- Email: [email protected]
Laser Ablation of Dielectric Layers and Formation of Local Al-BSF in Dielectric back Passivated Solar Cells
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 38 (2013) 670 – 676
SiliconPV: March 25-27, 2013, Hamelin, Germany
Laser ablation of dielectric layers and formation of local AlBSF in dielectric back passivated solar cells Lotfi Bounaasa*, Nicolas Auriaca, Bernadette Grangea, Rémi Monnaa, Marc Pirota, Sylvain De Vecchia Johann Jourdanb, Simon Mialonb, Marcel Pasquinellic, Damien Barakelc a CEA E --LITEN, INES, Le Bourget du Lac 73377, France MPO Energy, Domaine de Lorgerie, Averton 53700, France c IM2NP - Université Aix-Marseille III UMR 6242, Marseille 13397, France b
Abstract Dielectric back passivated solar cells with local Al-BSF (Back Surface Field) are investigated to lead the way on achieving higher efficiencies. However the formation of efficient and uniform local BSF needs a proper opening of the dielectric passivation layers. In this paper the laser ablation mechanisms with green and ultraviolet laser sources are studied to locally remove dielectric stacks such as SiO2/SiNx. Green laser process parameters are found to lower silicon surface damages but highlight the importance of using a post-laser cleaning step based on an alkaline etching solution. The impact of this additional step on BSF formation is studied by extracting the local internal quantum efficiency from LBIC mappings of a fabricated PERC solar cell.
© 2013 The Authors. Published by Elsevier Ltd.
© 2013 Theand/or Authors.peer-review Published byunder Elsevier Ltd. Selection responsibility of the scientific committee of the SiliconPV 2013 Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference
conference
Keywords: back passivated solar cells, PERC, laser ablation, dielectric passivation, local Al-BSF
1.
Introduction
Silicon solar cells integrating a full area screen-printed Al-BSF (Aluminum Back Surface Field) have widely been used in the photovoltaic industry over the last decades. However this back side structure displays several major issues preventing the device from reaching high electrical performances and using thinner wafers. Not only a strong bowing is observed on thin wafers, which is a restraint from an
* Lotfi Bounaas. Tel.: +33-479-792-050 E-mail address: [email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference doi:10.1016/j.egypro.2013.07.331
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
industrial production perspective, but most of all electrical losses are caused by low surface passivation (surface recombination velocity >500cm/s) and light trapping efficiencies [1]. Although some works have brought successful improvements on full area Al-BSF solar cells [2], the development of alternative solutions such as the Passivated Emitter and Rear Cell (PERC) [3] design providing a significantly improved surface passivation and optical reflectivity achieved by dielectric layers has intensively been explored [4], [5]. Nevertheless the complex fabrication process of PERC solar cells is still a limit towards mass production of low cost high efficiency solar panels and represents a great challenge for the PV industry. In the recent years, many R&D groups and companies have been working on the implementation of high throughput laser processing tools for the selective ablation of dielectric layers in back side passivated solar cells; a key process step that needs to be properly achieved in order to form a uniform local Al-BSF so that recombination under the contacts would be minimized [6]. There also should be a good isolation between the contacts and the inversion layer caused by the SiO2/SiNx passivation to prevent parasitic shunting [7]. Indeed a fixed charge density of 3.1012 cm-2 was reported in a previous work [15] for this dielectric stack and found to be sufficient to induce parasitic shunting effects. This study focuses on the impact laser processing using two wavelength on the silicon substrate and question the necessity of performing a subsequent KOH etching step to remove residual laser damages. The local Al-BSF formation is then investigated on screen printed solar cells regarding the defined laser parameters. 2.
Experimental
For the optimization of laser ablation parameters, symmetrical test samples are prepared on chemically polished 239 cm² pseudo-square boron-doped Cz wafers following the process sequence in figure 1 (right). A thin SiO2 layer is grown by thermal oxidation at high temperature and a thick silicon nitride on both sides by PECVD (Plasma Enhanced Vapor Deposition). In this work, 532 nm green and 355 nm UV laser sources with a 10ps pulse period are used to remove the layers. First, laser ablation is assessed with optical microscope observation. Then a characterization routine based on MW-PCD Semilab tool (MicroWave Photoconductance Decay) spatially-resolved lifetime measurements referring to [8] is presented to tune the laser parameters. Different couple of parameters (fluence/overlap) are tested in order to perform the ablation without damaging the Si surface or the surrounding passivation layer. To that end, square shaped openings consisting of 1mm separated lines are performed on the samples applying parameters summarized in Table 1. The overlap was defined as the percentage of irradiated areas between two successive laser pulses compared to the laser opening diameter, meaning that two isolated pulses will be represented by a negative overlap value. A post-laser etching treatment was carried out on samples in a 20% concentrated KOH solution heated at 80°C during 4 minutes. Screen printed solar cells were also fabricated on the same silicon substrate wafers following the process sequence shown in figure 1 (left) and testing the laser parameters listed in table I. In order to understand the Al-BSF formation, the samples were characterized by measuring the LBIC (Light Beam Induced Current) response using a Semilab tool. 3.
Results and discussion
3.1. Laser ablation Figure 2 shows the surface topography after isolated pulse irradiation of test samples at fluences defined in Table 1. These images suggest that surface damages are correlated with increasing fluence for both laser sources with different wavelength. Besides the threshold ablation fluence of the dielectric stack
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Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
with a green laser source is found around 0.10 J/cm², which is in accordance with observations in [9] for a silicon nitride layer alone. In parallel, severe damages to the silicon substrate are caused even at very low
Fig. 1 Process sequence steps for fabricated test solar cells (left) and laser ablation test samples (right).
fluence with the UV laser. The reason for such a behavior is explained in [10][11] by the distinct SiNx ablation mechanisms associated with each wavelength. In the case of a green laser source only a part of the irradiation is absorbed in the layer. The ablation process then results from a partial lift-off based on the co-generation of a heat-affected and a melting of the underlying silicon. Table I. List of the laser parameters used for the square-shaped openings of SiO2/SiNx layers. Green source fluence (J/cm²)
UV source fluence (J/cm²)
Laser pulse overlap (%)
0.09
0.30
0.29
1.88
50
12.5
0.16
0.40
0.80
2.55
37.5
0
0.24
1.79
1.53
25
-50
Fig. 2. Surface topography images of single pulse irradiated samples with green (above) and UV (below) laser sources at different fluences: from left to right, 0.09 J/cm² , 0.16 J/cm², 0.24 J/cm², 0.30 J/cm², 0.40 J/cm² for green laser; 0.29 J/cm² , 0.80 J/cm², 1.53 J/cm², 1.88 J/cm², 2.55 J/cm² for UV laser.
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
In contrary, no lift-off mechanism might occur in the case of UV irradiation since the absorption is higher which gives rise to the degradation of the passivation layer. For further characterization, lifetime mappings are carried out on test samples first after passivation and then after laser ablation with a new passivation. The achievement of a proper ablation layer is estimated by the lifetime ratio resulting from the division of these two maps. Figure 4 presents ratio maps of samples with and without applying a postlaser damage removal consisting of a 4 minutes KOH etching step as displayed in figure 3. The KOH treatment resulted in the removal of a 1.5μm thick silicon layer in low defect density regions. Although we observed that the etching rate was much lower in severely damaged areas indicating that the silicon original crystalline structure near the surface was somehow changed due to laser induced amorphization [10].
Fig. 3. 3D confocal microscopy images displaying the surface topography of a 50% overlap irradiated area with a green laser before (left) and after (right) 4 min. KOH etching treatment.
a)
b) Fig. 4. Lifetime ratio maps of samples processed with (left image) and without (right image) a cleaning step for green (a) and UV (b) laser sources. Square shaped openings applying parameters referenced in Table 1, as follows: each fluence is tested with all overlaps (increasing from right to left) from the lowest (up) to the highest (down) one.
Each lifetime mapping shows soft laser conditions (low fluence and overlap) in the upper right whereas strong impact ones are in the lower left. Notice that some non-irradiated areas display ratios larger than unity because of a better passivation after SiN x re-deposition and that inhomogeneity in lifetime distribution within the wafer must be accounted for the results analysis. Still some trends may be
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Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
extracted from this analysis, especially one can distinguish the strong impact of high overlap and fluence (50% and 1.79 J/cm²) lowering the lifetime by a factor of 2 and related to the generation of recombination itions show less detrimental surface damages but still remain insufficient to ensure a good surface passivation. In contrary, areas with ratio close to unity show evidence of a partial or complete damage removal if a post-laser KOH etching is achieved. Nevertheless incomplete recovered lifetime at 1.79 J/cm² may indicate that defects have been introduced more than 1.5μm deep in the bulk material (figure 3) and could also explain why no clear trend could be detected in the same experiment with a UV laser source. In conclusion, this additional etching step is therefore necessary for laser damage removal and allows using a large range of SiO 2/SiNx laser ablation fluence running from 90 mJ/cm² to 400 mJ/cm² associated with laser pulse overlaps from -50% to 50%. One must even consider enlarging this range by carrying out longer etching time on the samples. However there is a risk that Kirkendall voids [13] would be formed at the back contact location, hence directly impacting the fill factor by the absence of a thick enough BSF as described in [12].
a)
c)
b)
Fig. 5. a) Internal Quantum Efficiency map extracted at 976nm from LBIC measurements at fluences from top to bottom: 0.09 J/cm², 0.12 J/cm², 0.24 J/cm², 0.40 J/cm², 1.79 J/cm²; From right to left, overlaps at -50 %, 0 %, 12.5 %, 37.5 % and 50% are represented. b) Variation of the mean IQE according to the fluence at 12.5% (black squares) and 50% (red circles) overlap. c) Variation of the mean IQE according to the overlap at 0.24 J/cm² (black squares), 0.30 J/cm² (red circles) and 1.79 J/cm² (blue triangles) fluence.
3.2. Contact formation Laser ablation parameters summarized in table I were then tested on screen printed solar cells by high resolution mapping of the LBIC response at 976nm. The mapping shows again soft laser conditions (low fluence and overlap) in the upper right whereas strong impact ones are in the lower left. The Semilab
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
apparatus measures the short circuit current induced by light beams of a solar cell and the reflectance of the same light on the same cell, in this way allowing the extraction of the IQE (Internal Quantum Efficiency) distribution. This technique was used in order to assess the Al-BSF formation regarding the impact of a green laser ablation process after KOH etching, as represented in figure 5a. A first comment on this figure is that the overall inhomogeneous IQE distribution has small consequences on values measured at irradiated areas. In figure 5b and 5c are represented the variations of the mean IQE measured within each irradiated square according to the overlap and fluence. For instance the square irradiated at 1.79 J/cm² and 50% overlap display the lowest IQE, showing evidence of a strong impact of the silicon substrate damage on the current collection which might be related to a deficient BSF formation at this location. In comparison, we can assume that using 12.5% overlap and 300 mJ/cm² fluence gives rise to a better BSF formation since damages have been greatly reduced. Except at 1790 mJ/cm² fluence,,the -50% pulse overlap condition shows the lowest IQE which is related to a thin BSF formation. Indeed it has been discussed by Müller et al. [16] that remains of the dielectric layer will prevent Si to dissolve in the Al melt between single pulses. Therefore the interface area between Si and Al is small with a high Al mass over ablated surface area ratio, leading to the formation of thin BSF layers and stronger contact recombination. In addition the optimal fluence is shifted to lower values when increasing the overlap, which is consistent with results found in section 3.1. Even though trends have been extrapolated between 400 mJ/cm² and 1790 mJ/cm², a range of optimal laser ablation parameters can be found at each overlap. However these results displaying the correlation between laser damage and IQE are not in accordance with previous work of Ramanathan et al. [17] which determined that the effect of laser surface damage is not critical to the cell performance since it is consumed by the BSF formation. No details are given about the laser fluence range tested in this work but we can assume that the surface damage was somehow limited and had no major impact on the contact recombination. In contrary, with similar experimental conditions as our work it was found to be more dependent on the surface damage in [12]. Based on SEM observations of local contacts in a processed PERC solar cell, it was argued that surface damage caused by a ns laser source dielectric ablation gives rise to the formation of a thin or no BSF at all unless after post-laser KOH etching a uniform BSF was formed. Finally, this characterization routine gives hints for the opening of local contact and subsequent formation of Al-BSF. However, other criteria independent from the laser process must be taken into account to understand the formation a local Al-BSF. For instance, Urrejola et al. studied the influence of the firing temperature and the rear geometry pattern for rear passivated solar cells [13] [14], showing that the formation of a high quality local BSF is extremely dependent on the Si diffusion in Al and on the overlap of Al on each side of the dielectric opening. Meemongkolkiat et al. [15] justified the use of modified screen-printing Al paste because of the direct influence of Si content on the BSF formation during alloying of the Al-Si eutectic. 4.
Conclusion
This paper presents an experimental procedure to characterize laser processing regarding the influence of parameters such as fluence and overlap. It was demonstrated the necessity to use a KOH based chemical treatment to remove silicon surface damages. Lifetime based characterization was also correlated to LBIC measurements in order to compare the influence of laser parameters on the contact formation. As a following experimental part, these findings must be completed by SEM observations in order to correlate laser ablation process with local contact and BSF formation.
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Acknowledgements The authors would like to thank IM2NP, CEA and OSEO funding group for financial support. References [1] Narasimha S, Rohatgi A. An optimized rapid aluminum back surface field technique for silicon solar cells, Photovoltaic Specialists Conference, 1997, Conference Record of the Twenty-Sixth IEEE , vol., no., pp.63-66, 29 Sep-3 Oct 1997. [2] Ristow A. Screen-printed back surface reflector for light trapping in crystalline silicon solar cells, 2001. [3] Green M.A., Blakers A.W., Zhao J., Wang A. Characterization of 23-percent efficient silicon solar cells, Electron Devices, IEEE Transactions on , vol.37, no.2, pp.331-336, Feb 1990. [4] Mack S. . Towards 19% efficient industrial PERC devices using simultaneous front emitter and rear surface passivation by thermal oxidation, Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE , vol., no., pp.34-38, 20-25 June 2010. [5] Lai J., Upadhyaya A., Ramanathan S., Das A., Rohatgi A. High-Efficiency Large-Area Rear Passivated Silicon Solar Cells With Local Al-BSF and Screen-Printed Contacts, Photovoltaics, IEEE Journal of , vol.1, no.1, pp.16-21, July 2011. [6] Jaffrennou P. Laser Ablation of SiO2/SiNx and AlOx/SiNx Back Side Passivation Stacks for Advanced Cell Architectures, Proceedings of the 26th World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 2011. [7] Dauwe S. Ph.D Thesis, Low-Temperature Surface Passivation of Crystalline and its Application to the Rear Side of Solar Cells, ISFH, 2004. [8] Engelhart P. Laser ablation of passivation SiNx layers for locally contacting emitters of high-efficiency solar cells, Proceedings of the 4th IEEE Photovoltaic Specialists Conference , 2006, vol.1., pp.1024-1027. [9] Trusheim D. Investigation of the influence of pulse duration in laser processed solar cells, Energy Procedia, 12, pp. 278-285, 201. [10] Heinrich G. Investigation of ablation mechanisms for selective ablation of silicon nitride layers, Energy Procedia, 8, pp. 592-597, 2011. [11] Poulain G. Laser ablation mechanism of Silicon nitride layers in a nanosecond UV regime, Energy Procedia, 27, pp.516521, 2012. [12] Du Z.R. Impact of KOH etching on laser damage removal and contact formation for Al local back surface field silicon wafer solar cells, Proceedings of the 26th World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 2012. [13] Urrejola E. Silicon diffusion in Aluminum for rear passivated solar cells, Applied Physics Letters 98 (2011). [14] Urrejola E. Al-Si alloy formation in narrow p-type Si contact areas for rear passivated solar cells, Applied Physics Letters 107 (2010). [15] Bounaas L., Auriac N., Grange B. Advanced solar cell with back side engineering and implanted phosphorus emitter, Proceedings of the 26th World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 2012. [16] Müller J., Recombination at local aluminum-alloyed silicon solar cell base contacts by dynamic infrared lifetime mapping, Energy Procedia, 8, pp.337-342, 2011. [17] Ramanathan S., Das A., Rohatgi A. 20% efficient screen printed LBSF cell fabricated using UV laser for rear dielectric remova, Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE , vol., no., pp.678,682, 20-25 June 2010.
ScienceDirect Energy Procedia 38 (2013) 670 – 676
SiliconPV: March 25-27, 2013, Hamelin, Germany
Laser ablation of dielectric layers and formation of local AlBSF in dielectric back passivated solar cells Lotfi Bounaasa*, Nicolas Auriaca, Bernadette Grangea, Rémi Monnaa, Marc Pirota, Sylvain De Vecchia Johann Jourdanb, Simon Mialonb, Marcel Pasquinellic, Damien Barakelc a CEA E --LITEN, INES, Le Bourget du Lac 73377, France MPO Energy, Domaine de Lorgerie, Averton 53700, France c IM2NP - Université Aix-Marseille III UMR 6242, Marseille 13397, France b
Abstract Dielectric back passivated solar cells with local Al-BSF (Back Surface Field) are investigated to lead the way on achieving higher efficiencies. However the formation of efficient and uniform local BSF needs a proper opening of the dielectric passivation layers. In this paper the laser ablation mechanisms with green and ultraviolet laser sources are studied to locally remove dielectric stacks such as SiO2/SiNx. Green laser process parameters are found to lower silicon surface damages but highlight the importance of using a post-laser cleaning step based on an alkaline etching solution. The impact of this additional step on BSF formation is studied by extracting the local internal quantum efficiency from LBIC mappings of a fabricated PERC solar cell.
© 2013 The Authors. Published by Elsevier Ltd.
© 2013 Theand/or Authors.peer-review Published byunder Elsevier Ltd. Selection responsibility of the scientific committee of the SiliconPV 2013 Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference
conference
Keywords: back passivated solar cells, PERC, laser ablation, dielectric passivation, local Al-BSF
1.
Introduction
Silicon solar cells integrating a full area screen-printed Al-BSF (Aluminum Back Surface Field) have widely been used in the photovoltaic industry over the last decades. However this back side structure displays several major issues preventing the device from reaching high electrical performances and using thinner wafers. Not only a strong bowing is observed on thin wafers, which is a restraint from an
* Lotfi Bounaas. Tel.: +33-479-792-050 E-mail address: [email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference doi:10.1016/j.egypro.2013.07.331
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
industrial production perspective, but most of all electrical losses are caused by low surface passivation (surface recombination velocity >500cm/s) and light trapping efficiencies [1]. Although some works have brought successful improvements on full area Al-BSF solar cells [2], the development of alternative solutions such as the Passivated Emitter and Rear Cell (PERC) [3] design providing a significantly improved surface passivation and optical reflectivity achieved by dielectric layers has intensively been explored [4], [5]. Nevertheless the complex fabrication process of PERC solar cells is still a limit towards mass production of low cost high efficiency solar panels and represents a great challenge for the PV industry. In the recent years, many R&D groups and companies have been working on the implementation of high throughput laser processing tools for the selective ablation of dielectric layers in back side passivated solar cells; a key process step that needs to be properly achieved in order to form a uniform local Al-BSF so that recombination under the contacts would be minimized [6]. There also should be a good isolation between the contacts and the inversion layer caused by the SiO2/SiNx passivation to prevent parasitic shunting [7]. Indeed a fixed charge density of 3.1012 cm-2 was reported in a previous work [15] for this dielectric stack and found to be sufficient to induce parasitic shunting effects. This study focuses on the impact laser processing using two wavelength on the silicon substrate and question the necessity of performing a subsequent KOH etching step to remove residual laser damages. The local Al-BSF formation is then investigated on screen printed solar cells regarding the defined laser parameters. 2.
Experimental
For the optimization of laser ablation parameters, symmetrical test samples are prepared on chemically polished 239 cm² pseudo-square boron-doped Cz wafers following the process sequence in figure 1 (right). A thin SiO2 layer is grown by thermal oxidation at high temperature and a thick silicon nitride on both sides by PECVD (Plasma Enhanced Vapor Deposition). In this work, 532 nm green and 355 nm UV laser sources with a 10ps pulse period are used to remove the layers. First, laser ablation is assessed with optical microscope observation. Then a characterization routine based on MW-PCD Semilab tool (MicroWave Photoconductance Decay) spatially-resolved lifetime measurements referring to [8] is presented to tune the laser parameters. Different couple of parameters (fluence/overlap) are tested in order to perform the ablation without damaging the Si surface or the surrounding passivation layer. To that end, square shaped openings consisting of 1mm separated lines are performed on the samples applying parameters summarized in Table 1. The overlap was defined as the percentage of irradiated areas between two successive laser pulses compared to the laser opening diameter, meaning that two isolated pulses will be represented by a negative overlap value. A post-laser etching treatment was carried out on samples in a 20% concentrated KOH solution heated at 80°C during 4 minutes. Screen printed solar cells were also fabricated on the same silicon substrate wafers following the process sequence shown in figure 1 (left) and testing the laser parameters listed in table I. In order to understand the Al-BSF formation, the samples were characterized by measuring the LBIC (Light Beam Induced Current) response using a Semilab tool. 3.
Results and discussion
3.1. Laser ablation Figure 2 shows the surface topography after isolated pulse irradiation of test samples at fluences defined in Table 1. These images suggest that surface damages are correlated with increasing fluence for both laser sources with different wavelength. Besides the threshold ablation fluence of the dielectric stack
671
672
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
with a green laser source is found around 0.10 J/cm², which is in accordance with observations in [9] for a silicon nitride layer alone. In parallel, severe damages to the silicon substrate are caused even at very low
Fig. 1 Process sequence steps for fabricated test solar cells (left) and laser ablation test samples (right).
fluence with the UV laser. The reason for such a behavior is explained in [10][11] by the distinct SiNx ablation mechanisms associated with each wavelength. In the case of a green laser source only a part of the irradiation is absorbed in the layer. The ablation process then results from a partial lift-off based on the co-generation of a heat-affected and a melting of the underlying silicon. Table I. List of the laser parameters used for the square-shaped openings of SiO2/SiNx layers. Green source fluence (J/cm²)
UV source fluence (J/cm²)
Laser pulse overlap (%)
0.09
0.30
0.29
1.88
50
12.5
0.16
0.40
0.80
2.55
37.5
0
0.24
1.79
1.53
25
-50
Fig. 2. Surface topography images of single pulse irradiated samples with green (above) and UV (below) laser sources at different fluences: from left to right, 0.09 J/cm² , 0.16 J/cm², 0.24 J/cm², 0.30 J/cm², 0.40 J/cm² for green laser; 0.29 J/cm² , 0.80 J/cm², 1.53 J/cm², 1.88 J/cm², 2.55 J/cm² for UV laser.
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
In contrary, no lift-off mechanism might occur in the case of UV irradiation since the absorption is higher which gives rise to the degradation of the passivation layer. For further characterization, lifetime mappings are carried out on test samples first after passivation and then after laser ablation with a new passivation. The achievement of a proper ablation layer is estimated by the lifetime ratio resulting from the division of these two maps. Figure 4 presents ratio maps of samples with and without applying a postlaser damage removal consisting of a 4 minutes KOH etching step as displayed in figure 3. The KOH treatment resulted in the removal of a 1.5μm thick silicon layer in low defect density regions. Although we observed that the etching rate was much lower in severely damaged areas indicating that the silicon original crystalline structure near the surface was somehow changed due to laser induced amorphization [10].
Fig. 3. 3D confocal microscopy images displaying the surface topography of a 50% overlap irradiated area with a green laser before (left) and after (right) 4 min. KOH etching treatment.
a)
b) Fig. 4. Lifetime ratio maps of samples processed with (left image) and without (right image) a cleaning step for green (a) and UV (b) laser sources. Square shaped openings applying parameters referenced in Table 1, as follows: each fluence is tested with all overlaps (increasing from right to left) from the lowest (up) to the highest (down) one.
Each lifetime mapping shows soft laser conditions (low fluence and overlap) in the upper right whereas strong impact ones are in the lower left. Notice that some non-irradiated areas display ratios larger than unity because of a better passivation after SiN x re-deposition and that inhomogeneity in lifetime distribution within the wafer must be accounted for the results analysis. Still some trends may be
673
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Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
extracted from this analysis, especially one can distinguish the strong impact of high overlap and fluence (50% and 1.79 J/cm²) lowering the lifetime by a factor of 2 and related to the generation of recombination itions show less detrimental surface damages but still remain insufficient to ensure a good surface passivation. In contrary, areas with ratio close to unity show evidence of a partial or complete damage removal if a post-laser KOH etching is achieved. Nevertheless incomplete recovered lifetime at 1.79 J/cm² may indicate that defects have been introduced more than 1.5μm deep in the bulk material (figure 3) and could also explain why no clear trend could be detected in the same experiment with a UV laser source. In conclusion, this additional etching step is therefore necessary for laser damage removal and allows using a large range of SiO 2/SiNx laser ablation fluence running from 90 mJ/cm² to 400 mJ/cm² associated with laser pulse overlaps from -50% to 50%. One must even consider enlarging this range by carrying out longer etching time on the samples. However there is a risk that Kirkendall voids [13] would be formed at the back contact location, hence directly impacting the fill factor by the absence of a thick enough BSF as described in [12].
a)
c)
b)
Fig. 5. a) Internal Quantum Efficiency map extracted at 976nm from LBIC measurements at fluences from top to bottom: 0.09 J/cm², 0.12 J/cm², 0.24 J/cm², 0.40 J/cm², 1.79 J/cm²; From right to left, overlaps at -50 %, 0 %, 12.5 %, 37.5 % and 50% are represented. b) Variation of the mean IQE according to the fluence at 12.5% (black squares) and 50% (red circles) overlap. c) Variation of the mean IQE according to the overlap at 0.24 J/cm² (black squares), 0.30 J/cm² (red circles) and 1.79 J/cm² (blue triangles) fluence.
3.2. Contact formation Laser ablation parameters summarized in table I were then tested on screen printed solar cells by high resolution mapping of the LBIC response at 976nm. The mapping shows again soft laser conditions (low fluence and overlap) in the upper right whereas strong impact ones are in the lower left. The Semilab
Lotfi Bounaas et al. / Energy Procedia 38 (2013) 670 – 676
apparatus measures the short circuit current induced by light beams of a solar cell and the reflectance of the same light on the same cell, in this way allowing the extraction of the IQE (Internal Quantum Efficiency) distribution. This technique was used in order to assess the Al-BSF formation regarding the impact of a green laser ablation process after KOH etching, as represented in figure 5a. A first comment on this figure is that the overall inhomogeneous IQE distribution has small consequences on values measured at irradiated areas. In figure 5b and 5c are represented the variations of the mean IQE measured within each irradiated square according to the overlap and fluence. For instance the square irradiated at 1.79 J/cm² and 50% overlap display the lowest IQE, showing evidence of a strong impact of the silicon substrate damage on the current collection which might be related to a deficient BSF formation at this location. In comparison, we can assume that using 12.5% overlap and 300 mJ/cm² fluence gives rise to a better BSF formation since damages have been greatly reduced. Except at 1790 mJ/cm² fluence,,the -50% pulse overlap condition shows the lowest IQE which is related to a thin BSF formation. Indeed it has been discussed by Müller et al. [16] that remains of the dielectric layer will prevent Si to dissolve in the Al melt between single pulses. Therefore the interface area between Si and Al is small with a high Al mass over ablated surface area ratio, leading to the formation of thin BSF layers and stronger contact recombination. In addition the optimal fluence is shifted to lower values when increasing the overlap, which is consistent with results found in section 3.1. Even though trends have been extrapolated between 400 mJ/cm² and 1790 mJ/cm², a range of optimal laser ablation parameters can be found at each overlap. However these results displaying the correlation between laser damage and IQE are not in accordance with previous work of Ramanathan et al. [17] which determined that the effect of laser surface damage is not critical to the cell performance since it is consumed by the BSF formation. No details are given about the laser fluence range tested in this work but we can assume that the surface damage was somehow limited and had no major impact on the contact recombination. In contrary, with similar experimental conditions as our work it was found to be more dependent on the surface damage in [12]. Based on SEM observations of local contacts in a processed PERC solar cell, it was argued that surface damage caused by a ns laser source dielectric ablation gives rise to the formation of a thin or no BSF at all unless after post-laser KOH etching a uniform BSF was formed. Finally, this characterization routine gives hints for the opening of local contact and subsequent formation of Al-BSF. However, other criteria independent from the laser process must be taken into account to understand the formation a local Al-BSF. For instance, Urrejola et al. studied the influence of the firing temperature and the rear geometry pattern for rear passivated solar cells [13] [14], showing that the formation of a high quality local BSF is extremely dependent on the Si diffusion in Al and on the overlap of Al on each side of the dielectric opening. Meemongkolkiat et al. [15] justified the use of modified screen-printing Al paste because of the direct influence of Si content on the BSF formation during alloying of the Al-Si eutectic. 4.
Conclusion
This paper presents an experimental procedure to characterize laser processing regarding the influence of parameters such as fluence and overlap. It was demonstrated the necessity to use a KOH based chemical treatment to remove silicon surface damages. Lifetime based characterization was also correlated to LBIC measurements in order to compare the influence of laser parameters on the contact formation. As a following experimental part, these findings must be completed by SEM observations in order to correlate laser ablation process with local contact and BSF formation.
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