Improving Efficiency of Photovoltaic Cell Using Nanomaterials

Conventional solar cells are not economical and are recently too expensive to the manufacturers for extensive-scale electricity generation. Cost and efficiency is most vital factor in the accomplishment of any solar technology. In order to improve the conversion efficiency, the major research in third-generation photovoltaic (PV) cells is directed toward retaining more sunlight using nanotechnology. Advancement in nanotechnology solar cell via quantum dots (QDs) could reduce the cost of PV cell and additionally enhance cell conversion efficiency. Silicon quantum dots (Si-QDs) are semiconductor nano crystals of nanometers dimension whose electron-holes are confined in all three spatial dimensions. Quantum dots have discrete electronic states. Quantum dots have capacity to change band gap with the adjustment in size of quantum dot. As the quantum dots size fluctuates over a wide range that demonstrates the variety of band gap so it will assimilate or discharge light. In this paper, the generic mathematical models of PV cell are adopted and then I-V and P-V characteristic curves are obtained from selected parameters using MATLAB software. The essential parameters are taken from datasheets. I-V and P-V characteristics curves are obtained for selected model. Silicon quantum dots have the tunable band gap that is added to conventional PV cell and obtain the I-V and P-V curves. After simulation, efficiency and power of Conventional PV cell to quantum dots based PV cell is compared. The property of quantum dots is used in extending the band gap of solar cells and increasing the maximum proportion of incident sunlight absorbed, hence improving efficiency.

increasing dimension of Si-QDs [3]. Traditional PV cells where in one photon generates one electron whereas the quantum dots have the ability to convert high energy photon into more than one electrons [4].

MATHEMATICAL MODELING OF PV MODULE
The use of mathematical equations and appropriate electric circuits make it conceivable to display the performance of PV cell and simulate it. The identical model also can be modified for modeling and simulating a PV module and PV array. It is essential to create a generic model that is appropriate for scaling in any respect tiers, i.e. the PV cell, module, and array [8].

Single Diode Model of PV Cell
The Where I L is the light generated current because of sunlight, I D is a current across the diode, I s is a diode reverse saturation current, q is the charge of electron (1.6 *10 -19 ), K is a Boltzmann constant (1.38*10 -23 J/K), n is an ideality factor, V is a cell output voltage, I is a cell terminal current, T c is a working cell temperature in Kelvin. R s is the series resistance, and R sh is the shunt resistance, V t is terminal voltage. The light generated current relies upon the cell's temperature and solar radiation and can be expressed as Where I sc is the short-circuit current taken at 1 KW/m 2 and 25 ºC, k i is the temperature coefficient corresponding to short circuit current (3 mA/°C),  is the solar insolation which is measured in KW/m 2 and T ref is the reference signal for temperature. The expression for saturation current is represented by equation (5)  Where I rs is the opposing saturation-current at a reference temperature, q is the charge of an electron, and E g is the band gap energy of the semiconductor material. The expression of Irs is described as equation (6)  

Single Diode Model for PV Module
The power produced by a single PV cell is roughly less than 2 W at 0.5 V, therefore, the cells needs to be linked in series-parallel arrangements on a module to produce enough power. A PV cluster is a collection of a number of PV cells that are connected in series-parallel arrangements to create the necessary voltage and current. The overall output current of a PV panel with N p and N s is given as equation (7). Fig. 2 shows a PV panel with parallel (N p ) and series (N s ) cells [10].
The voltage-current expression of PV module is given by equation (7) 

Efficiency of PV Cell
Portion of output power of the solar cell to input energy from the sun is described as efficiency. The numerical equation for the efficiency of a PV cell can be expressed as equation (8) A I where, I mp represents the current at maximum power,V mp represents the voltage at maximum power, I is termed as intensity of solar radiatoin per square meter, and A is the area where the solar radiation strikes [11].

SILICON QUANTUM DOTS (SI-QDS)
Nanomaterials provide distinctive possibilities in engineering and green energies. The real photovoltaic market is driven by silicon solar collectors. In any case, to cover the upcoming demand of electric power in the order of ~ 30 Terawatts, value of solar electricity need to be considerably decreased with excessive performance.
In an effort to improve the efficiency and minimize the cost, research is focused to third generation cells in the last few years with respect to finding an effective opportunity for silicon-based solar cells. The third generation solar cell tends to include the nonsemiconductor solutions (including polymers) quantum dot technologies to higher size incident light [12]. Quantum dots are semiconductor nano crystals of nanometer size whose electron holes are constrained in all dimensions. They got quantum-optical properties that are not realized in bulk material due to the characteristics of quantum confinement exhibited by means the nanoscale structures. Quantum dots have the ability to change band gap with the change in the dimension of the quantum dot so that the wavelength at which it is going to absorb or emit light may be modified for a particular application.

FIG.2. SINGLE DIODE MODEL WITH N S AND N P [10].
This property of quantum dot is utilized in extending the band gap of solar cells and increasing the most percentage of incident light absorbed, subsequently enhancing efficiency [3,13]. Fig. 3 shows the relationship between the band gap and quantum dot size [14], by increasing the size of quantum dots the band gap decrease and it requires less energy to knock out an electron from its valence shell.

SIMULATION RESULTS AND DISCUSSION
In order to investigate the performance of PV cell model,

Simulation Results for Different Temperature Levels
In Fig. 4(a), the generic model of PV cell current in Equation (7) is plotted against voltage for different temperature levels. Since these results have been validated with the previous studies [10], therefore, this ensures that the mathematical model can be modified for incorporating the impact of silicon quantum dots (Si-QDs) [13]. Fig. 4(b) shows the output power to the voltage at various temperature levels to ensures the validated response of the generic model. Both I-V & P-V results at distinct temperature with constant irradiance (1 KW/m 2 ) are obtained as shown in Fig. 4(a) and 4(b) respectively. Fig. 4(b) depicts that the output power decreases with the increment of temperature values.

Simulation Results for Different Irradiance Levels
In order to ensure the validation of the developed simulation results, the I-V & P-V performance plots are generated for distinct irradiance levels while keeping the temperature same 25 o C in both cases, as shown in Fig.  5(a) and Fig. 5(b). This analysis helps in understanding

FIG.3. RELATIONSHIP BETWEEN THE SIZE OF QD AND BAND GAP [14]
Characteristics Specification    [10]. Fig. 5(b) displays that the output power expands with the increment of irradiance level.

Simulation Results for Si-QDs
The As the size of Si-QDs are increased the band gap is reduced as shown in fig 3. Solar cells are sensitive to temperature. Increase in temperature reduce the band gap of a semiconductor as increasing the energy of electrons in the material. In conventional PV cell the silicon semiconductor is used. The proposed modified module uses the the silicon quantum dots. By using the siliocn quantum dots the band gap is adjusted.The simulation results show that the output power increases with the increase in the size of the Si-QDs. Hence, the efficiency of the PV cell in flip will increases. The power of conventional PV module is decreases as the temperature rises as shown in fig.4(b), but as compared to the proposed modified PV module can produce more power for the same given input irradiance and temperature value. The energy gap, the corresponding output power and the corresponding performance of the PV module are presented in Table. 2. The efficiency has been calculated by using equation (8). It is evident from the outcomes that the addition of a nanomaterial that is by changing the band gap of QDs, can enhance the efficiency of conventional PV cells.

CONCLUSION
A generalized MATLAB PV model to illustrate the execution of PV cell has been created and been tested with a commercial module. In this research, A modified PV cell model is utilized to investigate the improvement in the performance of the PV cell. A PV cell is modified by adding the nanomaterial called Si-QDs with various diameter sizes such as 3.5 nm, 4.25 nm, and 6 nm. In order to validate the photovoltaic modeling and simulation method a reference module Solarex "MSX60" is used. Both I-V & P-V results are generated which show the dependence of output power on the temperature and the solar irradiance. By expanding the solar irradiance output current increases, output voltage increases, thus the net output power is increased. By expanding the temperature, The output current increases but the output voltage decreases, So the net power decreases thus the efficiency is decreased. It was observed that by adding the Si-QDs the energy bandgap of the solar cell can be adjusted. Table  2 shows the efficiency and energy band and power output for different sizes of Si-QDs. Nano-coated solar cells model has been analyzed at different temperatures by simulating their I-V& P-V performance curves. It can be concluded that by adding the nanomaterial such as Si-QDs in this case, the efficiency of a PV cell can be significantly increased. This research study can bring a great revolution in the solar energy conversion systems as the conventional PV modules are suffering from the serious solar energy conversion issues.