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Photoelectron spectroscopy investigation of thin metal films employed as top contacts in transparent organic solar cells
Thin Solid Films 519 (2011) 1872–1875
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Photoelectron spectroscopy investigation of thin metal films employed as top contacts in transparent organic solar cells S. Olthof ⁎, J. Meiss, B. Lüssem, M.K. Riede, K. Leo Institut für Angewandte Photophysik, Technische Universität Dresden, D-01062 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 21 February 2010 Received in revised form 27 September 2010 Accepted 20 October 2010 Available online 4 November 2010 Keywords: Organic solar cells Photoelectron spectroscopy Metal contact Transparent contact
a b s t r a c t The performance of transparent metal top contacts in organic solar cells can strongly be improved by employing surfactant layers. We use scanning electron microscopy to investigate the change in morphology upon insertion of an Al surfactant layer between 4,7-diphenyl-1,10-phenanthroline (BPhen) and a silver top contact. UV photoelectron spectroscopy measurements show the changes in energetic alignments at different steps of the organic/metal interface formation. Furthermore, using X-ray photoelectron spectroscopy depth profiling, we compare the differing intermixing processes happening within the two samples. Thereby, we can show that Al binds to BPhen molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic solar cells (OSC) have attracted considerable interest as a versatile and potential cost-efficient alternative to current inorganic solar cells, leading to efficiencies of 8.3% for a 1.1 cm2 small molecule device [1] and 8.1% for a 0.047 cm2 polymer device [2]. Current standard device architectures employ glass coated with tin-doped indium oxide (ITO) as transparent bottom electrode and a nontransparent metal layer as top electrode. Much effort is put into replacing ITO and developing OSC with transparent, ITO-free top electrodes. Possible materials include conductive polymers [3], aluminium-doped zinc oxide [4], Ag wires [5], or carbon nanotubes [6]. Recently, ultra-thin metal layers have emerged as alternative [7– 9]. It was found that top-illuminated OSC could be created using combinations of 12–14 nm silver deposited on 1 nm aluminium on the organic molecules as top contact [7,9]. Thereby, the morphology of noble metal layers is improved upon insertion of Al between organic materials and Ag or Au [9]. However, the details of this surfactant effect caused by the 1 nm Al have not been resolved. Previous theories assume doping of organic materials by Al, leading to increased conductivity [10], or to intermixing of noble metal and organic layers [11]. In this article, we present morphology and photoelectron spectroscopy (PES) studies to obtain detailed information on the interactions of these metals with an organic layer, employing material combinations that can be used to fabricate efficient OSC. We show that ⁎ Corresponding author. Tel.: +49 351 46339190. E-mail address: [email protected] (S. Olthof). URL: http://www.iapp.de (S. Olthof). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.10.051
Al binds to 4,7-diphenyl-1,10-phenanthroline (BPhen) molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. 2. Experimental details The samples for microscopy studies are fabricated in a custom-made ultra high vacuum (UHV) system (K.J. Lesker, UK) at a base pressure of 10− 8 mbar on Borofloat 33 glass substrates (Schott AG, Germany). These substrates are pre-cleaned using organic solvents followed by an oxygen plasma treatment. The organic material is evaporated out of a heated crucible and the rate and thickness are controlled by a quartz crystal. Only the topmost organic layer of the OSC stack with the corresponding top contact is fabricated, as the underlying organic layers have no further influence on the morphology. Therefore, the samples consist of 7 nm of BPhen (ABCR, Germany), three times sublimated, evaporated at a rate of 0.3 Å/s and the pure 15 nm Ag or 1 nm Al/14 nm Ag top contact evaporated at rates of 0.3 Å/s, as well. The morphology of the deposited top contacts is analyzed using scanning electron microscopy (SEM) (Zeiss DSM 982 Gemini). The samples for the X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS) measurements are prepared in a multichamber UHV evaporation tool (Bestec, Germany) having a base pressure of b 10− 8 mbar on glass coated with 90 nm of ITO (Thin Film Devices Inc., USA). The ITO is cleaned using organic solvents, followed by 4 minutes of sputter treatment with Ar ions at 1.3 kV under UHV conditions, to remove residual surface contamination and thereby improve the sticking behavior of the BPhen layer. The PES samples consist of 10 nm BPhen and the two different metal top
S. Olthof et al. / Thin Solid Films 519 (2011) 1872–1875
3. Results Fig. 1 shows SEM micrographs of two samples on glass, each consisting of 7 nm BPhen and either 15 nm Ag (left), or 1 nm Al surfactant and 14 nm Ag (right). It is obvious that the morphology is significantly different, with the sample containing no surfactant layer being rougher, while the 1 nm Al leads to more closed and smooth layers. Since the total metal layer thickness is the same (15 nm) for both cases and should be above the coalescence threshold [12], we attribute this to different processes on the atomic scale. This could be explained by the surfactant changing a Volmer–Weber [13] growth process (formation of large, isolated clusters or hillocks) to a smooth, Frank-van der Merve-like mechanism [14] where uniform and even layers of metal form and coalesce on the organics. Since these processes cannot be resolved by microscopy, PES is used for a more detailed understanding of the contact formation. The results of UPS measurements of sample 1 are shown in Fig. 2. Here, the left side shows the high binding energy cutoff (HBEC) that correlates to the position of the vacuum level, the middle graph shows the density of states (DOS) of the valence band region, and the right image is a magnification of the gap region of BPhen. The binding energy is with reference to the Fermi edge of the substrate. The BPhen layer (Fig. 2a) has a work function of 2.89 eV and a hole injection barrier of 3.76 eV, marked by the solid vertical line. After deposition of 1 nm of Ag (Fig. 2b), the DOS shows no significant change and the highest occupied molecular orbital (HOMO) of BPhen is still clearly visible, being just slightly shifted away from the Fermi energy by 200 meV. Two gap states show up at 0.36 eV and 1.65 eV that indicate the formation of charge transfer states between BPhen and Ag [15]. The Fermi edge of the Ag layer is not visible. This means that the Ag atoms corresponding to 1 nm layer thickness do not form a closed layer, which would shield the BPhen
HBEC
valence region
gap region
(a) BPhen (b) +1nm Ag (c) +14nm Ag (d) sputtered
1.81eV
intensity (arb.units)
contacts as described above. The PES measurements are performed with a Phoibos 100 setup (Specs, Germany) at a base pressure of 10− 10 mbar that is directly connected to the UHV evaporation tool. The setup is equipped with a helium discharge lamp with an excitation energy of 21.22 eV as well as an Al Kα X-ray source having an excitation of 1486.6 eV. To investigate the properties of the two different top contacts, PES is performed at different stages of the structure formation. First, the BPhen layer is characterized, followed by a deposition and measurement of either 1 nm Ag (sample 1) or 1 nm Al (sample 2). Subsequently, 14 nm Ag are evaporated onto both samples, which are then analyzed again. The data obtained by UPS and XPS are used to follow the evolution of the valence band features, the change in vacuum level and the core level shifts of the carbon 1s, aluminium 2p, and silver 3d states. After these measurements, the top layer is stepwise removed by Ar sputtering at 1.3 kV in intervals ranging from 30 to 90 s. Following each sputtering interval, XPS is used to track the relative intensities of the C, Al, and Ag core level peaks in order to gain insight into the chemical composition of the layer structure.
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(d) (c) (b) 0.2eV
0.39eV 20
18
16
14
6
gap states
(a) 4
2
0
4
2
0 =EF
binding energy (eV) Fig. 2. UPS spectra of sample 1. The left side shows the HBEC corresponding to the vacuum level. The middle graph displays the valence band features and the right side shows a magnification of the region within the BPhen gap. Shown are the measurements of 10 nm BPhen where the HOMO onset is marked by a solid vertical line (a), an additional 1 nm Ag (b), additional coverage by 14 nm Ag (c), and the same sample after a 30 s sputtering process (d).
signal in this very surface sensitive measurement technique, but almost fully penetrate into the BPhen bulk where they lead to an n-doping effect. After an additional coverage by 14 nm Ag (Fig. 2c) the DOS has changed, but still bears similarity to the BPhen features. The typical 4d valence states of Ag around 4.5 eV binding energy are not present, the Fermi energy is only weakly visible, and the work function of 2.59 eV is too low for an amorphous Ag bulk layer. Sample 2, where 1 nm of Al is inserted between the organic layer and the Ag top contact, exhibits a completely different behavior. The UPS spectra of BPhen in Fig. 3a are similar to the measurement in Fig. 2a. The evaporated thin Al interlayer shown in Fig. 3b leads to the formation of a Fermi edge and covers the BPhen well enough to suppress the HOMO features. This makes it difficult to quantify the shifting of its energy levels. From the C1s core level shift of 0.39 eV towards lower binding energy, measured by XPS, a p-type doping can be concluded. We do not expect the Al to have a p-doping effect in BPhen, thus the reason has to be the oxygen that is always incorporated when evaporating the very reactive Al, even at pressures of 10− 8 mbar. The 14 nm Ag evaporated onto this interlayer in curve (c) exhibits a work function of 3.8 eV, which is already much closer to the expected value of 4.26 eV [16] compared to sample 1 and shows the typical Ag states and a clear Fermi edge. To find out whether the reason for the differences in the two samples is an insufficient metal coverage, or a contamination of the surface, both samples were shortly sputtered for 30 s to remove the top few angstroms of material. The UPS spectra taken after this treatment are shown in Fig. 2d for sample 1 and in Fig. 3d for sample 2. Upon sputtering, a drastic modification is seen for sample 1; the work
Fig. 1. SEM micrographs of the two different metal overlayers on glass substrates covered by 7 nm thick BPhen layers. Left: covered by 15 nm Ag. Right: with 1 nm Al and 14 nm Ag.
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S. Olthof et al. / Thin Solid Films 519 (2011) 1872–1875
without 1 nm Al
with 1 nm Al
10000
peak intensity (counts/s)
1000
Fig. 3. UPS spectra of sample 2. The left side shows the HBEC corresponding to the vacuum level. The middle graph displays the valence band features and the right side shows a magnification of the region within the BPhen gap. Shown are the measurements of 10 nm BPhen where the HOMO onset is marked by a solid vertical line (a), an additional 1 nm Al (b), additional coverage by 14 nm Ag (c), and the same sample after a 30 s sputtering process (d).
function changes by 1.81 eV to reach 4.4 eV. The valence band features change as well, now resembling the valence band of Ag and the Fermi edge appears. We thus conclude that the reason for the reduced work function and suppression of the Ag DOS is mostly due to a surface contamination consisting of a thin layer of BPhen molecules. For sample 2, there is no change in the valence band features upon sputtering, as it already looked like a clean surface after the Ag deposition. However, a small amount of surface contamination is present as well, as the vacuum level changes by 0.5 eV, resulting in a work function of 4.31 eV. To obtain more information on the composition of both samples, the sputtering of the surface is continued. In this so called X-ray depth profiling, the topmost layers are successively removed by Argon sputtering and the relative peak intensities are measured by XPS depending on the distance from the former surface. It is, however, not straightforward to correlate the sputter time to an actual depth due to inhomogeneities in the sputter profile and the probing depth of a few nanometers when using XPS that can only give an average of the signal of the topmost layers. Furthermore, atoms are not always removed by the sputtering, but can partly be pushed into the underlying layers as well. However, even without exact knowledge of the actual depth, this method is well suited for a qualitative comparison of the composition between samples of similar structure. The results for the depth profiling of both samples are shown in Fig. 4 where the intensity axes show the peak areas after a Shirley background is subtracted from the measurement signal. For several sputter steps between 30 and 90 s duration, the intensity of the carbon and metal peaks is recorded. An estimation of when the metal top layer is removed can be achieved from the measurement of the Al signal; at 300 s sputter time this peak reaches its maximum, marked by the vertical line in Fig. 4. The intensity scale is chosen logarithmic in order to show the relevant behavior of the C1s peak intensity at low sputter depths. Before sputtering, sample 1 shows a higher C signal and lower Ag signal compared to sample 2 due to the layer of BPhen on top. After 30 s of sputtering, this layer is removed and both samples show nearly identical peak intensities of the Ag signal. However, the C intensity remains larger for the sample without the Al throughout the whole metal top contact. This suggests a contamination throughout the Ag layer by BPhen in sample 1 which is not present in sample 2. 4. Conclusion From the measurements we conclude that at low Ag coverages the metal atoms diffuse into the BPhen, leaving almost a bare BPhen
carbon peak 100 3000
aluminium peak
2000 1000 silver peak 100000
10000 0
50
100
150
200
250
300
350
400
450
sputter time (s) Fig. 4. X-ray depth profile measurement of sample 1 (without Al, empty circles) and sample 2 (with Al, filled triangles). The changes in C, Al and Ag peak intensities are plotted depending on the sputter time and therefore the distance from the initial surface.
surface behind. For larger thicknesses, a small concentration of BPhen molecules remains in the metal layer and a BPhen monolayer forms on top of the metal contact. Therefore, there is no abrupt interface from BPhen to Ag. In case of a thin Al interlayer, the interface is abrupt as the Al binds at the surface to the BPhen preventing a diffusion of the Ag atoms, such that a pure metal layer can form on top which is advantageous for the metallic behavior and smoothness as is shown by the SEM images. Therefore, organic solar cells employing Al surfactant layers between organic materials and ultra-thin Ag metal top electrodes exhibit superior performance compared to top electrodes consisting of only the Ag layer; previous experiments showed that improvements by over 50% are possible when comparing samples with and without the 1 nm Al surfactant [9]. We suppose that this intermixing between an organic layer and a noble metal top contact is a common effect. We observed the same kind of contamination by organic materials in PES investigations of Ag top contacts on MeO-TPD (N,N,N′,N′- tetrakis(4-methoxyphenyl)-benzidine) and α-NPD (N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine) as well. Acknowledgements The current work is supported by the Bundesministerium für Bildung und Forschung in the framework of the InnoProfile project (03IP602) and the R2flex project (13N8855). We thank Ellen Kern from the electrochemistry group of Technical University Dresden for assistance with SEM measurements.
References [1] 8.3% efficient small molecule OSC on 1.1 cm2 area, made by Heliatek/IAPP, certified at Fraunhofer ISE (Freiburg) (2010). See http://www.heliatek.com/ news-19. [2] 8.1% efficient polymer device made by Solarmer in 2010, certified by NREL. See http://www.solarmer.com/news.php (accessed Aug. 19th, 2010). [3] J. Meiss, C.L. Uhrich, K. Fehse, S. Pfuetzner, M.K. Riede, K. Leo, Proc. SPIE 7002 (2008) 700210. [4] K. Schulze, C. Uhrich, R. Schueppel, K. Leo, M. Pfeiffer, E. Brier, E. Reinhold, P. Baeuerle, Adv. Mater. 18 (2006) 2872. [5] J.Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Nano Lett. 8 (2) (2008) 689. [6] J. van de Lagemaat, T.M. Barnes, G. Rumbles, S.E. Shaneen, T.J. Coutts, C. Weeks, I. Levitsky, J. Peltola, P. Glatkowsky, Appl. Phys. Lett. 88 (2006) 233503. [7] J. Meiss, M.K. Riede, K. Leo, Appl. Phys. Lett. 94 (2009) 013303.
S. Olthof et al. / Thin Solid Films 519 (2011) 1872–1875 [8] T. Oyamada, Y. Sugawara, Y. Terao, H. Sasabe, C. Adachi, Jpn. J. Appl. Phys. 46 (4A) (2007) 1734. [9] J. Meiss, M.K. Riede, K. Leo, J. Appl. Phys. 105 (2009) 063108. [10] N.J. Watkins, L. Yan, Y. Gao, Appl. Phys. Lett. 80 (2002) 4384. [11] B. Jaeckel, J.B. Sambur, B.A. Parkinson, Langmuir 23 (2007) 11366.
[12] [13] [14] [15] [16]
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C. Granqvist, Sol. Energy Mater. Sol. Cells 91 (2007) 1529. M. Volmer, A. Weber, Z. Phys. Chem. 119 (1926) 277. F.C. Frank, J.H. van der Merwe, Proc. Roy. Sot. Lond. A 198 (1949) 205. C. Chan, W. Zhao, S. Barlow, S. Marder, A. Kahn, Org. Electron. 9 (2008) 575. H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Photoelectron spectroscopy investigation of thin metal films employed as top contacts in transparent organic solar cells S. Olthof ⁎, J. Meiss, B. Lüssem, M.K. Riede, K. Leo Institut für Angewandte Photophysik, Technische Universität Dresden, D-01062 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 21 February 2010 Received in revised form 27 September 2010 Accepted 20 October 2010 Available online 4 November 2010 Keywords: Organic solar cells Photoelectron spectroscopy Metal contact Transparent contact
a b s t r a c t The performance of transparent metal top contacts in organic solar cells can strongly be improved by employing surfactant layers. We use scanning electron microscopy to investigate the change in morphology upon insertion of an Al surfactant layer between 4,7-diphenyl-1,10-phenanthroline (BPhen) and a silver top contact. UV photoelectron spectroscopy measurements show the changes in energetic alignments at different steps of the organic/metal interface formation. Furthermore, using X-ray photoelectron spectroscopy depth profiling, we compare the differing intermixing processes happening within the two samples. Thereby, we can show that Al binds to BPhen molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic solar cells (OSC) have attracted considerable interest as a versatile and potential cost-efficient alternative to current inorganic solar cells, leading to efficiencies of 8.3% for a 1.1 cm2 small molecule device [1] and 8.1% for a 0.047 cm2 polymer device [2]. Current standard device architectures employ glass coated with tin-doped indium oxide (ITO) as transparent bottom electrode and a nontransparent metal layer as top electrode. Much effort is put into replacing ITO and developing OSC with transparent, ITO-free top electrodes. Possible materials include conductive polymers [3], aluminium-doped zinc oxide [4], Ag wires [5], or carbon nanotubes [6]. Recently, ultra-thin metal layers have emerged as alternative [7– 9]. It was found that top-illuminated OSC could be created using combinations of 12–14 nm silver deposited on 1 nm aluminium on the organic molecules as top contact [7,9]. Thereby, the morphology of noble metal layers is improved upon insertion of Al between organic materials and Ag or Au [9]. However, the details of this surfactant effect caused by the 1 nm Al have not been resolved. Previous theories assume doping of organic materials by Al, leading to increased conductivity [10], or to intermixing of noble metal and organic layers [11]. In this article, we present morphology and photoelectron spectroscopy (PES) studies to obtain detailed information on the interactions of these metals with an organic layer, employing material combinations that can be used to fabricate efficient OSC. We show that ⁎ Corresponding author. Tel.: +49 351 46339190. E-mail address: [email protected] (S. Olthof). URL: http://www.iapp.de (S. Olthof). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.10.051
Al binds to 4,7-diphenyl-1,10-phenanthroline (BPhen) molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. 2. Experimental details The samples for microscopy studies are fabricated in a custom-made ultra high vacuum (UHV) system (K.J. Lesker, UK) at a base pressure of 10− 8 mbar on Borofloat 33 glass substrates (Schott AG, Germany). These substrates are pre-cleaned using organic solvents followed by an oxygen plasma treatment. The organic material is evaporated out of a heated crucible and the rate and thickness are controlled by a quartz crystal. Only the topmost organic layer of the OSC stack with the corresponding top contact is fabricated, as the underlying organic layers have no further influence on the morphology. Therefore, the samples consist of 7 nm of BPhen (ABCR, Germany), three times sublimated, evaporated at a rate of 0.3 Å/s and the pure 15 nm Ag or 1 nm Al/14 nm Ag top contact evaporated at rates of 0.3 Å/s, as well. The morphology of the deposited top contacts is analyzed using scanning electron microscopy (SEM) (Zeiss DSM 982 Gemini). The samples for the X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS) measurements are prepared in a multichamber UHV evaporation tool (Bestec, Germany) having a base pressure of b 10− 8 mbar on glass coated with 90 nm of ITO (Thin Film Devices Inc., USA). The ITO is cleaned using organic solvents, followed by 4 minutes of sputter treatment with Ar ions at 1.3 kV under UHV conditions, to remove residual surface contamination and thereby improve the sticking behavior of the BPhen layer. The PES samples consist of 10 nm BPhen and the two different metal top
S. Olthof et al. / Thin Solid Films 519 (2011) 1872–1875
3. Results Fig. 1 shows SEM micrographs of two samples on glass, each consisting of 7 nm BPhen and either 15 nm Ag (left), or 1 nm Al surfactant and 14 nm Ag (right). It is obvious that the morphology is significantly different, with the sample containing no surfactant layer being rougher, while the 1 nm Al leads to more closed and smooth layers. Since the total metal layer thickness is the same (15 nm) for both cases and should be above the coalescence threshold [12], we attribute this to different processes on the atomic scale. This could be explained by the surfactant changing a Volmer–Weber [13] growth process (formation of large, isolated clusters or hillocks) to a smooth, Frank-van der Merve-like mechanism [14] where uniform and even layers of metal form and coalesce on the organics. Since these processes cannot be resolved by microscopy, PES is used for a more detailed understanding of the contact formation. The results of UPS measurements of sample 1 are shown in Fig. 2. Here, the left side shows the high binding energy cutoff (HBEC) that correlates to the position of the vacuum level, the middle graph shows the density of states (DOS) of the valence band region, and the right image is a magnification of the gap region of BPhen. The binding energy is with reference to the Fermi edge of the substrate. The BPhen layer (Fig. 2a) has a work function of 2.89 eV and a hole injection barrier of 3.76 eV, marked by the solid vertical line. After deposition of 1 nm of Ag (Fig. 2b), the DOS shows no significant change and the highest occupied molecular orbital (HOMO) of BPhen is still clearly visible, being just slightly shifted away from the Fermi energy by 200 meV. Two gap states show up at 0.36 eV and 1.65 eV that indicate the formation of charge transfer states between BPhen and Ag [15]. The Fermi edge of the Ag layer is not visible. This means that the Ag atoms corresponding to 1 nm layer thickness do not form a closed layer, which would shield the BPhen
HBEC
valence region
gap region
(a) BPhen (b) +1nm Ag (c) +14nm Ag (d) sputtered
1.81eV
intensity (arb.units)
contacts as described above. The PES measurements are performed with a Phoibos 100 setup (Specs, Germany) at a base pressure of 10− 10 mbar that is directly connected to the UHV evaporation tool. The setup is equipped with a helium discharge lamp with an excitation energy of 21.22 eV as well as an Al Kα X-ray source having an excitation of 1486.6 eV. To investigate the properties of the two different top contacts, PES is performed at different stages of the structure formation. First, the BPhen layer is characterized, followed by a deposition and measurement of either 1 nm Ag (sample 1) or 1 nm Al (sample 2). Subsequently, 14 nm Ag are evaporated onto both samples, which are then analyzed again. The data obtained by UPS and XPS are used to follow the evolution of the valence band features, the change in vacuum level and the core level shifts of the carbon 1s, aluminium 2p, and silver 3d states. After these measurements, the top layer is stepwise removed by Ar sputtering at 1.3 kV in intervals ranging from 30 to 90 s. Following each sputtering interval, XPS is used to track the relative intensities of the C, Al, and Ag core level peaks in order to gain insight into the chemical composition of the layer structure.
1873
(d) (c) (b) 0.2eV
0.39eV 20
18
16
14
6
gap states
(a) 4
2
0
4
2
0 =EF
binding energy (eV) Fig. 2. UPS spectra of sample 1. The left side shows the HBEC corresponding to the vacuum level. The middle graph displays the valence band features and the right side shows a magnification of the region within the BPhen gap. Shown are the measurements of 10 nm BPhen where the HOMO onset is marked by a solid vertical line (a), an additional 1 nm Ag (b), additional coverage by 14 nm Ag (c), and the same sample after a 30 s sputtering process (d).
signal in this very surface sensitive measurement technique, but almost fully penetrate into the BPhen bulk where they lead to an n-doping effect. After an additional coverage by 14 nm Ag (Fig. 2c) the DOS has changed, but still bears similarity to the BPhen features. The typical 4d valence states of Ag around 4.5 eV binding energy are not present, the Fermi energy is only weakly visible, and the work function of 2.59 eV is too low for an amorphous Ag bulk layer. Sample 2, where 1 nm of Al is inserted between the organic layer and the Ag top contact, exhibits a completely different behavior. The UPS spectra of BPhen in Fig. 3a are similar to the measurement in Fig. 2a. The evaporated thin Al interlayer shown in Fig. 3b leads to the formation of a Fermi edge and covers the BPhen well enough to suppress the HOMO features. This makes it difficult to quantify the shifting of its energy levels. From the C1s core level shift of 0.39 eV towards lower binding energy, measured by XPS, a p-type doping can be concluded. We do not expect the Al to have a p-doping effect in BPhen, thus the reason has to be the oxygen that is always incorporated when evaporating the very reactive Al, even at pressures of 10− 8 mbar. The 14 nm Ag evaporated onto this interlayer in curve (c) exhibits a work function of 3.8 eV, which is already much closer to the expected value of 4.26 eV [16] compared to sample 1 and shows the typical Ag states and a clear Fermi edge. To find out whether the reason for the differences in the two samples is an insufficient metal coverage, or a contamination of the surface, both samples were shortly sputtered for 30 s to remove the top few angstroms of material. The UPS spectra taken after this treatment are shown in Fig. 2d for sample 1 and in Fig. 3d for sample 2. Upon sputtering, a drastic modification is seen for sample 1; the work
Fig. 1. SEM micrographs of the two different metal overlayers on glass substrates covered by 7 nm thick BPhen layers. Left: covered by 15 nm Ag. Right: with 1 nm Al and 14 nm Ag.
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S. Olthof et al. / Thin Solid Films 519 (2011) 1872–1875
without 1 nm Al
with 1 nm Al
10000
peak intensity (counts/s)
1000
Fig. 3. UPS spectra of sample 2. The left side shows the HBEC corresponding to the vacuum level. The middle graph displays the valence band features and the right side shows a magnification of the region within the BPhen gap. Shown are the measurements of 10 nm BPhen where the HOMO onset is marked by a solid vertical line (a), an additional 1 nm Al (b), additional coverage by 14 nm Ag (c), and the same sample after a 30 s sputtering process (d).
function changes by 1.81 eV to reach 4.4 eV. The valence band features change as well, now resembling the valence band of Ag and the Fermi edge appears. We thus conclude that the reason for the reduced work function and suppression of the Ag DOS is mostly due to a surface contamination consisting of a thin layer of BPhen molecules. For sample 2, there is no change in the valence band features upon sputtering, as it already looked like a clean surface after the Ag deposition. However, a small amount of surface contamination is present as well, as the vacuum level changes by 0.5 eV, resulting in a work function of 4.31 eV. To obtain more information on the composition of both samples, the sputtering of the surface is continued. In this so called X-ray depth profiling, the topmost layers are successively removed by Argon sputtering and the relative peak intensities are measured by XPS depending on the distance from the former surface. It is, however, not straightforward to correlate the sputter time to an actual depth due to inhomogeneities in the sputter profile and the probing depth of a few nanometers when using XPS that can only give an average of the signal of the topmost layers. Furthermore, atoms are not always removed by the sputtering, but can partly be pushed into the underlying layers as well. However, even without exact knowledge of the actual depth, this method is well suited for a qualitative comparison of the composition between samples of similar structure. The results for the depth profiling of both samples are shown in Fig. 4 where the intensity axes show the peak areas after a Shirley background is subtracted from the measurement signal. For several sputter steps between 30 and 90 s duration, the intensity of the carbon and metal peaks is recorded. An estimation of when the metal top layer is removed can be achieved from the measurement of the Al signal; at 300 s sputter time this peak reaches its maximum, marked by the vertical line in Fig. 4. The intensity scale is chosen logarithmic in order to show the relevant behavior of the C1s peak intensity at low sputter depths. Before sputtering, sample 1 shows a higher C signal and lower Ag signal compared to sample 2 due to the layer of BPhen on top. After 30 s of sputtering, this layer is removed and both samples show nearly identical peak intensities of the Ag signal. However, the C intensity remains larger for the sample without the Al throughout the whole metal top contact. This suggests a contamination throughout the Ag layer by BPhen in sample 1 which is not present in sample 2. 4. Conclusion From the measurements we conclude that at low Ag coverages the metal atoms diffuse into the BPhen, leaving almost a bare BPhen
carbon peak 100 3000
aluminium peak
2000 1000 silver peak 100000
10000 0
50
100
150
200
250
300
350
400
450
sputter time (s) Fig. 4. X-ray depth profile measurement of sample 1 (without Al, empty circles) and sample 2 (with Al, filled triangles). The changes in C, Al and Ag peak intensities are plotted depending on the sputter time and therefore the distance from the initial surface.
surface behind. For larger thicknesses, a small concentration of BPhen molecules remains in the metal layer and a BPhen monolayer forms on top of the metal contact. Therefore, there is no abrupt interface from BPhen to Ag. In case of a thin Al interlayer, the interface is abrupt as the Al binds at the surface to the BPhen preventing a diffusion of the Ag atoms, such that a pure metal layer can form on top which is advantageous for the metallic behavior and smoothness as is shown by the SEM images. Therefore, organic solar cells employing Al surfactant layers between organic materials and ultra-thin Ag metal top electrodes exhibit superior performance compared to top electrodes consisting of only the Ag layer; previous experiments showed that improvements by over 50% are possible when comparing samples with and without the 1 nm Al surfactant [9]. We suppose that this intermixing between an organic layer and a noble metal top contact is a common effect. We observed the same kind of contamination by organic materials in PES investigations of Ag top contacts on MeO-TPD (N,N,N′,N′- tetrakis(4-methoxyphenyl)-benzidine) and α-NPD (N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine) as well. Acknowledgements The current work is supported by the Bundesministerium für Bildung und Forschung in the framework of the InnoProfile project (03IP602) and the R2flex project (13N8855). We thank Ellen Kern from the electrochemistry group of Technical University Dresden for assistance with SEM measurements.
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