The structure and optical properties of organic semiconductor bulk hetrojunction blend (NiPcTs/Alq3) thin films

The effect of heat treatment on the optical properties of the bulk heterojunction blend nickel (II) phthalocyanine tetrasulfonic acid tetrasodium salt and Tris (8-hydroxyquinolinato) Aluminum (NiPcTs/Alq3) thin films which prepared by spin coating was described in this study. The films coated on a glass substrate with speed of 1500 rpm for 1.5 min and treated with different annealing temperature (373, 423 and 473) K. The samples characterized using UV-Vis, X ray diffraction and Fourier transform Infrared (FTIR) spectra, XRD patterns indicated the presence of amorphous and polycrystalline blend (NiPcTs/Alq3). The results of UV visible shows that the band gap increase with increasing the annealing temperature up to 373 K and decreases with increase the annealing temperature to (423, 473)K respectively.


Introduction
Organic semiconductors are a portion of materials that gathering between the mechanical and chemical advantages of organic compounds like plastics with the electronic characteristics of semiconducting materials, and that the amendment by chemical synthesis it has the ability to absorb light, conduct electricity, and emit light is united with a material structure for example, to assign electronic properties such as the desired emission wavelength, to allow for mechanically strong or to becomes it soluble, or, lightweight, and flexible thin film. There are two types of organic semiconductors: low molecular weight materials and polymers [1].
The bulk heterojunctions blend is the two components are intimately mixed in one layer [2]. As show the Fig.1.
Today, the most commonly used architecture is the so-called bulkheterojunction blend (BHJ blend) the first BHJs was presented by the groups of Friend [2] (polymer-polymer solar cells) and Heeger [6], and it has been shown to be superior to earlier architectures. Compared to the bi-layer architecture, the BHJ provides a larger interfacial area between the donor and the acceptor material, which is beneficial for the formation of the charge-transfer state (CTstate) as well as charge separation.
Many BHJ blends are not compatible with elevated production and operation temperatures. Phase separation and crystallization occur once the blend is heated above its glass transition temperature. In many cases, this leads to a detrimental decrease in device performance. The blend transition temperatures can strongly differ from those of the neat components [7][8][9]. Nickel (II) phthalocyanine tetrasulfonic acid, tetrasodium salt (NiPcTs) is a conjugated molecule. The molecular structure of NiPcTs/Alq3 which used as quick materials in this work. Attached to the angles of benzene rings in molecular structure of NiPcTs polar NaO 3 S and make this compound water soluble as shown in Fig.2a.
Alq 3 is well localized on the quinolate {quinoline (organic compound) any of a class of aromatic heterocyclic compounds containing a benzene ring fused with a pyridine ring} connections but donor-acceptor characteristics are due to electron movement between a phenoxide donor side-located HOMO and apyridy1 acceptor side-located LOMO of the quinolate ligands. Alq 3 possesses comparatively high mobility of electrons and low mobility of holes. This difference in mobility is attributed to the distribution of the electronic on a neutral molecule of Alq 3 . Due to ionic bonding of aluminum-oxygen, a cloud of excessive electronic density forms of the oxygen as shown in Fig.2b Fig.2b At first, the glass substrate was cleaned in an ultrasonic bath for 10 min using distilled water, followed by rinsing with acetone and alcohol respectively and finally distilled water used again in an ultrasonic bath for 10min. The substrate was dried using nitrogen gas.
The two compound NiPcTs and Alq 3 were grinding using get mortar to get a fine powder, then the powder dissolved in Dimethyl sulfoxide (DMSO) 20 mg/ml, as 10 mg/ml respectively to obtain blend (NiPcTS/Alq 3 ) solution then putted on a magnetic stirrer for 5 hour by using heat 60 o C. After that using 0.2 µm filters. And it heat plate magnetic stirrer with temperature of 60 o C for 5 hours to get a homogenous solution.
The source of radiation was NiPcTS /Alq3 Cu(K α ) with wave length= 1.5405 Å, the current was 30.0 mA and the voltage 40 kV. The scanning angle 2θ varied in the range of (0-60 o ).
To examine the optical properties of the as-deposited and heat treated BHJ blend (NiTsPc/Alq 3 ) thin films, Split Beam (internal double beam design) beam Metertech SP -8001UV-Vis NIR spectrophotometer was used in the range (300-800) nm.

a b
The optical energy gap values (E g opt ) for NiPcTs/Alq 3 films have been determined by using Tauc Eq.(1). [14] αE=B(E-Eg) r (3) where B is a constant, r is a constant whose value depends on the type of transition, where r is equal to 1/2 and 3/2 for allowed and forbidden direct transition, respectively. The constant r is equal to 2 and 3 for allowed and forbidden indirect transition, respectively. While Fourier-transformed Infrared (FTIR) spectrum for as-deposited and annealed blend (NiTsPc/Alq 3 ) thin films were recorded the spectrum over the range of 400-4000 cm -1 with resolution 4, the spectra obtained at room temperature and recorded in the transmittance mode using Thermo Scientific™ Nicolet™ iS™10 FTIR Spectrometer.

Results and discussion
The X-Ray diffraction spectra of asdeposited and heat treated BHJ blend (NiPcTs/Alq 3 ) thin films which are prepared by spin coating at room temperature were recorded by X-ray diffraction for 2θ in the range between 0 to 60 o as shown Fig. 3.
For this figure, one can see that the as-deposited and film annealed at 373 K film show amorphous, structure while the films annealed at 423, 473 K have polymorphism structure, i.e mix between amorphous and polycrystalline structure. The value of the crystallite size was estimated using the Equation: where, L is the mean crystallite size, K is the Scherer constant with value 0.94, λ= 1.542 Angstroms is the X-ray source wavelength and β is the FWHM value.
The full width at half maximum (FWHM), interplane distance (d) and crystallites size (G.S) of as-deposited and annealed blend (NiPcTs/Alq 3 ) thin films are listed in Table 1. It is clear that annealing the sample to 423K decreases the crystallite size.

Fig.3: XRD pattern of as-deposited and annealed blend NiPcTs/Alq 3 thin films.
Absorption spectra of the asdeposited and annealed BHJ blend NiPcTs/Alq 3 thin films are shown in Fig. 4 The absorption spectra exhibit two peaks which are B and Q bands. In the Q-band, an intense absorption peak (RT, 373, 423 and 473) K around (614, 611, 613 and 615) nm respectively is due to the transition between the bonding and antibonding (π-π*) at the dimer part of the phthalocyanine molecule. While the B-band an intense absorption peaks (RT, 373, 423, 473)K around (343, 335, 342, 345) nm respectively [15].
The absorbance decrease strongly with annealing temperature. The sample to 373 K while returned to increase with father annealing to 423K.

K
Some researchers utilized similar research work to enhance the absorption capability of the thin films [16,17].
The values of E g opt has been determined using Tauc equation to find transition type by plotting (αhν) r where r=1/2 for direct allowed versus photon energy (hν) and select the optimum linear part intersect the hν axis.
Before heat treatment (E g opt ) belonged to Q and B band respectively is equal to (1.64) eV and (3.04) eV increases to (1.76) eV and (3.26) eV as a result of annealing the sample at 373K.
The optical energy gap for Q-band decreased to (1.74, 1.68) eV with increased annealing temperature to 423and 473 K as show Fig. 5; the same behavior observed in B-band, the optical energy gap increased from (3.26) eV when the temperature increases to 373 K, while the optical energy gap return to decreased to (3.1, 3.04) eV after increasing the temperature to 423, 473 K respectively although it still above the value of room temperature as show Fig. 6. The FT-IR spectra Fig. 7 for asdeposited and annealed BHJ blend NiPcTc/Alq 3 thin films were measured at different temperatures. FT-IR for blend NiPcTc/Alq 3 thin films shows the bond bending represented by the range (400-2000) cm -1 while the bond stretching represented by the range (2000-4000) cm -1 . One can see a weak peak in the range (400-600) cm -1 .

Fig.6: (αhʋ) 2 versus photon energy of incident radiation for as-deposited and annealed blend NiPcTs/Alq 3 thin films (B-band).
The FTIR absorption spectrum of Alq 3 displays all the characteristic absorbance of quinoline and the Al ion with ligands. For example, the characteristic bands centered at 600-800 cm -1 come from the vibrations of quinoline, and the band at 400-600 cm -1 can be attributed to the stretching vibration of Al ion with ligands. Compared with the normal FTIR absorption spectrum of quinoline, the intensive band of C-O at 1114.8 cm -1 and the weaken band at 1280.6 cm -1 indicated that the Al-O bond had formed, transformations of aromatic stretching (C=C, 1600-1450 cm -1 ) and aromatic amine resonance (C-N-C, 1370-1250 cm -1 ) [18].
The Table 2 shows the types of bonds to the bulk heterojunction blend (NiPcTs/Alq 3 ) and compare it each compound bonds.

Conclusions
 The heat treatment affected on the absorption and energy gap. The energy gap increase at 373K and return decrease at 423, 473K.  The X-Ray pattern of the prepared hertojunction blend (NiPcTs/Alq 3 ) also effect by annealing temperature where the structure changing from amorphous to polymorphism.  The FTIR spectra measurement was applied to know the type of the bonds of NiPcTs/Alq 3 bulk heterojunctions blend thin films.