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Enhanced photoconductivity in organic single-layered photoreceptors with bipolar charge transport materials
Materials Chemistry and Physics 82 (2003) 210–215
Enhanced photoconductivity in organic single-layered photoreceptors with bipolar charge transport materials Jian Ye, Hong-Zheng Chen∗ , Mang Wang Department of Polymer Science & Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 11 February 2003; received in revised form 18 March 2003; accepted 25 April 2003
Abstract The organic single-layered photoreceptors (SLPRs) consisting of polycarbonate (PC) matrix containing dispersed N,N -diethyl-4aminobenzaldehyde-1-phenyl-1 -(␣-naphthyl)-hydrazone (BAH) and 2,4,7-trinitrylfluorenone (TNF) as bipolar charge (hole and electron) transport materials, as well as oxotitanium phthalocyanine (TiOPc) as charge generation materials (CGMs), were prepared. The influence of organic electron transport material (ETM) on the photosensitivity in the photoreceptors was investigated. It was found that small amount of TNF (TNF/BAH ≤0.005) could improve the photosensitivity of photoreceptors greatly, i.e. the enhanced photoconductive effect, but the further increase of TNF concentration would lead to the decline of the photosensitivity. Differential scanning calorimeter (DSC), UV-Vis and cyclic voltammograms (CV) study showed that the enhanced photoconductivity might be resulted from the improvement of the separation efficiency of electron–hole pairs, and the decline of photosensitivity was due to the formation of the charge transfer complex (CTC) of TNF–BAH. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Photoconductive; Bipolar charge transport; Charge transfer complex
1. Introduction Organic photoconductive materials have been rapidly developed for their non-toxicity, magnitude and variability of development, and widely used in xerography, plate-making printings, laser printers and photovoltaic devices. Among these devices, the single-layered organic photoreceptor is a new particularly attractive subject in the research of photoreceptors for its simple structure and low cost. The charge transfer phenomena of organic molecules have also received much attention from chemists in the fields of organic solar cells [1], light-emitting diodes [2,3] and photoreceptors [4] during the past decade. Especially Sebastian et al. [5], Saito et al. [6], Tokura et al. [7] and Pan et al. [8] have found that charge transfer complexes (CTCs) had an obvious influence on photosensitivity of organic photoreceptors. However, most of investigations focused on CTCs between charge generation materials (CGMs) and charge transport materials (CTMs) [1,2,9]. There were few reports on CTCs between hole transport materials (HTMs) and electron transport materials (ETMs) in photoreceptors. ∗ Corresponding author. Tel.: +86-571-87952557; fax: +86-571-87951635. E-mail address: [email protected] (H.-Z. Chen).
We reported previously some negatively charged singlelayered photoreceptors (SLPRs) comprising CGMs and HTMs dispersed in polymer matrix within one layer [10–12]. Recently, some research groups designed a new kind of SLPRs with bipolar charge transport materials containing HTM and ETM, both of which function as the charge transport material, giving a SLPR where CGM, HTM, ETM three components dispersed in a polymer matrix within one layer [13–15]. The SLPR’s configuration and photoconductive mechanism are depicted in Fig. 1. CGM, HTM and ETM were used in the SLPRs in order to take advantage of the functions of individual components. Under the light exposure, electron–hole pairs were generated by CGM, holes were transported by HTM, and electrons were transported by ETM under the electric field (Fig. 1b). In this way the photoreceptors might be applied in many kinds of devices in the fields mentioned above. Generally, HTMs are strong electron donors, with high position of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels; while ETMs are strong electron acceptors with low position of LUMO and HOMO energy levels. According to the generation mechanism of CTCs, it is easier to form CTCs between HTMs and ETMs. But, it has not demonstrated so far whether CTCs can be formed between
0254-0584/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00198-6
J. Ye et al. / Materials Chemistry and Physics 82 (2003) 210–215
211
2.2. Fabrication of single-layered photoreceptors
Fig. 1. The configuration (a) and photoconductive mechanism (b) of an organic single-layered photoreceptor.
HTMs and ETMs in this novel photoreceptor. Especially, little is known about photoconductive mechanism in the HTM/ETM bipolar composites. So it is important to investigate the influence of photosensitivity from HTM/ETM bipolar composite system in organic single-layered photoreceptors. In this paper, TiOPc (Fig. 2a) was chosen as CGM for its excellent photoconductive properties, 2,4,7-trinitrylfluorenone (TNF) (Fig. 2b) was selected as ETM, N,N -diethyl4-aminobenzaldehyde-1-phenyl-1 -(␣-naphthyl)-hydrazone (BAH) (Fig. 2c) was used as HTM, and polycarbonate (PC) was applied as the polymer matrix to prepare the single-layered photoreceptors. The photoconductivity of the photoreceptors containing TNF–BAH bipolar CTMs was investigated, aiming at understanding the basic photoconductive process in the system.
2. Experimental 2.1. Materials TiOPc was synthesized and purified according to the literature [16]. TNF and BAH were commercially available and were recrystallized before use. The bisphenol A type polycarbonate was purchased from Aldrich and was purified by dissolving it in chloroform and precipitated with methanol. The black green solid powder of TNF–BAH bipolar composite was obtained by dissolving yellow TNF and brown yellow BAH (in the mol ratio of 1:1) in ethanol and precipitated in deionized water.
TiOPc and tetrahydrofuran (THF) were added into a flask with the presence of fine glass beads (1 mm in diameter), and the mixture was wet milled for 6 h. Then 1,4-dioxane, PC, TNF and BAH were added into the flask and were mixed uniformly. The resulting slurry was cast onto the Al substrate with dip coating method. The obtained single-layered photoreceptor was dried for 1 h at 353 K. The thickness of the formed film was ∼20 m, measured from ELEKTKOPHYSIK MINITEST 2000 thin-film-measuring apparatus.
3. Measurement Xerographic measurements were made on a GDT-II model photoconductivity measuring device, using a 5 W, 24 V incandescent lamp as a light source. The monochromatic wavelengths of 450, 500, 570, 605, 678, 702 and 762 nm were obtained by optical filters. In the measurement, the surface of the single-layered device was negatively charged in the dark, and the light intensity of the exposure (I) was controlled at 40 W. Upon the exposure, the surface potential decreased, and the photoinduced discharge curve of the device was recorded, from which the time from the original potential to half under exposure (t1/2 ) and the half decaying exposure energy (E1/2 , the product of t1/2 multiplied by I) were obtained [17]. The photosensitivity was characterized by the reciprocal of E1/2 . Differential scanning calorimeter (DSC) curves were recorded on a Perkin-Elmer Pyris 1 differential scanning calorimeter under N2 , scanning rate was 10 ◦ C min−1 . UV-Vis spectra were measured on a CARY 100 Bio spectrophotometer. Cyclic voltammograms (CV) were recorded with CHI600A model electrochemical analyzer. All electrochemical measurements were carried out in a three-electrode cell containing a working electrode of a platinum disk, a counter electrode of a platinum wire and a reference electrode of saturated calomel electrode (SCE). Solutions for cyclic voltammetry were prepared in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrode, deaerated by nitrogen and kept under nitrogen during all experiments. Scanning rate was 50 mV s−1 . All potentials were referred to SCE if not mentioned otherwise.
Fig. 2. Molecular structures of TiOPc (a), TNF (b) and BAH (c).
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4. Results and discussion 4.1. Xerographic properties of SLPRs with bipolar CTMs The xerographic properties of the SLPRs where TiOPc served as CGM and BAH and TNF acted as CTMs, when exposed to light with various wavelengths, were summarized in Table 1. We found that when only the hole transport material of BAH was used as CTM matched with TiOPc (no. 1 in Table 1), the photoreceptor showed good photosensitiv−1 ity in the near-IR region with E1/2 value of 0.769 cm2 J−1 at λ = 762 nm, but presented poor photosensitivity in the −1 visible region with E1/2 value of 0.429 cm2 J−1 at λ = 500 nm with the exception of the wavelength at 570 nm, of which the reason was not known yet. If small amount of the electron transport material of TNF was added in the SLPRs, the photosensitivity could be enhanced. For example at λ = 762 nm, when TNF/BAH = 0.001 (no. 2) and 0.005 (no. 3), the photoreceptors exhibited excellent photosensi−1 tivity with E1/2 values of 2.00 and 1.00 cm2 J−1 , respectively, 2.6 and 1.3 times higher compared to that with the −1 absence of TNF (E1/2 = 0.769 cm2 J−1 ). However, with the further increase of TNF concentration in the system, the photoconductivity somehow decreased (see nos. 4 and 5 in Table 1). The photosensitivity reached the best when the ratio of TNF and BAH equaled to 0.001. The same results were also obtained at various wavelengths. The preferable ratio of TNF and BAH was in the range of 0–0.005 for the photoreceptor with bipolar CTMs composite under our experimental conditions. The further study will be undergoing to find out what a still lower ratio of TNF/BAH would achieve an even large effect.
Fig. 3. DSC curves of BAH (a), TNF (b) and TNF–BAH (1:1, mol%) (c) in powder samples.
melting peak at about 140 ◦ C, and TNF had a melting peak at about 177 ◦ C. However, the DSC curve of TNF–BAH composite (1:1, mol%) presented two new endothermic peaks at about 129 and 147 ◦ C, respectively, with the above two melting peaks of TNF and BAH disappeared. It suggested that there was a strong interaction between TNF and BAH molecules, and the CTCs of TNF–BAH might be formed. 4.2.2. UV-Vis spectra UV-Vis spectra of TNF, BAH and TNF–BAH (1:1, mol%) dissolved in 1,4-dioxane with the concentration of 5 × 10−5 M were recorded and showed in Fig. 4A(a,b,c). There was no absorption peak for both TNF and BAH in the range of 400–900 nm, while there were strong absorptions in the range of 200–400 nm. TNF presented a strong peak and a broad peak, centered at 281 and 338 nm, respectively; and BAH presented two peaks, centered at 261 and 363 nm, respectively. The absorption spectrum of TNF–BAH composite could be fitted by superposition of the spectra of the individual components, indicating no formation of CTCs between the two components for the salvation in the solution with very low concentration. In order to investigate the agglomeration state of TNF and BAH molecules in organic single-layered photoreceptors, the thin films, cast
4.2. Photoconductive mechanism of SLPRs with bipolar CTMs In order to understand the basic photoconductive process in the single-layered photoreceptors, DSC, UV-Vis spectra and cyclic voltammograms were studied. 4.2.1. DSC measurements From the DSC curves of powder samples of TNF, BAH and TNF–BAH composite (Fig. 3), we found that BAH had a
Table 1 −1 Photosensitivity (E1/2 , cm2 J−1 ) of single-layered photoreceptors when exposed to various wavelengths No.
1 2 3 4 5
Composite (mol%)
BAH:PC:TiOPc (1:1:0.0125) TNF:BAH:PC:TiOPc (0.001:1:1:0.0125) TNF:BAH:PC:TiOPc (0.005:1:1:0.0125) TNF:BAH:PC:TiOPc (0.01:1:1:0.0125) TNF:BAH:PC:TiOPc (0.5:1:1:0.0125
Note: the light intensity of the exposure (I) is 40 W.
λ (nm) 450
500
570
605
678
702
762
0.148 0.157 0.148 0.127 0.119
0.429 0.459 0.429 0.370 0.172
0.725 1.000 1.000 0.885 0.222
0.242 0.380 0.364 0.275 0.160
0.296 0.347 0.296 0.258 0.163
0.347 0.500 0.364 0.364 0.145
0.769 2.000 1.000 0.500 0.385
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Fig. 5. Cyclic voltammograms of 1 × 10−3 M BAH (a), 1 × 10−3 M TNF (b), 1 × 10−3 M TNF–BAH (c), 0.02 M TNF–BAH (d) at a Pt electrode in acetonitrile, 0.1 mol/l TBAP. Scan rate = 50 mV s−1 .
Fig. 4. Absorption spectra of TNF (a), BAH (b) and TNF–BAH (c) in 1,4-dioxane solution (A) and in thin films (B).
from suspended 0.5 M 1,4-dioxane solutions of the composites containing TNF, BAH and PC matrix, were prepared, and their UV-Vis spectra were recorded (Fig. 4B). Because high concentration resulted in the absorbance exceeding the limit of the instrument, the absorption spectra only in the range of 350–900 nm were recorded. As we can see from Fig. 4B(c), a new absorption band for the TNF–BAH composite was observed at about 500 nm in the film. According to Wang’s report of [18], the absorption bands of CTCs always appear at the red-shift direction of maximum absorption wavelength of original components. Following this argument, the new absorption peak might be due to the CTCs formed from TNF and BAH in the solid state. 4.2.3. Cyclic voltammogram Fig. 5 showed the cyclic voltammograms of TNF, BAH and TNF–BAH with different concentrations in acetonitrile. The curves in the positive potential range reflected the oxidizing behaviors of the electron donor BAH, and in the
negative potential range the reducing behaviors of the electron acceptor TNF. From Fig. 5a, two couple of redox peaks were observed in the positive potential range, centering at 0.56 and 0.50, and 1.03 and 0.96 V, respectively, while no redox peak was found in the negative range, indicating BAH was a kind of hole transport materials with a strong ability of donating electrons. TNF showed two couple of redox peaks in the negative potential range (Fig. 5b), centered at −0.45 and −0.39, and −0.68 and −0.58 V, respectively, but no redox behaviors could be observed in the positive potential range, indicating TNF was an electron transport material with a strong ability of accepting electrons. From the above data, it could be concluded that the first redox couples of BAH or TNF were related to a reversible one-electron process with E of approximately 60 mV for both compounds, however, the second redox couples of BAH or TNF were a quasi-reversible one-electron process with E values ranging from 70 to 100 mV [19]. As shown in Fig. 5c and d, no redox peak shift was observed in the curve of 1 × 10−3 M TNF–BAH (1:1, mol%) in acetonitrile when compared to the individual components; while in the curve of 0.02 M TNF–BAH in acetonitrile, the oxidation peaks in the positive potential range had a positive-shift with values of 70 and 80 mV, and the reduction peaks in the negative potential range had a negative-shift with values of 70 and 110 mV. It suggested that there was no charge transfer between TNF and BAH molecules, due to the salvation in low concentration. While on the occasion of high concentration, part of charge transferred from BAH molecule to TNF molecule, the abilities of further donating electrons of BAH and further accepting electrons of TNF got decreased, confirming the formation of TNF–BAH CTCs. 4.2.4. Photoconductive mechanism According to the results of DSC measurement, TNF–BAH CTCs could be formed between TNF and BAH molecules.
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photoconductive effect, we should control the ratio of TNF and BAH in the range of 0–0.005 and avoid the forming of TNF–BAH CTCs. The enhanced photoconductive effect from small amount of TNF might facilitate the preparations of organic photoconductive device under the drive of low fields.
5. Conclusion
Fig. 6. Energy levels of TNF, TiOPc [21] and BAH.
UV-Vis spectra and cyclic voltammograms showed that there was no charge transfer between two molecules for the salvation in low concentration, however, CTCs formed in high concentration. So, the TNF–BAH CTCs are the key of understanding the photoconductive process in the single-layered photoreceptor. In order to investigate the photoconductive mechanism, the optical energy gap (the absorption onset energy) can be obtained from UV-Vis spectra, and the absolute positions of HOMO and LUMO levels versus the vacuum level were derived from the electrochemical redox and the optical energy gap potentials. For TNF and BAH, only reduction or oxidation potentials were successfully obtained (see Fig. 5), so their HOMO or LUMO levels were estimated by subtracting the apparent optical energy gap from the LUMO level or adding the apparent optical energy gap to their HOMO levels [20] (see Fig. 6). The HOMO and LUMO levels for TiOPc in Fig. 6 can be referred to [21]. To the question why small amount of TNF could improve the photosensitivity, we provide the following explanations. As can be seen from Fig. 6, after electrons in TiOPc jumped from HOMO to LUMO under light exposure, the electrons of BAH in HOMO could inject into HOMO of TiOPc, realizing the separation of electron–hole pairs. Furthermore, electrons in LUMO of TiOPc could inject into LUMO of TNF, hence also improving the separation efficiency of charge–carrier pairs, leading to the enhanced photoconductivity. However, with the increase of TNF concentration, charge might be transferred from BAH molecules to TNF ones, and three changes would be observed: (1) TNF–BAH CTCs have absorption in the visible region (Fig. 4B), minimizing the light intensity absorbed by CGMs, and leading to the decrease of CGMs’ charge generation efficiency; (2) TNF–BAH CTCs can lower the abilities of TNF’s accepting electrons and BAH’s donating electrons, i.e. LUMO energy level of TNF becomes higher and HOMO energy level of BAH becomes lower (Fig. 6, dashed line), resulting in the decrease of separation efficiency of charge–carrier pairs. All these three factors result in the decline of the photosensitivity with TNF content increase. So, in order to make use of this enhanced
In conclusion, small amount of TNF (TNF/BAH ≤0.005) can greatly improve the photosensitivity of the single-layered photoreceptors containing bipolar charge transport materials, i.e. the enhanced photoconductive effect. However, with the increase of TNF concentration, the charge transfer complexes of TNF–BAH can be formed, leading to the decline of photosensitivity in the single-layered photoreceptors. It is a great significance to study this enhanced photoconductive effect and formation of the charge transfer complex for designing new photoconductive materials and devices by making use of organic charge transport materials.
Acknowledgements The work was financed by the National Natural Science Foundation of China (Grant No. 50173021, 50225312), and by the Natural Science Foundation of Zhejiang Province, China (Grant No. ZC0101). References [1] T. Tsuzuki, Y. Shirota, J. Rostalski, D. Meissner, Solar Energy Mater. Solar Cells. 61 (2000) 1. [2] G. Yu, J. Gao, J.C. Hummelen, A.J. Heeger, Science 270 (1995) 1789. [3] S.A. Jenckhe, J.A. Osaheni, Science 265 (1994) 765. [4] M. Wang, H.-Z. Chen, J.L. Shen, S.L. Yang, Sci. Chin. A 37 (4) (1994) 497. [5] L. Sebastian, G. Weiser, G. Peter, H. Bassler, Chem. Phys. 75 (1983) 103. [6] T. Saito, W. Sisk, T. Kobayashi, S. Suzuki, T. Lwayanagi, J. Phys. Chem. 97 (30) (1993) 8026. [7] Y. Tokura, T. Koda, Y. Lyechika, H. Kuroda, Chem. Phys. Lett. 102 (1994) 174. [8] Y.L. Pan, L.B. Chen, Y. Wang, Y.Y. Zhao, F.M. Li, H.W. Zhou, A. Wagiki, M. Yamashita, Tako T. Appl. Phys. A 65 (1997) 425. [9] C.H. Lee, G. Yu, A.J. Heeger, Phys. Rev. B 15 (1993) 543. [10] H.-Z. Chen, R.S. Xu, M. Wang, J. Appl. Polym. Sci. 69 (1998) 2609. [11] H.-J. Chen, K.J. Jiang, M. Wang, J. Photochem. Photobiol. A: Chem. 117 (1998) 149. [12] H.-Z. Chen, C. Pan, M. Wang, Nanostruct. Mater. 11 (4) (1999) 523. [13] E. Kurimoto, T. Ikegami, S. Rokutanzono, JP 07 199 490 (1995). [14] T. Tanaka, T. Yamaguchi, H. Kondo, M. Yanagisawa, M. Yoshikawa, JP 08 314 168 (1997). [15] H. Sugimoto, T. Yamaguchi, JP 08 262 886 (1997).
J. Ye et al. / Materials Chemistry and Physics 82 (2003) 210–215 [16] F.H. Moser, A.L. Thomas, The Phthlocyanines, first ed., vol. 56, CRC Press, Boca Raton, FL, 1983. [17] K.Y. Law, I.W. Tarnawskyj, J. Photochem. Photobiol. A: Chem. 51 (1991) 217. [18] C.C. Wang, B.J. Deng, S.K. Fu, Chem. J. Chin. Univ. 15 (10) (1994) 1559.
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[19] S. Mantoa, N. Tebello, Polyhedron 15 (17) (1996) 2901. [20] C.C. Wu, J.C. Sturm, R.A. Register, J. Tian, E.P. Dana, M.E. Thompson, IEEE Trans. Electron. Dev. 44 (8) (1997) 1269. [21] N.B. McKeown, Phthalocyanine Materials, Synthesis, Structure and Function, University Press, Cambridge, 1998 (Chapter 6).
Enhanced photoconductivity in organic single-layered photoreceptors with bipolar charge transport materials Jian Ye, Hong-Zheng Chen∗ , Mang Wang Department of Polymer Science & Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 11 February 2003; received in revised form 18 March 2003; accepted 25 April 2003
Abstract The organic single-layered photoreceptors (SLPRs) consisting of polycarbonate (PC) matrix containing dispersed N,N -diethyl-4aminobenzaldehyde-1-phenyl-1 -(␣-naphthyl)-hydrazone (BAH) and 2,4,7-trinitrylfluorenone (TNF) as bipolar charge (hole and electron) transport materials, as well as oxotitanium phthalocyanine (TiOPc) as charge generation materials (CGMs), were prepared. The influence of organic electron transport material (ETM) on the photosensitivity in the photoreceptors was investigated. It was found that small amount of TNF (TNF/BAH ≤0.005) could improve the photosensitivity of photoreceptors greatly, i.e. the enhanced photoconductive effect, but the further increase of TNF concentration would lead to the decline of the photosensitivity. Differential scanning calorimeter (DSC), UV-Vis and cyclic voltammograms (CV) study showed that the enhanced photoconductivity might be resulted from the improvement of the separation efficiency of electron–hole pairs, and the decline of photosensitivity was due to the formation of the charge transfer complex (CTC) of TNF–BAH. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Photoconductive; Bipolar charge transport; Charge transfer complex
1. Introduction Organic photoconductive materials have been rapidly developed for their non-toxicity, magnitude and variability of development, and widely used in xerography, plate-making printings, laser printers and photovoltaic devices. Among these devices, the single-layered organic photoreceptor is a new particularly attractive subject in the research of photoreceptors for its simple structure and low cost. The charge transfer phenomena of organic molecules have also received much attention from chemists in the fields of organic solar cells [1], light-emitting diodes [2,3] and photoreceptors [4] during the past decade. Especially Sebastian et al. [5], Saito et al. [6], Tokura et al. [7] and Pan et al. [8] have found that charge transfer complexes (CTCs) had an obvious influence on photosensitivity of organic photoreceptors. However, most of investigations focused on CTCs between charge generation materials (CGMs) and charge transport materials (CTMs) [1,2,9]. There were few reports on CTCs between hole transport materials (HTMs) and electron transport materials (ETMs) in photoreceptors. ∗ Corresponding author. Tel.: +86-571-87952557; fax: +86-571-87951635. E-mail address: [email protected] (H.-Z. Chen).
We reported previously some negatively charged singlelayered photoreceptors (SLPRs) comprising CGMs and HTMs dispersed in polymer matrix within one layer [10–12]. Recently, some research groups designed a new kind of SLPRs with bipolar charge transport materials containing HTM and ETM, both of which function as the charge transport material, giving a SLPR where CGM, HTM, ETM three components dispersed in a polymer matrix within one layer [13–15]. The SLPR’s configuration and photoconductive mechanism are depicted in Fig. 1. CGM, HTM and ETM were used in the SLPRs in order to take advantage of the functions of individual components. Under the light exposure, electron–hole pairs were generated by CGM, holes were transported by HTM, and electrons were transported by ETM under the electric field (Fig. 1b). In this way the photoreceptors might be applied in many kinds of devices in the fields mentioned above. Generally, HTMs are strong electron donors, with high position of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels; while ETMs are strong electron acceptors with low position of LUMO and HOMO energy levels. According to the generation mechanism of CTCs, it is easier to form CTCs between HTMs and ETMs. But, it has not demonstrated so far whether CTCs can be formed between
0254-0584/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00198-6
J. Ye et al. / Materials Chemistry and Physics 82 (2003) 210–215
211
2.2. Fabrication of single-layered photoreceptors
Fig. 1. The configuration (a) and photoconductive mechanism (b) of an organic single-layered photoreceptor.
HTMs and ETMs in this novel photoreceptor. Especially, little is known about photoconductive mechanism in the HTM/ETM bipolar composites. So it is important to investigate the influence of photosensitivity from HTM/ETM bipolar composite system in organic single-layered photoreceptors. In this paper, TiOPc (Fig. 2a) was chosen as CGM for its excellent photoconductive properties, 2,4,7-trinitrylfluorenone (TNF) (Fig. 2b) was selected as ETM, N,N -diethyl4-aminobenzaldehyde-1-phenyl-1 -(␣-naphthyl)-hydrazone (BAH) (Fig. 2c) was used as HTM, and polycarbonate (PC) was applied as the polymer matrix to prepare the single-layered photoreceptors. The photoconductivity of the photoreceptors containing TNF–BAH bipolar CTMs was investigated, aiming at understanding the basic photoconductive process in the system.
2. Experimental 2.1. Materials TiOPc was synthesized and purified according to the literature [16]. TNF and BAH were commercially available and were recrystallized before use. The bisphenol A type polycarbonate was purchased from Aldrich and was purified by dissolving it in chloroform and precipitated with methanol. The black green solid powder of TNF–BAH bipolar composite was obtained by dissolving yellow TNF and brown yellow BAH (in the mol ratio of 1:1) in ethanol and precipitated in deionized water.
TiOPc and tetrahydrofuran (THF) were added into a flask with the presence of fine glass beads (1 mm in diameter), and the mixture was wet milled for 6 h. Then 1,4-dioxane, PC, TNF and BAH were added into the flask and were mixed uniformly. The resulting slurry was cast onto the Al substrate with dip coating method. The obtained single-layered photoreceptor was dried for 1 h at 353 K. The thickness of the formed film was ∼20 m, measured from ELEKTKOPHYSIK MINITEST 2000 thin-film-measuring apparatus.
3. Measurement Xerographic measurements were made on a GDT-II model photoconductivity measuring device, using a 5 W, 24 V incandescent lamp as a light source. The monochromatic wavelengths of 450, 500, 570, 605, 678, 702 and 762 nm were obtained by optical filters. In the measurement, the surface of the single-layered device was negatively charged in the dark, and the light intensity of the exposure (I) was controlled at 40 W. Upon the exposure, the surface potential decreased, and the photoinduced discharge curve of the device was recorded, from which the time from the original potential to half under exposure (t1/2 ) and the half decaying exposure energy (E1/2 , the product of t1/2 multiplied by I) were obtained [17]. The photosensitivity was characterized by the reciprocal of E1/2 . Differential scanning calorimeter (DSC) curves were recorded on a Perkin-Elmer Pyris 1 differential scanning calorimeter under N2 , scanning rate was 10 ◦ C min−1 . UV-Vis spectra were measured on a CARY 100 Bio spectrophotometer. Cyclic voltammograms (CV) were recorded with CHI600A model electrochemical analyzer. All electrochemical measurements were carried out in a three-electrode cell containing a working electrode of a platinum disk, a counter electrode of a platinum wire and a reference electrode of saturated calomel electrode (SCE). Solutions for cyclic voltammetry were prepared in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrode, deaerated by nitrogen and kept under nitrogen during all experiments. Scanning rate was 50 mV s−1 . All potentials were referred to SCE if not mentioned otherwise.
Fig. 2. Molecular structures of TiOPc (a), TNF (b) and BAH (c).
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4. Results and discussion 4.1. Xerographic properties of SLPRs with bipolar CTMs The xerographic properties of the SLPRs where TiOPc served as CGM and BAH and TNF acted as CTMs, when exposed to light with various wavelengths, were summarized in Table 1. We found that when only the hole transport material of BAH was used as CTM matched with TiOPc (no. 1 in Table 1), the photoreceptor showed good photosensitiv−1 ity in the near-IR region with E1/2 value of 0.769 cm2 J−1 at λ = 762 nm, but presented poor photosensitivity in the −1 visible region with E1/2 value of 0.429 cm2 J−1 at λ = 500 nm with the exception of the wavelength at 570 nm, of which the reason was not known yet. If small amount of the electron transport material of TNF was added in the SLPRs, the photosensitivity could be enhanced. For example at λ = 762 nm, when TNF/BAH = 0.001 (no. 2) and 0.005 (no. 3), the photoreceptors exhibited excellent photosensi−1 tivity with E1/2 values of 2.00 and 1.00 cm2 J−1 , respectively, 2.6 and 1.3 times higher compared to that with the −1 absence of TNF (E1/2 = 0.769 cm2 J−1 ). However, with the further increase of TNF concentration in the system, the photoconductivity somehow decreased (see nos. 4 and 5 in Table 1). The photosensitivity reached the best when the ratio of TNF and BAH equaled to 0.001. The same results were also obtained at various wavelengths. The preferable ratio of TNF and BAH was in the range of 0–0.005 for the photoreceptor with bipolar CTMs composite under our experimental conditions. The further study will be undergoing to find out what a still lower ratio of TNF/BAH would achieve an even large effect.
Fig. 3. DSC curves of BAH (a), TNF (b) and TNF–BAH (1:1, mol%) (c) in powder samples.
melting peak at about 140 ◦ C, and TNF had a melting peak at about 177 ◦ C. However, the DSC curve of TNF–BAH composite (1:1, mol%) presented two new endothermic peaks at about 129 and 147 ◦ C, respectively, with the above two melting peaks of TNF and BAH disappeared. It suggested that there was a strong interaction between TNF and BAH molecules, and the CTCs of TNF–BAH might be formed. 4.2.2. UV-Vis spectra UV-Vis spectra of TNF, BAH and TNF–BAH (1:1, mol%) dissolved in 1,4-dioxane with the concentration of 5 × 10−5 M were recorded and showed in Fig. 4A(a,b,c). There was no absorption peak for both TNF and BAH in the range of 400–900 nm, while there were strong absorptions in the range of 200–400 nm. TNF presented a strong peak and a broad peak, centered at 281 and 338 nm, respectively; and BAH presented two peaks, centered at 261 and 363 nm, respectively. The absorption spectrum of TNF–BAH composite could be fitted by superposition of the spectra of the individual components, indicating no formation of CTCs between the two components for the salvation in the solution with very low concentration. In order to investigate the agglomeration state of TNF and BAH molecules in organic single-layered photoreceptors, the thin films, cast
4.2. Photoconductive mechanism of SLPRs with bipolar CTMs In order to understand the basic photoconductive process in the single-layered photoreceptors, DSC, UV-Vis spectra and cyclic voltammograms were studied. 4.2.1. DSC measurements From the DSC curves of powder samples of TNF, BAH and TNF–BAH composite (Fig. 3), we found that BAH had a
Table 1 −1 Photosensitivity (E1/2 , cm2 J−1 ) of single-layered photoreceptors when exposed to various wavelengths No.
1 2 3 4 5
Composite (mol%)
BAH:PC:TiOPc (1:1:0.0125) TNF:BAH:PC:TiOPc (0.001:1:1:0.0125) TNF:BAH:PC:TiOPc (0.005:1:1:0.0125) TNF:BAH:PC:TiOPc (0.01:1:1:0.0125) TNF:BAH:PC:TiOPc (0.5:1:1:0.0125
Note: the light intensity of the exposure (I) is 40 W.
λ (nm) 450
500
570
605
678
702
762
0.148 0.157 0.148 0.127 0.119
0.429 0.459 0.429 0.370 0.172
0.725 1.000 1.000 0.885 0.222
0.242 0.380 0.364 0.275 0.160
0.296 0.347 0.296 0.258 0.163
0.347 0.500 0.364 0.364 0.145
0.769 2.000 1.000 0.500 0.385
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Fig. 5. Cyclic voltammograms of 1 × 10−3 M BAH (a), 1 × 10−3 M TNF (b), 1 × 10−3 M TNF–BAH (c), 0.02 M TNF–BAH (d) at a Pt electrode in acetonitrile, 0.1 mol/l TBAP. Scan rate = 50 mV s−1 .
Fig. 4. Absorption spectra of TNF (a), BAH (b) and TNF–BAH (c) in 1,4-dioxane solution (A) and in thin films (B).
from suspended 0.5 M 1,4-dioxane solutions of the composites containing TNF, BAH and PC matrix, were prepared, and their UV-Vis spectra were recorded (Fig. 4B). Because high concentration resulted in the absorbance exceeding the limit of the instrument, the absorption spectra only in the range of 350–900 nm were recorded. As we can see from Fig. 4B(c), a new absorption band for the TNF–BAH composite was observed at about 500 nm in the film. According to Wang’s report of [18], the absorption bands of CTCs always appear at the red-shift direction of maximum absorption wavelength of original components. Following this argument, the new absorption peak might be due to the CTCs formed from TNF and BAH in the solid state. 4.2.3. Cyclic voltammogram Fig. 5 showed the cyclic voltammograms of TNF, BAH and TNF–BAH with different concentrations in acetonitrile. The curves in the positive potential range reflected the oxidizing behaviors of the electron donor BAH, and in the
negative potential range the reducing behaviors of the electron acceptor TNF. From Fig. 5a, two couple of redox peaks were observed in the positive potential range, centering at 0.56 and 0.50, and 1.03 and 0.96 V, respectively, while no redox peak was found in the negative range, indicating BAH was a kind of hole transport materials with a strong ability of donating electrons. TNF showed two couple of redox peaks in the negative potential range (Fig. 5b), centered at −0.45 and −0.39, and −0.68 and −0.58 V, respectively, but no redox behaviors could be observed in the positive potential range, indicating TNF was an electron transport material with a strong ability of accepting electrons. From the above data, it could be concluded that the first redox couples of BAH or TNF were related to a reversible one-electron process with E of approximately 60 mV for both compounds, however, the second redox couples of BAH or TNF were a quasi-reversible one-electron process with E values ranging from 70 to 100 mV [19]. As shown in Fig. 5c and d, no redox peak shift was observed in the curve of 1 × 10−3 M TNF–BAH (1:1, mol%) in acetonitrile when compared to the individual components; while in the curve of 0.02 M TNF–BAH in acetonitrile, the oxidation peaks in the positive potential range had a positive-shift with values of 70 and 80 mV, and the reduction peaks in the negative potential range had a negative-shift with values of 70 and 110 mV. It suggested that there was no charge transfer between TNF and BAH molecules, due to the salvation in low concentration. While on the occasion of high concentration, part of charge transferred from BAH molecule to TNF molecule, the abilities of further donating electrons of BAH and further accepting electrons of TNF got decreased, confirming the formation of TNF–BAH CTCs. 4.2.4. Photoconductive mechanism According to the results of DSC measurement, TNF–BAH CTCs could be formed between TNF and BAH molecules.
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photoconductive effect, we should control the ratio of TNF and BAH in the range of 0–0.005 and avoid the forming of TNF–BAH CTCs. The enhanced photoconductive effect from small amount of TNF might facilitate the preparations of organic photoconductive device under the drive of low fields.
5. Conclusion
Fig. 6. Energy levels of TNF, TiOPc [21] and BAH.
UV-Vis spectra and cyclic voltammograms showed that there was no charge transfer between two molecules for the salvation in low concentration, however, CTCs formed in high concentration. So, the TNF–BAH CTCs are the key of understanding the photoconductive process in the single-layered photoreceptor. In order to investigate the photoconductive mechanism, the optical energy gap (the absorption onset energy) can be obtained from UV-Vis spectra, and the absolute positions of HOMO and LUMO levels versus the vacuum level were derived from the electrochemical redox and the optical energy gap potentials. For TNF and BAH, only reduction or oxidation potentials were successfully obtained (see Fig. 5), so their HOMO or LUMO levels were estimated by subtracting the apparent optical energy gap from the LUMO level or adding the apparent optical energy gap to their HOMO levels [20] (see Fig. 6). The HOMO and LUMO levels for TiOPc in Fig. 6 can be referred to [21]. To the question why small amount of TNF could improve the photosensitivity, we provide the following explanations. As can be seen from Fig. 6, after electrons in TiOPc jumped from HOMO to LUMO under light exposure, the electrons of BAH in HOMO could inject into HOMO of TiOPc, realizing the separation of electron–hole pairs. Furthermore, electrons in LUMO of TiOPc could inject into LUMO of TNF, hence also improving the separation efficiency of charge–carrier pairs, leading to the enhanced photoconductivity. However, with the increase of TNF concentration, charge might be transferred from BAH molecules to TNF ones, and three changes would be observed: (1) TNF–BAH CTCs have absorption in the visible region (Fig. 4B), minimizing the light intensity absorbed by CGMs, and leading to the decrease of CGMs’ charge generation efficiency; (2) TNF–BAH CTCs can lower the abilities of TNF’s accepting electrons and BAH’s donating electrons, i.e. LUMO energy level of TNF becomes higher and HOMO energy level of BAH becomes lower (Fig. 6, dashed line), resulting in the decrease of separation efficiency of charge–carrier pairs. All these three factors result in the decline of the photosensitivity with TNF content increase. So, in order to make use of this enhanced
In conclusion, small amount of TNF (TNF/BAH ≤0.005) can greatly improve the photosensitivity of the single-layered photoreceptors containing bipolar charge transport materials, i.e. the enhanced photoconductive effect. However, with the increase of TNF concentration, the charge transfer complexes of TNF–BAH can be formed, leading to the decline of photosensitivity in the single-layered photoreceptors. It is a great significance to study this enhanced photoconductive effect and formation of the charge transfer complex for designing new photoconductive materials and devices by making use of organic charge transport materials.
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