Letter

Ion-implanted and screen-printed large area 20% efficient N-type frontjunction Si solar cells

Young-Woo Ok a,n, Ajay D. Upadhyaya a, Yuguo Tao a, Francesco Zimbardi a, Kyungsun Ryu a,Moon-Hee Kang a, Ajeet Rohatgi a,b

a Georgia Institute of Technology, 777 Atlantic Drive, Atlanta, GA 30332-0250, USAb Suniva Inc., 5765 Peachtree Industrial Blvd., Norcross, GA 30092, USA

a r t i c l e i n f o

Article history:Received 5 August 2013Received in revised form20 November 2013Accepted 2 January 2014Available online 30 January 2014

Keywords:N-type Si Solar cellImplantationAl2O3 passivationScreen printing

a b s t r a c t

This paper reports on the fabrication of high efficiency (�20%) front junction n-type Si solar cells on239 cm2 Cz using ion implanted boron emitter and phosphorus back surface field (BSF) in combinationwith screen printed metallization. Cell efficiencies of 19.8% and 20.0% were achieved with SiO2/SiNx andAl2O3/SiNx passivation of boron implanted emitter, respectively, supporting the superiority of Al2O3

passivation. This is consistent with low boron emitter saturation current densities of 76 and 45 fA/cm2

achieved for boron emitter passivated with SiO2/SiNx and Al2O3/SiNx stacks, respectively. Saturationcurrent density in metal contact area of boron emitter and phosphorus BSF was measured directly byvarying the metal contact coverage. Detailed analysis of saturation current density showed that theperformance of our 20% is largely limited by saturation current density associated with recombination onmetal contact area of boron emitter and bulk of phosphorus BSF, which accounted for almost �50% ofthe total saturation current density.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

N-type Si solar cells are a strong candidate for high and stableefficiency because, unlike the counterpart p-base cells, they havemuch higher bulk lifetime and do not suffer from light induceddegradation (LID) associated with the formation of boron–oxygencomplex [1]. LID is known to reduce the efficiency of p-type cellsin the range of 0.2–0.7% (absolute) under illumination for couple ofdays. In addition, the LID increases in higher efficiency advancedcell structures with good surface passivation [2]. Finally, n-type Sihas higher tolerance for common metal impurities and defects dueto lower hole capture cross section [3,4].

In spite of many advantages of n-type Si, there are challenges inattaining low-cost and high-efficiency front junction n-type com-mercial ready Si solar cells because it requires boron (B) diffusionon one side and phosphorus (P) on the other. In addition, it isimportant to find a simple process for the formation of uniformB emitter without the formation of B rich layer (BRL) [5]. Con-ventional diffusion using the BBR3 source involves hazardouschemical, leads to the formation of BRL, and requires an extramasking step to form single side B emitter [6,7]. Simple andlow-cost B sources such as spin-on solution or ink-jet printed

paste often lead to wrap around diffusion and are sensitive tosource contamination and formation of thick BRL layer [5,8,9].Besides diffusion, the passivation of the B emitter presents anotherchallenge to achieving high efficiency cells. Recently, Al2O3 withlarge negative charge has been found to be very effective inpassivating the B emitter and is also stable after high temperaturecontact firing step. Several small area high efficiency cells havebeen reported with Al2O3 passivated B emitter [10–12]. However,very little has been done on production of large area (239 cm2)commercial scale screen printed n-type cells with Al2O3 passivatedB emitter. Recently, high throughput tools for plasma enhanceddeposition (PED) and atomic layer deposition (ALD) of Al2O3 havebeen developed which should help the mass production of n-typefront junction solar cells in the near future.

In this work, we report on the development of 239 cm2 largearea high efficiency n-type Cz Si solar cells using ion implantationof B emitter and P back surface filed (BSF) in conjunction withscreen printed contacts. Ion implantation simplifies the fabricationof advanced solar cells by allowing high quality, uniform andsingle side diffusion without the formation of glass and the needfor masking and junction edge isolation. These above advantagescan also simplify the fabrication of cell structures such as IBC andlocal BSF [13,14]. This paper reports on the successful fabricationof screen printed 20% efficient 239 cm2 n-type Cz cells with ionimplanted B emitter in conjunction with Al2O3/SiNx stack passiva-tion. A detailed cell analysis is presented to quantify surface

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Solar Energy Materials & Solar Cells

0927-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.solmat.2014.01.002

n Corresponding author.E-mail address: yok6@mail.gateh.edu (Y.-W. Ok).

Solar Energy Materials & Solar Cells 123 (2014) 92–96

recombination velocities and saturation current densities of theAl2O3/SiNx passivated cells and its comparison with the counter-part SiO2/SiNx passivated devices.

2. Experiment

Large-area (239 cm2) front junction cells [Fig. 1] were fabri-cated on 1–4Ω cm n-type Cz wafers with B emitter with screenprinted Ag/Al grid and screen printed local Ag contacts to P BSFconnected with full area metal. After standard saw damageetching, alkaline texturing and RCA clean, all samples wereimplanted with high B dose (�3e15/ cm2) and annealed at a hightemperature (4950 1C) in N2 ambient in a tube furnace to obtain�60 Ω/□ sheet resistance (4�1019 cm�3 surface doping concen-tration, 1 μm junction depth). This was followed by a short HFtreatment and chemical etching to remove B rich emitter surfacelayer using a mixture of HNO3, CH3COOH and HF, which signifi-cantly reduced the saturation current density of B emitter [15]. Thesheet resistance of B emitter increased from 60 to 70 Ω/□ after thechemical etching, but the pyramid shape of textured Si was notaltered, as confirmed by secondary electron microscopy (SEM).After the chemical etching, the samples received P implantation onthe rear side followed by a low temperature (o850 1C) annealresulted in �10 nm oxide on the B emitter and �70 Ω/□BSFcapped with �40 nm oxide capping layer. (P BSF—1�1020 cm�3

surface doping concentration, 0.5 μm junction depth). Afterannealing, the samples were divided into two groups. PECVD SiNx

was deposited on both sides of samples in group one to make SiO2/SiNx passivated B emitter cells. In the second group, a PECVD SiNx

was deposited only on the P BSF followed by etching the oxide onthe B emitter. After that, 80 Å Al2O3 was deposited on the Bemitter by plasma ALD and capped with SiNx to make Al2O3/SiNx

passivated B emitter cells. Thus both the groups had identical SiO2/SiNx passivation on the BSF, but different passivation schemes onthe B emitter. A conventional grid pattern with 74 lines was

screen-printed on the front using an Ag/Al paste. Point contactswith metal coverage of �4% were screen printed on the rear sideusing Ag paste. All samples were fired at a peak temperature of750 1C on a belt-furnace. Finally, a metal paste was screen-printedon the rear to connect the point contacts and followed by a lowtemperature curing.

For the direct measurement of saturation current density of thepassivated B emitter (J0e), SiNx/SiO2/pþ/n/pþ/SiO2/SiNx symmetrictest structures were prepared on textured Si surface. B emitter(pþ) was formed using the same process used in cell fabrication.B emitter was passivated with thermally grown SiO2 capped withPECVD SiNx. In selected cases, �80 Å Al2O3 was deposited on top ofthe B emitter using plasma ALD after removing thermal SiO2, followedby formation of SiNx/Al2O3/pþ/n/pþ/Al2O3/SiNx symmetric test struc-ture on textured Si surface for J0e analysis of Al2O3/SiNx passivated Bemitter. In addition, SiNx/SiO2/nþ/n/nþ/SiO2/SiNx symmetric teststructures were prepared by phosphorus implantation, oxidationand SiNx deposition to study the saturation current density of SiO2/SiNx passivated P BSF (J0bʹ). To study the impact of metallization on J0eand J0bʹ, metal dots with different coverage were screen printed onboth sides of the symmetric test structures. The metal contactcoverage was varied by using screens with different pitch while thevia opening was kept constant. All samples with and without metaldots were fired in a belt furnace at the same peak temperature usedfor cell fabrication. The quasi-steady state photoconductivity (QSSPC)tool was used to measure the J0e and J0bʹ [16] The metal dots wereremoved by HCl treatment before measurements. This was done tomaintain the accuracy of QSSPC measurement which can be affectedby the presence of metal. We also validated that, due to the very smallmetal coverage (0–10%), measured J0 values were in good agreementbefore and after the removal of metal dots.

3. Results and discussions

Table 1 shows the average and best efficiency of the cellsfabricated with SiO2/SiNx and Al2O3/SiNx passivation of the B emitter.The cell with the SiO2/SiNx passivation achieved 19.8% efficiency witha Voc of 647 mV, Jsc of 38.78 mA/cm2 and FF of 0.789, which is higherthan our previous report [14]. B emitter passivated with Al2O3/SiNx

gave a best efficiency of 20.0% with Voc of 655 mV, Jsc of 39.2 mA/cm2

and FF of 0.779, certified by Fraunhofer ISE (Fig. 2). This representsone of the highest-efficiency 239 cm2 n-type front junction cell withimplanted B emitter with Al2O3/SiNx passivation and screen printedcontacts. It is noteworthy that the cell with Al2O3/SiNx passivated Bemitter gave �8 mV higher Voc than the SiO2/SiNx passivated Bemitter cell. This indicates that Al2O3 layer with negative chargedensity provides much better passivation of B emitter than the SiO2

layer with positive charge density. This is supported by the IQEmeasurements in Fig. 3, which show a comparison of the SiO2/SiNx

and Al2O3/SiNx passivated cells. It is clear that Al2O3 passivation gavesuperior short wavelength response in the range of 300–600 nm,

Fig. 1. Schematic structure of n-type solar cells with B emitter passivated P BSFwith local contacts and full metal back.

Table 1Average and best cell efficiency from implanted B emitter passivated by SiO2/SiNx and Al2O3/SiNx.

Voc (mV) Jsc (mA/cm2) FF (%) n (%) Rs (ohm cm2) Rshunt (ohm cm2) n factor

SiO2/SiNx passivation on B emitterAverage efficiency(#10 cells) 64872 38.5670.22 78.970.4 19.770.1Best efficiency 647 38.78 78.9 19.8 0.80 7860 1.03

Al2O3/SiNx passivation on B emitterAverage efficiency (6 cells) 65474 38.7570.45 78.171.0 19.870.2Best efficiency 655 39.2 77.9 20.0n 0.88 5790 1.12

n Independently confirmed by Fraunhofer ISE.

Y.-W. Ok et al. / Solar Energy Materials & Solar Cells 123 (2014) 92–96 93

which has resulted in Jsc and Voc improvement. However, the cell withAl2O3/SiNx passivated B emitter showed lower fill factor (�77.9%)than the SiO2/SiNx passivated cell (�78.9%) due to slightly higher Rsand n factor. In order to further quantify the difference, J0e measure-ments were performed on symmetric structures fabricated during thecell processing.

Fig. 4 shows the trend in J0e for SiO2/SiNx and Al2O3/SiNx

passivated B emitter on textured Si during the process. Fig. 4a andb shows that the J0e values are very high (�400 fA/cm2) after theoxidation of B emitter. J0e value decreases a little (�300 fA/cm2) afterSiNx deposition but decreases rapidly from 295 to 76 fA/cm2 aftersimulated firing as shown in Fig. 4a. This is attributed to SiNx-inducedhydrogen passivation of the SiO2/Si interface during the firing cycle.However, when the implanted B emitter is passivated with Al2O3/SiNx, after removing the thermally grown oxide, J0e value decreasessharply from 400 to 53 fA/cm2. This is attributed to field inducedpassivation of Al2O3/SiNx interface due to the large negative chargedensity in Al2O3. It is well known that 450 1C anneal during SiNx

deposition creates negative charge in Al2O3 [10–12]. In addition,Al2O3/SiNx passivation was found to be stable after firing since theJ0e value essentially remained unchanged at 45 fA/cm2. Thus, bothSiO2/SiNx and Al2O3/SiNx passivation of chemically etched B emittergive high efficiency because of the low J0e values of 76 and 45 fA/cm2,prior to metallization, which correspond to an implied Voc of

approximately 670 and 685 mV, respectively. Since the final cell Vocis also affected by the metal recombination besides the bulk andinterface recombinations, we investigated the sensitivity of saturationcurrent density of B emitter and P BSF to metal coverage.

Fig. 5 shows the change in J0e and J0bʹ as a function of metalcontact coverage. This was determined by QSSPC measurementson the SiNx/Al2O3/pþ/n/pþ/Al2O3/SiNx and SiNx/SiO2/nþ/n/nþ/SiO2/SiNx symmetric test structure on the textured Si after firing.Screen printed and fired metal dots were removed using an HClsolution to expose the non-passivated Si surface with highrecombination. In this experiment, prior to metal deposition J0ewas �50 fA/cm2 and J0bʹ of the P BSF was �85 fA/cm2. Fig. 5ashows that the J0e gradually increased from �50 to �140 fA/cm2

when the metal coverage was increased to from 0% to 7%.Similarly, J0bʹ increased from 85 fA/cm2 to 180 fA/cm2 for the sameincrease in the metal coverage (Fig. 5(b)). Slopes in Fig. 5 reveal anincrease in J0e of 12.2 fA/cm2 and J0bʹ of 14.2 fA/cm2 for every 1%increase in metal contact coverage. This indicates that our P BSF ismore sensitive to metal recombination than the B emitter. Also,from the liner equation in Fig. 5a and b, metal induced emitter andBSF saturation current density were extracted: J0e, metal¼1269 andJ0bʹ, metal¼1505 fA/cm2 when metal coverage x¼100%.

For an in-depth understanding and characterization of 20%efficient Al2O3/SiNx passivated B emitter cell, total saturation currentdensity (J0, total) was calculated and subdivided into its five compo-nents. Since Voc–VTn ln(Jsc/J0, total), J0, total¼330 using the measuredJsc¼39.2 mA/cm2 and Voc¼655mV and VT¼25.7 mV at room tem-perature. Total J0 (J0, total) can be written as

J0; total ¼ J0eð J0e; metal f 1þ J0e; passð1� f 1ÞÞ

Fig. 2. I–V characteristics of the 20.0% n-type Si solar cells with SiNx/Al2O3

passivated B emitter verified by Fraunhofer.

Fig. 3. IQE and reflectance data of the n-type Si solar cells with SiNx/SiO2 and SiNx/Al2O3 passivated chemically etched B emitter.

Fig. 4. Measured J0e trend during cell processing from SiNx/SiO2/pþnpþ/SiO2/SiNx

and SiNx/Al2O3/pþnpþ/Al2O3/SiNx symmetric structure on textured high resistivityn-type Si.

Y.-W. Ok et al. / Solar Energy Materials & Solar Cells 123 (2014) 92–9694

þ J0bð J0nþ J0b0 ; metal f 2þ J0b0 ; pass ð1� f 2ÞÞ ð1Þ

where f1 (0.075) and f2 (0.04) are emitter and BSF metallizationfractions, respectively and J0, total is the sum of five recombinationcomponents.

Contribution from front metal contact region ( J0e, metal�f1¼1269�0.075¼95 fA/cm2).

a) Contribution from front passivated emitter region ( J0e, pass�(1� f1)¼45� (1�0.075)¼42 fA/cm2).

b) Contribution from back metal contact region ( J0bʹ, metal�f2¼1505�0.04¼60 fA/cm2).

c) Contribution from back passivated BSF region ( J0bʹ, pass�(1� f2)¼85� (1�0.04)¼82 fA/cm2).

d) Contribution from bulk wafer ( J0n¼ J0, total�(aþbþcþd)¼330�(95þ42þ60þ82)¼51 fA/cm2).

From the above analysis, J0e, metal (95 fA/cm2) and J0bʹ, pass(82 fA/cm2) are the major contributors to total J0. This indicatesthat the performance of our 20% is largely limited by the satura-tion current densities associated with metal contact to B emitterand the passivated P BSF region, which account for almost �50%of the total saturation current density.

Table 2 shows the PC 1D simulated parameters and five recombi-nation components of J0, total for the B emitter passivated with SiO2/SiNx and Al2O3/SiNx stack layer. PC 1D simulations showed a goodagreement between the measured and simulated efficiency and cellparameters. Matching the short wavelength response with the PC 1Dmodel gave an effective FSRV values of 6000 and 3000 cm/s for theSiO2/SiNx and Al2O3/SiNx passivated B emitters, respectively, in combi-nation with a BSRV of 25 cm/s at the nþ/n interface. PC 1D simula-tions indicate that the cell efficiency of n-base front junction cell canreach �21.5% if the BSRV at the nþ/n interface can be reduced to5 cm/s from 25 cm/s in these cells, in conjunction with 1 ms bulklifetime and �5% front metal coverage. In addition, in the 20% cellwith Al2O3/SiNx passivation, the cell Voc was found to is dictated by J0bsince J0b (�193 fA/cm2) is much greater than J0e (�137 fA/cm2). Thefive J0 components in the table reveal that J0e, metal (95 fA/cm2),recombination from front metal contact, and J0b ,́ pass (82 fA/cm2),recombination from P BSF, are the main contributors to total J0.

Fig. 5. Measured J0e and J0bʹ as a function of metal contact coverage obtained fromSiNx/Al2O3/pþnpþ/Al2O3/SiNx and SiNx/SiO2/nþnnþ/SiO2/SiNx symmetric teststructure on textured high resistivity n-type Si after firing.

Table 2PC 1D simulated parameters and five components of J0, total for the B emitter passivated with SiO2/SiNx and Al2O3/SiNx stack layers and 21.5% efficient cell.

Cell parameters SiO2/SiNx passivation Al2O3/SiNx passivation For 21.5%

Wafer thickness (mm) 200 200 200Base resistivity (Ω cm) �2 �2 �1RSERIES (Ω cm2) 0.85 0.9 0.5RSHUNT (Ω cm2) 7860 5790 5790Metal contact coverage Front/Back �7.5%/�4% �7.5%/�4% �5%/�1%Emitter sheet resistance (Ω/sq) 85 85 85tbulk (ms) �500 �550 1000BSRV (cm/s) 25 25 5FSRV (cm/s) 6000 3000 3000Rback (%) 93 95 95Modeled Voc (mV) 650 655 670Modeled Jsc (mA/cm2) 38.8 39.1 39.5Modeled FF (%) 78.6 78.1 81.2Modeled efficiency (%) 19.8 20.0 21.5J0e (fA/cm2)

J0e, pass 70 42 43J0e, metal 95 95 63

J0b (fA/cm2)J0bʹ, pass 82 82 20J0bʹ, metal 60 60 15J0n �70 �50 �40

Total J0 (fA/cm2) �380 �330 �185

Y.-W. Ok et al. / Solar Energy Materials & Solar Cells 123 (2014) 92–96 95

J0e, metal can be reduced by fine line printing or by selective B emitterwhile J0bʹ, pass can be reduced by replacing the full area BSF with localBSF or by lowering the doping concentration in the P BSF withoutcompromising the contact quality.

4. Conclusions

We reported that large area screen printed n-type front junc-tion cell efficiencies of 19.8% and 20.0% were achieved with SiO2/SiNx and Al2O3/SiNx passivation of a chemically etched B emitterformed by ion implantation. It was shown that very low emittersaturation current density (J0e) of 76 and 45 fA/cm2 can beachieved by a chemically etched B emitter with SiO2/SiNx andAl2O3/SiNx passivation, respectively. Our cell analysis showed thatthe main limitation of the 20% cell performance is the largesaturation current density contribution from front metal gridcontact area (J0e, metal) and the P BSF bulk (J0bʹ, pass)

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