Effect of vacuum annealing on the optoelectric and morphological properties of F 16 CuPc thin films

Brief Background : Teaching in Catholicate College , Pathanamthitta since March, 2013. Education Ph.D in Thin Film Physics, under the guidance of Prof.(Dr).C.S.Menon, (Former Dean of Science and Director of SPAP) at School of Pure and Applied Physics, M.G.University, Kottayam. M.Phil (Physics) at School of Pure and Applied Physics, M.G.University, Kottayam. M.Sc (Material Science) at Department of Physics, Catholicate College, Pathanamthitta, under M.G.University. B.Sc (Physics) at Department of Physics, Mar Ivanios College, Thiruvanathapuram, under University of Kerala Main Subject Studied Material Science Solid State Physics Laser Physics Research Interests Research related to amorphous semiconductor thin films involving their structural, optical and electrical characterization using various techniques. Research Experience Six Years PUBLICATIONS Research Papers Published:


Introduction
In recent years, organic semiconductors have attracted a great deal of attention due to their potential use in a wide range of technological applications.Due to their interesting physical properties combined with the low material cost make this particular family of organic compounds a strong candidate for the electronic and optoelectronic devices.Phthalocyanines (Pcs) represent one of the most promising candidates for ordered organic thin films.As these systems possess advantageous attributes such as chemical stability and excellent film growth which results in optimized electronic properties.Many of these materials incorporating different metals have been investigated for gas sensing 1 , photovoltaic devices 2,3 , thin film transistors 4 , organic light emitting diodes 5 and Schottky diodes 6 .The light absorbing properties of phthalocyanines in the visible and infra red regions are of significant importance because of the current interest in the conversion of solar to electrical energy 7 .Hexadecafluorophthalocyanines attracted interest as a possible n-type organic semiconductor with high electron mobility and good stability characteristics 8,9 .This suggests that hexadecafluorophthalocyanines are useful to develop organic bipolar devices such as transistors and electroluminescent diodes 10 .Among various phthalocyanines, copper hexadecafluoro phthalocyanine has received considerably less attention.This article brings out the results of the optical band gap, refractive index and dielectric constants, from the absorption co-efficient and extinction co-efficient of F 16 CuPc thin films, which is essential for the fabrication of electronic devices.From the electrical studies we estimate the values of activation energies in the intrinsic and extrinsic regions.In the paper we also investigate the morphological properties of the F 16 CuPc thin films.Results show that F 16 CuPc is highly stable against heat, high energy radiations and atmospheric air.

Experimental
Copper hexadecafluoro phthalocyanine, procured from Sigma-Aldrich Chemicals (USA), was purified by the train -sublimation technique using nitrogen as the carrier 11 and was used as the source material for thermal evaporation.Thin films of F 16 CuPc were prepared on a glass substrate using a Hind Hivac 12A4 evaporation plant.Glass slides with dimensions 5 cm x 1.15 cm x 0.1 cm were used as substrates.Evaporation of the material was done at a base pressure of 10 -5 Torr using a molybdenum boat.During evaporation the substrates are placed at a distance of 11 cm above the source and the deposition rate is controlled within a range of 10 -12 nm / min.The deposition was controlled at 10 -13 nm per minute.F 16 CuPc thin films 60±5 nm thick were annealed in vacuum for 1 h at 373,423 and 473 K for 1 h in vacuum at a base pressure of 10 -3 Torr.The thickness of the film is measured using a Dektac thickness profilometer and is cross checked with a Tolansky's multiple beam interference technique.The absorption spectra of F 16 CuPc thin films are recorded using the Shimadzu 160 A UV-Vis-NIR Spectrophotometer in the wavelength range of 200-900 nm.Absorption spectra of the films are analyzed to obtain the energy band gap.Reflectivity measured by using a Jasco V-570 Spectrophotometer.Electrical measurements are performed using a Programmable Keithley electrometer (Model No.617) in the constant current source mode.Evaporated silver electrodes are used for ohmic contacts.Each sample is mounted on the sample holder of the conductivity cell.Electrical contacts are made using copper strands of diameter 0.6 mm and are fixed to the specimen with a colloidal suspension of silver in alkadag.To avoid contamination the measurements are done in a subsidiary vacuum of 10 -3 Torr.The surface morphology of the thin films is studied using the JOEL JSM scanning electron microscopy images.

Optical properties
The optical properties of a material are important, as they provide information on the electronic structures, localized states and type of optical transitions.The optical absorption spectra of all the prepared samples of F 16 CuPc in the wavelength range 200-900 nm are recorded.Typical absorption spectrums of as-deposited and vacuum-annealed samples are given in Figure 1.The molecular spectra in Pc's originate essentially from the molecular orbital's within the aromatic 18 π electron system and from the overlapping orbital on the central metal atom 12 .All the absorption spectra consists of two main bands B band (Soret band) and Q band.The electronic transitions from the π-π * orbitals result in intense band in the UV region called B band positioned at 358 nm which gives the fundamental absorption edge, while broad band that appeared in the visible region called Q band, which split up into two, positioned at 638 nm and 792 nm gives the excitonic energy 13 .The fundamental absorption edge is analyzed within the framework of one electron theory of Bardeen et.al 14 .Using this theory, energy band gaps of a number of MPc's have been found out 13,15 .The absorption coefficient α is calculated and is related to the band gap Eg and photon energy hν according to the relation 16 .
α = α 0 (hν -E g ) n (1) Where n= ½ for direct allowed transitions and n=2 for indirect allowed transitions Figure 2 shows the plots of variation of α 2 with hν for F 16 CuPc thin film deposited at room temperature of the as-deposited and vacuum annealed temperatures.A satisfactory fit is obtained for α 2 vs. hν indicating the presence of a direct band gap.The intercept on the energy axis gives the energy band gap.A band gap of (3.06±0.01eV) is obtained.The absorption at lower energy side is related to singlet exactions and has been confirmed for many other phthalocyanines 17 .The variation in band gap energy with annealing temperature is given in Table 1.
Where, α is the absorption coefficient and λ is the wavelength.The reflectivity of an absorbing medium of indices n and k in air for normal incidence is given by R = ( 1) ( 1) Thus by knowing the R and k, n is calculated.The optical properties of the medium are characterized by a complex refractive index (N=n-ik) and complex dielectric constants (ε 1 and ε 2 ).The real part generally relates to dispersion, while the imaginary part provides measure of the dissipative rate of the wave in the medium 15 .
In this way the dependence of the real and imaginary part of the dielectric constant (ε 1 and ε 2 ) and on photon energy and dependence of n and k on photon energy for F 16 CuPc also are plotted.The real and the imaginary parts of the dielectric constants are calculated using equations 4&5 and the variations can be check.The two parameters are related to n and k through the equation

Electrical studies
Electrical conductivity studies on F 16 CuPc thin films of 60±5 nm thickness are used to determine thermal activation energy and to study the effect of vacuum annealing temperature on the activation energy.In inorganic semiconductors the semiconducting properties are brought about by thermal excitation, impurities, lattice defects and nonstochiometry.Holes in the valence band and electrons in the conduction band contribute the electrical conductivity.The electrical conductivity σ can be expressed as σ =σ 0 exp(-E a /k b T) (6) Where σ is the conductivity at temperature T, E a is the thermal activation energy, k b is the Boltzmann constant and σ 0 is a pre-exponential factor.Arrhenius plots of lnσ vs. 1000/T of F 16 CuPc thin films are made to study the dependence of the conduction mechanism on vacuum annealing temperature 473 K and as-deposited form are given Figure 6  The activation energy E 1 is associated with an intrinsic generation process.It is based on the assumption of electron hole pair production via thermal transition from a valance band to the conduction band and may occur in several organic solids.The phthalocyanine owes their intrinsic conductivity to the partial charge transfer from phthalocyanine ring to the central ion.E 2 and E 3 are the activation energy needed to excite the carriers from the corresponding trap levels to conduction band and are associated with the impurity conduction 18 .The activation energies E 1 , E 2 and E 3 obtained are collected in Table 2.

Surface morphological studies
Morphology and crystal growth in organic thin films have much influence on their recording efficiency and optical properties 19 .Surface topography and micro structural features of thin films are controlled by film preparation procedures 20 .It is well known that the morphology of vacuum sublimed organic films varies with deposition parameters 21 .The SEM micrographs shown in Figure 7   The size of the microcrystalline grains is found to decrease with increase of annealing temperature.As annealing temperature increases agglomerated forms of the particles and cloudy nature is obtained.The fine grain crystallite on the as-deposited films is transformed to a structure of nature with agglomerated form.The microcrystalline grain boundaries of F 16 CuPc thin films annealed in air increases with increase of temperature.These films are polycrystalline in nature.

Conclusion
Electrical, optical and surface morphological studies of F 16 CuPc thin films have been studied with respect to post deposition vacuum annealing.Fundamental optical parameters have been determined from both absorption and reflection data.A direct allowed transition is observed.The optical band gap energy is 3.06±0.01eV and excitonic energy gap also remains unchanged under different heat treatments.Electrical conductivity by thermal activation process is found to involve different conduction mechanisms.The activation energies in the intrinsic region are observed to increase with increase in vacuum annealing temperature.The increase in activation energy is due to the reversible change in molecular interactions observed in fluorinated phthalocyanines, which are n-type semiconductors.In F 16 CuPc thin films the fluorine atoms are help to block the penetration of moisture.The increase in activation energy is due to the decrease in interaction of molecular π-systems of the fluorinated phthalocyanine.This decrease in interaction leads to an increased barrier of charge transport in the film and hence the increase in activation energy.From the SEM study reveals that the size of the microcrystalline grains is found to decrease with increase of annealing temperature.As annealing temperature is increased a cloudy nature of film is observed.F 16 CuPc thin films are the potential candidates for the fabrication of molecular electronic devices.In future there is find application on the fabrication and characteristic study of p-n junction diodes using CuPc and F 16 CuPc thin films.

Figure 1 .
Figure 1.Plot of absorbance versus wavelength for F 16 CuPc thin films as-deposited form and different vacuum annealing temperatureThe molecular spectra in Pc's originate essentially from the molecular orbital's within the aromatic 18 π electron system and from the overlapping orbital on the central metal atom12 .All the absorption spectra consists of two main bands B band (Soret band) and Q band.The electronic transitions from the π-π * orbitals result in intense band in the UV region called B band positioned at 358 nm which gives the fundamental absorption edge, while broad band that appeared in the visible region called Q band, which split up into two, positioned at 638 nm and 792 nm gives the excitonic energy13 .The fundamental absorption edge is analyzed within the framework of one electron theory of Bardeen et.al14 .Using this theory, energy band gaps of a number of MPc's have been found out13,15 .The absorption coefficient α is calculated and is related to the band gap Eg and photon energy hν according to the relation16 .α= α 0 (hν -E g ) n (1) Where n= ½ for direct allowed transitions and n=2 for indirect allowed transitions Figure2shows the plots of variation of α 2 with hν for F 16 CuPc thin film deposited at room temperature of the as-deposited and vacuum annealed temperatures.A satisfactory fit is obtained for α 2 vs. hν indicating the presence of a direct band gap.The intercept on the energy axis gives the energy band gap.A band gap of (3.06±0.01eV) is obtained.The absorption at lower energy side is related to singlet exactions and has been confirmed for many other phthalocyanines17 .The variation in band gap energy with annealing temperature is given in Table1.

Figure 3 .
Figure 3. Reflectance versus wavelength for as-deposited F 16 CuPc thin film The extinction coefficient k is calculated using the equation.k = 4 αλ π (2)

Figure 4 .Figure 5 .
Figure 4. Plot of refractive index n and extinction coefficient k versus photon energy for as deposited F 16 CuPc thin film Figure 5. Plot of real and imaginary parts of the dielectric constant versus photon energy for as deposited F 16 CuPc thin film It is observed that these parameters vary with photon energy.The refractive index n has a maximum value of 1.793 at 2.797 eV for the F 16 CuPc thin film.The extinction coefficient k has a maximum value of 0.786 at 1.562 eV.The real part ε 1 shows a maximum value of 3.208 at 2.775 eV while imaginary part ε 2 has a maximum value of 2.414 at 1.958 eV.It is observed that these parameters vary with photon energy.
Figure 6(a).Plot of lnσ vs. 1000/T for the as-deposited film Figure 6(b).Plot of lnσ vs. 1000/T for the film annealed at 473 K a and b display the development of the surface morphology of 165 nm thick F 16 CuPc films as-deposited form and annealed at 473 K in vacuum.

Figure 7 .
Figure 7. (a) and (b) SEM images of as-deposited and annealed at 473 K in vacuum respectively.The size of the microcrystalline grains is found to decrease with increase of annealing temperature.As annealing temperature increases agglomerated forms of the particles and cloudy nature is obtained.The fine grain crystallite on the as-deposited films is transformed to a structure of nature with agglomerated form.The microcrystalline grain boundaries of F 16 CuPc thin films annealed in air increases with increase of temperature.These films are polycrystalline in nature.

Table 1 .
Variation in band gap energy with annealing temperature

Table 2 .
Variations of activation energies with annealing temperatures The intrinsic activation energy E 1 , increases with increase in annealing temperature.This is attributed to better ordering of the film due to annealing.The value of E 2 and E 3 and its corresponding increase with increase in annealing temperature are attributed to the different distribution of shallow trapping states.