Vailable online at www.sciencedirect.com Energy Procedia 27 (2012 ) 543 548 SiliconPV: April 03-05, 2012, Leuven, Belgium Simplified interdigitated back contact solar cells C.E. Chana*, B.J. Hallam, S.R. Wenham School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW, 2052 Abstract In this work, the fabrication of an interdigitated back contact solar cell is investigated on p-type Czochralski silicon wafers using a novel laser doping approach to form both polarities of rear contacts. Using only one conventional thermal diffusion which forms the n + active emitter on the rear and n + floating emitter on the front of the device, implied open circuit voltages exceeding 690 mv have been achieved on partly processed devices prior to metallisation, with virtually full-area emitter coverage and both polarities of contacts formed, indicating the potential of this structure for achieving high efficiencies with a simple process. Severe shunting post-metallisation due to the poor electrical isolation properties of the rear surface passivation layer currently limits the final device voltage to 625 mv. Further investigations must be undertaken in order to minimise the parasitic shunting effect and maintain a high open circuit voltage post-metallisation. 2012 Published by by Elsevier Ltd. Ltd. Selection and and peer-review under under responsibility of the of scientific the scientific committee of the committee SiliconPV of 2012 the SiliconPV conference. 2012 Open conference access under CC BY-NC-ND license. Keywords: Solar cells; interdigitated back contacts; laser doping 1. Introduction Interdigitated back contact (IBC) solar cells offer numerous advantages over conventional solar cells including significant improvement in short circuit current achieved from zero shading loss; simpler interconnection techniques and a higher packing density [1]; improved aesthetics; lower resistive losses and consequently higher efficiencies [2]. Despite these advantages, the complexities in the processing required to achieve such structures are often too costly for commercial production with typically two or more high temperature diffusion steps being required. For example, a processing sequence for a rear contacted rear junction cell can often include at a minimum: 1) rear emitter diffusion, 2) FSF diffusion and 3) rear base diffusion [3-5]. There are numerous disadvantages associated with multiple high * Corresponding author. Tel.: +61 2 9385 6053; fax: +61 2 9385 7212 E-mail address: catherine.chan@unsw.edu.au 1876-6102 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the SiliconPV 2012 conference. Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2012.07.107
544 C.E. Chan et al. / Energy Procedia 27 ( 2012 ) 543 548 temperature processing steps including: increased processing time; high costs; increased potential for introducing contaminants; and degradation in bulk lifetime, particularly when using lower quality substrates which are emerging onto the market. Moreover, one or more of these diffusions is performed through a diffusion mask, such as silicon dioxide, which needs to be grown at high temperatures and patterned using methods such as photolithography, laser ablation or inkjet printing. Laser doping has the potential to drastically simplify the processing sequence for rear contact solar cells as it can simultaneously pattern a dielectric layer and form heavily doped regions in the underlying silicon. The manufacture of laser doped selective emitter solar cells (LDSE) has already been shown to be achievable in a commercial environment [6, 7] and record cell results have recently been achieved by UNSW in industrial environments [8, 9]. However, little work has previously been done on laser doping through an emitter of opposite polarity. Recently, the authors of this work have demonstrated the formation of deep laser doped regions with junctions extending more than 10 μm into the wafer, as shown in Figure 1(a) [10]. The formation of such deep molten regions allows an existing doped surface layer (such as an n-type emitter) to be smeared out so that its dopant concentration is substantially reduced. At the same time, p-type dopants present in a dopant film on the surface of the wafer can be incorporated into the molten region. By controlling the concentration of dopants within the molten region as well as the heating and cooling regime, the authors have shown that it is possible to fully overcompensate the n-type emitter and form a direct contact to the buried p-type base material as shown in Figure 1(b) [10]. The applicability of this process to solar cells and ability to avoid junction tunneling between the n-type emitter and p-type compensated laser doped contacts has been investigated by the authors [11], with the finding that low dark saturation currents are introduced by the laser doping step and a high implied V oc can therefore be maintained after the formation of these contacts, without the need for isolation between the n + emitter and p ++ laser doped contact region. Wafer surface Wafer surface n-type emitter SiN Opening in SiON n-type emitter SiN Opening in SiON Junction p-type base Laser doped region (n-type) Junction p-type base Laser doped region (p-type) Fig. 1. Combined SEM/EBIC image of the cross section of (a) an n-type laser doped line and (b) a p-type laser doped line, both processed at 0.2 m/s on a p-type wafer with a thermally diffused phosphorus emitter at the surface [10]. The location of the junction is indicated by the bright regions In this work, we investigate the fabrication of a laser doped IBC (LD-IBC) solar cell using this laser doping technique to form both polarities of contacts. 2. Processing sequence Standard commercial grade 1 Ω.cm p-type CZ wafers were used in this work. For simplicity, planar
C.E. Chan et al. / Energy Procedia 27 ( 2012 ) 543 548 545 wafers were used with no texturing on the front surface, with the initial aim of achieving high open circuit voltages as proof of concept. Future work will focus on optimising the optics for current collection. The process flow is shown schematically in Figure 2. Wafers were saw damage etched in NaOH solution to a thickness of 150 μm. Following a full RCA clean and HF dip, wafers were thermally diffused at 860 C via solid source phosphorus diffusion in a back-to-back arrangement to obtain a final active emitter sheet resistivity of 150 Ω/ on the rear (nonlight receiving) side of the device and approximately 1000 Ω/ floating emitter on the front (light receiving) side of the device. Wafers then underwent an additional HF dip to remove PSG prior to PECVD SiON deposition on both sides. For the laser doping, firstly, a commercially available boron spin on dopant source was spun onto the wafer at 2000 rpm for 40 seconds and baked at 130 C for 10 mins. A high powered 532 nm wavelength laser with scanning optics was then scanned over the wafer to form p-type laser doped lines penetrating through the n-type emitter on a pitch of 1 mm. Following a rinse-off of the p-type dopant source, phosphoric acid (85% concentration) was spun onto the wafer as an n-type dopant source. The same laser was used to scan over the wafer to form n-type laser doped lines on a 1mm pitch in an interdigitated pattern (resulting in a spacing of 0.5 mm between contacts of opposite polarity). After removal of residual phosphoric acid, wafers were then annealed at 400 C for 15 minutes in nitrogen ambient followed by a 30 second HF dip to deglaze the laser doped lines. Metal contacts were formed via thermal evaporation of aluminium. Finally, cells were sintered in nitrogen ambient at 300 C for 5 minutes to reduce contact resistance. The final device structure is shown schematically in Figure 3. Saw damage removal + cleaning Solid source P diffusion (back to back) at 860 C + PSG removal PECVD SiON on both sides Laser doping of p- type and then n- type lines Anneal at 400 C for 15 mins followed by deglaze of LD lines Thermal aluminium evaporation to form metal contacts Sinter at 300 C for 5 mins Fig. 2. Process flow for LD-IBC solar cell SiON (75 nm) n-type front floating junction (1000 ohm/sq) p-type CZ bulk (1 Ohm-cm) n-type rear emitter (150 Ohm/sq) SiON (200 nm) Aluminium Fig. 3. Structure of the LD-IBC solar cell
546 C.E. Chan et al. / Energy Procedia 27 ( 2012 ) 543 548 Quasi-steady-state photoconductance (QSS-PC) was used to determine the implied 1-sun open circuit voltage (iv oc ) at three stages of processing: 1. after PECVD deposition, 2. after laser doping, and 3. after annealing. 3. Results High implied open circuit voltages of over 690 mv were obtained on partly-processed devices prior to metallisation, demonstrating the ability for both polarities of contacts to be formed using laser doping with minimal defect formation or parasitic shunting between contact polarities being induced. This shows the potential of the process to achieve high-efficiency devices provided this voltage can be maintained after metallisation. However, after metallisation, severe shunting between the n- and p-type silicon occurs where n-type silicon is exposed around the perimeter of the p-type laser doped lines where the dielectric layer is damaged, as shown in Figure 4(a) and (b). This reduces the final V oc to 625 mv and efficiency to 14.5%. The light J-V curve of the LD-IBC cell is shown in Figure 5. Note that the front surface is planar with high reflectance, resulting in low currents. As can be seen in Figure 1(b), the deep lateral diffusion of the p-type dopants should extend underneath this partially ablated dielectric region. However, optimisation of laser parameters to encourage deeper lateral diffusion extending beyond the damaged region could help minimise the shunting. Alternatively, the use of dielectric layers with a higher melting point or alternative laser beam profiles such as a top-hat beam could be investigated to reduce shunting by reducing the risk of partially ablating and forming pinholes in the dielectric layer. Table 1. One-sun implied open circuit voltages at each stage of processing and final open circuit voltage Stage of processing iv oc (mv) After PECVD 699 After laser doping After anneal After metallisation and sinter 675 692 625 (V oc) (a) Shunt path (b) p++ Al n+ SiON Fig. 4. (a) Microscope image of p-type laser doped line with partially ablated dielectric layer around the edge of the line and (b) schematic of shunt path through partially ablated dielectric
C.E. Chan et al. / Energy Procedia 27 ( 2012 ) 543 548 547 Fig. 5. Light J-V curve of LD-IBC cell 4. Conclusion A simple method for fabricating IBC solar cells has been demonstrated in which only one high temperature process is required. Patterning of the rear dielectric layer and heavy doping of the underlying silicon is performed simultaneously in a single laser doping step for each polarity of contact, eliminating the need for any further conventional high temperature diffusion steps after the initial emitter diffusion. For the p-type contact, laser parameters are selected such that the n-type emitter is locally overcompensated by p-type dopants, forming direct contact to the base. Virtually full emitter coverage can be maintained in this process, eliminating any electrical shading normally present in IBC solar cells caused by recombination above the base diffusion. This can enhance carrier collection and thereby relax the constraints placed on wafer quality in contrast to the tight constraints required by cell technologies which have only partial emitter coverage. The LD-IBC cell is capable of achieving very high implied open circuit voltages of 690 mv prior to metallization. Shunting post-metallisation limits the cell efficiency to 14.5 %, and future work must focus on achieving good electrical isolation between the n-type emitter and the p-type contacts to maintain high shunt resistance. Acknowledgements The authors would like to acknowledge the support of Tom Puzzer and the staff at the UNSW Electron Microscope Unit, a division of the Australian Microscope and Microanalysis Research Facility (AMMRF) for the assistance with the SEM and EBIC images produced in this work. The support of the Australian Solar Institute and Suntech are also acknowledged. References [1] Kress A, Breitenstein O, Glunz S, Fath P, Willeke G, Bucher E. Investigations on low-cost back-contact silicon solar cells. Solar Energy Materials and Solar Cells, 2001;65(1-4):555-560. [2] Kerschaver E, Beaucarne G. Back-contact solar cells: a review. Progress in Photovoltaics, 2006;14(2):107-123. [3] Guo J-H, Cotter J. Laser-grooved backside contact solar cells with 680 mv open-circuit voltage. IEEE Transactions on Electron Devices, 2004;51(12):2186-2192.
548 C.E. Chan et al. / Energy Procedia 27 ( 2012 ) 543 548 [4] Engelhart P, Harder N-P, Grischke R, Merkle A, Meyer R, Brendel R. Laser structuring for back junction silicon solar cells. Progress in Photovoltaics, 2007;15(3):237-243. [5] Mulligan W, Rose D, Cudzinovic M, De Ceuster D, McIntosh K, Smith D, Swanson R. Manufacture of solar cells with 21% efficiency. Proceedings of the 19 th European Photovoltaic Solar Energy Conference and Exhibition, Paris, France, 2004, pp. 387-390. [6] Tjahjono B, Yang M, Lan C, Ting J, Sugianto A, Ho H, Kuepper N, Beilby B, Szpitalak T, Wenham S. 18.9% efficient laser doped selective emitter solar cell on industrial grade p-type CZ wafer. Proceedings of the 25 th European Photovoltaic Solar Energy Conference, Valencia, Spain, 2010, pp.1396-1400. [7] Tjahjono B, Haverkamp H, Wu V, Anditsch HT, Jung W-H, Cheng J, Ting J, Yang MJ, Habermann D, Sziptalak T, Buchner C, Schmid C, Beilby B, Hsu K-C. Optimising selective emitter technology in one year of full scale production. Proceedings of the 26 th European Photovoltaic Solar Energy Conference, Hamburg, Germany, 2011, pp. 901-905. [8] Hallam B, Wenham S, Sugianto A, Mai L, Chong CM, Edwards M, Jordan D, Fath P. Record large area p-type CZ production cell efficiency of 19.3% based on LDSE technology. Journal of Photovoltaics, 2011;1:43-48. [9] Hallam B, Wenham S, Edwards M, Lee HS, Lee E, Lee HW, Kim J, Shin J. Record industrial cell efficiency fabricated on commercial grade p-type CZ substrates. Proceedings of the 21 st Photovoltaics Specialist Conference, Fukuoka, Japan, 2011. [10] Hallam B, Chan C, Wenham S. 2012, to be submitted [11] Chan C, Hallam B, Wenham S. 2012, to be submitted