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Web Site: www.ijettcs.org Email: editor@ijettcs.org, editorijettcs@gmail.com Volume 1, Issue 3, September – October 2012 ISSN 2278-6856 EFFECT OF IRRADIATION ON THE TRANSIENT RESPONSE OF A SILICON SOLAR CELL I. Gaye 1, R. Sam 2, A.D. Seré 2, I.F. Barro 1 , M.A. Ould El Moujtaba 1, R. Mané 1, G. Sissoko1 1 Faculté des Sciences et Technologies, Université Cheikh Anta Diop de Dakar BP 5005, Dakar-Fann, Sénégal 2 Département de Physique, Institut des Sciences Exactes et Appliquées, Université Polytechnique de Bobo Dioulasso, Burkina Faso Abstract: A theoretical study of a silicon solar cell under in which non-ionizing collisions between the incident multispectral illumination and particles (electrons, particle and the target atoms causes displacement of the protons…) irradiation is presented. The solar cell is placed in atoms in the lattice. It is the permanent displacement a fast switch interrupted circuit and the transient decay is produced by non-ionizing events incident (protons and obtained between two steady states. The transient variation of electrons) that degrades the performance of the minority carriers’ density is presented and we show how semiconductor devices [2]. diffusion length, transient photovoltage, transient The purpose of this study is to show the influence of the photocurrent, and transient capacitance depend on the irradiation energy and the nature of the radiation Kl on of particles). silicon solar cell, particularly on the following Keywords: Solar cell, Transient variation, parameters: minority carriers’ density, transient Irradiation. photovoltage, transient photocurrent density and transient capacitance. 1. INTRODUCTION 2. DEVICE OPERATION Photovoltaic solar energy is the main energy source for satellites and other space stations. Most solar panels are Figure 1 show the experimental setup used to obtain the embedded in silicon semiconductor materials. They are transient response of the solar cell. subjected to ionizing radiation, which are able to change their electrical behaviour. When there is absorption of a dose of ionizing radiation, the concentration of electrons and holes is modified and the solar cells operation could be strongly modified. The harmful radiation sources for semiconductors are of two sorts: natural phenomena and those related to human activity. The first one is mainly brought due to the space environment: solar flare, solar wind, cosmic radiation and the radiation belts. Phenomena resulting from human activity are similar but the energy and flow of radiation Figure 1: Experimental setup are higher. Radiative emission is found in civil (nuclear power This setup includes a square wave generator (BRI8500) plants) and military (nuclear explosion, etc ...) which drives a RFP50N06 MOSFET type, two adjustable applications. resistors R1 and R2, a silicon solar cell, a digital All these phenomena generate emissions of particles and oscilloscope, a computer and multi-spectral light source radiation which interact with matter and introduce [3], [4]. disturbances in the atomic structures and the electrical The I-V curve of the solar cell is given in Figure 2 [5]. balance in the solar cell. Particles that interact are charged particles (ions, electrons, protons etc ...), At time t < 0 (Figure 1), the solar cell is under neutrons and photons [1]. constant multispectral illumination, MOSFET T is When energetic particles go through the atomic lattice of turned off and the solar cell is loaded only by resistor the material, they transfer their energy to the network R2: this correspond to operating point F2 in steady through events in which ionizing electrons in the network state. are temporarily excited to higher energy levels and events Volume 1, Issue 3, September – October 2012 Page 210 Web Site: www.ijettcs.org Email: editor@ijettcs.org, editorijettcs@gmail.com Volume 1, Issue 3, September – October 2012 ISSN 2278-6856 At t = 0 (Figure 1), MOSFET T turning on and after L is the diffusion length of minority carriers in the base a very short time (600-800ns) it is fully turned on so depend on the irradiation energy and the damage that resistor R2 is in parallel with R1 + Rdson. Rdson coefficient Kl through the following expression [7], [2], is the drain (D) - source (S) resistance. For a [8]: sufficient Gate voltage, the value of Rdson is very low (4) (less than one ohm) and can be neglected compared L ( Kl , ) 1 to that of R1 (10 1 Kl 2 at the operating point F1 in steady state (Figure 2). L 0 L0 is the diffusion length without irradiation. We present on figure 3 the diffusion length versus particles energy for various damage coefficients. 0.01 3 9.5 10 3 9 10 Figure 2: I-V curve photovoltage of a silicon solar cell 3 Kl=5 cm^2/s 8.5 10 The transient decay occurs between the operating points Kl=10 cm^2/s Kl=15 cm^2/s F1 and F2. The transient voltage across the solar cell is Kl=25 cm^2/s recorded by a digital oscilloscope (Tektronix), coupled 0 0.5 1 1.5 2 with a computer for processing and analysis. Varying R1 and R2, lead to changing operating points F1 Figure 3: Profile of the variation of the diffusion and F2 respectively; this allow us to perform the transient length depending on the irradiation energy decay at any operating point of the solar cell. The diffusion length decreases as the particle energy 3. THEORY increases. The diffusion length also decreases with the coefficient damage, but this decrease is more marked for 3.1 Excess minority carrier density This study is done on a n+pp+ BSF silicon solar cell. higher irradiation energy. Since the diffusion length is strongly influenced by irradiation. It is clear that the Given that the base contribution is more important, our analysis will be conducted only in this region of the solar behaviour of the solar cell will also be influenced by irradiation. cell. Equation (1) is solved by taking into account the The solar cell is under constant multispectral boundary conditions at the junction and at the back side illumination. At time t and at the depth x in the base, the of the solar cell [9], [10]: distribution of the minority charge carriers is represented by n (x,t) in transient state. At the junction (x = 0): Let n (x) be the distribution of minority charge carriers in the steady state and (x, t) the excess minority carriers at ( x) (5) time t from the final state, we have [6]: D Sf (0) (1) x x 0 Distribution of minority carrier’s n(x, t) at time t satisfies the continuity equation on the charge carriers given by: At the back surface (x = H): (2) ( x) (5’) D Sb (H ) D is a diffusion constant and L is the diffusion length of x x H the minority carriers. Sf and Sb are respectively the minority carrier’s recombination velocities at the junction and at the back G (x) is the carrier generation rate at the depth x in the side of the cell. base. Expressions (1), (2) and (3) represent Sturm Liouville’s 3 bm x system. G ( x) n ame (3) The excess minority carrier’s density can be written in the m 1 following form: n is the illumination level, H is the thickness of the base, am and bm are coefficients tabulated from overall AM1.5 (6) solar radiation [2]. Volume 1, Issue 3, September – October 2012 Page 211 Web Site: www.ijettcs.org Email: editor@ijettcs.org, editorijettcs@gmail.com Volume 1, Issue 3, September – October 2012 ISSN 2278-6856 The solutions of these differential equations in X(x) and T(t) lead to the following general terms: (7) And (8) An , Bn and Tn(0) are constants. c,n is the decay time constant and is related to the minority carriers lifetime by the following expression. (9) n is the Eigen value of the transcendental equation below. Figure 8: Variation of the carrier density versus time for We can establish the following transcendental equation, different values of damage coefficient Kl taking into account the expression of L We note in Figures 7 and 8 that the particle irradiation (10) energy affects the transient variation of the carrier This equation is valid only if: density. When the irradiation energy increases, the carrier density decreases, and the variation in time is faster. Thus, for a given value of the energy of radiation, we feel (11) the same effects with the increase in the coefficient of We present on figure 6 the transient decay and also the damage. The passage of a charged particle, including an series expansion of equation (6) limited to one, two, and ion through the material generates a region damaged three terms. along its path; irradiation creates defects inherent in interactions between charged particles and electrons of silicon. The charged particles lose their energy in the material and the electron density decreases [12], [13]. We present in figure 9 the excess minority carriers density versus irradiation energy for various damage coefficients Figure 6: Transient decay versus time This figure show that the different terms of the series expansion decrease very quickly and after a certain amount of time t0, the fundamental mode corresponding to n=0 predominates and is equal to the excess minority We can write that [11]. Figure 9: Carrier density versus irradiation energy (12) various damage coefficients The excess minority carrier’s density versus time is We can observe that the density of minority carriers presented on figures 7 and 8 respectively for various decreases with the irradiation for a damage coefficient. irradiation energies and various damage coefficients. And it’s more perceptible for higher irradiation energy and higher damage coefficient. 3.2 Transient photovoltage The transient photovoltage is determined from the Boltzmann relation: (13) VT is the thermal voltage Or (13’) Figure 7: Variation of the carrier density versus time for Let for different values Volume 1, Issue 3, September – October 2012 Page 212 Web Site: www.ijettcs.org Email: editor@ijettcs.org, editorijettcs@gmail.com Volume 1, Issue 3, September – October 2012 ISSN 2278-6856 (14) transient photovoltage. Figures 14 and 15 below represent the transient photocurrent profiles for various radiation energies and damage coefficients Kl. Figure11: P Figure 14: Profile of the photocurrent density versus time We can observe that Transient voltage increases with the for different values of carriers are increasingly blocked at the junction. Figures 12 and 13 below present the transient coefficients Kl. Figure 15: Profile of the photocurrent density versus time for different values of the damage coefficient Kl The transient photocurrent density decreases when the amplitude level of radiation increases. Irradiation affects the density of the carriers, the carriers passing through Figure 12: Profile of the transient voltage between the space charge zone decreases, so it is a decrease of the two operating points for different values of the radiation amplitude of the transient photocurrent density and therefore. 3.4 Transient capacitance Transient capacitance is given by the following relation: (15) Figures 16 and 17 below present the transient capacitance for various irradiation energy and damage coefficient Kl respectively. Figure 13: Profile of the transient voltage between two operating points for different values of damage coefficient Kl We note in Figures 12 and 13 that photovoltage decreases in time. The magnitude of the transient voltage remains constant when the irradiation energy increases and also when the damage coefficient increases. This means that irradiation does not affect the carriers trapped at the Figure16: Profile of the transient capacity versus time for junction. different values of the irradiation energy 3.3 Transient Photocurrent The transient photocurrent is given by the equation below Volume 1, Issue 3, September – October 2012 Page 213 Web Site: www.ijettcs.org Email: editor@ijettcs.org, editorijettcs@gmail.com Volume 1, Issue 3, September – October 2012 ISSN 2278-6856 illuminated I-V characteristic. World Renewable Energy Congress, pp.1848-1851, 1998. [6] G. Sissoko, C. Museruka, A. Corréa, I. Gaye, A. L. Ndiaye. Light spectral effect on recombination parameters of silicon solar cell World Renewable Energy Congress, part III, pp.1487-1490, 1996 [7] M.A. Ould El Moujtaba, M. Ndiaye, A.Diao, M.Thiame, I.F. Barro and G. Sissoko. Theoretical Study of the Influence of Irradiation on a Silicon Solar Cell under Multispectral Illumination. Figure17: Profile of the transient capacity versus time for Research Journal of Applied Sciences, Engineering different values of damage coefficient Kl and Technology ISSN When the irradiation energy increases, ie when the [8] R. K. Ahrenkiel, D. J. Dunlavy, H. C. Hamaker, R. radiation level increases, as the increase in the coefficient T. Green, C. R. Lewis, R. E. Hayes, H. Fardi Time- of injury, the number of particles interactions increases, of-flight studies of minority-carrier diffusion in and the carrier density is affected which reduces the AlxGa1-xAs homojunctions J. Appl. Phys. 49(12) number of carriers stored on either side of the junction, 1986. and there is a widening of the space charge zone. This is [9] Mara Bruzzi Radiation Damage in Silicon Detectors what explains the decrease in the amplitude of the for High-Energy Physics Experiments IEEE transient capacitance with increasing irradiation energy transactions on nuclear science, vol. 48, no. 4, august and damage coefficient. 2001. [10] A. Ricaud Photopiles solaires. Presses 4. CONCLUSION polytechniques et universitaires romandes, 1997. This study based on a silicon solar cell irradiated by [11] Saïdou Madougou, Mohamadou Kaka and energetic particles shows that the diffusion length Gregoire Sissoko Silicon solar cells: recombination depends strongly on the irradiation energy but also on the and electrical parameters. Solar Energy, Book edited damage coefficient of these particles. The study also by: Radu D. Rugescu, ISBN 978-953-307-052-0, pp. showed that the minority carriers density, transient 432, February 2010, photovoltage, transient photocurrent density and transient [12] H. Mathieu, H. Fanet. Physique des capacitance, are influenced by both the irradiation energy semiconducteurs et des composants électroniques 6ème and the damage coefficient. We can also extend this study Ed, Dunod, 2009 to a bifacial solar cell. [13] L. Andricek, D. Hauff, J. Kemmer, P. LuKewille, G. Lutz, H.G. Moser, R.H. Richter, T. Rohe, K. REFERENCES Stolze, A. Viehl. Radiation hard strip detectors for [1] Helmuth Spieler Introduction to Radiation-Resistant large-scale silicon trackers Nuclear Instruments and semiconductor devices and circuits. Ernest Orlando Methods in Physics Research A 436 (1999) 262-271. Lawrence Berkeley National Laboratory, Physics Division, AUTHOR [2] R. J. Walters and G. P. Summers Space Radiation Since 2006: Professor of physical Effects in Advanced Solar Cell Materials and sciences in High School Malick Sy, Devices Thiès-Sénégal. 2010-2012: PhD in Mat. Res. Soc. Symp. Proc. Vol. 692 physics (2nd registration), University [3] P. Mialhe, G. Sissoko, F. Pelanchon, J. M. Cheikh Anta Diop Dakar-Sénégal Salagnon 2010-2011: Certificate in secondary Régimes transitoires des photopiles : durée de vie des education, University Cheikh Anta porteurs et vitesse de recombinaison. Journal de Diop Dakar-Sénégal 2009-2010: Master II in Physics and physique. III (Print) A. 1992, vol. 2, n° 12, pp. 2317- Applications, University Cheikh Anta Diop Dakar 2003- 2331. 2004: Master II in Systems, Networking and [4] F.I. Barro, A. Seidou Maïga, A Wereme, G. Sissoko Telecommunication, High School of Sciences computers, Determination of recombination parameters in the base of a bifacial silicon solar cell under constant Dakar-Sénégal 2002-2003: Master in Electronics multispectral light Phys. Chem. News 56(2010) 76- sciences, University Mohamed 1er Oujda-Morocco. 84. [5] G. Sissoko, E. Nanema, A. Correa, P. M. Biteye, M. Adj, A. L. Ndiaye. Silicon Solar cell recombination parameters determination using the Volume 1, Issue 3, September – October 2012 Page 214