Proceedings of the European Summer School of Photovoltaics 4 â 7 ...

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ature were simulated for solar cell with standard area (100 cm 2 ) and thickness – 210 µm – Fig. 6. The mechanism of current flow in internal structure of cell is linked with temperature. Increase of temperature leads to linear fall of open circuit voltage and almost linear growth of short circuit current. Heating reduces the size of energy gap. The increase in the short-circuit current is caused by rising the absorbed photon current [5]. The heating has a destructive effect on the opencircuit voltage. Finally, the efficiency of solar cell is reduced accordingly. It is caused by reducing of conductivity. The rise of temperature is also linked with high irradiation in areas with poor ventilation. It is very desirable to employ a good air circulation among PV systems. Summary The work referred to research on the factors which may contribute to decrease the efficiency of solar cells. Progressive degradation process for multi- and mono -crystalline solar cells was observed. Optical measurements showed that effective reflectance is worse for mc-Si than Cz-Si cells although they were coated antireflective layer. It is caused generally by their heterogeneous structure. The process of texturization in case of Cz-Si improves their optical parameters. On the other hand it contributes to faster oxidation their surfaces. The process of obsolescence was finally corroborated by electrical measurements. The simulation of impact of heating showed, that its high values cause decrease of solar cells efficiency. References [1] Swatowska B., A. Stańco, P. Panek: The causes of silicon solar cells degradation. 35th International Microelectronics and Packaging IMAPS – IEEE CPMT 21–24 September, Gdańsk–Sobieszewo, Poland, 2011. [2] http://www.globalmarket.com/ [3] Swatowska B., T. Stapiński, J. Chojnacki: Parameters of silicon solar cells in changeable external conditions. 23 International Conference of IMAPS – CPMT IEEE 21–24 September, Pszczyna, Poland, 2009. [4] Strehlke S., S. Bastide, J. Guillet, C. Lévy – Clément: Design of porous silicon antireflection coatings for silicon solar cells. Materials Science and Engineering B69 – 70 (2000) 81–86. [5] Würfel P.: Physics of Solar Cells – From Principles to New Concepts. WILEY – VCH Verlag GmbH & Co. KGaA, Weinheim, Germany 2005. Low-ohmic contacts on the basis of silver nanopowder dedicated to photovoltaic cells Joanna Kalbarczyk, Marian Teodorczyk, Konrad Krzyżak, Grzegorz Gawlik, Mateusz Jarosz, Anna Młożniak Institute of Electronic Materials Technology, Warszawa Obtaining satisfactory parameters of solar cell is largely dependent on mastering the technique of producing contacts metal – semiconductor. The m-s contact can be strictly mechanical connection (e.g. trough contact electrode with semiconductor) as well as inseparable, integral connection. The m-s contacts may be regarded as ideal if they generate no additional resistance for the current, do not react chemically with semiconductor material, do not change characterictics while changing illumination, temperature or value of applied electric filed and do not exhibit rectifying effects. Such contact can be described with Ohm’s law – its current-voltage characteristic is linear. It is obvious that ideal contacts do not exist, but the aim of the technology is to approach as close as possible to the imaginary ideal contact. To eliminate serious mistakes while creating ohmic contacts it is necessary to know the type of conduction. The material chosen for contacts should create impurity centers that comply with semiconductor’s type of conduction – it should contain donor impurities in case of contact with n – type semiconductor and acceptor impurities in case of p-type semiconductor. Except specified electrical properties there are many other physicochemical properties (such as good thermal conductivity, compatibility of metal and semiconductor coefficient of heat expansion, proper mechanical strength in different temperatures) that should characterize a useful m-s contact [1]. In the following thesis, two methods of creating contacts will be presented. The first method is the cathode sputtering. The silicon base is subjected to high vacuum ion bombardment in inert atmosphere, then silver layer is deposited. The method provides obtaining layer of high degree of purity. The second method of making contacts is applying the paste with silver nanopowder. The method is quite new and is an attempt of confrontation already well-known and bringing satisfactory effects in silver contact preparation procedure (sputtering) and a new method which uses up-to-date nanomaterial that may exhibit interesting properties for the technique of solar cells contact preparation [2, 4]. 82 Tabl. 1. Properties of nanosilver layers in Si substrates [4] Sample Polished side, firing in 300 0 C Mat side, firing in 300 0 C Polished side, firing in 600 0 C Mat side, firing in 600 0 C Resistance/□ [mΩ/□] Layer thickness [µm] Resistivity [Ω∙m×10 -8 ] Up to now silver pastes used in screen printing technique have been composed of silver powder (conductive phase), where grain size varied between 1 and 3 µm, and glaze (auxiliary phase) which has helped in the sintering process. The characteristic temperature of sintering silver layers is 850 0 C. If the composition of glaze has been appropriate – the temperature of sintering might have been reduced to about 650 0 C. The temperature of firing commercially used pastes (e.g. Du Pont) achieves about 750 0 C, which is temperature of silicon wafer when temperature setting is 920 0 C [3–5]. Silver nanopowder paste represents a new generation of thick-layer materials dedicated to screen printing. The conductive phase is silver powder where grains size range from a few to over a dozen nanometers. Methyl polimethacrylate has been used as the matrix of nanocomposite [4, 5]. Nanomaterials often manifest new specific properties – physicochemical, electrical, magnetic or catalytic. These new characteristics arise from the quantum effects and high share of surface atoms causing high reactivity. In consequence melting point of nanoparticles can be much lower than microcrystalline solid. Many other new phenomena may also occur in nanoscale [2-4]. Preliminary research in the field of silver nanopowder paste contact preparation for silicon solar cells has brought to wellconducting layer forming through firing in much lower tempera- Adhesion 12,2 2 2,44 + 12,0 2 2,40 + 14,35 2 2,87 + 14,7 2 2,94 + Elektronika 6/2012
tures in comparison to so far used pastes. Obtained resistance results for paste firing in 300 and 600 0 C are similar. Resistivity – firing temperature relationship has shown that already from 250 0 C layers exhibit very good electric conductivity which insignificantly fluctuates with the firing temperature escalation. Results of layer resistance after firing in different conditions have been shown in Table 1 [4]. The main advantages of silver nanopastes is that they do not contain the glaze phase, which impaired electric conduction of the layer, and the possibility of firing in lower temperatures. Obtained results demonstrates that layers made from pastes with silver nanopowder can be used as ohmic contacts for silicon solar cells [3]. Experiment The aim of the first stage of research was to determine electrical properties of contacts made with sputtering method. Such contacts have been already used to investigate electrical and optical properties of edge-illuminated silicon solar cells and enabled to acquire repeatable, satisfactory results. Because of that the sputtering-method-made contacts have been acknowledged as a good comparative base for new type of silver nanopowder contacts. Resistance has been measured with two point probe connected with multimeter. The silver layers have been applied trough a mask and three values of resistance have been measured. The scheme of the sample with applied contacts and measurement points have been shown on Fig. 1. a) b) c) Fig. 1. The scheme of measurement sample with “sputtered” contacts; R1, R2, R3 – values of resistance between marked silver layers The resistance measurement has to be made with comparative method, because obtained values do not define directly single contact resistance, but contain also resistances of particular meter circuit elements and foundation material. After sputtering process, the layer has been firing in RTP AG Heatpulse system (Rapid Thermal Processing) in nitrogen atmosphere. The parameters – temperature and time – of contacts’ firing have been varied to optimize the process. Resistances have been measured both directly after sputtering process and after the firing in different conditions. It enabled to determine the optimum processing parameters of the contact preparing. Analogically experimental work concerning applying nanosilver pastes has been barely begun. The screen printer and suitable masks has been prepared for applying the paste on basic silicon wafers. Experimental research schedule has been established. Results of sputtering-method-made contacts The results for these method have been elaborated for three kinds of base material: silicon wafer doped with boron (B), silicon wafer doped with phosphorus (P) and silicon wafer doped with antimony (Sb). The results are shown on graphs below. Resistances R1, R2 and R3 are resistances between points as shown on Fig. 1. Fig. 2. Dependence of resistance on firing time for silicon doped with boron samples for defined measuring points: a) R1, b) R2, c) R3 The results of resistance for boron-doped silicon samples have been presented only for measurements carried out after the firing process, because the values of resistance before that process varied between about 400 and 650 Ω. Putting such high values on the graphs would made them illegible. The improvement of contact resistance on boron doped basis after firing is tremendous. The best results have been obtained for firing in 600 0 C. The results of P and Sb doped silicon samples have been shown both for measuring before and after the firing. The improvement for resistances after firing is clearly apparent. The optimum temperature of the process is 500 0 C for phosphorus doped samples and 600 0 C for antimony doped samples. Results of silver nanopowder paste contacts Preliminary results have been obtained for contacts applied on alundum basis with screen printing method through a mask shown on Fig. 5. After printing contacts have been dried in a band drier in 120 0 C for 10 minutes. On that stage of research the contacts have been fired in Czylok chamber furnace in different temperatures for 1 hour. The results of resistance measurement have been shown in Table 2. During further research the firing process will be executed in RTP AG Heatpulse system. Elektronika 6/2012 83
Page 1 and 2: Proceedings of the European Summer
Page 3 and 4: Spis treści str. 1. Analysis of se
Page 5: Fig. 4. Spectral reflectance depend
Page 9 and 10: Summary The silver nanopowder paste
Page 11 and 12: B A Rys. 5. Modele cząsteczek stos
Page 13 and 14: Laser texturing and microtreatment
Page 15 and 16: Tab. 2. Wyniki średnie, odchylenia
Page 17 and 18: Tab. 1. Elementy składowe komercyj
Page 19 and 20: Rys. 2. Analizowane kształty tekst
Page 21 and 22: Podsumowanie W wyniku badań doświ
Page 23 and 24: a) b) Fig. 4. ITME image: a) graphe
Page 25 and 26: To obtain the total thickness of th
Page 27 and 28: Optical and electrical parameters o
Page 29 and 30: Antireflection coating with plasmon
Page 31 and 32: Technological issues and optimizati
Page 33 and 34: GaAsN as a photovoltaic material -
Page 35 and 36: AP-MOVPE technology of AIIIBV-N het
Page 37 and 38: optical and structural features wer
Page 39 and 40: chanism, as it provides alternative
Page 41 and 42: The experimental results showed in
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Page 45 and 46: pyrrolidone). To 0.04 g of polymer
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IV Elektronika 6/2012
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