Influence of Air Exposure on Photocarrier Generation in Amorphous

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Influence of Air Exposure on Photocarrier Generation in Amorphous and Phase-II Thin-Films of Titanyl Phthalocyanine Keitaro Eguchi, Yoshiaki Imai, Michio M. Matsushita, and Kunio Awaga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00290 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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The Journal of Physical Chemistry

Influence of Air Exposure on Photocarrier Generation in Amorphous and Phase-II Thin-Films of Titanyl Phthalocyanine

Keitaro Eguchi, Yoshiaki Imai, Michio M. Matsushita and Kunio Awaga* Department of Chemistry & Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Corresponding Author: [email protected]

ABSTRACT: We examined the influence of atmospheric gases on the photoelectric properties of titanyl phthalocyanine (TiOPc) thin films, using in situ photocurrent measurements under high vacuum without/with exposure to air. Prior to air exposure, the photocurrent of amorphous and Phase-II thin-films of TiOPc with thicknesses of 24 nm exhibited low external quantum efficiencies (EQE) of 0.05% and 0.52%, respectively, in the Q-band regions, and these values were enhanced to 0.21% and 12.0%, respectively, by air exposure. By contrast, this treatment resulted in a decrease in the photosensitivity, defined as the photo-to-dark current ratio, by one or two orders of magnitude. Once these thin films experienced air exposure, their photoelectric properties did not recover to the original ones even after restoration to high vacuum.

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INTRODUCTION In organic optoelectronics, photocarrier generation in organic materials is an important process for photoconduction and photovoltaic effects.1–5 Due to the low dielectric constants of organic media, e.g., ε = ~4 of phthalocyanines,6 the electron-hole pair (exciton) created by photon absorption is tightly bound by an attractive Coulomb force with a binding energy (Eb) of ~0.5– 1.0 eV.7–9 Because the values of Eb are significantly larger than the room-temperature thermal energy of 26 meV, single-component organic photocells exhibit poor quantum efficiencies10 and a high electric field is required to separate their excitons.11–13 However, photocarrier generation is known to be enhanced by gas adsorption onto material surfaces and/or impurities in molecular films.14, 15 In situ measurement is a reliable method to characterize the intrinsic properties of molecular thin films and the influence of air exposure on their electrical conductivities. Kazaoui et al. reported the effects of oxygen on the dark- and photo-conductivities of C60 thin films using an in situ technique16; both under dark and illuminated conditions, the conductivities of these thin films were reduced by several orders of magnitude after exposure to oxygen, due to the capture of conduction electrons by the adsorbed and intercalated oxygen molecules. Due to a strong interaction between oxygen and C60, the oxygen-exposed films never reproduced the electrical properties of the pristine samples even after annealing treatments. Thus, in situ measurements are necessary to precisely examine the air exposure effects on the electrical conductivities of molecular thin films. Titanyl phthalocyanine (TiOPc; Fig. 1) is in a class of highly photoconductive materials, and is widely used as a hole transport layer in organic photovoltaic (OPV) devices17-20 and as a photoreceptor in laser printers.3,

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TiOPc exhibits a characteristic variation in its optical


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absorption spectra, depending on its crystal forms, i.e., amorphous, Phase I (β-form),21, 22 Phase II (α-form),21, 22 Phase Υ (Υ-form),22 and others.23, 24 These different crystal forms strongly affect photocarrier-generation efficiency and solar-cell performance. It has been reported that Phases-II and -Υ exhibit higher photoconductivities than amorphous and Phase-I TiOPc,25 and further that the former provide better photovoltaic performance than the latter.19 In the present report, we describe the phase-selective preparations of amorphous and Phase-II thin films of TiOPc by control of the growth temperatures, and the results of the in situ photocurrent measurements to elucidate the effects of air exposure (see Fig. 1). We discuss the influence of air exposure on

photoresponsivity,

external

quantum

efficiency,

and

photosensitivity.

EXPERIMENTAL SECTION TiOPc powder (Sigma-Aldrich; purity > 95%) was purified by thermal gradient vacuum sublimation with a continuous N2 gas flow (40–50 ml min−1). In a deposition chamber (P < 10−4 Pa), the purified TiOPc was deposited onto quartz substrates, on which comb-shaped Pt electrodes with a length of 32 cm and an inter-electrode spacing of 5 µm had been patterned, by a home-made deposition cell. TiOPc films were also prepared under the same conditions on glass and Si substrates for optical and microscopic measurements, respectively. During deposition, the substrate temperatures were maintained at 300 or 400 K using a ceramic heater. The substrate temperatures were monitored using an alumel-chromel thermocouple that was directly attached to the substrate. We prepared thin films with thicknesses of 12 and 24 nm at a deposition rate of ~0.3 nm min−1 by means of a thickness monitor calibrated by scanning electron microscopy (SEM) (see Fig. S1).


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Ex situ optical measurements were performed on a JASCO V-570 UV-Vis-NIR spectrometer. SEM measurements were done on a Hitachi SEM SU6600 under high vacuum conditions (P < 10−3 Pa) in order to characterize the surface morphology. The in situ photocurrent measurements were performed for the pristine TiOPc thin films, as shown in Fig. 1, without/with exposure to air with a humidity of 40–50%. After the photocurrent measurements for the pristine thin films, atmospheric air was introduced into the chamber until the pressure reached ~105 Pa. Then, after keeping the samples under the atmospheric condition for 1min, the chamber was evacuated with a rotary and a turbomolecular pump, until the high vacuum (P < 10−4 Pa) condition was achieved. The samples were remained in a high vacuum (5–7 × 10−5 Pa) for ~12 h and the photocurrent measurements were performed. Finally, the photocurrent measurements were performed in air, after the samples were left in this condition for 10 min. In these electrical measurements, we applied a voltage of 10 V, which corresponded to an electric field of 2×104 V cm−1, using a source-measure unit (Advantest Corp. R6245A). We also used commercial light emitting diodes (LEDs; Optosupply) with photon energies (wavelengths) of 1.32 eV (940 nm), 1.46 eV (850 nm), 1.88 eV (660 nm), 2.05 eV (605 nm), 2.36 eV (525 nm), 2.64 eV (470 nm), and 3.14 eV (395 nm). The light intensities were calibrated using a Si photodiode (Hamamatsu Photonics K. K.).

RESULTS AND DISCUSSION Amorphous and Phase-II thin films of TiOPc were prepared on glass substrates at the substrate temperatures of 300 and 400 K, respectively. These temperatures were selected according to the literature.26 Figure 2(a) shows a comparison between the UV-vis-NIR absorptions for the thin films (24 nm) grown at these two temperatures. The thin film grown at 300 K (black curve)


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exhibits a main and a satellite peak at 1.71 eV (726 nm) and 1.88 eV (660 nm), respectively, in the Q-band region. These peaks are characteristic of the amorphous phase.19, 24–27 In contrast, the spectrum of the TiOPc deposited at 400 K (red curve) exhibits a broader band with a significant red shift. The main peak at 1.41 eV (877 nm) agrees with that of Phase II, as reported in the literature.19, 24–27 Although the absorption spectrum of Phase Υ is known to be similar to that of Phase II,24 the preparation condition for Phase Υ is very different; it requires chemical vapor annealing treatments.19, 25 Therefore, it is concluded that the TiOPc thin films grown at 300 and 400 K are in amorphous and Phase II, respectively. Figures 2(b) and 2(c) show the SEM images for the two samples with a thickness of 24 nm, indicating that the substrate temperature also affects the surface morphologies of thin films. The thin film prepared at 300 K has a smooth surface with a number of small grains with diameters of ~50 nm, while the film at 400 K features large crystalline grains with a size of several hundred nanometers. Figure S2 shows the SEM images of the TiOPc thin films of 12 nm, grown at 300 and 400 K. Their images are similar to those of the corresponding films of 24 nm, indicating that the surface morphology has little dependence on the thickness. To investigate the influence of air exposure on the electrical properties of the TiOPc thin films, we carried out in situ photocurrent measurements in the vacuum chamber previously used for the growth of the thin films (see Fig. 1). We measured the dark- and photo-currents by repeating the on-and-off illumination of a photon energy of Eph = 1.88 eV with an interval of 30 sec. In these measurements, the light intensity was stepwisely increased from 26 µW cm−2 to 325 µW cm−2 in order to examine the optical power dependence of the photocurrents. As shown in Fig. 1, the thin films of TiOPc were phase-selectively prepared on the comb-shaped Pt electrodes under the control of substrate temperature, and then, after transferring the thin films from the


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preparation to the measurement chamber, in situ photocurrent measurements were done for the pristine thin films without exposure to air. We then introduced air into the chamber for 1 min, and, after evacuating the air, we carried out the photocurrent measurements under high vacuum. Finally, we performed the photocurrent measurements again in ambient air. Figures 3(a) and 3(b) show the photocurrent of the amorphous thin films of TiOPc with thicknesses of 12 and 24 nm, respectively. In these figures, the time trajectories of the photocurrent are plotted in logarithmic scales, while Fig. S3 shows the same data in linear scales. The black curves in these figures show the results for the pristine samples before air exposure. Their OFF current Id—namely, the current in the dark—are on the order of 10−11 ~ 10−12 A. During illumination, the currents are enhanced by two orders of magnitude, reaching the order of 10−10 A. The red curves in Figs. 3(a) and 3(b) show the results after the 1 min exposure to air. Though these measurements were done in high vacuum, the OFF current increased by one or two orders of magnitude compared to the corresponding one before air exposure, while the increases in the ON current were less than one order of magnitude. The blue curves in these figures show the photocurrent, as measured in air. Due to the significant enhancement in the OFF currents, the ON/OFF ratios become less than 10. The enhancement of the OFF current after air exposure was probably caused by a charge transfer between the TiOPc thin films and oxygen and/or water molecules at the film surfaces.15, 28 After air exposure, the dark conductivities of the thin films do not return to those of the pristine samples even under high vacuum. Figures 3(c) and 3(d) show the results for the Phase-II thin films of 12 and 24 nm, respectively. They are essentially the same as those for the amorphous samples except that the ON/OFF ratios are still larger than 10 after air exposure. Table 1 indicates the calculated OFF electrical conductivity for the four


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samples. These values are comparable to those previously reported for TiOPc (σd ≈ 10−13 ~ 10−6 S cm−1).15, 21, 29 The results at the other photon energies are summarized in Fig. S4. We calculated the photocurrent Iph as a difference between the ON/OFF currents, namely Iph = ION − IOFF, for the above four samples at various excitation energies. The results for the amorphous and Phase-II TiOPc are shown in Figs. S5 and S6, respectively, in which the dependences of Iph on Popt are plotted in a double logarithmic scale. It is found that, in all cases, Iph monotonically increases with an increase in Popt, showing a linear relation between log Iph and log Popt without saturation. This means that these dependences can be explained as Iph = C·Poptγ, where C and γ are the fitting parameters. The optimized values for γ under the three conditions— namely, (i) under high vacuum before air exposure, (ii) under high vacuum after 1 min of air exposure, and (iii) in air—are listed in Table S1. It is concluded that the γ values for (i) are slightly larger than those for (ii) and (iii), while there is no meaningful difference between the values for (ii) and (iii). This decrease in γ after air exposure is probably caused by an increase in trap states in the energy gaps.30, 31 To understand the influence of air exposure on the two polymorphisms, we examined the action spectra of the photoresponsivity (PR) and the external quantum efficiency (EQE) at an optical power of ~1 µW. Note that PR and EQE are estimated using PR = Iph/Popt and EQE = (Iph/e)(Popt/Eph)−1 (Refs. 32, 33), respectively, where e is the elementary charge. The opticalpower dependence of PR and EQE for the four kinds of thin films are shown in Figs. S7~S10. Figures 4(a) and 4(b) show the action spectra of PR and EQE for the amorphous thin films of 24 nm, respectively, under the illumination of ~1 µW at room temperature. In these figures, the data obtained under the three conditions (i), (ii), and (iii) are presented using black, red, and blue circles, respectively, and the optical absorption spectrum of this thin film is indicated as a gray


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shadow. The action spectra of PR and EQE before air exposure under Condition (i) (black) are symbatic with the absorption spectra. After air exposure for 1 min, the action spectra under Condition (ii) (red) indicate that the values of PR and EQE are nearly doubled compared to those under Condition (i), maintaining the symbatic relations. In air under Condition (iii) (blue), PR and EQE are nearly twice as large as those under Condition (ii), while the symbatic relations are still maintained. The action spectra of PR and EQE for Phase-II thin films of 24 nm are shown in Figs. 4(c) and 4(d), respectively, in which the notations are the same as in Figs. 4(a) and 4(b). Note that, in Figs. 4(c) and 4(d), the PR and EQE values under Condition (i) are magnified 30 and 50 times, respectively, for comparison with the optical absorption spectrum. It can be clearly seen that the effects of the air exposure for Phase II are similar to those for the amorphous phase, but the PR and EQE values for the former are one or two orders of magnitude larger than those for the latter. This suggests an effective formation of charge-transfer excited states25 and/or a longer exciton diffusion length34 in crystalline Phase II. Moreover, the Phase-II thin films exhibit a strong photoresponse in the NIR (1.3–1.5 eV) region, probably due to a strong absorption in this range, while the photoresponse is negligibly small for the amorphous ones. There is also a symbatic relation between the optical absorption spectrum and PR or EQE for Phase II, though it is less clear for the amorphous phase. In addition, the air exposure of the Phase-II thin films results in the anomalies at around 2.0 eV in the PR and EQE, as indicated by the arrows in Figs. 4(c) and 4(d). One possible mechanism is a formation of an interface state at the edge of the Qband by gas adsorption, which is accessible by the electrons excited with the illumination of more than 2.0 eV. This process should enhance the exciton dissociations at the surface, because exciton dissociations occur at the surface.


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Next, we carried out the same experiments on the two phases using the 12 nm films. The results are shown in Fig. S10. Although the action spectra for the amorphous films exhibit little dependence on the film thickness (12 or 24 nm) under Condition (i), the photocurrents under Conditions (ii) and (iii) for the Phase II films of 12 nm were nearly ten times smaller than that of the 24 nm films. This reduction was probably caused by the worse crystallinity and the decrease in absorption for the 12 nm thin films. In addition, the PR and EQE values for the 12 nm thin films under Condition (i) were insensitive to their polymorphs. The values of PR and EQE for the TiOPc thin films in the Q-bands are summarized in Table 2. These data indicate that photocarrier generation in the TiOPc thin films is promoted, regardless of their polymorphs, as a result of the adsorption of atmospheric gases, and further that this promotion is more significant in Phase II than that in the amorphous thin films. The maximum value of EQE (24.0%) for the Phase-II TiOPc thin film of 24 nm in the Q-band region is comparable to that (~25%) for a TiOPc (Phase II)/C60 heterojunction solar-cell; by contrast, EQE for the amorphous TiOPc thin films (24 nm) was significantly smaller than that (~20%) for a TiOPc (amorphous)/C60 heterojunction solar cell.19 These facts suggest that, as in the case of heterojunction solar cells, the air-exposed TiOPc (Phase II) surface states effectively dissociate excitons into photocarriers. The mechanism of the promotion of photocarrier generation by air exposure has not been fully understood, but one possible mechanism is a formation of electric dipoles at the film surfaces due to a charge transfer interaction between the TiOPc thin film and the oxygen and/or water molecules on it, which enhance the exciton dissociation efficiency. 35–37 Finally, we calculated the photosensitivity (PS), using PS = Iph/Id for the amorphous and Phase-II thin films (24 nm), under Conditions (i), (ii), and (iii). The optical-power dependence of PS is shown in Fig. S7, and the action spectra of PS at an optical power of ~1 µW are shown in


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Fig. 5, where the results under the three conditions are indicated in black, red and blue, respectively. Before air exposure (black), the action spectra of PS were symbatic with their absorption spectra, and the maximum PS values for the amorphous and Phase II states were 85 and 246, respectively. However, these values were significantly reduced to 1.5 and 11, respectively, after air exposure. The 12-nm thin films also exhibit a similar tendency, as shown in Fig. S14. It is concluded that the pristine thin films exhibit high photosensitivities, but they become less sensitive by air exposure. CONCLUSION We systematically examined the influence of air exposure on the electric conductivity of the amorphous and Phase-II thin films of TiOPc by using in situ measurements. The results indicate that air exposure increases the dark conductivity, photoresponsivity and EQE, but decreases photosensitivity. The difference in the response to air exposure between the two polymorphs was not significant in the dark, but became crucial under illumination; Phase-II thin films exhibited significant enhancements in photoresponsivity and EQE. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. SEM images of a cross-section (Fig. S1) and surfaces (Fig. S2); electric current under ON/OFF illumination of Eph = 1.88 eV (Fig. S3); electric current under ON/OFF illumination of various photon energies (Fig. S4); light intensity dependence of IPh (Figs. S5 and S6); optical power dependence of PR (Figs. S7 and S8) and EQE (Figs. S9 and S10); PR and EQE for the thin films of 12 nm (Fig. S11); optical power dependence of PS (Figs. S12 and S13); PS for the thin films of 12 nm (Fig. S14); values of parameter γ (Table S1).


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ACKNOWLEDGEMENTS The authors would like to thank Dr. S. Nakao for his technical support with the scanning electron microscopic measurements. A part of this work was conducted at the Institute for Molecular Science (IMS), with support from the Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work was supported by a Japan Society for the Promotion of Science (JSPS) KAKENHI Grant numbers JP15J11122 and JP16H06353 and JSPS Core-to-Core Program (A. Advanced Research Networks).

AUTHOR INFORMATION Corresponding Author. * E-mail: [email protected] Notes. The authors declare no competing financial interest. REFERENCES (1) Ostroverkhova, O. Organic Optoelectronic Materials: Mechanisms and Applications. Chem. Rev. 2016, 116, 13279–13412. (2) Stoltzfus, D. M.; Donaghey, J. E.; Armin, A.; Shaw, P. E.; Burn, P. L.; Meredith, P. Charge Generation Pathways in Organic Solar Cells: Assessing the Contribution from the Electron Acceptor. Chem. Rev. 2016, 116, 12920–12955. (3) Law, K. Y. Organic Photoconductive Materials: Recent Trends and Developments. Chem. Rev. 1993, 93, 449–486. (4) Weiss, D. S.; Abkowitz, M. Advances in Organic Photoconductor Technology. Chem. Rev. 2010, 110, 479–526.


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Figure 1. Experimental set-up for in situ photocurrent measurements and the molecular structure of TiOPc. Thin-film growth (i), sample transfer (ii) and photocurrent measurement (iii) can be done under high vacuum without exposure to air.


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Figure 2. UV-vis-NIR absorption spectra of TiOPc thin films (24 nm) prepared on glass substrates at 300 K (black) and 400 K (red) (a). SEM images of TiOPc thin films prepared at 300 K (b) and 400 K (c).


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Figure 3. Electric current under ON/OFF illumination of Eph=1.88 eV for the TiOPc thin films [(a) amorphous 12 nm, (b) amorphous, 24 nm, (c) Phase II, 12 nm, (d) Phase II, 24 nm], measured at 300 K under high vacuum. The light intensity is stepwisely increased from 26 µW cm−2 to 325 µW cm−2 with the ON/OFF cycles. The black, red, and blue curves show the results under the three conditions: (i) under high vacuum before air exposure, (ii) under high vacuum after 1 min air exposure, and (iii) in air. See also Fig. S3.


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Figure 4. PR [(a) and (c)] and EQE [(b) and (d)] for the amorphous [(a) and (b)] and Phase II [(c) and (d)] TiOPc thin films of 24 nm, measured at an optical power of ~1 µW at room temperature under high vacuum. The black, red, and blue plots show the results under the three conditions: (i) under high vacuum before air exposure, (ii) under high vacuum after 1 min air exposure, and (iii) in air. The optical absorption spectra for the amorphous and Phase II thin films are shown as gray shadows.


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Figure 5. PS for the amorphous (a) and Phase II (b) TiOPc thin films of 24 nm, measured at an optical power of ~1 µW at room temperature under high vacuum using three conditions: (i) under high vacuum before air exposure, (ii) under high vacuum after 1 min air exposure, and (iii) in air. The optical absorption spectra for the amorphous and Phase II thin films are shown as gray shadows.


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Table 1. Dark conductivity of the amorphous and Phase II thin films of TiOPc measured at 300 K under three conditions: (i) under vacuum before air exposure, (ii) under vacuum after 1-min air exposure, and (iii) in air. Dark conductivity (σd) / S cm−1 (i)

(ii)

(iii)

Amorphous (12 nm)

4.0 × 10−12

2.1 × 10−10

5.0 × 10−9

Amorphous (24 nm)

2.8 × 10−12

1.4 × 10−11

1.2 × 10−9

Phase II (12 nm)

3.8 × 10−12

5.5 × 10−11

1.1 × 10−9

Phase II (24 nm)

5.9 × 10−12

3.4 × 10−10

3.1 × 10−9

Table 2. PR and EQE values of the amorphous and Phase II thin films of TiOPc at the photon energies of 1.88 eV (amorphous) and 1.46 eV (Phase II), under three conditions: (i) under vacuum before air exposure, (ii) under vacuum after 1 min air exposure, and (iii) in air. PR / mA W−1

EQE / %

(i)

(ii)

(iii)

(i)

(ii)

(iii)

Amorphous (12 nm)

0.28

0.75

1.6

0.05

0.14

0.30

Amorphous (24 nm)

0.27

0.43

1.1

0.05

0.08

0.21

Phase II (12 nm)

0.29

6.3

12.4

0.04

0.92

1.8

Phase II (24 nm)

3.6

21.3

82.4

0.52

3.1

12.0


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TOC Graphic


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Fig. 1 258x143mm (150 x 150 DPI)



Fig. 2 83x189mm (150 x 150 DPI)


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Fig. 3 134x204mm (150 x 150 DPI)



Fig. 4 177x165mm (150 x 150 DPI)


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Fig. 5 175x80mm (150 x 150 DPI)



TOC 215x115mm (150 x 150 DPI)


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Influence of Air Exposure on Photocarrier Generation in Amorphous

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