- Email: [email protected]
organic heterojunction and application to optical computing device combining with organic electroluminescence
ELSEVIER
Synthetic
Metals
91 ( 1997) 77-79
Photocurrent multiplication phenomenon at organic /organic heterojunction and application to optical computing device combining with organic electroluminescence
Abstract Photocurrrnt multiplication reaching 3000-fold wax newly observed at an organic/organic heterojunction between p-type phthalocyanine and n-type perylene pigment layers. Multiplied photocurrent caused upon the irradiation of red light exciting CuPc was effectively suppressed by irradiating blue light exciting Me-PTC simultaneously. A rudimentary optical computing device was successfully demonstrated combining organic electroluminescence with this organic heterojunction device. 0 1997 Elsevier Science S.A. I\‘c~~wo~~ls: Organic/organic
heterojunctions:
Photocurrent
multiplication;
Electroluminescence:
1. Introduction Photocurrent multiplication is a phenomenonin which the quantum efficiency of photocurrent exceeds unity. Such a phenomenon has a great potential in fabricating highly sen-
sitive photosenxorsor new types of optoelectronic devices. Recently. we have observed large photocurrent
multiplication
phenomena exceeding IO’-fold in the vacuum-deposited organic films such as perylene pigment [ 11, quinacridone pigment [ 21 and naphthalenetetracarboxylic anhydride [ 31. Thesephenomenahave been explained asunique interfacial phenomenaat organic/metal junctions. namely, tunneling electron injection from a metal electrodeto the pigmentlayer in a high electric field built up by the photoaccumulatedspace chargesof trapped holes near the interface [ l-31. We have
Optical computing
devices
of the photocurrent, we have fabricated a new type of optical computing device combining organic electroluminescence (EL) with this organic heterojunction device.
2. Experimental CuPc (Sumitomo ChemicalIndustry Co., Ltd., Fig. 1) and Me-PTC (Dainichiseika Color & ChemicalsManufacturing
Blue Light (480 nm)
Red Light (680 nm)
started the present study based on the idea that the photocur-
rent multiplication phenomenonwould alsooccur at the interface between different organic films, i.e., organic/organic heterojunctions. In this paper, we would like to report a photocurrent multiplication phenomenon exceeding 3000-fold. newly observeda.torganic/organic heterojunctionsbetweenp-type phthalocyanine ( CuPc1 and n-type pcrylene ( Me-PTC) pigment layers. An interestingfeature of this phenomenonis that multiplied
photocurrent
was reversibly
suppressed by super-
imposing a secondlight. Utilizing this uniquephoto-control * Corresponding
author
0379-6779/97/$17.00 PUSO379-6779(97)03979-9
0 1997 Elaecier Science
S.A. All rights reserved
IT0 glass CuPc Me-PTC (600 nm) (100 nm) /==a Pl
”
CUP?
Fig. 1, Structure of double-layered cell used in this study. Thicknesses of &PC, Me-PTC and Au films are 600. 100 and 20 nm, respectively. IT0 electrode was negatively biased with respect to Au electrode. Chemical structures of CuPc and Me-PTC are also shown.
78
M. Hirmnom
er al. /Symhrric
Co, Ltd., Fig. 1) at least twice purified by sublimation were used. The structure of double-layered cell consisting of a CuPc film and a Me-PTC film used in this study is depicted in Fig. 1. Organic layers were successively deposited on an iridium-tin oxide (ITO) glass substrate by the vacuum evaporation technique under 1 X lo-” Pa at the rate of 0.1 nm s- ‘. Finally, a semitransparent Au electrode was provided on the organic surface. Measurements were made in an optical cryostat (Technolo Kogyo, CN-3) evacuated up to about 0.1 Pa at room temperature. Photocurrent was measured using an electrometer { Keithley, model 485) with an applied voltage between the IT0 and Au electrodes. Red light (680 nm) which can excite only CuPc was irradiated on the Au electrode. The photocurrent quantum efficiency, i.e., multiplication rate, was calculated as the ratio of the number of carriers flowing through the device by the red light illumination to the number of photons absorbed by CuPc film. In some cases, blue light (480 nm) which can excite only Me-PTC was irradiated on the IT0 electrode simultaneously with red light.
3. Results and discussion Fig. 2 shows the dependence of the multiplication rate of photocurrent on applied voltage observed for a double-layered cell having a CuPciMe-PTC heterojunction (Fig. 1) . When the IT0 electrode was biased negatively, carrier injection from both metal electrodes was effectively prevented and the photocurrent multiplication phenomenon occurring at the CuPc/Me-PTC interface dominated. Multiplication rate exceeded 3000-fold. This means that 3000 carriers flowed across the device by one input-photon. The active interface can be identified from the photocurrent action spectrum in the region where only CuPc has the absorption. When the monochromatic light was irradiated on the IT0 electrode, multiplied photocurrent showed maxima in the absorption edges of the CuPc film (around 560 and 760 nm) where the incident photons can penetrate deeply into the CuPc film, and showed minima in the wavelength region of strong absorption (around 620 and 700 nm) where most of the incident photons were absorbed near the interface of the negatively biased IT0 electrode. On the other hand, when the positively biased Au electrode was illuminated,
Applied Voltage I V Fig. 2. Dependence of photocurrent quantum efficiency on applied voltage. IT0 electrode was nqgatively biased with respect to Au electrode. Monochromatic light of 680 nm exciting CuPc selectively was irradiated on Au electrode. Measurements were performed at room temperature.
Merck
91 (1997)
77-79
5
Red Light Blue Light on off
Oil
3 % ‘$ 0.5 %
0
1J
on off
on
11
\1
I
-I
2 3 4 5 Time / min Fig. 3. Typical multiplied photocurrent suppression by blue light (480 nm) irradiation on IT0 electrode. Red light (680 nm) was always irradiated on Au electrode.
such distinct peaks were hardly observed. These results suggest that the CuPc/Me-PTC interface is responsible for the multiplication process. When the deposition rate of organic layers was kept at a constant value of 0.1 nm s- ‘, namely, in the cases of usual ITO/CuPc (thickness 600 nm, deposition rate0.1 nm s-’ ) /Me-PTC ( 100 nm, 0.1 nm s- ‘) /Au cells, large multiplication was observed. On the other hand, when only a very thin CuPc layer (50 nm) next to the Me-PTC layer was evaporated at a very high deposition rate of 4 nm s-‘, namely, in the case of the ITO/CuPc (5.50 nm, 0.1 nm s- ‘) /CuPc (50 nm, 4 nm s- ‘) /Me-PTC( 100 nm, 0.1 nm s- ‘) /Au cell, little multiplication was observed. This is another strong support that the present multiplication occurs at the CuPciMe-PTC heterojunction. A very interesting phenomenon was observed when the two respective organic layers were excited simultaneously by two different wavelength lights. As clearly demonstrated in Fig. 3, under the condition of the occurrence of photocurrent multiplication by the irradiation of red light (680 nm) exciting CuPc, the superimposed irradiation of blue light (480 nm) from the opposite direction, which can selectively excite Me-PTC (see Fig. 1) , suppressed the multipliedphotocurrent effectively. Response was quite reversible. Suppression was observed in the wavelength region of the superimposed second light from 400 to 540 nm, corresponding well to the absorption of Me-PTC film. Multiplied photocurrent by red light exhibited rather slow transient response on the order of several tens of seconds (Fig. 3). This suggests that the present multiplication is closely related to some trapping events of photogenerated carriers. Therefore, we propose the following multiplication mechanism, assuming the neutral electron trap near the CuPc/Me-PTC interface, Fig. 4(a) illustrates the energy structure of the cell during photocurrent multiplication. When CuPc is excited by red light, electrons and holes are photogenerated in the CuPc layer and move along the potential gradient. Some of the photogenerated electrons reaching the CuPc/Me-PTC interface are trapped by the interfacial states between CuPc and Me-PTC. Accumulation of trapped electrons at the interfacial states builds up a high electric field across the Me-PTC layer. Due to this high field, the energy
M. Hiramm
(4
et al. /Sphetic
Metals 91 (1997)
77-79
79
(b) hv (red) EL
hv (blue) IT0 J
~
ITC
t-B&h-PTC 'IT'
CUPC
h4emACU
curt Me-Fe
Fig. 4. Energy structures of ITOICuPcIMe-PT C/Au cell under an applied electric field: t a) photocurrent multiplication under red light irradiation: (b) multiplication suppression by blue light superimposed on red light.
band is strongly inclined and. finally, the tunneling injection of electronsfrom the valenceband of CuPc to the conduction band of Me-PTC occurs, asshown in Fig. 4(a). This tunneling current continues to flow, once the strong field concentration to the Me-PTC layer is accomplished.As a result, even 3000 electronsor more flowed per absorbedphoton. We observed the large photocurrent due to this photoinduced tunneling current. We suspectthat the natureof electron traps causing the present photocurrent multiplication closely relateswith the molecularstacking of pigmentsat the organic heterojunction sincethe disturbanceof molecularstackingof CuPc nearthe interface by the high rate depositionsuppressed the multiplication effectively. Multiplication suppressioncan be reasonably explained basedon this model (Fig. 4(b) ). Extinction of accumulated electronsoccurs due to the recombination with holesphotogeneratedby superimposedblue light in the Me-PTC film. This effectively suppressesthe multiplication. It should be noted that the photocurrent suppressionby blue light itself strongly supportsthe proposedtunneling mechanismbased on the trapped electron accumulation at the CuPc/Me-PTC heterojunction. If we could observe the radiative recombination from CuPc during the multiplication suppression.it would be a strong proof of the proposedmodel. We have already reported a new type of light transducer fabricated by combining the organic EL diode and a photoresponsiveorganic pigment film acting as an electron photoinjecting electrode to the EL film, which has achieved light amplification [4]. Since the present multiplication and its suppressionprovide a new way of photo-modulation of the photocurrent, we tried to apply thesephenomenato develop another type of light transducercombining organic EL and the CuPc/Me-PTC heterojunction. Device structure is depicted in Fig. 5. Organic EL consisting of red fluorescent t-BuPh-PTC and hole transporting TPD (see Fig. 5) was
Electrical-Photo Conversion (Organic EL)
Photo-Electrical Conversion (Organic Heterojunction)
;~,“j~gB’ j&g t-BuPh-PTC (red fluorescent) Fig. 5. Structure of light transducer PTC heterojunction.
CH3
TPD combining
V
organic
EL with CuPc/Me-
used. In this device, the irradiation of red light causedthe photocurrent multiplication at the CuPc/Me-PTC heterojunction and holes in the CuPc layer generated by the multiplication process (see Fig. 4(a)) were injected to tBuPh-PTC through TPD. Recombination between these holesand electronsfrom the negatively biasedMg electrode resulted in red EL output. We observed that EL output was almost suppressedby superimposingblue light causing the multiplication suppression.Interestingly, when superimposedblue light having a spatial pattern was used,only the correspondingpartsof the EL output were erased.This means that ‘NOT’ optical operation is possible.Therefore, the present device can be regardedasan optical computing device. In conclusion, a photocurrent multiplication phenomenon reaching 3000-fold was newly observed at an organic/ organic heterojunction. Furthermore, the multiplied photocurrent causedby red light was found to be suppressedby additional blue light irradiation. A new type of light transducer combining organic EL with an organic heterojunction, which may be applicableasan optical computing device, was successfullydemonstrated. References [ I] M. Hiramoto, i 1994) 187. [2] M. Hiramoto, ( 1996) L319. [3] T. Katsume, ( 1996) 3722. [ 11 M. Hiramoto,
T. Imahigashi S. Kawase M. Hiramoto T. Katsume
and M. Yokoyama, and M. Yokoyama, and M. Yokoyama, and M. Yokoyama,
Appl. Phys. Lett., 64 Jpn. J. Appl. Phys., 35 Appl. Opt. Rev.,
Phys. Lett., 69 I ( 1994) 82.
Synthetic
Metals
91 ( 1997) 77-79
Photocurrent multiplication phenomenon at organic /organic heterojunction and application to optical computing device combining with organic electroluminescence
Abstract Photocurrrnt multiplication reaching 3000-fold wax newly observed at an organic/organic heterojunction between p-type phthalocyanine and n-type perylene pigment layers. Multiplied photocurrent caused upon the irradiation of red light exciting CuPc was effectively suppressed by irradiating blue light exciting Me-PTC simultaneously. A rudimentary optical computing device was successfully demonstrated combining organic electroluminescence with this organic heterojunction device. 0 1997 Elsevier Science S.A. I\‘c~~wo~~ls: Organic/organic
heterojunctions:
Photocurrent
multiplication;
Electroluminescence:
1. Introduction Photocurrent multiplication is a phenomenonin which the quantum efficiency of photocurrent exceeds unity. Such a phenomenon has a great potential in fabricating highly sen-
sitive photosenxorsor new types of optoelectronic devices. Recently. we have observed large photocurrent
multiplication
phenomena exceeding IO’-fold in the vacuum-deposited organic films such as perylene pigment [ 11, quinacridone pigment [ 21 and naphthalenetetracarboxylic anhydride [ 31. Thesephenomenahave been explained asunique interfacial phenomenaat organic/metal junctions. namely, tunneling electron injection from a metal electrodeto the pigmentlayer in a high electric field built up by the photoaccumulatedspace chargesof trapped holes near the interface [ l-31. We have
Optical computing
devices
of the photocurrent, we have fabricated a new type of optical computing device combining organic electroluminescence (EL) with this organic heterojunction device.
2. Experimental CuPc (Sumitomo ChemicalIndustry Co., Ltd., Fig. 1) and Me-PTC (Dainichiseika Color & ChemicalsManufacturing
Blue Light (480 nm)
Red Light (680 nm)
started the present study based on the idea that the photocur-
rent multiplication phenomenonwould alsooccur at the interface between different organic films, i.e., organic/organic heterojunctions. In this paper, we would like to report a photocurrent multiplication phenomenon exceeding 3000-fold. newly observeda.torganic/organic heterojunctionsbetweenp-type phthalocyanine ( CuPc1 and n-type pcrylene ( Me-PTC) pigment layers. An interestingfeature of this phenomenonis that multiplied
photocurrent
was reversibly
suppressed by super-
imposing a secondlight. Utilizing this uniquephoto-control * Corresponding
author
0379-6779/97/$17.00 PUSO379-6779(97)03979-9
0 1997 Elaecier Science
S.A. All rights reserved
IT0 glass CuPc Me-PTC (600 nm) (100 nm) /==a Pl
”
CUP?
Fig. 1, Structure of double-layered cell used in this study. Thicknesses of &PC, Me-PTC and Au films are 600. 100 and 20 nm, respectively. IT0 electrode was negatively biased with respect to Au electrode. Chemical structures of CuPc and Me-PTC are also shown.
78
M. Hirmnom
er al. /Symhrric
Co, Ltd., Fig. 1) at least twice purified by sublimation were used. The structure of double-layered cell consisting of a CuPc film and a Me-PTC film used in this study is depicted in Fig. 1. Organic layers were successively deposited on an iridium-tin oxide (ITO) glass substrate by the vacuum evaporation technique under 1 X lo-” Pa at the rate of 0.1 nm s- ‘. Finally, a semitransparent Au electrode was provided on the organic surface. Measurements were made in an optical cryostat (Technolo Kogyo, CN-3) evacuated up to about 0.1 Pa at room temperature. Photocurrent was measured using an electrometer { Keithley, model 485) with an applied voltage between the IT0 and Au electrodes. Red light (680 nm) which can excite only CuPc was irradiated on the Au electrode. The photocurrent quantum efficiency, i.e., multiplication rate, was calculated as the ratio of the number of carriers flowing through the device by the red light illumination to the number of photons absorbed by CuPc film. In some cases, blue light (480 nm) which can excite only Me-PTC was irradiated on the IT0 electrode simultaneously with red light.
3. Results and discussion Fig. 2 shows the dependence of the multiplication rate of photocurrent on applied voltage observed for a double-layered cell having a CuPciMe-PTC heterojunction (Fig. 1) . When the IT0 electrode was biased negatively, carrier injection from both metal electrodes was effectively prevented and the photocurrent multiplication phenomenon occurring at the CuPc/Me-PTC interface dominated. Multiplication rate exceeded 3000-fold. This means that 3000 carriers flowed across the device by one input-photon. The active interface can be identified from the photocurrent action spectrum in the region where only CuPc has the absorption. When the monochromatic light was irradiated on the IT0 electrode, multiplied photocurrent showed maxima in the absorption edges of the CuPc film (around 560 and 760 nm) where the incident photons can penetrate deeply into the CuPc film, and showed minima in the wavelength region of strong absorption (around 620 and 700 nm) where most of the incident photons were absorbed near the interface of the negatively biased IT0 electrode. On the other hand, when the positively biased Au electrode was illuminated,
Applied Voltage I V Fig. 2. Dependence of photocurrent quantum efficiency on applied voltage. IT0 electrode was nqgatively biased with respect to Au electrode. Monochromatic light of 680 nm exciting CuPc selectively was irradiated on Au electrode. Measurements were performed at room temperature.
Merck
91 (1997)
77-79
5
Red Light Blue Light on off
Oil
3 % ‘$ 0.5 %
0
1J
on off
on
11
\1
I
-I
2 3 4 5 Time / min Fig. 3. Typical multiplied photocurrent suppression by blue light (480 nm) irradiation on IT0 electrode. Red light (680 nm) was always irradiated on Au electrode.
such distinct peaks were hardly observed. These results suggest that the CuPc/Me-PTC interface is responsible for the multiplication process. When the deposition rate of organic layers was kept at a constant value of 0.1 nm s- ‘, namely, in the cases of usual ITO/CuPc (thickness 600 nm, deposition rate0.1 nm s-’ ) /Me-PTC ( 100 nm, 0.1 nm s- ‘) /Au cells, large multiplication was observed. On the other hand, when only a very thin CuPc layer (50 nm) next to the Me-PTC layer was evaporated at a very high deposition rate of 4 nm s-‘, namely, in the case of the ITO/CuPc (5.50 nm, 0.1 nm s- ‘) /CuPc (50 nm, 4 nm s- ‘) /Me-PTC( 100 nm, 0.1 nm s- ‘) /Au cell, little multiplication was observed. This is another strong support that the present multiplication occurs at the CuPciMe-PTC heterojunction. A very interesting phenomenon was observed when the two respective organic layers were excited simultaneously by two different wavelength lights. As clearly demonstrated in Fig. 3, under the condition of the occurrence of photocurrent multiplication by the irradiation of red light (680 nm) exciting CuPc, the superimposed irradiation of blue light (480 nm) from the opposite direction, which can selectively excite Me-PTC (see Fig. 1) , suppressed the multipliedphotocurrent effectively. Response was quite reversible. Suppression was observed in the wavelength region of the superimposed second light from 400 to 540 nm, corresponding well to the absorption of Me-PTC film. Multiplied photocurrent by red light exhibited rather slow transient response on the order of several tens of seconds (Fig. 3). This suggests that the present multiplication is closely related to some trapping events of photogenerated carriers. Therefore, we propose the following multiplication mechanism, assuming the neutral electron trap near the CuPc/Me-PTC interface, Fig. 4(a) illustrates the energy structure of the cell during photocurrent multiplication. When CuPc is excited by red light, electrons and holes are photogenerated in the CuPc layer and move along the potential gradient. Some of the photogenerated electrons reaching the CuPc/Me-PTC interface are trapped by the interfacial states between CuPc and Me-PTC. Accumulation of trapped electrons at the interfacial states builds up a high electric field across the Me-PTC layer. Due to this high field, the energy
M. Hiramm
(4
et al. /Sphetic
Metals 91 (1997)
77-79
79
(b) hv (red) EL
hv (blue) IT0 J
~
ITC
t-B&h-PTC 'IT'
CUPC
h4emACU
curt Me-Fe
Fig. 4. Energy structures of ITOICuPcIMe-PT C/Au cell under an applied electric field: t a) photocurrent multiplication under red light irradiation: (b) multiplication suppression by blue light superimposed on red light.
band is strongly inclined and. finally, the tunneling injection of electronsfrom the valenceband of CuPc to the conduction band of Me-PTC occurs, asshown in Fig. 4(a). This tunneling current continues to flow, once the strong field concentration to the Me-PTC layer is accomplished.As a result, even 3000 electronsor more flowed per absorbedphoton. We observed the large photocurrent due to this photoinduced tunneling current. We suspectthat the natureof electron traps causing the present photocurrent multiplication closely relateswith the molecularstacking of pigmentsat the organic heterojunction sincethe disturbanceof molecularstackingof CuPc nearthe interface by the high rate depositionsuppressed the multiplication effectively. Multiplication suppressioncan be reasonably explained basedon this model (Fig. 4(b) ). Extinction of accumulated electronsoccurs due to the recombination with holesphotogeneratedby superimposedblue light in the Me-PTC film. This effectively suppressesthe multiplication. It should be noted that the photocurrent suppressionby blue light itself strongly supportsthe proposedtunneling mechanismbased on the trapped electron accumulation at the CuPc/Me-PTC heterojunction. If we could observe the radiative recombination from CuPc during the multiplication suppression.it would be a strong proof of the proposedmodel. We have already reported a new type of light transducer fabricated by combining the organic EL diode and a photoresponsiveorganic pigment film acting as an electron photoinjecting electrode to the EL film, which has achieved light amplification [4]. Since the present multiplication and its suppressionprovide a new way of photo-modulation of the photocurrent, we tried to apply thesephenomenato develop another type of light transducercombining organic EL and the CuPc/Me-PTC heterojunction. Device structure is depicted in Fig. 5. Organic EL consisting of red fluorescent t-BuPh-PTC and hole transporting TPD (see Fig. 5) was
Electrical-Photo Conversion (Organic EL)
Photo-Electrical Conversion (Organic Heterojunction)
;~,“j~gB’ j&g t-BuPh-PTC (red fluorescent) Fig. 5. Structure of light transducer PTC heterojunction.
CH3
TPD combining
V
organic
EL with CuPc/Me-
used. In this device, the irradiation of red light causedthe photocurrent multiplication at the CuPc/Me-PTC heterojunction and holes in the CuPc layer generated by the multiplication process (see Fig. 4(a)) were injected to tBuPh-PTC through TPD. Recombination between these holesand electronsfrom the negatively biasedMg electrode resulted in red EL output. We observed that EL output was almost suppressedby superimposingblue light causing the multiplication suppression.Interestingly, when superimposedblue light having a spatial pattern was used,only the correspondingpartsof the EL output were erased.This means that ‘NOT’ optical operation is possible.Therefore, the present device can be regardedasan optical computing device. In conclusion, a photocurrent multiplication phenomenon reaching 3000-fold was newly observed at an organic/ organic heterojunction. Furthermore, the multiplied photocurrent causedby red light was found to be suppressedby additional blue light irradiation. A new type of light transducer combining organic EL with an organic heterojunction, which may be applicableasan optical computing device, was successfullydemonstrated. References [ I] M. Hiramoto, i 1994) 187. [2] M. Hiramoto, ( 1996) L319. [3] T. Katsume, ( 1996) 3722. [ 11 M. Hiramoto,
T. Imahigashi S. Kawase M. Hiramoto T. Katsume
and M. Yokoyama, and M. Yokoyama, and M. Yokoyama, and M. Yokoyama,
Appl. Phys. Lett., 64 Jpn. J. Appl. Phys., 35 Appl. Opt. Rev.,
Phys. Lett., 69 I ( 1994) 82.