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Photovoltaic properties of C60PPP heterojunction: molecular D-A photocell
ELSEVIER
Synthetic Metals 71 (1995) 2247-2248
Photovoltaic
properties S.B.Leea,
aDepartment bDepartment
of &()/PPP
P.K.Khabibullaeva,
heterojunction:
A.A.Zakhidova,
S.Moritab
molecular
D-A photocell
and K.Yoshinob
of Thermophysics, Uzbek Academy of Sci., Katartal 28, Tashkent 700135, Uzbekistan of Electronic Engineering., Osaka University, 2-2 Yamada-oka, Suita, Osaka, 56!3, Japan
Abstract We have found recently that thin film layered heterojunction between C60 and conduct\ng polymer, like poly(3-alkylthiophene) shows a photovoltaic effect due to photoinduced charge transfer at the interface. In this work we report the method of fabrication and characterization of AIIC601PPPISn02 junctions and examine its photoresponse. The PPP thin film have been vacuum deposited by discrete (or flash) evaporation of PPP (polyparaphenylene) powder, which allows to avoid the decomposition of polymer chains. Then C60 was evaporated on the top of PPP film. The heterostructure showed the assymmetric I-V dependence with the rectification ratio of about IO4 at 20V reverse bias (plus at Al-electrode). Upon illumination the drastic increase of the photocurrent has been observed at reverse bias, suggesting the possible application of this junction as a sensitive photodetector. Spectral response of the device was studied and the mechanisms of the photovoltaic effect are discussed. Contrary to conventional Inorganic semiconductor p-n junction photocell, the C 60.PPP junction can be rather viewed as molecular donor-acceptor (D-AI type photocell. 1. INTRODUCTION The recent discovery of photoinduced charge transfer from fullerenes to conducting polymers (CP) Ill has stimulated the studies of various heterojunctions between the semiconducting conjugated polymers and C60 which show photovoltaic effect 12-41. Contrary to conventional Inorganic semiconductor p-n junction photocell, the C6OCP junctions can be rather viewed as molecular donor-acceptor (D-A) type photocell [51. In a D-A molecular photocell mainly neutral excitons are first created by light, with charges being primarily separated at the narrow interface region due to D-A type intermolecular charge transfer interactions. In this paper we report the method of fabrication and characterization of Al/C6O/PPP/SnD2 junctions and examine its photoresponse. Upon illumination the drastic increase of the current has been observed at reverse bias, suggesting the possible applicatron of this junction as a sensitive photodetector. Spectral response of the junction was studied and the mechanisms of the photovoltaic effect are also discussed. We analyze how this processes depend on the wavelength of the pumping light and on the polarity of applied voltage. 2. EXPERIMENTAL For preparation of heterostructures we have used the PPP powder, obtained by the Kovacic method., and the fine purified C60 powder, provided by Science Laboratones Co., as starting materials. The Al/C6O/PPP/SnD2 heterostructure have been prepared according to the following procedure. The thin film of PPP were obtained by discrete vacuum evaporation of PPP-powder onto a glass slides with the tin oxide
0379-6779/95/$09.50
SSDI
0
1995 Elsevier
0379-6779(94)03243-Y
Science
S.A.
All
rights reserved
conducting transperant coating 141. Evaporation of C60 onto the PPP thin film was performed by heating the C6O powder in an MO crucible at 6OO’C. Then the procedure was finished by vacuum evaporation the front semitransparent Al electrode onto the obtained in Vacuum C6O/PPP/SnD2 heterostructure. the evaporation chamber was about 10m5 torr. The I-V characteristics of AIIC6OIPPPISn02 lheterostructure In the dark and upon UV-illumination were measured under vacuum at room temperature using U51 1 electrometeramplifier, V7-34A voltmeter and 85-45 dc source. Photoresponse was measured by irradiating with Xe arc lamp light passing through a monochromator on the sample placed in the vacuum cryostat. 3. RESULTS AND DISCUSSION The heterostructure has showed the asymmetric dark I-V dependence with rectification ratio about IO4 at +-20V (Fig.1). Upon illumination the increase of current by two order was observed at-20V reverse bias (Al electrode as positive electrode) and nearly by three orders upon increasing the reverse bias from 0 V to -28 V. Fig.2 shows the spectral dependence of photocurrent of AIICS0/PPP/Sn02 heterostructure at positive and negative bias. The illumination was from Sn02 side. In the case of the applied voltage of -1 V, a large photoresponse was observed dlJe to photoinduced charge transfer between PPP and C60. The most important feature of the spectral characteristic is the observation of small photoresponse peak at about 2 eV. Such spectral dependence can be explained by the following way. An electron photoexcited by z--‘II* transition in the PPP can be transfered to CSO, and a hole, photogenerated by hu-tl g transitions in CSO,
S.B. Lee et al. I Synthetic Metals 71 (1995) 2247-2248
spectra of C6O/C70 film [8] and C66-conducting polymer composite [9]. On the other hand, in the case of positive bias, the photoinduced charge transfer is not enhanced (Fir.21 because the electric field direction is inverse to the charge transfer direction from PPP to C60, resulting in the smaller photoresponse. The intensity dependence of open circuit voltage U,, and short circuit current lsc are shown at Fig.3. Open circuit voltage is fastly saturated by logarithmic way, but short circuit current is increased linearly.
0
0 0
B :: 0
::
0
t---,,
IO-‘_;-
IO Voltage,
Fig.1
6x10-l
20
V
Current vs. voltage characteristics of the AI/C60/PPP/Sn02 device in the dark and upon illumination with UV light source.
injected to PPP. Therefore, in the case of backward bias, the photoinduced charge transfer between PPP and C60 is activated and accelerated, resulting in the large photoresponse.
oL 0
5
10
Light intensity,
1
5 ti
’
__
Fig.3
negz
1
IT-
_! 200
rnW/crn2
U,, and J,, vs. light power Al/C6o/PPP/SnO2 device.
characteristics
of the
We are working on optimization of the photocell by changing the thickness of the C60 and PPP films and probing multilayered structures.
REFERENCES
I
I
1.5
I
2.0
I.
2.5
I
I
3.0
3.5
.I
4.0
Energy, eV Fig.2
The spectral dependence of photocurrent Af/C&PPP/SnO2 heterostructure.
of
Photoseparated positive polaron P+ in PPP and negative polaron Pm in C60 layers drift downward the potential and are collected at electrodes. The small peak is considered to be due to the h,-tlu transition of C60 which is the forbidden transition and was not observed in the absorption spectrum of C60 in solution. However it theoretically has been pointed that this dipole forbidden transition of a single C60 molecule should become dipole-allowed in some extent by latticefluctuation or intermolecular interactions [7]. This transition was clearly observed in the photoconduction
1. S. Morita, A.A. Zakhidov and K. Yoshino, Solid State Commun., 82 (I 992) 249 2. S.Morita, A.A.Zakhidov, K.Yoshino, Jpn J. Appl. Phys., 32 (1993) L873 3. N.S. Sariciftci, L. Smilowirz, A.J. Heeger and F.Wudl, Science, 258 (1992) 1474 4. N.S. Sariciftci, D. Braun, C. Zhang, V. Srdanov, A.J. Heeger and F. Wudl, Appl. Phys. I_ett., 62 (1993) 585 5. K.Yoshino, X.H.Yin, T.Akashi, K.Yoshimoto, S.Morita, and A.A.Zakhidov, Mol. Cryst. Liq. Cryst. (to be published) 6. Ya.L.Kogan, A.A.Abalyaeva, A.A.Zakhidov, S.B.Lee, Synth.Met., 41-43 (1991) 4016 7. K.Harigaya and S.Abe, private communication 8. J.Mort, er al., Chem.Phys. Lett., 186 (I 991) 281 9. S.Morita, S.Kiyomatsu, X.H.Yin, A.A. Zakhidov, T.Noguchi, T.Ohnishi and K.Yoshino, J.Appl.Phys., 74 (1993) 2860
Synthetic Metals 71 (1995) 2247-2248
Photovoltaic
properties S.B.Leea,
aDepartment bDepartment
of &()/PPP
P.K.Khabibullaeva,
heterojunction:
A.A.Zakhidova,
S.Moritab
molecular
D-A photocell
and K.Yoshinob
of Thermophysics, Uzbek Academy of Sci., Katartal 28, Tashkent 700135, Uzbekistan of Electronic Engineering., Osaka University, 2-2 Yamada-oka, Suita, Osaka, 56!3, Japan
Abstract We have found recently that thin film layered heterojunction between C60 and conduct\ng polymer, like poly(3-alkylthiophene) shows a photovoltaic effect due to photoinduced charge transfer at the interface. In this work we report the method of fabrication and characterization of AIIC601PPPISn02 junctions and examine its photoresponse. The PPP thin film have been vacuum deposited by discrete (or flash) evaporation of PPP (polyparaphenylene) powder, which allows to avoid the decomposition of polymer chains. Then C60 was evaporated on the top of PPP film. The heterostructure showed the assymmetric I-V dependence with the rectification ratio of about IO4 at 20V reverse bias (plus at Al-electrode). Upon illumination the drastic increase of the photocurrent has been observed at reverse bias, suggesting the possible application of this junction as a sensitive photodetector. Spectral response of the device was studied and the mechanisms of the photovoltaic effect are discussed. Contrary to conventional Inorganic semiconductor p-n junction photocell, the C 60.PPP junction can be rather viewed as molecular donor-acceptor (D-AI type photocell. 1. INTRODUCTION The recent discovery of photoinduced charge transfer from fullerenes to conducting polymers (CP) Ill has stimulated the studies of various heterojunctions between the semiconducting conjugated polymers and C60 which show photovoltaic effect 12-41. Contrary to conventional Inorganic semiconductor p-n junction photocell, the C6OCP junctions can be rather viewed as molecular donor-acceptor (D-A) type photocell [51. In a D-A molecular photocell mainly neutral excitons are first created by light, with charges being primarily separated at the narrow interface region due to D-A type intermolecular charge transfer interactions. In this paper we report the method of fabrication and characterization of Al/C6O/PPP/SnD2 junctions and examine its photoresponse. Upon illumination the drastic increase of the current has been observed at reverse bias, suggesting the possible applicatron of this junction as a sensitive photodetector. Spectral response of the junction was studied and the mechanisms of the photovoltaic effect are also discussed. We analyze how this processes depend on the wavelength of the pumping light and on the polarity of applied voltage. 2. EXPERIMENTAL For preparation of heterostructures we have used the PPP powder, obtained by the Kovacic method., and the fine purified C60 powder, provided by Science Laboratones Co., as starting materials. The Al/C6O/PPP/SnD2 heterostructure have been prepared according to the following procedure. The thin film of PPP were obtained by discrete vacuum evaporation of PPP-powder onto a glass slides with the tin oxide
0379-6779/95/$09.50
SSDI
0
1995 Elsevier
0379-6779(94)03243-Y
Science
S.A.
All
rights reserved
conducting transperant coating 141. Evaporation of C60 onto the PPP thin film was performed by heating the C6O powder in an MO crucible at 6OO’C. Then the procedure was finished by vacuum evaporation the front semitransparent Al electrode onto the obtained in Vacuum C6O/PPP/SnD2 heterostructure. the evaporation chamber was about 10m5 torr. The I-V characteristics of AIIC6OIPPPISn02 lheterostructure In the dark and upon UV-illumination were measured under vacuum at room temperature using U51 1 electrometeramplifier, V7-34A voltmeter and 85-45 dc source. Photoresponse was measured by irradiating with Xe arc lamp light passing through a monochromator on the sample placed in the vacuum cryostat. 3. RESULTS AND DISCUSSION The heterostructure has showed the asymmetric dark I-V dependence with rectification ratio about IO4 at +-20V (Fig.1). Upon illumination the increase of current by two order was observed at-20V reverse bias (Al electrode as positive electrode) and nearly by three orders upon increasing the reverse bias from 0 V to -28 V. Fig.2 shows the spectral dependence of photocurrent of AIICS0/PPP/Sn02 heterostructure at positive and negative bias. The illumination was from Sn02 side. In the case of the applied voltage of -1 V, a large photoresponse was observed dlJe to photoinduced charge transfer between PPP and C60. The most important feature of the spectral characteristic is the observation of small photoresponse peak at about 2 eV. Such spectral dependence can be explained by the following way. An electron photoexcited by z--‘II* transition in the PPP can be transfered to CSO, and a hole, photogenerated by hu-tl g transitions in CSO,
S.B. Lee et al. I Synthetic Metals 71 (1995) 2247-2248
spectra of C6O/C70 film [8] and C66-conducting polymer composite [9]. On the other hand, in the case of positive bias, the photoinduced charge transfer is not enhanced (Fir.21 because the electric field direction is inverse to the charge transfer direction from PPP to C60, resulting in the smaller photoresponse. The intensity dependence of open circuit voltage U,, and short circuit current lsc are shown at Fig.3. Open circuit voltage is fastly saturated by logarithmic way, but short circuit current is increased linearly.
0
0 0
B :: 0
::
0
t---,,
IO-‘_;-
IO Voltage,
Fig.1
6x10-l
20
V
Current vs. voltage characteristics of the AI/C60/PPP/Sn02 device in the dark and upon illumination with UV light source.
injected to PPP. Therefore, in the case of backward bias, the photoinduced charge transfer between PPP and C60 is activated and accelerated, resulting in the large photoresponse.
oL 0
5
10
Light intensity,
1
5 ti
’
__
Fig.3
negz
1
IT-
_! 200
rnW/crn2
U,, and J,, vs. light power Al/C6o/PPP/SnO2 device.
characteristics
of the
We are working on optimization of the photocell by changing the thickness of the C60 and PPP films and probing multilayered structures.
REFERENCES
I
I
1.5
I
2.0
I.
2.5
I
I
3.0
3.5
.I
4.0
Energy, eV Fig.2
The spectral dependence of photocurrent Af/C&PPP/SnO2 heterostructure.
of
Photoseparated positive polaron P+ in PPP and negative polaron Pm in C60 layers drift downward the potential and are collected at electrodes. The small peak is considered to be due to the h,-tlu transition of C60 which is the forbidden transition and was not observed in the absorption spectrum of C60 in solution. However it theoretically has been pointed that this dipole forbidden transition of a single C60 molecule should become dipole-allowed in some extent by latticefluctuation or intermolecular interactions [7]. This transition was clearly observed in the photoconduction
1. S. Morita, A.A. Zakhidov and K. Yoshino, Solid State Commun., 82 (I 992) 249 2. S.Morita, A.A.Zakhidov, K.Yoshino, Jpn J. Appl. Phys., 32 (1993) L873 3. N.S. Sariciftci, L. Smilowirz, A.J. Heeger and F.Wudl, Science, 258 (1992) 1474 4. N.S. Sariciftci, D. Braun, C. Zhang, V. Srdanov, A.J. Heeger and F. Wudl, Appl. Phys. I_ett., 62 (1993) 585 5. K.Yoshino, X.H.Yin, T.Akashi, K.Yoshimoto, S.Morita, and A.A.Zakhidov, Mol. Cryst. Liq. Cryst. (to be published) 6. Ya.L.Kogan, A.A.Abalyaeva, A.A.Zakhidov, S.B.Lee, Synth.Met., 41-43 (1991) 4016 7. K.Harigaya and S.Abe, private communication 8. J.Mort, er al., Chem.Phys. Lett., 186 (I 991) 281 9. S.Morita, S.Kiyomatsu, X.H.Yin, A.A. Zakhidov, T.Noguchi, T.Ohnishi and K.Yoshino, J.Appl.Phys., 74 (1993) 2860