Electronic properties of the iron phthalocyanine thin films UHV annealed and exposed to oxygen

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Electronic properties of the iron phthalocyanine films UHV annealed and exposed to oxygen

thin

J Szuber, B Szczepaniak”, S Kochowski and A Opilski, Institute of Physics, Silesian Technical University, 44-100 Gliwice, Krzywoustego 2, Poland

The electronic properties of the UHV annealed iron phthalocyanine (FePc) thin films exposed to oxygen have been studied using Photoemission Yield Spectroscopy (PYS). Depending on the annealing temperature as well as the oxygen exposure, the variations of the work function cpwere analyzed together with the effective density of the filled electronic states localized in the band gap below the Fermi level E, and in the upper part of the valence band. The possible origin of these electronic states was briefly discussed.

1. Introduction

In the last twenty years considerable efforts have been made in order to understand the electronic properties of phthalocyanine (PC) compounds, the well known organic materials with interesting semiconducting’and catalytic4 properties. Because of both their high temperature and high chemical stability’, they became widely used in thin film technology for specific optoelectronic devices6, solar cells’ and gas sensors*. A lot of experimental work on the electronic properties of various phthalocyanines has been done. For the case of iron phthalocyanine (FePc) thin films, up to now only ultraviolet photoelectron spectroscopy (UPS)%13 has been used for this purpose. In the earliest UPS studies of Vilesov et aL9, re-examined by Schechtman and Spicer”, only optical density of the states in the upper part of the valence band up to -7 eV have been determined. It was characteristic for the conjugated inner ring of the main molecule and found to be in rather weak agreement with the free-electron molecular orbital calculations”. Thereafter, Pong and Smith’* and Koch and Grobman13 have studied the iron phthalocyanine (FePc) photoemission spectra for the photon energy range of 7-23 eV, but only for the higher photon energy they observed the structure characteristic for the optical density of states. In addition, the electron mean free path for the energies larger than 1.5 eV above the valence band was estimated to be about 1 nm’*. Because of the limited energy range and sensitivity of the UPS method, many questions concerning the electronic properties of the iron phthalocyanine (FePc) thin films still remain unanswered. Investigations using other techniques are expected to give new and complementary information. One of the best and suitable methods for this purpose seems to be Photoemission Yield Spectroscopy (PYS), mainly

* Permanent address : Institute of Physics, Pedagogical University, 42200 Czestochowa, Al. Armii Krajowej 13/15, Poland.

developed by the group of Sebenne in Paris14. The possibilities of this method in studies of the electronic states localized in the band gap of organic semiconductors, were confirmed in our recent investigations of the electronic properties of the space charge layer of the copper phthalocyanine (CuPc) thin films UHV annealed and exposed to oxygen’s,‘6. In this paper we present the results of the Photoemission Yield Spectroscopy (PYS) investigations of the influence of UHV annealing as well as oxygen exposure on the electronic properties of the iron phthalocyanine (FePc) thin films.

2. Experimental procedure The iron phthalocyanine (FePc) thin films used in these studies were prepared by vacuum evaporation method (10-j Pa), from the iron phthalocyanine (FePc) powder manufactured by ALDRICH, onto a glass substrate kept at room temperature, similarly as in other previous studies’&‘*. The thickness of the obtained films was about 1 pm. After mounting of the sample manipulator in the measuring chamber of the home-built ‘multiple-technique’ surface spectrometer” with a base pressure of about lo-’ Pa, the sample was annealed for 3 hours at different temperatures up to 410 K, as in recent studies on the effects of annealing and exposure to oxygen on the electronic properties of the copper phthalocyanine (CuPc) thin films’s,‘6. After cooling the sample at room temperature the photoemission yield spectra Y(E) were recorded as the number of emitted photoelectrons per incident photon as a function of photon energy E up to 6.2 eV. Because of the large dynamic range (up to eight orders of magnitude) and high energy resolution (up to 0.03 eV), PYS allows us to determine, among other things, the values of work function cp and ionization energy CD.Moreover, with the debatable but usual assumption that the transition matrix elements and escape depth remain constant in a usually small photon energy range (about 3 eV), the derivative of the photoemission yield with respect to photon energy [dY(E)/dE] 547

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is supposed to be proportional to the effective density of the filled electronic states N(E) localized in the band gap below the Fermi level EF and in the upper part of the valence band (within the escape depth of photoelectrons). In our Photoemission Yield Spectroscopy (PYS) experiments a light from a 40 W deuterium lamp D,E was passing through a high resolution SPM-2 monochromator with quartz optics and focused onto the sample at normal incidence. Photoelectrons were collected with a channeltron placed in a special metallic screen at an angle of about 15” from the normal to the sample. A small part of the light beam was deflected by a thin quartz plate toward a M12FQC51 photomultiplier in order to measure the spectral intensity of the incident photon flux. Yield spectra were taken at 1 nm intervals up to 200 nm (6.2 eV) and recorded by a digital counting electronics, and transferred to IBM PC type microcomputer for storage and subsequent analysis, as in our previous studies’5,‘6. 3. Results and discussions Figure 1 shows the typical spectra of the effective density of filled electronic states N(E) (in semilog plot) of the freshly evaporated iron phthalocyanine (FePc) thin films (curve a) and after UHV annealing at different temperatures (curve b to e respectively), obtained numerically as the first derivatives of the experimental photoemission yield spectra Y(E). From these spectra, using the the work function q and ionrecently proposed procedure”, ization energy @ were determined together with the effective density of the filled electronic states N(E) localized in the band gap below the Fermi level EF and in the upper part of the valence band. The work function cp, which gives the position of Fermi level EF with respect to the vacuum level E,,, can be determined by fitting the low energy tail of the spectrum N(E) with the Fermi distribution function14. For the investigated iron phthalocyanine (FePc) thin films, with increasing annealing temperature, the work function cp decreased in the range 4.55 till 3.80 eV (with accuracy of &-0.03 eV) which was about 0.4 eV greater than for

FePc

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Figure 1. Typical spectra of the effective density of states N(E) [arb units] of the iron phthalocyanine (FePc) thin films freshly evaporated (curve a) and UHV annealed at different temperatures (curves b to e respectively).

the copper phthalocyanine (CuPc) thin films in similar temperature range]‘. These changes are probably connected with the desorption of the water vapour and oxygen from the surface sensitive lattice defects. It should be moreover pointed out that the value of work function rp of the iron phthalocyanine (FePc) thin films UHV annealed at 410 K was almost equal to that obtained by Koch and Grobman13 for the metal-free phthalocyanine (H,Pc) thin films. The ionization energy @, which gives the position of the valence band edge E, with respect to the vacuum level E,,,, can be determined from the valence band contribution by fitting the high energy part of the effective density of states spectra N(E) of the semiconductor valence band structure’4. For all the spectra the best fitting we obtained using Kane’s” theory with the law Y(E) = (E- EJsy2 that depicts indirect electron transitions with scattering on ‘real’ surfaces and a value @ = 5.00f0.30 eV was obtained, almost constant for the different annealing temperatures. This value was almost equal to that obtained recently by Vilesov et aL9. It is well known that commonly prepared iron phthalocyanine (FePc) thin films have the properties of the intrinsic semiconductor’.“. Since we observed the photoemission it means that our iron phthalocyanine (FePc) thin films are rather highly extrinsic with a large defect induced density of states in the band gap in the form of the wide defect band, since the impurity concentration was rather small. The existence of filled electronic states occupying a wide band in the band gap of the iron phthalocyanine (FePc) thin films confirms the effective density of states N(E) spectra of the iron phthalocyanine (FePc) thin films. They are easily seen in Figure 1 as the low energy part of the N(E) spectra between the Fermi level EF and the top of the valence band E,. For the highest annealing temperature we additionally observed a narrow band of the filled electronic states near the Fermi level EF. In order to obtain additional information on these observed filled gap states we performed additional studies of the reactivity of iron phthalocyanine (FePc) thin films to oxygen chemisorption. Figure 2 shows the evolution of the spectra of the effective density of states N(E) of the iron phthalocyanine (FePc) thin films UHV annealed at highest temperature of 410 K after different oxygen exposures. One can easily see that with increasing oxygen exposure up to 10’ L the spectra of the effective density of states N(E), mainly in their low energy part, are shifted to higher photon energy E. This means that oxygen chemisorption provokes an increase in the work function cp by about 0.2 eV. This is opposite to that which has been observed with increasing annealing temperature (Figure 1) and can be attributed to the oxygen chemisorption. The drastic difference between decrease in the value of the work function with increasing annealing temperature and increase in the value of work function with the oxygen exposure was probably connected with the existence of water vapor in the evaporated iron phthalocyanine (FePc) thin films that desorbs during its UHV annealing, which was observed with mass spectrometric studies. Up to the oxygen exposure of lo4 L the shape of the spectra of effective density of states N(E) changed rather slightly. Only for the oxygen exposure of 10’ L the shape of N(E) spectrum is drastically changed and a narrow band of the filled electronic state band near the Fermi level EF completely disappeared. It means that one cannot exclude the possibility of the existence of the two different types of lattice defects that are responsible for

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function value cp decreased by about 0.6 eV, which was attributed to the desorption of water vapor and oxygen from the surface sensitive lattice defects, -with increasing the oxygen exposure up to lo5 L the work function cp of the iron phthalocyanine (FePc) thin films UHV annealed at highest temperature 410 K slightly increased by 0.20 eV, -there are two filled electronic state bands localized in the band gap of the investigated iron phthalocyanine (FePc) thin films that can be attributed to the two kinds of lattice defects which were different to the previously studied copper phthalocyanine (CuPc) thin film16.

a - annealed at 410 K IO-+-

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Acknowledgements 10-7 -

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The work was sponsored by Institute of Physics, Silesian Technical University within the Research Project BW/RMF-l/1995.

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Figure 2. Typical spectra of the effective density of states N(E) [arb units] of the iron phthalocyanine (FePc) thin films UHV annealed at 410 K (curve a) and then subjected to different oxygen exposures (curves b to d respectively).

the filled electronic states observed in the band gap of the iron phthalocyanine (FePc) thin films. The defects corresponding to the electronic states localized near the Fermi level EF are probably the surface sensitive lattice defects. Such filled gap states have already been predicted theoretically for the thin films as well as crystalline surface of the copper phthalocyanine (Cu PC) by Hamann and Lehmann*‘.*‘, and were attributed to the surface sensitive lattice defects of high concentration. 4. Conclusions In conclusion one can note that our Photoemission Yield Spectroscopy (PYS) studies of the iron phthalocyanine (FePc) thin films UHV annealed and subsequently exposed to oxygen showed that : -with

increasing the annealing temperature to 410 K the work

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