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Thermally Stimulated Current Investigation of Copper Octaethylporphyrin Dimer Langmuir-Blodgett Films Antonio Serra,† Tiziana Siciliano,† Emanuela Filippo,† Gioacchino Micocci,† Antonio Tepore,† Dennis P. Arnold,‡ and Ludovico Valli*,§ Laboratorio di Fisica Applicata, Dipartimento di Scienza dei Materiali, Universita` degli Studi di Lecce and INFM UdR Lecce F1, Via Arnesano, I-73100 Lecce, Italy, Synthesis and Molecular Recognition Program, School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane, Australia 4001, and Dipartimento di Ingegneria dell’Innovazione, Universita` degli Studi di Lecce, Via Provinciale per Monteroni, I-73100 Lecce, Italy Received August 5, 2004. In Final Form: October 14, 2004 Langmuir-Blodgett films containing meso,meso′-buta-1,3-diyne-bridge Cu(II) octaethylporphyrin dimer have been deposited with the aim of carrying out a thermally stimulated current (TSC) investigation. This characterization was obtained upon cooling samples irradiated by light and others without irradiation: in this way TSC curves have been registered in the temperature range of 100-300 K. Analysis of experimental data has been performed through the application of three different approaches: the initial rate, the heating rate, and Chen’s methods. In particular, a trap center at about 0.38 eV has been evidenced and the three different methods have given results in close accordance.
Introduction Optical and electronic properties of porphyrin arrays have aroused much scientific interest owing to their potential applications as solar cells and molecular photonic and electronic devices.1-3 Over the past decade, the Langmuir-Blodgett (LB) method has been widely used to construct ordered molecular assemblies of porphyrins with well-controlled composition, thickness, and architecture.4,5 Thermal stability, rigidity, crystallinity, anisotropy, metal complex formation, gas adsorption, possible polymerization, and high dielectric constants are some desirable properties of such multilayers. The conjugated aromatic systems of porphyrins are good examples satisfying these requirements, and in addition they evidence other interesting properties typical of semiconductors, photoconductors, photovoltaics, and nonlinear optical * To whom correspondence should be addressed. Mail: Dipartimento di Ingegneria dell’Innovazione, Universita` degli Studi di Lecce, Via Provinciale per Monteroni, Edificio “La Stecca”, I-73100 Lecce, Italy. Tel: +39.0832.297325. Fax: +39.0832.325004. Email:
[email protected]. † Laboratorio di Fisica Applicata, Dipartimento di Scienza dei Materiali, Universita` degli Studi di Lecce and INFM UdR Lecce F1. ‡ Synthesis and Molecular Recognition Program, School of Physical and Chemical Sciences, Queensland University of Technology. § Dipartimento di Ingegneria dell’Innovazione, Universita ` degli Studi di Lecce. (1) Lammi, R. K., et al. Quenching of porphyrin excited states by adjacent or distant porphyrin cation radicals in molecular arrays. Chem. Phys. Lett. 2001, 341 (1,2), 35-44. (2) Debreczeny, M. P., et al. Optical control of photogenerated ion pair lifetimes: An approach to a molecular switch. Science 1996, 274 (5287), 584-587. (3) Joran, A. D., et al. Effect of exothermicity on electron-transfer rates in photosynthetic molecular models. Nature 1987, 327 (6122), 508-511. (4) Petty, M. C. Langmuir-Blodgett Films, an introduction; Cambridge University Press: Cambridge, U.K., 1996. (5) Arnold, D. P., et al. Porphyrin Dimers Linked by a Conjugated Alkyne Bridge: Novel Moieties for the Growth of Langmuir-Blodgett Films and Their Applications in Gas Sensors. Langmuir 1997, 13 (22), 5951-5956.
materials. Therefore, they have huge potential applications as molecular devices.6-12 Even though porphyrin dimers and oligomers linked by covalent bridges also match in an excellent way such requirements,13 most porphyrin oligomers are not conjugated because deviation from coplanarity leads to poor π-overlap; for example, in meso-aryl porphyrins there is generally a twist of about 70° between the plane of the aryl ring and that of the porphyrin. The butadiyne bridge overcomes this problem,14-16 and it offers some of these interesting properties, in that it is rigid, linear, and sterically (6) Bao, Z.; Yu, L. Recent progress in the synthesis and applications of porphyrin-containing polymers as electronic and photonic materials. Trends Polym. Sci. 1995, 3 (5), 159-164. (7) Redmore, N. P., et al. Synthesis, Electronic Structure, and Electron-Transfer Dynamics of (Aryl)ethynyl-Bridged Donor-Acceptor Systems. J. Am. Chem. Soc. 2003, 125 (29), 8769-8778. (8) LeCours, S. M., et al. Push-Pull Arylethynyl Porphyrins: New Chromophores That Exhibit Large Molecular First-Order Hyperpolarizabilities. J. Am. Chem. Soc. 1996, 118 (6), 1504-1510. (9) Priyadarshy, S., et al. Acetylenyl-Linked, Porphyrin-Bridged, Donor-Acceptor Molecules: A Theoretical Analysis of the Molecular First Hyperpolarizability in Highly Conjugated Push-Pull Chromophore Structures. J. Am. Chem. Soc. 1996, 118 (6), 1497-1503. (10) Lin, V. S.-Y.; Therien, M. J. The role of porphyrin-to-porphyrin linkage topology in the extensive modulation of the absorptive and emissive properties of a series of ethynyl- and butadiynyl-bridged bisand tris(porphinato)zinc chromophores. Chem.sEur. J. 1996, 1 (9), 645651. (11) Lin, V. S.-Y., et al. Highly conjugated, acetylenyl bridged porphyrins: New models for light-harvesting antenna systems. Science 1994, 264 (5162), 1105-1111. (12) Hyslop, A. G.; Therien, M. J. Electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers shows a weak distance dependence. Chemtracts: Inorg. Chem. 1993, 5 (2), 88-91. (13) Anderson, H. L. Building molecular wires from the colours of life: Conjugated porphyrin oligomers. Chem. Commun. 1999, (23), 2323-2330. (14) Arnold, D. P., et al. Dimeric porphyrins linked by conjugated groups containing triple bonds: The crystal structure of the nickel octaethylporphyrin dimer bridged by 2,5-diethynylthiophene. J. Chem. Soc., Chem. Commun. 1994, (18), 2131-2132. (15) Arnold, D. P.; Heath, G. A. Voltammetric and UV to near-IR spectroelectrochemical characterization of the meso,meso′-buta-1,3diyne-bridged octaethylporphyrin dimers {[M(OEP)](µ-C4)[M(OEP)]} (M2 ) H4, Co2, Ni2, Cu2, Zn2, Pd2, Pt2, Co/Ni, and Ni/Zn), in their neutral, mononegative, and dinegative oxidation states. J. Am. Chem. Soc. 1993, 115 (25), 12197-12198.
10.1021/la048020s CCC: $30.25 © 2005 American Chemical Society Published on Web 12/02/2004
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Chart 1. Structure of meso,meso′-Buta-1,3-diyne-bridge Cu(II) Octaethylporphyrin Dimer (M ) Cu)
nondemanding and allows the formation of a wider and highly conjugated molecular structure. These properties depend on the electron interaction between porphyrin macrocycles in the films, and the macrocycle orientation and distance are important parameters for electron interactions. Among the electronic investigation methods, only very rarely has thermally stimulated current (TSC) analysis been carried out on porphyrin monomer and dimer Langmuir-Blodgett films. The TSC technique is a powerful tool for the characterization of trapping centers in solids and has been extensively employed for characterization of inorganic semiconductors, but scarcely used for organic films.17 In principle, this method consists of filling the traps after cooling the material to low temperature and then heating the device at a constant rate and observing the thermally stimulated current as the traps are emptied. The energy level within the band gap of a particular trapping center is related to the temperature at which it is emptied, while the number of traps contributing to the observed current peak depends on the amount of stored charges that are released. The dynamics of charge transport in porphyrin dimer films is of key importance to the performance of the devices. For technological employment, several studies are needed. Although the details of the charge transport are not well understood on the molecular scale, it is generally accepted that it occurs through thermally activated hopping between electronic levels.18 In this paper, we have investigated the localized electronic states in LangmuirBlodgett films of meso,meso′-buta-1,3-diyne-bridge Cu(II) octaethylporphyrin dimer (CuOEPD, Chart 1) which has the two metalated macrocycles connected by a butadiyne bridge, by using the TSC method. The obtained results suggest a semiconducting behavior and are analyzed in terms of an intrinsic semiconductor model. Experimental Details The compound CuOEPD was available from our previous studies.5 Langmuir experiments were carried out using a KSV5000 System 3 apparatus (subphase surface of 850 cm2). A 1:4 molar mixture of porphyrin and arachidic acid was dissolved in chloroform as the spreading solvent. The porphyrin and arachidic acid concentrations were 7.5 × 10-5 and 3.0 × 10-4 M, respectively, in all cases. A 200 µL aliquot of the spreading solution was spread onto the subphase, whose temperature was kept at 20 °C by a Haake GH D8 apparatus. The subphase was ultrapure water (Millipore, Milli-Q) with resistivity of 18.2 MΩ cm, containing 3 × 10-4 M CdCl2 and buffered with 10-5 M KHCO3 to pH 6.0. After solvent evaporation, the floating films were (16) Arnold, D. P.; James, D. A. Dimers and Model Monomers of Nickel(II) Octaethylporphyrin Substituted by Conjugated Groups Comprising Combinations of Triple Bonds with Double Bonds and Arenes. 1. Synthesis and Electronic Spectra. J. Org. Chem. 1997, 62 (11), 3460-3469. (17) Iwamoto, M., et al. Analysis of thermally stimulated Maxwelldisplacement current across organic monolayers. J. Appl. Phys. 1998, 83 (9), 4891-4896. (18) Gattinger, P., et al. Mechanism of Charge Transport in Anisotropic Layers of a Phthalocyanine Polymer. J. Phys. Chem. B 1999, 103 (16), 3179-3186.
Figure 1. Absorption spectra of the spreading solution (dotted line) and of a 100 layer LB film (solid line). compressed at a speed of 5 mm min-1. During the different depositions, the transfer surface pressure was fixed at 25 mN m-1 for CuOEPD. The transfer onto hydrophobic substrates was performed at a speed of 5 mm min-1 for both withdrawing and lowering the substrate through the monolayer on the water surface. Y-Type depositions were always obtained with transfer ratios close to unity for both dipping directions. For optical characterization, transparent quartz substrates were employed, while for electrical characterization gold-coated silicon substrates were used. After deposition of the organic layers, the gold cathode was deposited by thermal evaporation in an adjacent vacuum chamber without external contamination. The electrodes were patterned by a lithographic process. Polarized angle resolved optical transmission studies were performed on LB films by attaching film-covered (one side only) slides to a rotation stage assembly. This allows variation of the incidence angle of the film plane relative to the electric vector of the source light beam. A clean quartz slide at the same angle values was used as the reference. Absorption and transmission spectra were obtained with the rotation-stage assembly in a Varian Cary 5 double beam spectrophotometer. Linear polarization of the source beam was achieved with a visible wavelength sheet, which maintains a polarizing extinction ratio larger than 500:1 throughout the range 350-800 nm. The TSC measurements were taken and recorded by means of a Keithley 6517A electrometer in a Galileo cryostat under an Ar atmosphere at 10-3 mbar. For trap filling, the devices were first cooled by liquid nitrogen and then irradiated for 5 min by a 100 W filtered tungsten lamp. Afterward the samples were kept in the dark under a constant bias of 5 V for 5 min before the TSC sweep was carried out with a constant heating rate β under a driving voltage.
Results and Discussion The electronic absorption spectra of the spreading solution (10-4 M) and of a 100 layer LB film of ′CuOEPD are reported in Figure 1. As is apparent, they essentially consist of strong transitions to the second excited states (S0 f S2) in the 400-500 nm spectral region (the Soret or B band) and weak transitions to the first excited states (S0 f S1) at about 600 nm (the Q band); see the inset in Figure 1. Therefore, the Soret band has two perpendicular components that are degenerate in a simple monomer, but in porphyrin dimer they couple differently and cause a splitting. The splitting is strongly dependent on twist of the planes of the porphyrin as is well explained by the simple point-dipole exciton coupling theory developed by Kasha.19 The splitting in the Soret band vanishes in an orthogonal dimer, and inter-ring angles up to ca. 30° do not affect the splitting significantly.20 (19) Kasha, M. Exciton model in molecular spectroscopy. Pure Appl. Chem. 1965, 11 (3-4), 371-392. (20) Stranger, R., et al. Communication between Porphyrin Rings in the Butadiyne-Bridged Dimer Ni(OEP)(µ-C4)Ni(OEP): A Density Functional Study. Inorg. Chem. 1996, 35 (26), 7791-7797.
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The absorption spectrum obtained for the LB film is not grossly different from that found for the spreading solution. In solution, the Soret band displays three major components at 430, 448, and 481 nm, and Q components appear at about 567 and 606 nm. As already observed for Ni(II) octaethylporphyrin dimer5 in Langmuir-Blodgett films, the most significant variations are the loss of relative intensity of the band at ca. 480 nm (that suggests a twisting phenomenon between the planes of the dimers and a broadening and loss of resolution in the Soret region). On the other hand, the Q band region in the film spectra does not exhibit great changes from those registered in solution, except for a small blue shift for the major component. The loss of resolution in the Soret region is consistent with formation of aggregates of dimer molecules, even though no sharp bands are exhibited in the spectra. This is probably evidence that aggregation is not specific, but molecular association could take place from both plane to plane and edge to edge interactions. Moreover, in this connection, the red shift of the Soret band normally denotes preferential side by side association while face to face association causes a blue shift. Also the spectrum of Cu(II) octaethylporphyrin dimer, like the one observed for Ni(II) octaethylporphyrin dimer, may therefore be a reflection of weak aggregation in the porphyrin layer.21,22 The B and Q bands both arise from πfπ* transitions and can be explained by considering the four frontier orbitals:23 two π orbitals (a1u and a2u) and a degenerate pair of π* orbitals (egx and egy). The two highest occupied π orbitals happen to have about the same energy and constitute the highest occupied molecular orbital (HOMO) levels, while the lowest unoccupied π* orbitals constitute the lowest unoccupied molecular orbital (LUMO) levels. The average position of the a1u and a2u HOMO orbitals with respect to the eg LUMO orbitals is reflected in the positions of the Soret and Q bands. The absorption bands due to the a1u f eg and a2u f eg transitions translate to two different bands with very different intensities and wavelengths because configurational interaction takes place. Constructive interference leads to the intense shortwavelength B band, while the weak long-wavelength Q band results from destructive combinations. According to Gouterman’s four-orbital model,24,25 the conjugation of porphyrin dimers causes splitting in the π and π* levels, thus reducing the HOMO-LUMO gap. The HOMO-LUMO gap can be approximated to the energy of the longest wavelength absorption band. This is referred to as the optical energy gap Eg. So, the optical energy gap was obtained from the absorption spectrum of LB films analyzing the center of the Q absorption bands.26 In this way a HOMO-LUMO separation of about 2 eV was obtained. Determination of the Activation Energy of Trap. Figure 2 shows a pair of TSC curves in the 100-300 K (21) Arnold, D. P., et al. Porphyrin dimers linked by conjugated butadiyne bridges: Preparations, spectra, voltammetry and reductive spectroelectrochemistry of {(M(OEP))(µ-C4)(M(OEP))} (M2 ) H4, Co2, Ni2, Cu2, Zn2, Pd2, Pt2, Co/Ni, Ni/Cu, Ni/Zn). J. Porphyrins Phthalocyanines 1999, 3 (1), 5-31. (22) Arnold, D. P., et al. Conjugated dimers of nickel(II) octaethylporphyrin linked by extended meso,meso-alkynyl bridges. II. Redox properties and electronic spectra of electrogenerated anions and dianions. New J. Chem. 1998, 22 (12), 1377-1387. (23) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: San Diego, CA, 1978; Vol. 3, pp 1-165. (24) Gouterman, M. Spectra of Porphyrins. J. Mol. Spectrosc. 1961, 6, 138-163. (25) Gouterman, M. Effects of substitution on the absorption spectra of porphin. J. Chem. Phys. 1959, 30, 1139-1161. (26) Taylor, P. N., et al. Conjugated porphyrin oligomers from monomer to hexamer. Chem. Commun. 1998, (8), 909-910.
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Figure 2. TSC curves obtained upon heating a cooled sample irradiated by light (solid line) and a sample without irradiation (dotted line).
Figure 3. Standard simple energy level model used to analyze the TSC curves.
temperature range. The temperature was swept at a rate β ) 0.1 K s-1. One curve was obtained from a sample irradiated by light when cooled (solid line), and the other one from a sample not irradiated when cooled (dotted line). Current peaks are present only in the sample irradiated by light. Therefore, it is concluded that the TSC peaks are due to photoexcited carrier trapping. As may be seen, one well-separated peak at the temperature of 210 K is revealed, indicating the presence of one trapping center. A large number of models for the analysis of experimental TSC spectra have been described in the literature. For a single shallow trap Et, these models substantially assume the same single energy level scheme as shown in Figure 3. In this figure, Ed is a deeper trapping level and Er is a recombination center. Transition 1 corresponds to the thermal release of electrons from trap Et; transitions 2 and 2′ correspond to the trapping of electrons in traps Et and/or Ed, while transitions 3 and 3′ correspond to recombination of LUMO electrons with holes in centers Er and in the HOMO energy levels, respectively. The differences among the various methods of analysis of TSC curves are due to the different assumptions about the relative balance of the transitions reported in Figure 3, arising from the densities of the levels, their capture cross sections, and their activation energies. To evaluate the activation energy of traps from our experimental TSC spectra, we have chosen first-order recombination kinetics. The “Initial Rise” Method. Garlick and Gibson27 showed that when the traps begin to empty as the (27) Garlick, G. F. J.; Gibson, A. F. The electron-trap mechanism of luminescence in sulfide and silicate phosphors. Proc. Phys. Soc. 1948, 60, 574-590.
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Table 1. Summary of Results Obtained on Different CuOEPD LB Films Applying Chen’s Equations 4-6 films
τ (K)
δ (K)
ω (K)
Tm (K)
µg
Eτ (eV)
Eδ (eV)
Eω (eV)
1 2 3 4 5 6
13.0 13.1 13.0 13.2 12.5 12.5
9.0 9.0 9.5 8.9 9.1 10.0
22.0 22.1 22.5 22.1 21.5 22.5
211.0 209.0 209.5 210.0 211.5 212.0
0.410 0.409 0.411 0.404 0.418 0.440
0.386 0.380 0.382 0.381 0.406 0.410
0.416 0.409 0.405 0.416 0.418 0.378
0.403 0.395 0.395 0.399 0.415 0.397
Figure 4. Plot of ITSC vs 1/kT for the activation energy analysis by using the initial rise method.
temperature is increased, the TSC current can be expressed by
ITSC ) A exp(-Et/kT)
(1)
where k is Boltzmann’s constant, and A is a constant that depends on the density of initially full traps and the probabilities of the transitions in the TSC process. Thus, a plot of the logarithm of the current intensity against 1/kT should yield a straight line with a slope -Et. This is shown in Figure 4. The progressive shifting from the linear behavior at high current is due to overcoming the critical temperature Tc, after which the exponential law is no more valid, as was reported by Haake.28 This author, in fact, has investigated in detail the initial rise method and discussed the range of temperatures over which eq 1 is valid. The activation energy calculated by this procedure is Et ) (0.38 ( 0.02) eV. Heating Rate Method. According to Bube,29 the trap activation energy is related to the temperature Tm corresponding to the TSC peak by
Et ) kTm ln(νkTm2/βEt)
(2)
where ν is termed the attempt to escape frequency factor. In a semi-insulator with simple energy band surfaces,30
ν ) Ncvσt
Figure 5. TSC curves as a function of heating rate: (a) β ) 0.1 K s-1; (b) β ) 0.2 K s-1; (c) β ) 0.3 K s-1; (d) β ) 0.5 K s-1; (e) β ) 1.0 K s-1.
(3)
where Nc is the effective density of states in the LUMO levels, σt is the electron capture cross section, and v is the carrier thermally velocity. As shown by Hoogenstraten,31 if different heating rates are used and Tm is determined as a function of β, it is possible to derive Et from the slope of the straight line which should be obtained by plotting ln(Tm2/β) versus 1/kT. The measured TSC curves are shown in Figure 5; as is (28) Haake, C. H. Trapping action in electroluminescent zinc sulfide phosphors. J. Opt. Soc. Am. 1957, 47, 881-887. (29) Bube, R. H. Photoconductivity of solids; Wiley: New York, 1960. (30) Blakemore, J. S. Solid State Physics; Cambridge University Press: Cambridge, U.K., 1993. (31) Kroger, F. A.; Hoogenstraten, W. The location of dissipative transitions in luminescent systems. Physica (The Hague) 1950, 16, 3032.
Figure 6. Plot of ln(Tm2/β) vs 1/kT for the activation energy analysis by using the heating rate method.
evident, the TSC peak shifts to higher temperatures with increasing heating rates. Figure 6 shows the results of this analysis. The derived trap activation energy is given by Et ) (0.39 ( 0.02) eV. Knowing this, an attempt to escape frequency factor ν ) (4.1 ( 0.5) × 108 s-1 is obtained by using eq 2. Chen’s Method.32,33 This author makes use of numerical approximation in order to derive the activation energy of traps from experimental TSC curves. This method requires the measurement of the low-temperature Tl and/ or the high temperature Th at which the TSC signal is equal to half of its maximum value. The activation energy of trap is then given by
kTm2 Eτ ) [1.51 + 3.0(µg - 0.42)] τ [1.58 + 4.2(µg - 0.42)]kTm (4) Eδ ) [0.976 + 7.3(µg - 0.42)]kTm2/δ kTm2
Eω ) [2.52 + 10.2(µg - 0.42)]
ω
- 2kTm
(5) (6)
where τ ) Tm - Tl, δ ) Th - Tm, ω ) τ + δ, and µg ) δ/ω. (32) Chen, R. J. Electrochem. Soc. 1969, 116 (9), 1254-1257. (33) Chen, R. J. Appl. Phys. 1969, 40 (2), 570-595.
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In Table 1, the results obtained by this method on various samples are reported. As can be seen, the values of µg are in the range 0.404-0.440, which are close to Chen’s predicted value of µg ) 0.42 for first-order kinetics. This indicates that the trapping process is negligible for this center. The values of energies obtained with this method of analysis agree very well with those obtained by the two previous methods. Conclusions The samples that we have investigated by the TSC technique exhibit at least a trap center at about 0.38 eV from the LUMO levels with a value of about 4 × 108 s-1 for the attempt to escape frequency. The trap center was determined by different methods of analysis, and they well agree with each other. The retrapping process is negligible for this level. Studies performed on several layered organic compounds34-37 attribute trap states to structural defects and chemical impurities in the organic
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layers. However, further systematic studies are necessary in order to find reliable connections between the presence of traps and the transport properties in the CuOEPD LB films. Thermally stimulated current analysis is a powerful investigation tool in studying the energy level characteristics in the organic layered Langmuir-Blodgett films; furthermore, comparison of samples prepared under different arrangement conditions must be done in order to relate the energy level localization to structural features and/or chemical impurities of the films. LA048020S (34) von Malm, N., et al. Trap engineering in organic hole transport materials. J. Appl. Phys. 2001, 89 (10), 5559-5563. (35) Forsythe, E. W., et al. Trap states of tris-8-(hydroxyquinoline) aluminum and naphthyl-substituted benzidine derivative using thermally stimulated luminescence. Appl. Phys. Lett. 1998, 73 (11), 14571459. (36) Karg, S., et al. Determination of trap energies in Alq3 and TPD. Synth. Met. 2000, 111-112, 277-280. (37) Imperia, P., et al. Thermally stimulated processes in heterocyclic materials suitable for heterolayer organic light emitting diodes. Synth. Met. 2001, 124 (1), 83-85.
