Effect of dye aggregation on the photogeneration efficiency of organic

Jul 1, 1988 - Susan Spencer , Jeremy Cody , Scott Misture , Brandon Cona , Patrick Heaphy , Garry Rumbles , John Andersen , and Christopher Collison...
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J. Phys. Chem. 1988, 92. 4226-4231

4226

the differentiation of eq 28. Since the numerical value off(zj) is obtained from eq 20, which does not involve a, b, sl, and s2, the derivative of the left-hand side is zero. Therefore, we can solve for t h e y derivative and substitute this back into the derivative of eq 27. A similar procedure is used for eq 29 and 30. After some algebra we obtain the following results: For the ternary system (i = 1, 2) a?* - -

As with the Rayleigh derivatives, the T i should be replaced in the actual least-squares equations by their expressions in terms of (a, b, ai)for ternary and ( a , bi, ai) for four-component systems.

B: Summary of Ternary Equations For completeness, we present the expressions for a, b, si, s2in terms of Dij, and conversely.6

Appendix

d/2K[erf ( s l y j )- erf ( s t v j ) ] 2Yj

aa

a?*

aI;* aa

CUI----

(A-10)

- - - mi e ~ p [ - ( s p ~ ) ~ ]

(A-11)

-=

db

a yj* asi

a =

('4-9)

(

s12D- DZ2-

[D(s12- s22)] (B-1)

For the four-component system (i = 1, 2 , 3) a?* - - d/2Kai[erf (slyj) - erf (s3yj)]

aai

2Yj

a?* - -

(A-12) 03-71

d/2Kai[erf (stvj) - erf ( s g j ) ]

abi

2Yj

-a?*-

asi

- mi e ~ p [ - ( s p ~ ) ~ ]

(A- 13)

(A-14)

D22 = [ ( a + b)(l - a)s12 - ~ ( -l u - b ) ~ 2 ~ ] / ( b ~ I ~ (B-9) ~2~)

Effect of Dye Aggregation on the Photogeneration Efficiency of Organic Photoconductors Kock-Yee Law Xerox Webster Research Center, 800 Phillips Road, 01 14-390, Webster, New York 14580 (Received: November 30, 1987)

The photoconductivities of a soluble vanadyl phthalocyanine dye, ~-Bu,,~VOPC, and model squaraines bis[4-(dimethylamino)phenyl]squaraine(1) and bis(4-methoxypheny1)squaraine (2) have been studied in single-layerand bilayer photoreceptor devices, respectively, by xerographic photodischarge technique. The aggregational behavior of these materials was studied by absorption spectroscopy and X-ray diffraction. Results show that t-Bul,4VOPccan exist as a glassy phase I and a crystalline phase I1 in polystyrene matrix and that 1 and 2 form different aggregates in solid. Xerographic results showed that the phase I1 of ~ - B U ~ , ~ VisO>300 P C times more sensitive than the phase I and the aggregate of 1 is >lo0 times more sensitive than that of 2. These results are attributable to crystallization effect and aggregational effect, respectively. Analysis of the data on the xerographic properties of 4-[p-(dimethylamino)phenyl]-2,6-diphenylthiapyrylium perchlorate (4) reveals that, despite the wide structural variation among the phase I1 of ~ - B U ~ , ~ V Oaggregate PC, of 1, and aggregate of 4, these high-efficiency organic photogenerators share remarkable similarity in electronic and solid-state properties. The use of these properties as a guide for the design and the synthesis of future high-performance organic photoconductors is recommended.

Introduction

Organic pigments, which have a small band gap and absorb strongly in the visible, are often found to be useful as photoconductors for xerographic photoreceptor and organic solar cell applications. Useful classes of organic photoconductors are metallophthalocyanines,'-" squaraines,12-21thiapyrylium salts,22-24 (1) Loutfy, R. 0.;Hsiao, C. K.; Hor, A. M.; Baranyi, G. D. J . Imaging Sci. 1985, 29, 148. Loutfy, R. 0.;Hor, A. M.; Rucklidge, A. J. Imaging Sci. 1986, 31, 31. (2) Loutfy, R. 0.;Hor, A. M.; Baranyi, G. D.; Hsiao, C. K. J. Imaging Sci. 1985, 29, 116. (3) Kakuta, A.; Mori, Y . ;Takano, S.;Sawada, M.; Shibuya, I. J . Imaging Techno!. 1985, I ! , 7. (4) Yanishita, T.; Ikegami, K.; Narusawa, T.; Okuyama, H. IEEE Trans. Ind; Applr 1984, lA-20,-1642. (5) Arishima, K.; Hiratsuka, H.; Tate, A,; Okada, T. App1. Phys. Lett. 1982, 40, 219.

0022-3654/88/2092-4226$01.50/0

p e r y l e n e ~ , azo ~ ~ - compound^,^^*^*-^^ ~~ etc. Among these organic photoconductors, metallophthalocyanines and squaraines are ~~~~~~~

~

(6) Grammatica, S.; Mort, J. Appl. Phys. Lett. 1981, 38, 445. (7) Minami, N.; Sasaki, K.; Tsuda, K. J . Appl. Phys. 1985, 54, 6764. (8) Loutfy, R. 0.;McIntyre, L. F. Can. J . Chem. 1983, 61, 72. (9) Dodelet, J. P. J . Appl. Phys. 1982, 53, 4270. (10) Martin, M.; Andre, J. J.; Simon, J. Nouu. J. Chim. 1981, 5 , 485. (11) Loutfy, R. 0.;Sharp, J. H. J . Chem. Phys. 1979, 71, 1211. (12) Law, K. Y.; Bailey, F. C. J . Imaging Sci. 1987, 31, 172. (13) Tam, A. C.; Balanson, R. D. I E M J . Res. Deu. 1982, 26, 186. (14) Wingard, R. E. IEEE Ind. Appl. 1982, 1251. (15) Tam, A. C. Appl. Phys. Lett. 1980, 37, 978. (!6) Melz, R. J.; Champ, R. B.; Chang, L. S.;Chiou, C.; Keller, G. S.; Liclican, 1.C.;Neiman, R. B.; Shattuck, M. D.; Weiche, W. J. Photogr. Sci. Eng. 1977, 21, 73. (17) Loutfy, R. 0.;Hsiao, C. K.; Kazmaier, P. M. Photogr. Sci. Eng. 1983, 27, 5. (18) Morel, D.L. Mol. Cryst. Liq. Cryst. 1979, 50, 127.

0 1988 American Chemical Society

Photogeneration Efficiency of Organic Photoconductors

The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4227

a)

10% t-Bul.4 VOPC/PS

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b)

,

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60% MARKROLONQ

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molecular structure and device performance is important, extreme caution should be taken in the interpretation of these results. Due to dye aggregation (or intermolecular interaction) in solid, the identity of a molecule in solution and in solid is often different. Correlation between solution properties and device performance is thus indirect and may not be appropriate, especially when the solid-state property is differed from the solution property due to aggregation. In this paper, we report an investigation of the effect of dye aggregation on the photogeneration efficiency of organic photoconductors. Two organic photoconductive systems were studied. In the first system, we studied the photogeneration efficiency of the phase I and the phase I1 of t-Bu,,,VOPc in single-layer photoreceptor devices. In the second system, we studied the photogeneration efficiency of two types of squaraine aggregates using model squaraines 1 and 2 in bilayer photoreceptor devices. Findings relative to the effect of dye aggregation on the photogeneration efficiency will be presented and discussed. ~

N

AI - SUBSTRATE

Figure 1. Schematics of (a) single-layer and (b) bilayer photoreceptors.

6 t-h,

particularly attractive because of their high thermal and high photochemical stabilities. Recent advances in solid-state GaAs diode lasers” have further elevated their utilities as photoconductors for future device application because most of these compounds exhibit high photosensitivity in the near-IR region (-800 nm) where the solid-state diode lasers emit. Indeed, squaraines and metallophthalocyanines have been reported to be useful as IR photoconductors for diode laser printer applications.1-6*12-16 Scientific understanding of factors that influence photoconductivity is of particular value because only with such knowledge can the rational design and the synthesis of novel structures with optimal performance characteristics be achieved. Basic studies of photoconductive processes of particular note in the literature include those by Loutfy and Sharp,32who studied the fluorescence of furanquinones in solution in which they found a correlation between the quantum yield of fluorescence and the photoimaging sensitivity, and by Piechowski and c o - w o r k e r ~who , ~ ~ obtained a general relationship between the performance of a large number of organic dyes in photovoltaic cells and their fluorescence quantum yield in solution. These authors proposed that fluorescent dyes should have a very low quantum yield of internal conversion from the first singlet excited state to the ground state due to minimal molecular flexibility. Thus, the quantum yield of photogeneration should be high in devices. While correlation between (19) Merritt, V. Y . I B M J . Res. Deu. 1978, 22, 353. (20) Morel, D. L.; Ghosh, A. K.; Feng, T.; Stogryn, E. L.; Purwin, P. E.; Shaw, R. F.; Fishman, C. Appl. Phys. Lett. 1978, 32, 495. (21) Merritt, V. Y.; Hovel, H. J. Appl. Phys. Lett. 1976, 29, 414. (22) Dulmage, W. J.; Light, W. A,; Marino, S. J.; Salzberg, C. D.; Smith, D. L.; Staudenmayer, W. J. J. Appl. Phys. 1978, 49, 5543. (23) Borsenberger, P. M.; Chowdry, A,; Hoesterey, D. C.; Mey, W. J . Appl. Phys. 1978, 49, 5555. (24) Tang, C. W. US.Patent 4281 053, 1981. (25) Khe, N. C.; Yokota, S.; Takahashi, K. Photogr. Sci. Eng. 1984, 28, 191. (26) Schlosser, E. G. J . Appl. Photogr. Eng. 1978, 4 , 118. (27) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (28) Khe, N. C.; Takenouchi, 0.;Kawara, T.; Tanaka, H.; Yokota, S. Phorogr. Sci. Eng. 1984, 28, 196. (29) Tsutsui, K.; Hashimoto, M.; Ohta, M.; Sasaki, M. U S . Patent 4 596 754, 1986. (30) Hiro, M.; Takasu, Y.; Ishikawa, S.; Katagirim, K.; Takahashi, H. US.Patent 4 551 404, 1985. (31) Hug, W. F. Laser Focus 1986, Aug, 92. (32) Loutfy, R. 0.; Sharp, J. H. J . Phys. Chem. 1979, 8 3 , 1208. (33) Morel, D. L.; Stogryn, E. L.; Ghosh, A. K.; Feng, T.;Purwin, P. E.; Shaw, R. F.; Fishman, C.; Bird, G. R.; Piechowski, A. P. J. Phys. Chem. 1984, 88, 923.

VOPC

Experimental Section Materials. Soluble vanadyl phthalocyanine dye was prepared as described previously34 and was purified by an acid-pasting p r o c e d ~ r e . ~Bis[4-(dimethylamino)phenyl]squaraine ~ (1) was synthesized from di-n-butyl squarate and N,N-dimeth~laniline~~ and was used without further purification. Bis(4-dimethoxypheny1)squaraine (2) was prepared according to a procedure published by Farnum and co-workers3’ and was purified by Soxhlet extraction with chloroform before use. Polystyrene (PS), trade name Lustrex H F 7 7 7 , was from Goodyear Chemicals and Makrolon 5705 was a polycarbonate from Mobay Chemical Co.; these polymers were used as received. Hole transporting molecule (for bilayer device) N,N’-diphenyl-N,N’-bis(3-methylphenyl)[ l,l’-biphenyl]-4,4’-diamine(TPD) was obtained from an internal source. The synthesis of TPD had been reported elsewhere.38 All solvents used in this work were analyzed reagent grade from Baker and were routinely stored over molecular sieves ( 3 A). Device Fabrication. Two device configurations, namely, single-layer and bilayer photoreceptors, were used in this work, and schematics of the device configurations are given in Figure 1. (a) t-Bu,,,VOPc/PS Single-Layer Photoreceptor The coating solution was prepared by dissolving 0 . 7 5 g of PS in 5.5 mL of methylene chloride in a 1-oz brown bottle. t-Bul,4VOPc, 0.084 g, and steel shots, 50 g, were added and the mixture was homogenized on a Red Devil Paint Shaker (Model 5100X) for about 0.5 h. The resulting solution was coated onto a 7 . 5 in. X 10 in. precleaned brush-grained aluminum substrate (from Ron Ink Co.) using a Gardner Mechanical Drive Film Applicator with a 3.5 in.-wide, 3-mil wet gap draw bar inside a humidity-controlled (34) Law, K. Y . Inorg. Chem. 1985, 24, 1778. (35) Germano, N . J.; Nealey, R. H. Xerox D i d . J . 1978, 3, 377. (36) Law, K. Y.; Bailey, F. C. Can. J. Chem. 1986, 64, 2267. (37) Farnum, D. G.; Webster, B.; Wolf, A . D. Tetrahedron Lett. 1968, 5003. (38) Stolka, M.; Yanus, J. F.;Pai, D. M. J. Phys. Chem. 1984,88, 4707. (39) p e to the high stability of t-Bu,,,VOPc in organic solvents and to the requirement of vapor-induced crystallization to produce its photoactive form, the photoconductivity of t-Bu, ,VOPc was studied in the single-layer device to avoid fabrication problems.

4228

Law

The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 CHARGING

DARK- DISCHARGE

PHOTODISCHARGE

-

In

a

400

1000

600 h(nm)

TIME

f

Figure 2. Schematics of photodischarge curves.

-

glove box (relative humidity 125%). The resulting single-layer device was 10 pm thick as estimated from TEM micrographs and was vacuum-dried at 100 OC for >16 h before electrical testing. ( b ) Squaraine Bilayer Photoreceptor Device. The fabrication procedure for squaraine bilayer photoreceptor devices has been detailed in an earlier report.40 Typically, the charge generation layer (CGL) was prepared by coating a squaraine coating dispersion containing 0.2 g of Makrolon and 0.096 g of squaraine in 10 mL of methylene chloride onto an aluminum substrate using a 6-in.-wide, 0.5-mil wet gap draw bar. The CGL was -0.5 pm in thickness and was dried at -100 OC for 1 h before further coating. The complete bilayer device was prepared by coating a TPD solution, which consists of 4.2 g of Makrolon and 2.8 g of TPD in 3 1 mL of methylene chloride, onto the CGL using a 3.5-in-wide, 5-mil wet gap draw bar. The thickness of the CTL was -30 pm as determined by a Permascope Instrument. The resulting bilayer device was air-dried for 0.5-1 h and vacuum-dried for > 16 h before electrical testing. Xerographic measurement^.^^ Xerographic measurements were made on a flate plate scanner using 2 in. X 2.5 in. samples. Details of the schematic of the apparatus and the measurement procedure were reported earlier.40 Typically, the device was charged up (positively for single-layer device and negatively for bilayer device) to 1000 V by a corotron device. The surface potential of the device was monitored with a capacitively coupled ring probe connected to a Keithley electrometer (Model 610C) in the Coulomb mode. The output of the electrometer was displayed on a strip chart recorder ( H P Model 740A) which was calibrated by applying known voltage on an uncoated aluminum substrate. The exposure wavelength and the intensity were selected and adjusted by using interference and neutral density filters, respectively. With the shutter closed, the dark decay of the device (AV/At) was measured. With the shutter open, the device could be exposed to an intense erase light to determine the residual potential (V,) or to a monochromatic radiation of known intensity ( I in erg/(cm2-s)) to determine the photosensitivity.of the device. The photosensitivity of the device is expressed as Eo.5,the energy required to photodischarge half of the initial potential (vi). Eo,5 is the product of I and t where t is the time for I to photodischarge the device from vi to (1/2)v; the lower the Eo,svalue, the higher the photosensitivity. Schematics of the photodischarge curves are given in Figure 2.

Figure 3. Absorption spectra of C-BU,,~VOPC in polystyrene ((-) and (- - -) after ethyl acetate vapor treatment).

before

SCHEME I

-

-

Results and Discussion ~ - B U ~ , ~ VSingle-Layer OPC Photoreceptors. (a) Characterization of t-Bu,,,VOPc Dye Aggregates. Figure 3 (solid curve) shows the absorption spectrum of a thin film of t-Bu,,,VOPc in PS (10% by weight loading) on a transparent Mylar substrate. Absorption (40) Law, K. Y. J. ImagingSci. 1987, 31, 83. (41) For a discussion on the principle of the photodischarge of photoreceptor devices and the measurement techniques, see Schaffert, R. M. Electrophotography; Focal Press: London, 1980; also see ref 16.

0

maxima at 650 and 694 nm are observed in the visible region. By (log comparison with its absorption in chloroform solution (A, e ) : 697.4 (5.13), -665 (shoulder), and 626.5 (4.43)),34the absorption spectra of the polymorphs of t-Bu,,,VOPc (phases I and II),"* as well as the spectral data of VOPC:~ we assign the visible absorption spectrum to the phase I of t-Bul,,VOPc. A small absorption shoulder at -800 nm was also observed. Since IR absorption is the characteristics of the phases I1 of VOPc and ~ - B u , , , V O P Cwe , ~ ~attribute ~ ~ ~ the near-IR absorption shoulder to the existence of a small amount of phase I1 of ~ - B u , , ~ V O in PC the polymer matrix. The formation of the phase I1 is not surprising because phase I1 is a thermodynamically more stable phase.43 The interesting feature of the results is the greater tendency for tBul,,VOPc to precipitate as the metastable phase I rather than the more stable phase I1 during the film formation. This observation may be due in part to the steric stabilization of the metastable phase I by the tert-butyl groups in the solid state4, and in part to the fast solvent evaporation during the film formation, which produces a quenching effect and in turn generates the thermodynamically less stable dye aggregate^.^^^^^ Ethyl acetate vapor treatment (24 h) of the above tBul,,VOPc/PS film causes a change in color of the film from blue-green to blue-blue. In the absorption spectrum (Figure 3, dashed curve), a decrease in absorption in the visible region and an increase in absorption in the near-IR region (Amx -810 nm) are observed. Since the near-IR absorption is the unique characteristic of the phases I1 of VOPc and t-Bul,4VOPc,the absorption spectral data in Figure 3 suggest that the phase I of t-Bu,,,VOPc is converted to the phase I1 upon exposure to ethyl acetate vapor. Details on the chemistry of this vapor induced crystallization process has been addressed elsewhere.42 Figure 4 (curves a and b) shows the X-ray powder diffraction patterns of PS films of t-Bul,,VOPc before and after the vapor treatment. Results showed that there is an increase in crystallinity of the sample after the vapor treatment. Relatively sharp dif(42) Law, K. Y. J . Phys. Chem. 1985, 89, 2652. (43) Griffiths, C. H.; Walker, M. S.; Goldstein, P. Mol. Cryst. Liq. Cryst. 1976,33, 149. Huang, T. H.; Sharp, J. H.; Wagner, H. J. Chem. Phys. 1982, 65, 205. (44) Very similar steric stabilization of metastable polymorph was also observed rn copper phthalocyanine; see Stepp, J. D. U.S. Patent 4289698, 1981. (45) Law, K. Y. Polymer 1982, 23, 1627.

Photogeneration Efficiency of Organic Photoconductors

The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4229

r

ERASE Ll$HT

TIME LIGHT 6OQ nm n

/

'

2S E C . 4

,,

l

l

4

l

8

l

l

l

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l

16

20

l

I

I

24

l

1

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32

28

28

Figure 4. X-ray powder diffraction patterns of (A) t-Bu,,,VOPc in P S before vapor treatment, (B) ~ - B U ~ . ~ V O inPPS C after vapor treatment, and ( C ) phase I1 of VOPc in PS.

TABLE I: Xerographic Data of ~ - B u , , ~ V O Pand C VOPc Single-Layer Photoreceptor Devicesd t-Bul.4VOPc untreated

treatedb

VOPc

corotron, kV

+5.5

v,, v AV/At, v,, v

+900 +10 +250

+5.6 +920

+6.0 +800

+35

+30 +20 21

V/s

E O Sa t 600 nm, erg/cm2

>lo4

+10 33

10% by weight VOPc or t-Bu, ,VOPc in polystyrene, thick. bEthyl acetate vapor treated for 8 h.

-

10 r m

fraction lines at d-spacings of 3.25, 4.0, and 4.9 A are observed. As pointed out in our earlier publication, t-Bu, ,VOPc consists of a mixture of VOPc dyes of varying degree of tert-butyl sub~ t i t u t i o n .The ~ ~ fact that crystallinity is observed suggests that t-Bu, ,VOPc molecules must be packed in an orderly fashion in phase 11. Comparison of the X-ray diffraction pattern of the phase I1 of VOPc in polystyrene (Figure 4,curve c), where diffraction lines at d-spacings of 3.1, 3.92, 3.99, and 4.85 A are observed, suggests that the molecular arrangements of the phases I1 of t-Bu, ,VOPc and VOPc are very similar. A schematic of the molecular rearrangement of t-Bul ,VOPc molecules in phase I1 can be depicted in Scheme I, in accordance to the crystal structure of the phase I1 of V O P C . ~ ~ ( b ) Xerographic measurement^.^^ Parts a and b of Figure 5 show the photodischarge curves of a t-Bu, ,VOPc/PS single-layer photoreceptor before and after ethyl acetate vapor treatment. The xerographic data are tabulated in Table I. The data for an analogous VOPc device are also listed for comparison. Results show that there is a slight increase in dark decay, a significant decrease in residual potential, and a dramatic increase in photosensitivity after the ethyl acetate vapor treatment. Transmission electron microscopy results reveal that t-Bu, ,VOPc particles (46) Ziolo, R.; Griffiths, C . H.; Troup, J. M. J . Chem. SOC.,Dalton Trans. 1980, 2300.

1500

b

Figure 5. Photodischarge curves of t-Bu, ,VOPc/PS single-layer photoreceptor: (a) dark decay measurement; (b) photosensitivity measurement ((- - -) before and (-) after vapor treatment).

(-0.1-0.15 pm) distribute uniformly within the layer and there is neither any change in particle distribution nor any change in particle size after the vapor treatment. Since absorption spectral data and X-ray results show that t-Bu, ,VOPc crystallizes from the glassy phase I to the crystalline phase I1 upon the vapor treatment, the change in xerographic properties is thus attributable to this phase change. The increase in dark conductivity (decay) and reduction in residual potential are consistent with the increased crystallinity because crystalline materials are known to have higher conductivity than glassy material^.,^ The most striking result in Table I is the >300 times improvement in photosensitivity (lower Eo value) after the ethyl acetate vapor treatment. The Eo value of the vapor-treated t-Bu, ,VOPc device is 33 erg/cm2 and is -50% larger than that of a VOPc device. However, as seen in Figure 5, there is a long induction period (tJa in the photodischarge curve. This observation is not unusual; very similar photodischarge curves had been reported earlier by Wiegl and co-workers on their study of metal-free phthalocyanine-binder p h o t ~ r e c e p t o r . ~These ~ authors studied the effect of phthalocyanine concentration and the effect of electric field applied to the device on t, and found that t, decreases as the phthalocyanine concentration decreases and as the electrical field decreases. They proposed that t, is caused by limitations in charge transport across the device, which primarily relies on particle-particle contacts. We obtained analogous results in various t-Bu, ,VOPc devices in this Since the t, values of the t-Bu, ,VOPc and the VOPc device (at 10%loading) are -0.76 and -0.25 s, respectively, and since the slopes of the linear portion of the photodischarge curves are about the same (60 and 66 V.cm*/erg), our results suggest that the slightly larger Eo value of the t-Bu, ,VOPc device is primarily due to the relatively slow charge transport in the device and that the quantum efficiencies of photogeneration of the phases I1 of VOPc and t-Bul ,VOPc are actually very similar. In our previous paper, we reported that we were able to reproduce the solid-state properties of VOPc while increasing its solubility in organic solvents by introducing an average of 1.4 tert-butyl groups on the VOPc ring.34 This work further demonstrates that we are able to reproduce not only the solid-state (47) Gutman, F.;Lyons, L. E. Organic Semiconductors; Krieger: Malabar, FL, 1981; p 168. (48) Induction period (II) defines as the delay time observed in the photodischarge curve after light exposure. For 10-rm-thick t-Bu, ,VOPc/PS devices, t, decreases from 0.76 to 0.66 s when the concentration of t-Bu, ,VOPc increases from 10% to 14% by weight. For a 10-rm-thick device (10% dye loading), t, decreases from 0.76 0.72 0.53 0.48 s as V,decreases from 900 840 750 660 V. Law, K. Y . , unpublished observation. (49) Weigl, J. W.; Mammino, J.; Whittaker, G. L.; Radler, R. W.; Byrne, J. F In Current Problems in Electrophotography; Berg, W. F., Hauffe, K., Eds.; de Gruyter: Berlin, 1972.

- - -

- - -

4230 The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 I

I

1

I

a)

\

Law SCHEME I1 I

/

-I

'. "'\

1

400

600

I

1

800

1000

SCHEME I11

A (nm)

a a W

\ \

a

\

a

I

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TABLE 11: Xerographic Data of 1 and 2 in Bilayer Photoreceptor Device

\

m

\

\

'.'.

600

1 corotron, kV

I

I

800

1000

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(nm) Figure 6. Solid-state ((- - -) in KBr) and solution absorption spectra (-) of (a) squaraine 1 and (b) squaraine 2. A

properties but the photoconductivity as well. Squaraine Bilayer Photoreceptors. ( a ) Characterization of Squaraine Aggregates. The solution absorption of squaraine 1 in CH2C12and the solid-state absorption of 1 in KBr are given in Figure 6a. The solution absorption is sharp and intense (A,, 627 nm and tmax -3 X lo5 cm-I M-I). I n the solid state, the absorption is very panchromatic (broad) and extended to the near-IR. We attribute the significant red-shift in the absorption edge (from -640 to -900 nm) and the broad absorption of 1 in solid to the absorption of an aggregate of 1. Recently Wingard reported a crystal structure of bis[2methyl-4-(dimethylamino)phenyl]squaraine (3), where he showed that there exist extensive intermolecular interactions between the anilino moieities and the four-membered ring of squaraine and Since recent MO that the interplanar distance is -3.5 calculations on 1 show that both the ground and the excited state of squaraine are intramolecular charge-transfer states,50the intermolecular interactions in the solid state of 3 is thus charge transfer in nature. In fact, Wingard proposed that the broad absorption of 3 in the solid state is due to intermolecular charge-transfer interactions between the anilino moieties and the four-membered ring of squaraine. A very similar conclusion was also reached by Tristani-Kendra and Eckhardt in their study of the structure and the solid-state absorption of bis[2-hydroxy4-(diethylamino)phenyl]~quaraine.~~Since the X-ray powder diffraction patterns and the solid-state absorption of 1 and 3 are nearly identical, we believe that very similar intermolecular charge-transfer interactions also occur in the aggregate of 1. A schematic of the intermolecular donor-acceptor (D-A) chargetransfer interactions is given in Scheme 11. The solution absorption and the solid-state absorption spectra of bis(4-methoxypheny1)squaraine (2) are presented in Figure 6b. The A,, of 2 in CH2CI2is at 538 nm and is blue-shifted by -90 (50) Bigelow, R. W.; Freund, H . J . Chem. Phys. 1986, 107, 159. (51) Tristani-Kendra, M.; Eckhardt, C. J. J . Chem. Phys. 1984,8/, 1160.

v,,v

AV/At, V/s VR, v E , a t 600 nm,erg/"

-6.1 -970 -90

-50 3.3

2 -4.8 -920

-45 -20 390

nm from that of 1, but the basic feature between the solution absorption spectra of 1 and 2 is the same. In contrast to that seen in 1, the solid-state absorption band of 2 is narrow and is blueshifted from its solution absorption. We assign the solid-state absorption to the aggregate of 2. Recently Ziolo and Law redetermined the single-crystal structure of Structural results show that the interplanar distance between 2 is -3.5 A; however, the major intermolecular interactions come from the C-0 dipoles rather than the intermolecular charge-transfer interactions as in 1 (Scheme 111). The change in molecular packing is presumably due to the decrease in polarization of the a-electrons in 2 as compared to 1. On the basis of the structural result, we conclude that the difference in solid-state absorption spectra between 1 and 2 is an aggregational effect. ( b )Xerographic measurement^.^^ The photoconductivities of 1 and 2 were studied by the xerographic photodischarge technique using bilayer photoreceptor devices. Results (Table 11) show that the device of 1 exhibits higher dark decay and higher V, values as compared to that of 2. These differences can be caused by intrinsic or extrinsic factors (e.g., impurities). More detailed experimentation is needed to elucidate all the possibilities. While dark decay and VRvalues differ only by a factor of 2, the difference in photosensitivity between 1 and 2 is over a factor of 100. The low sensitivity (large E o s value) of 2 is not due to impurity effect because impurities primarily affect the dark decay of the device.53 The low photosensitivity should not be due to any energy mismatch between the CGL and the CTL because the oxidation potential of 2 is expected to be higher than that of l.54It is also not due to any fabrication effect because similar ( 5 2 ) Redetermination of the crystal structure of 2 provides us accurate structural and packing data of 2 in solid. For the original report, see Farnum, D. G.; Neuman, M. A,; Suggs, W . T . J . Cryst. Mol. Struct. 1974, 4, 199. (53) Law, K. Y.; Bailey, F. C. Dyes Pigm. 1988, 9, 85. (54)Due to the electrochemical instability of 2 in solution, the relative energy level between 2 and TPD could not be assessed. However, the oxidation potential of 2 should be more positive than that of 1 because removal of an electron from an anisyl group is expected to be more difficult than from an anilino group.

Photogeneration Efficiency of Organic Photoconductors

The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4231

differences in photosensitivity are observed when 1 and 2 are tested in different device configurations. We thus attribute the low photosensitivity of 2 to aggregational effect. Discussions on the Effect of Dye Aggregation on the Photogeneration Efficiency. Xerographic results on ~ - B u , , ~ V O P C / P S single-layer photoreceptor devices show that there is a drastic increase in photogeneration efficiency when the glassy phase I of t-Bu,,,VOPc is crystallized to phase 11. A very similar improvement in photosensitivity was also observed by Dulmage and co-workers22in an analogous single-layer photoreceptor based on 4- [p-(dimethylamino)phenyl]-2,6-diphenylthiapyryliumsalt (4) and bis[p-(diethylamino)-o-methylphenyl]phenylmethane(5) in CH3,

p

3

N

polycarbonate. These authors reported that the as-coated device is glassy and homogeneous. Upon exposure of the device to methylene chloride vapor, 4 crystallizes to form an aggregate in the polymer matrix. This is accompanied by an increase of photosensitivity (by a factor of 150 at the absorption maximum). Subsequent spectroscopic, electrochemical, and structural studies suggest that the electronic states of 4 are intramolecular charge-transfer states and that there exists extensive intermolecular charge-transfer interactions in the aggregate. The interplanar distance of 4 in the aggregate is -3.4 A.55Since xerographic measurements by Borsenberger at aL2j showed that electron-hole pairs are generated upon excitation of the aggregate, the enhanced photoresponse observed after the vapor treatment of 4 was attributed to the extensive intermolecular charge-transfer interactions in the aggregate of 4 resulting from the cry~tallization.~~ Although it remains unanswered as to whether the low sensitivity of the glassy state of 4 is due to the low quantum efficiency of photogeneration or to the high rate of electron-hole pair recombination, there is little doubt that intermolecular charge-transfer interaction in the aggregate of 4 are responsible for the high efficiency of photogeneration observed. The t-Bu,,,VOPc system bears a remarkably similarity to 4 in addition to the drastic increase in photoresponse upon crystallization. Theoretical calculations by Schaffer, Gouterman, and DavidsonS6showed that the 7-orbital of VOPc is localized in the outer benzene ring and the **-orbital is localized in the inner macrocyclic ring. The K,T* state of VOPc is consequently an intramolecular charge-transfer state also.57 The close interactions between the outer benzene ring and the inner macrocyclic ring in the phase I1 form of t-Bu, ,VOPc (Scheme I) suggest that ( 5 5 ) Perlstein, J. H. Presented at the Ninth Annual Summer Institute in Polymer Science and Technology-Polymers in Electron Applications, June 11, 1979. ( 5 6 ) Schaffer, A. M.; Gouterman, M.; Davidson, E. R. Theor. Chim.Acfa 1973, 30, 9. ( 5 7 ) The absorption maximum of f-Bu,VOPc (n = 1-4) shifts to longer wavelengths as n increases. This alkyl substituent effect, coupled with the

solvent effect study on tlie absorption spectra of t-Bu,VOPc, where a qualitative relationship between A,, and solvent acceptor number (AN) was observed.. sueeests that the *.P* state of VOPc should be a charge-transfer state.

--

-

interactions reminiscent of those in the aggregate of 4 should occur upon optical excitation. The high photogeneration efficiency of the phase I1 of t-Bu, 4VOPc can be rationalized as the high efficiency of photogeneration in the crystalline aggregated state. In the squaraine system, 1 and 2 form different aggregates in solid and different photosensitivities are observed. The difference is attributable to an aggregational effect on the photogeneration efficiency. The most fascinating observation here is the high photosensitivity of 1 because 1 is shown to form aggregates having extensive intermolecular charge-transfer interactions with an interplanar distance of -3.5 A (Scheme 11). Since the excited state of squaraine is a charge-transfer the kind of intermolecular charge-transfer interactions in 1 is actually identical with those of the phase I1 of t-Bu, ,VOPc and the aggregate of 4. The high photogeneration efficiency of 1 is thus rational. Our results here also illustrate that, although dye aggregation is a prerequisite for the high photogeneration efficiency, the precise molecular arrangement in the aggregate is as critically important. Effect of dye aggregation on the photosensitivity are not uncommon in organic photoreceptors, especially among devices where various crystalline phases (polymorphs) of metallophthalocyanines are possible. Weigl and co-workers reported that the x-form of H2Pc is more sensitive than the a-form and the P-form of H2Pc in their phthalocyanine-binder photoreceptor Yagishita et al. preferred the use of the eform of CuPc over the a-form and the @form in their photoreceptor device., Arishi and co-workers showed that a crystalline, IR-absorbing, highly photosensitive CGL can be obtained by exposing a polycrystalline thin film .(by evaporation) of ClPcAlCl to tetrahydrofuran vapor.5 Very similar observations were also reported by Loutfy et a1.* Detailed structures of these highly sensitive aggregates are, however, not available due to experimental difficulties. The common feature of these aggregates is their strong absorption in the near-IR region (-780-850 nm), which is red-shifted from their solution absorption maxima by 100-150 nm. The spectral changes of these aggregates from their solution absorption are identical with that of t-Bu, ,VQPc. Thus, if the near-IR absorption is an indication of the similarity in intermolecular interactions between these aggregates and the phase I1 of t-Bul ,VOPc, their high photosensitivities might provide further support of this viewpoint.

Concluding Remarks This work demonstrates the general importance of dye aggregation on the photogeneration efficiency of organic photoconductors. Xerographic measurements showed that the phase I1 of t-Bu, ,VOPc, the aggregate of squaraine 1, and the aggregate of thiapyrylium salt 4 are highly efficient photogenerators. Despite the wide structural variation among them, their electronic properties and their intermolecular interactions in solid are remarkably similar. The observation in this work suggests that an efficient photogenerator should have (1) a large charge mobility upon optical excitation (charge-transfer states), ( 2 ) short interplanar distances ( 3.5 A), and (3) strong intermolecular charge-transfer interactions in solid. We believe that the knowledge gained in this work should be of value in the design and the synthesis of future high-performance organic photoconductors.

-

Acknowledgment. I am indebted to Dr. R . Ziolo for the collaboration on the structure analysis of squaraine 2 and Dr. A. Melnyk for continuous helpful discussion on xerography. Registry No. 1, 43134-09-4; 2, 20868-01-3.