Optical and Electrical properties of organic Aluminum phthalocyanine based thin films for organic electronics

— This research emphasizes the optical and electrical properties of organic aluminum phthalocyanine (AlPc) molecule based thin films. The transmittance of such materials presents a narrow peak and valley within a range of 446 nm. Dielectric parameters such as refractive index and extinction coefficient are studied within the UV-Vis-IR range. Optical band gap (Eg) and Urbach energy (Eu) values are determined as 2.6 eV and 33 meV. Based on single oscillator dispersion energy and related parameters such as E 0 , E d , S 0 ,  0 , n 0 ,  L , N/m* and   .The nonlinear optic (NLO) properties are emphasized and electrical parameters are extracted from current-voltage characteristics of the Au/AlPc/Si/Al heterojunction in dark and room temperature conditions.


I. INTRODUCTION
Organic materials gained a considerable part in electronic devices for solar cells and microelectronics and optoelectronics applications [1].The Phthalocyanines (Pcs) and metalophthalocyanines (MPcs) can be used in light emitting diodes [2].Fascinating characteristics of Pcs and MPcs include their strong coloring properties, chemical inertness, thermal and chemical stability, photochemical properties, and semiconductivity and photoconductivity.Consequently, they find several technical uses, including solar cells.Indium phthalocyanine chloride/p-Si solar cell is previously fabricated [3].The ZnPc-based metal-semiconductor Schottky diode Al/ZnPc/ITO is fabricated by an effusion cell coating unit [4].Absorption is one of the most important optical properties of the active layer in solar cells.Throughout this research, optical properties of AlPc deposited onto glass substrate are investigated.Transmittance, reflectance and absorbance versus photon wavelength are measured, plotted and parameters like optical bandgap are extracted.Based on Wemple-DiDomenico (WDD) single oscillator model , various dielectric and dispersion parameters are evidenced and related parameters are determined, discussed and compared.

II. EXPERIMENTAL SET UP
The organic material AlPc thin film is used to fabricate the organic heterojunction diode.The Au/AlPc/p-Si/Al organic heterojunction diode is made-up by thermal evaporation technique using the homemade vacuum NVBJ-300 NANOVAK thermal evaporation system.Firstly, p-type Si substrate with (100) crystal orientation is washed in 5H 2 SO 4 + H 2 O 2 + H 2 O solution for 1 min to eliminate organic contamination and then cleaned by H 2 O + HCl solution.The substrate is then washed in deionized water and dried by nitrogen (N2).After formation of ohmic contact, 0.02 g of the AlPc powder bought from Sigma-Aldrich are thermally evaporated in vacuum at low pressure.The gold metal is thermally evaporated on AlPc layer as front contact through a shadow mask, having a diameter of 1.5 mm.The evaporation process is carried out in chamber of the NVBJ-300 NANOVAK® vacuum thermal evaporation system.The architecture of molecule showing Al in the center of molecule Pc and the design of our heterojunction diode are depicted in Fig. 1.UV-Vis measurements are carried out using Shimadzu UV-3100 PC.Using Keithely 2400 sourcemeter in dark, the current-voltage (I-V) measurement of Au/AlPc/p-Si/Al heterojunction diode is accomplished from within −3 V, +3 V bias voltage range at room temperature.

A. Optical and dispersion properties
Figure 2 shows the variation of both transmittance T(%) and reflectance R(%) of AlPc organic material based thin film as a function of photon wavelength within the 200-2500 nm range.There is a peak and valley occurrence in the T curve as indicated by rectangle inside a spectral band of 446 nm.At 315 nm, reflectance of AlPc layer coincides with transmittance at 2.85% as indicated by arrow in inset where the reflectance declines rapidly while transmittance starts to increase from this point inside UV range to attain a peak of 79% at 486 nm then drops till 7% at 790 nm limit of visible band then continues to a highest value of 81 % at 2500 inside IR range.It is observed that change is comprise in a 400-846 nm band.This characteristic might due to optical filter properties as mentioned elsewhere.The reflectance of AlPc thin layer exhibits a decline from UV to VIS and then to IR ranges.It shows a slight peak that looks like a bump as seen in inset of Figure 3. Reflectance R in terms of n and k is expressed as [5][6]; Where R (%) is the reflectance, λ is photon wavelength, t is the film thickness and α is the absorption coefficient.The value of t is 673 nm.
where E 0 is oscillator energy, E d is the dispersion energy, and hν is the photon's energy, h=6.62x10 -34 J.s is Planck constant [8].
The n 2 in term of  2 and N/m* are written as [8][9]; where e is the electron's charge,  is the vacuum's permittivity, N is the free charge carrier concentration and m* is the charge carrier effective mass.Dielectric constant  is given by [10]; 1 − 2 (9) where ε 1 and ε 2 are the real and imaginary parts of complex dielectric constant ε.
where n and k are the refractive index and extinction coefficient expressed in eqns.(3)(4).
The optical band gap of AlPc thin film is determined by Tauc's law as shown in Figure 4 according to the following equation [6]; Where α is absorption coefficient, A is a constant and Eg is the optical band gap, n is a constant exponent which calculates the optical transitions type.For indirect allowed transition, n=2; for indirect forbidden transition, n=3, for direct allowed transition, n= 1/2; for direct forbidden transition, n=3/2.A medium value of bandgap, Eg=2.6 eV, compared to Si (1.12 eV) value and to ZnO (3.5 eV) is obtained.A value greater of Eg, 3 eV, is obtained for N,N′ -Bis(3-methylphenyl)-N,N′ -diphenylbenzidine (TPD) film [11].Urbach energy Eu is determined by plotting ln against energy h according to the following equation [6]; Plotting the linear portion in the ln vs. curve (not shown here) yields the Urbach energy Eu (meV).Eu is found by taking the reciprocal of the slope of the linear component of lnα vs. hv within 3.3 eV-4 eV energy range.
Eu is much less than Eg, and the ratio of Eu =33 meV to Eg is approximately Eu/Eg  0.012.
To explain the dispersion behavior of AlPc organic molecule based thin layer the mean oscillator power, Thus, using Sellmeier representation of the real dielectric constant in the single oscillator model, the average oscillator strength, So, and the average oscillator wavelength, λo, are computed as follows [9,[11][12]; In order to explore the nonlinear optic properties of organic APc molecule containing π-conjugated bonds.The static value of χ (1) is considered as [9,11,13,14]; (1) In transparent area E<<E 0 ; (1)   4  0 (16) 3 rd order nonlinear optical susceptibility is expressed as [13]; (3) where A is a constant around 1.7 × 10 −10 esu.The nonlinear refractive n 2 is determined in terms of  (3)  as [8,12]; Figure 5 shows the refractive index variation as a function of photon wavelength of AlPc film.A decay of n is observed from 4 at 200 nm in UV range to 1.24 at 2500 nm in IR band.A similar decay of n- is reported for indigo organic semiconductor film.It is about 1.82 at 900 nm and decreases to 1.68 at 2500 nm [15].The n- curve presents many peaks within 200-500 nm range are revealed by coumarin dye-based metal-free thin films as reported prior [16].Besides, the optical parameters are recorded as E 0 =3.2 eV, E d = 6.5 eV,  L =3.2,   = 2.99.Extinction coefficient versus photon wavelength is described in Figure 6 Real and imaginary parts of dielectric constant  1 and  2 of AlPc thin film against photon wavelength are demonstrated in Figures 7-8.It drops from 16.7 to lower values along 500-2500 nm range obeying an exponential decay.A similar decay of  2 is followed along 200-2500 nm range from 1.6 to lower values recording weak peaks indicated by arrows as portrayed in Figure 8.The quantity (n 2 -1) − 1 versus (hυ) 2 of AlPc thin film is plotted as shown in Figure 9. Two regions are separated fluctuations occurrence in low energy range (A) and a decay looks like a rough straight line (B).By fitting the linear part of 1/(n 2 -1)-1 against (h) for the Indigo films as mentioned earlier [15].Lower values, 0.078 and 1.75 are previously cited for MnTPPCl [17] but ZnTPyP exhibits 0.5 eV and 2.84 eV as cited by Shehata et al. [18].Figure 11 depicts the n 2 versus  2 of AlPc organic film plot.An exponential decay is followed by n 2 within 1x10 5 -4x10 5  2 range.By fitting the linear part of n 2 vs.  2 curve the parameters  L and (N/m*) are determined according to n 2 = L -(e 2 N/ 4π 2 c 2  0 m*) 2 eqn.where the slope is e 2 N/ 4π 2 c 2  0 m*and intercept value is  L .So, the values of  L and N/m* are easily extracted.So,  L =1.73, N/m*=36x10 47 kg -1 m -3 are extracted for AlPc organic thin film.The 1/(n 2 -1) versus 1/ 2 of AlPc organic film is drawn as seen in Figure 10.A fluctuation with a decline of 1/(n 2 -1) is recorded inside region 1/ 2 < 10 nm -2 , beyond this value a linear decay is followed by 1/(n 2 -1) quantity.By fitting the linear part of 1/(n 2 -1)-1 versus / 2 curve, the parameters S 0 and  0 are determined according to eqn.

B. Electrical measurement
Current-voltage characteristics of our device based AlPc organic thin layer in dark and room temperature conditions are displayed within -3V, +3V bias voltage range as portrayed in Figure 14.Rectifying behavior is important assessed by RR rectifying ratio (RR=I(+3V/-3V)) found to be 80482.Ideality factor is extracted by using of slope in the linear portion expressed as [19]; where Iand V are the measured current and bias voltage, T is absolute temperature and n is ideality factor which exceeds 1 for non-ideal diode.Electron charge is q and Boltzmann constant is k.Our heterojunction exhibits an ideality factor of 2.35 determined by the slope according to eqn.19 of linear part of I-V curve.This finding reveals that our device is not ideal due to series resistance and interface state.This is shown by curvature of I-V for high voltage as indicated by arrow in Figure 14.Besides, series resistance is around 10 k and barrier height is of 0.74 eV.As reported before, Au/TPD/n-Si/Al heterojunction, TPD is N,N′ -Bis(3-methylphenyl)-N,N′ diphenylbenzidine, demonstrated lower n1.4,a barrier height of 0.82 eV and Rs of 20k [11].Brilliant green dye deposited onto Si heterojunction presents n of 4.3, barrier height of 0.68 in dark [20].

IV. CONCLUSIONS
Throughout this research, a complete study of optical, dielectric, and dispersive properties of AlPc organic material based thin film are investigated.Several optical and dispersion parameters are extracted.Nonlinear properties of AlPc are also studied.As seen from the profile of T/R vs. , an exponential decay of R is revealed and both peak and valley within a narrow range are occurred.A medium value of optical bandgap (2.6 eV) and minor Urbach (33 meV) value are easily recorded.A rapid decay of refractive index within UV and approximately a plateau in Vis-IR bands.The sketch of k- displays three peaks located at 224, 483 and 798 nm inside UV-Vis ranges.Using (n 2 -1) − 1 vs. (hυ) 2 plot, 3.7 eV and 6.2 eV are the obtained energies values, oscillator and dispersion energies, of AlPc based thin film.It is concluded a strong decay of both parts of complex dielectric constant  1 and  2 .Organic material Alpc based thin film exhibits values of S 0 and  0 respectively of 1.5x10 -5 nm -2 and 201.25 nm using 1/(n 2 -1) versus 1/ 2 curve of AlPc organic film, while the parameters of L and N/m* are respectively obtained around 1.7 and 36x10 47 usi via the sketch of n 2 vs.  2 .Nonlinear optic behavior of AlPc organic film is confirmed by the static value of  (1) and 3 rd order NLO susceptibly  (3) values.Both of them exhibit roughly a plateau within 1-6 eV and a rapid growth.Deposited onto Si substrate the heterojunction presents a non-ideal behavior due to resistance and interface occurrence.High rectifying, >80000, of diode based AlPc organic layer is revealed.Such material can meet several applications in solar cell, OLED, sensors, microelectronic and optoelectronic devices and even organic electronic device applications.Nonlinear properties of AlPc can enlarge its applications.

CONFLICT OF INTEREST
Authors declare that they have no conflict of interest.

Figure 2 : 4 (
Figure 2: The plot of transmittance and reflectance versus photon wavelength of AlPc layer in double Y axis graph.Arrows spot the peak

Figure 3 :
Figure 3: plot of reflectance versus photon wavelength of AlPc layers comparative graph.Inset displays the fluctuation of reflectance within the 400-930 nm visible-near infrared range.

Figure 4 :
Figure 4: Tauc's plot of AlPc organic layers showing an optical band gap of 2.6 eV as displays by an arrow.

Figure 5 :
Figure 5: Plot of refractive index of AlPc thin film versus photon wavelength.Inset shows n vs. photon energy and peak occurrence around 500 nm.

Figure 6 :
photon energy dependence of refractive index.This latter records a growth looks like an exponential progress.
[8]rve the parameters E d and E 0 are determined according to eqn.(n 2 -1) -1 =E 0 /E d -(h) 2 / E 0 E d where the slope is 1/E 0 E d and intercept value is E 0 /E d .This latter is 1.66 and 1/E 0 E d is 0.043.Consequently, E 0 and E d are respectively found to be 6.2 eV and 3.7 eV.Where   =(E 0 + E d )/E 0 , is found to be 1.59 and n 0 is 1.26.Similar decay of (n 2 -1) − 1 versus (hυ) 2 is revealed by Khan for PDS material[8].PEDOT: PSS exhibits E0 and Ed of 2 and 3.11 eV respectively as Khan et al. reported[8].2.75 eV and 4.81 eV are the reported values of E 0 and E d