ARTICLE pubs.acs.org/JPCC
Decay Mechanism of Spontaneously Built-up Surface Potential in a Thin Film of a Zwitterionic Molecule Having Noncentrosymmetric Crystal Structure Jun'ya Tsutsumi,*,† Hiroyuki Yoshida,‡,§ Richard Murdey,^ and Naoki Sato*,‡ †
Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan § PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ^ Pioneering Research Unit for Next Generation, Kyoto University, Uji, Kyoto 611-0011, Japan ‡
ABSTRACT: The spontaneously built-up surface potential in a vacuum-deposited thin film of a zwitterionic molecule, pyridinium 5,7-dihydro-5,7-dioxo-6H-cyclopenta[b]pyridin-6-ylide (4N-PI), was observed to decay upon illumination with a photon energy above the photoconduction threshold. The decay rate increased with increasing film crystallinity. The current-voltage relationship obtained from the surface potential decay can be explained by a combination of ohmic and space-charge-limited current conduction mechanisms, and the dependence of the fitting parameters on film crystallinity provided further strong evidence that photogenerated charge carriers contribute to the disappearance of the surface potential.
’ INTRODUCTION Organic insulators with gross electric polarization have potential applications in pyroelectric, piezoelectric, and/or ferroelectric devices. Such gross polarization is usually induced by an external electric field.1 It has recently been reported, however, that some organic materials show a spontaneous buildup of the electric polarization in the course of film deposition.2,3 This phenomenon was first observed in a vacuum-deposited film of an organic electroluminescent material, tris(8-hydroxyquinolinato)aluminum (Alq3).2 The surface potential, measured with the Kelvin probe technique, increased with the film thickness and reached 28 V at 560 nm when the film was deposited in the dark.2 The observed surface potential is an order of magnitude higher than, for instance, the interfacial energy difference at metal-organic contacts.4 Such spontaneous buildup of the surface potential is believed to arise from the preferential ordering of the molecular dipole moments during film growth.2,3,5-7 The surface potential on the Alq3 film grown in the dark decayed irreversibly to nearly 0 when illuminated with visible light.2 Two mechanisms have been proposed so far to explain the decay process, relaxation of preferentially ordered molecular dipole moments by photoinduced molecular rotation2,6,8,9 and cancellation of the built-up potential through the displacement of photogenerated charge carriers.10,11 The experimental evidence is contradictory. It was reported that the optical second-harmonic generation (SHG) intensity of the Alq3 film decreases with illumination, interpreted as an increase in randomization in the orientation of molecular dipole moments.2,9 In contrast, another research group claimed that the time profile of the potential decay could be explained by the drift of photogenerated charge carriers according r 2011 American Chemical Society
to the Poole-Frenkel conduction model.10,11 It is therefore difficult to ascertain which explanation is correct. We have recently observed a large surface potential for a polycrystalline thin film of pyridinium 5,7-dihydro-5,7-dioxo-6Hcyclopenta[b]pyridin-6-ylide (4N-PI) vacuum-deposited in the dark.12 The potential decayed upon illumination by visible light.12 This molecule is significantly different from Alq3 in terms of both its structure and properties. The molecular arrangement in the single crystal is noncentrosymmetric,13 in contrast to the centrosymmetric one of Alq3.14 This crystal structure is also observed in the polycrystalline film, where the crystallites orient with their (110) axes normal to the substrate surface. In contrast with the largely amorphous Alq3 films, the origin of the spontaneously built-up surface potential in 4N-PI is thought to be the uniaxial orientation of the crystallites.12 To elucidate the decay mechanism of the surface potential observed for the vacuum-deposited 4N-PI film, we first confirmed the crystal structure and estimated the crystallinity for films prepared at room temperature and 343 K. Then, we examined the wavelength dependence of the surface potential decay upon light illumination and investigated the time profile of the potential decay.
’ EXPERIMENTAL SECTION 4N-PI was synthesized according to the reported procedure15 and purified with silica gel column chromatography. After further Received: August 19, 2010 Revised: December 7, 2010 Published: January 7, 2011 2356
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Figure 1. GIXD and θ-2θ diffraction patterns of a 4N-PI film shown in comparison with the simulated pattern calculated from the single-crystal structure.
purification by vacuum sublimation, the sample material was vacuum-deposited on the native oxide surface of a silicon wafer at the rate of 1 nm min-1. Films were prepared in the dark and transferred to the measurement chamber without being exposed to air. The surface potential of the films was measured using a McAllister 6500 Kelvin probe both in the dark and under illumination. For the illumination, two different light-emitting diodes (LEDs) were used, hν = 2.8 (1.6 mW cm-2) and 1.9 eV (0.9 mW cm-2). The ambient pressure during the film preparation and Kelvin probe measurements was about 10-5 Pa. The structure of the films was examined in air with a RIGAKU ATX-G X-ray diffractometer; both θ-2θ diffraction and grazing incidence X-ray diffraction (GIXD) methods were used. Angular resolutions for the respective setups were 0.2 and 0.4. The photocurrent spectrum was measured for a 1 μm thick 4NPI film sandwiched by two gold electrodes separated by 1.4 mm laterally. The applied electric field was 18 kV m-1. A monochromatic light source (Bunkokeiki model SM-250) was used for the photoexcitation, and the photocurrent was measured with a Keithley 6487 picoammeter.
’ RESULTS AND DISCUSSION Thin Film Structure. Figure 1 shows X-ray diffraction patterns of a 300 nm thick film of 4N-PI. Both the θ-2θ and GIXD patterns show peaks corresponding to those simulated from the singlecrystal structure, indicating that the vacuum-deposited thin film, while polycrystalline, has the same crystallographic structure as the single crystal.13 The strong intensity of the (110) diffraction peaks in the θ-2θ pattern indicates that the crystallites are preferentially oriented such that the (110) axes are normal to the substrate surface.12 The dependence of the θ-2θ diffraction patterns at around 2θ = 26 on the substrate temperature for two 4N-PI films is shown in Figure 2. Whereas the diffraction profiles have almost the same shape, the (110) diffraction peak for the sample deposited at 343 K shows a smaller width (0.24 vs 0.34) and double the intensity of the film prepared at room temperature. The size of the crystallites in the film can be estimated from the diffraction peak width using Scherrer's equation.16 Taking the resolution of the X-ray diffractometer into consideration, we estimated the crystallite dimension along the (110) direction as 30 and 40 nm for the film deposited on the substrates at room temperature and 343 K, respectively. Performing the deposition on substrates at elevated temperature therefore improves the crystallinity in the film.
Figure 2. θ-2θ diffraction patterns, indicated by the open circles, of 4N-PI films prepared by vacuum deposition onto substrates at (a) room temperature and (b) 343 K. The solid and dashed lines indicate the simulated sum and component patterns, respectively, obtained from a Gaussian peak fit analysis. The width of the (110) diffraction peak varies with substrate temperature.
Photoconductivity. The photocurrent spectrum of the 4N-PI film is shown in Figure 3, with the incident photon-to-current conversion efficiency (IPCE) plotted against photon energy hν. The photoabsorption spectrum, also shown, is considerably different in shape. The photocurrent spectrum shows a sharp peak at 2.5 eV and a broad feature at around 3.5-5.0 eV, where the absorption intensity is relatively weak. This inverse relation is typically observed when photocarriers are predominantly generated in the bulk region of the film where strongly absorbing wavelengths of light cannot reach.17 The energy gap of the 4N-PI film is estimated to be 2.2 ( 0.1 eV from the onset of the photocurrent spectrum. This estimate corresponds to one (2.20 eV) reported in the earlier work of the photoconductivity experiment.18 Arrows shown in Figure 3 indicate the emission energies of the different LEDs used. The photocurrent spectrum demonstrates moderate intensity at the emission energy of the blue LED (hν = 2.8 eV), but the emission energy of the red LED (hν = 1.9 eV) lies below the estimated energy gap. Photocarrier generation in 4N-PI is therefore anticipated for the blue LED only. While molecular rotation under illumination is unlikely to occur in the closely packed crystal structure of 4N-PI,13 we cannot exclude the possibility of the molecular rotation upon illumination because of the large absorbance above 2.5 eV. Photoconduction and light-induced molecular rotation are, in principle, both possible with the blue LED. In the next subsection, we will analyze the decay curve in detail to distinguish which decay mechanism is most appropriate. Decay of the Spontaneous Surface Potential upon Illumination. We previously measured the surface potential of the 2357
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Figure 3. The photocurrent (solid line) and photoabsorption spectra (dashed line) observed for a 4N-PI film. Arrows indicate the emission energies of the LEDs used to irradiate the sample.
4N-PI thin film as a function of its thickness in the dark.12 At 300 nm, the potential reached 5.5 V, about one-third of the maximum value observed for Alq3 films of comparable thickness.2 The surface potential decayed when the film was illuminated by the blue LED, as shown in Figure 4. As anticipated, no decay was observed when using the red LED. It is clearly seen in Figure 4 that the surface potential decays more rapidly for the film prepared at 343 K than for the film prepared at room temperature. This is qualitatively consistent with a decay mechanism based on charge transport as the film prepared at 343 K has higher crystallinity and is therefore expected to have higher conductance. Neither decay curve can be fitted to a single exponential or power law function; however, though the decay is approximately exponential for both films at longer time scales in excess of about 5 min. The complexity of the decay curve means that additional analysis is required in order to properly assess the validity of the charge-transport model. The I-V transforms simplify the analysis by allowing various models for the current relation I = f(V) to be evaluated directly without having to derive time-dependent solutions as described later. To model into decay mechanism, the film is approximated as having a constant, ideal capacitance C with the standard definition Qo - Q ¼ CV
ð1Þ
The actual surface charge, Q, is offset by an effective charge, Qo, proportional to the net dipole moment in the film. The capacitor voltage is 0 when the surface charge exactly balances the net dipole moment. The surface potential VK measured by the Kelvin method did not fall to 0 but converged to a negative value V¥ estimated at ∼0.55 and ∼0.44 V for the films deposited on the substrates at room temperature and 343 K, respectively. This small voltage offset likely originates from the contact potential difference between the 4N-PI film and the substrate. The capacitor voltage is defined in relation to this offset potential as V = (VK - V¥). In the dark, Q = 0, and the capacitor voltage reaches the maximum value Qo/C. When the film is illuminated with light above a given energy threshold, the film becomes conductive, and photogenerated charge carriers drift toward the surfaces to cancel the net dipole moment. V decreases, and the rate of charge arriving at the surface of the film, taken as the current flow into the capacitor, is found by differentiating eq 1, dQ dV ¼ -C I ¼ dt dt
ð2Þ
Figure 4. Time profiles of the surface potential decay upon illumination of hν = 2.8 eV light for 4N-PI films prepared at (a) room temperature and (b) 343 K. The dashed lines indicate the estimated convergence at t = ¥.
The current is proportional to the derivative of the decay data in Figure 4. The I-V curves, with the current expressed as a reduced variable I/C, can be derived using eq 2, and these results are shown in Figure 5. The current varies linearly with the voltage up to the crossover voltage, Vc, above which a higher-order exponential dependence is observed. The crossover voltages determined from Figure 5 are 2.2 and 0.8 V for the films deposited on the substrates at room temperature and 343 K, respectively. The linear current-voltage relationship below Vc implies ohmic conduction, with a conductivity proportional to the slope. From the slope of the fits shown, it is noted that the film prepared at 343 K has a higher conductivity, consistent with the higher observed crystallinity and higher expected charge carrier mobility. The current is proportional to V6 and V4 above Vc for the respective films. These exponential dependences are consistent with spacecharge-limited current (SCLC) conduction in a high trap density environment.19 It is known that an exponential distribution of the density of states for charge carrier traps can give the relation, I Vlþ1, where l is an empirical parameter. A large value of l corresponds to a wide trap distribution over an extended energy region. For films prepared at room temperature and 343 K, l = 5 and 3, respectively. This indicates the broader energetic distribution of trap states in the film prepared at room temperature. In this case, trap density N is expressed as19 Nt Et exp NðEt Þ ¼ ð3Þ lkB T lkB T In this equation, Et and Nt are an energy depth and a volume density of the trap states, respectively. Vc is proportional to the trap density Nt;17,20 the larger crossover voltage observed for the film 2358
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Figure 5. I-V characteristics converted from the time decay data using eq 2 for 4N-PI films prepared at (a) room temperature and (b) 343 K. The current is ohmic at low voltages but increases exponentially with the order indicated for voltages exceeding the crossover voltage Vc.
prepared at room temperature implies a higher trap density, a result consistent with the difference in film crystallinity. The conductance of the film in the ohmic region, the magnitude of the crossover voltage, and the value of exponent measured in the SCLC region observed for the films prepared at room temperature and 343 K all trend in the expected direction given the increased crystallinity of the film prepared at high temperature and the concomitant changes expected in the carrier mobility and trap density.
’ CONCLUSION In this work, the light-induced decay of spontaneously built-up surface potential was investigated for a vacuum-deposited polycrystalline thin film of a zwitterionic molecule, 4N-PI. The excitation wavelength required to induce the decay lies above the energy gap of 2.2 eV. Using a fixed excitation wavelength of 2.8 eV, the decay rate was examined for two films for which different crystallinity was observed. The decay profile can be accurately modeled as a photogenerated current having ohmic behavior at low voltages and traplimited SCLC at high voltages. The electrical parameters derived from the fitted I-V curves trend appropriately with the change in film crystallinity. We therefore conclude that photogenerated carriers, rather than molecular rotations, contribute essentially to the disappearance of the surface potential of 4N-PI films under illumination.
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’ ACKNOWLEDGMENT This work was partially supported by a MEXT Joint Project of Chemical Synthesis Core Research Institutions and by a Grant-in-Aid for Scientific Research No. 22550121 from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). ’ REFERENCES (1) Dvey-Aharon, H.; Sluckin, T. J.; Taylor, P. L. Phys. Rev. B 1980, 21, 3700. (2) Ito, E.; Washizu, Y.; Hayashi, N.; Ishii, H.; Matsuie, N.; Tsuboi, K.; Ouchi, Y.; Harima, Y.; Yamashita, K.; Seki, K. J. Appl. Phys. 2002, 92, 7306. (3) Hayashi, N.; Imai, K.; Suzuki, T.; Kanai, K.; Ouchi, Y.; Seki, K. Proceedings of the International Symposium on Super-Functionality Organic Devices, IPAP Conference Series; The Institute of Pure and Applied Physics: Tokyo, 2005; Vol. 6, p 69. 2359
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