PHOTOCONDUCTIVITY OF N-ISOPROPYLCARBAZOLE
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Photoconductivity of N-Isopropylcarbazole and Its Picryl Chloride Complex‘
by James H. Sharp Research Laboratories, Xerox Corporation, Rochester, New York
(Received January 16, 1967)
The bulk photoconductive properties of single crystals of N-isopropylcarbazole (NIPC) and its picryl chloride charge-transfer complexes (1% and 1 : 1, mole %) have been studied. The introduction of these charge-transfer states into the parent NIPC crystal has little or no effect on either the photoconductive threshold or the efficiency of the carrier generation step. The band gap for NIPC is 4.7 ev and the carrier generation step is interpreted as a direct photogeneration process. The quantum efficiency of carrier generation, deterPicryl chloride was found mined from pulsed light experiments, is of the order of to act as a weak electron trap. The electron mobilities in NIPC and the 1% and 1:l complexes were found to be 1.0, 0.43, and 0.12 cmz/v sec, respectively. The hole currents, arising from pulsed photoconductive measurements, exhibited space-charge-limited characteristics. The hole mobility was found to be 0.33 cmz/v sec and the introduction of chargetransfer states into the NIPC crystal had little effect on this value. The temperature dependence of the electron and hole mobilities and the quantum efficiency of carrier generation were determined.
The photoconductive properties of organic crystals, particularly anthracene, have been extensively studied. Generally, the action spectra for photoconductivity closely resemble the absorption ~ p e c t r a . ~ -In~ the case of anthracene, the absorption edge (4000 A) corresponds to the photoconduction threshold ( ~ 3 . 1ev). Although the “band gap” for anthracene has not been unambiguously established, it can be safely argued to be considerably greater than 3.1 e ~ . ~ The mechanism of charge carrier production is believed to involve exciton interactions. These excitons are mobile and by interacting with impurities,6 each other,’ or photons,* free carriers are formed. Experimental evidence for bimolecular exciton interaction leading to charge carrier production has been found by Silver, et al.9 Because of competing processes, such as fluorescence and radiationless transitions, the quantum yields of charge carrier production are very small (4 Until recently, there was little or no evidence for direct band-to-band transitions in organic crystals. Castro and Hornig’O have reported the observation of a direct band-to-band transition in anthracene corresponding to 4.4 ev and Chaiken and Kearnslo have confirmed the photocurrent peak at 4.4 ev corresponding to the direct
io15 >io15 >io15
3.9 3.9 3.3
2. Cy s t a l Structure. The crystallographic properties of the crystals have been studied using Weissenberg, precession, and diff ractometry techniques18 and are summarized in Table 11. There is a remarkable correlation between the crystal structure of the parent NIPC molecule and the 1:1 complex. This suggests that PC molecules easily replace NIPC molecules in the unit cell of the latter without serious distortion.
WAVELENGTH, (i)
Figure 2. (A) The absorption spectrum of N-isopropylcarbazole dissolved in dichloromethane; (B) the absorption spectrum of the N-isopropylcarbazole-picryl chloride charge-transfer ) and picryl chloride complex in dichloromethane (absorption edge (- - -); (C) the polarized absorption spectra of the 1% picryl chloride-N-isopropylcarbazole single crystal.
N- ISOPROPYL CARBAZOLE
PICRYL CHLORIDE
Figure 1. The molecular structure of N-isopropylcarbazole and picryl chloride.
The Journal of Physical Chemistry
(13) M.Pope, J. Burzos, and J. Giachino, J. Chem. Phya., 43, 3367 (1965). (14) H.Akamatu and H.Kuroda, ibid., 39,3364 (1964). (15) H. Hoegl, J . Phya. C h m . , 69,755 (1965). (16) M.Lardon, E.Lell, and J. Weigl, unpublished results. (17) J. H.Sharp, J . Phys. Chem., 70, 584 (1966). (18) P.Cherin and M. Burak, ibid., 70, 1470 (1966).
PHOTOCONDUCTIVITY OF N-ISOPROPYLCARBAZOLE
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Table 11: Unit, Cell Dimensions aobed
PX
8, OC
z
dcms
g/cma
90
8
1,14
1.15
100.1 f 0.1 102.5
8
1.47
1.46
4
...
...
Space
Cryctal
group
NIPC
Ic2a
PC-XIPC (1:l) PC
P21/c P21/c
a, .4
b, A
18.01 f 0.02 18.12 i 0.07 11.10
7.963 f 0.006 6.962 i 0.013 6.83
Both crystals cleave readily parallel to the ab planes and the molecules appear to be stacked in the c direction. Polarized absorption measurements also indicate that the c axis is the “stacking” axis in the complex (see Figure 2c). 3. Absorption Spectra. The absorption spectrum of highly purified S I P C dissolved in dichloromethane is shown in Figure 2A. The lowest T-T* state, corresponding to the ’So + ’& electronic transition, lies a t 28,900 ern-’ (3.57 ev). The absorption edge of a single crystal of KIPC is almost identical with that of the 1% PC-NIPC single crystal absorption (Ito the c axis) which is shown in Figure 2C. The absorption spectrum of the 1: 1 complex in solution is shown in Figure 2B. The characteristic charge-transfer band has a maximum at) 4150 A (24,100 cm-’) and there is no evidence of any structure. The thermodynamics of the complex formation1’ show that a weak complex, having a heat of formation AH of -1.41 kcal/mole, is involved. An infrared analysis of the charge-transfer complex showed that it was composed entirely of a combination of the observed bands for the parerit NIPC and PC molecules. Thus, little or no evidence of charge-transfer interaction in the ground electronic state of the complex was detected by this method. Figure 2C illustrates the polarized absorption spectra of the 1% PC-NIPC single crystal. The absorption is strongly polarized along the c direction or stacking axis of the crystal. 4. Photoconductivity Measurements. 4a(i) SteadyState Instrumentation. In order to determine the action spectra of the photogenerated carriers, steadystate dc measurements were made. All measurements were made perpendicular to the major cleavage plane, i.e., the ab plane. A typical crystal had a surface area of 0.5 cm2 and a thickness of 0.20 cm. A semitransparent silver electrode was evaporated onto the top surface and the crystal was mounted on a 2-in. X %in. Nesa glass plate by means of a silver paint electrode. A grounded silver paint guard ring was carefully painted around the edges of the crystal to ensure that bulk measurements were being made.
c. A
16.82 i 0.01 16.70 i 0.08 12.62
The mounted crystals were housed in an aluminum box which was equipped with a window and a shutter. The applied voltage, supplied by a Keithley Model 241 voltage supply, was connected to the Nesa plate. A Keithley 610B electrometer was used to measure the photocurrent and was connected to the top electrode via a fine tungsten wire. The output of the electrometer was fed to an Esterline Angus chart recorder. The irradiating system consisted of a Bausch and Lomb 250-mm grating monochromator which was equipped with a 100-w quartz mercury arc lamp. The monochromatic light was passed through a crystal quartz-fluoride achromatic condenser lens and the entrance and exit slits of the monochromator were adjusted so that a parallel and uniform light beam was incident upon the semitransparent electrode of the crystal. The incident light flux from 2000 to 4000 A was measured with a Reeder thermopile designed for use with a Bausch and Lomb monochromator. It had been previously calibrated with a NBS standard light source. 4a(ii) Voltage and Intensity Dependence. The voltage dependence of the electron steady-state photocurrent (negative electrode illuminated) for NIPC is shown in Figure 3. A similar response was observed for both the 1% and the 1 : l complexes. Such a response is strongly indicative of trap modulated spacecharge-limited photocurrents. The intensity-voltage curves exhibit two main features; there is a steep portion at low voltages and a saturation region a t higher applied voltages. Figure 3 also shows the effect of varying the light intensity. In the saturation region, the photocurrent shows a linear dependence on the light intensity. Although the data are not complete, it appears that the photocurrent is insensitive to the light intensity at low applied voltages. 4a(iii) Action Spectra. The action spectra of the NIPC, the 1% PC-NIPC, and the 1 : l complex are shown in Figure 4a. The incident light intensity has been corrected for wavelength variation in both lamp emission and silver electrode transmission and has been normalized t o a constant flux of lo1*quanta/ cm2/sec. It is significant that the spectra are almost Volume 71 Number 8 J u l y 1967 ~
JAMESH. SHARP
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I
I
I
I I l 1 1 (
I
I
1
1 I
I
I I Ill1
I
I
I
I I 1 1 1 1
Ill!
12.0.1 Io
I
I
100 VOLTAGE
lo00
identical in shape for all three crystals. Little or no photocarrier generation resulted when the 1% or the 1:l complexes were irradiated in the region of the charge-transfer absorption band. Furthermore, there was no photogeneration of carriers resulting from the population of the lower T-T* electronic states of any of the crystals. In each case, the threshold of bulk photoconductivity occurs well into the ultraviolet region of the spectra. The shape of the spectra are independent of the polarity of the illuminated electrode. However, the current densit'ies were much higher (3040X) when the negative electrode was illuminated. A comparison of the electron and hole photocurrents for the NIPC crystal is shown in Figure 5 . A similar effect, although not as pronounced, was noted for the 1% and the 1 : l complexes.
t
4
1
Figure 3. The steady-state electron photocurrent dependence for the N-isopropylcarbazole crystal on the applied field and the incident light intensity.
3
8 HOLE PHOTO CURRENT 0 FLUORESENCE INTENSITY
1
I
IO-I~~.~
,
I
I
j , I d LL
I
5.6 4.8 EXCITING IRRADIATION,e.v.
4.0
Figure 5. A comparison of the electron and hole photocurrents for the N-isopropylcarbazole single crystal and a comparison of the fluorescence efficiency as a function of the exciting wavelength. Figure 4. (a) The action spectra of N-isopropylcarbazole, the 1% picryl chloride-N-isopropylcarbazole, and the 1 : 1 complex. Applied field is 100 v/cm (positive voltage applied a t the bottom electrode) and the photocurrent has been normalized to a constant incident flux of 10" quanta/cma/sec. (b) Typical photocurrent response.
The Journal of Physical Chemistry
The observed rise and decay times of the steadystate photocurrent are very fast and are limited by the response time of the electrometer and the recorder ( T = 0.5 see). The photocurrent response, typical for all crystals, is shown in Figure 4b.
PHOTOCONDUCTIVITY OF N-ISOPROPYLCARBAZOLE
4b(i) Transient Photocurrent Instrumentation. Kepler2b and LeBlanclg were the first workers to use pulsed photoconductivity techniques to measure carrier mobilities in organic crystals. One advantage of this technique is that it enables one to overcome polarization and electrode effects in the study of organic photoconductors. A block diagram of the experimental arrangement used in this work is shown in Figure 6. A quartz xenon flash lamp was used as the light source and the triggering and charging units were obtained from the Amglo Co., Chicago, 111. The flash lamp was discharged from a 5.1-pf low-inductance capacitor. The flash time was 5-10 psec and the total power output varied between 5 and 30 joules depending on the charged capacitor voltage. The incident light intensity was also varied by the insertion of metal screens between the flash lamp and the crystals. The crystals were mounted on n'esa glass plates and the light pulse was incident on them through the top electrode of semitransparent evaporated silver. Several experiments were conducted using semitransparent silver or aluminum-coated quartz electrodes. No difference in the results was obtained under these conditions. The light pulse was monitored by a silicon photodiode and the response was displayed simultaneously with the transient pulse on the dual beam oscilloscope. Both the photodiode response and the transient pulse sweep times were triggered from the trigger transformer. The RC time constant of the measuring circuit was approximately 50 psec. Only the ultraviolet component of the
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Figure 7 . Electron transients in the crystals studied: (A) NIPC, applied field is -50 v (top electrode); crystal thickness, d, is 0.25 cm, 0.5 msec/div, 0.5 mv/div; (B) lY0 PC-NIPC, applied field is -200 v (top electrode); crystal thickness, d, is 0.20 cm, 0.2 msec/div, 10 mv/div; (C) 1: 1 PC-NIPC, applied field is -800 v (top electrode); crystal thickness, d, is 0.20 cm, 0.2 msec/div, 10 mv/div. (The top trace in each figure represents the time of the light pulse.)
flash emission was effective in producing transient photocurrents. When a clear Pyrex filter (Atrans >3000 A) was placed over the sample, no transient pulse was observed. 4b(ii) Electron Mobilities. Figure 7 depicts typical electron transients for the NIPC, 1%, and 1: 1 crystals. The transit time is taken at the break in the flat top of the pulse (see Figures 7A and 7B). The shape of the transient pulse is very similar to those reported for holes in anthracene by I
