Effect of Growth Process on Polycrystalline Silicon Solar Cells Efficiency. ZOHRA BENMOHAMED, MOHAMED REMRAM* Electronic department university of Guelma Département d Electronique Université mai 195 BP 01Guelma 2000 *Electronic department university of Constantine Département d Electronique Université Mantouri Constantine25000 ALGERIA Abstract In this paper, we present the effect of the dislocation density over the minority-carrier diffusion length for polycrystalline samples grown by PHOTOWATT POLIX and SUMITOMO SITIX.The analyzes presented shows that the dislocation is not a dominating factor which limit the electric properties of material. The presence of impurities and their Interactions with dislocations and other intragrain defects affect the minoritycarrier lifetime. Simulator PC1D is used to calculate the solar cells performances according to the last technological processes developed. A comparison between photovoltaic properties in particular polycrystalline silicon solar cells efficiency grown by POLIX and SITIX is studied taking account of the structural parameters of these materials. The results shown that an efficiency of over 1% has been obtained for SITIX solar cells, whereas an efficiency not exceeding 13% has been obtained in the case of POLIX solar cells. Key words:polycrystalline -silicon - dislocation impurities- efficiency-interaction 1 Introduction The potential and value of polycrystalline silicon solar cells have already been proven for photovoltaic applications.large grained polycrystalline silicon solar cells are actually obtained with wafers made by conventional casting method supplied from Photowatt Polix or Sumitomo Sitix. Once fabricated the material, which is characterised by a columnar structure, is revealed to be competing with monocrystalline silicon for terrestrial photovoltaic applications(1]. Due to the various growth methods involved, structural defects, such as dislocations, grain boundaries and intragrain defects, can behave differently on the electric properties of material. The performance-limiting impurities, such as oxygen, carbon and transition metals can be present in different concentrations in wafers cut out of ingots. The impurities interactions with dislocations and intragrain defects limit the minority- carrier diffusion length then they decrease the solar Cell efficiency. However, the new technological approaches suggested to realize low cost and high efficiency are insufficient. The detailed comprehension of the parameters which influence the carrier transport remains important in order to improve the polycrystalline silicon solar cells performances. The harmful effect of impurities and their interactions with structural defects on polycrystalline silicon solar cells electrical and photovoltaic properties must be considered. In this paper, correlations between minoritycarrier diffusion lengths and dislocation density have been analysed. A comparison between photovoltaic properties in particular polycrystalline silicon solar cells efficiency grown by Photowatt Polix (France) and Sumitomo Sitix (Japan) is presented and discussed.the usual electrical and structural characterization techniques have been used along with the (SPV) method to evaluate minority- carrier diffusion length. Simulator PC1D is used in order to calculate solar cells performances under standard illumination (AM1.5G, 0mW/cm 2 ).At this stage, we carried out the calculation on the cell performance of polycrystalline silicon solar cells using practical device parameters which have recently been reported. The technological parameters are selected according to the recent development in solar cells fabrication
2 Experimental procedure In order to study the effect of structural and electrical parameters on the polycristalline solar cells photovoltaic conversion, we have used two series of polycrystalline silicon wafers cut out of ingots grown respectively by Photowatt Polix and Sumitomo Sitix. The wafers are P type; the carrier concentration was 1x 1 Cm -3. To evaluate minority-carrier diffusion length, we have used the surface photovoltage"spv"[2,3];that consists in subjecting the polycrystalline silicon wafers to an optical excitation, under this excitation, minoritycarrier diffuse through surface establishing a potential (SPV). The potential thus established is proportional to the excess carriers density; that one being in direct relationship to the minority- carrier diffusion length. The Polix samples were treated by rapid thermal processing (RTP) during 20s in an FV furnace of JIPILEC France, with a cooling speed of -50 C/s at a temperature range from 00 C to 00 C. The minority carrier diffusion length for SITIX samples was evaluated at room temperature.the Table 1 and Table 2 respectively gather the results of this characterization. Wafers diffusionlength (µm) SS1 2 SS2 7 SS3 0 SS 72 SS5 5 SS 7 SS7 2 SS 2 SS9 53 SS 0 SS11 2 SS 53 Table 1: values of minority-carrier diffusion length (µm) at ambient temperature for Sitix wafers. 3 Effect of the dislocation density on diffusion length We have evaluated the minority-carrier diffusion length versus dislocations density for the two types of materials in particular at ambient temperature. Fig.1 illustrates the variation of diffusion length versus dislocations density. We notice that the diffusion length decrease when the dislocation density increases from 3 cm -2 at 5 cm -2 for to two series of wafers, furthermore, we notice that SITIX samples presents a diffusion length larger than that of POLIX samples. The accentuated degradation in the case of POLIX samples can be explained by the fact that dislocations affect the electronic structure and induce dangling bonds. In addition, impurities precipitation or the segregation (transitions metals, oxygen and carbon) in the vicinity of dislocations increases their activity recombining by the means of the deep centres generated by the impurities [, 5]. These have a direct consequence on the decreasing of minority-carrier diffusion length and their lifetime. Table 2: values of minority-carrier diffusion length (µm) after RTP in Polix wafers. Diffusion length(µm) longueur de diffusion(µm) 90 5 0 75 70 5 0 55 50 5 0 35 30 25 20 15 25 C 00 C 700 00 SP1 30 20 15 0 SP2 35 30 22 20 SP3 3 30 20 1 SP 33 30 2 2 SP5 3 35 32 30 SP 0 35 27 2 5 Dislocation density (cm -2 ) densité de dislocation(cm -2 ) Fig. 1: Minority-carrier diffusion length versus dislocations density. ln sitix ln polix
Mathematical models describing the diffusion length according to the structural parameters are in accordance with the results which we obtained. Indeed, M. Yamaguchi et al. [] have described minority carrier diffusion length by the relation (1) given below: 1/L 2 = 1/L 0 2 + 1/L D 2 +1/Li 2 (1) Were: L 0 is the bulk minority-carrier diffusion length. In addition to the effect of dislocations expressed by L D, Li describes the various possible interactions (grain boundaries - dislocations, dislocationimpurities) which can occur in material and deteriorate the minority carrier diffusion length. In conclusion, our opinion is that the parameter dislocation remains limited to describe the transport of the carriers in polycrystalline silicon. However the quantification of such a phenomena remains difficult to realize, given the evolution of the unstable state of various interactions. The results of electrical and structural characterization that we have obtained are used for the calculation of the polycrystalline solar cells photovoltaic performances.however the determination of the optimal parameters remains an important stage before passing to the realization. Thus we used the PC1D version 5 [7] to simulate the solar cells photovoltaic properties. 3 Calculation Fig.2 shows the schematic polycrystalline silicon unit cell used here for simulation. The structure assumes an N + PP + junction, the cell area is set to be 1 cm 2 which consist of a thin n emitter region (0.15µm) doped at 20 cm -3 a p base region with thickness of 20µm and a 5µm thick P + layer serving as a back surface field (BSF).the doping of p and p+ regions is fixed at 1 cm -3 and 5. 1 cm -3, respectively. The reflectance of % and the rear and the front surface recombination velocities of 500cm/s and 0cm/s are respectively used. The technological parameters are selected according to the recent development in solar cells fabrication [,9]. The numerical simulation of this structure is carried out using the simulator PC1D, under global irradiance conditions AM1.5 (0mW/cm) at room temperature. The material parameters such as the dielectric constant, refractive index, absorption coefficient, etc., used in this calculation are the default settings in the model. Fig.2: Schematic of poly-si cell unit used for simulation. Results and Discussions Fig.3 and Fig. shows the polycrystalline silicon solar cells efficiency evolution versus cell thickness as a function of dislocation density, elaborate respectively by Sumitomo and Photowatt. As expected, the increase of the dislocation density from 3 cm -2 to 5. 5 cm -2 reduces significantly the energetic efficiency. This degradation is more important for Photowatt solar cells compared to Sumitomo solar cells. Moreover for the two series solar cells there is an optimum cell thickness for energetic efficiency and it increases as the dislocation density decreases. For this result, a polycrystalline Sitix solar cells conversion efficiency of 1% is feasible and 13% for Polix solar cells even at a cell thickness of 20µm if the dislocation density is less then 5 cm -2. Efficiency(%) rendement de conversion(%) 1 1 1 sitix 1 0 Cell épaisseur thickness(µm) de la cellule(µm) BaseP N + diffused emitter ND= 3cm-2 ND= cm-2 ND= 5cm-2 ND=5. 5 cm -2 Fig.3 : Sitix solar cells conversion efficiency vs dislocation density. P region P + BSF Contacts
We also notice that conversion efficiency profile shows different paces for the two series of samples.indeed one remarkable point is that for a given thickness; the SITIX solar cells efficiency is more importantly reduced than that for POLIX solar cells when the dislocation density increases. That s confirms the non linearity of the photovoltaic conversion versus the minority carrier diffusion length. Efficiency(% rendement de conversion (%) Efficiency(%) 1 polix 2 0 1 0 épaisseur de la cellule (µm) Fig. : Polix solar cells conversion efficiency vs dislocation density. 1 1 1 2 0 Cell thickness(µm) 3 5 5. 5 Dislocation density(cm -2 ) ND= 3cm-2 ND= cm-2 ND= 5cm-2 ND=5. 5cm-2 3 5 5. 5 Dislocation density (cm -2 ) polix sitix Fig.5: comparison of conversion efficiency for the two solar cells series. A comparison between solar cells conversion efficiency for the two series of samples is illustrated by the Fig.5. An efficiency of varying from 11% to 1% is reached for polycrystalline silicon solar cells grown by Sumitomo sitix whereas that obtained for solar cells based of Photowatt polix samples vary from % to 13%. We notice that the efficiency obtained for samples POLIX decrease little when the density of dislocation increases of two decades, contrary with that obtained for SITIX samples where the efficiency variation is larger. Conclusion In this article we presented and discuss the influence of the dislocation density over the minority-carrier diffusion length in polycrystalline silicon. A comparison of polycrystalline silicon solar cells photovoltaic performances grown by Photowatt POLIX and Sumitomo SITIX is analyzed. The results obtained show that the behaviour of dislocations in the two series of samples differs according to the technique growth.this can be certainly to explain by the fact why the cooling process is certainly responsible for the inhomogeneous formation of microscopic structures in particular the growth of oxygen precipitates. This confirms that the dislocation density is not the only factor which influences the solar cell conversion efficiency, the possible interaction of the impurities with the intragrain defects is also the seat of the limitation of polycrystalline silicon solar cells performances. References: [1] G. Beaucarne, S. Bourdais, A. Slaoui and J.Poortmans, Thin film polysilicon solar cells on foreign substrates using direct thermal CVD, Material and Solar Cell Design, Thin Solid Films, 2002, pp.03-0. [2] I. Baser, Semiconductor material and devices characterization, edition wiley intersciences, 199. [3] S.C. Choo, L.S.Tan, Theory of the photovoltage at semiconductor surfaces and its applications to diffusion length measurements, Solid State Elect.., Vol.35, N 3,1992,pp.29-23. [] Z. Benmohamed,Etude de l influence des impuretés sur les performances des cellules solaires au silicium multicristallin,thèse de Magistère, Université de Constantine, Algérie,1999. [5] Z.Benmohamed,M.Remram,A.Laugier, Effect of oxygen and carbon on efficiency of multicristalline silicon solar cells, World
Renewable Energie Network Brighton, 1-7 July, 2000, UK. [] S.Pizini, P.Cagnoni, Grain Boundary Segregation of Oxygen and Carbon, Applied Physics Letters, Vol 51,pp.7-77 [7] M. Yamaguchi and C. Amano, Efficiency calculations of thin-film GaAs solar cells on Si substrates, Journal of applied physics,vol.5, 195,pp.301-30. [] P.A.Basore, Proceedings of the 25 th IEEE Photovoltaic Specialists Conference,199,(unpublished),pp.377. [9] A. Zerga, E. Christoffel, A. Slaoui, Twodimensional modelling of polycrystalline silicon thin film solar cells, 3 rd world conference on photovoltaic energy conversion, may 11-1, 2003,pp.53-5. Osaka. Japan.