OPTIMUM THICKNESS DETERMINATION TECHNIQUE AS APPLIED TO A SERIES VERTICAL JUNCTION SILICON SOLAR CELL UNDER POLYCHROMATIC ILLUMINATION: EFFECT OF IRRADIATION

Omar Dia, Mohamed Abderrahim Ould El Moujtaba, Sega Gueye, Mamadou Lamine Ba, Ibrahima Diatta, Gora Diop, Marcel Sitor Diouf and Gregoire Sissoko Laboratory of Semiconductors and Solar Energy, Physics Department, Faculty of Science and Technology, University Cheikh Anta Diop, Dakar, Senegal. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 12 March 2020 Final Accepted: 14 April 2020 Published: May 2020

:-Vertical series junction silicon solar cell to the n + -p-p + type under irradiation and polychromatic illumination.
The incidental polychromatic illumination is parallel to the surface of the space charge region (SCR), which separates the emitter(n + ) from the base(p). The zone (p + ) allows the creation of a rear electric field area that allows the minority carriers to be returned towards the space charge region, and then to be collected.This solar cell unit is connected to several others in series, in order to increase the phototension.
represents the excess minority carrier density in the base of the solar cell at the zposition, dependent of p  the irradiation energy flux, and kl the damage coefficient intensity.
is the diffusion coefficient of excess minority carriers in the base of the solar cell.

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and is the lifetime of excess minority carriers in in the base defined by the following Einsteinrelationship: the excess minority carriers diffusion lengthin the base and also both, irradiation energy flux (ϕp) and damage coefficient intensity (kl) dependent. It is related to the diffusion length (L 0 )without irradiation by the following empirical relation [24] [25] [26]: Where: 0 L is the diffusion length of the excess minority carriers in the base before irradiation, p  is the irradiation energy flux, and kl the damage coefficient intensity. G(z) is the carrier's generation rate for a polychromatic illumination in the base. Its expression, for a series vertical junction solar cell is given by: The resolution of continuity equation (1) gives the expression of the excess minority carrier density in the base as: The coefficients A and B are determined from the boundary conditions: At the junction (x = 0): Sf is the recombination velocity of the minority carriers at the junction imposed by the external charge and thus characterizes the operating point of the solar cell varying from the open circuit to the short-circuit [27] [28] [29].

At the rear face (x = H):
Sb is the recombination velocity of the excess minority carriers on the backsurface [22], [30], [31], [32]. It is the consequence of the electric field created by the p/p+ (low-high) junction and characterizes the behavior of excess minority carriers at the base-rear junction [33]. The photocurrent density is defined by the following relation: Where q is the electrical charge of the electron. At high recombination velocity of exess minority carriers at the junction (Sf), the photocurrent density is constant and corresponds to (Jsc) the short-circuit current density. This yields to establish the following equation : The resolution of the equation (10)

Conclusion:-
The influence of irradiation (flow φp and intensity kl) has been studied on the density of minority carriers of excess charge in the base, on the photocurrent density and on the determination of the optimum thickness of the solar cell. The optimum thickness of the base is deduced from the intersection of the back surface recombination velocity curves for differents values of the radiation energy flow and the intensity of the damage coefficient. The variation of these two elements induces a slight decrease in the optimum thickness of the base of the solar cell. This technique for optimizing the thickness of the base contributes to reducing the quantity of material (Si) to be used for the manufacture of solar cells which could operate in a radiative medium while retaining a good conversion efficiency, to minimize the manufacturing costs and reduce the resale price.