SCREEN PRINTED N-TYPE SILICON SOLAR CELLS FOR INDUSTRIAL APPLICATION

Valentin D. Mihailetchi1, Johann Jourdan1, Alexander Edler1, Radovan Kopecek1, Rudolf Harney1,

Daniel Stichtenoth2, Jan Lossen2, Tim S. Böscke2, Hans-Joachim Krokoszinski2

1International Solar Energy Research Center (ISC), Rudolf-Diesel-Str. 15, D-78467 Konstanz, Germany,

Phone: +49 7531 3618348; Fax: +49 7531 3618311; Email: valentin.mihailetchi@isc-konstanz.de. 2Bosch Solar Energy AG, Wilhelm-Wolff-Str. 23, D-99099 Erfurt, Germany

ABSTRACT: The rapid fall in module prices demands researchers to come up with substantial efficiency

improvement and cost reductions in the solar cell and module processes. This paper presents a solar cell

development on n-type Cz-Si substrates with homogeneous diffusion of boron emitter and phosphorous back-

surface-field. The resulting solar cell is bifacial and it is fabricated using only industrial compatible techniques.

The best achieved solar cell efficiency, using a screen printed and firing through metallization, on 241 cm2 (total

area) wafers is 18.6%. Moreover, we present a new method to passivate boron emitters which substantially

reduces surface recombination resulting in improved VOC of the cells. We show that internal quantum efficiencies

of these n-type cells when illuminated from the front- and from the back side are identical when good quality Cz-

Si substrates are used in the fabrication.

1 INTRODUCTION

The rapid fall in module prices leads researchers to

intensify their research on efficiency improvement of

solar cells and to further reduce the costs of the

fabrication process. Additionally, new cell concepts are

currently preparing to enter into production. One of these

cell concepts that recently receive much attention is

based on n-type silicon substrates that feature a front

boron emitter, a rear phosphorous back-surface-field

(BSF), as well as front- and rear screen printed

metallization grid. Efficiencies up to 18.5% have been

reported on large area for such n-type solar cells

fabricated using industrial applicable techniques [1], thus

exceeding the typical efficiencies currently achieved by a

similar process based on p-type substrates.

N-type silicon has long been proven to have higher

tolerance to common transition metal impurities,

potentially resulting in higher minority carrier diffusion

lengths compared to p-type substrates [2,3]. Additionally,

the minority carrier lifetime does not suffer from light

induced degradation due to the Boron-Oxygen related

defect which is commonly present in p-type Cz-Si [4]. In

spite of these fundamental material advantages, n-type

cells comprising homogeneous front emitter and rear BSF

are not readily produced industrially due to insufficient

understanding and the lack of a cost-effective production

process allowing for high conversion efficiencies. It is

crucial to improve the boron diffusion to form the

emitter, passivation of boron emitters, and metallization.

This paper presents a solar cell process based on n-

type monocrystalline wafers with boron front emitter, and

low cost fabrication processes ready for industrial use

(such as screen printing and microwave plasma enhanced

chemical vapour deposition (PECVD) of hydrogenated

silicon nitride (SiNx)). Using this process we demonstrate

efficiencies of 18.6% on 156×156 mm2 monocrystalline

Cz-Si wafers. Additionally a new method to passivate

boron emitters is presented which results in an excellent

passivation quality. This new method is simpler and more

cost-effective compared to well-established methods,

such as those based on atomic layer deposition of Al2O3

[5] and thermally or chemically grown SiO2 layers [6-8].

We also show that internal quantum efficiencies of these

n-type cells when illuminated from the front- and from

the back side are identical, if the process conditions are

adjusted with the aim to obtain bifacial cells.

2 EXPERIMENTAL DESIGN

We developed our n-type process on 156×156 mm2

phosphorous-doped monocrystalline Czochralski (Cz) Si

wafers with base resistivity ranging between 1 and 10

Ωcm. The n-type Cz crystals are grown and sliced into

wafers at Bosch Solar Energy AG in Arnstadt, Germany.

The crystal growth and wafering is performed on

industrial equipment. Figure 1 shows a schematic cross-

section of our complete fabricated n-type solar cell. The

solar cell process comprises an alkaline texture on the

front side and a polished back side. On the front side

(light receiving side), a p+ emitter with a sheet resistance

of typical 60 Ω/square was diffused from a boron

tribromide (BBr3) source in a quartz tube furnace of an

industrial scale. The BSF on the back side is formed in a

separate diffusion step by diffusing phosphorous in a

similar quartz-tube furnace using a POCl3 source. The

passivation of the BSF diffusion region is achieved by a

PECVD SiNx deposition while for the front boron emitter

the passivation is achieved using an in-house developed

method consisting in a passivating layer and a SiNx anti-

reflection coating stack. The fabrication process includes

also the necessary cleaning steps using baths containing

HCl, HF, H2O2/H2SO4, and rising water. The

metallization on the front and back side was applied by

screen printing

Figure 1: Schematic cross-section of the fabricated n-

type solar cells.

and firing through of metal pastes using an H-pattern

screen design. The cell area is 241 cm2 which equals to

the area of our pseudo squared 156×156 mm2

monocrystalline wafer with a diagonal of 205 mm. Hence

no edge isolation by snapping the edges or similar tricks

was applied.

3 RESULTS AND DISCUSSION

3.1 Passivation of boron emitters

One of the drawbacks in the development of an

industrially compatible n-type cell process is the

passivation of the boron emitter. As PECVD-SiNx, which

is commonly used to passivate phosphorous emitters in

the standard p-type process, does not passivate boron

emitters, a considerable amount of effort has been put to

find new methods to passivate boron emitters. Currently

the most successful methods used are stacks in which the

top layer is SiNx used as antireflection coating layer and

as hydrogen source for the passivating layer(s)

underneath. Among the passivation layers used in

combination with SiNx are: a thin atomic layer deposition

of Al2O3 [5], amorphous silicon deposition [9], thermally

grown SiO2 [6,7], chemically grown SiO2 [8]. However,

some of these methods are quite costly, other yield only

layers of limited quality. In our quest to continuously

improve the passivation quality of boron emitter and to

reduce the costs, we have developed a new method for

passivation. The method consists of a thin passivating

layer and a SiNx anti-reflection coating stack. Details

about the properties of the passivating layer and its

application method will be the subject of a later

publication. Herein we have designed an experiment to

test the passivation quality of our new method compared

with other known methods used to passivate industrial-

type boron diffused emitters. The test structure consists

of a polished n-type monocrystalline Cz substrate

symmetrically diffused with 60 Ω/square boron emitter

on both sides. After the necessary cleaning steps the

passivation layer(s) are applied on p+ diffused regions.

The samples were subsequently subjected to a firing step

followed by a QSSPC measurement. From this the

effective recombination lifetime τeff , the implied VOC at 1

sun intensity, and the emitter saturation current density

JoE at high injection level (∆n=2×1016 cm-3) were

extracted. A schematic cross-section of such a test

structure is shown in figure 2.

Figure 3 compares the τeff and JoE of our passivation

method against several other passivation methods that

includes a wet thermal SiO2/SiNx stack, a chemical

SiO2/SiNx stack (NAOS method), and just a SiNx layer.

The results in figure 3 clearly demonstrate the excellent

passivation potential of our developed method, with τeff

=517 µs, implied VOC=683 mV, and JoE = 14 fA/cm2 per

side.

Figure 2: Schematic cross-section of the fabricated test

structures to investigate the passivation of boron emitters.

Figure 3: Effective minority carrier lifetime τeff measured

on a p+np+ test structure at 1 sun illumination and emitter

saturation current density JoE measured at an injection

level of 2×1016 cm-3 as a function of different methods to

passivate the p+ (boron emitter) region. The passivation

methods tested in this study are: a silicon nitride layer

(SiNx), a stack of chemically grown SiO2 in a nitric acid

solution (NAOS) and SiNx, a stack of wet thermally

grown SiO2 (20 nm) in a quartz tube furnace and SiNx,

and our new developed method consisting in a stack of a

passivating layer and SiNx.

These results are comparable to the best achieved

passivation of p+ surfaces up to now, namely the atomic

layer deposition of Al2O3 and SiNx stack [5]. Although

thermal SiO2/SiNx stack produce very good passivation

as well, as can be seen from the results of figure 3, it

requires, however, an extra high temperature step in the

solar cell process.

3.1 Solar cell results and discussion

The excellent passivation results obtained on boron

emitters were then applied to fabricate n-type cells using

the process described above and schematically shown in

figure 1. On the front boron emitter we have applied our

passivation method while on the phosphorous BSF we

deposited the commonly used SiNx passivating layer. The

metallization was applied on both sides of the cells by

screen printing metal pastes. The cells went through a

firing step to complete the contact formation. Table 1

shows the solar cell parameters of our best fabricated cell

and a solar cell with the best bifacial efficiencies ratio.

The best achieved efficiency is 18.6%, and it is certified

by Fraunhofer ISE CalLab in Germany.

It should be noted that all solar cells that we have

processed so far have a polished rear side and a random

pyramid texture on the front side. This results in different

short-circuit current densities (JSC) under front and back

side illumination. However, this effect could be

eliminated by comparing the internal quantum

efficiencies (IQE) measured under front and back side

illumination conditions.

Table I: Comparison of the best solar cell parameters measured under standard test conditions (AM1.5G, 100 mW/cm2, 25

oC). The solar cells were fabricated on 241 cm2 monocrystalline n-type Cz Si with base resistivity of 8 Ωcm. The result for

the best efficiency of a bifacial fabricated cell is also shown. The implied VOC value was measured using QSSPC on a similar

cell but without the metal contacts on front and back side.

Substrate,

illumination

Surface

texture

Jsc

[mA/cm2]

Voc

[mV]

implied Voc

[mV]

FF

[%]

pseudo-FF

[%]

ηηηη [%]

Cz, best, front Rand. pyramids 38.3* 637* 664 76.5* 82.5 18.6*

Cz, front Rand. pyramids 38.5 634 75.2 18.3

Cz, back alkaline polished 34.2 631 75.9 16.4

*values measured by ISE CalLab.

Figure 4 shows the IQE measurement of our best cell

(black circles) illuminated from the front side. The

response observed at short wavelengths (below 0.5 µm)

confirms the excellent passivation of boron emitter (60

Ω/square) demonstrated in figure 3. Moreover, figure 4

also shows the IQE data of an optimized bifacial cell,

which have different BSF sheet resistance and

metallization grid as compared with cell optimized only

for the front side. The IQE results of the bifacial cell

show that independent from which side the cell is

illuminated the response of the cell is the same. This

demonstrates on one hand the outstanding passivation of

the diffused surfaces and on the other hand the very high

lifetime of the material, which obviously does not suffer

substantially during the process. A symmetrical IQE from

both sides is an excellent characteristic because it allows

us to fabricate bifacial cells with the same power

conversion efficiencies from the front- or back side

illumination, providing that the back surface is also

textured. Based on the experimental data shown in table

1 and figure 4 the major difference between front side

and back side performance of the cells is in the JSC.

Therefore work is under way to adapt our cell process to

allow for both sides to be textured in order to match the

JSC.

An advantage of realizing cells with equal

performance from the front- or back side (fully bifacial)

can be taken in the module interconnection. If such a

performance match is desired for certain applications,

then equal performance match will allow for the

introduction and fabrication of planar interconnected n-

type cells in a module. A schematic representation of our

proposed Planar Interconnected N-type Cells (PINC) in a

module is shown in figure 5.

Figure 4: Internal quantum efficiency (IQE) of our best

fabricated cell (confirmed by ISE CalLab), and of a

bifacial cell illuminated from the front- and back side.

Figure 5: Schematic representation of our proposed

Planar Interconnected N-type Cells (PINC) in a module

of our fully bifacial cells. In the PINC module the boron

emitter side of a cell is planar connected with the BSF

side of an adjacent cell.

4 CONCLUSSION

We have presented a simple industrial process to

fabricate n-type solar cells with boron front emitter and

phosphorous back-surface-field which leads to an

efficiency of 18.6% for large area (241 cm2)

monocrystalline Cz-Si substrate. We demonstrated that

this cell process results in identical internal quantum

efficiencies when a cell is illuminated from the emitter

side or from the back-surface-field side. This feature

allows for the fabrication of fully bifacial cells, with

identical efficiencies from both sides, and to introduce an

innovative cells interconnection in the module.

Additionally, we introduce a new method to passivate

boron emitters for industrial application with excellent

passivation quality.

ACKNOWLEDGEMENTS

This work is financially supported by German

government (BMU) within EnSol project, contract

number 0325120A.

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