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Substrate dependence of energy level alignment at the donor–acceptor interface in organic photovoltaic devices
Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 35–39
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Substrate dependence of energy level alignment at the donor–acceptor interface in organic photovoltaic devices Y.C. Zhou a , Z.T. Liu a , J.X. Tang b , C.S. Lee a,∗ , S.T. Lee a a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, PR China b Functional Nano & Soft Materials Laboratory (FUNSOM), Soochow University, Suzhou 215006, Jiangsu, PR China
a r t i c l e
i n f o
Article history: Available online 13 March 2009 Keywords: Organic photovoltaic device OPV Interface Energy level alignment
a b s t r a c t The interface energy level alignment between copper phthalocyanine (CuPC) and fullerene (C60), the widely studied donor–acceptor pair in organic photovoltaics (OPVs), on indium–tin oxide (ITO) and Mg substrate was investigated. The CuPC/C60 interface formed on ITO shows a nearly common vacuum level, but a dipole and band bending exist, resulting in a 0.8 eV band offset at the same interface on Mg. This observation indicates that the energy difference between the highest occupied molecular orbital of CuPC and the lowest unoccupied molecular orbital of C60, which dictates the open circuit voltage of the CuPC/C60 OPV, can be tuned by the work function of the substrate. Furthermore, the substrate effect on the energy alignment at the donor/acceptor interface can satisfactorily explain that a device with an anode of a smaller work function can provide a higher open circuit voltage. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past three decades, the interest on organic electronics has been expanding from organic light-emitting devices (OLEDs) to a wide range of applications including transistors, memory and photovoltaic devices, etc. In all these applications, interfaces of organic semiconductors have important influences on the device performance. Prof. W.R. Salaneck and Prof. K. Seki have made significant contributions on our current understanding on the interfaces of organic semiconductors. Of their many works, Seki and co-workers have pointed out that an organic material can have different Fermi level positions when it is deposited on different metal substrates [1]. While the importance of this work has been underestimated for many years, it is expected to have profound implications on performance of various organic electronic devices. Based on this work, Tang et al. [2,3] and Braun et al. [4] have shown that properties of organic heterojunction can be controlled via the substrates’ work function. Using organic photovoltaic (OPV) devices as an example, the present work demonstrates how the substrate effect can be used to tune the device characteristics. To maximize solar light absorption, organic semiconductors used as photoactive materials typically have a band gap value of about 2 eV, but the observed open circuit voltages (Voc ) of OPV devices are only in the range of 0.5–1 V. Analysis of the state-of-
∗ Corresponding author. E-mail address: [email protected] (C.S. Lee). 0368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2009.02.018
the-art OPV devices reveals that the efficiency is limited by the low Voc [5,6]. Following the classical thin-film solar cell concept or the metal–insulator–metal (MIM) model, Voc is simply equal to the work function difference between the two electrodes [7]. However, the validity of this model is debatable for OPVs based on fullerene (C60) and its derivatives, considering the strong Fermi level pinning [8] at the interface between C60 and the metal cathode. Via a model based on the Marcus theory for electron transfer, and the generalized Shockley theory for the dark current density, Rand et al. [9] suggested that the maximum value of Voc is a function of the difference between the donor’s ionization potential (IP) and the acceptor’s electron affinity (EA), minus the binding energy of the electron-hole pair. Based on this concept, boron subphthalocyanine chloride (SubPC) with a high IP than CuPC was used to obtain a larger Voc for an OPV device based on the SubPC- or CuPC–C60 [10] configuration. Scharber et al. [6] studied the relation between Voc and the energy levels of the donor–acceptor blend of 26 different polymeric OPV devices, and obtained a similar equation Voc =
1 [EHOMO (Donor) − ELUMO (PCBM)] − 0.3V e
(1)
However, in contrast to the strong Fermi level pinning at the cathode side, the anode work function does have some influence on the Voc of devices. For example, Bhosle et al. [11] found that devices based on GaZnO, which has a work function lower than ITO, yielded a higher Voc . Similar phenomena were also reported by Destruel et al. [12] and Hong et al. [13]. Devices fabricated on ozone-treated ITO with a higher work function were found to have a smaller Voc than those devices fabricated on untreated ITO with
36
Y.C. Zhou et al. / Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 35–39
a lower work function. These observations are inconsistent with the IPDonor –EAAcceptor model, which suggests that the Voc should be independent of the electrode work function, whereas the MIM model would predict just the opposite trend to the experimental observations. It is thus evident that the mechanism of the dependence of Voc on the anode work function remains unclear. In the (IPDonor –EAAcceptor ) model for estimating the Voc , alignment of vacuum levels at the donor–acceptor interface (i.e. the Schottky–Mott description) is implicitly assumed. However, such an assumption is at variance with the results of ultraviolet photoemission spectroscopy (UPS) and Kelvin Probe measurements showing that the vacuum level offset at metal–organic interfaces can be as much as 1 eV, clearly revealing the invalidity of the Schottky–Mott relation. On the other hand, the weak intermolecular van der Waals interaction between organic molecules has led to the belief that the majority of undoped organic/organic (O–O) interfaces are well-described by the vacuum level alignment model. However, recent works have shown that in some cases, such as for tris(8-hydroxyquinoline) aluminum (Alq3)–CuPC [2,3] and (4,4 N,N -dicarbazolyl-biphenyl) (CBP)–(4,4,4 -tris [3-methyl-phenyl (phenyl) amino]-triphenylamine) (m-MTDATA) [4], the energy level alignment at the O–O interface is strongly influenced by the substrate work function. Therefore, it is of interest to study the energy level alignment for a wider range of donor–acceptor interfaces in order to obtain a better perspective on the influence of the substrate work function on different organic electronic devices. In this work, the interface energy level alignment between CuPC and C60, the widely studied donor–acceptor pair in OPV devices, on ITO and Mg was investigated by combined X-ray photoemission spectroscopy (XPS) and UPS measurements. On an ITO substrate with a high work function of 4.95 eV, the C60/CuPC interface shows nearly a common vacuum level, whereas on a Mg substrate with a low work function of 3.68 eV, a considerable dipole and band bending exist at the interface, resulting in a 0.8 eV shift of the vacuum level. These show that the change of C60/CuPC interface energy level alignment induced by the substrate work function can lead to a tunable value of energy difference between HOMOCuPC and LUMOC60 . 2. Experimental Experiments were carried out in a VG ESCALAB 220i-XL ultrahigh vacuum (UHV) surface analysis system. Details concerning the interconnected analysis and deposition vacuum systems and the sample preparation procedure have been published elsewhere [2,3]. Solvent-cleaned ITO anode was treated in UV-ozone for 25 min immediately before loading into the ultrahigh vacuum chamber. Fresh Mg surface was obtained by depositing Mg in situ on Si substrates in the deposition chamber. Organics were deposited in situ onto ITO or Mg substrates, respectively, with the film thickness and evaporation rate monitored via a quartz crystal microbalance. The samples were transferred to the analysis chamber for measurements after each deposition step. UPS using a He I radiation (21.22 eV) source was performed to measure the valence states and the vacuum level positions with an energy resolution of 90 meV. To observe the secondary electron cutoff, samples were biased at −5 V. With a monochromatic Al K␣ source (1486.6 eV), XPS measurements were applied to study the possible interfacial chemical reactions and the development of molecular level bending across the interfaces. The XPS high-binding-energy cutoff spectra were measured, also with −5 V bias to observe the secondary electron cutoff. 3. Results and discussion To investigate the CuPC–C60 interface on Mg, a 60 Å thick CuPC film was first deposited onto the Mg substrate, then a 100 Å thick
Fig. 1. (a) Secondary electron cutoff, (b) HOMO peak of UPS He I spectra as a function of increasing C60 coverage on CuPC on the Mg substrate. With increasing C60 thickness, energy levels shift to lower binding energy. The corresponding XPS secondary electrons cutoffs as shown in (c). The agreement between UPS and XPS secondary electrons cutoffs suggests the observed energy level shift are contributions of interface dipole or band bending and not due to sample charging effect.
C60 layer was deposited on the CuPC layer in several steps. Fig. 1(a) and (b) shows the secondary cutoff and the HOMO region of UPS spectra from the Mg substrate, the CuPC layer, and the C60 layer with a thickness ranging from 2 to 100 Å. In the spectrum of the Mg substrate, the Fermi edge can clearly be seen. Typical CuPC features emerge after depositing 60 Å CuPC layer, where the features of the Mg substrate beneath are completely suppressed. Upon deposition of the first 2 Å of C60, the secondary electron cutoff shifts by 0.13 eV to lower binding energy corresponding to an increase in the work function of the sample surface, which is mainly caused by the appearance of an interface dipole. During this initial work function increase, the shape of the entire UPS spectrum changed slightly, indicating a small C60 coverage can build up an interface dipole. Upon increasing C60 coverage, the characteristic features of CuPc and C60 show progressive shifts towards the lower binding energy (BE) region, accompanied by a 0.81 eV rising of the vacuum level. Since organics are usually poor conductors, an organic layer of about 200 Å could lead to significant charging when performing photoelectron spectroscopies [14]. Charging effect should be prevented, and energy level shift due to the charging should be distinguished from the interface electronic structures, like interface dipole or band bending. At metal–organic (M–O) interfaces, charging effect usually is minimal in the intermediate interface at M–O contact, ranging from sub-monolayer to a few monolayers. However, when dealing with organic–organic (O–O) interfaces, precaution should be paid to confirm that the energy level shift at the interface is not caused by charging, because we typically work on samples with organic layers twice as thick as that in the M–O interface researches. Using an electron flood gun is not an ideal means for eliminating the charging effect due to possible irradiation damages to the organic molecules. Similar to the high BE cutoff edges in UPS spectra, the high BE cutoff edges in XPS spectra also give the vacuum energy level. Due to the several orders of magnitudes of intensity difference between the X-ray and the ultraviolet photon sources, differences in the positions of the high BE cutoff edges in the UPS and XPS spectra would sensitively indicate charging [14]. Fig. 1(c) depicts the measured XPS cutoffs, which show nearly the same shifts with the UPS cutoffs. The agreement of vacuum levels from both UPS and XPS measurements suggests that there was little or no observable charging. Thus, the observed vacuum level shifts can be reliably attributed to the interface dipole and band bending at the CuPC–C60 interface.
Y.C. Zhou et al. / Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 35–39
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Fig. 2. (a) Cu2p, (b) N1s, and (c) C1s XPS spectra as a function of increasing C60 coverage on CuPC on the Mg substrate. With increasing C60 thickness, energy levels shift to lower binding energy with the same magnitude. The C1s peak of CuPC and C60 are obtained by de-convoluting the measured spectra into the two contributions, and the de-convolution of 10 Å C60 on CuPC is shown in (d) as an example.
Due to the large IP difference between CuPC and C60, their HOMO peaks do not overlap and can be distinguished. Upon increasing C60 coverage, HOMOs of CuPC and C60 shift respectively by 0.14 and 0.32 eV to lower binding energy, indicating an upward band bending in both layers. These band bendings are reasonable because the CuPC layer works as donor and the C60 layer as acceptor respectively, hence at the interface CuPC molecules are expected
Fig. 3. Comparison of energy level shifts with the increasing C60 thickness on CuPC on Mg. All the measured energy levels of CuPC, including HOMO (CuPC), C1s (CuPC), N1s, and Cu2p, show nearly the same shift with the increase of C60 coverage; so do the energy levels of C60, implying the presence of band bending in the organic layers. The vacuum levels obtained via UPS and XPS are consistent with each other. The difference between the shift of vacuum level and the band bending is regarded as interface dipole.
to be positively charged and C60 molecules negatively charged. The difference in the magnitude of band bending in the two layers may be ascribed to different charge density. According to the model of band bending, energy level shifts come from the space charge effect, and both valance levels and core levels are expected to shift with the same magnitude. Fig. 2 shows the evolution of Cu2p, N1s, and C1s core levels with increasing C60 coverage. Cu2p and N1s peaks (from the CuPC layer) show a shift to lower binding energy; but they do not show new component or broadening, except for increasing intensity attenuation with increasing C60 coverage, indicating little chemical reaction between CuPC and C60. The C1s spectra have several components.
Fig. 4. (a) Secondary electron cutoff, (b) HOMO peak of UPS He I spectra as a function of increasing C60 coverage on CuPC on the ITO substrate. With increasing C60 thickness, energy levels shifts are negligible.
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Fig. 5. Energy level diagrams of C60/CuPC interfaces on (a) Mg and (b) ITO. All the numbers are in eV.
Adopting the approach suggested by Molodtsova and Knupfer [15], the C1s spectra were least-square fitted in order to analyze contributions to the photoemission signal stemming from the CuPc and C60 components. The fitting was carried out with the XPSPEAK program [16]. Fig. 2d shows the de-convoluted peaks of the C1s spectrum of 10 Å C60 on CuPC as an example. The C1s spectrum of a pure CuPC layer consists of three peaks attributed respectively to the aromatic carbon of the benzene rings, the pyrrole carbon linked to nitrogen, and the p–p* satellite of the latter [17]. The peak positions and relative intensities of both the CuPC and the C60 components were thus obtained by fitting while keeping the peak shape of C1s (CuPC) and C1s (C60) unchanged. For the C1s levels, both the CuPC and the C60 components show shifts to lower binding energy, as observed in Cu2p, N1s core levels and valence levels like HOMO. In Fig. 3, variation of all the energy levels with thickness are plotted, including vacuum level (UPS), vacuum level (XPS), HOMO peak (CuPC), HOMO peak (C60), C1s (CuPC), Cu2p (CuPC), N1s (CuPC), and C1s (C60) to help understand the alignment of energy levels. It can be seen that the energy levels from CuPC are parallel to each other, so do those from C60, clearly indicating the existence of band bending of 0.14 eV in CuPC and 0.32 eV in C60. The difference between vacuum level shift (0.81 eV) and band bending can be ascribed to an interface dipole of 0.35 eV at the CuPC–C60 contact. A detailed energy level diagram is shown in Fig. 5a. Similar experiments are performed on an UV-ozone-treated ITO substrate (˚ = 4.95 eV). In sharp contrast to that on the Mg substrate, the increase of C60 coverage does not result in a shift of vacuum level or HOMO, as shown in Fig. 4, and the related core levels of CuPC and C60 are not shifted (not shown here) either. In this case, the Schottky–Mott model describes the interface well. We also observe that the exact amount of vacuum level offset at the CuPu/C60 interface is sensitive to the ITO surface condition. On ITO with a lower work function (˚ = 4.65 eV), a vacuum level offset of 0.45 eV was observed at the CuPC–C60 interface, in the same direction as on Mg substrates [18]. It is remarkable that the energy level alignment at the CuPC/C60 interface is substrate dependent. As shown in Fig. 5, on both Mg and ITO substrates, the Fermi level is pinned at about 0.2–0.3 eV below the LUMO of C60; however the energy levels of CuPC shift as the substrate work function changes. Note that the transition band gap of CuPC and C60 is taken from Ref. [19]. The extent of Fermi level movement in the energy band gap can be described by the interface slope parameter [10–22], defined as s = −dh /dm , where m is the metal work function, h the hole injection barrier at the
M–O interface. The Schottky–Mott model corresponds to the case when s = 1, but for the extreme case of Fermi level pinning, s = 0. It has been shown that strong Fermi level pinning occurs at the metal–C60 interface (s = 0), consistent with the present results [23]. However for the metal–CuPC interface, the slope parameter is not zero (∼0.5) [21]. The different slope parameters may be responsible for the dependence of the energy level alignment at the CuPC–C60 interface on the substrate work function; so that when the substrate work function changes, the energy levels of CuPC are shifted (to some extent), while the energy levels of C60 are pinned. The energy difference between the HOMO of CuPC (donor) and LUMO of C60 (acceptor), which is believed to dominantly determine the open circuit voltage of the CuPC/C60 OPV, increases from 0.66 to 1.03 eV when the substrate work function decreases from 4.95 eV (ITO) to 3.68 eV (Mg). This tendency is consistent with Voc dependence on the anode work function previously reported. Therefore, we conclude that in an OPV device vacuum levels are not necessarily aligned between the donor and acceptor layer, and may be influenced by the substrate work function, i.e., a lower work function tends to result in a downward shift of donor energy levels. This substrate effect in the IPDonor –EAAcceptor model can offer a satisfactory explanation for the Voc dependence on the anode work function, and a potential way for tuning the Voc via choosing the substrate. 4. Conclusions In conclusion, the interface energy level alignment at the CuPC/C60 interface on ITO and Mg substrates was investigated by combined XPS and UPS measurements. On ITO the CuPC/C60 interface shows nearly a common vacuum level; whereas the same interface on Mg shows a considerable dipole and band bending, resulting in a 0.8 eV shift of the vacuum level. The substrate work function dependence of C60/CuPC energy level alignment is understandable in terms of the different slope parameters of CuPC and C60. This substrate dependence leads to a tunable value of energy difference between HOMOCuPC and LUMOC60 , and can satisfactorily explain the observed dependence of Voc on the substrate work function. Acknowledgement This work is supported by the Research Grants Council of Hong Kong, Hong Kong SAR (CERG Project: CityU 101707).
Y.C. Zhou et al. / Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 35–39
References [1] H. Ishii, N. Hayashi, E. Ito, Y. Washizu, K. Sugi, Y. Kimura, M. Niwano, Y. Ouchi, K. Seki, Phys. Stat. Sol. A 201 (2004) 1075. [2] J.X. Tang, K.M. Lau, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 88 (2007) 232103. [3] J.X. Tang, C.S. Lee, S.T. Lee, J. Appl. Phys. 101 (2007) 064504. [4] S. Braun, M.P. de Jong, W. Osikowicz, W.R. Salaneck, Appl. Phys. Lett. 91 (2007) 202108. [5] P. Schilinsky, C. Waldauf, C.J. Brabec, Appl. Phys. Lett. 81 (2002) 3885. [6] M.C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J. Brabec, Adv. Mater. 18 (2006) 789. [7] S. Karg, W. Riess, V. Dyakonov, M. Schwoerer, Synth. Met. 54 (1993) 427. [8] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanchez, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 374. [9] B.P. Rand, P.D. Burk, S.R. Forrest, Phys. Rev. B 75 (2007) 115327. [10] K.L. Mutolo, E.I. Mayo, B.P. Rand, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 128 (2006) 8108. [11] V. Bhosle, J.T. Prater, F. Yang, D. Burk, S.R. Forrest, J. Narayan, J. Appl. Phys. 102 (2007) 023501.
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[12] P. Destruel, H. Bock, I. Séguy, P. Jolinat, M. Oukachmih, E. Bedel-Pereira, Polym. Int. 55 (2005) 601. [13] Z.R. Hong, C.J. Liang, X.Y. Sun, X.T. Zeng, J. Appl. Phys. 100 (2006) 093711. [14] R. Schlaf, C.D. Merritt, L.A. Crisafulli, Z.H. Kafafi, J. Appl. Phys. 86 (1999) 5678. [15] O.V. Molodtsova, M. Knupfer, J. Appl. Phys. 99 (2006) 053704. [16] http://www.phy.cuhk.edu.hk/∼surface/XPSPEAK/. [17] H. Peisert, M. Knupfer, T. Schwieger, J.M. Auerhammer, M.S. Golden, J. Fink, J. Appl. Phys. 91 (2002) 4872. [18] J.X. Tang, Y.C. Zhou, Z.T. Liu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 93 (2008) 043512. [19] W.R. Salaneck, K. Seki, A. Kahn, J.-J. Pireaux (Eds.), Conjugated Polymers and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications, Marcel Dekker, New York, 2002. [20] A. Kahn, N. Koch, W.Y. Gao, J. Polym. Sci. Part B 41 (2003) 2529. [21] J.X. Tang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 87 (2005) 252110. [22] W. Mönch, Appl. Phys. Lett. 88 (2006) 112116. [23] N. Hayashi, H. Ishii, Y. Ouchi, K. Seki, J. Appl. Phys. 92 (2002) 3784.
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Substrate dependence of energy level alignment at the donor–acceptor interface in organic photovoltaic devices Y.C. Zhou a , Z.T. Liu a , J.X. Tang b , C.S. Lee a,∗ , S.T. Lee a a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, PR China b Functional Nano & Soft Materials Laboratory (FUNSOM), Soochow University, Suzhou 215006, Jiangsu, PR China
a r t i c l e
i n f o
Article history: Available online 13 March 2009 Keywords: Organic photovoltaic device OPV Interface Energy level alignment
a b s t r a c t The interface energy level alignment between copper phthalocyanine (CuPC) and fullerene (C60), the widely studied donor–acceptor pair in organic photovoltaics (OPVs), on indium–tin oxide (ITO) and Mg substrate was investigated. The CuPC/C60 interface formed on ITO shows a nearly common vacuum level, but a dipole and band bending exist, resulting in a 0.8 eV band offset at the same interface on Mg. This observation indicates that the energy difference between the highest occupied molecular orbital of CuPC and the lowest unoccupied molecular orbital of C60, which dictates the open circuit voltage of the CuPC/C60 OPV, can be tuned by the work function of the substrate. Furthermore, the substrate effect on the energy alignment at the donor/acceptor interface can satisfactorily explain that a device with an anode of a smaller work function can provide a higher open circuit voltage. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past three decades, the interest on organic electronics has been expanding from organic light-emitting devices (OLEDs) to a wide range of applications including transistors, memory and photovoltaic devices, etc. In all these applications, interfaces of organic semiconductors have important influences on the device performance. Prof. W.R. Salaneck and Prof. K. Seki have made significant contributions on our current understanding on the interfaces of organic semiconductors. Of their many works, Seki and co-workers have pointed out that an organic material can have different Fermi level positions when it is deposited on different metal substrates [1]. While the importance of this work has been underestimated for many years, it is expected to have profound implications on performance of various organic electronic devices. Based on this work, Tang et al. [2,3] and Braun et al. [4] have shown that properties of organic heterojunction can be controlled via the substrates’ work function. Using organic photovoltaic (OPV) devices as an example, the present work demonstrates how the substrate effect can be used to tune the device characteristics. To maximize solar light absorption, organic semiconductors used as photoactive materials typically have a band gap value of about 2 eV, but the observed open circuit voltages (Voc ) of OPV devices are only in the range of 0.5–1 V. Analysis of the state-of-
∗ Corresponding author. E-mail address: [email protected] (C.S. Lee). 0368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2009.02.018
the-art OPV devices reveals that the efficiency is limited by the low Voc [5,6]. Following the classical thin-film solar cell concept or the metal–insulator–metal (MIM) model, Voc is simply equal to the work function difference between the two electrodes [7]. However, the validity of this model is debatable for OPVs based on fullerene (C60) and its derivatives, considering the strong Fermi level pinning [8] at the interface between C60 and the metal cathode. Via a model based on the Marcus theory for electron transfer, and the generalized Shockley theory for the dark current density, Rand et al. [9] suggested that the maximum value of Voc is a function of the difference between the donor’s ionization potential (IP) and the acceptor’s electron affinity (EA), minus the binding energy of the electron-hole pair. Based on this concept, boron subphthalocyanine chloride (SubPC) with a high IP than CuPC was used to obtain a larger Voc for an OPV device based on the SubPC- or CuPC–C60 [10] configuration. Scharber et al. [6] studied the relation between Voc and the energy levels of the donor–acceptor blend of 26 different polymeric OPV devices, and obtained a similar equation Voc =
1 [EHOMO (Donor) − ELUMO (PCBM)] − 0.3V e
(1)
However, in contrast to the strong Fermi level pinning at the cathode side, the anode work function does have some influence on the Voc of devices. For example, Bhosle et al. [11] found that devices based on GaZnO, which has a work function lower than ITO, yielded a higher Voc . Similar phenomena were also reported by Destruel et al. [12] and Hong et al. [13]. Devices fabricated on ozone-treated ITO with a higher work function were found to have a smaller Voc than those devices fabricated on untreated ITO with
36
Y.C. Zhou et al. / Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 35–39
a lower work function. These observations are inconsistent with the IPDonor –EAAcceptor model, which suggests that the Voc should be independent of the electrode work function, whereas the MIM model would predict just the opposite trend to the experimental observations. It is thus evident that the mechanism of the dependence of Voc on the anode work function remains unclear. In the (IPDonor –EAAcceptor ) model for estimating the Voc , alignment of vacuum levels at the donor–acceptor interface (i.e. the Schottky–Mott description) is implicitly assumed. However, such an assumption is at variance with the results of ultraviolet photoemission spectroscopy (UPS) and Kelvin Probe measurements showing that the vacuum level offset at metal–organic interfaces can be as much as 1 eV, clearly revealing the invalidity of the Schottky–Mott relation. On the other hand, the weak intermolecular van der Waals interaction between organic molecules has led to the belief that the majority of undoped organic/organic (O–O) interfaces are well-described by the vacuum level alignment model. However, recent works have shown that in some cases, such as for tris(8-hydroxyquinoline) aluminum (Alq3)–CuPC [2,3] and (4,4 N,N -dicarbazolyl-biphenyl) (CBP)–(4,4,4 -tris [3-methyl-phenyl (phenyl) amino]-triphenylamine) (m-MTDATA) [4], the energy level alignment at the O–O interface is strongly influenced by the substrate work function. Therefore, it is of interest to study the energy level alignment for a wider range of donor–acceptor interfaces in order to obtain a better perspective on the influence of the substrate work function on different organic electronic devices. In this work, the interface energy level alignment between CuPC and C60, the widely studied donor–acceptor pair in OPV devices, on ITO and Mg was investigated by combined X-ray photoemission spectroscopy (XPS) and UPS measurements. On an ITO substrate with a high work function of 4.95 eV, the C60/CuPC interface shows nearly a common vacuum level, whereas on a Mg substrate with a low work function of 3.68 eV, a considerable dipole and band bending exist at the interface, resulting in a 0.8 eV shift of the vacuum level. These show that the change of C60/CuPC interface energy level alignment induced by the substrate work function can lead to a tunable value of energy difference between HOMOCuPC and LUMOC60 . 2. Experimental Experiments were carried out in a VG ESCALAB 220i-XL ultrahigh vacuum (UHV) surface analysis system. Details concerning the interconnected analysis and deposition vacuum systems and the sample preparation procedure have been published elsewhere [2,3]. Solvent-cleaned ITO anode was treated in UV-ozone for 25 min immediately before loading into the ultrahigh vacuum chamber. Fresh Mg surface was obtained by depositing Mg in situ on Si substrates in the deposition chamber. Organics were deposited in situ onto ITO or Mg substrates, respectively, with the film thickness and evaporation rate monitored via a quartz crystal microbalance. The samples were transferred to the analysis chamber for measurements after each deposition step. UPS using a He I radiation (21.22 eV) source was performed to measure the valence states and the vacuum level positions with an energy resolution of 90 meV. To observe the secondary electron cutoff, samples were biased at −5 V. With a monochromatic Al K␣ source (1486.6 eV), XPS measurements were applied to study the possible interfacial chemical reactions and the development of molecular level bending across the interfaces. The XPS high-binding-energy cutoff spectra were measured, also with −5 V bias to observe the secondary electron cutoff. 3. Results and discussion To investigate the CuPC–C60 interface on Mg, a 60 Å thick CuPC film was first deposited onto the Mg substrate, then a 100 Å thick
Fig. 1. (a) Secondary electron cutoff, (b) HOMO peak of UPS He I spectra as a function of increasing C60 coverage on CuPC on the Mg substrate. With increasing C60 thickness, energy levels shift to lower binding energy. The corresponding XPS secondary electrons cutoffs as shown in (c). The agreement between UPS and XPS secondary electrons cutoffs suggests the observed energy level shift are contributions of interface dipole or band bending and not due to sample charging effect.
C60 layer was deposited on the CuPC layer in several steps. Fig. 1(a) and (b) shows the secondary cutoff and the HOMO region of UPS spectra from the Mg substrate, the CuPC layer, and the C60 layer with a thickness ranging from 2 to 100 Å. In the spectrum of the Mg substrate, the Fermi edge can clearly be seen. Typical CuPC features emerge after depositing 60 Å CuPC layer, where the features of the Mg substrate beneath are completely suppressed. Upon deposition of the first 2 Å of C60, the secondary electron cutoff shifts by 0.13 eV to lower binding energy corresponding to an increase in the work function of the sample surface, which is mainly caused by the appearance of an interface dipole. During this initial work function increase, the shape of the entire UPS spectrum changed slightly, indicating a small C60 coverage can build up an interface dipole. Upon increasing C60 coverage, the characteristic features of CuPc and C60 show progressive shifts towards the lower binding energy (BE) region, accompanied by a 0.81 eV rising of the vacuum level. Since organics are usually poor conductors, an organic layer of about 200 Å could lead to significant charging when performing photoelectron spectroscopies [14]. Charging effect should be prevented, and energy level shift due to the charging should be distinguished from the interface electronic structures, like interface dipole or band bending. At metal–organic (M–O) interfaces, charging effect usually is minimal in the intermediate interface at M–O contact, ranging from sub-monolayer to a few monolayers. However, when dealing with organic–organic (O–O) interfaces, precaution should be paid to confirm that the energy level shift at the interface is not caused by charging, because we typically work on samples with organic layers twice as thick as that in the M–O interface researches. Using an electron flood gun is not an ideal means for eliminating the charging effect due to possible irradiation damages to the organic molecules. Similar to the high BE cutoff edges in UPS spectra, the high BE cutoff edges in XPS spectra also give the vacuum energy level. Due to the several orders of magnitudes of intensity difference between the X-ray and the ultraviolet photon sources, differences in the positions of the high BE cutoff edges in the UPS and XPS spectra would sensitively indicate charging [14]. Fig. 1(c) depicts the measured XPS cutoffs, which show nearly the same shifts with the UPS cutoffs. The agreement of vacuum levels from both UPS and XPS measurements suggests that there was little or no observable charging. Thus, the observed vacuum level shifts can be reliably attributed to the interface dipole and band bending at the CuPC–C60 interface.
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Fig. 2. (a) Cu2p, (b) N1s, and (c) C1s XPS spectra as a function of increasing C60 coverage on CuPC on the Mg substrate. With increasing C60 thickness, energy levels shift to lower binding energy with the same magnitude. The C1s peak of CuPC and C60 are obtained by de-convoluting the measured spectra into the two contributions, and the de-convolution of 10 Å C60 on CuPC is shown in (d) as an example.
Due to the large IP difference between CuPC and C60, their HOMO peaks do not overlap and can be distinguished. Upon increasing C60 coverage, HOMOs of CuPC and C60 shift respectively by 0.14 and 0.32 eV to lower binding energy, indicating an upward band bending in both layers. These band bendings are reasonable because the CuPC layer works as donor and the C60 layer as acceptor respectively, hence at the interface CuPC molecules are expected
Fig. 3. Comparison of energy level shifts with the increasing C60 thickness on CuPC on Mg. All the measured energy levels of CuPC, including HOMO (CuPC), C1s (CuPC), N1s, and Cu2p, show nearly the same shift with the increase of C60 coverage; so do the energy levels of C60, implying the presence of band bending in the organic layers. The vacuum levels obtained via UPS and XPS are consistent with each other. The difference between the shift of vacuum level and the band bending is regarded as interface dipole.
to be positively charged and C60 molecules negatively charged. The difference in the magnitude of band bending in the two layers may be ascribed to different charge density. According to the model of band bending, energy level shifts come from the space charge effect, and both valance levels and core levels are expected to shift with the same magnitude. Fig. 2 shows the evolution of Cu2p, N1s, and C1s core levels with increasing C60 coverage. Cu2p and N1s peaks (from the CuPC layer) show a shift to lower binding energy; but they do not show new component or broadening, except for increasing intensity attenuation with increasing C60 coverage, indicating little chemical reaction between CuPC and C60. The C1s spectra have several components.
Fig. 4. (a) Secondary electron cutoff, (b) HOMO peak of UPS He I spectra as a function of increasing C60 coverage on CuPC on the ITO substrate. With increasing C60 thickness, energy levels shifts are negligible.
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Fig. 5. Energy level diagrams of C60/CuPC interfaces on (a) Mg and (b) ITO. All the numbers are in eV.
Adopting the approach suggested by Molodtsova and Knupfer [15], the C1s spectra were least-square fitted in order to analyze contributions to the photoemission signal stemming from the CuPc and C60 components. The fitting was carried out with the XPSPEAK program [16]. Fig. 2d shows the de-convoluted peaks of the C1s spectrum of 10 Å C60 on CuPC as an example. The C1s spectrum of a pure CuPC layer consists of three peaks attributed respectively to the aromatic carbon of the benzene rings, the pyrrole carbon linked to nitrogen, and the p–p* satellite of the latter [17]. The peak positions and relative intensities of both the CuPC and the C60 components were thus obtained by fitting while keeping the peak shape of C1s (CuPC) and C1s (C60) unchanged. For the C1s levels, both the CuPC and the C60 components show shifts to lower binding energy, as observed in Cu2p, N1s core levels and valence levels like HOMO. In Fig. 3, variation of all the energy levels with thickness are plotted, including vacuum level (UPS), vacuum level (XPS), HOMO peak (CuPC), HOMO peak (C60), C1s (CuPC), Cu2p (CuPC), N1s (CuPC), and C1s (C60) to help understand the alignment of energy levels. It can be seen that the energy levels from CuPC are parallel to each other, so do those from C60, clearly indicating the existence of band bending of 0.14 eV in CuPC and 0.32 eV in C60. The difference between vacuum level shift (0.81 eV) and band bending can be ascribed to an interface dipole of 0.35 eV at the CuPC–C60 contact. A detailed energy level diagram is shown in Fig. 5a. Similar experiments are performed on an UV-ozone-treated ITO substrate (˚ = 4.95 eV). In sharp contrast to that on the Mg substrate, the increase of C60 coverage does not result in a shift of vacuum level or HOMO, as shown in Fig. 4, and the related core levels of CuPC and C60 are not shifted (not shown here) either. In this case, the Schottky–Mott model describes the interface well. We also observe that the exact amount of vacuum level offset at the CuPu/C60 interface is sensitive to the ITO surface condition. On ITO with a lower work function (˚ = 4.65 eV), a vacuum level offset of 0.45 eV was observed at the CuPC–C60 interface, in the same direction as on Mg substrates [18]. It is remarkable that the energy level alignment at the CuPC/C60 interface is substrate dependent. As shown in Fig. 5, on both Mg and ITO substrates, the Fermi level is pinned at about 0.2–0.3 eV below the LUMO of C60; however the energy levels of CuPC shift as the substrate work function changes. Note that the transition band gap of CuPC and C60 is taken from Ref. [19]. The extent of Fermi level movement in the energy band gap can be described by the interface slope parameter [10–22], defined as s = −dh /dm , where m is the metal work function, h the hole injection barrier at the
M–O interface. The Schottky–Mott model corresponds to the case when s = 1, but for the extreme case of Fermi level pinning, s = 0. It has been shown that strong Fermi level pinning occurs at the metal–C60 interface (s = 0), consistent with the present results [23]. However for the metal–CuPC interface, the slope parameter is not zero (∼0.5) [21]. The different slope parameters may be responsible for the dependence of the energy level alignment at the CuPC–C60 interface on the substrate work function; so that when the substrate work function changes, the energy levels of CuPC are shifted (to some extent), while the energy levels of C60 are pinned. The energy difference between the HOMO of CuPC (donor) and LUMO of C60 (acceptor), which is believed to dominantly determine the open circuit voltage of the CuPC/C60 OPV, increases from 0.66 to 1.03 eV when the substrate work function decreases from 4.95 eV (ITO) to 3.68 eV (Mg). This tendency is consistent with Voc dependence on the anode work function previously reported. Therefore, we conclude that in an OPV device vacuum levels are not necessarily aligned between the donor and acceptor layer, and may be influenced by the substrate work function, i.e., a lower work function tends to result in a downward shift of donor energy levels. This substrate effect in the IPDonor –EAAcceptor model can offer a satisfactory explanation for the Voc dependence on the anode work function, and a potential way for tuning the Voc via choosing the substrate. 4. Conclusions In conclusion, the interface energy level alignment at the CuPC/C60 interface on ITO and Mg substrates was investigated by combined XPS and UPS measurements. On ITO the CuPC/C60 interface shows nearly a common vacuum level; whereas the same interface on Mg shows a considerable dipole and band bending, resulting in a 0.8 eV shift of the vacuum level. The substrate work function dependence of C60/CuPC energy level alignment is understandable in terms of the different slope parameters of CuPC and C60. This substrate dependence leads to a tunable value of energy difference between HOMOCuPC and LUMOC60 , and can satisfactorily explain the observed dependence of Voc on the substrate work function. Acknowledgement This work is supported by the Research Grants Council of Hong Kong, Hong Kong SAR (CERG Project: CityU 101707).
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