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C60 heterojunction
Solid State Communications,
Vol.
I
15,
104, No. 9, pp. 5 l-5 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00
PII: s0038-1098(!q00344-x
PHOTOVOLTAIC
EFFECT
AND ELECTRICALLY DETECTED ELECTRON HTPHTHALOCYANINE/CW HETEROJUNCTION I. Hiromitsu,
Department
SPIN RESONANCE
OF A
Y. Kaimori and T. Ito
of Material Science, Interdisciplinary Faculty of Science and Engineering, Matsue 690, Japan
Shimane University,
(Received 30 May 1997; accepted 29 July 1997 by A. Okiji) The Au/H*-phthalocyanine(HZPc)/C&Au heterojunction shows a photovoltaic effect with the conversion efficiency of 0.02%. An electrically detected electron spin resonance (EDESR) study is done for this system. The observed EDESR signal represents a decrease of the photocurrent due to the spin-dependent recombination of the localized electron-hole pair which is created by the photoinduced charge transfer between the HzPc and the CeO. The microwave-power-saturation behavior of the EDESR signal is not explained by the formula generally used. 0 1997 Elsevier Science Ltd Keywords: A. fullerenes, A. heterojunctions, trapping, E. electron paramagnetic resonance.
and
resembles to that of the chlorophyll, the natural photosynthetic pigment. Because of its good stability, the phthalocyanine is one of the materials suitable for the organic opto-electric devices. In this communication, first reported is the photovoltaic characteristics of the H2Pc/Ca heterojunction. Then, the EDESR results are presented and the origin of the EDESR signal is discussed. A new problem comes out that the microwave-power-saturation behavior of the EDESR signal is not explained by the formula generally used.
1. INTRODUCTION Recently, the photovoltaic effects of conductive-polymer/CGO heterojunctions have been reported [l, 21. The basic mechanism of the photovoltaic effect in these junctions is the photo-induced charge transfer across the donor-acceptor interface [2]. These heterojunctions can be a model system to study the charge transfer properties in the conductive polymers and the fullerenes. For the study of the charge transfer mechanism in the heterojunctions, the EDESR has a great possibility to be a powerful experimental technique which provides microscopic information about the transfer. In the EDESR measurement, detected is the change of the photocurrent or of the photovoltage caused by the electron spin resonance (ESR). The sensitivity of the EDESR is 3-5’orders higher [3, 41 than that of the conventional ESR, so that the EDESR has an advantage to be applied to the small volume systems like the heterojunctions. Only very limited number of the EDESR studies, however, have been done for the heterojunctions [4, 51, so that the relevance of the EDESR to the heterojunction systems has not yet been established. In the present study, the EDESR is applied to a new Cho-based heterojunction, which is Hz-phthalocyanine(HzPc)/C, double layer. The phthalocyanine is an organic pigment whose optical absorption spectrum
D. recombination
2. EXPERIMENTAL The H2Pc was purchased from Nacalai Tesque Inc. and was used after subliming twice in vacuum. The COO (99.95%) was purchased from MER Corp. and was used without further purification. The arrangement of the thin films in the heterojunction system is shown in Fig. 1. A gold film of about 20 nm thickness was evaporated on a quartz substrate of 4 X 24 X 1 mm3 under a pressure below 1 X 10m4 Pa, followed by the HzPc and the C60 films each of about 100 nm thickness. On top of the Cm film, another gold electrode was evaporated of about 20 nm thickness. The active area for the photovoltaic effect is about 0.1 cm*. In all the measurements, the sample was kept in vacuum in a quartz tube of 6 mm inner diameter.
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PHOTOVOLTAIC
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EFFECT
OF H 2-PHTHALOCY ANINE/C 60
&PC
Vol. 104, No. 9
cso
(a> Fig. 1. Scheme system.
of the Au/H2Pc/C&Au
heterojunction
Light illumination was performed with a 500 W Xe lamp either from the H2Pc side or from the C6a side as shown in Fig. 1. The photocurrent was detected by using a 1 kS2 load. The EDESR measurement was done on a home built X-band ESR spectrometer using an 80 Hz modulation of the microwave intensity. The voltage across the 1 k8t load was fed to a lock-in amplifier which was synchronous to the 80 Hz modulation. A TEO 8 1 cylindrical cavity was used which had a grid for the illumination of the sample. The maximum microwave power incident to the cavity was 10 mW. 3. RESULTS 3.1. Photovoltaic
AND DISCUSSION
efSect
Figure 2 shows the current density vs the applied voltage of the Au/H2Pc/C6dAu heterojunction system in the dark and under illumination. The positive direction of the applied voltage is defined in Fig. 1. In the dark, a recti~cation ratio of only 1.5 is observed. This is in contrast to the case of Au/HzPc/C6dAl in which a rectification ratio of 6.2 is observed. The difference is attributed to the larger work function of the Au electrode which is 5.2 eV [6] compared to that of the Al which is
Fig. 2. The current-density J vs the applied voltage V of the Au/HzPc/C&Au heterojunction. The illumination is (a) from the H2Pc side and (b) from the Co-, side. The illumination light power is 5.9 mW crnp2 for 420 nm, 9.0 mW cme2 for 560 nm and 6.1 mW crnT2 for 700 nm. X shows the dark current density.
Fig. 3. The mechanism of the photovoltaic effect of the H~Pc/C& double layer. (a) The H2Pc is photoexcited. (b) The C6a is photoexcited. 4.2 eV [6]. All the measurements in the present study were done for the Au/HzPc/CbdAu because of its higher stability compared to the Au/HzPc/C&Al. By the illumination, the current increases d~atic~y especially for V 5 0 as is seen in Fig. 2. This is a characteristic feature of the donor-acceptor type photocell [2]. The mechanism to cause the photocurrent for V 5 0 is as follows. It is assumed [7] that the top of the HOMO band and the bottom of the LUMO band of the HzPc are higher than those of the C6a (Fig. 3). When the valence electron of the HzPc is photoexcited [Fig. 3(a)], the excited electron in the conduction band of the HzPc flows into the conduction band of the G,o. When the valence electron of the C60 is photoexcited [Fig. 3(b)], on the other hand, the excited hole in the valence band of the ChO flows into the valence band of the H2Pc. Thus, the separated electron-hole pair is created in both cases of the H*Pc-excitation and the ~~*-excitation. In the case of the 560 nm-illumination in Fig. 2(b), the fill factor (Ffl defined by FF = J,V,/J,,V,, becomes 0.16, where J, and V, are the current density and the applied voltage for maximum power output and V, - 0.12 V) and JsC and V,, (Jr,,- 17pAcm-* are the short-circuit current density and the open circuit voltage (Js, = 39 PA cm-’ and V, = 0.32 V). The power conversion efficiency defined by q = FFJ~~V~~/Pi~ becomes 0.02%, where P, is the incident light power density which is 9 mW cm-’ for h = 560 nm. This conversion efficiency is in the same order as that reported for ITO/MEM-PPV/C~dAu cell 111. Figure 4 shows the action spectra, i.e. (the photocurrent per photon number) vs (the illumination wavelength), of the Au/H2Pc/C&Au with no bias voltage, i.e. V = 0 V, for the two illumination directions from the H2Pc side and the Cm side. Comparing the two spectra in Fig. 4, the photocurrent with the illumination from the ChO side is substantially larger in the wavelength region between 520 and 770 nm. In this wavelength region, the H2Pc has a large optical absorption band while the Cm shows only weak abso~tion (Fig. 5), so that the incident light from the CbO side reaches the H2Pc/Co, interface
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Fig. 4. The action spectra of the Au/HzPclC&Au heterojunction. The illumination direction is from the HzPc side (solid line) and from the C6a side (broken line). more effectively yielding a larger photocurrent. Similarly, in the wavelength region between 400 and 500 nm, the photocurrent with the illumination from the HzPc side is larger. In this wavelength region, the C,ja has a relatively strong absorption band while the H2Pc shows only weak absorption. Thus, the incident light of X = 400-500 nm reaches the interface more efficiently with the illumination from the HzPc side. These results indicate that the photocurrent is generated by the photoexcitation in the HzPc/C~~ interface region [S]. For X 2 550 nm illumination, the photocurrent is mainly attributed to the HzPc-photoexcitation, while, for X = 400-500 nm illumination, it is attributed mainly to the &a-excitation. 3.2. EDESR Figure 6 shows the EDESR spectra of the Au/HzPc/ C&Au heterojunction with the bias voltage V = 0 V for three different wavelength regions of the illumination, the light of the desired wavelength having been obtained by using appropriate glass filters. With the white-light illumination, it follows from the discussion in Section 3.1 that the photoexcitations of both the H2Pc and the C60 contribute to the photocurrent. On the other hand,
I
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400
500
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600
700
800
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Fig. 5. The optical absorption spectra of the thin films of HzPc (solid line) and CeO (dotted line).
.,.I,
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Fig. 6. EDESR spectra of the Au/H2Pc/C&Au heterojunction. (a) With white-light illumination. (b) With X = 400-500 nm light. (c) With h z 600 nm light. The illumination direction is from the H2Pc side. The bias voltage V = 0 V. with the h = 400-500 nm and with the X > 600 nm illumination, most of the photocurrent is attributed to the C6a- and the HzPc-excitations, respectively. For each of the three spectra in Fig. 6, the light intensity was controlled so as to obtain the same photocurrent density of 20 @A cm-‘. Comparing the three spectra, the g-values are identical and the integrated signal intensity is nearly the same, while the line width for (c), i.e. for the H2Pc excitation, is 20% smaller compared to the other two spectra. The line shape is nearly Lorentzian for the three cases. From the signal intensity and the phase in the lock-in detection, it was found that the photocurrent decreases by the order of 0.01% at the center of the EDESR signal. It is noted that the EDESR signal intensity strongly depends on samples. The intensity varies by a factor of 5 from one sample to another. The spectra shown in Fig. 6 are for a sample which gave the largest signal in the present study. The basic mechanism of the decrease in the photocurrent by the ESR is an increase in the recombination probability between the photogenerated electron-hole pair as a result of the disturbance of the spin configuration of the pair. In the present EDESR study, the same g-value and the same signal intensity were observed for the two cases of the H2Pc- and the CeO-photoexcitations. This indicates that the EDESR detects the same electron-hole pair for the two cases. This agrees with the model shown in Fig. 3, in which the electron and the hole are produced in the CeO and the H2Pc layers, respectively, in both cases. In order for the recombination of the electron-hole pair to take place, the pair must have the singlet spin configuration, The ESR increases the probability to take the singlet configuration so that the recombination probability is increased. Two models have been proposed in order to explain the increase of the singlet probability. One model is proposed by Lepine [9] in which the ESR decreases the population difference between the
514
PHOTOVOLTAIC
EFFECT
OF HrPHTHALOCYANINE/C60
Z.eeman-split S, = t i levels of the electron and/or the hole, which originally obeys the Boltzmann distribution at off-resonance and, as a result, the probability increases for an electron to find a hole in the singlet configuration. In this model, the maximum change of the photocurrent due to the ESR is by the factor of 10T6 at ambient temperature [lo], which is much smaller than 10d4 observed in the present study. The other model is originally proposed by Kaplan et al. [ 101 which considers a localized electron-hole pair: If the spins orient randomly, the probability to find a singlet pair is l/4. However, the actual probability to find a singlet pair becomes less than l/4, since the singlet pairs can disappear by the recombination. By the ESR, the singlet state is populated from the triplet states and the recombination probability increases. The model of Kaplan et al. has been revised as to consider a nonBoltzmann distribution in the triplet sublevels due to selective intersystem crossing rates [3,4], which may be a situation when a small but finite exchange interaction exists between the electron and the hole. The present 10e4 change of the photocurrent by the ESR is explained by this localized pair model. The strong sample dependence of the EDESR signal intensity may also support this model: It is interpreted that the electron and the hole are trapped in localized states in the HzPc/C& interface region and that the number and the depth of the localized states depend on samples. By applying a bias voltage of V = - 0.4 V, the EDESR signal intensity decreases to 70% of that with V = 0 V although the photocurrent increases to 200%. The decrease in the EDESR signal intensity is attributed to an increase in the dissociation rate and a decrease in the recombination rate of the localized electron-hole pair by the application of the negative voltage. Kaplan et al. showed [lo] that in the localized pair model such a change in the EDESR intensity occurs when the ratio WD/Ws is larger than 0.3, where W, is the dissociation rate and Ws is the recombination rate for a pair in the singlet configuration. The EDESR spectra in Fig. 6 consist of only single line although the present system contains two paramagnetic species, i.e. the electron and the hole. The following three explanations may be possible. The first one is that the g-values of the electron and the hole are exactly the same. Although the detailed features of the localized electron and the hole are unknown, they are most probably the C, and the H*Pc+, respectively. Since the g-value of the C, is 1.9998 [5] and that of the H2Pc+ is assumed to be 2.003 [ll], the first explanation is unlikely. The second explanation is that the spin-lattice relaxation time T1 of either the localized electron or the hole is much shorter than the TI of the other, so that the ESR disturbs the spin configuration only through the
Vol. 104, No. 9
resonance of the longer-T1 species. This is also unlikely, however, because neither the HzPc nor the C60 contains heavy atoms that make the TI shorter through the spinorbit interaction. The third explanation is that, although the electron and the hole have different resonance frequencies, the two EDESR lines join into a single line because of an exchange interaction between them [12]. Such a joining of the two lines occurs when the condition J/ho0 2 l/G is fulfilled, where J is the exchange coupling constant and wa is the frequency difference between the two lines. Assuming that the g-values of the electron and the hole are 1.9998 and 2.003, respectively, as mentioned above, the joining occurs if J is larger than 0.001 K. Since the electron and the hole that take part in the recombination should be in close proximity, it is expected that such a weak exchange interaction can exist. Furthermore, the observed g-value of 2.0015 is in the middle of 1.9998 and 2.003, which is also consistent with this explanation. We propose that this is the most probable explanation of the single EDESR line observed in the present system. Generally, the EDESR signal intensity V is given by the following formula [3, 91,
v=c*,
(1)
where W is the ESR transition probability between the spin levels and C and fl are constants. It is assumed that the W has a Lorentzian distribution with a half width of UT2, i.e. 1 pw = (YP,, (2) 1 + (w - w~)~T; where P,, is the microwave power and cy is a constant. Then, the P,, dependence of the signal intensity at the center of the resonance, Vo, is proportional to aP,,l(l + aP,,). Fig. 7 shows the experimental and 10 -
Fig. 7. The microwave-power (P,,) dependences of the EDESR signal intensity at the center of the resonance (VO) and of the full width at half maximum (FWHM). The solid lines show the theoretical curves for the homogeneously broadened case. The broken lines for the inhomogeneously broadened case.
Vol. 104, No. 9
PHOTOVOLTAIC
EFFECT OF HrPHTHALOCYANINE&
the theoretical P,, dependence of the VO. A good agreement is seen. However, the P,, dependence of the line width (FWHM) is not explained with these equations: From equations (1) and (2), the theoretical line width is proportional to J-/T*, which is also shown in Fig. 7. The experimental line width is much less sensitive to the increase of the P,, compared to the theory. This may suggest that the observed EDESR line is inhomogeneously broadened. In the limiting case when the spin packet width is much smaller than the total line width [ 131, the V0 is proportional to IZYP~,I~~ and the line width becomes constant. The theoretical saturation curves for the V, and the FWHM are shown in Fig. 7 in this limiting case. As is seen in Fig. 7, the P,, dependence of the V0 is not explained, although a fairly good agreement is seen for the FWHM. Thus, the assumption of the inhomogeneity does not overcome the problem. These analyses demonstrate that a new formalism is necessary in order to explain the EDESR power-saturation behavior in Fig. 7. Finally, the number of the spins detected in the present EDESR measurement is estimated to be smaller than lo6 as follows. The number of the localized electron-hole pairs N obeys the rate equation dN
,,=G-(~+w~)N, where G is the pair-generation rate, W, and WR the dissociation and the recombination rates, respectively. Assuming that no multiple scattering occurs between the electrons and the holes, the photocurrent is given by I = WDNe,
(4)
where e is the electron charge. Equation (3) implies that the N varies with a decay rate W, + W, after a sudden change of the G or the WR. In order to estimate the W, + W,, two types of measurements have been done. One is the measurement of the decay rate of the photocurrent after shutting off the light and the other the dependence of the EDESR intensity on the modulation frequency of the microwave intensity [14]. Both the measurements showed that the decay rate of the carrier number N, is lo3 s-‘. This decay rate determines the lower limit for the W, + W,, the reason of which is as follows. The N, depends on the N so that the N, contains a term with the rate W, + W, as well as terms with different rates which may come, for example, from the carrier trapping and the detrapping processes. The observed decay rate lo3 s-’ for the N, is ascribed to a term with the slowest rate. Thus, W, + W, Z lo3 s-l.
515
Using the relation W, > WR [15], the W. becomes 2 lo” s-‘. Since I = low6 A in the present EDESR measurement, the N is estimated to be
Sariciftci, N.S., Braun, D., Zhang, C., Srdanov, V.I., Heeger, A.J., Stucky, G. and Wudl, F., Apply. Phys. Lett., 62, 1993, 585. 2. Morita, S., Lee, S.B., Zakhidov, A.A. and Yoshino, K., Mol. Cryst. Liq. Cryst., 256, 1994, 839. S. and Spaeth, J.-M., J. 3. Stich, B., Greulich-Weber, Appl. Phys., 77, 1995, 1546. 4. Dyakonov, V., Gauss, N., Rosier, G., Karg, S., Riel3, W. and Schwoerer, M., Chem. Phys., 189, 1994,687. 5. Maier, A., Grupp, A. and Mehring, M., Solid State Commun., 99, 1996, 623. 6. Wang, H.L., Huang, F., MacDiarmid, A.G., Wang, Y.Z., Gebler, D.D. and Epstein, A.J., Synthetic Metals, 80, 1996, 97. 7. The ionization potential (I,,) of H2Pc is 5.2 eV [S], and the band gap (E,) is estimated to be 1.5 eV from the absorption edge of the H2Pc in Fig. 5. On the other hand, the Z, and the E, of the Cba are 6.0 eV and 1.6 eV, respectively, although there is an ambiguity in these values because of a large electron correlation in the C60. See Weaver, J.H., in The Fullerenes (Edited by H.W. Kroto, J.E. Fischer and D.E. Cox), p. 263. Pergamon Press, Oxford, 1993. 8. Fan, F.-R. and Faulkner, L.R., J. Chem. Phys., 69, 1978, 3341. 9. Lepine, D.J., Phys. Rev., B6, 1972,436. 10. Kaplan, D., Solomon, I. and Mott, N.F., J. Physique Lett., 39, 1978, L5 1. 11. Raynor, J.B., Robson, M. and Ton-ens-Burton, A.S.M., J. Chem. Sot. Dalton, 1977, 2360. 12. Anderson, P.W., J. Phys. Sot. Jpn, 9, 1954, 3 16. 13. Portis, A.M., Phys. Rev., 91, 1953, 1071. the decay rate of the 14. By the latter measurement, EDESR signal is obtained which is governed by the decay rate of the carrier number since the EDESR measures the variation of the photocurrent. on the bias-voltage 15. As has been discussed dependence of the EDESR signal intensity, W,/Ws is larger than 0.3, where Ws is the recombination rate for the singlet configuration. Since W, in equation (3) is the average recombination rate for the singlet and the triplet configurations, WR is equal to Ws/4 [IO]. Thus, the relation W,/W, > 1 holds.
Vol.
I
15,
104, No. 9, pp. 5 l-5 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00
PII: s0038-1098(!q00344-x
PHOTOVOLTAIC
EFFECT
AND ELECTRICALLY DETECTED ELECTRON HTPHTHALOCYANINE/CW HETEROJUNCTION I. Hiromitsu,
Department
SPIN RESONANCE
OF A
Y. Kaimori and T. Ito
of Material Science, Interdisciplinary Faculty of Science and Engineering, Matsue 690, Japan
Shimane University,
(Received 30 May 1997; accepted 29 July 1997 by A. Okiji) The Au/H*-phthalocyanine(HZPc)/C&Au heterojunction shows a photovoltaic effect with the conversion efficiency of 0.02%. An electrically detected electron spin resonance (EDESR) study is done for this system. The observed EDESR signal represents a decrease of the photocurrent due to the spin-dependent recombination of the localized electron-hole pair which is created by the photoinduced charge transfer between the HzPc and the CeO. The microwave-power-saturation behavior of the EDESR signal is not explained by the formula generally used. 0 1997 Elsevier Science Ltd Keywords: A. fullerenes, A. heterojunctions, trapping, E. electron paramagnetic resonance.
and
resembles to that of the chlorophyll, the natural photosynthetic pigment. Because of its good stability, the phthalocyanine is one of the materials suitable for the organic opto-electric devices. In this communication, first reported is the photovoltaic characteristics of the H2Pc/Ca heterojunction. Then, the EDESR results are presented and the origin of the EDESR signal is discussed. A new problem comes out that the microwave-power-saturation behavior of the EDESR signal is not explained by the formula generally used.
1. INTRODUCTION Recently, the photovoltaic effects of conductive-polymer/CGO heterojunctions have been reported [l, 21. The basic mechanism of the photovoltaic effect in these junctions is the photo-induced charge transfer across the donor-acceptor interface [2]. These heterojunctions can be a model system to study the charge transfer properties in the conductive polymers and the fullerenes. For the study of the charge transfer mechanism in the heterojunctions, the EDESR has a great possibility to be a powerful experimental technique which provides microscopic information about the transfer. In the EDESR measurement, detected is the change of the photocurrent or of the photovoltage caused by the electron spin resonance (ESR). The sensitivity of the EDESR is 3-5’orders higher [3, 41 than that of the conventional ESR, so that the EDESR has an advantage to be applied to the small volume systems like the heterojunctions. Only very limited number of the EDESR studies, however, have been done for the heterojunctions [4, 51, so that the relevance of the EDESR to the heterojunction systems has not yet been established. In the present study, the EDESR is applied to a new Cho-based heterojunction, which is Hz-phthalocyanine(HzPc)/C, double layer. The phthalocyanine is an organic pigment whose optical absorption spectrum
D. recombination
2. EXPERIMENTAL The H2Pc was purchased from Nacalai Tesque Inc. and was used after subliming twice in vacuum. The COO (99.95%) was purchased from MER Corp. and was used without further purification. The arrangement of the thin films in the heterojunction system is shown in Fig. 1. A gold film of about 20 nm thickness was evaporated on a quartz substrate of 4 X 24 X 1 mm3 under a pressure below 1 X 10m4 Pa, followed by the HzPc and the C60 films each of about 100 nm thickness. On top of the Cm film, another gold electrode was evaporated of about 20 nm thickness. The active area for the photovoltaic effect is about 0.1 cm*. In all the measurements, the sample was kept in vacuum in a quartz tube of 6 mm inner diameter.
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(a> Fig. 1. Scheme system.
of the Au/H2Pc/C&Au
heterojunction
Light illumination was performed with a 500 W Xe lamp either from the H2Pc side or from the C6a side as shown in Fig. 1. The photocurrent was detected by using a 1 kS2 load. The EDESR measurement was done on a home built X-band ESR spectrometer using an 80 Hz modulation of the microwave intensity. The voltage across the 1 k8t load was fed to a lock-in amplifier which was synchronous to the 80 Hz modulation. A TEO 8 1 cylindrical cavity was used which had a grid for the illumination of the sample. The maximum microwave power incident to the cavity was 10 mW. 3. RESULTS 3.1. Photovoltaic
AND DISCUSSION
efSect
Figure 2 shows the current density vs the applied voltage of the Au/H2Pc/C6dAu heterojunction system in the dark and under illumination. The positive direction of the applied voltage is defined in Fig. 1. In the dark, a recti~cation ratio of only 1.5 is observed. This is in contrast to the case of Au/HzPc/C6dAl in which a rectification ratio of 6.2 is observed. The difference is attributed to the larger work function of the Au electrode which is 5.2 eV [6] compared to that of the Al which is
Fig. 2. The current-density J vs the applied voltage V of the Au/HzPc/C&Au heterojunction. The illumination is (a) from the H2Pc side and (b) from the Co-, side. The illumination light power is 5.9 mW crnp2 for 420 nm, 9.0 mW cme2 for 560 nm and 6.1 mW crnT2 for 700 nm. X shows the dark current density.
Fig. 3. The mechanism of the photovoltaic effect of the H~Pc/C& double layer. (a) The H2Pc is photoexcited. (b) The C6a is photoexcited. 4.2 eV [6]. All the measurements in the present study were done for the Au/HzPc/CbdAu because of its higher stability compared to the Au/HzPc/C&Al. By the illumination, the current increases d~atic~y especially for V 5 0 as is seen in Fig. 2. This is a characteristic feature of the donor-acceptor type photocell [2]. The mechanism to cause the photocurrent for V 5 0 is as follows. It is assumed [7] that the top of the HOMO band and the bottom of the LUMO band of the HzPc are higher than those of the C6a (Fig. 3). When the valence electron of the HzPc is photoexcited [Fig. 3(a)], the excited electron in the conduction band of the HzPc flows into the conduction band of the G,o. When the valence electron of the C60 is photoexcited [Fig. 3(b)], on the other hand, the excited hole in the valence band of the ChO flows into the valence band of the H2Pc. Thus, the separated electron-hole pair is created in both cases of the H*Pc-excitation and the ~~*-excitation. In the case of the 560 nm-illumination in Fig. 2(b), the fill factor (Ffl defined by FF = J,V,/J,,V,, becomes 0.16, where J, and V, are the current density and the applied voltage for maximum power output and V, - 0.12 V) and JsC and V,, (Jr,,- 17pAcm-* are the short-circuit current density and the open circuit voltage (Js, = 39 PA cm-’ and V, = 0.32 V). The power conversion efficiency defined by q = FFJ~~V~~/Pi~ becomes 0.02%, where P, is the incident light power density which is 9 mW cm-’ for h = 560 nm. This conversion efficiency is in the same order as that reported for ITO/MEM-PPV/C~dAu cell 111. Figure 4 shows the action spectra, i.e. (the photocurrent per photon number) vs (the illumination wavelength), of the Au/H2Pc/C&Au with no bias voltage, i.e. V = 0 V, for the two illumination directions from the H2Pc side and the Cm side. Comparing the two spectra in Fig. 4, the photocurrent with the illumination from the ChO side is substantially larger in the wavelength region between 520 and 770 nm. In this wavelength region, the H2Pc has a large optical absorption band while the Cm shows only weak abso~tion (Fig. 5), so that the incident light from the CbO side reaches the H2Pc/Co, interface
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Fig. 4. The action spectra of the Au/HzPclC&Au heterojunction. The illumination direction is from the HzPc side (solid line) and from the C6a side (broken line). more effectively yielding a larger photocurrent. Similarly, in the wavelength region between 400 and 500 nm, the photocurrent with the illumination from the HzPc side is larger. In this wavelength region, the C,ja has a relatively strong absorption band while the H2Pc shows only weak absorption. Thus, the incident light of X = 400-500 nm reaches the interface more efficiently with the illumination from the HzPc side. These results indicate that the photocurrent is generated by the photoexcitation in the HzPc/C~~ interface region [S]. For X 2 550 nm illumination, the photocurrent is mainly attributed to the HzPc-photoexcitation, while, for X = 400-500 nm illumination, it is attributed mainly to the &a-excitation. 3.2. EDESR Figure 6 shows the EDESR spectra of the Au/HzPc/ C&Au heterojunction with the bias voltage V = 0 V for three different wavelength regions of the illumination, the light of the desired wavelength having been obtained by using appropriate glass filters. With the white-light illumination, it follows from the discussion in Section 3.1 that the photoexcitations of both the H2Pc and the C60 contribute to the photocurrent. On the other hand,
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Fig. 5. The optical absorption spectra of the thin films of HzPc (solid line) and CeO (dotted line).
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I,
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Fig. 6. EDESR spectra of the Au/H2Pc/C&Au heterojunction. (a) With white-light illumination. (b) With X = 400-500 nm light. (c) With h z 600 nm light. The illumination direction is from the H2Pc side. The bias voltage V = 0 V. with the h = 400-500 nm and with the X > 600 nm illumination, most of the photocurrent is attributed to the C6a- and the HzPc-excitations, respectively. For each of the three spectra in Fig. 6, the light intensity was controlled so as to obtain the same photocurrent density of 20 @A cm-‘. Comparing the three spectra, the g-values are identical and the integrated signal intensity is nearly the same, while the line width for (c), i.e. for the H2Pc excitation, is 20% smaller compared to the other two spectra. The line shape is nearly Lorentzian for the three cases. From the signal intensity and the phase in the lock-in detection, it was found that the photocurrent decreases by the order of 0.01% at the center of the EDESR signal. It is noted that the EDESR signal intensity strongly depends on samples. The intensity varies by a factor of 5 from one sample to another. The spectra shown in Fig. 6 are for a sample which gave the largest signal in the present study. The basic mechanism of the decrease in the photocurrent by the ESR is an increase in the recombination probability between the photogenerated electron-hole pair as a result of the disturbance of the spin configuration of the pair. In the present EDESR study, the same g-value and the same signal intensity were observed for the two cases of the H2Pc- and the CeO-photoexcitations. This indicates that the EDESR detects the same electron-hole pair for the two cases. This agrees with the model shown in Fig. 3, in which the electron and the hole are produced in the CeO and the H2Pc layers, respectively, in both cases. In order for the recombination of the electron-hole pair to take place, the pair must have the singlet spin configuration, The ESR increases the probability to take the singlet configuration so that the recombination probability is increased. Two models have been proposed in order to explain the increase of the singlet probability. One model is proposed by Lepine [9] in which the ESR decreases the population difference between the
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Z.eeman-split S, = t i levels of the electron and/or the hole, which originally obeys the Boltzmann distribution at off-resonance and, as a result, the probability increases for an electron to find a hole in the singlet configuration. In this model, the maximum change of the photocurrent due to the ESR is by the factor of 10T6 at ambient temperature [lo], which is much smaller than 10d4 observed in the present study. The other model is originally proposed by Kaplan et al. [ 101 which considers a localized electron-hole pair: If the spins orient randomly, the probability to find a singlet pair is l/4. However, the actual probability to find a singlet pair becomes less than l/4, since the singlet pairs can disappear by the recombination. By the ESR, the singlet state is populated from the triplet states and the recombination probability increases. The model of Kaplan et al. has been revised as to consider a nonBoltzmann distribution in the triplet sublevels due to selective intersystem crossing rates [3,4], which may be a situation when a small but finite exchange interaction exists between the electron and the hole. The present 10e4 change of the photocurrent by the ESR is explained by this localized pair model. The strong sample dependence of the EDESR signal intensity may also support this model: It is interpreted that the electron and the hole are trapped in localized states in the HzPc/C& interface region and that the number and the depth of the localized states depend on samples. By applying a bias voltage of V = - 0.4 V, the EDESR signal intensity decreases to 70% of that with V = 0 V although the photocurrent increases to 200%. The decrease in the EDESR signal intensity is attributed to an increase in the dissociation rate and a decrease in the recombination rate of the localized electron-hole pair by the application of the negative voltage. Kaplan et al. showed [lo] that in the localized pair model such a change in the EDESR intensity occurs when the ratio WD/Ws is larger than 0.3, where W, is the dissociation rate and Ws is the recombination rate for a pair in the singlet configuration. The EDESR spectra in Fig. 6 consist of only single line although the present system contains two paramagnetic species, i.e. the electron and the hole. The following three explanations may be possible. The first one is that the g-values of the electron and the hole are exactly the same. Although the detailed features of the localized electron and the hole are unknown, they are most probably the C, and the H*Pc+, respectively. Since the g-value of the C, is 1.9998 [5] and that of the H2Pc+ is assumed to be 2.003 [ll], the first explanation is unlikely. The second explanation is that the spin-lattice relaxation time T1 of either the localized electron or the hole is much shorter than the TI of the other, so that the ESR disturbs the spin configuration only through the
Vol. 104, No. 9
resonance of the longer-T1 species. This is also unlikely, however, because neither the HzPc nor the C60 contains heavy atoms that make the TI shorter through the spinorbit interaction. The third explanation is that, although the electron and the hole have different resonance frequencies, the two EDESR lines join into a single line because of an exchange interaction between them [12]. Such a joining of the two lines occurs when the condition J/ho0 2 l/G is fulfilled, where J is the exchange coupling constant and wa is the frequency difference between the two lines. Assuming that the g-values of the electron and the hole are 1.9998 and 2.003, respectively, as mentioned above, the joining occurs if J is larger than 0.001 K. Since the electron and the hole that take part in the recombination should be in close proximity, it is expected that such a weak exchange interaction can exist. Furthermore, the observed g-value of 2.0015 is in the middle of 1.9998 and 2.003, which is also consistent with this explanation. We propose that this is the most probable explanation of the single EDESR line observed in the present system. Generally, the EDESR signal intensity V is given by the following formula [3, 91,
v=c*,
(1)
where W is the ESR transition probability between the spin levels and C and fl are constants. It is assumed that the W has a Lorentzian distribution with a half width of UT2, i.e. 1 pw = (YP,, (2) 1 + (w - w~)~T; where P,, is the microwave power and cy is a constant. Then, the P,, dependence of the signal intensity at the center of the resonance, Vo, is proportional to aP,,l(l + aP,,). Fig. 7 shows the experimental and 10 -
Fig. 7. The microwave-power (P,,) dependences of the EDESR signal intensity at the center of the resonance (VO) and of the full width at half maximum (FWHM). The solid lines show the theoretical curves for the homogeneously broadened case. The broken lines for the inhomogeneously broadened case.
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EFFECT OF HrPHTHALOCYANINE&
the theoretical P,, dependence of the VO. A good agreement is seen. However, the P,, dependence of the line width (FWHM) is not explained with these equations: From equations (1) and (2), the theoretical line width is proportional to J-/T*, which is also shown in Fig. 7. The experimental line width is much less sensitive to the increase of the P,, compared to the theory. This may suggest that the observed EDESR line is inhomogeneously broadened. In the limiting case when the spin packet width is much smaller than the total line width [ 131, the V0 is proportional to IZYP~,I~~ and the line width becomes constant. The theoretical saturation curves for the V, and the FWHM are shown in Fig. 7 in this limiting case. As is seen in Fig. 7, the P,, dependence of the V0 is not explained, although a fairly good agreement is seen for the FWHM. Thus, the assumption of the inhomogeneity does not overcome the problem. These analyses demonstrate that a new formalism is necessary in order to explain the EDESR power-saturation behavior in Fig. 7. Finally, the number of the spins detected in the present EDESR measurement is estimated to be smaller than lo6 as follows. The number of the localized electron-hole pairs N obeys the rate equation dN
,,=G-(~+w~)N, where G is the pair-generation rate, W, and WR the dissociation and the recombination rates, respectively. Assuming that no multiple scattering occurs between the electrons and the holes, the photocurrent is given by I = WDNe,
(4)
where e is the electron charge. Equation (3) implies that the N varies with a decay rate W, + W, after a sudden change of the G or the WR. In order to estimate the W, + W,, two types of measurements have been done. One is the measurement of the decay rate of the photocurrent after shutting off the light and the other the dependence of the EDESR intensity on the modulation frequency of the microwave intensity [14]. Both the measurements showed that the decay rate of the carrier number N, is lo3 s-‘. This decay rate determines the lower limit for the W, + W,, the reason of which is as follows. The N, depends on the N so that the N, contains a term with the rate W, + W, as well as terms with different rates which may come, for example, from the carrier trapping and the detrapping processes. The observed decay rate lo3 s-’ for the N, is ascribed to a term with the slowest rate. Thus, W, + W, Z lo3 s-l.
515
Using the relation W, > WR [15], the W. becomes 2 lo” s-‘. Since I = low6 A in the present EDESR measurement, the N is estimated to be
Sariciftci, N.S., Braun, D., Zhang, C., Srdanov, V.I., Heeger, A.J., Stucky, G. and Wudl, F., Apply. Phys. Lett., 62, 1993, 585. 2. Morita, S., Lee, S.B., Zakhidov, A.A. and Yoshino, K., Mol. Cryst. Liq. Cryst., 256, 1994, 839. S. and Spaeth, J.-M., J. 3. Stich, B., Greulich-Weber, Appl. Phys., 77, 1995, 1546. 4. Dyakonov, V., Gauss, N., Rosier, G., Karg, S., Riel3, W. and Schwoerer, M., Chem. Phys., 189, 1994,687. 5. Maier, A., Grupp, A. and Mehring, M., Solid State Commun., 99, 1996, 623. 6. Wang, H.L., Huang, F., MacDiarmid, A.G., Wang, Y.Z., Gebler, D.D. and Epstein, A.J., Synthetic Metals, 80, 1996, 97. 7. The ionization potential (I,,) of H2Pc is 5.2 eV [S], and the band gap (E,) is estimated to be 1.5 eV from the absorption edge of the H2Pc in Fig. 5. On the other hand, the Z, and the E, of the Cba are 6.0 eV and 1.6 eV, respectively, although there is an ambiguity in these values because of a large electron correlation in the C60. See Weaver, J.H., in The Fullerenes (Edited by H.W. Kroto, J.E. Fischer and D.E. Cox), p. 263. Pergamon Press, Oxford, 1993. 8. Fan, F.-R. and Faulkner, L.R., J. Chem. Phys., 69, 1978, 3341. 9. Lepine, D.J., Phys. Rev., B6, 1972,436. 10. Kaplan, D., Solomon, I. and Mott, N.F., J. Physique Lett., 39, 1978, L5 1. 11. Raynor, J.B., Robson, M. and Ton-ens-Burton, A.S.M., J. Chem. Sot. Dalton, 1977, 2360. 12. Anderson, P.W., J. Phys. Sot. Jpn, 9, 1954, 3 16. 13. Portis, A.M., Phys. Rev., 91, 1953, 1071. the decay rate of the 14. By the latter measurement, EDESR signal is obtained which is governed by the decay rate of the carrier number since the EDESR measures the variation of the photocurrent. on the bias-voltage 15. As has been discussed dependence of the EDESR signal intensity, W,/Ws is larger than 0.3, where Ws is the recombination rate for the singlet configuration. Since W, in equation (3) is the average recombination rate for the singlet and the triplet configurations, WR is equal to Ws/4 [IO]. Thus, the relation W,/W, > 1 holds.