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Photovoltaic properties of a sandwich cell consisting of bromophosphorus phthalocyanine and titanium dioxide layers
Optical Materials 27 (2005) 1480–1483 www.elsevier.com/locate/optmat
Photovoltaic properties of a sandwich cell consisting of bromophosphorus phthalocyanine and titanium dioxide layers R. Signerski *, B. Kos´cielska Faculty of Applied Physics and Mathematics, Gdansk University of Technology, G. Narutowicza 11/12, 80-952 Gdan´sk, Poland Available online 17 February 2005
Abstract The photovoltaic properties of sandwich structures formed from TiO2 films prepared on ITO coated glass substrates using a spin coating technique and vapour-deposited layers of bromophosphorus phthalocyanine, tetracene and Au electrode (ITO/TiO2/PBrPc/ Tc/Au) were investigated. The system exhibits a strong rectification effect in the dark, while illumination through the ITO leads to a noticeable photovoltaic effect connected with exciton dissociation within the TiO2/PBrPc heterojunction. 2005 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Bromophosphorus phthalocyanine; Photovoltaic effect
1. Introduction Phthalocyanines and titanium dioxide (TiO2) are attractive materials for photovoltaic cells. Phthalocyanines exhibit high light absorption in the visible range, a relatively high efficiency of charge carrier photogeneration and form well ordered layers by vapour deposition [1]. The latter, namely dye-sensitised nanocrystalline TiO2, is often used in photoelectrochemical and solid state solar cells [2]. There have also been several studies dealing with the photovoltaic properties of p–n heterojunctions in which the TiO2 layer plays the role of n-type semiconductor and the p-type semiconductor is an organic material such as merocyanine [3], copper phthalocyanine CuPc [4,5], zinc phthalocyanine ZnPc [5,6], polythiophene [7], poly(phenylene-vinylene): MEHPPV [8–11], PA-PPV [12], MDMO-PPV [13], or porphyrins [5,14,15]. The main focus of these works is the investigation of photovoltaic structures that will link the beneficial properties of organic materials (high optical absorption) with those of inorganic ones (well prop*
Corresponding author. Tel.: +4858 347 1358; fax: +4858 347 2821. E-mail address: [email protected] (R. Signerski).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.006
erties of electronic transport). In particular the works mentioned above indicate that the junction of an organic material and TiO2 is a region with efficient exciton dissociation resulting in the transfer of an electron to the conduction band of TiO2 and a hole moving in the organic layer. This is the most common mechanism of charge carrier generation in such cells and this work deals with this issue and presents data obtained on a sandwich structure consisting of the following layers: indium-tin-oxide (ITO), titanium dioxide (TiO2), bromophosphorus phthalocyanine (PBrPc), tetracene (Tc) and gold (Au). Such a structure, noted hereafter as ITO/ TiO2/PBrPc/Tc/Au, has not yet been investigated and the photoelectric properties of PBrPc are unknown. Nevertheless it can anticipated that PBrPc will be a good photovoltaic material due to its high light absorption connected with the fact that not only the Q band but also the Soret band occur in the visible range [16]. Additional research performed by us on the ITO/PBrPc/Al system indicates, that PBrPc, like other typical metallophthalocyanines, may be treated as a p-type material. This implies that the system of TiO2/PBrPc layers existing in our structure forms a n–p heterojunction, because the TiO2 layer exhibits n-type conductivity [5,17]. An
R. Signerski, B. Kos´cielska / Optical Materials 27 (2005) 1480–1483
additional Tc layer is included in this system in order to: (i) prevent the PBrPc layer from being damaged during Au deposition and (ii) eliminate dissociation of excitons excited in PBrPc on a metal electrode. A similar role is played by bathocuproine in other photovoltaic structures [18]. The aim of this work is to present the results of research on the electrical and photovoltaic properties of the ITO/TiO2/PBrPc/Tc/Au system. In particular attention is paid to the heterojunction formed from the TiO2 and PBrPc layers. 2. Experimental Fig. 1 shows the chemical structure of the molecules used and the structure of the resulting device. Films of TiO2 were deposited on indium tin oxide substrate (ITO: thickness 35 nm, 100 X/square, AWAT) by a spin coating technique. A starting solution for coating was prepared by mixing titanium butoxide with ethanol (EtOH) in molar ratio 1:5 with acetyloacetone (AcAc) as the complexing agent. The resulting gel layers were dried and then heated at 500 C for 1 h in order to obtain a smooth homogeneous TiO2 films. The TiO2 films were measured to be 300 nm thick. Next the system of glassjITOjTiO2 was placed under a vacuum of 3 · 104 Pa (Auto 306 Turbo, Edwards) and layers of PBrPc (thickness 80 nm), Tc (thickness 25 nm) and Au electrodes (thickness 20 nm) were subsequently evapo˚/ rated using thermal sublimation (deposition rate 0.5 A s). Bromophosphorus phthalocyanine was provided by Dr. A. Kempa, who synthesized it according to [16] and tetracene (Tc) was purchased from Aldrich. Both organic materials were purified locally by sublimation in a stream of N2 (train sublimation). To check repro-
1481
ducibility in one cycle four samples were produced, each of them had 6 mm2-common surface of electrodes. The absorption spectra were obtained using the Perkin Elmer Lambda 10 spectrometer. For the electric measurements a Keithley 6517 electrometer was used with a computer controlling voltage source. The light source consisted of an Xe (ILC 201) lamp with solar light simulated by the use of an AMO filter (Oriel). Spectra of photocurrents were obtained using a CM110 monochromator (CVI) with a constant flux of photons [photons/ (cm2 s)] within the whole wavelength range. Measurements were performed under ambient air at room temperature.
3. Results and discussion The current–voltage characteristics of the sample is presented in Fig. 2. Positive voltage refers to the higher potential on ITO. Curve 1 corresponds to data obtained in the dark while curve 2 was recorded when the sample was illuminated with a white light of 20 mW/cm2 through the ITO. The dark curve exhibits a strong current rectification (see inset). The ratio of rectification is about 180 at U = 0.8 V. The shape of curve 1 is fairly typical of an organic p-n junction. Therefore it is believed that the dark current is mainly determined by the following processes occurring in the junction region: thermal generation for reversed bias (+ITO/Au), and recombination of charge carriers injected from electrodes for forward bias (ITO/+Au). The illumination of the sample explicitly results in a photovoltaic effect (curve 2). For a measured short-circuit current density of jsc = 37 lA/cm2 and an open-circuit voltage of Uoc = 0.47 V the fill factor could be calculated as FF = 0.27 from which a power conversion efficiency of
N 140 N N
M N
100
N
80
Tc
N
M = P-Br : PBrPc Au
J [µA/cm2]
N
ITO/TiO2/PBrPc/Tc/Au
120
60
Jsc
40 20 0 -20
-1.0
-60
Voc 2
ITO
glass Fig. 1. Chemical structure of molecules and device structure used.
0.5
1.0
U[V]
-1.0 -0.8 -0.6 -0.4 -0.2
-2 -3 -4
1
-5 -6
-80
TiO2
0.0
-1
-40
Tc PBrPc
-0.5
0
1 J [µA/cm2]
N
0.0
0.2
0.4
0.6
0.8
1.0
U [V] Fig. 2. Current density versus applied voltage in the dark (curve 1 and inset) and under illumination through the ITO with a white light of 20 mW/cm2 (curve 2).
R. Signerski, B. Kos´cielska / Optical Materials 27 (2005) 1480–1483
Fig. 3. Energy-level diagram of the ITO/TiO2/PBrPc/Tc/Au cell in short-circuit mode and interfacial exciton dissociation. Ef—Fermi energy of electrodes, Vac—vacuum energy level, VB, CB—levels transporting, respectively, holes or electrons.
1.0 ITO/TiO2/PBrPc/Tc/Au
100
1 10 1
0.5
0.1
PBrPc
2 TiO2
0.01
optical density
g = 0.02% was calculated. Such values make this system attractive and worthy of further investigation to optimise the structure. It is worth noting that curve 2 clearly departs from the characteristic typical of an illuminated inorganic diode of high power conversion efficiency. Following other research in this area it is suggested that this results from a high value of the series resistance of organic layers and from the influence of electric field on charge carrier photogeneration in a junction. In the analysis of photovoltaic phenomena an energyband diagram of an investigated structure is often helpful. Fig. 3 shows such a diagram for the this system in a short-circuit mode. The location of VB and CB levels transporting, respectively, holes or electrons for PBrPc are assumed to be the same as for CuPc [1,19,20]. In the case of the other materials the relevant data was taken from the literature [12,21]. This figure does not include band bending which can exist in these system. Additionally, the process of charge carrier generation connected with the exciton dissociation in the TiO2/ PBrPc junction is sketched. This process of photogeneration will be responsible for the symbatic relation between absorption spectrum of PBrPc and the photocurrent spectrum (it means that peaks of photocurrent and absorption occur at the same wavelength) for illumination through the ITO, while illumination through the Au it will result in the antibatic relation (minima of photocurrent within the strong absorption of PBrPc). Spectra of short-circuit currents and absorption presented in Fig. 4 indeed confirm the occurrence of exciton dissociation in theTiO2/PBrPc junction. Curve 1 (illumination through the ITO) is symbatic in respect to the absorption spectrum of PBrPc, while curve 2 (illumination through the Au) is antibatic. It is noticeable within the Q band (550–750 nm), as well as within the Soret band (370–500 nm) of PBrPc. The filtration effect of
Jsc[nA/cm2]
1482
Tc 0.0 300
400
500
600
700
800
900
λ [nm] Fig. 4. Spectra of the short-circuit currents of the ITO/TiO2/PBrPc/Tc/ Au device: (i) curve 1—under illumination through the ITO (I0 = 1014 photons/(cm2 s)), (ii) curve 2—under illumination through the Au (I0 = 1013 photons/(cm2 s)). The optical density spectra of TiO2, Tc and PBrPc layers are included for a comparison.
the light beam by the Tc layer is not noticeable in curve 2. Similarly the influence of the light absorption by TiO2 on curve 1 at k > 380 nm is not observed and only in the shorter wavelength range the changes in jsc be referred to band to band transitions in TiO2. Measurements of the spectral dependence of the open-circuit voltage, Uoc (k) were performed but they exhibited the same features as jsc (k) and as a result they are not presented here. Fig. 5 shows the short-circuit current (jsc) and the open-circuit voltage versus light intensity. The sample was illuminated through the ITO with monochromatic light of a wavelength 430 nm. It is seen that the shortcircuit current is directly proportional to a light intensity jsc I0, and the open-circuit voltage fulfils the typical relationship of an p–n junction: U oc ¼ ðnkT =eÞ lnðjsc =jo Þ; in which n is a diode quality factor and jo is the saturation current of a junction [1,22]. Taking kT/e = 25 mV we can estimate n = 1.7. Such a value of the diode quality factor indicates the occurrence of generation and recombination processes in the junction.
Fig. 5. Light intensity dependence of the short-circuit current (curve 1) and open-circuit voltage (curve 2) for the device when illuminated through the ITO with monochromatic light of wavelength 430 nm.
R. Signerski, B. Kos´cielska / Optical Materials 27 (2005) 1480–1483
4. Conclusions From the investigations performed into the photovoltaic properties of the ITO/TiO2/PBrPc/Tc/Au structure the following conclusions can be drawn: • A TiO2 film prepared on an ITO glass substrate using the sol–gel method constitutes a good optical window and it effectively transports electrons in our organic– inorganic solar cell. • Bromophosphorus phthalocyanine is a p-type organic material which exhibits interesting photoelectric properties. • The investigated system exhibits a strong rectification effect in the dark and an explicit photovoltaic effect under illumination. • Modulation of the layer thicknesses used may result in a significant improvement in the photovoltaic parameters of the system.
Acknowledgment The authors are grateful to Dr. A. Kempa for the synthesis of bromophosphorus phthalocyanine. This work was supported in part by KBN under Program No. 4T11B05722. References [1] J. Simon, J.-J. Andre´, Molecular Semiconductors, Springer, Berlin, 1985.
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[2] M. Gra¨zel, Prog. Photovolt. Res. Appl. 8 (2000) 171. [3] T. Skotheim, J.-M. Yang, J. Otvos, M.P. Klein, J. Chem. Phys. 77 (1982) 6144. [4] S.M. Tracey, A.K. Ray, T.S. Shishiyanu, JEE Proc. Circuits Devices Syst. 145 (1995) 383. [5] E. Itoh, Y. Ohmori, K. Miyairi, Jpn. J. Appl. Phys. 43 (2004) 817. [6] K. Kajihara, K. Tanaka, K. Hirao, N. Soga, Jpn. J. Appl. Phys. 35 (1996) 6110. [7] K. Kajihara, K. Tanaka, K. Hirao, N. Soga, Jpn. J. Appl. Phys. 36 (1997) 5537. [8] T.J. Savenije, J.M. Warman, A. Goossens, Chem. Phys. Lett. 287 (1998) 148. [9] A.C. Arango, S.A. Carter, P.J. Brock, Appl. Phys. Lett. 74 (1999) 1698. [10] A.J. Breeze, Z. Schlesinger, S.A. Carter, P.J. Brock, Phys. Rev. B. 64 (2001) 125205. [11] M.Y. Song, J.K. Kim, K.-J. Kim, D.Y. Kim, Synth. Metals 137 (2003) 1387. [12] A.C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, S.A. Carter, H.-H. Ho¨rhold, Adv. Mater. 12 (2000) 1689. [13] B. van der Zanden, A. Goossens, J. Appl. Phys. 94 (2003) 6959. [14] J. Wienke, T.J. Schaafsma, A. Goossens, J. Phys. Chem. B. 103 (1999) 2702. [15] K. Miyairi, E. Itoh, Y. Hashimoto, Thin Solid Films 438–439 (2003) 147. [16] M. Gouterman, P. Sayer, E. Shankland, J.P. Smith, Inorg. Chem. 20 (1981) 87. [17] A.K. Hassan, N.B. Chaure, A.K. Ray, A.V. Nabok, S. Habesch, J. Phys. D.: Appl. Phys. 36 (2003) 1120. [18] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [19] I.G. Hill, A. Kahn, Z.G. Soos, R.A. Pascal Jr., Chem. Phys. Lett. 327 (2000) 181. [20] S.M. Tadayyon, H.M. Grandin, K. Griffiths, L.L. Coatsworth, P.R. Norton, H. Aziz, Z.D. Popovic, Org. Electron. 5 (2004) 199. ˇ a´pek, Organic Molecular Crystals, AIP Press, [21] E.A. Silinsh, V. C New York, 1994. [22] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.
Photovoltaic properties of a sandwich cell consisting of bromophosphorus phthalocyanine and titanium dioxide layers R. Signerski *, B. Kos´cielska Faculty of Applied Physics and Mathematics, Gdansk University of Technology, G. Narutowicza 11/12, 80-952 Gdan´sk, Poland Available online 17 February 2005
Abstract The photovoltaic properties of sandwich structures formed from TiO2 films prepared on ITO coated glass substrates using a spin coating technique and vapour-deposited layers of bromophosphorus phthalocyanine, tetracene and Au electrode (ITO/TiO2/PBrPc/ Tc/Au) were investigated. The system exhibits a strong rectification effect in the dark, while illumination through the ITO leads to a noticeable photovoltaic effect connected with exciton dissociation within the TiO2/PBrPc heterojunction. 2005 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Bromophosphorus phthalocyanine; Photovoltaic effect
1. Introduction Phthalocyanines and titanium dioxide (TiO2) are attractive materials for photovoltaic cells. Phthalocyanines exhibit high light absorption in the visible range, a relatively high efficiency of charge carrier photogeneration and form well ordered layers by vapour deposition [1]. The latter, namely dye-sensitised nanocrystalline TiO2, is often used in photoelectrochemical and solid state solar cells [2]. There have also been several studies dealing with the photovoltaic properties of p–n heterojunctions in which the TiO2 layer plays the role of n-type semiconductor and the p-type semiconductor is an organic material such as merocyanine [3], copper phthalocyanine CuPc [4,5], zinc phthalocyanine ZnPc [5,6], polythiophene [7], poly(phenylene-vinylene): MEHPPV [8–11], PA-PPV [12], MDMO-PPV [13], or porphyrins [5,14,15]. The main focus of these works is the investigation of photovoltaic structures that will link the beneficial properties of organic materials (high optical absorption) with those of inorganic ones (well prop*
Corresponding author. Tel.: +4858 347 1358; fax: +4858 347 2821. E-mail address: [email protected] (R. Signerski).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.006
erties of electronic transport). In particular the works mentioned above indicate that the junction of an organic material and TiO2 is a region with efficient exciton dissociation resulting in the transfer of an electron to the conduction band of TiO2 and a hole moving in the organic layer. This is the most common mechanism of charge carrier generation in such cells and this work deals with this issue and presents data obtained on a sandwich structure consisting of the following layers: indium-tin-oxide (ITO), titanium dioxide (TiO2), bromophosphorus phthalocyanine (PBrPc), tetracene (Tc) and gold (Au). Such a structure, noted hereafter as ITO/ TiO2/PBrPc/Tc/Au, has not yet been investigated and the photoelectric properties of PBrPc are unknown. Nevertheless it can anticipated that PBrPc will be a good photovoltaic material due to its high light absorption connected with the fact that not only the Q band but also the Soret band occur in the visible range [16]. Additional research performed by us on the ITO/PBrPc/Al system indicates, that PBrPc, like other typical metallophthalocyanines, may be treated as a p-type material. This implies that the system of TiO2/PBrPc layers existing in our structure forms a n–p heterojunction, because the TiO2 layer exhibits n-type conductivity [5,17]. An
R. Signerski, B. Kos´cielska / Optical Materials 27 (2005) 1480–1483
additional Tc layer is included in this system in order to: (i) prevent the PBrPc layer from being damaged during Au deposition and (ii) eliminate dissociation of excitons excited in PBrPc on a metal electrode. A similar role is played by bathocuproine in other photovoltaic structures [18]. The aim of this work is to present the results of research on the electrical and photovoltaic properties of the ITO/TiO2/PBrPc/Tc/Au system. In particular attention is paid to the heterojunction formed from the TiO2 and PBrPc layers. 2. Experimental Fig. 1 shows the chemical structure of the molecules used and the structure of the resulting device. Films of TiO2 were deposited on indium tin oxide substrate (ITO: thickness 35 nm, 100 X/square, AWAT) by a spin coating technique. A starting solution for coating was prepared by mixing titanium butoxide with ethanol (EtOH) in molar ratio 1:5 with acetyloacetone (AcAc) as the complexing agent. The resulting gel layers were dried and then heated at 500 C for 1 h in order to obtain a smooth homogeneous TiO2 films. The TiO2 films were measured to be 300 nm thick. Next the system of glassjITOjTiO2 was placed under a vacuum of 3 · 104 Pa (Auto 306 Turbo, Edwards) and layers of PBrPc (thickness 80 nm), Tc (thickness 25 nm) and Au electrodes (thickness 20 nm) were subsequently evapo˚/ rated using thermal sublimation (deposition rate 0.5 A s). Bromophosphorus phthalocyanine was provided by Dr. A. Kempa, who synthesized it according to [16] and tetracene (Tc) was purchased from Aldrich. Both organic materials were purified locally by sublimation in a stream of N2 (train sublimation). To check repro-
1481
ducibility in one cycle four samples were produced, each of them had 6 mm2-common surface of electrodes. The absorption spectra were obtained using the Perkin Elmer Lambda 10 spectrometer. For the electric measurements a Keithley 6517 electrometer was used with a computer controlling voltage source. The light source consisted of an Xe (ILC 201) lamp with solar light simulated by the use of an AMO filter (Oriel). Spectra of photocurrents were obtained using a CM110 monochromator (CVI) with a constant flux of photons [photons/ (cm2 s)] within the whole wavelength range. Measurements were performed under ambient air at room temperature.
3. Results and discussion The current–voltage characteristics of the sample is presented in Fig. 2. Positive voltage refers to the higher potential on ITO. Curve 1 corresponds to data obtained in the dark while curve 2 was recorded when the sample was illuminated with a white light of 20 mW/cm2 through the ITO. The dark curve exhibits a strong current rectification (see inset). The ratio of rectification is about 180 at U = 0.8 V. The shape of curve 1 is fairly typical of an organic p-n junction. Therefore it is believed that the dark current is mainly determined by the following processes occurring in the junction region: thermal generation for reversed bias (+ITO/Au), and recombination of charge carriers injected from electrodes for forward bias (ITO/+Au). The illumination of the sample explicitly results in a photovoltaic effect (curve 2). For a measured short-circuit current density of jsc = 37 lA/cm2 and an open-circuit voltage of Uoc = 0.47 V the fill factor could be calculated as FF = 0.27 from which a power conversion efficiency of
N 140 N N
M N
100
N
80
Tc
N
M = P-Br : PBrPc Au
J [µA/cm2]
N
ITO/TiO2/PBrPc/Tc/Au
120
60
Jsc
40 20 0 -20
-1.0
-60
Voc 2
ITO
glass Fig. 1. Chemical structure of molecules and device structure used.
0.5
1.0
U[V]
-1.0 -0.8 -0.6 -0.4 -0.2
-2 -3 -4
1
-5 -6
-80
TiO2
0.0
-1
-40
Tc PBrPc
-0.5
0
1 J [µA/cm2]
N
0.0
0.2
0.4
0.6
0.8
1.0
U [V] Fig. 2. Current density versus applied voltage in the dark (curve 1 and inset) and under illumination through the ITO with a white light of 20 mW/cm2 (curve 2).
R. Signerski, B. Kos´cielska / Optical Materials 27 (2005) 1480–1483
Fig. 3. Energy-level diagram of the ITO/TiO2/PBrPc/Tc/Au cell in short-circuit mode and interfacial exciton dissociation. Ef—Fermi energy of electrodes, Vac—vacuum energy level, VB, CB—levels transporting, respectively, holes or electrons.
1.0 ITO/TiO2/PBrPc/Tc/Au
100
1 10 1
0.5
0.1
PBrPc
2 TiO2
0.01
optical density
g = 0.02% was calculated. Such values make this system attractive and worthy of further investigation to optimise the structure. It is worth noting that curve 2 clearly departs from the characteristic typical of an illuminated inorganic diode of high power conversion efficiency. Following other research in this area it is suggested that this results from a high value of the series resistance of organic layers and from the influence of electric field on charge carrier photogeneration in a junction. In the analysis of photovoltaic phenomena an energyband diagram of an investigated structure is often helpful. Fig. 3 shows such a diagram for the this system in a short-circuit mode. The location of VB and CB levels transporting, respectively, holes or electrons for PBrPc are assumed to be the same as for CuPc [1,19,20]. In the case of the other materials the relevant data was taken from the literature [12,21]. This figure does not include band bending which can exist in these system. Additionally, the process of charge carrier generation connected with the exciton dissociation in the TiO2/ PBrPc junction is sketched. This process of photogeneration will be responsible for the symbatic relation between absorption spectrum of PBrPc and the photocurrent spectrum (it means that peaks of photocurrent and absorption occur at the same wavelength) for illumination through the ITO, while illumination through the Au it will result in the antibatic relation (minima of photocurrent within the strong absorption of PBrPc). Spectra of short-circuit currents and absorption presented in Fig. 4 indeed confirm the occurrence of exciton dissociation in theTiO2/PBrPc junction. Curve 1 (illumination through the ITO) is symbatic in respect to the absorption spectrum of PBrPc, while curve 2 (illumination through the Au) is antibatic. It is noticeable within the Q band (550–750 nm), as well as within the Soret band (370–500 nm) of PBrPc. The filtration effect of
Jsc[nA/cm2]
1482
Tc 0.0 300
400
500
600
700
800
900
λ [nm] Fig. 4. Spectra of the short-circuit currents of the ITO/TiO2/PBrPc/Tc/ Au device: (i) curve 1—under illumination through the ITO (I0 = 1014 photons/(cm2 s)), (ii) curve 2—under illumination through the Au (I0 = 1013 photons/(cm2 s)). The optical density spectra of TiO2, Tc and PBrPc layers are included for a comparison.
the light beam by the Tc layer is not noticeable in curve 2. Similarly the influence of the light absorption by TiO2 on curve 1 at k > 380 nm is not observed and only in the shorter wavelength range the changes in jsc be referred to band to band transitions in TiO2. Measurements of the spectral dependence of the open-circuit voltage, Uoc (k) were performed but they exhibited the same features as jsc (k) and as a result they are not presented here. Fig. 5 shows the short-circuit current (jsc) and the open-circuit voltage versus light intensity. The sample was illuminated through the ITO with monochromatic light of a wavelength 430 nm. It is seen that the shortcircuit current is directly proportional to a light intensity jsc I0, and the open-circuit voltage fulfils the typical relationship of an p–n junction: U oc ¼ ðnkT =eÞ lnðjsc =jo Þ; in which n is a diode quality factor and jo is the saturation current of a junction [1,22]. Taking kT/e = 25 mV we can estimate n = 1.7. Such a value of the diode quality factor indicates the occurrence of generation and recombination processes in the junction.
Fig. 5. Light intensity dependence of the short-circuit current (curve 1) and open-circuit voltage (curve 2) for the device when illuminated through the ITO with monochromatic light of wavelength 430 nm.
R. Signerski, B. Kos´cielska / Optical Materials 27 (2005) 1480–1483
4. Conclusions From the investigations performed into the photovoltaic properties of the ITO/TiO2/PBrPc/Tc/Au structure the following conclusions can be drawn: • A TiO2 film prepared on an ITO glass substrate using the sol–gel method constitutes a good optical window and it effectively transports electrons in our organic– inorganic solar cell. • Bromophosphorus phthalocyanine is a p-type organic material which exhibits interesting photoelectric properties. • The investigated system exhibits a strong rectification effect in the dark and an explicit photovoltaic effect under illumination. • Modulation of the layer thicknesses used may result in a significant improvement in the photovoltaic parameters of the system.
Acknowledgment The authors are grateful to Dr. A. Kempa for the synthesis of bromophosphorus phthalocyanine. This work was supported in part by KBN under Program No. 4T11B05722. References [1] J. Simon, J.-J. Andre´, Molecular Semiconductors, Springer, Berlin, 1985.
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