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Effects of water in phthalocyanine layers
Solar Energy Materials & Solar Cells 63 (2000) 15}21
E!ects of water in phthalocyanine layers H.R. Kerp*, E.E. van Faassen Section Interface Physics, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, Netherlands
Abstract The molecular water concentration inside zinc phthalocyanine (ZnPc) thin "lms was measured. After exposure to air, gas e!usion experiments show that the ZnPc layers contain (1.7$0.4);10 water molecules per cm, which corresponds to 1 H O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H O in ZnPc "lms. The mobile part e!uses out at room temperature when exposing the "lms to a low pressure of 10\ mbar, whereas temperature activation is needed to reach a complete out-di!usion of water. The e!usion process was observed to proceed with a di!usion coe$cient D of & (1.3$0.3);10\ cm s\ at 296 K. The rate of water e!usion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is re"lled from the bulk of water molecules during the e!usion process. 2000 Elsevier Science B.V. All rights reserved. Keywords: Phthalocyanine; Water; Di!usion process; Conductivity
1. Introduction Two important conditions for organic pigments, used in photovoltaic devices, are long-range energy transfer and e!ective charge carrier separation. Phthalocyanines (Pc's) are promising candidates for the active part of organic solar cells, because they exhibit a characteristic structural self-organization [1], which is re#ected in an e$cient energy migration in the form of exciton transport. In earlier work, we had determined the exciton di!usion length in zinc phthalocyanine (ZnPc) thin "lms [2,3] by analysing the photocurrent response of solar cells, consisting of n-type perylene
* Corresponding author. Tel.: #31-30-2532468; fax: #31-30-2543165. E-mail address: [email protected] (H.R. Kerp). 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 0 4 0 - 4
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H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
tetracarboxy diimide (PTCDI), and ZnPc as the p-type conducting antenna layer. Under normal working conditions, the electrical properties of the "lms are known to be largely a!ected by the in#uence of gaseous molecules entering the layers from ambient air. The intrinsic conductance of Pc layers is very low (&10\}10\ S cm\). It has been demonstrated that under ambient conditions, the conductivity of undoped metal Pc "lms can be raised several orders of magnitude [4,5]. This e!ect has been mainly attributed to molecular oxygen entering the "lms acting as a p-type dopant. The in#uence of water has been discussed less frequently, but should not be left out of consideration. For example, electrical analysis of p-type poly(3-methylthiophene) (P3MT) thin "lms [6] pointed out that the presence of water vapour leads to the chemical reduction of doping molecules. In this paper, we have applied gas e!usion measurements in order to determine the bulk concentration of water inside ZnPc "lms, as well as possible surface contributions. We have performed conductivity measurements at low pressure in order to relate the amount of e!used water to the "lm conductivity. Finally, we will discuss characteristic properties of the water di!usion process, like time scales and temperature dependence.
2. Experimental The water di!usion inside ZnPc thin "lms was investigated using gas e!usion and conductivity measurements. ZnPc was evaporated in vacuum (p&10\ Torr) by resistive heating of a molybdenum crucible. Before deposition, the substrates (silicon for the e!usion samples and Corning glass for the conductivity samples) were thoroughly rinsed with isopropanol, deionized water and blown dry with nitrogen. The thickness of the "lms was measured with a step pro"ler after deposition. The e!usion measurements took place in a two-chamber system (see Fig. 1), consisting of a sample chamber and a reference chamber, interconnected with a valve. The whole apparatus was connected to a turbo pump. After pumping down the reference chamber, the valve between the sample chamber and the reference chamber was brie#y opened. This resulted in a sudden pressure drop inside the sample chamber to a "nal pressure of ca. 10\ mbar, established within approximately 10 s. A di!erential pressure gauge measured the pressure di!erence between the chambers, which builds up upon degassing a sample in the sample chamber. Due to the small dimensions of the system (chamber volumes (1 cm), relatively small amounts of e!used molecules can be detected. The detection limit corresponds to approximately 5;10 e!used particles. For the conductivity measurements, two parallel silver contacts of 2;0.2 cm were evaporated onto the ZnPc "lms with a spacing of 0.5 mm between them. In this con"guration the surface conductivity p parallel to the sample surface could be , measured [7]. The junction between ZnPc and Ag was veri"ed to be ohmic. This is due to nearly equal workfunctions of the materials, which prevents a dominant contribution of the contact regions. The samples were placed in a vacuum chamber, where at 10 V bias potential the dark current through the electrodes was measured as
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
17
Fig. 1. Measurement set-up for e!usion experiments: di!erential pressure gauge measures pressure di!erence *p of reference chamber, relative to sample chamber after pumping down to &10\ mbar and closing the interconnecting valve.
a function of time after pumping down from 1 bar to 10\ mbar, approximately the same value of the pressure at the start of the e!usion experiments.
3. Results 3.1. Ewusion experiments In Fig. 2, the e!usion from a 2.9 lm thick ZnPc "lm is shown. The di!erential pressure *p on the vertical axis is converted to the number of e!used molecules n through the gas law *p<"nk¹, where < represents the volume of the sample chamber minus the volume of the sample, k is the Boltzmann constant and ¹ the absolute temperature. At room temperature, more than 2 mbar pressure builds up due to e!usion from an air-saturated sample. However, the amount of water e!usion from the ZnPc layer must be corrected for the systematic e!ect of non-completed evacuation of the sample chamber, which is re#ected by the "rst rapid component of *p(t). This was done by disregarding the initial pressure rise of 0.6 mbar, which was reproducibly measured as the residual pressure present in an empty sample chamber after 10 s of evacuation time. After ca. 20 min, the e!usion from the sample gradually ceases, indicating that the remaining gas molecules inside the sample have reached an
18
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
Fig. 2. Pressure di!erence of reference chamber relative to sample chamber as a result of exposing a 2.9 lm ZnPc "lm to &10\ mbar pressure. Before the measurement, the "lm had been exposed to ambient air with a relative humidity of ca. 50% for several days.
equilibrium distribution. Mass spectrometer analysis indicated that the e!used atmosphere, resulting from gas e!usion at this timescale, consists of water molecules. However, the total amount of water remaining inside the sample has not become very low. E!usion measurements at elevated temperatures point out that a signi"cant part of the water remains inside the ZnPc "lm at room temperature and comes out at elevated temperatures. In Fig. 2 it is seen that for the ZnPc layer, which had been degassed already two times consecutively at room temperature, leading to poor residual e!usion signals, a large e!usion takes place when increasing the temperature to 423 K. At around 360 K, saturation takes place, indicating that all water has di!used out of the sample. Apparently, a large part of the water in the ZnPc is "xed to a certain degree and only comes out of the "lm at elevated temperatures. The ratio of immobile water to the mobile fraction, di!using freely at room temperature, is about 1 : 1. After characterizing several "lms in the range of 1.8}7 lm and taking the average for the total water concentration, taking into account all e!usion steps, it follows that the bulk concentration of H O is (1.7$0.4);10 cm\. A surface contribution of &10 molecules cm\ was determined, which would imply considerably more than
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
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Fig. 3. Surface conductivity p as a function of time of ZnPc layers with variable thickness, during exposure to 10\ mbar air (starting at t"0) at 296 K. Before the measurements, the "lms had been exposed to ambient air for several days.
a monolayer coverage. The completely degassed "lm was exposed to pure nitrogen for several hours. As a result of this, no e!usion could be detected for temperatures between 296 and 423 K. From this, we conclude that N does not enter the layer. The di!usion coe$cient of water was found by performing numerical simulations of the e!usion as a function of time, using Fick's law for describing the di!usion of water inside the bulk of the layers. It was observed that this bulk di!usion coe$cient of water D determines the shape of the curves. The e!usion from all "lm thicknesses &was found to be described accurately by the same value for D of (1.3$0.3); &10\ cm s\. 3.2. Conductivity experiments The surface conductivity p was measured as a function of time of ZnPc layers with , di!erent thicknesses when being exposed to a stabilized pressure of ca. 10\ mbar ambient air. In Fig. 3 the changes in surface conductivity are plotted. Typically, p dropped by an order of magnitude during the experiment (from the 10\ to the , 10\ S range). For thin ZnPc "lms ((500 nm), the decreasing surface conductivity saturated within several minutes, whereas a 7 lm thick "lm showed electrical degradation during more than half an hour.
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H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
4. Conclusions Temperature-dependent e!usion measurements were used to determine the amount of water inside ZnPc layers. We found a high bulk H O concentration of (1.7$0.4); 10 molecules cm\, which corresponds to approximately one H O molecule per ten ZnPc molecules. In the following, we will discuss the obtained results and, in particular, focus on the implications for the surface conductivity of ZnPc layers. First of all, a mobile and an immobilized population of water inside the bulk of ZnPc "lms were observed. The mobile part, about one-half of the total, e!uses out at room temperature when exposing the "lms to a low pressure of 10\ mbar, whereas elevated temperatures are needed to reach a complete out-di!usion of water. This indicates the presence of di!erently bound species of water in the Pc "lms. We have observed that nitrogen does not di!use into the ZnPc layers. On the other hand, oxygen molecules are known to enter the material and establish a p-type doping [5,7,8]. As the values of the van der Waals radius of the three molecules N , O and H O are nearly equal, the possibility of gas di!usion through crystal voids and grain boundaries can be omitted. On the other hand, the selective admittance of O and H O in the layer points into the direction of electronic interactions between O and H O on the one hand and ZnPc on the other. The relatively high electronegativity of oxygen might play a role in this mechanism. We have demonstrated that it is possible to monitor the velocity of e!usion of H O from ZnPc "lms by means of surface conductivity changes during the e!usion process. Under low-pressure conditions, the rate at which the surface conductivity drops exhibits the same timescale as the rate of water e!usion from the same layers. This indicates the existence of an electrically active surface layer of water molecules, which is replenished from the pool of water molecules di!using inside the bulk of the layer as long as there is a negative density gradient of H O molecules towards the "lm surface. This is con"rmed by the fact that a surface contribution of ca. &10 molecules cm\ was determined for water. This obviously exceeds a monolayer coverage, which typically corresponds to 1;10 molecules cm\ for a clean surface. The water di!usion process inside the layer proceeds with a di!usion coe$cient D of (1.3$0.3);10\ cm s\ at room temperature. This was found by simula&ting the e!usion curves using Fick's law. The fact that the same bulk di!usion coe$cient was found to describe accurately the water e!usion from all layer thicknesses, indicates that the escape rate at the sample surface does not determine the velocity of the process. In particular, the e!usion seems unhindered by the water layer covering the ZnPc surface. Based on the results in this paper, it has become obvious that the presence of water in phthalocyanine layers has a pronounced e!ect on the conductive properties of the "lms, in particular at interfaces. Apparently, apart from oxygen, water also in#uences the electrical properties of the layers and both have to be taken into account in future research. Further investigations will be needed in order to completely monitor the e!ect of water on the interface properties of the layers. For application in solar cells, it will be important to investigate, e.g. the role of water in the formation of the barrier potential at the p}n junction.
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
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Acknowledgements We are indebted to R. Heller and R. Giliamse for using the set-up for e!usion measurements and for providing the "gure of the e!usion instrument. We thank the Netherlands Agency for Energy and the Environment (NOVEM) for "nancial support.
References [1] J. Kroon, R. Koehorst, M. van Dijk, G. Sanders, E. SudhoK lter, J. Mater. Chem. 7 (4) (1997) 615. [2] H. Kerp, E. van Faassen, Phys. Chem. Chem. Phys. 1 (1999) 1761}1763. [3] H. Kerp, H. Donker, R. Koehorst, T. Schaafsma, E. van Faassen, Chem. Phys. Lett. 298 (4}6) (1998) 302. [4] K.-Y. Law, Chem. Rev. 93 (1993) 449. [5] J. Simon, J.J. Andre, Molecular Semiconductors, Springer, Berlin, 1985. [6] S. Tagmouti, A. Oueriagli, A. Outzourhit, M. Khaidar, El.L. Ameziane, A. Yassar, H.K. Youssou", F. Garnier, Synth. Met. 88 (1997) 109. [7] H. Laurs, G. Heiland, Thin Solid Films 149 (1987) 129. [8] H. Yasunaga, K. Kojima, H. Yohda, K. Takeya, J. Phys. Soc. Jpn. 37 (4) (1974) 1024.
E!ects of water in phthalocyanine layers H.R. Kerp*, E.E. van Faassen Section Interface Physics, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, Netherlands
Abstract The molecular water concentration inside zinc phthalocyanine (ZnPc) thin "lms was measured. After exposure to air, gas e!usion experiments show that the ZnPc layers contain (1.7$0.4);10 water molecules per cm, which corresponds to 1 H O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H O in ZnPc "lms. The mobile part e!uses out at room temperature when exposing the "lms to a low pressure of 10\ mbar, whereas temperature activation is needed to reach a complete out-di!usion of water. The e!usion process was observed to proceed with a di!usion coe$cient D of & (1.3$0.3);10\ cm s\ at 296 K. The rate of water e!usion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is re"lled from the bulk of water molecules during the e!usion process. 2000 Elsevier Science B.V. All rights reserved. Keywords: Phthalocyanine; Water; Di!usion process; Conductivity
1. Introduction Two important conditions for organic pigments, used in photovoltaic devices, are long-range energy transfer and e!ective charge carrier separation. Phthalocyanines (Pc's) are promising candidates for the active part of organic solar cells, because they exhibit a characteristic structural self-organization [1], which is re#ected in an e$cient energy migration in the form of exciton transport. In earlier work, we had determined the exciton di!usion length in zinc phthalocyanine (ZnPc) thin "lms [2,3] by analysing the photocurrent response of solar cells, consisting of n-type perylene
* Corresponding author. Tel.: #31-30-2532468; fax: #31-30-2543165. E-mail address: [email protected] (H.R. Kerp). 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 0 4 0 - 4
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H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
tetracarboxy diimide (PTCDI), and ZnPc as the p-type conducting antenna layer. Under normal working conditions, the electrical properties of the "lms are known to be largely a!ected by the in#uence of gaseous molecules entering the layers from ambient air. The intrinsic conductance of Pc layers is very low (&10\}10\ S cm\). It has been demonstrated that under ambient conditions, the conductivity of undoped metal Pc "lms can be raised several orders of magnitude [4,5]. This e!ect has been mainly attributed to molecular oxygen entering the "lms acting as a p-type dopant. The in#uence of water has been discussed less frequently, but should not be left out of consideration. For example, electrical analysis of p-type poly(3-methylthiophene) (P3MT) thin "lms [6] pointed out that the presence of water vapour leads to the chemical reduction of doping molecules. In this paper, we have applied gas e!usion measurements in order to determine the bulk concentration of water inside ZnPc "lms, as well as possible surface contributions. We have performed conductivity measurements at low pressure in order to relate the amount of e!used water to the "lm conductivity. Finally, we will discuss characteristic properties of the water di!usion process, like time scales and temperature dependence.
2. Experimental The water di!usion inside ZnPc thin "lms was investigated using gas e!usion and conductivity measurements. ZnPc was evaporated in vacuum (p&10\ Torr) by resistive heating of a molybdenum crucible. Before deposition, the substrates (silicon for the e!usion samples and Corning glass for the conductivity samples) were thoroughly rinsed with isopropanol, deionized water and blown dry with nitrogen. The thickness of the "lms was measured with a step pro"ler after deposition. The e!usion measurements took place in a two-chamber system (see Fig. 1), consisting of a sample chamber and a reference chamber, interconnected with a valve. The whole apparatus was connected to a turbo pump. After pumping down the reference chamber, the valve between the sample chamber and the reference chamber was brie#y opened. This resulted in a sudden pressure drop inside the sample chamber to a "nal pressure of ca. 10\ mbar, established within approximately 10 s. A di!erential pressure gauge measured the pressure di!erence between the chambers, which builds up upon degassing a sample in the sample chamber. Due to the small dimensions of the system (chamber volumes (1 cm), relatively small amounts of e!used molecules can be detected. The detection limit corresponds to approximately 5;10 e!used particles. For the conductivity measurements, two parallel silver contacts of 2;0.2 cm were evaporated onto the ZnPc "lms with a spacing of 0.5 mm between them. In this con"guration the surface conductivity p parallel to the sample surface could be , measured [7]. The junction between ZnPc and Ag was veri"ed to be ohmic. This is due to nearly equal workfunctions of the materials, which prevents a dominant contribution of the contact regions. The samples were placed in a vacuum chamber, where at 10 V bias potential the dark current through the electrodes was measured as
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
17
Fig. 1. Measurement set-up for e!usion experiments: di!erential pressure gauge measures pressure di!erence *p of reference chamber, relative to sample chamber after pumping down to &10\ mbar and closing the interconnecting valve.
a function of time after pumping down from 1 bar to 10\ mbar, approximately the same value of the pressure at the start of the e!usion experiments.
3. Results 3.1. Ewusion experiments In Fig. 2, the e!usion from a 2.9 lm thick ZnPc "lm is shown. The di!erential pressure *p on the vertical axis is converted to the number of e!used molecules n through the gas law *p<"nk¹, where < represents the volume of the sample chamber minus the volume of the sample, k is the Boltzmann constant and ¹ the absolute temperature. At room temperature, more than 2 mbar pressure builds up due to e!usion from an air-saturated sample. However, the amount of water e!usion from the ZnPc layer must be corrected for the systematic e!ect of non-completed evacuation of the sample chamber, which is re#ected by the "rst rapid component of *p(t). This was done by disregarding the initial pressure rise of 0.6 mbar, which was reproducibly measured as the residual pressure present in an empty sample chamber after 10 s of evacuation time. After ca. 20 min, the e!usion from the sample gradually ceases, indicating that the remaining gas molecules inside the sample have reached an
18
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
Fig. 2. Pressure di!erence of reference chamber relative to sample chamber as a result of exposing a 2.9 lm ZnPc "lm to &10\ mbar pressure. Before the measurement, the "lm had been exposed to ambient air with a relative humidity of ca. 50% for several days.
equilibrium distribution. Mass spectrometer analysis indicated that the e!used atmosphere, resulting from gas e!usion at this timescale, consists of water molecules. However, the total amount of water remaining inside the sample has not become very low. E!usion measurements at elevated temperatures point out that a signi"cant part of the water remains inside the ZnPc "lm at room temperature and comes out at elevated temperatures. In Fig. 2 it is seen that for the ZnPc layer, which had been degassed already two times consecutively at room temperature, leading to poor residual e!usion signals, a large e!usion takes place when increasing the temperature to 423 K. At around 360 K, saturation takes place, indicating that all water has di!used out of the sample. Apparently, a large part of the water in the ZnPc is "xed to a certain degree and only comes out of the "lm at elevated temperatures. The ratio of immobile water to the mobile fraction, di!using freely at room temperature, is about 1 : 1. After characterizing several "lms in the range of 1.8}7 lm and taking the average for the total water concentration, taking into account all e!usion steps, it follows that the bulk concentration of H O is (1.7$0.4);10 cm\. A surface contribution of &10 molecules cm\ was determined, which would imply considerably more than
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
19
Fig. 3. Surface conductivity p as a function of time of ZnPc layers with variable thickness, during exposure to 10\ mbar air (starting at t"0) at 296 K. Before the measurements, the "lms had been exposed to ambient air for several days.
a monolayer coverage. The completely degassed "lm was exposed to pure nitrogen for several hours. As a result of this, no e!usion could be detected for temperatures between 296 and 423 K. From this, we conclude that N does not enter the layer. The di!usion coe$cient of water was found by performing numerical simulations of the e!usion as a function of time, using Fick's law for describing the di!usion of water inside the bulk of the layers. It was observed that this bulk di!usion coe$cient of water D determines the shape of the curves. The e!usion from all "lm thicknesses &was found to be described accurately by the same value for D of (1.3$0.3); &10\ cm s\. 3.2. Conductivity experiments The surface conductivity p was measured as a function of time of ZnPc layers with , di!erent thicknesses when being exposed to a stabilized pressure of ca. 10\ mbar ambient air. In Fig. 3 the changes in surface conductivity are plotted. Typically, p dropped by an order of magnitude during the experiment (from the 10\ to the , 10\ S range). For thin ZnPc "lms ((500 nm), the decreasing surface conductivity saturated within several minutes, whereas a 7 lm thick "lm showed electrical degradation during more than half an hour.
20
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
4. Conclusions Temperature-dependent e!usion measurements were used to determine the amount of water inside ZnPc layers. We found a high bulk H O concentration of (1.7$0.4); 10 molecules cm\, which corresponds to approximately one H O molecule per ten ZnPc molecules. In the following, we will discuss the obtained results and, in particular, focus on the implications for the surface conductivity of ZnPc layers. First of all, a mobile and an immobilized population of water inside the bulk of ZnPc "lms were observed. The mobile part, about one-half of the total, e!uses out at room temperature when exposing the "lms to a low pressure of 10\ mbar, whereas elevated temperatures are needed to reach a complete out-di!usion of water. This indicates the presence of di!erently bound species of water in the Pc "lms. We have observed that nitrogen does not di!use into the ZnPc layers. On the other hand, oxygen molecules are known to enter the material and establish a p-type doping [5,7,8]. As the values of the van der Waals radius of the three molecules N , O and H O are nearly equal, the possibility of gas di!usion through crystal voids and grain boundaries can be omitted. On the other hand, the selective admittance of O and H O in the layer points into the direction of electronic interactions between O and H O on the one hand and ZnPc on the other. The relatively high electronegativity of oxygen might play a role in this mechanism. We have demonstrated that it is possible to monitor the velocity of e!usion of H O from ZnPc "lms by means of surface conductivity changes during the e!usion process. Under low-pressure conditions, the rate at which the surface conductivity drops exhibits the same timescale as the rate of water e!usion from the same layers. This indicates the existence of an electrically active surface layer of water molecules, which is replenished from the pool of water molecules di!using inside the bulk of the layer as long as there is a negative density gradient of H O molecules towards the "lm surface. This is con"rmed by the fact that a surface contribution of ca. &10 molecules cm\ was determined for water. This obviously exceeds a monolayer coverage, which typically corresponds to 1;10 molecules cm\ for a clean surface. The water di!usion process inside the layer proceeds with a di!usion coe$cient D of (1.3$0.3);10\ cm s\ at room temperature. This was found by simula&ting the e!usion curves using Fick's law. The fact that the same bulk di!usion coe$cient was found to describe accurately the water e!usion from all layer thicknesses, indicates that the escape rate at the sample surface does not determine the velocity of the process. In particular, the e!usion seems unhindered by the water layer covering the ZnPc surface. Based on the results in this paper, it has become obvious that the presence of water in phthalocyanine layers has a pronounced e!ect on the conductive properties of the "lms, in particular at interfaces. Apparently, apart from oxygen, water also in#uences the electrical properties of the layers and both have to be taken into account in future research. Further investigations will be needed in order to completely monitor the e!ect of water on the interface properties of the layers. For application in solar cells, it will be important to investigate, e.g. the role of water in the formation of the barrier potential at the p}n junction.
H.R. Kerp, E.E. van Faassen / Solar Energy Materials & Solar Cells 63 (2000) 15}21
21
Acknowledgements We are indebted to R. Heller and R. Giliamse for using the set-up for e!usion measurements and for providing the "gure of the e!usion instrument. We thank the Netherlands Agency for Energy and the Environment (NOVEM) for "nancial support.
References [1] J. Kroon, R. Koehorst, M. van Dijk, G. Sanders, E. SudhoK lter, J. Mater. Chem. 7 (4) (1997) 615. [2] H. Kerp, E. van Faassen, Phys. Chem. Chem. Phys. 1 (1999) 1761}1763. [3] H. Kerp, H. Donker, R. Koehorst, T. Schaafsma, E. van Faassen, Chem. Phys. Lett. 298 (4}6) (1998) 302. [4] K.-Y. Law, Chem. Rev. 93 (1993) 449. [5] J. Simon, J.J. Andre, Molecular Semiconductors, Springer, Berlin, 1985. [6] S. Tagmouti, A. Oueriagli, A. Outzourhit, M. Khaidar, El.L. Ameziane, A. Yassar, H.K. Youssou", F. Garnier, Synth. Met. 88 (1997) 109. [7] H. Laurs, G. Heiland, Thin Solid Films 149 (1987) 129. [8] H. Yasunaga, K. Kojima, H. Yohda, K. Takeya, J. Phys. Soc. Jpn. 37 (4) (1974) 1024.