applied sciences Article
Performance Improvement of High Efficiency Mono-Crystalline Silicon Solar Cells by Modifying Rear-Side Morphology Chung-Yuan Kung 1 , Chih-Hsiang Yang 1 , Chun-Wei Huang 2 , Shui-Yang Lien 3, *, Wen-Zhang Zhu 4 , Hai-Jun Lin 4 and Xiao-Ying Zhang 2,4 1 2 3 4
*
Department of Electrical Engineering, National Chung-Hsing University, Taichung 40227, Taiwan;
[email protected] (C.-Y.K.);
[email protected] (C.-H.Y.) Department of Electrical Engineering, Da-Yeh University, Chunghua 51591, Taiwan;
[email protected] (C.-W.H.);
[email protected] (X.-Y.Z.) Department of Materials Science and Engineering, Da-Yeh University, Chunghua 51591, Taiwan School of Opto-Electronic and Communication Engineering, Fujian Provincial Key Laboratory of Optoelectronic Technology and Devices, Xiamen University of Technology, Xiamen 361024, China;
[email protected] (W.-Z.Z.);
[email protected] (H.-J.L.) Correspondence:
[email protected]; Tel.: +886-04-851-1888 (ext. 1760)
Academic Editor: Alejandro Pérez-Rodríguez Received: 15 February 2017; Accepted: 17 April 2017; Published: 18 April 2017
Abstract: In this work, aluminum oxide films with excellent passivation effects were prepared on the rear-side surface of passivated emitter and rear cells (PERCs) using a self-developed spatial atomic layer deposition system. Various rear-side surface morphologies were obtained through different etching treatments. We compared the PERCs with standard etching treatment and further polishing processes on rear-side surfaces. Experimental results show that compared with the unpolished cell, the polished cell attained superior electrical performance, particularly in open-circuit voltage (V oc ) and short-circuit current density (Jsc ), because of the more effective rear-side surface passivation and reabsorption of long-wavelength light. The improvement in V oc and Jsc raised the conversion efficiency to 19.27%. This study verifies that despite polished cells requiring complex processes, the polishing treatment displays application potential for achieving high efficiency in the solar industry. Keywords: passivated emitter and rear cells; atomic layer deposition; surface passivation
1. Introduction The principal goals of recent solar cell processing based on crystalline Si (c-Si) are increasing the conversion efficiency under long-term stability and reducing the solar cell cost per watt. To improve the conversion efficiency, different structural modifications of solar cells have been investigated [1,2]. A special solar structure, passivated emitter and rear cells (PERCs), is globally drawing increasing interest owing to its high efficiency and appropriate production cost, and could be a good candidate for the future development of solar cells [3,4]. One of the most renowned features of PERCs is its rear-side passivation based on a dielectric layer or stack of layers, which can either chemically saturate the surface dangling bonds or offer a strong surface field, thus playing a critical role in reducing surface recombination. Several dielectric materials such as silicon dioxide, silicon nitride, aluminum oxide (Al2 O3 ), and amorphous silicon are regarded as appropriate choices because of their superior passivation ability [5]. The Al2 O3 has a very low interface defect density with strong field-effect passivation at the Al2 O3 /Si interface [6]. Hence, applying Al2 O3 to PERCs has become more prevalent. The quality of the interface between Si and Al2 O3 is crucial for improving the performance
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of PERCs. The damaged interfaces, which can cause considerable defects and absorption losses of long-wavelength light, should be avoided. In this study, four Si wafers with distinct rear-side surface morphologies were fabricated. Three of the wafers displayed uniform and continuous pyramids with various sizes, and the other has a relatively planar rear-side surface similar to a mirror. All samples were then completed to form entire PERCs in order to compare their open-circuit voltage (V oc ), short-circuit current density (Jsc ), and fill factor (FF). 2. Materials and Methods The PERCs were fabricated on mono-crystalline p-type commercial-grade CZ silicon wafers with a thickness of 230 ± 20 µm and a resistivity of 0.5 to 3 Ω-cm. The original lifetime of a bare wafer is under 5 µs. Initially all wafers were cleaned using a standard Radio Corporation of America (New York City, NY, USA) cleaning process and were textured using 5% KOH solution. A rear-side polishing process with the same alkaline solution was applied to three of the samples for 10, 20, and 30 min after the first etching. The p-n junction and antireflection coatings (ARC) were made by thermal diffusion and plasma-enhanced chemical vapor deposition (PECVD). The ARC is a 75 nm-thick Si3 N4 film that helps trap more incident light. A 25 nm Al2 O3 layer and a 135 nm silicon nitride (SiNx ) layer were then deposited by self-developed spatial atomic layer deposition (ALD) at room temperature and by PECVD at 120 ◦ C on rear-side wafers, respectively. The post-annealing at 450 ◦ C was performed on all samples. The rear openings were created by a 532 nm laser after the post-annealing of the stack Al2 O3 /SiNx films. Then the front Ag grid screen-printing and Al local contacts to p-type Si were formed to complete the PERCs [7]. Finally, all samples were co-fired in a belt furnace for front contacts and local Al back surface field [8]. The surface morphologies were characterized using a field emission scanning electron microscope (FE-SEM). The orientation of the Al2 O3 films was confirmed by X-ray diffraction (XRD) using 45 kV-40 mA CuKα radiation. The effective minority carrier lifetime and implied V oc were measured by the quasi-steady-state photoconductivity method (WCT-120) using lifetime samples with only Al2 O3 /SiNx stack film deposition on both sides (without emitter layers). The stack films were annealed at 450 ◦ C for 20 min. On the solar cell samples, quantum efficiency measurements were also performed to obtain the internal quantum efficiency (IQE) and rear-side reflectance. We measured the current density-voltage (J-V) characteristics of the cells using a solar simulator under AM1.5G (100 mW/cm2 ) conditions. 3. Results and Discussion The XRD patterns of the Al2 O3 films after annealing treatment from 350 ◦ C to 650 ◦ C are shown in Figure 1. The only (100) peaks, which represent crystalline silicon substrates, are observed in all samples. This result indicates that all the Al2 O3 films are amorphous phase, and the thermal stability of the Al2 O3 films are outstanding. In this study, Al2 O3 films annealed at 450 ◦ C are used, due to their longer lifetimes compared with Al2 O3 films annealed at other temperatures. As the annealing temperature increases over 500 ◦ C, the lifetime starts to decrease. Lifetime reduction at high annealing temperature has also been obtained by other research groups [9,10]. The rear-side surface morphologies for the samples treated with the standard texture process and further polishing processes for 10, 20, and 30 min are shown in Figure 2. The typical textured surface morphology containing uniformly sized pyramids of around 5 to 8 µm (width of the base) can be seen in Figure 2a. In Figure 2b, large and small pyramids start to coexist with the additional polishing process as the duration approaches 10 min. By extending the duration to 20 min, all the pyramids become uniformly smaller and thinner as shown in Figure 2c. Increasing the duration to 30 min produces a smoothed surface like a mirror; most pyramids almost disappear as seen in Figure 2d. Overall these four stages constitute a substantially texturing cycle.
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Figure1.1.XRD XRDpatterns patternsof ofthe theAl Al22O O33 films films annealed annealed at at various various temperatures. temperatures. Figure Figure 1. XRD patterns of the Al2O3 films annealed at various temperatures.
Figure 2. SEM images of rear-side of the silicon wafers with further polishing duration for: (a) 0; (b) 10; 2. (c) 20; (d)images 30 min. Figure 2.EM SEM of rear-side of silicon the silicon wafers further polishing duration (a)10; 0; Figure images of rear-side of the wafers with with further polishing duration for: (a)for: 0; (b) (b)20; 10;(d) (c) 30 20;min. (d) 30 min. (c)
Figure 3a shows the injection level dependent effective minority carrier lifetime and Auger lifetime for 3a theshows four samples. Thelevel effective lifetimeeffective is calculated from the lifetime photoluminescence Figure injection dependent minority andlifetime Auger Figure 3a shows thethe injection level dependent effective minority carriercarrier lifetime and Auger intensityfor by the calibration method, and the Auger lifetime calculated by the model lifetime the self-consistent fourThe samples. effective lifetime is calculated fromis the photoluminescence for the four samples. effectiveThe lifetime is calculated from the photoluminescence intensity by the proposedby bythe Richter et al. [11]. The effective lifetime for samples A, B, C, and D areby 174.4, 163.7, intensity self-consistent calibration method, and the Auger lifetime is calculated the model self-consistent calibration method, and the Auger lifetime is calculated by the model proposed by 200.1, andby 225.3 μs atet theal.injection level of 5 ×lifetime 1015 cm−3 , respectively. TheC,1-Sun lifetime values for proposed Richter [11]. The effective for samples A, B, and D are 174.4, 163.7, Richter et al. [11]. The effective lifetime for samples A, B, C, and D are 174.4, 163.7, 200.1, and 225.3 µs 15 −3 sampleand A, B, C, and D are injection 170, 159.9,3 194,ofand respectively. As the time increases, the 200.1, 225.3 μs at 5 ×212 10 μs, cm , respectively. Theetching 1-Sun lifetime for at the injection level of the 5 × 1015 cm−level , respectively. The 1-Sun lifetime values for sample A,values B, C, and lifetimeA, decreases firstly and then increases to212 its μs, saturation value As when the wafertime is fully polished. sample B, C, and D are 170, 159.9, 194, and respectively. the etching increases, the D are 170, 159.9, 194, and 212 µs, respectively. As the etching time increases, the lifetime decreases A lifetime drop in firstly sampleand B can beincreases attributed the non-uniform pyramid with many lifetime decreases to to its saturation value whenpolished. thestructure waferAislifetime fully too polished. firstly and then increases to itsthen saturation value when the wafer is fully drop in turning points (peaks andBvalleys of pyramids), which may cause film cracks. Thiswith damage traps A lifetime drop in sample can be attributed to the non-uniform pyramid structure too many sample B can be attributed to the non-uniform pyramid structure with too many turning points (peaks carriers and reduces the lifetime. Another reason which for themay lower lifetime iscracks. related to adamage less efficient turning points (peaks and valleys of pyramids), cause film This traps and valleys of pyramids), which may cause film cracks. This damage traps carriers and reduces the cleaningand of the surface to theAnother presencereason of narrow valleys between pyramids. Two factors can carriers reduces thedue lifetime. for the lifetimecleaning is related to asurface less efficient lifetime. Another reason for the lower lifetime is related to alower less efficient of the due to account for thesurface better due performance of samples C and D. First, the polishing process not only cleaning of the to the presence of narrow valleys between pyramids. Two factors can the presence of narrow valleys between pyramids. Two factors can account for the better performance smooths for the wafer surface but also removes sawing damages, thereby reducing thenot surface account performance samples andsmooths D. First, polishing only of samples C the and better D. First, the polishingofprocess notConly thethe wafer surface process but also removes recombination velocity. Second, sample A and B both have larger rear-side surfacethe areas than smooths the wafer surface butsince also removes sawing damages, thereby reducing surface sawing damages, thereby reducing the surface recombination velocity. Second, since sample A and that of sample velocity. C and D,Second, more defects are present at Bthe Si/Al 2O3 larger interface, actingsurface as recombination recombination since sample A and both have rear-side areas than B both have larger rear-side surface areas than that of sample C and D, more defects are present centers. This result wellare with the observations the SEM Sample D that sample C andcorresponds D, more defects present at the Si/Al2O3from interface, actingimages. as recombination at theofSi/Al result corresponds well with the 2 O3 interface, acting as recombination centers. This 15 −3 exhibits aThis slight increase in lifetime at anwith injection of 5 × 10 from cm ,the which is more apparent than centers. result the level observations SEM images. Sample D observations from thecorresponds SEM images. well Sample D exhibits a slight increase in lifetime at an injection level 15 cm−3, which that observed for the other cells due to the superior field-effect passivation for the Al 2 O 3 film on the exhibits a slight increase in lifetime at an injection level of 5 × 10 is more apparent than of 5 × 1015 cm−3 , which is more apparent than that observed for the other cells due to the superior mirror-like polished compared with that onfield-effect the other rougher surfaces. injection that observed for the surface other cells due to the superior passivation for theAt Ala2Olow 3 film on the field-effect passivation for the Al2 O3 film on the mirror-like polished surface compared with that level, negligible minority carriers are generated charge the inversion andAt a shorter lifetime mirror-like polished surface compared with thattoon the other rougher layer, surfaces. a low injection on the other rougher surfaces. At a low injection level, negligible minority carriers are generated to is gained due tominority the asymmetry in generated the electron and hole cross sections at the Si/Al 2O3 level, negligible carriers are to charge thecapture inversion layer, and a shorter lifetime charge the inversion layer, and a shorter lifetime is gained due to the asymmetry in the electron and interface to higher injection levels, lifetime starts to sections shorten with is gained [12]. due With to theregard asymmetry in the electron andthe hole capture cross at theincreasing Si/Al2O3 hole capture cross sections at the Si/Al2 O3 interface [12]. With regard to higher injection levels, the Auger recombination. Theto implied Voc exported the Sinton-120 lifetimewith tester (Sinton interface [12]. With regard higher injection levels, from the lifetime starts to shorten increasing lifetime starts to shorten with increasing Auger recombination. The implied V oc exported from the Instruments, Boulder, CO, USA) for the four samples are shown in Figure 3b to evaluate the Auger recombination. The implied Voc exported from the Sinton-120 lifetime tester (Sinton Sinton-120 lifetime tester (Sinton Instruments, Boulder, CO, USA) for the four samples are shown in maximum VocBoulder, before metal deposition. implied Voc are is extracted the relation by Instruments, CO, USA) for theThe four samples shown inbyFigure 3b to proposed evaluate the Sinton [13]:Voc before metal deposition. The implied Voc is extracted by the relation proposed by maximum Sinton [13]: ∆ ( ) (1) (∆ ) = −∆∆ ( () )/∆ (1) (∆ ) = − ∆ ( )/∆
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Figure 3b to evaluate the maximum V oc before metal deposition. The implied V oc is extracted by the relation proposed by Sinton [13]: τe f f (∆n) =
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∆n(t) G − ∆n(t)/∆t
4 of(1) 6
∆ ( ∆ ) = kT (∆n( NA + ∆n)) (2) implied Voc = ln (2) 2 q ni where G is the generation rate; τeff is the effective minority carrier lifetime; ∆n is the excess carrier where G is the generation rate; τ eff is the effective minority carrier lifetime; ∆n is the excess carrier concentration; ∆t is the time taken in seconds; k is the Boltzmann constant; T is the temperature; concentration; ∆t is the time taken in seconds; k is the Boltzmann constant; T is the temperature; NA is NA is the acceptor concentration of the wafer; and ni is the intrinsic carrier concentration. The the acceptor concentration of the wafer; and ni is the intrinsic carrier concentration. The implied V oc implied Voc following the same trend as the effective minority carrier lifetime with respect to the following the same trend as the effective minority carrier lifetime with respect to the further polishing further polishing time can exactly predict the performance of the surface passivation for the time can exactly predict the performance of the surface passivation for the fabricated PERCs, especially fabricated PERCs, especially in Voc. in V oc .
Figure3.3.(a) (a)Effective Effectivelifetime lifetimeas asaafunction function of of injection injection level; (b) Implied Voc forthe theAl Al22OO3 3films filmswith with Figure ocfor differentrear-side rear-sidesurface surfacemorphologies. morphologies. different
Figure Figure44illustrates illustratesthe thereproducible reproducibleilluminated illuminatedJ-V J-Vcharacteristics characteristicsfor forall allthe thePERCs. PERCs.All Allof ofthe the curves curvesare arevery veryclose closein inthis this figure. figure. The Thedetailed detailedsolar solarcell cellexternal externalparameters parametersare aresummarized summarizedin in Table 1. Both V oc and J sc are obviously affected by the rear-side surface, whereas the FF changes little Table 1. Both V oc and Jsc are obviously affected by the rear-side surface, whereas the FF changes little in in range of 0.777 to 0.793, is attributed to the probable experimental thethe range of 0.777 to 0.793, whichwhich is attributed to the probable experimental scattering. scattering. This revealsThis that reveals that the polishing processfor is V beneficial for V oc better and J scsurface due to better surface passivation and the polishing process is beneficial and J due to passivation and long-wavelength oc sc long-wavelength reflection, respectively, rarelyresistance influences the series resistance or obvious shunt reflection, respectively, but rarely influences but the series or shunt resistance. The most resistance. The most obvious variation of the electrical performance for each cell is in the Voc, variation of the electrical performance for each cell is in the V oc , proving that the polishing treatment proving that the polishing treatment a much morerecombination important role in preventing carrier plays a much more important role in plays preventing carrier at the rear-side Si/A 2 O3 recombination at the rear-side Si/A 2 O 3 interface than gaining the long-wavelength light. BothD interface than gaining the long-wavelength light. Both relatively planar surfaces of samples C and relatively planar surfaces samples C and D 2have Voc over 0.66 V and Jsc over 36.6 mA/cm2, while have V oc over 0.66 V and Jof sc over 36.6 mA/cm , while the further etching time exceeded 20 min. Note the exceeded 20 min. Note trend that the realimplied Voc for all following same trend thatfurther the realetching V oc fortime all cells following the same as the V occells is evidence forthe reproducibility as the implied V oc is evidence for reproducibility in this work. Overall, sample D demonstrates in this work. Overall, sample D demonstrates the optimized efficiency of 19.27%, with the Vocthe of 2 , and optimized of 19.27%, with Voc of 0.662 V, Jsc of 36.69 mA/cm2, and FF of 0.793. 0.662 V, Jscefficiency of 36.69 mA/cm FFthe of 0.793. 40 Table 1. The detailed electrical external parameters for all PERCs (passivated emitter and rear cells). 35
Sample 30 A 25 B 20 C D 15
V oc (V) 0.655 0.648 0.660 0.662
Jsc (mA/cm2 ) 36.47 36.43 36.64 sample36.69 A
FF
Efficiency (%)
0.779 0.789 0.777 0.793
18.62 18.64 18.80 19.27
sample B
10
sample C
5
sample D
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
proving that the polishing treatment plays a much more important role in preventing carrier recombination at the rear-side Si/A2O3 interface than gaining the long-wavelength light. Both relatively planar surfaces of samples C and D have Voc over 0.66 V and Jsc over 36.6 mA/cm2, while the further etching time exceeded 20 min. Note that the real Voc for all cells following the same trend as Voc is evidence for reproducibility in this work. Overall, sample D demonstrates5 ofthe Appl.the Sci. implied 2017, 7, 410 6 optimized efficiency of 19.27%, with the Voc of 0.662 V, Jsc of 36.69 mA/cm2, and FF of 0.793. 40 35 30 25 Appl. Sci. 2017, 7, 410
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sample A
sample B for all PERCs (passivated emitter and rear cells). Table 1. The detailed electrical external parameters 10
sample C
2) Sample Voc (V)5 Jscsample (mA/cm FF Efficiency (%) D A 0.655 36.47 0.779 18.62 0 B 0.648 0 0.1 0.2 36.43 18.64 0.3 0.4 0.5 0.60.789 0.7 C 0.660 36.64 0.777 18.80 Figure4.4.DJ-V J-V characteristics characteristics for both both the the PERCs PERCs (passivated emitter emitter and rear rear cells) cells)19.27 withdifferent different Figure with 0.662for 36.69(passivated 0.793and
rear-sidesurface surfacemorphologies. morphologies. rear-side
The IQE and reflectance for the PERCs with different surface morphologies are shown in Figure In the wavelength 500 nm,surface the performance for are each curveinisFigure similar. The5.IQE andshort reflectance for the range PERCsbefore with different morphologies shown 5. Samples C wavelength and D represent better500 spectral compared other samples atand the In the short range before nm, theresponses performance for eachwith curvethe is similar. Samples C middle wavelength range from 500 to 900 with nm. the Thisother maysamples be dueat to relatively exceptional D represent better spectral responses compared thethe middle wavelength range rear-side provided by the Al2relatively O3 films on the flatterrear-side surfaces; photogenerated electrons from 500 topassivation 900 nm. This may be due to the exceptional passivation provided by the at2the bottom of wafer therefore easily repelled to theatp-n collection. This can be Al O3 films on the flatter are surfaces; photogenerated electrons the junction bottom offor wafer are therefore easily proven by calculating the effective diffusion length (Leff) using the method proposed by P. A. Basore repelled to the p-n junction for collection. This can be proven by calculating the effective diffusion [14]. The Leffusing for allthe samples is more thanby 1450 μm. As for[14]. the wavelengths over 900 nm, obvious length (Leff) method proposed P. A. Basore The Leff for all samples is an more than difference spectral response occurs as nm, well.anThe smoothed surface of sample D that is functionally 1450 µm. Asinfor the wavelengths over 900 obvious difference in spectral response occurs as well. analogous to asurface mirroroffacilitates reflection of more unabsorbed long-wavelength light back to The smoothed sample Dthe that is functionally analogous to a mirror facilitates the reflection themore active region of the Si bulk. Both reasons mentioned aboveregion make contributions the higher Jsc of unabsorbed long-wavelength light back to the active of the Si bulk.toBoth reasons 2 for sample D. of 36.69 mA/cm mentioned above make contributions to the higher Jsc of 36.69 mA/cm2 for sample D.
Figure 5. IQE (internal quantum efficiency) and reflectance for the PERCs deposited on Si wafers Figure 5. IQE (internal quantum efficiency) and reflectance for the PERCs deposited on Si wafers with with different rear-side surface morphologies. different rear-side surface morphologies.
4. Conclusions 4. Conclusions In this study, the performance of PERCs with different rear-side morphologies are investigated. In this study, the performance of PERCs with different rear-side morphologies are investigated. Due to superior field-effect passivation and reabsorption of long-wavelength light, the fully polished Due to superior field-effect passivation and reabsorption of long-wavelength light, the fully polished sample D attains a longer effective lifetime (225.3 μs) and higher averaged rear-side reflectance sample D attains a longer effective lifetime (225.3 µs) and higher averaged rear-side reflectance (7.6%) (7.6%) in the long-wavelength range than the other PERCs. The polishing process improves the in the long-wavelength range than the other PERCs. The polishing process improves the conversion conversion efficiency from 18.62% to 19.27%, revealing that the polishing process enhances the efficiency from 18.62% to 19.27%, revealing that the polishing process enhances the performance of the performance of the PERCs. This study demonstrates the importance of an optimized rear-side polishing process for the industrial fabrication of PERC solar cells with higher efficiencies. Acknowledgments: This work is sponsored by the Ministry of Science and Technology of the Republic of China under the grants No. 104-2632-E-212-002—Project supported by the Science and Technology Program of the Educational Office of Fujian Province, China (No. JB12182).
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PERCs. This study demonstrates the importance of an optimized rear-side polishing process for the industrial fabrication of PERC solar cells with higher efficiencies. Acknowledgments: This work is sponsored by the Ministry of Science and Technology of the Republic of China under the grants No. 104-2632-E-212-002—Project supported by the Science and Technology Program of the Educational Office of Fujian Province, China (No. JB12182). Author Contributions: Chih-Hsiang Yang and Shui-Yang Lien designed the experiments; Chih-Hsiang Yang, Chun-Wei Huang, Wen-Zhang Zhu, Hai-Jun Lin, and Xiao-Ying Zhang performed the experiments and measurements; Chung-Yuan Kung and Chih-Hsiang Yang analyzed the results; Chih-Hsiang Yang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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