Photogeneration mechanism of charged carriers in copper-phthalocyanine thin films

~

Solid State Communications, Vol. 85, No. 1, pp. 5-10, 1993. Printed in Great Britain.

0038-1098/9356.00+.00 Pergamon Press Ltd

PHOTOGENERATION MECHANISM OF CHARGED CARRIERS IN COPPERPHTHALOCYANINE THIN FILMS K.Yamamoto, S.Egnsa, M.Sugiuchi, and A.Miura Tosh~a R&D Center, Komukai, Saiwai-ku, Kawasaki 210, Japan (Received on 12 September 1992 by H.Kamimura) The photogeneration properties of copper-phthalocyanine (CuPc) thin films were investigated by measuring photo-induced displacement currents for metal/CuPc/SiO2/Si devices. The impurity effect that the quantum efficiency (QE) of photogeneration in the layer including impurities is larger than that in the intrinsic layer was quantitatively estimated from this method. The QE increase has been found to be due to the charge transfer reaction from an electron-hole pair generated by photon absorption to an impurity. The QE difference between two layers is discussed in relation to the formation process of bound pairs of opposite charges.

1. INTRODUCTION The photoconductive properties of organic solids have been extensively investigated in relation to applications to organic electronic devices1). The quantum efficiency (QE) of carrier photogeneration, which is defined as the ratio of the number of generated photocarriers to that of photons absorbed in the bulk, is a key parameter affecting photoconductivity. In molecular solids and amorphous semiconductors, the photogeneration processes have been generally considered to fall into two primary steps 2"5) although rather complicated bypaths are concerned. The first is the bound electron-hole pair formation just after photoabsorption, and the second is the pair dissociation. Thus, the individual efficiencies in these steps should affect the overall QE of photogeneration. A considerable effort to elucidate the photogeneration mechanism has been made from theoretical and experimental viewpoints. As for the latter, the pair dissociation process has been appropriately explained on the basis of the Onsager theoq/s,7), describing the dissociation probability of bound pairs under the combined influence of thermal excitation and applied electric field. Many plaus~le mechanisms on the pair formation process of the former have been also proposed s:2). However, detailed understanding has not yet been well established. It has been well known that the photocurrents markedly increased when some impurities are present in the bulk, giving rise to a higher QE 1'13). These impurities effects, by which the pair formation process may be greatly influenced, are important to elucidate the photogeneration mechanism of molecular solids. A method which can accurately measure the number of photocarriers is indispensable for that purpose. So far, the xerographic discharge technique 2'4), transient

photoconductive response 3), delayed-collection field technique n'n), and field-induced fluorescence quenchings'11,n) have been chiefly used to determine QE. However, these are not suitable, since there is a fear of the measured signals reflecting not only the photocarriers but also the carrier transport properties and other relaxation processes of photoexcited states. This paper reports a novel approach to elucidate the photogeneration processes in organic thin filrn~ by measuring the photo-induced displacement currents (Iph) for metaForganic/SiOJSi (M/Org/OS) deviceS. Copper-phthalocyanine (CuPc) was used as the organic layer, because CuPc was one of typical molecular solids and has been known to include 0 2 molecules1,14). Displacement current (Id) measurement 15) for the M/CuPc/OS devices was carried out in both the dark and fight. The electrical and photoelectrical properties of an impurity were investigated. As a result, the impurity effects on QE were quantitatively estimated from this method. The QE increase has been found to be due to the charge transfer reaction from an electron-hole pair created by photon absorption to an impurity. The QE difference between two layers is discussed in relation to the formation process of bound pairs of opposite charges. 2. EXPERIMENTAL CuPc purified by temperature graduated sublimation was evaporated onto a SIO2(20 nm)/p-Si(10 flcm) substrate. The CuPc film was t h e ~ c a l l y metastable a-form 16), determined by FT-IR measurement, and its thickness was about 50 run. Au was then evaporated, yielding a Au/CuPc/OS device, as shown in Fig.1. Id measurement in the dark was carried out, by applying triangular wave biases (± 10 V, 100 Hz) to the d~ at room temperature.

6

PHOTOGENERATION MECHANISM Id (×10"5A) 10

bv

Au CuPc SiO 2 Si Au/Cr

Fig.1

Vol. 85 No. 1

V (V)

Au/CuPc/OS device structure.(Schematic)

IS is given by Hg.2 IS characteristics for Au/CuPe/OS devices in vacuum (dashed line) and in air (solid line) at fffi 100 Hz.

(1) where Ctot is the capacitance for the Au/Ckd~c/OS device and V is the bias applied to the device. The photogeneration properties in CuPe were investigated by measuring I O for the device in fight. A 500 W Xe lamp was used as an excitation light source, and interference filters were individually applied to select the excitation wavelength. The Q E of photogeneration in the organic layer was determined as follows. When the CuPe layer in the device was photoexcited, photoearriers were generated, and then current increase was observed. The number of photoearriers per unit area (Nph), generated in the voltage region from 0 to TV, is given by Eq.(2).

N~=N~-N,#

(2)

where Nut and N d a r e the carrier densities generated in the light and dark, respectively. Eq.(2) is represented by using I ~ and IS.

N. =(q-~)-'foY(/.- I ~

(3)

where q is the elementary charge. Therefore, QE can be given by

OE=p

(4)

where P is the number of photons per unit area absorbed in the organic layer. P was estimated from the individual transmission spectra of Au and CuPe evaporated onto the SiO 2 substrate and from the reflection spectrum of the Si substrate 173. 3. EXPERIMENTAL RESULTS Displacement Ckn'rent Characteristics The individual IS characteristics for the CuPc layers in high vacuum (about 10~ torr) and in air were measured. In Fig.2, the dashed lines show the IS curves

for the Au/CuPc/OS device in vacuum. The IS curves were symmetric with respect to the horizontal voltage axis, and IS increased at positive biases. The threshold voltage (Vm) for this IS increase was about 0 V. At positive biases, the values of I d were in good agreement with the product of the capacitance for the SiO 2 layer and dV/dt. This result shows that the charged carriers accumulate in the organic region near the CuPc/SiO 2 interface is). On the contrary, the small IS at negative biases indicates that the CuPc layer is a dielectric. This is because the carriers are injected at positive biases from the Au electrode into the CuPc layer but they are not at negative biases. The ionization potential for CuPc measured by UPS was about 5 eV Is),and this value coincides with the work function for A u (4.8 to 5 eV) determined by U P S 19), photoresponsez°), and C-V measurements z°). Hence, the injected carriersare holes, and the small value of Vth reflects the lower potential barrier of Au/CuPc interface for hole injection. In vacuum, the injected carriersfrom the electrode were observed on the IS curves, while the intrinsic carriersin the organic layerwere not. Hence, the CuPc layer was essentiallyan intrinsicsemiconductor. The measurement in air for the device gave the solid line curves in Fig.2. At positivebiases, an Is increase was observed as well as in vacuum. However, at negative biases where the injected carriers from the Au electrode were not observed, IS was larger than that in vacuum. When O 2 gas or air was introduced into the chamber during the measurement in vacuum, the IS curves changed into the solid line curves in Fig.2. It has been found that O z impurities caused extrinsic carrier generation in the Ctff'c layer, and consequently the IS increase at negative biases. Furthermore, in air, a linear relationship between the voltage and 1/Co~; Cots is the capacitan..cefor the CuPc layer,was observed at small negative bmses, This indicates that the extrinsic carriers are depleted at negative biases. In case that the CuPc layer including O 2 molecules is an impurity semiconductor, the IS characteristic in

PHOTOGENERATION MECHANISM "

Vol. 85 No. 1

a)

b)

"-" °

7

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1.5

O

o

N

o

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0.5

o

v

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b)

AulCuPc/Si021Si

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< Energy-band diagrams for Au/CuPc/OS devices in air under applying (a) low and (b) high biases. (Schematic)

the negative voltage region are interpreted as follows. Since an O 2 molecule can be classified as an acceptor for donor molecules such as CuPc, the extrinsic carriers are considered to be holes. At a small negative voltage, the depletion of holes first takes place in the organic region near the CuPc/SiO 2 interface, as shown in Fig.3a. This is because the quasi-Fermi level in that region is located on the 02 impurity level within the forbidden-gap of CuPc. The impurities are negatively ionized, and then holes are generated. Thus, Id is larger than that in vacuum. It should be noted that a negative voltage increase causes this depletion layer to widen. Hence, at a sufficiently large voltage, all impurities in the CuPc layer are ionized, as shown in Fig.3b. The organic layer behaves as a dielectric, and then Id decreases to that in vacuum. The 0 2 concentration in the CuPc layer was estimated to be from 1017 to 101S/cm3 from the area of the shadow region in Fig.2. Photo-Induced Displacemem Current Characteristics In the I ~ measurement, Q E was determined from the negat/ve voltage region (-10 to 0 V) in which

a)

Iph

x10"SA) -10

b)

Iph

IV(V) 1.10~ QE;Nph/P

-10

(x10"SA) -10

~10

~v (v)

-10

FIgA I ~ characteristics for Au/CuPc/OS devices (a) in vacuum and (b) in air at f= 100 Hz under white light irradiation. Dashed curves in both figures are for devices in dark.

o 25:i:j 0~-'Y "

500

i I

]

600 700 ~, (nm)

"r--800

)~g.$ Wavelength dependence of (a) N o and Co) QE. Closed and open symbols in both figures are for devices in vacuum and in air, respectively. carrier injection was not observed. The solid lines in Figure 4a shows the I ~ curve in vacuum for the Au/CuPc/OS device under white light irradiation. A current increase was observed in the negative voltage region of dV/dt<0 (I~<0), as mentioned in Section 2. However, in the high negative voltage region of dV/dt>0 (I~>0), the I ~ values were apparently smaller than Id. ' IO is proportional to d~/dx; ~ is the quasi-Fermi level for the organic layer. When dV/dt<0 changes into dV/dt>0, d~/dx for the CuPc layer, that is, I ~ also varies from a negative sign to positive. Hence, this current decrease is due to the lower response of d~/dx with the sign change of dV/dt. Furthermore, in Q E measurement, the I ~ < 0 region irrespective of the d~/dx response was used. Measurement in air under white light irradiation gave the solid line curves in Fig.4b. In the low negative voltage region, the value of I ~ was larger than that in the vacuum, while in the high voltage region I~ was obviously smaller than that in vacuum or that at the lower biases in air. Furthermore, N O was larger than those in vacuum. This result reveals that 0 2 impurities into the bulk causes the QE of photogeneration to increase. Figure 5 shows the individual excitation wavelength dependences of N o and QE, measured under the condition that the-number of incident photons is constant at the individual wavelengths. The N O spectra were relatively similar to the absorption spectrum of CuPc in both vacuum and air. This shows that the current increase induced by illumination is due to the photocarriers generated in the CuPc layer. It has been found that the photoabsorption first caused

PHOTOGENERATION MECHANISM photoexcitation of the CuPc layer irrespective of 0 2 impurities in the CuPc layer. The QE values of photogeneration in vacuum were nearly independent of the excitation wavelength and showed a constant value of 2 to 3x10 -s. On the other hand, those in air were apparently wavelength dependent. In particular, QE markedly increased, at the wavelength regions of ~<550 nm and ~.>700 nm where the absorption coefficient for CuPc had a smaller value. Figure 6 shows the respective light intensity dependences of Np~ and QE at ~.=610 nm. A linear relationship between N~a and light intensity was observed in vacuum. In air, the N~ values had almost the same values as those in vacuum at the high intensity region, while the Nph values were much larger than those in vacuum at the low intensity. The QE values in vacuum, which were independent of the light intensity, had a constant value of 2 to 3x10 "3. On the contrary, those in the air continuously increased from 10"s to 10-2 with decreasing light intensity. Considering both this result and the excitation wavelength dependence of QE, it is more remarkable that the impurity affects the absolute value of QE, as the density of excited states in the CuPe layer generated by photoabsorption is smaller.

a)

hv

kr :

Vol. 85 No. 1

b)

hv

kr( ~ket

[A']

Fig.7 Energy-band diagrams for the CuPc layers (a) in vacuum and (b) in air under illuminated condition. (Schematic)

between Nph and light intensity was observed. In intrinsic semiconductors, photoabsorption usually results in the creation of bound electron-hole pairs. These results, hence, show that the photocarriers are due to the dissociation of these bound pairs, as shown in Fig.7a. In the intrinsic process, the QE of photogeneration can be approximated by t,

t,+t,

(5)

4. DISCUSSION The I d characteristics in the dark indicated that the CuPc layer in vacuum behaved as an intrinsic semiconductor. From the measurement in light, the action spectrum on Nph was relatively similar to the absorption spectrum of CuPc, and a linear relationship

=)

,~12

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O0

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11 I

-2 A

I

I I IIIll

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•

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-3 15

I

I

I I IIII1

I

I

16

log I

(cm'2$ee -1)

Fig.6 Light intensity dependence of (a) Nph and (b) QE. Closed and open symbols in both figures are for device= in vacuum and in air, respectively.

where I~ and kr are the carrier generation and electron-hole pair recombination rates,respectively. Furthermore, the result in fight revealed that intrinsic CuPc had a small constant Q E of about 10-s. From ~..(5), the carrier generation rate kg is only about 10''" of the recombination rate kr As mentioned in Section 1, the photogeneration process can be generally separated into a first step for electron-hole pair formation and a second step for pair dissociation. The former and the latter correspond to the photoexcitation and the carrier generation processes in the figure, respectively. In the intrinsic process, the efficiency of pair formation in the former is obviously unity so that l~/(k~+l~) is considered to be the dissociation probability in the latter. Hence, the small QE is probably caused by the lower efficiency in the second step. On the other hand, in the CuPc layer including 0 2 impurities ([A]), the value of QE was not only larger than that in vacuum but also apparently dependent on the excitation wavelength and light intensity.These indicate that the photogeneration of CuPc including [A]s does not result from the intrinsic process mentioned above. Furthermore, the N w spectrum wns relatively coincident with the absorption spectrum, as well as in vacuum. From this point, it is clear that the photocarriers are not caused by electron transitions from the valence band to the impurity level and from the impurity level to the conduction band, neither. For this reason, a third photogeneration process due to the interaction between the excited states of CuPc and [A]s should he considered. From the Id characteristics, the [A]s are considered to be an acceptor for CuPc and form the electron

PHOTOGENERATION MECHANISM

Vol. 85 No. 1

capture states within the forbidden-gap. When an electron-hole pair is generated by photoabsorption, intrinsic photogeneration, recombination, and electron transfer from a pair to a neutralized [A°] is considered as the relaxation process of the pair, as shown in Fig.7b. In case that the electron transfer rate ket is relatively similar to the recombination rate k r or faster than that, a collision between a pair and [A°] causes an electron tramfer reaction, and a bound pair of positively ionized CuPe and negatively [A] ([Org+-A-]) is newly formed. The lifetime of [Org+-A -] will be longer than that of the electron-hole pair as long as the individual rates of the thermal electron reexcitation from the [A] level to the conduction band and the [Org+-A -] recombination are sufficiently small. Hence, it is expected that [Org+-A "] dissociation causes efficient hole generation and QE becomes much larger than that in the intrinsic process. The authors have verified this process from a different viewpoint. The I a characteristic in the dark revealed that the ionized [A-] concentration in the CuPc layer was enhanced with increasing negative voltage. If neutralized [A°]s closely participate in photogeneration, 1~ in the high negative voltage region should be necessarily smaller than that in the low voltage region. In fact, the I ~ curve in air had such a characteristic, as shown in Fig.4b. Hence, the QE increase in the CuPc layer is due to the electron transfer reactions from electrun-hole pairs produced by photon absorption to neutralized [A~]s. The dependences of QE on the excitation wavelength and the light intensity are interpreted as follows. In case that the number of photons absorbed in the organic layer is smaller than that of the neutralized [A°]s, the electron transfer reaction takes precedence over intrinsic photogeneration due to lqt~kg. In the opposite case, the photocarriers are generated by the intrinsic process. This is because the probability of collision between an electron-hole pair and an impurity is low. Therefore, a large QE was observed at low light intensity or at an excitation wavelength where CuPe had a small absorption coefficient. On the contrary, the absolute value of QE decreased to that in the intrinsic process at the high intensity or at the wavelength where an absorption coefficient showed a large value. Finally, the kinetics in the charge transfer process is considered. Differing from the intrinsic process, [Org +A-] formation is the first step in photogeneration, and its dissociation becomes the second step. When the efficiency of [Org+-A -] dissociation is expressed by QEom and intrinsic photogeneration is negligible from the assumption of igaMr~, QE can be approximated by

t,, Q£~ ka+k,+krQ£'="

(6)

where l ~ / ( k = + ~ + k r ) is the efficiency of [Org+-A "] formation. As previously stated, Q E in air was much larger than that in vacuum. Thus, a comparison between Eqs.(5) and (6) leads to the result that the dissociation probability of a bound pair of [Org+-A -]

becomes larger than that of an electron-hole pair in the intrinsic process. Such a characteristic of the efficiency in the second step is certainly caused by the difference between [Org+-A-] and the electron-hole pair generated in the individual first steps. Therefore, it has been clarified that the overall Q E of photogeneration is strongly dependent not only on the dissociation process but also on the formation process of oppositely charged pairs. The authors are now studying the electronic states of impurities in order to obtain knowledge on [Org+-A']. Such an approach is expected to lead to a detailed understanding on photogeneration in organic solids. 5. SUMMARY The photogeneration properties of CuPc thin films have been investigated by measuring the photo-induced displacement currents for M/CuPc/OS devices. As a result, the impurity effects on the absolute value of QE were quantitatively estimated by this method. The Q E increase has been found to be due to the charge transfer reactions from electron-hole pairs to impurities. Furthermore, it has been found that Q E is strongly dependent on the formation process of oppositely charged pairs. It is expected from the scientific and technological points of view that a deeper understanding on photogeneration will be established.

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1. J.Simon and J.-J. Andre., Molecular Semiconductors (Springer-Verlag, Berlin, 1985). 2. P.J. Melz, J. Chem. Phys. $7, 1694 (1972). 3. D. M. Pal and R. C. Enck, Phys. Rev. Bll, 5163 (1975). 4. P. M. Borsenberger and A. I. Ateya, J. Appl. Phys. 49, 4035 (1978). 5. M. Yokoyama, Y. Endo, A. Matsubara, and H. Mikawa, J. Chem. Phys. 75, 3006 (1981). 6. L Onsager, J. Chem. Phys. 2, 599 (1934). 7. g Omager, Phys. Rev. 54, 554 (1938). 8. J. G. Knight and E. A. Davis, J. Phys. Chem. Solids 3S, 543 (1974). 9. J. Noolandi and K. M. Hong, J. Chem. Phys. 70, 3230 (1979). 10. E . A . S'tlinsh, V. A. Kolesnikov, L J. Muzikante, and D. R. Balode, Phys. Stat. Sol. (b)l13, 379 (1982). 11. Z. D. Popovic, J. Chem. Phys. 78, 1552 (1983). 12. Z. D. Popovic, Chem. Phys. 86, 311 (1984). 13. P. Day and M. G. Price, J. Chem. Soc. A, 236 (1969). 14. J. Mizuguchi, Jpn. J. Appl. Phys. 20, 713 (1981). 15. S. Egusa, N. Gemma, A. Miura, K. Mizushima, and M. Azuma, a. Appl. Phys. 71, 2042 (1992). 16. J. H. Sharp and M. Abkowilz, J. Phys. Chem. 77, 477 (1973). 17. H. W. Verleur, J. Opt. SOc. Am. $8, 1356 (1968). 18. F. I. Vilesov, A. A. Zagrubsldi, and D. Garbuzov,

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