Annealing Effect on (SnO2)0.3:(In2O3)0.7 Solar Cell Prepared by PLD Technique

Indium tin oxide (ITO) thin films were produced by Qswitched Nd:YAG laser with wavelength (1064 nm) has 800 mJ peak energy on In2O3 :SnO2 target with 0.3 ratio on p-type Si and on porous Si to fabricated solar cell. The as deposited and annealed thin films on glass substrates were characterized by X-ray diffraction Atomic force microscopy (AFM), UV-visible absorbance and Hall effect measurements. Then the fabricated solar cells examined in the dark and under illumination for SnO2:In2O3 /p-Si and SnO2:In2O3/Psi/p-Si annealed and as deposited samples.


Introduction
Now a day there are many techniques utilized to enhance solar cell efficiency enhance solar cells efficiency [1].
The generic solar cell is a structure consisting of two active layers, thin heavily doped top layer called the emitter or window layer and thick moderately doped bottom layer, called the base or absorber with opposite doping [2]. We need to study the optical, structural [3], morphological [4] and electrical properties for used material as window in solar cells on the side where light enters, to reach optimum efficiency [5] X-ray diffraction used to characterize obtained films crystallinety. The d hkl spacing between crystals planes can deduced by X-ray diffraction using Bragg's law [6] hkl n 2d sin   (1) where Ө is diffraction angle and λ is the used XRD wavelength. Scherrer equation formula used to calculate crystalline size utilize the peaks broadening [7].
Where λ is the x-ray wavelength for k α transition from Cu target (1.5406 Å ), FWHM is full width at half maximum and θ is the angle of diffraction. Several parameters are used to characterize solar cells. A solar cell under illumination is characterized by the following parameters: the Short-circuit current, the open-circuit voltage, the fill factor and the power conversion efficiency [8]. Fill Factor indicates the quality of current-voltage characteristics and it depends on the ability of charges to reach the electrodes which is governed by recombination processes. The full factor is directly affected by the values of the cell's series and shunt resistances [9]. The solar cell efficiency is the ratio for outcome power to incident light power density [10].

Experimental Part
A porous silicon structure is prepared by electrochemical etching of silicon substrate of (111) orientation with a resistivity (4 -10) Ω. cm and the etching cell is made of Teflon. The electrochemical cell used has two electrode configurations with a platinum electrode as cathode and silicon wafer as anode. The etching times is chosen to be 10 min and current (30) mA. The samples are dipped in (48%) concentration of (HF) acid in mixing ratio (1 : 1) HF: ethanol with the aid of diode laser source as an illumination sources. However, the addition of ethanol to HF eliminates hydrogen and ensures complete infiltration of HF solution which further improves the uniform distribution of porosity and thickness.
In 2 O 3 with purity (99.9 %) and SnO 2 powder purity (99.98 %) by FERAK company, England were mixed with In 2 O 3 :SnO 2 ratio 30 % in gate mortar to use it to make pellet target as a disk of 1.5cm diameter using hydraulic piston under pressure of 6 tons. SnO 2 : In 2 O 3 thin films were prepared on glass, p-Si and porous-Si substrates by Q switched pulsed laser (Huafei Tongda Technology-DIAMOND-288 pattern EPLS) λ = 1064 nm with 800 mJ peak power inside a vacuum chamber at vacuum (10 -3 Torr) using double stage rotary pump. The incident beam coming through a window making an angle of 45° with the target surface. The substrate is placed International Journal of Physics 111 parallel target surface. The produced films were characterized by X-ray diffraction (XRD), DC conductivity, Hall effect measurements and UV-visible absorption Metal electrodes of aluminum are formed both back surface of the Si wafer and top of the SnO 2 :In 2 O 3 films by thermal evaporation, carried out at a pressure of (10 -5 mbar) in a vacuum (Edwards Auto 306). Annealing at 773K for 2 hours to enhance the films properties and its adhesion.
I-V measurements were made for fabricated solar cell heterojunctions in case of dark and under illumination by Halogen lamp light Philips (120W) with 100 mW/cm 2 intensity using Keithley Digital Electrometer 616, voltmeter and D.C. power supply under reverse and forward bias voltage in the range (-2 to 2) Volt to study the effect of annealing and porous formation on solar cells efficiency.   Figure 2 shows the X-ray diffraction for as deposited In 2 O 3 :SnO 2 with 0.3 ratio films deposited on glass substrates by pulse laser deposition and annealed at different temperatures (573, 673and 773) K. We can observe from this figure that as deposited film has amorphous structure at RT, while annealed one at 573 K, convert to polycrystalline structure, and with annealing at 673 K many peaks corresponding to In 2 O 3 and one peaks for SnO 2 . The crystallinety increase with increasing annealing to 773 K and their location shift toward higher values indicate on increase atomic backing with annealing. The FWHM decrease with increasing annealing temperature indicate on increasing G.S. Table 1 shows the experiment and the standard peaks from international center for diffraction data JCPD cards number (96-210-4744 and 96-101-0589). Figure 3 shows the AFM images and their granularity accumulation distribution for as deposited In 2 O 3 :SnO 2 with 0.3 ratio films deposited on glass substrates by pulse laser deposition and annealed at different temperatures (573, 673and 773) K. This figure and Table 2 illustrate that the average diameter and RMS roughness decrease with increasing annealing temperature.    Figure 4 shows the transmission spectra for In 2 O 3 :SnO 2 annealed at different temperature. It shows that the transmission increase with increasing annealing temperature reaching 85% in visible range, as a result of enhance the crystallinety [11].

Results and Discussion
The optical energy gap values (E g opt ) for In 2 O 3 :SnO 2 composite films at different annealing temperature have been determined by using Tauc equation as shown in Figure 5. From this figure seems that the energy gap increase from (2.8 to 3.7) eV with increasing annealing temperature as a result of increasing atomic backing with annealing [12]. Hall effect measurements show that all films were (n-type). The variation of n H and μ H with annealing temperature were shown in Table 3. The values of (n) increases with increasing annealing temperature reaching the maximum value at 673 K then decrease with more annealing. While the maximum value of electron mobility at 573 annealing temperature.
The I-V characteristics of SnO 2 :In 2 O 3 /p-Si and SnO 2 :In 2 O 3 /porous-Si solar cell with 0.3 In 2 O 3 content, in the dark and under illumination using power densities equal to 100 mW/cm 2 with the applied forward and reverse bias voltage for RT and annealed at (773)K were shown in Figure 6. The solar cells parameters were shown in Table 4. This table shows that in general the annealed ones best than as deposited films and the samples deposited on porous Si best than that deposited on Si wafers.
From the plot of the forward current (I f ) versus applied forward bias voltage (V f ), the ideality factor (β) is obtained according to the relation [13] [ln( where I s is the saturation current and it can be obtained by extrapolating the forward current curves to zero voltage.

Conclusions
The results of examination of In 2 O 3 :SnO 2 thin films deposited on glass substrates by XRD by pulse laser deposition show that annealing convert amorphous structure to polycrystalline structure with peaks corresponding to In 2 O 3 and SnO 2 . The crystallinety and the crystalline size increase with increasing annealing to 773 K.
The UV visible easements shows that the annealing leads to increase transparency and increase energy gap from 2.8 to 3.7 eV.
Hall effect measurements shows n-type for all films. The charge carrier concentration has maximum value at 673 K, while the maximum value of electron mobility at 573 annealing temperature.
The results for fabricated solar cells ( SnO 2 :In 2 O 3 /p-Si and SnO 2 :In 2 O 3 /Psi/p-Si) by pulse laser deposition technique annealed and as deposited samples shows that the annealed ones best than as deposited films and the samples deposited on porous Si best than that deposited on Si wafers.