-31G(d) calculations using the crystal structure geometry reported previously (see the Supporting Information).35 A 45-line spectrum is reported in the literature for the rubrene radical cation in frozen solution.36 The lack of resolution in our spectra is believed to be due to line broadening resulting from rapid exchange of the hole between rubrene sites as expected for this high mobility semiconductor.36-40 The observed line shape is symmetrical and the lack of asymmetry is consistent with previous ESR studies of AsF5 doped polyacetylene, in which asymmetric (Dysonian) line shape was only observed at very high doping levels that corresponded to metallic behavior in this system.41 As such, we conclude that the density of rubrene radical cations produced is low enough that the material is not exhibiting metallic properties. After continued irradiation, a finely structured spectrum (containing at least 17 lines) appears superimposed on the broad peak (Figure 2c). Under our irradiation conditions, the maximum intensity of overall signal was observed after an irradiation time of 30 min. Continuing irradiation caused a decrease of the EPR signal. The finely structured spectrum with a 20% contribution was extracted from the broad signal by scaled subtraction of spectrum b (Figure 2) from spectrum c and is shown in Figure 2d. This finely structured spectrum superimposed on the broad peak is not accounted for by the coupling constants calculated for the rubrene radical cation. The resolved features are satisfactorily reproduced (using the SimFonia program by Bruker) by considering 8 protons with a coupling constant aH = 3 G and 8 protons with a (35) Bulgaroskava, I.; Vozzhennikov, V.; Aleksandrov, S.; Belsky, V. Latv. PSR Zinat. Akad. Vestis, Kim. Ser. 1983, 4, 53. (36) Wheeler, L.; Bard, A. J. Phys. Chem. 1967, 13, 4513–4517. (37) da Silva, D. A.; Kim, E. G.; Bredas, J. L. Adv. Mater. 2005, 17, 1072–þ. (38) Olivier, Y.; Lemaur, V.; Bredas, J. L.; Cornil, J. J. Phys. Chem. A 2006, 110, 6356–6364. (39) Valeev, E. F.; Coropceanu, V.; da Silva, D. A.; Salman, S.; Bredas, J. L. J. Am. Chem. Soc. 2006, 128, 9882–9886. (40) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644–1646. (41) Goldberg, I.; Crowe, H.; Newman, P.; Heeger, A.; MacDiarmid, A. J. Chem. Phys. 1979, 70, 1132–1136.
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Figure 3. Structure of rubrene cation (gray)-superoxide anion (red) radical complex that gives rise to structured signal in Figure 2d based on calculated hyperfine constants and Simfonia EPR spectra simulation.
Figure 4. EPR spectrum upon illumination of rubrene in the presence of O2 at 120 K. Experimental spectrum (black), simulated spectrum (red) (microwave power 6.4 mW).
coupling constant aH = 1.6 G (see Figure 2g). We have attempted numerous B3LYP/6-31G(d) computations of the coupling constants of possible paramagnetic impurities and byproducts of rubrene reaction with oxygen including rubrene cation/O2 radical and rubrene/O2 (triplet state) diradical systems. Coupling constant values (of the protons of the phenyl groups) consistent with those used in the simulation were found for a (gas-phase) rubrene/O2 system (represented in Figure 3) with a net charge = 0 and multiplicity = 3, as expected for a rubrene cation/superoxide anion diradical species. The same rubrene geometry found35 in the crystal, an O-O distance of 1.32 A˚ (close to that (1.35 A˚) calculated for the O2- anion) and an O-C distance of 1.80 A˚ were used. Although other possibilities cannot be ruled out, these results indicate that the structured signal can be accounted for by close proximity of superoxide anion to rubrene radical cation. Irradiation for extended periods of time caused photodegradation of the rubrene crystal and both reduction in EPR signal intensity as well as disappearance of the finely structured signal. Removal of the sample from the EPR cavity and visual inspection indicate significant darkening of the rubrene sample as a result of illumination in the presence of oxygen. In the case of rubrene illuminated without oxygen, this darkening is not observed. These
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Figure 5. (a, b) Kinetic traces (red lines) of the EPR signal at (a) 3471.31 G, (b) 3479.13 of rubrene in the presence of 1 atm O2. The irradiation was interrupted at time = 0. The blue lines show the fits of the experimental data to a first order kinetics. The black spectrum is equivalent to Figure 2c and arrows indicate the field positions of the kinetic traces. (c) Transient photoconductivity measurement on rubrene single crystal at 500 nm. The light was modulated by a shutter, and the transient conductivity was measured by HP4155A at intervals of 100 ms. (d) Rescaling of trace in c to emphasize long τ decay.
observations demonstrate that the rubrene crystal clearly undergoes degradation in the presence of oxygen upon extended illumination. To further define the role of oxygen in photochemically induced conductivity of rubrene, we sought direct evidence for the other putative partner in the electron transfer process, i.e., the superoxide radical anion. The superoxide radical has been studied under various conditions in low temperature glasses, absorbed on various media.42,43 It is well-known that the superoxide signal is typically too broad to be observed by EPR at room temperature. However, at low temperature (120 K), the superoxide signal sharpens and the spectrum becomes observable. Thus we have measured the EPR spectra of photoirradiated rubrene at 120 K (Figure 4). The superoxide anion radical is reported to be long-lived and distinguished by the three g components gxx= 2.022, gyy = 2.009, gzz = 2.0035.44,45 The simulated spectrum (Figure 4 bottom), which was computed by using these reported g values shows good agreement with the experimental spectrum (Figure 4 top). Therefore we assigned the observed EPR signal to the superoxide anion radical. Computation of the experimental spectrum also revealed the presence of an unresolved signal with g = 2.0038, which is attributed to the free rubrene radical cation. Kinetic studies of the disappearance of the room temperature EPR spectra can be used to identify the lifetime (42) Bagchi, R.; Bond, A.; Scholz, F.; Stosser, R. J. Am. Chem. Soc. 1989, 111, 8270–8271. (43) Qiao, X.; Chen, S.; Tan, L.; Zheng, H.; Ding, Y.; Ping, Z. Magn. Reson. Chem. 2001, 39, 207–211. (44) Murphy, D. M.; Chiesa, M. In Electron Paramagnetic Resonance; Gilbert, B. C., Davies, M. J., Murphy, D. M., Eds.; The Royal Society of Chemistry: London, 2004; Vol. 19, pp 279-317. (45) Anpo, M.; Che, M.; Fubini, B.; Garrone, E.; Giamello, E.; Paganini, M. Top. Catal. 1999, 8, 189–198.
Table 2. Transient Photoconductivity for Rubrene Crystal at Various Wavelengths with Fitting Parameters for Double Exponential Decay wavelength (nm)
τ1 (s)
τ2 (s)
350 400 450 500 545
14.6 15.3 13.9 16.9 10.9
0.7 0.8 0.7 1.0 0.5
R2 0.98576 0.98869 0.98665 0.99239 0.98112
of the rubrene radical cation within the crystal. For these studies, crystalline rubrene in the presence of 1 atm O2 was irradiated for several minutes. After interruption of irradiation, the EPR signal intensity was recorded at a fixed magnetic field (Figure 5a and 5b). The signal decays fitted well to a first order kinetics with lifetimes of approximately 18 s. This value suggests a relatively slow recombination of radical anion and cation for regeneration of the rubrene and oxygen molecules presumably due to the time required for a rubrene “hole” to migrate via hopping within the proximity of the superoxide anion to permit recombination. Recombination of rubrene cations and superoxide anions is likely the primary mechanism of radical loss; however, alternative decomposition pathways are also operative. This observation of kinetic decay correlates, reasonably well with literature reports of persistent photoconductivity in rubrene.12,17 Transient photoconductivity (PC) measurements on crystalline rubrene samples indicate a double exponential decay (see Figure 5c,d and Table 2). The shorter lifetime is on the order of 1 s and is believed to be due to the removal of transient charge that is swept to the electrodes by the applied field, and is no longer regenerated in the absence of light. The longer lifetime is found to be between 11 and 17 s and is believed to result from recombination of charge, i.e., reverse electron transfer
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from the rubrene cation to the superoxide anion. This longer lifetime agrees well with the EPR lifetime measurements for the radical cation. This correlation suggests that the EPR spectra and PC response have the same microscopic origin. Conclusion The direct observation of both rubrene radical cation and superoxide radical anion via EPR spectroscopy gives strong evidence for our hypothesis of an electron transfer mechanism of oxygen doping in rubrene single crystals: Photoexcitation of the rubrene molecule produces an electronically excited singlet state that reacts with available molecular oxygen to form a rubrene radical cation-superoxide radical anion pair (R•þ/O2•-) (see eq 1). Kinetic studies provide a means of estimating the lifetime of the R•þ and O2•- radicals within the crystal. Detailed chemical knowledge of the mechanism of oxygen photodoping of rubrene will permit the development of processes for controlled doping of rubrene crystal similar to those developed by us.13 Beyond the knowledge of the precise doping mechanism operating in rubrene, this study suggests that other organic semiconductors can be understood at a similar fundamental level. Ultimately, precise understanding of
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oxygen doping in organics will open up a pathway to generate devices with specific conductivity profiles analogous to p-type doping of silicon. Furthermore we believe that this electron transfer mechanism of doping is present in oxygen treated rubrene field effect transistors, in which case the electric field is used to provide the appropriate energetics for charge transfer rather than light. We are currently extending our studies of oxygen doping via EPR spectroscopy to electrochemical doping. Acknowledgment. N.J.T., S.J., and J.Y.-C.C. thank the NSF for its generous support of this research through Grant CHE 07-17518. A.J.M, W.-Y.S., A.P.R., C.N., and B.K. thank the DOE for its support through Grant DE-FG0204ER46118. A.M. thanks the Italian Ministero dell0 Istruzione, a e della Ricerca, for financial support. The dell0 Universit authors are grateful to the reviewers for their comments and suggestions, which have substantially improved the quality of this manuscript. Supporting Information Available: EPR spectra of rubrene in the absence of oxygen at various irradiation times and details of calculation of oxygen diffusivity in rubrene based on photoconductivity data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
