21.1 % EFFICIENT PERC SILICON SOLAR CELLS ON LARGE SCALE BY USING INLINE SPUTTERING FOR METALLIZATION

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1 21.1 % EFFICIENT PERC SILICON SOLAR CELLS ON LARGE SCALE BY USING INLINE SPUTTERING FOR METALLIZATION Dirk Reinwand 1, Jan Specht 1, David Stüwe 1, Sonja Seitz 1, Jan-Frederik Nekarda 1, Daniel Biro 1, Ral Preu 1, Roland Trassl 2 1 Fraunhoer Institute or Solar Energy Systems (ISE), Heidenhostrasse 2, D Freiburg, Germany 2 Applied Materials GmbH & Co. KG, Siemensstrasse 100, D Alzenau, Germany ABSTRACT This work presents the irst results or the production o highly eicient solar cells industrial processes using the PERC structure. Two batches o FZ and Cz waers were conducted to prove the applicability o a transer rom the clean room to an industrial standard. The ront and rear side metallization was done by a new pilot system using the sputtering technology [1] The investigation o solar cell results, series resistance, contact resistance and a optical analysis are discussed later on. INTRODUCTION Passivated emitter and rear cell (PERC) concepts are very well known or highly eicient crystalline silicon solar cells [2]. One important purpose is to transer this promising cell concept into an industrial production environment. The Photovoltaic Technology Evaluation Center () [3] at the Fraunhoer Institute or Solar Energy Systems (Germany) oers the possibility to manuacture PERC silicon solar cells on large scale industrial related pilot systems. One o the key technologies or the metallization o these cell concepts eatures Physical Vapor Deposition (PVD) technologies instead o the standard screen printing o metal pastes or establishing either the complete metallization or a seed layer or subsequent electroplating. Less shadowing and a lower contact resistance are compared to screen printed contacts the two major advantages o sputtered contacts. Sputtering, instead o the standard thermal evaporation process, or metallization is interesting due to low costs or thin layer deposition o most metals. With a new pilot system rom Applied Materials (AMAT) it is possible to sputter various metal layers a high throughput. (Cz) waers a bulk resistivity ρ Si,Bulk = 1 Ωcm were abricated. Because o the complexity o the process, on every waer 16 single cells were created to increase the number o cells per batch. Thereore dierent process steps or structuring were necessary. A masking oxidation o an approximately 250 µm thick wet SiO 2 was created. Aterwards the emitter windows (2 2 cm²) were deined by using inkjet hotmelt resist. The opening o the windows (selective oxide removal) was done by a wet chemical process. During the subsequent texturing, random pyramids were created. Aterwards the phosphorous emitter diusion in a tube urnace (inal sheet resistance R SH = 120 Ω/sq. ) was perormed prior to the removal o the remaining diusion barrier (SiO 2) and the phosphorussilicate glass rom the waer in a wet chemical process. For the passivation layer on the ront and rear side a 105 nm thick oxide was used. Within this step the emitter was driven in. For the rear side metallization a 2 µm thick aluminum layer was sputtered, ollowed by the laser-ired contact (LFC) process [4] (pitch 750 µm) and the annealing under orming gas (425 C, 25 min.). Subsequent the photolithographical deinition o the ront side pattern was done. 1. Laser cutting 125*125 cm²) and wet chemical cleaning 2. Masking Oxidation 3. Windows deinition (Inkjet hotmelt resist, area 4 cm²) 6. POCl 3 diusion (R SH = 120 Ω/sq.) 7. PSG and oxide removal 8. Thermal growing o SiO 2 (both sides 100 nm) Clean room 11. De. o ront side pattern, slective oxide removal 12. Inline sputtering o metal seed layer Clean room 13. Lit-o process, EXPERIMENTAL The process low o the investigated PERC cells is shown in Figure 1. For a detailed analysis o the manuactured high eicient solar cell structure inline sputtered ront and rear side contacts, six inch shiny etched boron doped loat zone (FZ) waers (ρ Si,Bulk = 1 Ωcm, 250 µm thick) were cut down to a size o 125*125 mm², because o the used carrier and tray systems o the equipment. Additionally in a second batch which was processed simultaneously the FZ waers monocrystalline Czochralsky 4. Selective oxide removal 5. Front surace texture 9.Inline sputtering o Al (rear side) 10. Laser ired contacts, annealing 14. Annealing 15.Silver plating, characterization Figure 1: Process overview or manuactured PERC silicon solar cells. Processes perormed in and in clean room are indicated.

2 The ront side contacts were deposited inline sputtering technology. A two layer system o titanium (50 µm) and silver (100 µm) was deposited. Through a lit-o process the photo resist was removed. Ater that the ront side contacts were annealed. The ront side contacts were thickened by silver plating to ensure a suicient conductivity. All processes (except the deinition o the ront side pattern and the lit-o process ater the metal deposition) have been established in the a high reliability and quality. With the new inline sputtering pilot system ATON 500 Ev + Sp (Figure 2) rom Applied Materials (AMAT) it is possible to sputter ront side contacts industrial throughput. Depending on the inal layer thickness the pilot system is capable to process 540 waers per hour. Two rotatable targets in the process chamber allow sputtering dierent metal layers out breaking the vacuum. The tool installed in the PVTEC is in the moment able to sputter Ti, Ag, Al, NiCr and NiV. Figure 2: Schematic o the pilot line PVD metallization system or thermal evaporation and DC sputtering. The homogeneity o the sputtered metal layers has been determined by using the our-point-probe measurement [5]. For sputtered titanium on every waer o one tray (consists o 3x3 waers) 10x10 measuring points were taken which results in a total number o 900 measuring points or one tray. The deviation was determined to 5.41 % over the whole eective sputtering area or deposited titanium. ANALYSIS OF INDUSTRIALLY FABRICATED PERC SILICON SOLAR CELLS The experimental ocus o the abricated solar cells was on a variation o the annealing process and on the electrical losses due to the dierent contact ormation o the ront side contacts. Thereore the IV-parameters, the series resistance and the contact resistivity were determined. Solar cell results In the irst batch 64 silicon solar cells (FZ) a seed layer inger width o 15 µm and 64 solar cells (FZ) a line width o 45 µm were produced. In a second batch 64 PERC cells (Cz) a Ti-Ag seed layer o 15 µm inger width were abricated (in total 192 solar cells). The results o the illuminated IV-curve or manuactured PERC cells show that both or Cz and FZ waers constant high eiciencies are possible over a wide temperature range or the annealing process. Nevertheless, a non-negligible eect o the annealing step can be observed. The reason or that seems to be a slight dierence o the silicide ormation, depending on the chosen temperature and time. The depth o the titanium silicide silicon interace depends on the chosen annealing parameters [1], [6]. However, the overall high eiciencies show that the equipment allows abricating PERC cells industrially related throughput high reliability. eiciency η [%] = 15 µm, t = 10 min, Cz 8 = 15 µm, t = 5 min, Cz 6 = 15 µm, t = 10 min, FZ = 15 µm, t = 5 min, FZ 4 = 45 µm, t = 10 min, FZ 2 = 45 µm, t = 5 min, FZ Figure 3: Results o processed solar cells sputtered ront and rear side contacts. In Figure 3 the results rom IV-measurements or Cz and FZ waers, annealed between T min = 100 C and T max = 500 C under orming gas or t 1 = 5 min and t 2 = 10 min, are shown. The eiciency increases continuously rom 100 C to 400 C or the FZ waers and up to 450 C or the Cz waers. The highest eiciencies η max,fz = 21.1 % and η max,cz = 19.4 % are observed at 400 C (t = 10 min) and at 450 C (t = 5 min). The eiciency o the abricated silicon solar cells the 45 µm Ti-Ag seed layer decreases rapidly between 400 C and 450 C (i.e. or t = 10 min the eiciency drops rom η 400 C,10 min = 19.5 % to η 450 C,10 min = 4.6 %). A urther investigation o the ront and rear side metallization was done later on by determining the series resistance and the contact resistance. The detailed results o the optimized annealing process or FZ and Cz waers are listed in Table 1. The open circuit voltage V OC or the best PERC cell sputtered 15 µm thick Ti-Ag seed layer is mv at a current density o j SC = 39.1 ma/cm² and an eiciency o 21.1 percent. The abricated Cz waers seem to be more stable at higher temperatures, because although FZ and Cz waers were produced simultaneously the highest eiciencies were reached at 450 C at a illactor o FF = 77.6 %.

3 Table 1: IV-parameters or the investigated PERC cells at the optimized annealing process. T t V OC J SC FF η [ C] [min] [mv] [ma/cm²] [%] [%] Best cell FZ Avg. o 4 cells FZ Best cell Cz Avg. o 4 cells Cz In Figure 4 it is shown that the blue response curve o the internal quantum eiciency o an industrial abricated PERC cell a Ti-Ag sputtered seed layer compared to a reerence cell (clean room) shows no dierence, only or longer wavelengths the IQE is slightly reduced, because o the lower ρ Si,Bulk = 0.5 Ωcm IQE 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 reerence cell (FZ, ρ Si,Bulk = 0.5 Ω cm, clean room) industrially abricated PERC cell 0,1 (FZ, ρ Si,Bulk = 1 Ω cm, ) 0, , c:\dokumenteundeinstellungen\reinwand\eigenedateien\dissertation\ptq09_0346\02_sr\vergleich\090525_vergleich.opj λ (nm) Figure 4: Comparison o the Internal quantum eiciency (IQE) o one industrial abricated PERC cell and a reerence cell prepared in the clean room. The good results o the IQE o the abricated PERC cells underline the capability o producing highly eicient solar cells the pilot systems o the. Investigation o the series resistance The series resistance was determined by using the method [7], [8] o comparing the results o the SunsVoc measurement, introduced by Sinton [9] and the illuminated IV-curve. The total series resistance R S was calculated rom [10]: R ΔV V V mpp( SunsVOC ) mpp S, SunsV = = (Eq. 1) OC J mpp J mpp Whereas V mpp(sunsvoc) is the voltage at the maximum power point rom series resistance ree SunsVoc measurement and V mpp and j mpp the voltage drop and current density rom IV-measurements. In Figure 5 the calculated series resistance R S or highly eicient silicon solar cells a seed layer inger width o 15 µm is plotted versus the annealing temperature. All cells were annealed or ten minutes under orming gas atmosphere. From 100 C to 250 C the series resistance increases rom R S,100 C = 0.37 Ω cm² to R S,250 C = 0.49 Ω cm² which can not be explained until now. The lowest R S was determined at 400 C (R S,400 C = 0.16 Ω cm²) which was also the annealing step leading to the highest cell perormance (compare to Table 1). series resistance R S [Ω cm²] 1,25 1,00 0,75 0,50 0,25 0,00 R S,min = 0,16 Ω cm² (T = 400 C, t = 10 min) Figure 5: Series resistance o PERC cells 15 µm thick Ti-Ag seed layer, annealed between 100 C and 450 C or ten minutes. The highest series resistance R S,SunsVoc = 0.95 Ω cm² was determined at T = 450 C. Contact resistivity and normalized contact resistance The contact resistivity o the sputtered ront side contacts was determined the transmission line model (TLM) [11], [12] by using the our point probe technique. Isolated ingers o the cells served as the TLM structure. For the determination o the contact resistivity and the width weighted contact resistance, the ingers o the ront side contacts were isolated mechanically by sawing. Every measurement was done at eight isolated ingers. Because o the mechanical stress during the isolation process only cells rom the second batch the seed layer o 45 µm could be used or the TLM measurements. Figure 6 a sketch o the isolated area or the measurement (let) and the principle o extracting the contact resistance rom the TLM measurement (right).

4 total resisatnce R tot slope R SH } 2xR c 1d 2d 3d 4d contact distance d Figure 6: Drawing o the isolated area or TLM measurements (let) and the principle o extracting the contact resistance rom the measurement. The inger line width was approximately 65 µm (45 µm seed layer thickness and a urther 20 µm rom the silver plating process) and the contact width o 10 mm. The contact distance d was 0.8 mm. The contact resistivity ρ C versus the annealing temperature is plotted in Figure 7. R C w [Ω cm] 2,2 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Rc*w [Ohm*cm] 45T10FZ Figure 8: Determined width weighted contact resistance or dierent annealing temperatures. contact resistivity ρ C [mω cm 2 ] = 45 µm, t = 10 min (FZ) The contact resistivity o the solar cells, annealed at 450 C, could not be determined because the structure has been destroyed during the isolation process. Optical analysis An optical analysis scanning electron microscopy (SEM) was conducted to determine the aspect ratio (height / width) or sputtered and subsequent silver plated ront side contacts. Figure 7: Contact resistivity plotted versus the annealing temperature. The contact resistivity ρ C,100 C = 7.96 mω cm² continuously decreased to ρ C,400 C = 0.36 mω cm² or the investigated solar cells. Because o a possible dierent contact width w (due to the mechanical isolation o the ingers) the width weighted contact resistance was determined too (Figure 8). In comparison the behavior o the contact resistivity (Figure 7) the normalized contact resistance R C w shows only a slightly dierent characteristic between 250 C and 300 C. The normalized contact resistance R C w = 043 Ω cm remains the same or both temperatures. The reason is an already mentioned dierence in the eective contact width. Figure 9: SEM picture o a sputtered (Ti-Ag seed layer 45 µm) and subsequent silver plated inger; surace (let) and detailed cross section (right). In Figure 9 (let) the cross section area (right) and the surace o a sputtered (Ti-Ag seed layer 45 µm) and subsequent silver plated contact is shown. The height o approximately 11 µm and the width o ~60 µm lead to an aspect ratio o 1/5, or 15 µm thick seed layer deposition and subsequent silver plating the aspect ratio changed to 1/3 and a urther reduction o shading losses. CONCLUSION The major objective o this work was to investigate the easibility o abricating highly eicient solar cells industrial equipment o the. The ront and rear side metallization was conducted a new inline sputtering pilot system rom AMAT. The ront end and the most cru-

5 cial processes o the back end were perormed in the PV- TEC, only the deinition o the ront side pattern and the lit-o process were done in the clean room. Over 190 PERC cells (FZ and Cz material) were manuactured reaching eiciencies up to 21.1 percent. The sputtering technology allows depositing homogenous metal layers (i.e. the deviation o sputtered titanium is only 5.41 %). The investigated PERC cells had a phosphorous emitter a sheet resistance R SH = 120 Ω/ and 105 nm SiO 2 passivation / antirelection layer, sputtered aluminum on the rear side and a Ti-Ag (50 nm / 100 nm) seed layer on top prior the silver plating. The highly eicient solar cells were annealed between 100 C and 450 C or ive and ten minutes under orming gas. For the optimized annealing temperature the highest eiciency or FZ waers was determined to η max,fz = 21.1 % and or the second batch Cz material at 450 C to η max,cz = 19.4 %. Furthermore the investigation o the series resistance and the contact resistivity underline the applicability o abricating highly eicient silicon solar cells industrial equipment. The lowest contact resistivity ρ C,400 C = 0.36 mω cm² was observed at T = 400 C and t = 10 min. The aspect ratio o the ront side contacts was determined by SEM pictures. Over a broad temperature range PERC solar cells high eiciencies were manuactured. These results in combination the high throughput o the pilot systems (or example the inline sputtering tool it is possible to process 540 waers per hour) underline the easibility o producing PERC silicon solar cells industrial related machines. ACKNOWLEDGEMENT The authors would like to thank Elisabeth Schaeer or IVmeasurements, the whole team or antastic work and the Reiner Lemoine oundation or support. The inancial unding by the Bundesministerium ür Umwelt, Naturschutz und Reaktorsicherheit (German ministry or environment, nature conservation and reactor saety) under the contract no B and B is greatly appreciated. [4.] Schneiderlöchner, E., et al. "Investigations on laserired contacts or passivated rear solar cells". in Proceedings o the 29th IEEE Photovoltaics Specialists Conerence New Orleans, Louisiana, USA. [5.] Schroder, D.K., "Semiconductor material device characterization". 2. ed. Vol : John Wiley & Sons, Inc. [6.] Maex, K. and M. Van Rossum, "Properties o metal silicides". 1995, London: INSPEC, the Institution o Electrical Engineers [7.] Mette, A., et al., "Series resistance characterization o industrial silicon solar cells screen-printed contacts using hotmelt paste" Progress in Photovoltaics: Research and Applications, (6): p [8.] Pysch, D., A. Mette, and S.W. Glunz, "A review and comparison o dierent methods to determine the series resistance o solar cells" Solar Energy Materials & Solar Cells, : p [9.] Sinton, R.A. and A. Cuevas. "A quasi-steady-state open-circuit voltage method or solar cell characterization". in Proceedings o the 16th European Photovoltaic Solar Energy Conerence Glasgow, UK: James & James, London, UK, [10.] Wol, M. and H. Rauschenbach, Advanced Energy Conversion, 1963: p [11.] Berger, H.H. "Contact resistance on diused resistors". in IEEE International Solid-State Circuits Conerence [12.] Berger, H.H., "Models or contacts to planar devices" Solid-State Electronics, : p REFERENCES [1.] Reinwand, D., et al. "Industrial sputtering metallization technology or crystalline silicon solar cells". in Proceedings o the 24th European Photovoltaic Solar Energy Conerence Hamburg, Germany. [2.] Blakers, A.W., et al., "22.8% eicient silicon solar cell" Applied Physics Letters, (13): p [3.] Biro, D., et al. "PV-Tec: Photovoltaic technology evaluation center - design and implementation o a production research unit". in Proceedings o the 21st European Photovoltaic Solar Energy Conerence Dresden, Germany.