1. RIKEN KEIKI CO., LTD1 Photo-electron spectrometer in air Model AC-2 The counting mechanism of the photoelectron and The application to studies of The Organic Light Emitting Diode (OLED)

2. RIKEN KEIKI CO., LTD2 5. Conclusion 4. The application to the OLED T he measurements of the IPs and WFs of the Organic materials. 3. The data analysis What do the photoelectron spectrums mean? 2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air? 1. The outline of AC-2 applications, features Contents 1. The outline of AC-2 applications, features 2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air? 3. The data analysis What do the photoelectron spectrums mean? 4. The application to the OLED T he measurements of the IPs and WFs of the Organic materials. 5. Conclusion

3. RIKEN KEIKI CO., LTD3 Contents 1. The outline of AC-2 applications, features 2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air? 3. The data analysis What do the photoelectron spectrums mean? 4. The application to the OLED T he measurements of the IPs and WFs of the Organic materials. 5. Conclusion

4. RIKEN KEIKI CO., LTD4 1. Outline of Photo-Electron Spectrometer in Air (PESA) Light source part Measuring part Personal computer More than 170sets are used in Japan and world market. MODEL AC-2 120cm 45cm 36cm Mainly applications Material research of the OLED. The quality check of an ITO cleaning. The surface research of an MgO film for the PDP. Latest applications Organic transistor Organic solar battery Catalyst of fuel cell

5. RIKEN KEIKI CO., LTD5 The features of AC-2 Measurements can be done in the air. Usually a photoelectron spectrometer needs a vacuum. Because it is very difficult to detect and to count electrons in air. Easy operation & short measuring time. The work function and ionization potential can be measured in the air in very high resolution. Measurement of contamination or film thickness on a sample surface of 1mono layer-20nm.

6. RIKEN KEIKI CO., LTD6 Contents 1. The outline of AC-2 applications, features 2. The mechanism of counting photoelectrons We employed the open counter for the detector of AC-2. 3. The data and the analysis What do the photoelectron spectrums mean? 4. The application to the OLED T he measurements of the IPs and WFs of the Organic materials. 5. Conclusion

7. RIKEN KEIKI CO., LTD7 UV Light Sample HV Pulse counter Vacuum chamber A photoelectron is detected as an electric pulse. Conventional methods for detecting electrons 1pA (10 -12 A ) 6.24x10 6 cps (62 million/s) Photoelectrons are detected as quite small amount of electric current. Sample UV light Collector e e Picoammeter Channeltron e A pulse of 10 million electrons The electrons strike the channel walls and produce additional electrons. This method is not sensitive enough for photoelectron spectroscopy. This device works only in a vacuum.

8. RIKEN KEIKI CO., LTD8 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample O2O2 e The open counter The open counter is employed the photoelectron spectrometer in air. The open counter is unique counter which can detect and count photoelectrons in the air. e Display 1 count Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68. Open counter was invented in 1979 by Uda and Kirihata.

9. RIKEN KEIKI CO., LTD9 Anode Quenching Grid Suppresser Grid Structure The open counter is composed of an anode, a quenching grid, suppresser grid and electric circuits. Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Display 0 count Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68. 14mm 80mm

10. RIKEN KEIKI CO., LTD10 Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample The test sample is mounted on the sample stage which is kept at earth potential (0V). Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Display 0 count Structure Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68. 15mm

11. RIKEN KEIKI CO., LTD11 Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V Suppresser Grid is kept at +80V, Quenching Grid is +100V, Anode is +2900V. Display 0 count Structure Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

12. RIKEN KEIKI CO., LTD12 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V Measurement Monochromatized UV photons are used to excite photoelectrons from the sample surface. Display 0 count Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

13. RIKEN KEIKI CO., LTD13 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V Electron e If the energy of an UV photon(=h ) becomes bigger than a work function of a sample, a photoelectron is emitted from the sample surface. Display 0 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

14. RIKEN KEIKI CO., LTD14 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V Electron e The electron is accelerated by a weak electric field applied between the sample (0V) and the suppresser grid kept at +80V. Display 0 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

15. RIKEN KEIKI CO., LTD15 Sample surface O2O2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 O2O2 e N2N2 N2N2 N2N2 O2O2 N2N2 N2N2 O 2 - ion The electron becomes attached to an oxygen molecule to form O 2 - ion in the air during drift to the counter. Form the O 2 - ion Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

16. RIKEN KEIKI CO., LTD16 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V O2O2 e When an O 2 - ion reaches the inner cylinder of the open counter, the ion is accelerated again by a strong electric field applied between the quenching grid ( +100V) and the anode kept at +2900V. Display 0 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

17. RIKEN KEIKI CO., LTD17 Anode surface O2O2 O2O2 O2O2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 O2O2 N2N2 N2N2 N2N2 O2O2 Electric Field N2N2 N2N2 N2N2 N2N2 O2O2 e O 2 - ion O2O2 e The electron avalanche When the O 2 - ion arrives near the anode, the electron is detached from the O 2 - ion and then the electron is accelerated again to the anode. Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

18. RIKEN KEIKI CO., LTD18 Anode surface O2O2 O2O2 O2O2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 N2N2 O2O2 N2N2 N2N2 N2N2 O2O2 Electric Field N2N2 N2N2 N2N2 N2N2 O2O2 e O 2 - ion O2O2 e + e e + + + + + + + + + + e e e e e e e e ee e e The electron avalanche This causes an electron avalanche, which produces many electrons and positive ions around the anode wire, where only the electrons are collected on the anode. Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

19. RIKEN KEIKI CO., LTD19 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V The electron avalanche makes a small electric pulse on anode. This pulse is detected and counted as one electron. Display 0 count Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

20. RIKEN KEIKI CO., LTD20 Quenching Grid UV Light Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +2900V Quenching Grid +400V When the quenching circuit detects the electric pulse, +400V is applied in place of +100V to the quenching grid. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

21. RIKEN KEIKI CO., LTD21 Quenching Grid UV Light Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +2900V Quenching Grid +400V This reduction of the electric field around the anode causes the electron avalanche to be quenched. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

22. RIKEN KEIKI CO., LTD22 Suppresser Grid UV Light Quenching Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample Suppresser Grid -30V +400V +2900V On the other hand, -30V is applied in place of +80V to the suppressor grid. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

23. RIKEN KEIKI CO., LTD23 Suppresser Grid UV Light Quenching Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample Suppresser Grid -30V +400V +2900V Ion This voltage switch prevents leaving of the positive ions to the sample surface, and entering of the next O 2 - ions into the counter during quenching. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

24. RIKEN KEIKI CO., LTD24 Suppresser Grid UV Light Quenching Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample Suppresser Grid -30V +400V +2900V 3ms Such voltages applied to the quenching grid and suppressor grid are kept for 3msec i.e. the quenching time. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

25. RIKEN KEIKI CO., LTD25 Suppresser Grid UV Light Quenching Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample Suppresser Grid -30V +400V +2900V 3ms All positive ions produced around the anode wire arrive and neutralize at the quenching grid or suppressor grid during quenching time. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

26. RIKEN KEIKI CO., LTD26 Suppresser Grid UV Light Quenching Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample Suppresser Grid -30V +400V +2900V 3ms If the next electron has emitted from the sample surface, the suppresser grid prevents entering of the electron during quenching time. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

27. RIKEN KEIKI CO., LTD27 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample +80V +100V +2900V After the quenching time (3ms), the initial voltages (+100V and +80V, respectively) are restored and the quenching procedure is over. Display 1 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

28. RIKEN KEIKI CO., LTD28 UV Light Quenching Grid Suppresser Grid Anode Suppresser circuit Quenching circuit High Voltage Supply Scaling circuit and Rate meter Preamplifier Sample holder Sample O2O2 e Now the counter is ready to count the next electron. e Display 1 count Display 2 count Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

29. RIKEN KEIKI CO., LTD29 The calculation of counts Electron counts may be lost during the quenching time. The counting rates of electrons are estimated based on the counting rates of counter pulses by calculation. NONO 1-tN O N = N O : Counting rate of counter pulses N : Counting rate of photo electrons t :Quenching time Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

30. RIKEN KEIKI CO., LTD30 Measuring part open counter Light source part grating monochromator deuterium lamp e : UV light : photoelectron e sample sample stage Structure and functions of AC-2 open counter controller personal computer optical fiber deuterium lamp grating monochromator open counter 3.4eV 6.2eV photodiode

31. RIKEN KEIKI CO., LTD31 Measuring part open counter Light source part grating monochromator deuterium lamp : UV light : photoelectron e sample stage Structure and functions of AC-2 open counter controller personal computer optical fiber deuterium lamp grating monochromator open counter 3.4eV 6.2eV photodiode

32. RIKEN KEIKI CO., LTD32 Contents 1. The outline of AC-2 applications, features 2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air? 3. The data and the analysis What do the photoelectron spectrums mean? 4. The application to the OLED T he measurements of the IPs and WFs of the Organic materials. 5. Conclusion

33. RIKEN KEIKI CO., LTD33 Yield : counting rate after calibtation counting rate of photoelectrons (E) / intensity of UV-ray (E) x intensity of UV-ray (5.9eV) (Yield[cps]) 1/2 Incident Photon Energy [eV] Photoelectron Spectrum Photoemission Threshold Energy[eV] Metal : the relationship between the photon energy and yield 1/2 looks like a linear line. Semiconductor : yield 1/3 gives a linear line.

34. RIKEN KEIKI CO., LTD34 The relationship between the threshold energy and the energy diagrams Conduction Band Valence Band Fermi Level The energy level diagrams of metals, semiconductors and general materials Energy Vacuum level General material Metal Semiconductor Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)

35. RIKEN KEIKI CO., LTD35 The relationship between the threshold energy and the energy diagrams Conduction Band Valence Band Fermi Level Energy Vacuum level General material Metal Semiconductor Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO) A work function is an energy difference between a vacuum level and a Fermi level. On the other hand, the ionization potential is an energy difference between a vacuum level and highest occupied molecular orbital. Ionization potential Work function

36. RIKEN KEIKI CO., LTD36 The relationship between the threshold energy and the energy diagrams Conduction Band Valence Band Fermi Level Energy Vacuum level General material Metal Semiconductor Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO) A UV photon excites an electron from the occupied states to the higher energy states than the vacuum level. And this electron can be emitted from the sample surface. The electron is called the photoelectron. Photoelectron e e e UV photon

37. RIKEN KEIKI CO., LTD37 The relationship between the threshold energy and the energy diagrams Conduction Band Valence Band Fermi Level Energy Vacuum level General material Metal Semiconductor Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO) Therefore, the photoemission threshold energy is the ionization potential. Ionization potential Photoelectron UV photon e e e

38. RIKEN KEIKI CO., LTD38 The relationship between the threshold energy and the energy diagrams Conduction Band Valence Band Fermi Level Energy Vacuum level Ionization potential General material Metal Semiconductor Ionization potential Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO) The metal is special material. Because, the energy of highest occupied molecular orbital and the Fermi level are same. Therefore the photoemission threshold energy of a metal is the work function. Ionization potential Metal Work function Photoelectron UV photon e e e We can estimate the work functions or ionization potentials of the materials from the photoemission threshold energy.

39. RIKEN KEIKI CO., LTD39 The shape of a photoelectron spectrum Potential energy DOS Reference: M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767. Energy level diagram Energy Vacuum level unoccupied molecular orbital occupied molecular orbital

40. RIKEN KEIKI CO., LTD40 Potential energy Energy level diagram DOS the relationship between an energy level diagram and the photoelectron spectrum Reference: M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767. Photoemission yield (Y) Photoelectron spectrum Photon energy 0 dY dE = The DOS is estimated by the differential of the photoelectron spectrum with respect to the photon energy E.

41. RIKEN KEIKI CO., LTD41 WF f > WF s The difference of a photoelectron spectrum caused by the thickness of a surface film Contamination or Oxide film (0-20nm) Substrate Incident photon (E) WF f WF s - - - - - - - - The cross section of the sample surface covered with a thin film WF f > E > WF s

42. RIKEN KEIKI CO., LTD42 WF f > WF s The difference of a photoelectron spectrum caused by the thickness of a surface film Contamination or Oxide film (0-20nm) Substrate Incident photon (E) WF f WF s - - - - - - - - When photoelectrons pass through a surface film, some electrons collide with a molecule forming the surface film, and the photoelectrons are scattered. So some of photoelectrons can not escape from the sample surface. WF f > E > WF s

43. RIKEN KEIKI CO., LTD43 WF f > WF s The difference of a photoelectron spectrum caused by the thickness of a surface film Contamination or Oxide film (0-20nm) Substrate Incident photon (E) WF f WF s - - - - - - - - So, when the surface film is thick, many photoelectrons can not be emitted from the sample surface. WF f > E > WF s

44. RIKEN KEIKI CO., LTD44 The relationship between the slope and film thickness Large slope Thin contamination film - - - - - - - - (Yield[cps]) n Incident Photon Energy [eV] - - - - - - - - Thick contamination film (Yield[cps]) n Small slope WF f WF s WF f WF s WF f WF s - - - - - - - - - -

45. RIKEN KEIKI CO., LTD45 Contents 1. The outline of AC-2 applications, features 2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air? 3. The data and the analysis What do the photoelectron spectrums mean? 4. The application to the OLED T he measurements of the IPs and WFs of the Organic materials. 5. Conclusion

46. RIKEN KEIKI CO., LTD46 Glass Substrate ITO transparence anode Organic layers Metal cathode The applications to the OLEDs + - OLED device Photon WF Ionization Potential HOMO ITO Vacuum Level Organic LUMO Fermi Level Energy diagram of ITO and Organic Layer HOMO Barrier Fermi Level Hole +

47. RIKEN KEIKI CO., LTD47 Work functions of several metals In Air 1) [eV]In UHV 2) [eV] Fe4.354.50 Ni4.255.15 Cu4.454.65 Al3.604.20 Zn3.80- Au4.785.10 1)M. Uda ; Jpn. J.Appl.Phys. 24,284 (1985) 2)D.E. Eastman ; Phys.Rev. B2, 1 (1970)

48. RIKEN KEIKI CO., LTD48 Work functions or Ionization potentials of EL materials AC-2 [eV]UPS 1) [eV] Alq35.845.8  -NPD 5.505.4 CuPc4.995.2 1) I.G.Hill and A.Kahn, J.Appl.Phys. 86,4515 (1999)

49. RIKEN KEIKI CO., LTD49 4.7 4.9 5.1 5.3 5.5 0102030405060 duration time after treatment (min) Work function (eV) before cleaning UV-ozone 10min UV-ozone 20min UV-ozone 60min UV-ozone 0min (boiling in IPA ) The temporal change in the work function of ITO treated with UV-ozone Reference: Y. Nakajima, T. Wakimoto, T. Tuji, T. Watanabe, M.Uda, The 10th International Workshop on Inorganic and Organic Electroluminescence (2000.12.4), Hamamatu, Japan.

50. RIKEN KEIKI CO., LTD50 Change in WF of Al by Cl 2 Contamination Al in air : 4.05eV Al exposed mixed gas 20sec (Cl 2 1.34ppm + air) : 4.33eV (Yield[cps]) 0.5 Incident Photon Energy [eV]

51. RIKEN KEIKI CO., LTD51 5. Conclusion 1. We can detect and count photoelectrons in the air by the open counter. 2. We can measure the work function or the ionization potential of the OLED materials easily. 3. We can estimate the amount of the contamination on the ITO surface from 1mono-layer to 20nm in the thick. 4. AC-2 is the de facto standard equipment of the work function measurement on the OLED development.

52. RIKEN KEIKI CO., LTD52 References [1] H.Kirihata, and M.Uda “Externally quenched air counter for low-energy electron emission measurements”, Rev. Sci. Instrum. 52 (1981) 68. [2] M.Uda “Open counter for low energy electron detection”, Jpn. J.Appl. Phys. 24 (1985) 284. [3] T. Noguch, S. Nagashima and M. Uda “An electron counting mechanism for the open counter operated in air”, Nucl. Instr. Meth. A342 (1994) 521. [4] S. Nagashima, T. Tsunekawa, N, Shiroguchi, H. Zenba, M. Uda “Double cylindrical open counter of pocket size”, Nucl. Instr. Meth. A373 (1996) 148. [5] A. Koyama, M. Kawai, H. Zenba, Y. Nakajima, A. Yoneda and M. Uda “Electron counting by a double cylindrical open counter in mixtures of N2 and inert gases of various concentrations”, Nucl. Instr. and Meth. in Phys. Res. A422 (1999) 309.

53. RIKEN KEIKI CO., LTD53 References [6] M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767. [7] Y. Nakajima, M. Hoshino, D. Yamashita and M. Uda “ Near Edge Structures of Tetraphenylporphyrins Measured by PESA and Calculated with DV-Xα”, Adv. Quantum Chem. 42 (2003) 399. [8] Y. Nakajima, T. Wakimoto, T. Tuji, T. Watanabe, M.Uda “Measurements of the work function of ITO Using an electron spectroscopy in air and a contact potential difference method”,The 10th International Workshop on Inorganic and Organic Electroluminescence (2000.12.4), Hamamatu, Japan.