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Impact of pre-fabrication treatments on n-type UMG wafers for 21% efficient silicon heterojunction solar cells
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Impact of pre-fabrication treatments on n-type UMG wafers for 21% efficient silicon heterojunction solar cells Rabin Basnet a, *, William Weigand b, Zhengshan J. Yu b, Chang Sun a, Sieu P. Phang a, Hang C. Sio a, Fiacre E. Rougieux c, Zachary C. Holman b, Daniel Macdonald a a b c
Research School of Electrical, Energy and Material Engineering, The Australian National University, Canberra, ACT, 2601, Australia School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ8528, USA School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, Sydney, New South Wales, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Tabula rasa Hydrogenation Phosphorus diffusion gettering Silicon heterojunction solar cells Solar-grade silicon Czochralski silicon
Silicon heterojunction solar cells achieve high conversion efficiency due to the excellent surface passivation provided by the hydrogenated intrinsic amorphous silicon films. However, they require a high-quality wafer as a starting material because their low-temperature processing does not allow for gettering. Czochralski-grown upgraded metallurgical-grade (UMG-Cz) silicon is a low-cost alternative to electronic-grade silicon for silicon solar cells, but is often limited in lifetime by grown-in defects. We have previously shown that pre-fabrication treatments, namely tabula rasa, phosphorus diffusion gettering, and hydrogenation, can significantly improve the bulk quality of UMG-Cz wafers. These help to mitigate the impact of grown-in oxygen precipitate nuclei and metallic impurities. In this work, we fabricate rear-junction silicon heterojunction solar cells on both as-grown and pre-treated UMG-Cz and electronic-grade wafers. We show that pre-fabrication treatments have a marked impact on solar cell efficiencies. With pre-fabrication treatment, the efficiency improves from 18.0% to 21.2% for the UMG-Cz cells and 21.2%–22.7% for the electronic-grade cells. Comparison of the open-circuit voltages of the as-grown and pre-treated UMG-Cz and electronic-grade cells using Quokka simulations reveals that the bulk lifetime remains the primary limiting factor for the UMG-Cz wafers.
1. Introduction Silicon heterojunction (SHJ) solar cell technologies are capable of achieving very high efficiencies [1–3], and they currently hold the ef ficiency world record for single-junction silicon solar cells [1]. SHJ solar cells benefit from the excellent surface passivation provided by hydro genated intrinsic amorphous silicon (a-Si:H) films on crystalline silicon [4,5]. SHJ technology avoids the use of high-temperature steps to form junctions and provide electrical contact to the device, as the fabrication steps are all performed at temperatures below 250 � C. This limits the possibilities to improve the bulk quality of wafers during SHJ cell fabrication through high-temperature processes such as impurity get tering. Therefore, SHJ solar cells have been fabricated primarily with high-quality n-type Czochralski (Cz) grown or Float Zone (FZ) wafers using electronic-grade (EG) silicon feedstocks, with millisecond carrier lifetimes in the as-cut state. Upgraded metallurgical-grade (UMG) silicon is a promising solargrade feedstock material for silicon solar cells [6–8]. However, UMG
wafers tend to have relatively high concentrations of metallic impurities (e.g., Fe, Cu, and Cr) and non-metallic impurities (e.g., C and O) [7, 9–11] compared to EG-Cz wafers purified via the Siemens process [12]. Hence UMG wafers are susceptible to high-temperature degradation, usually during oxidation or boron diffusion steps, because of the acti vation of defects related to oxygen precipitates and metallic impurities. In this respect, UMG silicon may be well suited to SHJ technology because of its low thermal budget. However, we have previously found that the as-grown lifetime of UMG-Cz wafers is also limited by the presence of the grown-in oxygen precipitate nuclei as well as metallic impurities [13]. As a result, the performance of SHJ solar cells fabricated on as-grown UMG-Cz wafers will be limited by their bulk quality. Nevertheless, as we have previously demonstrated, the bulk quality of UMG-Cz wafers can be significantly improved through pre-fabrication treatments such as tabula rasa (TR), phosphorus diffusion gettering (PDG), and hydrogenation (H), either in isolation or in combination [13]. Tabula rasa, a high-temperature pre-fabrication annealing step, can be used to dissolve grown-in oxygen precipitate nuclei [14–16].
* Corresponding author. Research School of Electrical, Energy and Materials Engineering, The Australian National University, Canberra, ACT, 2601, Australia. E-mail address: [email protected] (R. Basnet). https://doi.org/10.1016/j.solmat.2019.110287 Received 6 June 2019; Received in revised form 29 October 2019; Accepted 8 November 2019 0927-0248/© 2019 Published by Elsevier B.V.
Please cite this article as: Rabin Basnet, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2019.110287
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Phosphorus diffusion gettering is a powerful method to remove mobile metallic impurities [17,18] and hydrogenation is an efficient process to passivate residual oxygen-related defects in silicon [19,20]. Further, Hallam et al. [21] have shown that pre-fabrication gettering and hy drogenation treatments can be beneficial for low-lifetime p-type EG-Cz silicon wafers, enabling SHJ solar cells with open-circuit voltages (Voc) greater than 700 mV. Importantly, successful hydrogenation as a pre-fabrication treatment is possible due to the low thermal budget of the SHJ fabrication steps; otherwise, subsequent high-temperature processing steps could out-diffuse hydrogen from the silicon bulk [22]. In this work, we demonstrate the benefits of these pre-fabrication treatments in increasing the performance of SHJ solar cells fabricated on n-type UMG-Cz and EG-Cz wafers.
with tetramethylammonium hydroxide (TMAH) solution to remove 10–12 μm from each side. Wafers were cleaned using standard Radio Corporation of America (RCA) cleaning steps prior to each hightemperature step. The TR step in this work was performed in an oxy gen ambient at 1000 � C for 30 min with loading and unloading at 700 � C and ramp up and down rates of 15 � C/min. Some of the TR and as-grown samples then underwent a PDG step after removing the oxide layer with a hydrofluoric acid (HF) dip. The PDG step was performed for 30 min at 785 � C with phosphorus oxychloride (POCl3) as the dopant source, resulting in a sheet resistance of 90–100 Ω/□. The diffused layers were then removed by a short HF dip and TMAH etch. For hydrogenation, some of the PDG samples were passivated with silicon nitride (SiNx:H) layers using plasma-enhanced chemical vapor deposition (PECVD) (Roth and Rau AK400). Hydrogenation of the SiNx:H coated samples was performed using a rapid thermal annealing system (Unitemp UTP-1100) under a nitrogen ambient at a sample temperature of 500 � C for 10 s with a ramp rate of 25 � C/s. Hydrogenated samples were stripped of their SiNx:H layers by dipping in an HF solution and briefly etched in TMAH to remove a few micrometers of silicon.
2. Materials and methods 2.1. Materials The cells in this work were fabricated on two different types of n-type silicon wafers. The first were UMG-Cz wafers grown by Apollon Solar (ingot number ISO10) in the framework of the PHOTOSIL project, based on UMG feedstock produced by FerroPem [6]. The length of the UMG-Cz ingot was 573 mm after cropping, and the weight was 67 kg. The second types of wafers used were commercially available EG-Cz wafers, selected as controls. The as-grown effective lifetime of the EG-Cz wafer was 1240 μs (discussed in detail in section 3.1). The dopant densities of both phosphorus and boron within the compensated UMG-Cz wafers were measured by secondary ion mass spectrometry (SIMS). The interstitial oxygen concentrations [Oi] were measured by Fourier transform infrared spectroscopy (FTIR), using a Bruker Vertex 80 tool and cali brated using SEMI MF standard 1188–1107. The resistivities, dopant concentrations, and [Oi] are summarized in Table 1.
2.3. Silicon heterojunction solar cell fabrication and characterization For the subsequent SHJ solar cell fabrication process, the UMG-Cz and EG-Cz wafers were random-pyramid textured in a potassium hy droxide (KOH) and surfactant additive (GP Solar, ALKA-Tex) solution. The textured wafers were chemically cleaned in solutions of piranha etch (H2O2/H2SO4) and H2O2/HCl followed by a buffered oxide etch (BOE) until the surfaces were hydrophobic. Then a-Si:H layers were deposited on the cleaned wafers using an Applied Materials P-5000 multi-chamber PECVD tool. A hole-contact was realized on the rear side (rear-junction device) by depositing 8 nm of intrinsic a-Si:H(i) using a mixture of silane and hydrogen followed by 11 nm of boron-doped a:Si: H(p) using trimethylboron as a doping gas. Similarly, an electron contact was formed on the front side by depositing, 8 nm of a-Si:H(i) and 4 nm of phosphorus-doped a-Si:H(n) using phosphine as a doping gas. Note that to improve the passivation of the a-Si:H(i)/c-Si interface, an in-situ, 15-slong hydrogen plasma treatment was performed immediately after the aSi:H(i) layer deposition on both sides [23–25]. Carrier lifetimes were measured after the a-Si:H stacks were deposited on both sides of the wafers. Subsequently, 75 nm of indium tin oxide (ITO) with a sheet resistance of 70 Ω/□ was deposited using DC magnetron sputtering on the front side. Similarly, 150 nm of ITO and 200 nm of silver were sputtered on the rear side. Note that four cells, each 2 cm � 2 cm in size, were defined within each wafer using a shadow mask during the ITO and silver sputtering processes. For current collection, grids were screen printed on the front surface using a low-temperature silver paste and cured at 200 � C for 20 min on a hotplate. A schematic of the SHJ solar cell used in this work is presented in Fig. 1. A rear-junction cell structure was chosen to minimize losses in the short-circuit current density (Jsc) due to parasitic absorption in the a-Si:H(p) layer, which must be thicker than the a-Si:H(n) layer to achieve a high fill factor (FF) [3,26,27]. Carrier lifetimes were measured using the quasi-steady state photo conductance (QSSPC) and transient photoconductance decay (PCD) techniques with a WCT-120 tool from Sinton Instruments [28]. Cell current density-voltage (J-V) measurements were performed under standard conditions of 1000 W/m2 and 25 � C with an AM1.5G spectrum. J-V and Suns-Voc measurements were performed using an FCT-450 tool from Sinton instruments. External quantum efficiency (EQE) spectra were measured using a PV measurements QEX10 tool. The cell reflec tance (R) was measured using a Lambda1050 spectrophotometer from PerkinElmer.
2.2. Pre-fabrication treatment methods Previously, we demonstrated that the bulk lifetimes of the UMG-Cz wafers are limited by both grown-in oxygen precipitate nuclei and mo bile metallic impurities [13]. As a result, pre-fabrication treatments for these UMG-Cz wafers must be designed to mitigate the effect of both types of defects to obtain the highest bulk lifetimes. This was achieved by using complementary methods of defect engineering: a tabula rasa step prior to a phosphorus diffusion gettering step (TR þ PDG) or a hydrogenation step after phosphorus diffusion gettering (PDG þ H) [13]. In the TR þ PDG treatment, the lifetime limiting grown-in oxygen precipitates are dissolved during the tabula rasa step, and metallic im purities are removed during the phosphorus diffusion gettering process. On the other hand, during the PDG þ H treatment, gettering removes metallic impurities, and hydrogenation subsequently passivates recombination-active oxygen precipitates induced by the high thermal budget of the phosphorus diffusion gettering step. To investigate the impact of these two pre-fabrication treatment combinations on SHJ cells, the wafers were divided into three groups: as-grown (no pre-treatment), TR þ PDG, and PDG þ H. For processing convenience, 4-inch round wafers were laser cut from the 6-inch pseudo-square wafers. All wafers were saw-damage-etched Table 1 Properties of the n-type wafers used for fabrication of SHJ cells. Parameters
UMG-Cz
EG-Cz
Resistivity (Ω⋅cm) [B] (cm 3)
2.4 1.1 � 1016
1.0 –
1.0 � 1015
5.0 � 1015
[P] (cm
3
)
Net doping (cm [Oi] (cm 3)
3
Thickness, t (μm)
)
1.2 � 1016
5.0 � 1015
6.5 � 1017
5.7 � 1017
150
250
2
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fabrication, the pre-fabrication treatments could be ineffective or even detrimental in some cases. Fig. 2(b) illustrates the 1-sun implied open-circuit voltage (i-Voc) and the implied fill factor (i-FF) of the wafers. These were derived from the injection-dependent carrier lifetime measurements shown in Fig. 2(a), for which the excess carrier densities corresponding to open-circuit and maximum power conditions are shown. The as-grown UMG-Cz and EGCz wafers have i-Voc values of 672 mV and 715 mV. Note that the UMGCz and EG-Cz wafers have different net doping and thicknesses, as shown in Table 1. These differences would affect the direct comparison of i-Voc between the UMG-Cz and EG-Cz wafers. This will be considered in detail in section 3.3. The PDG step, which is present in both prefabrication treatment sequences, is critical to significantly increase the lifetime in the UMG-Cz wafers. The PDG step leads to a 40 mV (UMG-Cz) or 11 mV (EG-Cz) i-Voc gain when combined with a tabula rasa step, and a similar 35 mV (UMG-Cz) or 11 mV (EG-Cz) gain when performed prior to hydrogenation. On its own, however, the phosphorus diffusion get tering step was found to result in oxygen-related ring defects which significantly limit the bulk lifetimes in these UMG-Cz wafers [13]. The i-FF values increased slightly for all wafers after the prefabrication treatments. These small increases are largely due to the increased lifetimes shifting the open-circuit and maximum power points to higher excess carrier densities, as shown in Fig. 2(a). At higher excess densities, the lifetime at open-circuit is more strongly affected by Auger recombination and recombination at the doped surfaces. This, in turn, leads to a larger relative lifetime at maximum power compared to opencircuit conditions, causing an increase in i-FF.
Fig. 1. Schematic diagram of the rear-junction SHJ solar cells fabricated in this work.
3. Results and discussion 3.1. Effective lifetimes, implied open-circuit voltages, and implied fill factors Fig. 2(a) presents injection-dependent effective minority-carrier lifetimes of the UMG-Cz and EG-Cz wafers passivated with a-Si:H stacks (before ITO deposition). The carrier mobilities of the compen sated UMG-Cz wafers, which were required to calculate lifetimes from photoconductance measurements, were determined using Schindler’s model [29]. As seen in Fig. 2(a), pre-fabrication treatments improved the lifetime of both the UMG-Cz and the EG-Cz wafers. At an injection level of Δn ¼ 1 � 1015 cm 3 , the lifetime of the as-grown UMG-Cz wafer was 290 μs and increased more than three-fold to 970 μs and 1130 μs after the PDG þ H and TR þ PDG treatments, respectively. Similarly, the lifetime of the as-grown EG-Cz wafer was 1240 μs and increased to 2635 μs and 2365 μs after PDG þ H and TR þ PDG treatments, respec tively. Note that benefit of pre-fabrication treatments in the EG-Cz wa fers is conditional. For the superior quality EG-Cz wafers used in the SHJ
3.2. Measured solar cell results Fig. 3 illustrates the impact of pre-fabrication treatments on the light J-V curves of champion SHJ solar cells fabricated on both UMG-Cz and EG-Cz wafers. The Voc for the as-grown UMG-Cz cell was approximately 678 mV, which is too low to achieve efficiencies above 20% for typical ntype SHJ solar cells. The UMG-Cz cells achieved best Voc values of 717 mV and 715 mV after TR þ PDG and PDG þ H, respectively, consistent
Fig. 2. (a) Injection-dependent lifetime curves illustrating the impact of pre-fabrication treatments on n-type UMG-Cz and EG-Cz wafers passivated by a-Si:H stacks. The injection levels corresponding to 1-sun open-circuit (OC) and maximum power point (MPP) conditions are marked by solid squares and solid circles, respectively. Also shown (red line) is the intrinsic lifetime parameterization by Richter et al. [30] (b) The 1-sun implied i-Voc and i-FF extracted from the effective minority-carrier lifetime measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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silver depositions [31,32]. However, a high Voc than i-Voc shown in Fig. 2 (b) was observed in the UMG-Cz cells. Note that the i-Voc values in Fig. 2 (b) averages across the wafers and the Voc in Fig. 3 are only for the best cells (2 � 2 cm2). The measured J-V parameters of all the SHJ solar cells, including the champion cells from Fig. 3, are presented in Fig. 4. The Jsc of the SHJ cells is limited by parasitic absorption in both the ITO and front a-Si:H layers [26,33]. The as-grown UMG-Cz cell had the lowest Jsc due to relatively lower minority-carrier diffusion length (LD), as shown in Table 2. Overall, the EG-Cz cells achieved higher Jsc than the UMG-Cz cells due to thicker wafers, as LD/t ratios for the cells were greater than 4.5, as shown in Table 2. Therefore, the UMG-Cz cells suffered from poor carrier collection due to bulk defects. Further, The estimated LD values presented in Table 2 were calculated using the measured minority-carrier lifetimes and simulated minority-carrier mobilities from Schindler’s model for compensated silicon for the UMG-Cz cells [29], and Klaassen’s model for the EG-Cz cells [34]. We note that the slightly lower than expected Jsc for the UMG-Cz (TR þ PDG) cells in Fig. 4 could be due to processing variations, as they were fabricated in a different batch from the other cells. Fig. 5 shows the measured EQE and total absorption (1-R) curves for the champion cells. The as-grown UMG-Cz cell exhibits no significant differences in reflection compared to the PDG þ H UMG-Cz cell. However, the EQE of the as-grown UMG-Cz
Table 2 Carrier mobilities, bulk lifetimes, minority-carrier diffusion lengths, and thick nesses for the n-type wafers.
Fig. 3. In-house-measured J-V curves of the champion SHJ cells fabricated on as-grown, TR þ PDG, and PDG þ H n-type UMG-Cz and EG-Cz wafers. The J-V parameters of the champion cells are also shown.
Parameters
with the i-Voc values shown in Fig. 2(b). Thus, the TR þ PDG and PDG þ H pre-fabrication treatments result in very similar improvement in the bulk quality of the UMG-Cz wafers. Similarly, for the as-grown EG-Cz solar cells, the best Voc value was 708 mV, and this increased to 720 mV and 721 mV after TR þ PDG and PDG þ H, respectively, as shown in Fig. 3. This indicates that the EG-Cz wafers used in this work benefited from pre-fabrication treatment steps. As expected, we observed some losses in Voc (e10 mV) in EG-Cz cells in comparison to the i-Voc values shown in Fig. 2(b). This is due to sputter damage during the ITO and
Hole mobility, μh (cm2V 1s 1) Electron mobility, μe (cm2V 1s 1) Measured bulk lifetimes, τbulk (μs) Calculated diffusion length, LD (μm) Thickness, t (μm) LD/t
UMG-Cz
EG-Cz
Asgrown
PDG þ H
Asgrown
PDG þ H
377 971
377 971
431 1242
431 1242
290 540
980 989
1240 1184
2635 1862
150 3.6
150 6.5
250 4.7
250 7.4
Fig. 4. In-house-measured illuminated J-V parameters of SHJ cells fabricated on as-grown, TR þ PDG, and PDG þ H n-type UMG-Cz and EG-Cz wafers. The box plots represent the average, 25th percentile, and 75th percentile, excluding outliers. Each cell was 2 cm � 2 cm in size. 4
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property. We have not included simulated Jsc, FF, and efficiency here, as they also depend on optical and electrical factors that are not directly related to the bulk quality of the wafers. Fig. 6 shows the simulated Voc for the as-grown and PDG þ H SHJ solar cells for both the UMG-Cz and EG-Cz wafers. The simulated Voc values are within 2–3 mV of the measured values in each case. We now consider the effects of changing the parameters of the EG-Cz cells to those of the UMG-Cz cells. After reducing t to 150 μm, the Voc values increased for both sets of cells (Voc(as-grown) ¼ 716 mV and Voc (PDGþH) ¼ 723 mV), as expected [40]. Subsequently, lowering n0 to 1 � 1015 cm 3, Voc increased further by 1 mV (Voc(as-grown) ¼ 717 mV and Voc (PDGþH) ¼ 724 mV). This occurs because SHJ cells reach high injection at open-circuit, as shown in Fig. 2(a). As a result, Auger and surface recombination are significant, and the excess carrier density (Δn) tends to either remain constant or increase as n0 decreases, in contrast to the expected behavior at lower injection levels. Conversely, the voltage at the maximum power point (VMPP) decreased with n0 (not shown) because the maximum power point occurs at a lower injection level, as shown in Fig. 2(a), at which Auger recombination is less important. Finally, μe and μh were reduced, as shown in Table 2. At this stage, the simulated Voc is greater than the measured Voc for the as-grown and PDG þ H UMG-Cz cells by 35 mV and 8 mV, respectively. The bulk lifetimes were then adjusted to the measured values of the UMG-Cz cells from Table 2. The simulated Voc values are then within 1–3 mV of the measured values of the UMG-Cz cells, as shown in Fig. 6. These results confirm that the measured Voc differences between the UMG-Cz and EG-Cz cells are not dominated by variations in t, n0, μe, and μh, but rather by the differences in their bulk lifetimes, and that improving bulk quality either by TR þ PDG or PDG þ H is critical to achieve higher efficiencies.
Fig. 5. Measured EQE and 1-R curves for the UMG-Cz and EG-Cz cells fabri cated on as-grown and PDG þ H wafers.
cell is significantly lower between 400 nm and 1000 nm in comparison to the PDG þ H UMG-Cz cell, reflecting the impact of the lower minority-carrier diffusion length in these rear-junction cells. The highest in-house-measured efficiencies on the as-grown UMG-Cz and EG-Cz wafers were 18.0% and 21.2%, respectively. The best effi ciencies achieved for the UMG-Cz and EG-Cz cells after TR þ PDG were 21.0% and 22.4%, respectively, and these further increased to 21.2% and 22.7% after PDG þ H. These results clearly demonstrate the benefits of pre-fabrication treatments on both the UMG-Cz and EG-Cz silicon wafers used in this work for SHJ cells. We did not observe significant differences in the SHJ cell results due to the two different combinations of pre-fabrication treatments (TR þ PDG or PDG þ H). However, selecting one or the other for integration into a SHJ fabrication sequence requires careful consideration of their cost and complexity, which we do not address here. The long-term stability of the cells after treatment is also an impor tant consideration. The compensated nature of these n-type UMG-Cz wafers can result in strong degradation under illumination, due to the presence of boron–oxygen-related defects [35–37]. However, Sun et al. [38] have recently demonstrated complete regeneration of boron-oxygen-related defects in SHJ solar cells fabricated on these UMG-Cz wafers by annealing under illumination. Therefore, this disadvantage can be overcome by appropriate post-treatment of the SHJ devices. All UMG-Cz based solar cell J-V characteristics in this work are reported in the non-degraded state prior to the activation of boron-oxygen-related defects.
4. Conclusion This work has demonstrated that as-grown UMG-Cz wafers are not suitable to achieve high-efficiency solar cells with conventional SHJ
3.3. Simulated Voc To confirm the impact of the pre-fabrication treatments observed experimentally in section 3.2, we performed Quokka [39] simulations in which we sequentially adjusted the wafer properties such as thickness (t), net doping (n0), mobility (μe and μh), and bulk lifetime (τbulk) from the values of the EG-Cz cells to those of the UMG-Cz cells, for both the as-grown and PDG þ H conditions. This allows for a direct comparison between the UMG-Cz and EG-Cz control cells. The thickness and net doping values in Table 1, and mobilities and bulk lifetime values in Table 2, were used in the simulations. The recombination parameter J0 of the textured front and rear surfaces were extracted from lifetime measurements from the baseline SHJ process and J0 (front) and J0(rear) were 8 fA/cm2 and 14 fA/cm2, respectively. For each simulation, we extracted the cell Voc to identify the influence of each wafer bulk
Fig. 6. Measured and simulated Voc of as-grown and PDG þ H solar cells fabricated on the UMG-Cz and EG-Cz wafers. The wafer properties-t, n0, μh, μe, and τbulk were sequentially adjusted from the values of the EG-Cz wafers to those of the UMG-Cz wafers. 5
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solar cells processing. However, utilizing pre-fabrication treatments, namely tabula rasa together with phosphorus diffusion gettering, or phosphorus diffusion gettering with hydrogenation, the bulk lifetimes of the UMG-Cz wafers can be increased significantly, promoting higher efficiencies. In addition, EG-Cz silicon wafers used in this work also benefited from pre-fabrication treatments, although to a lesser degree. After these pre-fabrication treatments, we measured absolute gains in efficiency of around 3.0% and 1.2%, for the UMG-Cz and EG-Cz SHJ solar cells, respectively, resulting in a champion cell of 21.2% for the UMG-Cz wafers. More broadly, the results highlight the potential of prefabrication treatments to enable SHJ solar cell technology to be applied to lower quality as-grown wafers. This could be accomplished, for example, through a hybrid solar cell structure utilizing both homojunction and heterojunction features to naturally incorporate such treatments during the solar cell fabrication process [41,42].
[7]
[8] [9]
[10] [11] [12]
Declaration of competing interest
[13]
� We wish to confirm that there are no known conflicts of interest associated with this publication. � All of the sources of funding for the work are acknowledged in the manuscript. � We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. � All listed authors have contributed to complete this paper.
[14] [15] [16]
[17] [18]
Acknowledgments This work was supported by the Australian Renewable Energy Agency (ARENA) through the Australian Centre for Advanced Photo voltaics (ACAP) projects RND009 and RND017. Support was also pro vided by the Engineering Research Center Program of the National Science Foundation and the Office of Energy Efficiency and Renewable Energy of the Department of Energy under NSF Cooperative Agreement No. EEC-1041895, USA. Apollon Solar is acknowledged for providing the UMG wafers for this work. Also, Dr. Di Yan is acknowledged for constructive discussions.
[19] [20] [21]
[22]
Appendix A. Supplementary data
[23]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110287.
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Impact of pre-fabrication treatments on n-type UMG wafers for 21% efficient silicon heterojunction solar cells Rabin Basnet a, *, William Weigand b, Zhengshan J. Yu b, Chang Sun a, Sieu P. Phang a, Hang C. Sio a, Fiacre E. Rougieux c, Zachary C. Holman b, Daniel Macdonald a a b c
Research School of Electrical, Energy and Material Engineering, The Australian National University, Canberra, ACT, 2601, Australia School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ8528, USA School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, Sydney, New South Wales, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Tabula rasa Hydrogenation Phosphorus diffusion gettering Silicon heterojunction solar cells Solar-grade silicon Czochralski silicon
Silicon heterojunction solar cells achieve high conversion efficiency due to the excellent surface passivation provided by the hydrogenated intrinsic amorphous silicon films. However, they require a high-quality wafer as a starting material because their low-temperature processing does not allow for gettering. Czochralski-grown upgraded metallurgical-grade (UMG-Cz) silicon is a low-cost alternative to electronic-grade silicon for silicon solar cells, but is often limited in lifetime by grown-in defects. We have previously shown that pre-fabrication treatments, namely tabula rasa, phosphorus diffusion gettering, and hydrogenation, can significantly improve the bulk quality of UMG-Cz wafers. These help to mitigate the impact of grown-in oxygen precipitate nuclei and metallic impurities. In this work, we fabricate rear-junction silicon heterojunction solar cells on both as-grown and pre-treated UMG-Cz and electronic-grade wafers. We show that pre-fabrication treatments have a marked impact on solar cell efficiencies. With pre-fabrication treatment, the efficiency improves from 18.0% to 21.2% for the UMG-Cz cells and 21.2%–22.7% for the electronic-grade cells. Comparison of the open-circuit voltages of the as-grown and pre-treated UMG-Cz and electronic-grade cells using Quokka simulations reveals that the bulk lifetime remains the primary limiting factor for the UMG-Cz wafers.
1. Introduction Silicon heterojunction (SHJ) solar cell technologies are capable of achieving very high efficiencies [1–3], and they currently hold the ef ficiency world record for single-junction silicon solar cells [1]. SHJ solar cells benefit from the excellent surface passivation provided by hydro genated intrinsic amorphous silicon (a-Si:H) films on crystalline silicon [4,5]. SHJ technology avoids the use of high-temperature steps to form junctions and provide electrical contact to the device, as the fabrication steps are all performed at temperatures below 250 � C. This limits the possibilities to improve the bulk quality of wafers during SHJ cell fabrication through high-temperature processes such as impurity get tering. Therefore, SHJ solar cells have been fabricated primarily with high-quality n-type Czochralski (Cz) grown or Float Zone (FZ) wafers using electronic-grade (EG) silicon feedstocks, with millisecond carrier lifetimes in the as-cut state. Upgraded metallurgical-grade (UMG) silicon is a promising solargrade feedstock material for silicon solar cells [6–8]. However, UMG
wafers tend to have relatively high concentrations of metallic impurities (e.g., Fe, Cu, and Cr) and non-metallic impurities (e.g., C and O) [7, 9–11] compared to EG-Cz wafers purified via the Siemens process [12]. Hence UMG wafers are susceptible to high-temperature degradation, usually during oxidation or boron diffusion steps, because of the acti vation of defects related to oxygen precipitates and metallic impurities. In this respect, UMG silicon may be well suited to SHJ technology because of its low thermal budget. However, we have previously found that the as-grown lifetime of UMG-Cz wafers is also limited by the presence of the grown-in oxygen precipitate nuclei as well as metallic impurities [13]. As a result, the performance of SHJ solar cells fabricated on as-grown UMG-Cz wafers will be limited by their bulk quality. Nevertheless, as we have previously demonstrated, the bulk quality of UMG-Cz wafers can be significantly improved through pre-fabrication treatments such as tabula rasa (TR), phosphorus diffusion gettering (PDG), and hydrogenation (H), either in isolation or in combination [13]. Tabula rasa, a high-temperature pre-fabrication annealing step, can be used to dissolve grown-in oxygen precipitate nuclei [14–16].
* Corresponding author. Research School of Electrical, Energy and Materials Engineering, The Australian National University, Canberra, ACT, 2601, Australia. E-mail address: [email protected] (R. Basnet). https://doi.org/10.1016/j.solmat.2019.110287 Received 6 June 2019; Received in revised form 29 October 2019; Accepted 8 November 2019 0927-0248/© 2019 Published by Elsevier B.V.
Please cite this article as: Rabin Basnet, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2019.110287
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Phosphorus diffusion gettering is a powerful method to remove mobile metallic impurities [17,18] and hydrogenation is an efficient process to passivate residual oxygen-related defects in silicon [19,20]. Further, Hallam et al. [21] have shown that pre-fabrication gettering and hy drogenation treatments can be beneficial for low-lifetime p-type EG-Cz silicon wafers, enabling SHJ solar cells with open-circuit voltages (Voc) greater than 700 mV. Importantly, successful hydrogenation as a pre-fabrication treatment is possible due to the low thermal budget of the SHJ fabrication steps; otherwise, subsequent high-temperature processing steps could out-diffuse hydrogen from the silicon bulk [22]. In this work, we demonstrate the benefits of these pre-fabrication treatments in increasing the performance of SHJ solar cells fabricated on n-type UMG-Cz and EG-Cz wafers.
with tetramethylammonium hydroxide (TMAH) solution to remove 10–12 μm from each side. Wafers were cleaned using standard Radio Corporation of America (RCA) cleaning steps prior to each hightemperature step. The TR step in this work was performed in an oxy gen ambient at 1000 � C for 30 min with loading and unloading at 700 � C and ramp up and down rates of 15 � C/min. Some of the TR and as-grown samples then underwent a PDG step after removing the oxide layer with a hydrofluoric acid (HF) dip. The PDG step was performed for 30 min at 785 � C with phosphorus oxychloride (POCl3) as the dopant source, resulting in a sheet resistance of 90–100 Ω/□. The diffused layers were then removed by a short HF dip and TMAH etch. For hydrogenation, some of the PDG samples were passivated with silicon nitride (SiNx:H) layers using plasma-enhanced chemical vapor deposition (PECVD) (Roth and Rau AK400). Hydrogenation of the SiNx:H coated samples was performed using a rapid thermal annealing system (Unitemp UTP-1100) under a nitrogen ambient at a sample temperature of 500 � C for 10 s with a ramp rate of 25 � C/s. Hydrogenated samples were stripped of their SiNx:H layers by dipping in an HF solution and briefly etched in TMAH to remove a few micrometers of silicon.
2. Materials and methods 2.1. Materials The cells in this work were fabricated on two different types of n-type silicon wafers. The first were UMG-Cz wafers grown by Apollon Solar (ingot number ISO10) in the framework of the PHOTOSIL project, based on UMG feedstock produced by FerroPem [6]. The length of the UMG-Cz ingot was 573 mm after cropping, and the weight was 67 kg. The second types of wafers used were commercially available EG-Cz wafers, selected as controls. The as-grown effective lifetime of the EG-Cz wafer was 1240 μs (discussed in detail in section 3.1). The dopant densities of both phosphorus and boron within the compensated UMG-Cz wafers were measured by secondary ion mass spectrometry (SIMS). The interstitial oxygen concentrations [Oi] were measured by Fourier transform infrared spectroscopy (FTIR), using a Bruker Vertex 80 tool and cali brated using SEMI MF standard 1188–1107. The resistivities, dopant concentrations, and [Oi] are summarized in Table 1.
2.3. Silicon heterojunction solar cell fabrication and characterization For the subsequent SHJ solar cell fabrication process, the UMG-Cz and EG-Cz wafers were random-pyramid textured in a potassium hy droxide (KOH) and surfactant additive (GP Solar, ALKA-Tex) solution. The textured wafers were chemically cleaned in solutions of piranha etch (H2O2/H2SO4) and H2O2/HCl followed by a buffered oxide etch (BOE) until the surfaces were hydrophobic. Then a-Si:H layers were deposited on the cleaned wafers using an Applied Materials P-5000 multi-chamber PECVD tool. A hole-contact was realized on the rear side (rear-junction device) by depositing 8 nm of intrinsic a-Si:H(i) using a mixture of silane and hydrogen followed by 11 nm of boron-doped a:Si: H(p) using trimethylboron as a doping gas. Similarly, an electron contact was formed on the front side by depositing, 8 nm of a-Si:H(i) and 4 nm of phosphorus-doped a-Si:H(n) using phosphine as a doping gas. Note that to improve the passivation of the a-Si:H(i)/c-Si interface, an in-situ, 15-slong hydrogen plasma treatment was performed immediately after the aSi:H(i) layer deposition on both sides [23–25]. Carrier lifetimes were measured after the a-Si:H stacks were deposited on both sides of the wafers. Subsequently, 75 nm of indium tin oxide (ITO) with a sheet resistance of 70 Ω/□ was deposited using DC magnetron sputtering on the front side. Similarly, 150 nm of ITO and 200 nm of silver were sputtered on the rear side. Note that four cells, each 2 cm � 2 cm in size, were defined within each wafer using a shadow mask during the ITO and silver sputtering processes. For current collection, grids were screen printed on the front surface using a low-temperature silver paste and cured at 200 � C for 20 min on a hotplate. A schematic of the SHJ solar cell used in this work is presented in Fig. 1. A rear-junction cell structure was chosen to minimize losses in the short-circuit current density (Jsc) due to parasitic absorption in the a-Si:H(p) layer, which must be thicker than the a-Si:H(n) layer to achieve a high fill factor (FF) [3,26,27]. Carrier lifetimes were measured using the quasi-steady state photo conductance (QSSPC) and transient photoconductance decay (PCD) techniques with a WCT-120 tool from Sinton Instruments [28]. Cell current density-voltage (J-V) measurements were performed under standard conditions of 1000 W/m2 and 25 � C with an AM1.5G spectrum. J-V and Suns-Voc measurements were performed using an FCT-450 tool from Sinton instruments. External quantum efficiency (EQE) spectra were measured using a PV measurements QEX10 tool. The cell reflec tance (R) was measured using a Lambda1050 spectrophotometer from PerkinElmer.
2.2. Pre-fabrication treatment methods Previously, we demonstrated that the bulk lifetimes of the UMG-Cz wafers are limited by both grown-in oxygen precipitate nuclei and mo bile metallic impurities [13]. As a result, pre-fabrication treatments for these UMG-Cz wafers must be designed to mitigate the effect of both types of defects to obtain the highest bulk lifetimes. This was achieved by using complementary methods of defect engineering: a tabula rasa step prior to a phosphorus diffusion gettering step (TR þ PDG) or a hydrogenation step after phosphorus diffusion gettering (PDG þ H) [13]. In the TR þ PDG treatment, the lifetime limiting grown-in oxygen precipitates are dissolved during the tabula rasa step, and metallic im purities are removed during the phosphorus diffusion gettering process. On the other hand, during the PDG þ H treatment, gettering removes metallic impurities, and hydrogenation subsequently passivates recombination-active oxygen precipitates induced by the high thermal budget of the phosphorus diffusion gettering step. To investigate the impact of these two pre-fabrication treatment combinations on SHJ cells, the wafers were divided into three groups: as-grown (no pre-treatment), TR þ PDG, and PDG þ H. For processing convenience, 4-inch round wafers were laser cut from the 6-inch pseudo-square wafers. All wafers were saw-damage-etched Table 1 Properties of the n-type wafers used for fabrication of SHJ cells. Parameters
UMG-Cz
EG-Cz
Resistivity (Ω⋅cm) [B] (cm 3)
2.4 1.1 � 1016
1.0 –
1.0 � 1015
5.0 � 1015
[P] (cm
3
)
Net doping (cm [Oi] (cm 3)
3
Thickness, t (μm)
)
1.2 � 1016
5.0 � 1015
6.5 � 1017
5.7 � 1017
150
250
2
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fabrication, the pre-fabrication treatments could be ineffective or even detrimental in some cases. Fig. 2(b) illustrates the 1-sun implied open-circuit voltage (i-Voc) and the implied fill factor (i-FF) of the wafers. These were derived from the injection-dependent carrier lifetime measurements shown in Fig. 2(a), for which the excess carrier densities corresponding to open-circuit and maximum power conditions are shown. The as-grown UMG-Cz and EGCz wafers have i-Voc values of 672 mV and 715 mV. Note that the UMGCz and EG-Cz wafers have different net doping and thicknesses, as shown in Table 1. These differences would affect the direct comparison of i-Voc between the UMG-Cz and EG-Cz wafers. This will be considered in detail in section 3.3. The PDG step, which is present in both prefabrication treatment sequences, is critical to significantly increase the lifetime in the UMG-Cz wafers. The PDG step leads to a 40 mV (UMG-Cz) or 11 mV (EG-Cz) i-Voc gain when combined with a tabula rasa step, and a similar 35 mV (UMG-Cz) or 11 mV (EG-Cz) gain when performed prior to hydrogenation. On its own, however, the phosphorus diffusion get tering step was found to result in oxygen-related ring defects which significantly limit the bulk lifetimes in these UMG-Cz wafers [13]. The i-FF values increased slightly for all wafers after the prefabrication treatments. These small increases are largely due to the increased lifetimes shifting the open-circuit and maximum power points to higher excess carrier densities, as shown in Fig. 2(a). At higher excess densities, the lifetime at open-circuit is more strongly affected by Auger recombination and recombination at the doped surfaces. This, in turn, leads to a larger relative lifetime at maximum power compared to opencircuit conditions, causing an increase in i-FF.
Fig. 1. Schematic diagram of the rear-junction SHJ solar cells fabricated in this work.
3. Results and discussion 3.1. Effective lifetimes, implied open-circuit voltages, and implied fill factors Fig. 2(a) presents injection-dependent effective minority-carrier lifetimes of the UMG-Cz and EG-Cz wafers passivated with a-Si:H stacks (before ITO deposition). The carrier mobilities of the compen sated UMG-Cz wafers, which were required to calculate lifetimes from photoconductance measurements, were determined using Schindler’s model [29]. As seen in Fig. 2(a), pre-fabrication treatments improved the lifetime of both the UMG-Cz and the EG-Cz wafers. At an injection level of Δn ¼ 1 � 1015 cm 3 , the lifetime of the as-grown UMG-Cz wafer was 290 μs and increased more than three-fold to 970 μs and 1130 μs after the PDG þ H and TR þ PDG treatments, respectively. Similarly, the lifetime of the as-grown EG-Cz wafer was 1240 μs and increased to 2635 μs and 2365 μs after PDG þ H and TR þ PDG treatments, respec tively. Note that benefit of pre-fabrication treatments in the EG-Cz wa fers is conditional. For the superior quality EG-Cz wafers used in the SHJ
3.2. Measured solar cell results Fig. 3 illustrates the impact of pre-fabrication treatments on the light J-V curves of champion SHJ solar cells fabricated on both UMG-Cz and EG-Cz wafers. The Voc for the as-grown UMG-Cz cell was approximately 678 mV, which is too low to achieve efficiencies above 20% for typical ntype SHJ solar cells. The UMG-Cz cells achieved best Voc values of 717 mV and 715 mV after TR þ PDG and PDG þ H, respectively, consistent
Fig. 2. (a) Injection-dependent lifetime curves illustrating the impact of pre-fabrication treatments on n-type UMG-Cz and EG-Cz wafers passivated by a-Si:H stacks. The injection levels corresponding to 1-sun open-circuit (OC) and maximum power point (MPP) conditions are marked by solid squares and solid circles, respectively. Also shown (red line) is the intrinsic lifetime parameterization by Richter et al. [30] (b) The 1-sun implied i-Voc and i-FF extracted from the effective minority-carrier lifetime measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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silver depositions [31,32]. However, a high Voc than i-Voc shown in Fig. 2 (b) was observed in the UMG-Cz cells. Note that the i-Voc values in Fig. 2 (b) averages across the wafers and the Voc in Fig. 3 are only for the best cells (2 � 2 cm2). The measured J-V parameters of all the SHJ solar cells, including the champion cells from Fig. 3, are presented in Fig. 4. The Jsc of the SHJ cells is limited by parasitic absorption in both the ITO and front a-Si:H layers [26,33]. The as-grown UMG-Cz cell had the lowest Jsc due to relatively lower minority-carrier diffusion length (LD), as shown in Table 2. Overall, the EG-Cz cells achieved higher Jsc than the UMG-Cz cells due to thicker wafers, as LD/t ratios for the cells were greater than 4.5, as shown in Table 2. Therefore, the UMG-Cz cells suffered from poor carrier collection due to bulk defects. Further, The estimated LD values presented in Table 2 were calculated using the measured minority-carrier lifetimes and simulated minority-carrier mobilities from Schindler’s model for compensated silicon for the UMG-Cz cells [29], and Klaassen’s model for the EG-Cz cells [34]. We note that the slightly lower than expected Jsc for the UMG-Cz (TR þ PDG) cells in Fig. 4 could be due to processing variations, as they were fabricated in a different batch from the other cells. Fig. 5 shows the measured EQE and total absorption (1-R) curves for the champion cells. The as-grown UMG-Cz cell exhibits no significant differences in reflection compared to the PDG þ H UMG-Cz cell. However, the EQE of the as-grown UMG-Cz
Table 2 Carrier mobilities, bulk lifetimes, minority-carrier diffusion lengths, and thick nesses for the n-type wafers.
Fig. 3. In-house-measured J-V curves of the champion SHJ cells fabricated on as-grown, TR þ PDG, and PDG þ H n-type UMG-Cz and EG-Cz wafers. The J-V parameters of the champion cells are also shown.
Parameters
with the i-Voc values shown in Fig. 2(b). Thus, the TR þ PDG and PDG þ H pre-fabrication treatments result in very similar improvement in the bulk quality of the UMG-Cz wafers. Similarly, for the as-grown EG-Cz solar cells, the best Voc value was 708 mV, and this increased to 720 mV and 721 mV after TR þ PDG and PDG þ H, respectively, as shown in Fig. 3. This indicates that the EG-Cz wafers used in this work benefited from pre-fabrication treatment steps. As expected, we observed some losses in Voc (e10 mV) in EG-Cz cells in comparison to the i-Voc values shown in Fig. 2(b). This is due to sputter damage during the ITO and
Hole mobility, μh (cm2V 1s 1) Electron mobility, μe (cm2V 1s 1) Measured bulk lifetimes, τbulk (μs) Calculated diffusion length, LD (μm) Thickness, t (μm) LD/t
UMG-Cz
EG-Cz
Asgrown
PDG þ H
Asgrown
PDG þ H
377 971
377 971
431 1242
431 1242
290 540
980 989
1240 1184
2635 1862
150 3.6
150 6.5
250 4.7
250 7.4
Fig. 4. In-house-measured illuminated J-V parameters of SHJ cells fabricated on as-grown, TR þ PDG, and PDG þ H n-type UMG-Cz and EG-Cz wafers. The box plots represent the average, 25th percentile, and 75th percentile, excluding outliers. Each cell was 2 cm � 2 cm in size. 4
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property. We have not included simulated Jsc, FF, and efficiency here, as they also depend on optical and electrical factors that are not directly related to the bulk quality of the wafers. Fig. 6 shows the simulated Voc for the as-grown and PDG þ H SHJ solar cells for both the UMG-Cz and EG-Cz wafers. The simulated Voc values are within 2–3 mV of the measured values in each case. We now consider the effects of changing the parameters of the EG-Cz cells to those of the UMG-Cz cells. After reducing t to 150 μm, the Voc values increased for both sets of cells (Voc(as-grown) ¼ 716 mV and Voc (PDGþH) ¼ 723 mV), as expected [40]. Subsequently, lowering n0 to 1 � 1015 cm 3, Voc increased further by 1 mV (Voc(as-grown) ¼ 717 mV and Voc (PDGþH) ¼ 724 mV). This occurs because SHJ cells reach high injection at open-circuit, as shown in Fig. 2(a). As a result, Auger and surface recombination are significant, and the excess carrier density (Δn) tends to either remain constant or increase as n0 decreases, in contrast to the expected behavior at lower injection levels. Conversely, the voltage at the maximum power point (VMPP) decreased with n0 (not shown) because the maximum power point occurs at a lower injection level, as shown in Fig. 2(a), at which Auger recombination is less important. Finally, μe and μh were reduced, as shown in Table 2. At this stage, the simulated Voc is greater than the measured Voc for the as-grown and PDG þ H UMG-Cz cells by 35 mV and 8 mV, respectively. The bulk lifetimes were then adjusted to the measured values of the UMG-Cz cells from Table 2. The simulated Voc values are then within 1–3 mV of the measured values of the UMG-Cz cells, as shown in Fig. 6. These results confirm that the measured Voc differences between the UMG-Cz and EG-Cz cells are not dominated by variations in t, n0, μe, and μh, but rather by the differences in their bulk lifetimes, and that improving bulk quality either by TR þ PDG or PDG þ H is critical to achieve higher efficiencies.
Fig. 5. Measured EQE and 1-R curves for the UMG-Cz and EG-Cz cells fabri cated on as-grown and PDG þ H wafers.
cell is significantly lower between 400 nm and 1000 nm in comparison to the PDG þ H UMG-Cz cell, reflecting the impact of the lower minority-carrier diffusion length in these rear-junction cells. The highest in-house-measured efficiencies on the as-grown UMG-Cz and EG-Cz wafers were 18.0% and 21.2%, respectively. The best effi ciencies achieved for the UMG-Cz and EG-Cz cells after TR þ PDG were 21.0% and 22.4%, respectively, and these further increased to 21.2% and 22.7% after PDG þ H. These results clearly demonstrate the benefits of pre-fabrication treatments on both the UMG-Cz and EG-Cz silicon wafers used in this work for SHJ cells. We did not observe significant differences in the SHJ cell results due to the two different combinations of pre-fabrication treatments (TR þ PDG or PDG þ H). However, selecting one or the other for integration into a SHJ fabrication sequence requires careful consideration of their cost and complexity, which we do not address here. The long-term stability of the cells after treatment is also an impor tant consideration. The compensated nature of these n-type UMG-Cz wafers can result in strong degradation under illumination, due to the presence of boron–oxygen-related defects [35–37]. However, Sun et al. [38] have recently demonstrated complete regeneration of boron-oxygen-related defects in SHJ solar cells fabricated on these UMG-Cz wafers by annealing under illumination. Therefore, this disadvantage can be overcome by appropriate post-treatment of the SHJ devices. All UMG-Cz based solar cell J-V characteristics in this work are reported in the non-degraded state prior to the activation of boron-oxygen-related defects.
4. Conclusion This work has demonstrated that as-grown UMG-Cz wafers are not suitable to achieve high-efficiency solar cells with conventional SHJ
3.3. Simulated Voc To confirm the impact of the pre-fabrication treatments observed experimentally in section 3.2, we performed Quokka [39] simulations in which we sequentially adjusted the wafer properties such as thickness (t), net doping (n0), mobility (μe and μh), and bulk lifetime (τbulk) from the values of the EG-Cz cells to those of the UMG-Cz cells, for both the as-grown and PDG þ H conditions. This allows for a direct comparison between the UMG-Cz and EG-Cz control cells. The thickness and net doping values in Table 1, and mobilities and bulk lifetime values in Table 2, were used in the simulations. The recombination parameter J0 of the textured front and rear surfaces were extracted from lifetime measurements from the baseline SHJ process and J0 (front) and J0(rear) were 8 fA/cm2 and 14 fA/cm2, respectively. For each simulation, we extracted the cell Voc to identify the influence of each wafer bulk
Fig. 6. Measured and simulated Voc of as-grown and PDG þ H solar cells fabricated on the UMG-Cz and EG-Cz wafers. The wafer properties-t, n0, μh, μe, and τbulk were sequentially adjusted from the values of the EG-Cz wafers to those of the UMG-Cz wafers. 5
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solar cells processing. However, utilizing pre-fabrication treatments, namely tabula rasa together with phosphorus diffusion gettering, or phosphorus diffusion gettering with hydrogenation, the bulk lifetimes of the UMG-Cz wafers can be increased significantly, promoting higher efficiencies. In addition, EG-Cz silicon wafers used in this work also benefited from pre-fabrication treatments, although to a lesser degree. After these pre-fabrication treatments, we measured absolute gains in efficiency of around 3.0% and 1.2%, for the UMG-Cz and EG-Cz SHJ solar cells, respectively, resulting in a champion cell of 21.2% for the UMG-Cz wafers. More broadly, the results highlight the potential of prefabrication treatments to enable SHJ solar cell technology to be applied to lower quality as-grown wafers. This could be accomplished, for example, through a hybrid solar cell structure utilizing both homojunction and heterojunction features to naturally incorporate such treatments during the solar cell fabrication process [41,42].
[7]
[8] [9]
[10] [11] [12]
Declaration of competing interest
[13]
� We wish to confirm that there are no known conflicts of interest associated with this publication. � All of the sources of funding for the work are acknowledged in the manuscript. � We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. � All listed authors have contributed to complete this paper.
[14] [15] [16]
[17] [18]
Acknowledgments This work was supported by the Australian Renewable Energy Agency (ARENA) through the Australian Centre for Advanced Photo voltaics (ACAP) projects RND009 and RND017. Support was also pro vided by the Engineering Research Center Program of the National Science Foundation and the Office of Energy Efficiency and Renewable Energy of the Department of Energy under NSF Cooperative Agreement No. EEC-1041895, USA. Apollon Solar is acknowledged for providing the UMG wafers for this work. Also, Dr. Di Yan is acknowledged for constructive discussions.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110287.
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