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Photovoltaic Properties of Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS Hetero junction Solar Cell

Abdul Kareem D. Ali

Department of Physics, University of Tikrit/College of Education for Pure Science

Abstract: Fabrication Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS Hetero junction as a Solar Cell by deposited Pb(Zrx,Ti1-x)O₃ (PZT) thin films with various content of Zr (x=0.1, 0.3, 0.5, 0.7, 0.9) on n- type Silicon (/n-Si) and on Porous Silicon (/PS) respectively by pulsed laser deposition technique to investigate the ideality factor and the efficiency in these devices. The films were deposited at room temperature. The ideality factor decreases with the increasing of Zr concentration. The efficiency increases with the increasing of Zr content to reach the highest value of Pb(Zrx,Ti1-x)O₃ /n-Si and for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.5 and then decreases. The devices were enhanced and gained larger efficiency with porous silicon.

Keywords: Zr concentration, PZT, solar cell

Citation: Abdul Kareem D. Ali, Photovoltaic Properties of Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS Hetero junction Solar Cell. International Journal of Physics, 5 (3). 82-86. January 2017. doi:10.12691/ijp-5-3-3

1. Introduction

Definition of photonic detectors or optical voltaic detectors as a device working on converting an optical signal to an electrical signal, there is several types of optical or the photonic voltaic including reagents (p-n), Schottky, break down and hetero junctions reagents [1,2].

Hybrid Junctions reagents named photo voltaic reagents because it works with optical voltaic and optical connectivity pattern as a result of the generation of an internal electric field due to the shift of the carrier high concentration areas to the low concentration regions [3]. These reagents operate in the presence or absence of an external electric field, to the existence of an internal electric field it operates with the existence of reverse bias so as to improve its parameters.

When the fall of the light card on a bilateral link semiconductor is absorbed photons to generate pairs of electron - hole. When the falling photon energy is greater than the energy gap of the semiconductor exhibition of light, the electron pairs - hole generated while the additional energy dissipated as heat. But if less than Eg there is not the presence of absorption mattresses forbidden energy gap caused by chemical impurities and physical defects [3,4].

Each semiconductor material has special absorption coefficient depends on the energy of the incident photon which is considered an important quality of the semiconductor materials was used in photoelectronic devices. Absorption coefficient decreases rapidly when the cutting wave length λC, which is given to the relationship [5]:

[picture] | (2-53)

Where λc: cutting wavelength of the semiconductor.

Several parameters are used to characterize solar cells. In this section we will briefly review some of these parameters and how they influence the performance of the device [6]. Figure 1 shows the I-V characteristics of a solar cell in the dark and under illumination. The product of voltage and current in the fourth quadrant is negative indicating that the cell puts out power, and this is the region of interest in solar cells. One of the parameters is the short circuit current Isc which is defined as the current flowing in the circuit when the load is shortened [7].

Figure 1. I-V Curve of the solar cell in dark and under light [7]

The open circuit voltage Voc another important parameter, is the voltage output of the cell when the attached load is infinite and is given by the relation:

[picture] | (2-46)

Where Io is the reverse saturation current or the dark current at the junction, Ao constant.

The expression for the total current is,

[picture] | (2-47)

Where IL is the light generated current.

Both Io and Ao provide important information on the predominant current transport mechanism in a device. There are a number of ways for obtaining these parameters, although the most consistent method involves their determination using the dark J-V curves. The slope of the J-V curve gives the Ao, whereas the y-intercept gives the Jo. They are usually considered together as there is great deal of correlation between the two parameters. It is desirable that Jo was as low as possible, so that higher voltages are required for the dark current to equal the light generated current. This increases the Voc of the devices. A lower value of Ao reduces Voc but helps increase the fill factors of devices. The discussion of Ao and Jo is more complex and needs a more careful analysis. Oman et al analyzes these parameters and their effect on cell behavior in greater detail [8]. We shall conclude our discussion here as it is beyond the scope of this research.

Isc and Voc are the short circuit current and the open circuit voltage and Im and Vm are the current and voltage corresponding to the maximum power point. This is the point where maximum power can be generated by the device.

Another parameter of interest is the Fill factor (F.F) which is given by the relation. [9]

[picture] | (2-48)

The photovoltaic conversion efficiency is another important parameter. It is a measure of the amount of light energy that is converted into electrical energy and is given by: [10]

[picture] | (2-49)

Where Pm is the the maximum power determined from area of rectangle and Pin is the incident power.

2. Experimental Procedure

Bulk samples of Pb(Zrx,Ti1-x)O₃ has been prepared by solid state reaction process. The powder of Lead dioxide, Zirconium dioxide and Titanium dioxide with a purity of 99.99% were Grinded and mixed together at a different concentration of ( x = 0.1, 0.3, 0.5, 0.7, 0.9 ) wt. % of the formula Pb(Zrx,Ti1-x)O₃ in a mixture machine for (10 minute). After that it was pressed on pellets with (1.2 cm) diameter and (0.2 cm) thick, using hydraulic piston type (SPECAC), under the pressure of 6 tons/cm² for 10 minutes. The pellets were sintered in the air to temperature (1073 K) for 2 hours then cool to room temperature. The temperature of the furnace was raised at a rate of 250°C/hour.

The substrate used in this work for deposit of Pb(Zrx,Ti1-x)O₃ thin films are the n-type silicon wafers of (111) orientation and porous Silicon to prepared photovoltaic device includes solar cell. Silicon wafers of dimensions of about (2x2) cm² are cleaned to remove any pollution on the surface. The n-type Si wafer of (0.015 Ω.cm) resistivity is used as a starting substrate in the photo electrochemical etching. The samples of (2 x 2 cm²) dimensions are cut from the wafer and rinsed with acetone and methanol to remove dirt. In order to remove the oxide layer on the samples, they are etched in diluted (50 %) Hydrofluoric HF acid. After cleaning the samples, they are immersed in HF acid of 50 % concentration and ethanol 50 % in a container.

The main parts of the photo electrochemical etching are a lamp of the laser with fixed output power (40mW/cm²), two millimeters, and container made from Teflon of high HF resistant in order to avoid any chemical reaction with HF acid. The container is consisting of three parts, the First piece of Teflon with a cupper placed on the top for contact and the second piece of Teflon of a cylinder shape has a center hole of 1×1cm² to allow the HF acid to touch the silicon wafer and a rubber O-ring.

Using thermal evaporation system (Blazer BAE 370) to evaporate Al on the back of the wafer to make an ohmic contact thick film. The low resistivity of the wafer enables a good contact between the wafer and its backside metallic connection. The silicon wafer is placed and a rubber O-ring is used before placing the upper part as shown in Figure 2.

Figure 2. The experimental set-up of photo electrochemical etching

An electrolyte of the (1:2) composition of (50%) concentration HF acid and purity ethanol (C₂H₅OH) (99.99%) is used in the etching cell in order to prepare the porous structures. The ethanol solution is very necessary to achieve lateral homogeneous structure and helps to decrease the hydrogen gases bubbles during the etching process.

Two electrodes are used to apply the voltage across the sample, The lower electrode is the Al foil below the wafer and the upper electrode are made of platinum mesh in order to produce a uniform current distribution of the sample. The electrical circuit is completed by DC power supply (WAIBK. PS-1502DD) and a millimeter (Keithly-616 EAM) to measure the passing current. The samples are etched with fixed etching time of (10min) and etching current density (J) of (20mA/cm²).

The pulsed laser deposition experiment is fulfilled inside a vacuum chamber under (10⁻³ mbar) vacuum conditions. The focused Nd:YAG laser beam at 800 mJ with a frequency second radiation at 1064 nm (pulse width 9 ns) frequency (6 Hz), for 400 laser pulses incident on the target surface makes an angle of 45° with it. The distance between the target and the laser gun is set to 15 cm, and between the target and the substrate is (2cm).

3. Results and Discussion

Figure (3-a,b,c,d,e) and Figure (4-a,b,c,d,e) show the current - voltage properties in the case of the illumination of the hybrid junction Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS fabricated by pulsed laser deposition with different Zr concentration (x= 0.1, 0.3, 0.5, 0.7, 0.9) at room temperature, which represents behavioral mainstream with the voltage in the case of the front and reverse bias. The current behavior confirm that the junctions hybrid manufacturer is a kind of asymmetric where the front aligned current different of reverse current bias behavior, in the case of front-Aligned behavior is roughly comparable "to the exponential function, but in the reverse bias, the current increases gradually with a reverse bias voltage of any it gives the breakdown voltage gradually and this behavior is a general recipe in hybrid junctions asymmetric. This non –linear feature indicates that the prevalent conduction mechanism is non-ohmic in nature but it may reveal the existence of different kinds of conduction mechanisms. in the forward direction of the diode, the current is generated via the flow of majority carriers and the applied voltage injects majority carriers which lead to the decrease of the potential barrier, in addition to the width of the depletion region. Figures (3-a,b,c,d,e) and (4-a,b,c,d,e) show the behavior of the photocurrent with the forward and reverse bias voltage. From this figure it is obvious that the photocurrent increases with the increment of the bias voltage. It can be noticed that the photocurrent (Iph) increases with increment the Zr content in Pb(Zrx,Ti1-x)O₃. These results can be interpreted as, with increasing the Zr atoms the free carrier find more field to move in and reduce the grain boundaries which hinder current flow. This increasing in photocurrent doesn't still continue with the increasing of Zr ratio, but it arrives to a certain point and start to get down. For the hetero junction under illumination, the width of the depletion region increases with the increase in the applied reverse bias voltage which causes an increase in the absorption through it and the creation of the electron-hole pairs. From figures it can be determining the values of current – voltage parameters (Voc, Isc, Vmax, Imax, F.F and η) which are tabulated in table (1). It can be observed that, the Isc increases with the increasing of Zr content and then decreases at the high substitution. The ideality factor decreases with the increment of Zr concentration which are between (1.06 – 2.73) and (1 – 2.96) for Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS respectively. The efficiency increases with the increasing of Zr content to reach the highest value (15%) for Pb(Zrx,Ti1-x)O₃ /n-Si and (17.5%) for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.5 and then decreases. This behavior is due to penetration depth of light into Pb(Zrx,Ti1-x)O₃ layer. This depth changes proportional to the wave length of illuminated light. It is noticed that the efficiency of Pb(Zrx,Ti1-x)O3 thin film with porous is enhanced as shown in Table 1, this agreement with Hadi [11]. The using a porous structure enables to receive light by partitioning it over a wider spectral region, thereby using the light more effectively and it is possible to obtain a higher photocurrent.

Table 1. I-V parameters under dark and illumination for Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS at different Zr concentration at RT

Figure 3-a. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-Si at x=0.1

Figure 3-b. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-Si at x=0.3

Figure 3-c. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-Si at x=0.5

Figure 3-d. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-Si at x=0.7

Figure 3-e. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-Si at x=0.9

Figure 4-a. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.1 at RT.

Figure 4-b. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.3.

Figure 4-c. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.5

Figure 4-d. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.7

Figure 4-e. I-V characteristics under illumination for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.9

4. Conclusions

In this work, fabrication Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS Hetero junction as a Solar Cell, it's found that The Isc increases with the increasing of Zr content and then decreases at the high substitution. The ideality factor decreases with the increment of Zr concentration which are between (1.06 – 2.73) and (1 – 2.96) for Pb(Zrx,Ti1-x)O₃ /n-Si and Pb(Zrx,Ti1-x)O₃ /n-PS respectively. The efficiency increases with the increasing of Zr content to reach the highest value (15%) for Pb(Zrx,Ti1-x)O₃ /n-Si and (17.5%) for Pb(Zrx,Ti1-x)O₃ /n-PS at x=0.5 and then decreases. the efficiency of Pb(Zrx,Ti1-x)O3 thin film with porous is enhanced and gained larger values.

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