EF FECTS OF RA DI A TION ON SO LAR CELLS AS PHOTOVOLTAIC GENERATORS

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28 Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2012, Vol. 27, No. 1, pp EF FECTS OF RA DI A TION ON SO LAR CELLS AS PHOTOVOLTAIC GENERATORS by Radovan Lj. RADOSAVLJEVI] 1 and Aleksandra
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28 Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2012, Vol. 27, No. 1, pp EF FECTS OF RA DI A TION ON SO LAR CELLS AS PHOTOVOLTAIC GENERATORS by Radovan Lj. RADOSAVLJEVI] 1 and Aleksandra I. VASI] 2* 1 Faculty of Electrical Engineering, University of Belgrade, Serbia 2 Faculty of Mechanical Engineering, University of Belgrade, Serbia Sci en tific pa per DOI: /NTRP R The grow ing need for ob tain ing elec tri cal en ergy through re new able en ergy sources such as solar energy have lead to significant technological developments in the production of the basic el e ment of PV con ver sion, the so lar cell. Ba si cally, a so lar cell is a p-n junc tion whose char ac - ter is tics have a great in flu ence on its out put pa ram e ters, pri mar ily ef fi ciency. De fects and im - pu ri ties in the ba sic ma te rial, es pe cially if lo cated within the en ergy gap, may be ac ti vated dur - ing its life time, be com ing traps for op ti cally pro duced elec tron-hole pairs and, thus, de creas ing the out put power of the cell. All of the said ef fects could be in duced in many ways over a life time of a so lar cell and are con sis tent with the ef fects that ra di a tion pro duces in semi con duc tor de vices. The aim of this pa per is to in ves ti gate changes in the main char ac ter is - tics of so lar cells, such as ef fi ciency, out put cur rent and power, due to the ex po sure of so lar systems to different (hostile) radiation environments. Key words: out put power, so lar cell, ra diation environment, dose, efficiency IN TRO DUC TION * Cor re spond ing au thor; The fundamental structural element of solar (PV) sys tems, the so lar cell, is usu ally based on the p-n junction device that, exposed to solar radiation, gives power as its out put char ac ter is tic. Due to the un avoid - able market competitiveness with conventional energy sources, PV tech nol ogy is ori ented to wards the pro duc - tion of high ef fi ciency so lar cells that could pro duce more en ergy and, thus, serve as a fea si ble re new able en - ergy source of the fu ture. On go ing re search is fo cused on the ef fects of the de fects and im pu ri ties that in flu - ence the main pa ram e ters, such as the life time of charge carriers, better understanding of transport processes, the cre ation of elec tron-hole pairs, etc. [1-4]. Analytical connections between fundamental and output characteristics of solar cells are a matter of theoretical analysis, but experimentally obtained results are more complex than theoretical suppositions. Both in the production of so lar cells and their per for mance, the dis tri bu tion of dopants, impurities, and especially, defects, is usually not uni form and pre dict able and could, thus, di rectly in - flu ence the pro cesses tak ing place in the cells them - selves. The empirically obtained influence of fundamental parameters is usually mathematically defined by the for mal in tro duc tion of the ideality fac tor, n, in the exponent of the current voltage characteristics of solar cells. The said ideality fac tor com bines all the vari a tions of the cur rent flow of the ideal case, in duced by var i ous internal and external influences of physical parameters of the man u fac tur ing pro cess, or those which are a con - se quence of ag ing. It has al ready been es tab lished 5, 6 that an in crease in the ideality fac tor has a sub stan tial negative influence on the output characteristics of solar cells, pri mar ily on the fill fac tor and ef fi ciency. The main char ac ter is tics of a so lar cell are its ef fi ciency, h, open cir cuit volt age, V oc, short cir cuit cur rent, J sc, max i - mum power, P m, and fill fac tor, ff. These parameters strongly depend on the fundamental parameters of a real so lar cell (or, on a real p-n junc tion in gen eral), such as the se ries and par al lel (shunt) re sis tance (R s and R sh ), ideality fac tor (n) and saturation current density (J 0 ). The changes in these so lar cell pa ram e ters caused by the process of aging are consistent with radiation induced effects in semiconductor devices. Due to their wide area of ap pli ca tion, so lar cells are of ten ex posed to a variety of radiation effects (nat - u ral space en vi ron ment, at mo spheric en vi ron ment, mil i tary and civil nu clear en vi ron ment). Since spent nu clear fu els, in ad di tion, si mul ta neously emit gamma rays, sev eral neu trons, semi con duc tor de vices such as R. Lj. Radosavljevi}, et al.: Ef fects of Ra di a tion on So lar Cells as Pho to vol taic... Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2012, Vol. 27, No. 1, pp so lar cells, when placed in the vi cin ity of these fu els, sus tain dif fer ent kinds of ra di a tion dam age, both from gamma and neu tron ir ra di a tion. There fore, ex ten sive stud ies on the de vel op ment of semi con duc tor de vices that could op er ate nor mally in a ra di a tion en vi ron ment have been car ried out. The con clu sion reached was that ra di a tion in duced de fects sig nif i cantly de grade the performance of photo detectors and solar cells From a tech no log i cal point of view, it is im por - tant to study the vari a tions in duced by the irradiation of semiconductor junction characteristic parameters (reverse saturation current, ideality factor etc.) that af - fect the per for mance of the so lar cells. The aim of this pa per is to in ves ti gate changes in the main char ac ter is tics of so lar cells, such as ef fi - ciency, out put cur rent and power, due to the ex po sure of so lar sys tems to dif fer ent (hos tile) ra di a tion en vi - ron ments. THEORETICAL ANALYSIS Out put char ac ter is tics of all semi con duc tor de vices are pri mar ily de fined by fun da men tal pa - ram e ters (re sis tance, life time and mo bil ity of charge car ri ers, dif fu sion length, etc.), and pro cesses in them. In polycrystalline and monocrystalline so lar cells, the in her ent pres ence of de fects and im pu ri ties in the ba sic ma te rial could, over time, pro duce cer - tain neg a tive ef fects. This is es pe cially em pha sized if those states are lo cated within the en ergy gap and are ac ti vated dur ing use. In such a case, they be come traps for op ti cally pro duced elec tron-hole pairs and thus de crease the num ber of col lected charge car ri - ers. Mac ro scop i cally, this ef fect may be ob served as a de crease in the out put cur rent and volt age and, might, ul ti mately, lead to the de crease of the ef fi - ciency of the so lar cell. Lower val ues of the short-cir cuit cur rent in di cate the ex is tence of re - com bi na tion cen ters that de crease the mo bil ity and dif fu sion length of charge car ri ers, mak ing re com bi - na tion in the de ple tion re gion a dom i nant mode of trans port in such so lar cells. Also, volt age de crease in the max i mum power point (P m ) has a great in flu - ence on ef fi ciency. One of the main rea sons for this de crease is the in crease of the ideality fac tor, so it can be said that the in flu ence of the ideality fac tor on so lar cell ef fi ciency is through volt age. A set of ex - per i men tally ob tained h = f (n) de pend en cies for dif - fer ent so lar cells is shown in fig. 1. All of these ef fects could be in duced in many ways over a life time of a so lar cell (op ti cally, by tem - per a ture changes in the en vi ron ment, etc.), and are con sis tent with the ef fects that ra di a tion pro duces in the semi con duc tor de vices. Many ex per i ments have shown that the short-cir cuit cur rent and out put power de crease grad u ally with the in crease in the ra di a tion dose [10-14], while the open cir cuit volt age can be se - Fig ure 1. Dependence of ef fi ciency on the ideality fac tor verely de graded even at low doses [14]. The change in the short-cir cuit cur rent was mainly re lated to the life - time of mi nor ity car ri ers, the change in the open cir cuit volt age, to the dam age of p-n junc tions. The pres ence of im pu rity at oms that are ei ther added to the base ma - te rial as do nors or dur ing the man u fac tur ing pro cess, has in di cated the pos si bil ity that some of the pro duced elec trons might be trapped by those at oms be tween the va lence and con duc tion band [15-17]. There fore, the power out put of the so lar cells ex posed to g radiation is also re duced. EX PER I MEN TAL PRO CE DURE All experimental measurements were performed on monocrystalline, non-en cap su lated Si so lar cells man u fac tured by Siemens. Cur rent-volt age data were used for the char ac ter iza tion of their prop er ties. Stan - dard mea sure ment equip ment was used to mea sure the I-V curve un der two dif fer ent il lu mi na tion lev els (32 W/m 2 and 58 W/m 2 ). A re flec tive lamp was used as a so lar sim u la tor. From the ob tained curves, all rel e vant parameters were obtained. Two types of ra di a tion (gamma and neu tron) were used for the mea sure ments. Be fore and af ter each step of the irradiation, current-voltage characteristics of the di odes were mea sured in highly con trolled con - di tions, at room tem per a ture, with com bined mea sure - ment un cer tainty less than 5% [18-21]. One group of so lar cells were ir ra di ated with a 60 Co gamma source with dif fer ent doses. The ir ra di a tion was per formed through glass, in a con trolled en vi ron ment. An other group of so lar cells were ir ra di ated with a Pu-Be point neu tron source. This point source is a mix ture of 238 Pu, with be ryl lium as a good source of neu trons (through a nu clear re ac tion in which Be ab sorbs an al pha par ti cle from Pu and forms 12 C, with the emis sion of a neu - tron). A di rect con tact of the sam ples and the source was es tab lished, and the max i mum dose rate was es - tab lished as dd/dt = 0.36 mgy/h. 30 Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2012, Vol. 27, No. 1, pp RE SULTS AND DIS CUS SION As is known, de pend ing on the en ergy of gamma rays, their in ter ac tion with the ma te rial is through the pho to elec tric and Compton ef fects that may pro duce ion iza tion in duced changes in the ma te rial. These changes usu ally re sult in an in crease of the sur face re - com bi na tion ve loc ity and the den sity of sur face states. If those states cor re spond to deep en ergy lev els in the sil i con en ergy gap, they act as ef fi cient sur face recombination cen ters for charge car ri ers. The generation of elec tron-hole pairs due to ion iza tion ef - fects usu ally re sults in the generation and increase of noise and the min i mum sig nal that can be de tected. All of these ef fects lead to the de crease of the out put current, as can be seen in fig. 2, for the cur rent at the max i mum power point (J m ). Sim i lar be hav iour could be ob served in the de - pend ence of max i mum power (P m ) on doses for the two illumination levels (fig. 3). Be sides the ex pected de crease of the out put char ac ter - is tics with an in crease in doses, a slight in crease of J m and P m was de tected at the higher il lu mi na tion level. This in crease is very sig nif i cant from the stand point of so - lar cells as power gen er a tors, be cause it in di cates a pos si ble ben e fi cial in flu ence of low doses of ir ra di a - tion. A pos si ble ex pla na tion could be the fact that dur - ing the fab ri ca tion pro cesses, un avoid able struc tural de fects and im pu ri ties pro duce ten sion in the crys tal lattice. The interaction of irradiation with such a material could act sim i larly to an neal ing, re lax ing the lat - tice struc ture and de creas ing the se ries re sis tance, ul ti - mately lead ing to a in crease in ef fi ciency (fig. 4). Fig ure 4. De pend ence of ef fi ciency on doses (gamma irradiation) Fig ure 2. De pend ence of the cur rent at the max i mum power point on doses (gamma ir ra di a tion) Fig ure 3. De pend ence of the max i mum power on doses (gamma ir ra di a tion) Charge car rier life time de creases due to radia - tion damage in duced by neu trons, causes the deg ra da - tion of electrical pa ram e ters of the cell such as se ries resistance (R s ), out put power and, fi nally, efficiency (h). A high level of series resistance usually indicates the pres ence of im pu rity at oms and de fects lo cal ized in the de ple tion re gion, act ing as traps for re com bi na tion or tun nel ing ef fects, in creas ing the dark cur rent of the cell. More over, shal low re com bi na tion cen ters in the vicinity of the con duct ing zone en hance the tun nel ing effect, further degrading the output characteristics of the cell by in creas ing the noise level (es pe cially that of burst noise, con nected to the pres ence of an ex cess cur rent). Such a neg a tive im pact of neu tron ra di a tion was ob served at a higher illumination level, as can be seen in fig. 5 But, an interesting phe nom e non an in crease of the max i mum power was ob served for lower val ues of il lu mi na tion. (Dif fer ent be hav iors for dif fer ent il lu - mination levels are due to the pres ence of fi nite se ries and par al lel re sis tance in the cell.) A pos si ble ex pla na tion for the ob served ef fect could be sim i lar to that of gamma ir ra di a tion, namely: small doses of ra di a tion may pro duce a de crease in se - ries resistance. Subsequently, this will lead to a low er - R. Lj. Radosavljevi}, et al.: Ef fects of Ra di a tion on So lar Cells as Pho to vol taic... Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2012, Vol. 27, No. 1, pp and, in par tic u lar, the life time of the mi nor ity car rier. This de crease in the life time en hances fur ther deg ra - dation of the cell s parameters. Many experiments have shown that out put power and ef fi ciency de crease grad u ally with the in crease of the ra di a tion dose. How - ever, though com monly re ferred to as a source of noise in semi con duct ing de vices, ra di a tion in duced ef fects could, in some cases, have a positive effect on the main electrical characteristics (J m, P m, h). Ini tial im prove - ment of such characteristics observed at small doses of neu tron radiation and low illumination levels, in di - cates that there is a pos si bil ity of us ing ir ra di a tion for the en hance ment of so lar cell qual ity. Fig ure 5. De pend ence of the max i mum power on doses.(neutrons) ing of the noise level and an in crease in the out put cur - rent since, as is well known, high se ries re sis tance of a so lar cell is one of the main lim it ing fac tors of its effi - ciency. Finally, the im prove ment of out put char ac ter is - tics af ter the first irradiation step at the low illumina - tion level has also been reg is tered for ef fi ciency, fig. 6. Al though higher doses of neu tron ra di a tion un doubt - edly have a neg a tive im pact on the per for mance of solar cells, ob served phe nom ena of fer pos si bil i ties for the use of radiation as a means in im prov ing solar cell characteristics. Fig ure 6. De pend ence of ef fi ciency on doses (neu trons) CONCLUSION So lar cells, the ba sic el e ments of the pho to vol - taic con ver sion of so lar en ergy, are es pe cially sus cep - ti ble to ra di a tion dam age, pri mar ily due to their large surface. Permanent damage in solar cell material is caused by the col li sion of in ci dent ra di a tion par ti cles with the at oms in the crys tal line lat tice. The re sult ing de fects de grade the trans port prop er ties of the ma te rial AC KNOWL EDGE MENTS The Ministry of Ed u ca tion and Sci ence of the Re pub lic of Ser bia has sup ported this work un der con - tract REFERENCES [1] Sze, S. M., Phys ics of Semi con duc tor De vices, 2 nd ed., John Wiley & Sons, New York, USA, 1981 [2] Fahrenbruck, A. L., Bube, R. 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R., Tran sient Ion iz ing Ra di a tion Efects in De vices and Cir cuits, IEEE Trans ac tion on Nu - clear Sci ences, 50 (2003), 3, pp [18] Vujisi}, M., Stankovi}, K., Osmokrovi}, P., A Sta tis ti - cal Anal y sis of Mea sure ment Re sults Ob tained from Non lin ear Phys i cal Laws, Ap plied Math ematical Mod el ing, 35 (2011), 7, pp [19] Stankovi}, K., et al., Sta tis ti cal Anal y sis of the Char - ac ter is tics of Some Ba sic Mass-Pro duced Pas sive Elec tri cal Cir cuits Used in Mea sure ments, Measure - ment, 44 (2011), 9, pp [20] Stankovi}, K., Influence of the Plain-Par al lel Elec - trode Sur face Di men sions on the Type A Mea sure - ment Un cer tainty of GM Coun ter, Nucl Technol Radiat, 26 (2011), 1, pp [21] Stankovi}, K., Vujisi}, M., Deli}, Lj., In flu ence of Tube Vol ume on Mea sure ment Un cer tainty of GM Counters, Nucl Technol Radiat, 25 (2010), 1, pp Re ceived on De cem ber 19, 2011 Ac cepted on Jan u ary 24, 2012 Radovan Q. RADOSAVQEVI], Aleksandra I. VASI] UTICAJ ZRA^EWA NA SOLARNE ]ELIJE KAO FOTONAPONSKE GENERATORE Rastu}a potreba za dobijawem elektri~ne energije iz obnovqivih izvora energije kao {to je sun~eva energija, dovela je do zna~ajnog tehnolo{kog razvoja u proizvodwi osnovnog elementa fotonaponske konverzije, solarne }elije. Solarna }elija je u osnovi p-n spoj ~ije karakteristike imaju veliki uticaj na izlazne parametre, prvenstveno na efikasnost. Defekti i ne~isto}e u osnovnom materijalu, posebno ako se nalaze unutar energetskog procepa, mogu tokom rada da se aktiviraju, postaju}i zamke za opti~ki generisane parove elektron-{upqina i na taj na~in smawuju}i izlaz
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