Anal Bioanal Chem DOI 10.1007/s00216-014-7659-1
TRENDS
Measuring the electron affinity of organic solids: an indispensable new tool for organic electronics Hiroyuki Yoshida
Received: 9 November 2013 / Revised: 22 January 2014 / Accepted: 23 January 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Electron affinity is a fundamental energy parameter of materials. In organic semiconductors, the electron affinity is closely related to electron conduction. It is not only important to understand fundamental electronic processes in organic solids, but it is also indispensable for research and development of organic semiconductor devices such as organic lightemitting diodes and organic photovoltaic cells. However, there has been no experimental technique for examining the electron affinity of organic materials that meets the requirements of such research. Recently, a new method, called lowenergy inverse-photoemission spectroscopy, has been developed. A beam of low-energy electrons is focused onto the sample surface, and photons emitted owing to the radiative transition to unoccupied states are then detected. From the onset of the spectral intensity, the electron affinity is determined within an uncertainty of 0.1 eV. Unlike in conventional inverse-photoemission spectroscopy, sample damage is negligible and the resolution is improved by a factor of 2. The principle of the method and several applications are reported. Keywords Electron affinity . Ionization energy . Organic semiconductor . Low-energy inverse-photoemission spectroscopy . Unoccupied states . Energy gap
Introduction Ionization energy and electron affinity are fundamental energy parameters that characterize the properties of materials. The ionization energy is defined as the minimum energy required to remove an electron from a neutral atom or molecule in its ground H. Yoshida (*) Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan e-mail: [email protected]
state, and the electron affinity is the energy released when an additional electron is added to a neutral atom or molecule. They are closely related to the reactivity and bonding nature of atoms and molecules [1]. The ionization energy and electron affinity of solid organic materials are also defined in a similar way [2]. Around 1950, some organic materials were discovered to have semiconducting properties [3–5]. Systematic studies of organic semiconductors started, and about four decades later, practical devices using organic semiconductors such as organic light-emitting diodes [6] and organic photovoltaic cells [7] were reported. Now organic light-emitting diodes are used as displays for cell phones, portable digital media players, car radios, and digital cameras. The power conversion efficiency of organic photovoltaic cells has rapidly increased in the last few years and exceeded 10 % in 2011, which was considered to be a milestone for the practical application of the technology [8]. In these devices, both holes and electrons, which carry positive and negative charges, respectively, play a crucial role. When the one-electron approximation is applied, the hole moves at the top of the valence levels and the electron moves at the bottom of the unoccupied states; the energies of these levels with respect to the vacuum level are the ionization energy and the electron affinity, respectively. Figure 1 shows the energy level diagram of semiconductors and the experimental techniques used to examine their electronic structure. Electrons occupy the core levels and the valence states. The states which are not occupied by electrons are called unoccupied states. The difference in energy between the bottom of the unoccupied states and the top of the occupied states is called the energy gap. The Fermi level lies somewhere in this energy gap.1 1
In organic semiconductors, the valence states (unoccupied states) correspond to the valence bands (conduction bands) in inorganic semiconductors. Since the edges of the valence states (unoccupied states) originate from the highest occupied molecular orbital, HOMO, (the lowest unoccupied molecular orbital, LUMO) of the constituent molecules, they are often referred to the HOMO level (LUMO level).
H. Yoshida Fig. 1 Energy level diagrams of experimental techniques for determining the ionization energy and electron affinity: photoemission spectroscopy (PES), ultraviolet–visible spectroscopy (photoabsorption spectroscopy; PAS), X-ray absorption spectroscopy (XAS), inverse-photoemission spectroscopy (IPES), and lowenergy inverse-photoemission spectroscopy (LEIPS)
The core and valence states have been extensively studied by photoemission spectroscopy (PES). In this technique, the sample is irradiated with a monochromatic beam of photons (energy hν) and the kinetic energies of the ejected electrons (Ek) are analyzed. From the energy conservation rule, the binding energy of the electron (Eb) is determined. The ionization energy is determined from the onset of the PES spectral intensity with respect to the vacuum level. In contrast, the determination of unoccupied states and electron affinity is more difficult [9, 10]. Often the energy gap is estimated from the onset of the optical transition between the valence and unoccupied states using ultraviolet– visible spectroscopy (photoabsorption spectroscopy; PAS). The electron affinity is calculated by adding the energy gap and the ionization energy. However, the energy gap determined by PAS is often smaller than the actual energy gap by 0.2–1 eV, and difference is interpreted as the exciton binding energy [11, 12]. Thus, the electron affinity is overestimated by PAS. X-ray absorption spectroscopy uses the electronic transition from a core to an unoccupied state. The interaction between the excited electron and the core-hole generated by high-energy X-rays largely affects the electronic states (core excitonic effect), preventing quantitative determination of the electron affinities. In inverse-photoemission spectroscopy (IPES), an electron having kinetic energy Ek is incident on the sample, and photons emitted owing to radiative transitions to the unoccupied states are detected. The energy released by adding an electron to the sample is directly measured as the photon energy, which fits the definition of the electron affinity.2 Since IPES is the most versatile and direct method to examine the unoccupied states and determine the electron affinities [9, 10,
2 Strictly speaking, the energy released depends on the timescale. On the timescale of the processes in IPES, only the electronic process is usually considered.
13], the history and limitations of IPES are discussed in the next section. There are other techniques such as internal photoemission spectroscopy and electron transmission spectroscopy [10] that can be used. Although these are historically important, they can be applied only under limited conditions. For example, electron transmission spectroscopy can be applied only to material with a negative electron affinity. Scanning tunneling spectroscopy can also access the valence and unoccupied states of organic films [10, 14]; in the scanning tunneling microscopy setup, the tip is fixed at the position of interest and the tunneling current is measured as a function of bias voltage to examine the density of states. The advantage of this technique is its extremely high spatial resolution down to submolecular size [15]. On the other hand, this method is applicable to only submonolayer films, and the damage to organic samples from the high electron current density induced from the tip has not yet been clarified.
Previous inverse-photoemission spectroscopy Although IPES is an ideal tool to examine the unoccupied states of solid materials in principle, it is used far less frequently than PES, mostly because the cross section of the process in IPES is three or five orders of magnitude smaller than that in PES [13, 16]. In practical experiments, an intense electron beam is required and weak photon signals are detected. IPES was first conducted in the X-ray range [9]. IPES became more widely used after the development of bandpass photon detectors for vacuum ultraviolet (VUV) light in the late 1970s by Dose [17, 18]. This type of photon detector has high sensitivity, high collection efficiency, and a simple construction. Figure 2a and b shows a typical IPES apparatus and the sensitivity curves for the bandpass photon detector,
Measuring the electron affinity of organic solids
a
b
vacuum
CaF2
Geiger-Müller tube
electron gun
Intenisty(arb.unit)
Fig. 2 Comparison of experimental apparatus and the sensitivity curves of the photon detectors for IPES and LEIPS: a typical experimental setup for IPES and b sensitivity curve of the bandpass detector using an iodine-filled Geiger–Müller tube [17, 18]; c schematic diagram of the apparatus [26] and d transmittance of bandpass filters used for LEIPS, where the center energies of the bandpass filters are indicated
I2 vapor
I2
SrF2
CaF2 or SrF2
5
15 eV
e9.0 9.5 10.0 10.5 Photon Energy/ eV
c
d
vacuum
photomultiplier
100
window
electron gun
0
bandpass filter
4 eV e-
sample
respectively. The detector consists of an iodine-filled Geiger– Müller tube and a CaF2 plate. As shown in Fig. 2b, the ionization of iodine above 9.2 eV serves as a high-pass filter (black line) and the transmittance of CaF2 serves as a low-pass filter (blue line), making a bandpass at 9.7 eV with a width of 0.7 eV (blue region). When the CaF2 plate is replaced with an SrF2 plate (red line), the resolution is improved to 0.4 eV (red region). Significant efforts have been devoted to improving the resolution by changing the filling gas, the filter material, etc. So far the best resolution achieved is 0.084 eV for the combination of acetone gas and an Sr0.7Ca0.3 F2 plate [19]. The drawback of these bandpass detectors is that the resolution and center energy are inherently determined by the materials used. Further, an increase in resolution is accompanied by a loss in sensitivity, limiting the resolution of practically useful bandpass detectors to about 0.4 eV. Another drawback, which may be more serious, is damage to organic samples caused by electron bombardment. The center energies of these bandpass detectors are always around 10 eV, whereas the electron affinities of most organic semiconductors fall in the range between 0 and 5 eV [20, 21]. Under these conditions, the electron kinetic energy is scanned from 5 to 15 eV for IPES measurements as shown in Fig. 1. Electron irradiation in this energy range causes serious damage to organic samples [22]. Surprisingly, such IPES apparatus has been used for nearly four decades without fundamental improvement.
lens
Transmittance %
80 60
3.71 eV 4.46 eV 4.97 eV 4.38 eV
4.88 eV
40 20 0 3.5 4.0 4.5 5.0 Photon Energy / eV
Low-energy inverse-photoemission spectroscopy (LEIPS) Recently, LEIPS has been reported [23], and this simultaneously solves the two problems of conventional IPES. As shown in Fig. 1, the electron energy is lowered below 4 eV, below the damage threshold of most organic molecules (about 5 eV [24]). From energy conservation, the energy of emitted photons falls in the range between 2 and 5 eV [wavelengths of 600 nm and 250 nm, i.e., the near-ultraviolet (NUV) or visible range]. The NUV or visible photons can be analyzed far more easily than the VUV photons detected in conventional IPES. The photons can be detected with high resolution and sensitivity using, for example, a grating spectrometer [25] or a multilayer interference bandpass filter [26]. A grating spectrometer allows more freedom in choosing the photon energy and resolution, whereas the bandpass filter has an order of magnitude higher throughput. Since the low signal intensity is the main concern in IPES experiments, the bandpass filter is preferable in most cases. A schematic diagram of the LEIPS apparatus with the bandpass filter is shown in Fig. 2c [26]. Electrons from an electron source are focused onto the sample. The emitted NUVor visible photons are efficiently collected using a quartz lens. Since the NUV or visible photons are not absorbed by oxygen, photon detectors are placed in air; this greatly facilitates the construction and maintenance of the apparatus. The
H. Yoshida
2.5 CuPc
3.0 ZnPc
3.5
PCBM C60
4.0 PTCDA Fig. 4 Electron affinities determined by LEIPS and molecular structures of pentacene [30], CuPc [23], zinc phthalocyanine (ZnPc), [6, 6]-phenylC61-butyric acid methyl ester (PCBM) [29], C60 [29], and perylene tetracarboxylic dianhydride (PTCDA)
by the measurement to 1.04 eV. This clearly indicates that the overall resolution is high enough to determine the electron affinity within an accuracy of 0.1 eV. The radiation damage to CuPc is assessed by making prolonged measurements. In Fig. 3b, showing repeated 1-h scans, the spectrum stays unchanged even after 14 h, showing that sample damage is negligible in LEIPS. In contrast, the spectral line shape is completely different after 30 min under the electron irradiation condition similar to those in conventional IPES. Phthalocyanines are among the most durable
LEIPS
b Intensity (arb. units)
4.97 eV
4.46 eV
Conventional IPES Intensity (arb. units)
a
Intensity (arb. units)
0h 14 h
0 1 2 3 4 Electron Energy / eV
30 min
3.71 eV
0
1 2 3 4 5 Electron Kinetic Energy / eV
0 1 2 3 4 Electron Energy / eV
2.0 1.5 1.0
electron 0.5 affinity 0.0
3.0
3.5
4.0
4.5
Photon Energy / eV
0 min
60 min
c Onset Energy / eV
Fig. 3 LEIPS spectra and evaluation of electron affinity of copper phthalocyanine (CuPc; [23]). a Spectra of CuPc taken at photon energies of 4.97 eV, 4.46 eV, and 3.71 eV. b Time dependence of the spectra measured under typical conditions of LEIPS (left) and conditions similar to those in conventional IPES (right) to investigate the damage to the sample. c The onset energies of the spectra versus the photon energy, fitted to a line with a slope of unity. The electron affinity of CuPc is determined from the intercept of the line
pentacene
Electron Affinity/ eV
photons are analyzed using a multilayer interference bandpass filter and photomultiplier. The sensitivity curves of the bandpass filters are shown in Fig. 2d. The center energy can be chosen from the NUV to the near-infrared range, and the resolution is between 0.1 and 0.25 eV. The resolution of the bandpass filter for LEIPS is two to seven times better than that of the VUV bandpass detector shown in Fig. 2b. The transmittance of photons is between 50 and 80 %, resulting in highly sensitive photon detection. The overall resolution of LEIPS is determined by the resolution of the photon detector and the energy spread of the electrons. It is estimated to be 0.27 eV from the spectrum of the Fermi edge of silver film when the resolution of the bandpass filter is 0.15 eV [26]. This value is about a factor of 2 better than that obtained with the previous IPES apparatus used for organic materials (about 0.5 eV). The LEIPS spectra of a typical organic semiconductor, copper phthalocyanine (CuPc), are shown in Fig. 3a (the molecular structure of CuPc is shown in Fig. 4). The spectra were taken at several photon energies: 3.71 eV (with a resolution of 0.09 eV), 4.46 eV (with a resolution of 0.17 eV), and 4.97 eV (with a resolution of 0.23 eV). The spectrum shifts according to the photon energy, meaning that the spectrum reflects the density of unoccupied states and that the effect of the initial state is negligible. The spectra are also consistent with earlier IPES results [12, 27]. The observed first peak, derived from the LUMO of CuPc, is about 1 eV wide. The observed width arises from the convolution of the true spectrum and the instrumental function (or the overall resolution of the instrument). When the overall resolution is 0.3 eV, the true width of 1 eV is only broadened
5.0
Measuring the electron affinity of organic solids
molecules. Molecules such as polyacene are one order of magnitude more sensitive to electron bombardment, and molecules with an alkyl chain are an additional two orders of magnitude more sensitive to electron bombardment [28]. This means that most organic semiconductors will be damaged during conventional IPES measurements. Since the resolution is improved and sample damage is negligible, the electron affinity can be determined precisely from the series of spectra in Fig. 3a. The electron affinity is determined as the onset energy (indicated by the arrows in Fig. 3a) with respect to the vacuum level (the dotted vertical lines). In Fig. 3c, the onset energies are plotted as a function of the photon energy, and the electron affinity is determined as the intercept of the linear relation with a slope of unity. By this procedure, systematic errors can be suppressed. The electron affinity of CuPc is determined to be 3.09±0.05 eV.
The energy gap of pentacene (its molecular structure is shown in Fig. 4) has been reported to be 1.8 eV by PAS, 2.2 eV by photoconductivity measurements [36, 37], and 2.8 eV if the charge-transfer (CT) exciton is assumed to interpret the electric-field-modulated absorption spectrum [38]. The LEIPS spectra of a 10-nm-thick pentacene film were measured at five different photon energies, and the electron affinity was determined to be 2.70 eV [30]. This gives an energy gap of 2.2 eV using 4.9 eV for the ionization energy. This value is in good agreement with that obtained by photoconductivity measurements, but it is 0.4 eV higher than the onset of photoabsorption because of the exciton binding energy. The discrepancy with the CT exciton results suggests that the assignment of the CT exciton peak should be reconsidered. Electron injection barrier between an electron-conducting polymer and gold electrodes [39]
Examples of low-energy inverse-photoemission spectroscopy Electron affinities of typical organic semiconductors By means of LEIPS, the electron affinities of widely studied organic semiconductors (perylene tetracarboxylic dianhydride (PTCDA), C60 [29], [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) [29], zinc phthalocyanine (ZnPc), CuPc [23], and pentacene [30]) were determined. The films of [6, 6]phenyl-C61-butyric acid methyl ester were prepared by spincasting, and films of the other compounds were prepared by vacuum deposition on indium tin oxide. The measurements were performed at a minimum of three different photon energies. The electron affinities are shown in Fig. 4 together with the molecular structures. In LEIPS, the electron affinity can be determined with uncertainties as low as 0.1 eV. This accuracy of the value is much higher than for conventional IPES, where the uncertainties are assumed to be as much as 0.7 eV [21]. Note that the ionization energy depends (normally on the order of 0.1 eV) on the film structures, such as crystallinity, polymorphs [31, 32], and the orientation of molecules [33]. Similar dependences are also expected for the electron affinities, although research into this has just started. Energy gap of pentacene Since the ionization energies of organic solids have been extensively studied, the energy gap can be determined when the electron affinity is determined precisely [34]. The energy gap has been experimentally examined by various optical techniques. By comparing these values, one can discuss electronic processes in organic solids in detail. The experimentally determined energy gaps also provide stringent tests for theoretical calculations [35].
The energy barriers at the interface of an organic semiconductor device govern its performance. For the hole injection barriers, the interfacial electronic structure has been extensively studied [40, 41]. A similar study for the electrons can be done using LEIPS. In an organic field-effect transistor made using an e l ec t r o n - c o n d u c t i n g p o l y m e r, p o l y { [ N ,N ′ - b i s ( 2 octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′-bithiophene)}, P(NDI2OD-T2), the activation energy was found to depend on the orientation of the polymer and was higher in a film with an edge-on orientation with respect to the substrate surface. The reason for this was elucidated by measuring the electron affinities of polymers with face-on and edge-on orientations. Although the electron affinities of the face-on and the edge-on films are about 4.1 eV, the vacuum levels are different, leading to a difference in the electron injection barriers [40].
Outlook LEIPS enables us to observe unoccupied states of solid materials with a resolution of 0.3 eV and without radiation damage to the samples. The electron affinity of solid samples can be determined for the first time with accuracy better than 0.1 eV, which meets the requirements in the research and development of organic electronics. A great advantage of organic electronics is that new materials can be tailored to the required physical and electronic properties by organic synthesis. This method will be used immediately in the development of new organic materials. In basic research, information about the valence states has been obtained using PES, and the behavior of the hole in organic semiconductors has been clarified. Similar
H. Yoshida
experiments for the unoccupied states and the electrons will be conducted using LEIPS. For example, the energy level alignment at the interface for unoccupied states and the electron injection barrier will be examined as described in the previous section [40, 41]. The intermolecular band dispersion [42] is certainly the next target of LEIPS. The band dispersion of unoccupied states is closely related to electron transport [43, 44], but conventional IPES cannot be applied because of sample damage and low resolution. For such fundamental studies, even higher energy resolution is desirable. Although the energy resolution is limited by the VUV bandpass detector and further improvement is difficult in conventional IPES, the energy resolution of current LEIPS apparatus is limited by the energy spread of electrons. The electron energy spread will soon be narrowed to below 0.1 eV by using an electron energy analyzer [45], resulting in an overall resolution of 0.1 eV. Such improvement of the experimental apparatus is expected to continue. So far, LEIPS has been applied to organic materials in connection with organic electronics. However, LEIPS is a more versatile technique in principle. It can be applied to adsorbates and surfaces of catalysts to elucidate catalytic behavior [43]. Since the method is nondestructive, biomolecules can be examined. The only requirements for the sample materials are sufficient conductivity and vacuum compatibility.
References 1. Pauling L (1960) The nature of the chemical bond and the structure of molecules and crystals: an introduction to modern structural chemistry. Cornell University Press, New York 2. Pope M, Swenberg CE (1999) Electronic processes in organic crystals and polymers. Oxford University Press, New York 3. Akamatu H, Inokuchi H (1950) On the electrical conductivity of violanthrone, iso-violanthrone, and pyranthrone. J Chem Phys 18(6):810–811 4. Eley DD (1948) Phthalocyanines as semiconductors. Nature 162(4125):819–819 5. Vartanyan AT (1948) Poluprovodnikovye Svoistva Organicheskikh Krasitelei .1. Ftalotsianiny. Zh Fiz Khim 22(7):769–782 6. Tang CW, Vanslyke SA (1987) Organic electroluminescent diodes. Appl Phys Lett 51(12):913–915 7. Tang CW (1986) 2-layer organic photovoltaic cell. Appl Phys Lett 48(2):183–185 8. Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED (2012) Solar cell efficiency tables (version 39). Prog Photovoltaics 20(1):12–20 9. Fuggle JC, Inglesfield JE (1992) Unoccupied electronic states fundamentals for XANES, EELS, IPS and BIS - introduction. Top Appl Phys 69:1–23 10. Seki K, Kanai K (2006) Development of experimental methods for determining the electronic structure of organic materials. Mol Cryst Liq Cryst 455:145–181 11. Bredas JL, Cornil J, Heeger AJ (1996) The exciton binding energy in luminescent conjugated polymers. Adv Mater 8(5):447–452 12. Hill IG, Kahn A, Soos ZG, Pascal RA (2000) Charge-separation energy in films of pi-conjugated organic molecules. Chem Phys Lett 327(3–4):181–188
13. Johnson PD, Hulbert SL (1990) Inverse photoemission. Rev Sci Instrum 61(9):2277–2288 14. Hipps KW (2006) In: Vij DR (ed) Handbook of applied solid state spectroscopy. Springer, New York 15. Soubatch S, Weiss C, Temirov R, Tautz FS (2009) Site-specific polarization screening in organic thin films. Phys Rev Lett 102(17): 177405 16. Pendry JB (1980) New probe for unoccupied bands at surfaces. Phys Rev Lett 45(16):1356–1358 17. Dose V (1977) VUV isochromat spectroscopy. Appl Phys 14(1):117– 118 18. Dose V (1983) Ultraviolet Bremsstrahlung spectroscopy. Prog Surf Sci 13(3):225–284 19. Maniraj M, D’Souza SW, Nayak J, Rai A, Singh S, Sekhar BNR, Barman SR (2011) High energy resolution bandpass photon detector for inverse photoemission spectroscopy. Rev Sci Instrum 82(9): 093901 20. Kahn A, Koch N, Gao WY (2003) Electronic structure and electrical properties of interfaces between metals and pi-conjugated molecular films. J Polym Sci B Polym Phys 41(21):2529–2548 21. Djurovich PI, Mayo EI, Forrest SR, Thompson ME (2009) Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors. Org Electron 10(3):515– 520 22. Tsutsumi K, Yoshida H, Sato N (2002) Unoccupied electronic states in a hexatriacontane thin film studied by inverse photoemission spectroscopy. Chem Phys Lett 361(5–6):367–373 23. Yoshida H (2012) Near-ultraviolet inverse photoemission spectroscopy using ultra-low energy electrons. Chem Phys Lett 539–540:180–185 24. Boudaiffa B, Cloutier P, Hunting D, Huels MA, Sanche L (2000) Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons. Science 287(5458):1658–1660 25. Yoshida H (2013) Low-energy inverse photoemission spectroscopy using a high-resolution grating spectrometer in the near ultraviolet range. Rev Sci Instrum 84(10):103901 26. Yoshida H (2014) Note: Low energy inverse photoemission spectroscopy apparatus. Rev Sci Instrum 85:016101 27. Yoshida H, Tsutsumi K, Sato N (2001) Unoccupied electronic states of 3d-transition metal phthalocyanines (MPc: M=Mn, Fe, Co, Ni, Cu and Zn) studied by inverse photoemission spectroscopy. J Electron Spectrosc Relat Phenom 121(1–3):83–91 28. Reimer L (1997) In: Hawkes PW (ed) Transmission electron microscopy: physics of image formation and microanalysis, Springer series in optical sciences, vol 36. Springer, Berlin 29. Yoshida H (2012) New experimental method to precisely examine the LUMO levels of organic semiconductors and application to the fullerene derivatives. MRS Symp Proc 1493:295–301 30. Han W, Yoshida H, Ueno N, Kera S (2013) Electron affinity of pentacene thin film studied by radiation-damage free inverse photoemission spectroscopy. Appl Phys Lett 103:123303 31. Sato N, Seki K, Inokuchi H, Harada Y (1986) Photoemission from an amorphous pentacene film. Chem Phys 109(1):157–162 32. Fukagawa H, Yamane H, Kataoka T, Kera S, Nakamura M, Kudo K, Ueno N (2006) Origin of the highest occupied band position in pentacene films from ultraviolet photoelectron spectroscopy: hole stabilization versus band dispersion. Phys Rev B 73(24):245310 33. Duhm S, Heimel G, Salzmann I, Glowatzki H, Johnson RL, Vollmer A, Rabe JP, Koch N (2008) Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies. Nat Mater 7(4):326–332 34. Krause S, Casu MB, Scholl A, Umbach E (2008) Determination of transport levels of organic semiconductors by UPS and IPS. New J Phys 10:085001 35. Refaely-Abramson S, Baer R, Kronik L (2011) Fundamental and excitation gaps in molecules of relevance for organic photovoltaics
Measuring the electron affinity of organic solids
36.
37.
38.
39.
from an optimally tuned range-separated hybrid functional. Phys Rev B 84(7):075144 Silinsh EA, Belkind AI, Balode DR, Brseniece AJ, Grechov VV, Taure LF, Kurik MV, Vertzymacha JI, Bok I (1974) Photoelectrical properties, energy level spectra, and photogeneration mechanisms of pentacene. Phys Stat Sol (a) 25:339 Lang DV, Chi X, Siegrist T, Sergent AM, Ramirez AP (2004) Amorphouslike density of gap states in single-crystal pentacene. Phys Rev Lett 93(8):086802 Sebastian L, Weiser G, Bassler H (1981) Charge transfer transitions in solid tetracene and pentacene studied by electroabsorption. Chem Phys 61:125 Fabiano S, Yoshida H, Chen ZH, Facchetti A, Loi MA (2013) Orientation-dependent electronic structures and charge transport mechanisms in ultrathin polymeric n-channel field-effect transistors. ACS Appl Mater Interfaces 5(10):4417–4422
40. Ishii H, Sugiyama K, Ito E, Seki K (1999) Energy level alignment and interfacial electronic structures at organic metal and organic organic interfaces. Adv Mater 11(8):605–625 41. Braun S, Salaneck WR, Fahlman M (2009) Energy-level alignment at organic/metal and organic/organic interfaces. Adv Mater 21(14–15): 1450–1472 42. Ueno N, Kera S (2008) Electron spectroscopy of functional organic thin films: deep insights into valence electronic structure in relation to charge transport property. Prog Surf Sci 83(10–12):490–557 43. Hoffmann R (1988) Solids and surfaces: a chemist's view of bonding in extended structures. VCH Publishers, New York 44. Ashcroft NW, Mermin ND (1976) Solid state physics. Thomson Learning, London 45. BudkeM, Renken V, LieblH, Rangelov G, DonathM (2007) Inverse photoemission with energy resolution better than 200 meV. Rev Sci Instrum 78(8):083903
TRENDS
Measuring the electron affinity of organic solids: an indispensable new tool for organic electronics Hiroyuki Yoshida
Received: 9 November 2013 / Revised: 22 January 2014 / Accepted: 23 January 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Electron affinity is a fundamental energy parameter of materials. In organic semiconductors, the electron affinity is closely related to electron conduction. It is not only important to understand fundamental electronic processes in organic solids, but it is also indispensable for research and development of organic semiconductor devices such as organic lightemitting diodes and organic photovoltaic cells. However, there has been no experimental technique for examining the electron affinity of organic materials that meets the requirements of such research. Recently, a new method, called lowenergy inverse-photoemission spectroscopy, has been developed. A beam of low-energy electrons is focused onto the sample surface, and photons emitted owing to the radiative transition to unoccupied states are then detected. From the onset of the spectral intensity, the electron affinity is determined within an uncertainty of 0.1 eV. Unlike in conventional inverse-photoemission spectroscopy, sample damage is negligible and the resolution is improved by a factor of 2. The principle of the method and several applications are reported. Keywords Electron affinity . Ionization energy . Organic semiconductor . Low-energy inverse-photoemission spectroscopy . Unoccupied states . Energy gap
Introduction Ionization energy and electron affinity are fundamental energy parameters that characterize the properties of materials. The ionization energy is defined as the minimum energy required to remove an electron from a neutral atom or molecule in its ground H. Yoshida (*) Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan e-mail: [email protected]
state, and the electron affinity is the energy released when an additional electron is added to a neutral atom or molecule. They are closely related to the reactivity and bonding nature of atoms and molecules [1]. The ionization energy and electron affinity of solid organic materials are also defined in a similar way [2]. Around 1950, some organic materials were discovered to have semiconducting properties [3–5]. Systematic studies of organic semiconductors started, and about four decades later, practical devices using organic semiconductors such as organic light-emitting diodes [6] and organic photovoltaic cells [7] were reported. Now organic light-emitting diodes are used as displays for cell phones, portable digital media players, car radios, and digital cameras. The power conversion efficiency of organic photovoltaic cells has rapidly increased in the last few years and exceeded 10 % in 2011, which was considered to be a milestone for the practical application of the technology [8]. In these devices, both holes and electrons, which carry positive and negative charges, respectively, play a crucial role. When the one-electron approximation is applied, the hole moves at the top of the valence levels and the electron moves at the bottom of the unoccupied states; the energies of these levels with respect to the vacuum level are the ionization energy and the electron affinity, respectively. Figure 1 shows the energy level diagram of semiconductors and the experimental techniques used to examine their electronic structure. Electrons occupy the core levels and the valence states. The states which are not occupied by electrons are called unoccupied states. The difference in energy between the bottom of the unoccupied states and the top of the occupied states is called the energy gap. The Fermi level lies somewhere in this energy gap.1 1
In organic semiconductors, the valence states (unoccupied states) correspond to the valence bands (conduction bands) in inorganic semiconductors. Since the edges of the valence states (unoccupied states) originate from the highest occupied molecular orbital, HOMO, (the lowest unoccupied molecular orbital, LUMO) of the constituent molecules, they are often referred to the HOMO level (LUMO level).
H. Yoshida Fig. 1 Energy level diagrams of experimental techniques for determining the ionization energy and electron affinity: photoemission spectroscopy (PES), ultraviolet–visible spectroscopy (photoabsorption spectroscopy; PAS), X-ray absorption spectroscopy (XAS), inverse-photoemission spectroscopy (IPES), and lowenergy inverse-photoemission spectroscopy (LEIPS)
The core and valence states have been extensively studied by photoemission spectroscopy (PES). In this technique, the sample is irradiated with a monochromatic beam of photons (energy hν) and the kinetic energies of the ejected electrons (Ek) are analyzed. From the energy conservation rule, the binding energy of the electron (Eb) is determined. The ionization energy is determined from the onset of the PES spectral intensity with respect to the vacuum level. In contrast, the determination of unoccupied states and electron affinity is more difficult [9, 10]. Often the energy gap is estimated from the onset of the optical transition between the valence and unoccupied states using ultraviolet– visible spectroscopy (photoabsorption spectroscopy; PAS). The electron affinity is calculated by adding the energy gap and the ionization energy. However, the energy gap determined by PAS is often smaller than the actual energy gap by 0.2–1 eV, and difference is interpreted as the exciton binding energy [11, 12]. Thus, the electron affinity is overestimated by PAS. X-ray absorption spectroscopy uses the electronic transition from a core to an unoccupied state. The interaction between the excited electron and the core-hole generated by high-energy X-rays largely affects the electronic states (core excitonic effect), preventing quantitative determination of the electron affinities. In inverse-photoemission spectroscopy (IPES), an electron having kinetic energy Ek is incident on the sample, and photons emitted owing to radiative transitions to the unoccupied states are detected. The energy released by adding an electron to the sample is directly measured as the photon energy, which fits the definition of the electron affinity.2 Since IPES is the most versatile and direct method to examine the unoccupied states and determine the electron affinities [9, 10,
2 Strictly speaking, the energy released depends on the timescale. On the timescale of the processes in IPES, only the electronic process is usually considered.
13], the history and limitations of IPES are discussed in the next section. There are other techniques such as internal photoemission spectroscopy and electron transmission spectroscopy [10] that can be used. Although these are historically important, they can be applied only under limited conditions. For example, electron transmission spectroscopy can be applied only to material with a negative electron affinity. Scanning tunneling spectroscopy can also access the valence and unoccupied states of organic films [10, 14]; in the scanning tunneling microscopy setup, the tip is fixed at the position of interest and the tunneling current is measured as a function of bias voltage to examine the density of states. The advantage of this technique is its extremely high spatial resolution down to submolecular size [15]. On the other hand, this method is applicable to only submonolayer films, and the damage to organic samples from the high electron current density induced from the tip has not yet been clarified.
Previous inverse-photoemission spectroscopy Although IPES is an ideal tool to examine the unoccupied states of solid materials in principle, it is used far less frequently than PES, mostly because the cross section of the process in IPES is three or five orders of magnitude smaller than that in PES [13, 16]. In practical experiments, an intense electron beam is required and weak photon signals are detected. IPES was first conducted in the X-ray range [9]. IPES became more widely used after the development of bandpass photon detectors for vacuum ultraviolet (VUV) light in the late 1970s by Dose [17, 18]. This type of photon detector has high sensitivity, high collection efficiency, and a simple construction. Figure 2a and b shows a typical IPES apparatus and the sensitivity curves for the bandpass photon detector,
Measuring the electron affinity of organic solids
a
b
vacuum
CaF2
Geiger-Müller tube
electron gun
Intenisty(arb.unit)
Fig. 2 Comparison of experimental apparatus and the sensitivity curves of the photon detectors for IPES and LEIPS: a typical experimental setup for IPES and b sensitivity curve of the bandpass detector using an iodine-filled Geiger–Müller tube [17, 18]; c schematic diagram of the apparatus [26] and d transmittance of bandpass filters used for LEIPS, where the center energies of the bandpass filters are indicated
I2 vapor
I2
SrF2
CaF2 or SrF2
5
15 eV
e9.0 9.5 10.0 10.5 Photon Energy/ eV
c
d
vacuum
photomultiplier
100
window
electron gun
0
bandpass filter
4 eV e-
sample
respectively. The detector consists of an iodine-filled Geiger– Müller tube and a CaF2 plate. As shown in Fig. 2b, the ionization of iodine above 9.2 eV serves as a high-pass filter (black line) and the transmittance of CaF2 serves as a low-pass filter (blue line), making a bandpass at 9.7 eV with a width of 0.7 eV (blue region). When the CaF2 plate is replaced with an SrF2 plate (red line), the resolution is improved to 0.4 eV (red region). Significant efforts have been devoted to improving the resolution by changing the filling gas, the filter material, etc. So far the best resolution achieved is 0.084 eV for the combination of acetone gas and an Sr0.7Ca0.3 F2 plate [19]. The drawback of these bandpass detectors is that the resolution and center energy are inherently determined by the materials used. Further, an increase in resolution is accompanied by a loss in sensitivity, limiting the resolution of practically useful bandpass detectors to about 0.4 eV. Another drawback, which may be more serious, is damage to organic samples caused by electron bombardment. The center energies of these bandpass detectors are always around 10 eV, whereas the electron affinities of most organic semiconductors fall in the range between 0 and 5 eV [20, 21]. Under these conditions, the electron kinetic energy is scanned from 5 to 15 eV for IPES measurements as shown in Fig. 1. Electron irradiation in this energy range causes serious damage to organic samples [22]. Surprisingly, such IPES apparatus has been used for nearly four decades without fundamental improvement.
lens
Transmittance %
80 60
3.71 eV 4.46 eV 4.97 eV 4.38 eV
4.88 eV
40 20 0 3.5 4.0 4.5 5.0 Photon Energy / eV
Low-energy inverse-photoemission spectroscopy (LEIPS) Recently, LEIPS has been reported [23], and this simultaneously solves the two problems of conventional IPES. As shown in Fig. 1, the electron energy is lowered below 4 eV, below the damage threshold of most organic molecules (about 5 eV [24]). From energy conservation, the energy of emitted photons falls in the range between 2 and 5 eV [wavelengths of 600 nm and 250 nm, i.e., the near-ultraviolet (NUV) or visible range]. The NUV or visible photons can be analyzed far more easily than the VUV photons detected in conventional IPES. The photons can be detected with high resolution and sensitivity using, for example, a grating spectrometer [25] or a multilayer interference bandpass filter [26]. A grating spectrometer allows more freedom in choosing the photon energy and resolution, whereas the bandpass filter has an order of magnitude higher throughput. Since the low signal intensity is the main concern in IPES experiments, the bandpass filter is preferable in most cases. A schematic diagram of the LEIPS apparatus with the bandpass filter is shown in Fig. 2c [26]. Electrons from an electron source are focused onto the sample. The emitted NUVor visible photons are efficiently collected using a quartz lens. Since the NUV or visible photons are not absorbed by oxygen, photon detectors are placed in air; this greatly facilitates the construction and maintenance of the apparatus. The
H. Yoshida
2.5 CuPc
3.0 ZnPc
3.5
PCBM C60
4.0 PTCDA Fig. 4 Electron affinities determined by LEIPS and molecular structures of pentacene [30], CuPc [23], zinc phthalocyanine (ZnPc), [6, 6]-phenylC61-butyric acid methyl ester (PCBM) [29], C60 [29], and perylene tetracarboxylic dianhydride (PTCDA)
by the measurement to 1.04 eV. This clearly indicates that the overall resolution is high enough to determine the electron affinity within an accuracy of 0.1 eV. The radiation damage to CuPc is assessed by making prolonged measurements. In Fig. 3b, showing repeated 1-h scans, the spectrum stays unchanged even after 14 h, showing that sample damage is negligible in LEIPS. In contrast, the spectral line shape is completely different after 30 min under the electron irradiation condition similar to those in conventional IPES. Phthalocyanines are among the most durable
LEIPS
b Intensity (arb. units)
4.97 eV
4.46 eV
Conventional IPES Intensity (arb. units)
a
Intensity (arb. units)
0h 14 h
0 1 2 3 4 Electron Energy / eV
30 min
3.71 eV
0
1 2 3 4 5 Electron Kinetic Energy / eV
0 1 2 3 4 Electron Energy / eV
2.0 1.5 1.0
electron 0.5 affinity 0.0
3.0
3.5
4.0
4.5
Photon Energy / eV
0 min
60 min
c Onset Energy / eV
Fig. 3 LEIPS spectra and evaluation of electron affinity of copper phthalocyanine (CuPc; [23]). a Spectra of CuPc taken at photon energies of 4.97 eV, 4.46 eV, and 3.71 eV. b Time dependence of the spectra measured under typical conditions of LEIPS (left) and conditions similar to those in conventional IPES (right) to investigate the damage to the sample. c The onset energies of the spectra versus the photon energy, fitted to a line with a slope of unity. The electron affinity of CuPc is determined from the intercept of the line
pentacene
Electron Affinity/ eV
photons are analyzed using a multilayer interference bandpass filter and photomultiplier. The sensitivity curves of the bandpass filters are shown in Fig. 2d. The center energy can be chosen from the NUV to the near-infrared range, and the resolution is between 0.1 and 0.25 eV. The resolution of the bandpass filter for LEIPS is two to seven times better than that of the VUV bandpass detector shown in Fig. 2b. The transmittance of photons is between 50 and 80 %, resulting in highly sensitive photon detection. The overall resolution of LEIPS is determined by the resolution of the photon detector and the energy spread of the electrons. It is estimated to be 0.27 eV from the spectrum of the Fermi edge of silver film when the resolution of the bandpass filter is 0.15 eV [26]. This value is about a factor of 2 better than that obtained with the previous IPES apparatus used for organic materials (about 0.5 eV). The LEIPS spectra of a typical organic semiconductor, copper phthalocyanine (CuPc), are shown in Fig. 3a (the molecular structure of CuPc is shown in Fig. 4). The spectra were taken at several photon energies: 3.71 eV (with a resolution of 0.09 eV), 4.46 eV (with a resolution of 0.17 eV), and 4.97 eV (with a resolution of 0.23 eV). The spectrum shifts according to the photon energy, meaning that the spectrum reflects the density of unoccupied states and that the effect of the initial state is negligible. The spectra are also consistent with earlier IPES results [12, 27]. The observed first peak, derived from the LUMO of CuPc, is about 1 eV wide. The observed width arises from the convolution of the true spectrum and the instrumental function (or the overall resolution of the instrument). When the overall resolution is 0.3 eV, the true width of 1 eV is only broadened
5.0
Measuring the electron affinity of organic solids
molecules. Molecules such as polyacene are one order of magnitude more sensitive to electron bombardment, and molecules with an alkyl chain are an additional two orders of magnitude more sensitive to electron bombardment [28]. This means that most organic semiconductors will be damaged during conventional IPES measurements. Since the resolution is improved and sample damage is negligible, the electron affinity can be determined precisely from the series of spectra in Fig. 3a. The electron affinity is determined as the onset energy (indicated by the arrows in Fig. 3a) with respect to the vacuum level (the dotted vertical lines). In Fig. 3c, the onset energies are plotted as a function of the photon energy, and the electron affinity is determined as the intercept of the linear relation with a slope of unity. By this procedure, systematic errors can be suppressed. The electron affinity of CuPc is determined to be 3.09±0.05 eV.
The energy gap of pentacene (its molecular structure is shown in Fig. 4) has been reported to be 1.8 eV by PAS, 2.2 eV by photoconductivity measurements [36, 37], and 2.8 eV if the charge-transfer (CT) exciton is assumed to interpret the electric-field-modulated absorption spectrum [38]. The LEIPS spectra of a 10-nm-thick pentacene film were measured at five different photon energies, and the electron affinity was determined to be 2.70 eV [30]. This gives an energy gap of 2.2 eV using 4.9 eV for the ionization energy. This value is in good agreement with that obtained by photoconductivity measurements, but it is 0.4 eV higher than the onset of photoabsorption because of the exciton binding energy. The discrepancy with the CT exciton results suggests that the assignment of the CT exciton peak should be reconsidered. Electron injection barrier between an electron-conducting polymer and gold electrodes [39]
Examples of low-energy inverse-photoemission spectroscopy Electron affinities of typical organic semiconductors By means of LEIPS, the electron affinities of widely studied organic semiconductors (perylene tetracarboxylic dianhydride (PTCDA), C60 [29], [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) [29], zinc phthalocyanine (ZnPc), CuPc [23], and pentacene [30]) were determined. The films of [6, 6]phenyl-C61-butyric acid methyl ester were prepared by spincasting, and films of the other compounds were prepared by vacuum deposition on indium tin oxide. The measurements were performed at a minimum of three different photon energies. The electron affinities are shown in Fig. 4 together with the molecular structures. In LEIPS, the electron affinity can be determined with uncertainties as low as 0.1 eV. This accuracy of the value is much higher than for conventional IPES, where the uncertainties are assumed to be as much as 0.7 eV [21]. Note that the ionization energy depends (normally on the order of 0.1 eV) on the film structures, such as crystallinity, polymorphs [31, 32], and the orientation of molecules [33]. Similar dependences are also expected for the electron affinities, although research into this has just started. Energy gap of pentacene Since the ionization energies of organic solids have been extensively studied, the energy gap can be determined when the electron affinity is determined precisely [34]. The energy gap has been experimentally examined by various optical techniques. By comparing these values, one can discuss electronic processes in organic solids in detail. The experimentally determined energy gaps also provide stringent tests for theoretical calculations [35].
The energy barriers at the interface of an organic semiconductor device govern its performance. For the hole injection barriers, the interfacial electronic structure has been extensively studied [40, 41]. A similar study for the electrons can be done using LEIPS. In an organic field-effect transistor made using an e l ec t r o n - c o n d u c t i n g p o l y m e r, p o l y { [ N ,N ′ - b i s ( 2 octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′-bithiophene)}, P(NDI2OD-T2), the activation energy was found to depend on the orientation of the polymer and was higher in a film with an edge-on orientation with respect to the substrate surface. The reason for this was elucidated by measuring the electron affinities of polymers with face-on and edge-on orientations. Although the electron affinities of the face-on and the edge-on films are about 4.1 eV, the vacuum levels are different, leading to a difference in the electron injection barriers [40].
Outlook LEIPS enables us to observe unoccupied states of solid materials with a resolution of 0.3 eV and without radiation damage to the samples. The electron affinity of solid samples can be determined for the first time with accuracy better than 0.1 eV, which meets the requirements in the research and development of organic electronics. A great advantage of organic electronics is that new materials can be tailored to the required physical and electronic properties by organic synthesis. This method will be used immediately in the development of new organic materials. In basic research, information about the valence states has been obtained using PES, and the behavior of the hole in organic semiconductors has been clarified. Similar
H. Yoshida
experiments for the unoccupied states and the electrons will be conducted using LEIPS. For example, the energy level alignment at the interface for unoccupied states and the electron injection barrier will be examined as described in the previous section [40, 41]. The intermolecular band dispersion [42] is certainly the next target of LEIPS. The band dispersion of unoccupied states is closely related to electron transport [43, 44], but conventional IPES cannot be applied because of sample damage and low resolution. For such fundamental studies, even higher energy resolution is desirable. Although the energy resolution is limited by the VUV bandpass detector and further improvement is difficult in conventional IPES, the energy resolution of current LEIPS apparatus is limited by the energy spread of electrons. The electron energy spread will soon be narrowed to below 0.1 eV by using an electron energy analyzer [45], resulting in an overall resolution of 0.1 eV. Such improvement of the experimental apparatus is expected to continue. So far, LEIPS has been applied to organic materials in connection with organic electronics. However, LEIPS is a more versatile technique in principle. It can be applied to adsorbates and surfaces of catalysts to elucidate catalytic behavior [43]. Since the method is nondestructive, biomolecules can be examined. The only requirements for the sample materials are sufficient conductivity and vacuum compatibility.
References 1. Pauling L (1960) The nature of the chemical bond and the structure of molecules and crystals: an introduction to modern structural chemistry. Cornell University Press, New York 2. Pope M, Swenberg CE (1999) Electronic processes in organic crystals and polymers. Oxford University Press, New York 3. Akamatu H, Inokuchi H (1950) On the electrical conductivity of violanthrone, iso-violanthrone, and pyranthrone. J Chem Phys 18(6):810–811 4. Eley DD (1948) Phthalocyanines as semiconductors. Nature 162(4125):819–819 5. Vartanyan AT (1948) Poluprovodnikovye Svoistva Organicheskikh Krasitelei .1. Ftalotsianiny. Zh Fiz Khim 22(7):769–782 6. Tang CW, Vanslyke SA (1987) Organic electroluminescent diodes. Appl Phys Lett 51(12):913–915 7. Tang CW (1986) 2-layer organic photovoltaic cell. Appl Phys Lett 48(2):183–185 8. Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED (2012) Solar cell efficiency tables (version 39). Prog Photovoltaics 20(1):12–20 9. Fuggle JC, Inglesfield JE (1992) Unoccupied electronic states fundamentals for XANES, EELS, IPS and BIS - introduction. Top Appl Phys 69:1–23 10. Seki K, Kanai K (2006) Development of experimental methods for determining the electronic structure of organic materials. Mol Cryst Liq Cryst 455:145–181 11. Bredas JL, Cornil J, Heeger AJ (1996) The exciton binding energy in luminescent conjugated polymers. Adv Mater 8(5):447–452 12. Hill IG, Kahn A, Soos ZG, Pascal RA (2000) Charge-separation energy in films of pi-conjugated organic molecules. Chem Phys Lett 327(3–4):181–188
13. Johnson PD, Hulbert SL (1990) Inverse photoemission. Rev Sci Instrum 61(9):2277–2288 14. Hipps KW (2006) In: Vij DR (ed) Handbook of applied solid state spectroscopy. Springer, New York 15. Soubatch S, Weiss C, Temirov R, Tautz FS (2009) Site-specific polarization screening in organic thin films. Phys Rev Lett 102(17): 177405 16. Pendry JB (1980) New probe for unoccupied bands at surfaces. Phys Rev Lett 45(16):1356–1358 17. Dose V (1977) VUV isochromat spectroscopy. Appl Phys 14(1):117– 118 18. Dose V (1983) Ultraviolet Bremsstrahlung spectroscopy. Prog Surf Sci 13(3):225–284 19. Maniraj M, D’Souza SW, Nayak J, Rai A, Singh S, Sekhar BNR, Barman SR (2011) High energy resolution bandpass photon detector for inverse photoemission spectroscopy. Rev Sci Instrum 82(9): 093901 20. Kahn A, Koch N, Gao WY (2003) Electronic structure and electrical properties of interfaces between metals and pi-conjugated molecular films. J Polym Sci B Polym Phys 41(21):2529–2548 21. Djurovich PI, Mayo EI, Forrest SR, Thompson ME (2009) Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors. Org Electron 10(3):515– 520 22. Tsutsumi K, Yoshida H, Sato N (2002) Unoccupied electronic states in a hexatriacontane thin film studied by inverse photoemission spectroscopy. Chem Phys Lett 361(5–6):367–373 23. Yoshida H (2012) Near-ultraviolet inverse photoemission spectroscopy using ultra-low energy electrons. Chem Phys Lett 539–540:180–185 24. Boudaiffa B, Cloutier P, Hunting D, Huels MA, Sanche L (2000) Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons. Science 287(5458):1658–1660 25. Yoshida H (2013) Low-energy inverse photoemission spectroscopy using a high-resolution grating spectrometer in the near ultraviolet range. Rev Sci Instrum 84(10):103901 26. Yoshida H (2014) Note: Low energy inverse photoemission spectroscopy apparatus. Rev Sci Instrum 85:016101 27. Yoshida H, Tsutsumi K, Sato N (2001) Unoccupied electronic states of 3d-transition metal phthalocyanines (MPc: M=Mn, Fe, Co, Ni, Cu and Zn) studied by inverse photoemission spectroscopy. J Electron Spectrosc Relat Phenom 121(1–3):83–91 28. Reimer L (1997) In: Hawkes PW (ed) Transmission electron microscopy: physics of image formation and microanalysis, Springer series in optical sciences, vol 36. Springer, Berlin 29. Yoshida H (2012) New experimental method to precisely examine the LUMO levels of organic semiconductors and application to the fullerene derivatives. MRS Symp Proc 1493:295–301 30. Han W, Yoshida H, Ueno N, Kera S (2013) Electron affinity of pentacene thin film studied by radiation-damage free inverse photoemission spectroscopy. Appl Phys Lett 103:123303 31. Sato N, Seki K, Inokuchi H, Harada Y (1986) Photoemission from an amorphous pentacene film. Chem Phys 109(1):157–162 32. Fukagawa H, Yamane H, Kataoka T, Kera S, Nakamura M, Kudo K, Ueno N (2006) Origin of the highest occupied band position in pentacene films from ultraviolet photoelectron spectroscopy: hole stabilization versus band dispersion. Phys Rev B 73(24):245310 33. Duhm S, Heimel G, Salzmann I, Glowatzki H, Johnson RL, Vollmer A, Rabe JP, Koch N (2008) Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies. Nat Mater 7(4):326–332 34. Krause S, Casu MB, Scholl A, Umbach E (2008) Determination of transport levels of organic semiconductors by UPS and IPS. New J Phys 10:085001 35. Refaely-Abramson S, Baer R, Kronik L (2011) Fundamental and excitation gaps in molecules of relevance for organic photovoltaics
Measuring the electron affinity of organic solids
36.
37.
38.
39.
from an optimally tuned range-separated hybrid functional. Phys Rev B 84(7):075144 Silinsh EA, Belkind AI, Balode DR, Brseniece AJ, Grechov VV, Taure LF, Kurik MV, Vertzymacha JI, Bok I (1974) Photoelectrical properties, energy level spectra, and photogeneration mechanisms of pentacene. Phys Stat Sol (a) 25:339 Lang DV, Chi X, Siegrist T, Sergent AM, Ramirez AP (2004) Amorphouslike density of gap states in single-crystal pentacene. Phys Rev Lett 93(8):086802 Sebastian L, Weiser G, Bassler H (1981) Charge transfer transitions in solid tetracene and pentacene studied by electroabsorption. Chem Phys 61:125 Fabiano S, Yoshida H, Chen ZH, Facchetti A, Loi MA (2013) Orientation-dependent electronic structures and charge transport mechanisms in ultrathin polymeric n-channel field-effect transistors. ACS Appl Mater Interfaces 5(10):4417–4422
40. Ishii H, Sugiyama K, Ito E, Seki K (1999) Energy level alignment and interfacial electronic structures at organic metal and organic organic interfaces. Adv Mater 11(8):605–625 41. Braun S, Salaneck WR, Fahlman M (2009) Energy-level alignment at organic/metal and organic/organic interfaces. Adv Mater 21(14–15): 1450–1472 42. Ueno N, Kera S (2008) Electron spectroscopy of functional organic thin films: deep insights into valence electronic structure in relation to charge transport property. Prog Surf Sci 83(10–12):490–557 43. Hoffmann R (1988) Solids and surfaces: a chemist's view of bonding in extended structures. VCH Publishers, New York 44. Ashcroft NW, Mermin ND (1976) Solid state physics. Thomson Learning, London 45. BudkeM, Renken V, LieblH, Rangelov G, DonathM (2007) Inverse photoemission with energy resolution better than 200 meV. Rev Sci Instrum 78(8):083903
