20.7% efficient ion-implanted large area n -type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

ACCELERATED PUBLICATION

20.7% efficient ion-implanted large area n-type frontjunction silicon solar cells with rear point contactsformed by laser opening and physical vapor depositionYuguo Tao1*, Adam Payne2, Vijaykumar D. Upadhyaya1 and Ajeet Rohatgi1,2

1 School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Drive, Atlanta, GA 30332-0250, USA2 Department of R&D, Suniva Inc., 5765 Peachtree Industrial Blvd., Norcross, GA 30092, USA

ABSTRACT

In this work, we report on ion-implanted, high-efficiency n-type silicon solar cells fabricated on large area pseudosquareCzochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser-patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and aphosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide duringthe implant anneal. Both front and back oxides were capped with SiNx, followed by screen-printed metal grid formationon the front side. An ultraviolet laser was used to selectively ablate the SiO2/SiNx passivation stack on the back to formthe pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO2/SiNx passivation layers,without inducing appreciable damage or defects on the surface of the n+ back surface field layer. It was also found that alow temperature annealing for less than 3min after PVD Al provided an excellent charge collecting contact on the back. Inorder to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essentialfor etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the sizeand pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n-type passivated emitter, reartotally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.

KEYWORDS

n-type wafer; laser pulse energy; PVD; annealing; plasma etching; pitch

*Correspondence

Yuguo Tao, School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Drive, Atlanta, GA30332–0250, USA.E-mail: [email protected]

Received 22 January 2014; Revised 12 May 2014; Accepted 17 July 2014

1. INTRODUCTION

Ion-implanted n-type silicon (Si) solar cells are attractingconsiderable interest in photovoltaics because of theirpotential for stabilized high efficiency and low cost. Inaddition, ion implantation can produce advanced high-efficiency cell structures with fewer processing steps[1–3]. It is well known that n-type silicon provides severaladvantages over p-type, including better tolerance tocommon impurities (e.g., Fe), high bulk lifetime, and nolight-induced degradation due to the boron-oxygen com-plex formation [4,5]. Benick et al. [6] have demonstratedthe high quality of boron and phosphorus implantation byfabricating a 22.3% small area efficient n-type passivatedemitter, rear totally diffused cell on a float-zone Si wafer.In addition, it has also been reported by Lai et al. [7] that

high-quality oxide with a low density of interface statescan be grown during the post-implantation anneal. Finally,it has been shown that low-cost high-quality screen-printedcontacts can be made to boron-doped emitters with a fillfactor (FF) approaching ~0.800 [8,9]. Therefore, in thisstudy, an ion-implanted boron emitter and a phosphorusback surface field (BSF) are implemented to produce largearea manufacturable n-type front junction Czochralski (Cz)Si cells with in situ oxide passivation on both front and backsurfaces along with a screen-printed Ag/Al grid on front. Ina previous work [9], we reported fully ion-implanted andscreen-printed 20.2% efficient n-type cells on large areaCz substrates with screen-printed Ag dots on the rearconnected with full metal, as shown in Figure 1 (left). Toeliminate both two (relatively expensive) Ag pastes on therear side, and to achieve higher efficiency, in this paper,

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONSProg. Photovolt: Res. Appl. 2014; 22:1030–1039

Published online 31 July 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2545

Copyright © 2014 John Wiley & Sons, Ltd.1030

we present 20.7% efficient cells through the developmentof rear point contacts to an n+ BSF formed by sputteringAl onto the laser-opened vias through the SiO2/SiNx stack,as shown in Figure 1 (right).

Because of its high throughput and precision, galvo-scanner laser systems have been used extensively in the in-dustrial manufacturing of solar cells, including laser edgeisolation [10], laser-fired contacts [11], laser doping toform selective emitter or local BSF [12], and drilling ofholes through Si to fabricate emitter wrap-through or metalwrap-through cells [13]. Various lasers have been investi-gated to selectively remove the SiNx-based passivatingantireflection coating layers without damaging the emittersurface [14–16]. It has also been reported that to selec-tively open the rear dielectric stack for local contacts, apost-laser etching step is required to reduce the level ofsurface damage on the ablated region when using a nano-second laser, while this etching step can be eliminatedwhen using a picosecond laser [17,18]. Furthermore, byapplying the laser ablation for local contact openings,promising cell performance of industrial p-type passivatedemitter and rear cell solar cells with improved back surfacerecombination velocity and increased back surfacereflectance has been demonstrated [7,19,20], comparedwith the standard full-area screen-printed aluminum backfield (Al-BSF) p-type Si solar cells. In this work, a diode-pumped solid state vanadate laser system with ultravioletwavelength (third harmonic) and nanosecond pulse dura-tion was applied to selectively open the SiO2/SiNx passiv-ation layers on the rear side to define the contact pattern.

As a non-contacting process, physical vapor deposition(PVD) is attractive because of its potential advantages ofreduced wafer breakage and allowing for processing ofthinner wafers. In terms of lower material price for Al thanAg, replacing Ag paste by Al has a significant cost reduc-tion capability, especially for our previous cells [9] withthe low-temperature Ag paste on full rear area to connectthe Ag dots. In addition, lower specific contact resistancefor Al/Si (<0.5mΩ∙cm2) [21,22] has also been reported,compared with the Ag/Si (1–2mΩ∙cm2) [23,24]. Becausethe sheet resistance of a 2-μm-thick PVD Al layer is just

~0.015Ω/□ [25], a thin (1 ~ 2μm) PVD Al layer on theentire rear area is sufficient to meet the required electricalconductance for large-area Si solar cells, which furtherleads to less wafer bow and less Al consumption [26].On the other hand, in order to obtain a good solder contactto the PVD Al side, depositing a double layer of Ni : V/Agwith respective thickness of 200 and 25 nm on top of PDAl layer can offer an excellent solderability with a peelforce greater than 3N/mm and long-term stability[27,28]. Moreover, to improve the mass production costof PVD Al process, the industrial pilot PVD machine withthroughput of over 500 wafers per hour has also beendemonstrated [29,30]. Another motivation of this studywas to investigate and simplify the post-PVD Al anneal.Lengthy Al anneals (~40min) at ~400 °C in forming gashave been used to achieve a record of high efficiency onsmall area Si cells [31–33]. Such an anneal improvesohmic contact, provides atomic hydrogen passivation,and is also known as “alneal” (aluminum anneal) [34,35];however, because of the high solubility and diffusivity ofSi in Al, Al spiking may occur during the contact formationannealing at a high temperature, which can degrade the de-vice performance [36]. For our cell structure with n/n+

high-low junction on the rear side [9], Al spiking can reducethe area fraction of the highly doped n+ region, hence candegrade the passivation quality [22]. So, the Al spiking hasto be avoided by optimizing the deeper doping profile, orusing a sufficiently thick spiking barrier such as Al–Si orTi, or a post-PVD anneal at lower temperatures [22,36].Therefore, in order to improve the throughput and have amore manufacturable process, we have developed a shorterand novel process for annealing PVD Al in an air ambient.

2. EXPERIMENTS

Figure 2 shows the comparison of process flow for ourfront junction n-type cell on a 239 cm2 Cz wafer with rearcontacts formed by screen printing (left), and laser openingand PVD Al (right). The fabrication process of both cellstructures involves saw damage removal in heated

Figure 1. Schematic of the front junction n-type Si solar cell structures with rear contacts formed by screen printing (left), and laseropening and physical vapor deposition (PVD) Al (right).

20.7% efficient ion-implanted n-type PERT cells with PVD AlY. Tao et al.

1031Prog. Photovolt: Res. Appl. 2014; 22:1030–1039 © 2014 John Wiley & Sons, Ltd.DOI: 10.1002/pip

potassium hydroxide (KOH) solution followed by alkalinetexturing of both sides of the starting 200μm thick waferswith a bulk resistivity of ~5Ω∙cm. To obtain a planar rearsurface, the double-side textured wafers were coated withSiNx as a barrier layer on the front side, and then, a heatedKOH solution was used to planarize the back. Afterplanarization, the wafer thickness reduced to about175μm. The boron and phosphorus implantations wereperformed on a production-line implanter at Suniva Inc.Two different anneals were performed in this study—thefirst right after the boron implantation and the second afterthe phosphorus implantation. To reduce surface recombi-nation velocity on the boron-doped emitter and obtain alow Joe, a chemical etching treatment was performed afterthe first anneal [37,38], and a thin SiO2 passivation layerwas grown on both sides as a by-product of the secondanneal [7,9]. The implantation, annealing, and chemicaletching treatments were optimized to obtain sheet resis-tance of ~80Ω/□ for the boron emitter and ~60Ω/□ forthe phosphorus BSF. The resulting thickness of thermallygrown oxide was ~10 nm on the front and ~20 nm on therear side. Then the SiNx films of appropriate thicknesswere deposited by plasma-enhanced chemical vapor depo-sition on the front and rear surfaces.

The subsequent steps are the metallization. The processflow for the cells with rear contacts formed by screen print-ing is briefly summarized in Figure 2 (left), and its detaileddescription is included in [9]. Figure 2 (right) shows that toform the rear contacts by laser opening and PVD Al, theSiO2/SiNx passivation stack on rear side was selectivelyopened to create vias by the Nd :YVO4 laser with appro-priate pulse energy, followed by a short time cleaning ina buffered oxide etch solution. Note that 3 ~ 4 sampleswere involved in each condition to determine each datapoint. In order to simplify the process and obtain higher

throughput, vias were formed by single laser pulse perpoint. In order to maintain good series resistance and highFF, the coverage of screen-printed front Ag/Al grid waslimited to ~7.5%, and a high temperature firing wasperformed in an industrial-style belt furnace. Then, anapproximately 1μm thick Al film was sputtered by PVDon the entire rear side with a small exclusion zone alongthe wafer edge. An in situ plasma etching process wasapplied just prior to depositing the PVD Al in the samechamber. Finally, the cells were annealed at low tempera-ture for less than 3min in an industrial-style belt furnaceto form a good ohmic contact on the rear side. The cellperformance, including the cell efficiency and other param-eters, was measured at AM 1.5G, 100mW/cm2, 25 °C,using a Fraunhofer-certified full-area (239.5 cm2) 20.2%efficient n-type cell [9] as a reference.

3. RESULTS AND DISCUSSIONS

3.1. Effect of laser pulse energy on theformation of vias and implied Voc

In order to achieve excellent contact between the heavilydoped (n+) BSF layer and the PVD Al and to minimizethe minority carrier recombination, the vias through thedielectric SiO2/SiNx stack had to be formed withoutdamaging the BSF (n+) surface. Therefore, technologydevelopment in this paper begins with the investigationand optimization of laser output energy. Because each spotis irradiated by a single pulse in this study, the total energyirradiated on each spot equals the laser output energy ofeach pulse (pulse energy). Figure 3 displays the opticalmicrographs of vias formed by various pulse energies. Itclearly shows that higher laser pulse energy results in

Figure 2. Comparison of process flow for the rear contacts formation by screen printing (left), and laser opening and physical vapordeposition (PVD) Al (right).

20.7% efficient ion-implanted n-type PERT cells with PVD Al Y. Tao et al.

1032 Prog. Photovolt: Res. Appl. 2014; 22:1030–1039 © 2014 John Wiley & Sons, Ltd.DOI: 10.1002/pip

larger ablation area with more damage in the center becauseof the near-Gaussian distribution of the energy in the laserpulse. The irradiated area with the lowest pulse energy(10μJ) is barely visible, as indicated by the arrow in Figure 3.On the other hand, a clear crater-like contact hole with abright yellow central region is observed at pulse energy of32μJ. This bright appearance of the irradiated spots, com-pared with the non-irradiated surface, indicates completeopening of the SiO2/SiNx layers stack [14,16]. Furthermore,the appearance of the central crater region turns into a silver-gray color at the pulse energy of 56μJ, in conjunction with adarker ring inside of the ablated region. This is attributed to adeeper and crater-like contact hole. As the pulse energy in-creases to 83μJ, more surface damage inside the ablated re-gion becomes visible along with cracks around the ablatedarea. In addition, a change in the appearance of the SiO2/SiNx

stack around the ablated area is visible in Figure 3. It has alsobeen reported that such damage and cracks can be generatedby a thermally induced stress during the laser ablation be-cause of the difference in thermal expansion coefficients ofthe dielectric films and silicon [15,39].

Figure 4 shows that as the laser pulse energy increasesfrom 10 to 83μJ, the diameter of the ablated area getsenlarged from 13 to over 50μm, while the implied open-circuit voltage (implied Voc), measured at one sun usingthe quasi-steady-state photoconductance method [40],decreases monotonically. The implied Voc degradation isthe difference between two implied Voc measurements

(before and after laser opening). In this study, the absoluterange of implied Voc is 648 ~ 653mV before laser ablationand can reach to 663 ~ 668mV after a simulated co-firingcycle (without laser ablation), which is similar to thosereported in a previous work [9]. It is worth noting thatthe pitch of rear point array is 300μm in this study. Noimplied Voc degradation was observed at the laser pulseenergy of 10μJ, which indicates that the passivation ofthe SiO2/SiNx stack remains intact. This is consistent withFigure 2, which shows that the SiO2/SiNx stack is hardlyperturbed at a low laser pulse energy (10μJ). As the pulseenergy increases to 32μJ, the sample begins to demon-strate a slight degradation (~1mV) in the implied Voc.The implied Voc degradation reaches ~5mV at 56 μJ andfinally becomes substantial (15mV) at 83μJ. The degrada-tion in implied Voc is attributed to the exposed area (1 ~ 2%coverage of the entire rear surface) as well as the surfacedamage and cracks observed at pulse energies above32μJ, which act as electrically active defects [15,41].Therefore, an optimum laser pulse energy is critical tofabricate high-efficiency solar cells.

3.2. Effect of post physical vapor depositionAl anneal on the formation of rear contactand cell performance

Figure 5 displays the impact of post-PVD Al annealingtemperature on the cell performance. In this paper, the

Figure 3. Optical microscope images of laser irradiated regions with various laser pulse energies of 10, 32, 56, and 83μJ. Note thatthe arrow points to a small region irradiated with laser pulse energy of 10μJ.



duration of the post-PVD anneal was less than 3min in airon an industrial belt furnace. The laser pulse energy of32μJ was applied for the cells used in this study. Becausea thin SiO2 layer is thermally grown in the vias during thehigh temperature firing (~800 °C) of the screen-printed gridon the front, an in situ plasma etching prior to PVD Aldeposition was performed. The impact of plasma etchingtime on cell performance will be discussed later. Figure 5shows that the cell efficiency (Eff) significantly increasesto 20.1% after post-PVD Al annealing at 300 °C, comparedwith just 18% right after the PVD process. This is reflectedin open-circuit voltage (Voc), short-circuit current density

(Jsc), and FF and is attributed to post-PVD anneal-inducedimprovement of the Si/Al interface and contact quality.Because the sheet resistance of PVD Al is barely changed,0.024Ω/□ after sputtering and 0.022Ω/□ after anneal at300 °C, hence the increase in FF is mainly due to theanneal-induced improvement of Si/Al contact. As theannealing temperature increases, both Voc and Jsc reachtheir peak values (656mV and 39.1mA/cm2) at 400 °C,while the FF (~80%) peaks around 400 ~ 450 °C. Thisresulted in a cell efficiency close to 20.5% around400 ~ 450 °C. Notice that not only Voc decreases slightlyat 500 °C, but the FF also drops, possibly because of theimpaired Si/Al contact quality at higher annealing temper-ature. Moreover, Eff drops dramatically to 20.0% at anannealing temperature of 550 °C as a result of the degrada-tion in Voc, Jsc, and FF. Note that the sheet resistance ofPVD Al still stays at 0.022Ω/□ even after anneal at550 °C. The observed degradation of cell performanceindicates that the Si/Al contact and interface quality havedeteriorated possibly because of Al spiking at higherannealing temperature [22,36].

3.3. Effect of in situ Argon plasma etchingon vias on cell performance

Figure 6 presents the effect of Argon plasma etching timeprior to PVD Al deposition on the cell performance. Notethat the cell efficiency increases dramatically by ~5%absolute, from 14.8% to 19.8%, after 5min Argon plasma

Figure 4. One sun implied Voc and the diameter of ablated areaas a function of the laser pulse energy. Dashed lines are given

only as a guide to the eyes.

Figure 5. Impact of annealing temperature in ambient air post physical vapor deposition on cell performance. Dashed lines are givenonly as a guide to the eyes.



etching, compared with the case without the Argon plasmaetching step. This efficiency enhancement is mainly attrib-uted to the improved FF (from ~58% to 78.4%), becausethe Jsc remains stable at ~39.1mA/cm2 for all etchingtimes; however, the Voc drops by about 10mV. The exactreason for the decrease in Voc is still unknown. It couldbe the result of passivated-contact formation because ofthe very thin SiO2 layer between the n+ and the Al [42],which breaks down after 5min etching. The increased FFdue to Argon plasma etching is the result of a significantdecrease in series resistance (Rs), from 4.73 to0.65Ω∙cm2, which also supports the hypothesis that theSiO2 layer between the n+ and the Al has been etchedaway. Notice that the cell ideality factor (n-factor) alsodecreases slightly, from 1.23 to 1.16, which also contrib-utes to the increase in FF. As the plasma etching timegradually increases from 5 to 30min, the Voc rises gradu-ally to ~656mV, the FF also improves from 78.4% furtherup to 80.2%, and the cell efficiency increases from 19.8%to 20.6%. The etching rate of SiO2 in our experiment isapproximately ~1Å/min, suggesting that a 2 ~ 3 nm thicknative SiO2 film may be present in the vias after the frontcontact formation.

3.4. Impact of laser pulse energy on cellperformance

Figure 7 shows the impact of pulse energy on cell perfor-mance. As the pulse energy increases from 10 to 32μJ,the cell efficiency increases from 19.4% to 20.6%, becauseof the increase in FF from ~76% to 80.1%. This is the

result of the improved n-factor from 1.05 to 1.02, and thedecrease in Rs from 1.34 to ~0.6Ω∙cm2, indicating a thor-ough removal of the SiO2/SiNx stack at a pulse energy of32μJ. At higher pulse energies (32 to 83μJ), a gradualdecrease is seen in Voc, from 655mV to below 640mV,because of the induced BSF damage and defects. This isalso reflected in a decrease in FF from 80% to 79% andan increase in n-factor from 1.02 up to 1.09. The Jsc alsosuffers, showing a decrease of ~0.3mA/cm2. As a result,the cell efficiency gradually drops from 20.6% to 19.6%when pulse energy is increased from 32 to 80μJ.

3.5. Effect of rear contact pitch onoptimized cell efficiency

Figure 8 shows the impact of rear contact pitch on ourcells fabricated with the optimized process parametersdiscussed earlier. Note that our cell efficiency (Figure 8d)peaks at a pitch of 300μm and then steadily declines asthe pitch of vias increases. This is mainly due to thedecrease in FF (Figure 8b), resulting from increased Rs

and the n-factor (Figure 8c), in spite of the increase inVoc (Figure 8a) due to reduced rear metal coverage ormetal-induced recombination.

Figure 9 shows the I–V data of the best cell achieved ona 239 cm2 Cz wafer (~4Ω∙cm after annealing), with a Voc

of 658.3mV, Jsc of 39.1mA/cm2, FF of 80.4%, and cellefficiency of 20.7%.

For comparison purposes, we have plotted Hermle’ssimulation results [2] on our experimental data in Figure 8dfor a comparable cell structure. The simulation results

Figure 6. Effect of the plasma etching time prior to physical vapor deposition on the cell performance. Dashed lines are given only as aguide to the eyes.



show higher absolute cell efficiency because of (i) loweremitter saturation current density (Joe) due to Al2O3

passivation, (ii) less front metal shading (6% as opposedto 7.5% for our cells), and (iii) lower series resistance

(0.5Ω∙cm2). Also note that Hermle’s simulations showthat efficiency is nearly independent of the pitch. That isbecause he used a more conductive BSF (~36Ω/□), com-pared with our cells (~62Ω/□). Therefore, aforementioned

Figure 7. Effect of the laser pulse energy on the cell performance. Dashed lines are given only as a guide to the eyes.

Figure 8. Effect of the rear contact pitch on the cell performance. Dashed lines are given only as a guide to the eyes. Note that theinfluence of rear contact pitch on simulated efficiency by Hermle et al. [2] is also included for comparison purposes.



modifications to our current cells can drive the large areacell efficiency above 21%, which is the target of ourfuture investigation.

3.6. Advantages and disadvantages of rearcontact scheme by laser opening andphysical vapor deposition Al

In comparison with the rear contacts formed by screen-printed and fired-through Ag dots plus low-temperatureAg paste in our previous work [9], PVD Al through selec-tively laser-opened rear passivation stack studied in thiswork provides several advantages. Firstly, PVD Al metal-lization has less metal contact fraction on the rear side,about 1.4% with the via diameter of ~40μm and pitch of300μm, against ~4.5% for screen-printed Ag dots withdiameter of ~120μm and pitch of 500μm. Hence, lessmetal-induced recombination comes from the rear side,so the resulting cells demonstrate lower back surfacerecombination velocity, which is supported by the higherinternal quantum efficiency in the long wavelength range,as shown in Figure 10. Consequently, less recombinationon the rear side results in an increase in Voc. Secondly,PVD Al metallization provides narrower point contactpitch (300μm, compared with 500μm for printed Ag dots)and hence lower lateral resistance resulting from the BSFlayer. Moreover, for similar doping profiles, the specificcontact resistance is lower for PVD Al (<0.5mΩ∙cm2)[21,22], compared with the screen-printed Ag (1 ~ 2mΩ∙cm2) [23,24]. As a result, in combination with sufficientlylow sheet resistance of 0.022Ω/□ for 1-μm-thick PVD Al(0.020Ω/□ for screen-printed low-temperature Ag paste),PVD Al metallization contributes ~0.1Ω∙cm2 less resis-tance to the cell series resistance, compared with thescreen-printed Ag contact formation, which leads toremarkably higher FF. However, more process steps areinvolved for PVD Al metallization, as shown in Figure 2.Moreover, in order to achieve a good solder contact tothe PVD Al on full rear side, depositing a double layer ofNi : V/Ag with respective thickness of 200 and 25 nm on

top of PVD Al layer is necessary [27,28]. To simplify itsfabrication process, the future work will focus on eliminat-ing: (i) the short time buffered oxide etch cleaning, forexample, by further reducing surface damages with opti-mized laser parameters and performing the laser patterningin an inert ambient to avoid the thin SiO2 layer grown inthe vias during ablation; (2) the in situ Ar plasma etching,for example, by processing the front contact firing in N2

ambient to avoid the thin SiO2 layer grown in the vias;and (iii) the post-PVD anneal, for example, by increasingthe deposition temperature during the PVD.

4. SUMMARY

This paper demonstrates large area ion-implanted n-typefront junction Si solar cells with rear point contacts formedby depositing ~1μm thick PVD Al layer on a selectivelylaser-opened SiO2/SiNx passivation stack on the back.SiO2 grown during the implant anneal was used for frontand back passivation. It was found that a post-PVD annealin an air ambient for less than 3min accomplished thedesired conductivity of Al layer in conjunction with theexcellent Si/Al contact through the vias. However, the thinSiO2 layer formed in the vias during the front contact firinghad to be etched by an in situ Argon plasma treatment priorto depositing Al in order to achieve excellent rear ohmicpoint contacts with a FF approaching 0.800. Laser energywas also found to be critical in achieving good pointcontacts. For example, low laser pulse energy is unableto fully open the vias through SiO2/SiNx passivation stack,which resulted in unacceptably high series resistance,while higher laser pulse energy induces damage to the n+

BSF surface, resulting in appreciable loss in Voc. Finally,it is shown that the pitch of the rear point contacts is alsocritical. Larger pitch leads to higher Voc because of reducedmetal-induced recombination, but the efficiency dropsbecause of a significant decrease in FF due to an increasein Rs. In-depth optimization of laser pulse energy, point

Figure 9. 20.7% efficient n-type passivated emitter, rear totallydiffused (PERT) cell (measured in-house by using the Fraunhofer-certified 20.2% efficient large area n-type cell [9] as a reference).

Figure 10. Comparison of internal quantum efficiency (IQE) andreflectance of the cell with rear contacts formed by screen-

printed Ag and laser opening plus PVD Al.



contact pitch, PVD Al deposition and anneal, and in situplasma etching of vias, resulted in 20.7% efficient largearea n-type front junction ion-implanted cells withscreen-printed front contacts.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Atul Gupta and otherR&D group members of Suniva Inc. for their support inion implantation. The authors would also like to thank allother group members at UCEP/GIT for their invaluablesupport and discussion.

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20.7% efficient ion-implanted large area n -type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition