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Contract No. DE-AC36-99-GO10337 

Q. Wang, M. Page, Y. Yan, and T. Wang 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37439 

High-Throughput Approaches to 

Optimization of Crystal Silicon 

Surface Passivation and 

Heterojunction Solar Cells 
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HIGH-THROUGHPUT APPROACHES TO OPTIMIZATION OF CRYSTAL SILICON 

SURFACE PASSIVATION AND HETEROJUNCTION SOLAR CELLS 

Qi Wanga

, Matt Page, Yanfa Yan, and Tihu Wang 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA a

 contact information: qi_wang@nrel.gov 

ABSTRACT

We use a high-throughput (combinatorial) hot-wire

chemical vapor deposition system to passivate the crystal

silicon surface and to grow heterojunction silicon solar

cells. We study the effectiveness of crystal surface

treatments by atomic H or/and NHx radicals, followed by

the growth of thin hydrogenated amorphous silicon

(a-Si:H) films. Treatment and layer properties such as

times, thicknesses and gas mixtures can be continuously

graded, creating a two-dimensional sample with each

variable varying in one direction. This results in highthroughput optimization of the processes. Effective carrier

lifetime is measured by photoconductive decay to evaluate

the effectiveness of the surface passivation by surface

treatments. The effective carrier lifetime increases from

about 5 μs without passivation to about 24 μs with an

optimized surface treatment and thickness a-Si:H on

single-sided c-Si. Transmission electron microscopy

reveals that a-Si:H, a mixed phase, or epitaxial growth of

thin-film Si depending upon the surface treatment. 

Improvement in effective carrier lifetime correlates to with

an immediate a-Si:H growth on c-Si, rather than a mixed

phase and epitaxial Si growth. We have obtained an

efficiency of 13.4% on a non-textured single-sided

heterojunction solar cell on a p-type CZ-Si processed with

optimized surface treatment. 

INTRODUCTION 

Surface passivation of c-Si is a critical process for

many applications, especially heterojunction solar cells.

The surface states often act as recombination centers for

the charge carriers. The reduction of excess carrier loss at

the c-Si surface, which is the part of heterojunction of an

a-Si/c-Si solar cell, is one of the key issues to improve the

device performances [1]. Hydrogenated amorphous

silicon (a-Si:H) has been widely used for thin-film 

photovoltaic [2] and many other applications. It has 

unique properties of high H content and higher bandgap

than c-Si. Therefore, it is a promising candidate for surface

passivation to c-Si than other traditional dielectrics such as

SiO2 and Si3N4. Sanyo’s HIT structure cell [3] proves that

a thin a-Si:H layer not only effectively passivates the c-Si

surface but also can be part of junction layer. However,

direct deposition of a-Si:H on a hydrophobic c-Si surface 

often leads to a mixed phase or epitaxial Si growth with

some degree of structure defects, especially by hot-wire

chemical vapor deposition (HWCVD) [4]. This epi-Si-like 

growth has prevented us from achieving a better surface 

passivation and heterojunction. A high-throughput 

(combinatorial) hot-wire chemical vapor deposition system

 [5] is an ideal tool to study thin-film Si growth on c-Si. We

believe that surface treatments before a-Si:H deposition

can lead to a sharp transition from c-Si to a-Si:H and a

better heterojunction for solar cell. In this paper, we will

demonstrate how we use this powerful combinatorial tool

to screen many surface treatments and successfully lead 

to the sharp transition, which improves the effective carrier

lifetime and solar cell performance. 

Motor

Motorized shutter

Mask

Wedge Film

Substrate

Hot filament

SiH3

SiH3

Si H

Figure 1. Schematic of high-throughput HWCVD of

making thickness grading film with a mask and a 

motorized shutter which is very close to the surface of 

the substrate. 

EXPERIMENTAL

A final 5% HF dipped clean c-Si substrate is loaded

into a high-throughput HWCVD system for surface

treatments, thin-film Si growth, or junction depositions.

Figure 1 shows the schematics of the high-throughput

HWCVD system and the mechanism of making thicknessgraded film. The detail description of the system has been

published elsewhere [5]. The substrate temperature is set 

at 250°C for surface passivation study and 200°C for

heterojunction solar cells. In HWCVD [6], a hot tungsten

filament (~2050°C) acts as a thermal source to

decompose the process gases. We use, as an example,

H2 for the surface treatment and pure SiH4 for the thin-film 

Si layer. When H2 passes through the hot filament, it

decomposes into H atoms. Therefore, the effects of

chemical treatment by atomic H on the surface of c-Si can

be rapidly examined using a physical mask. When SiH4

flows through the hot filament, it decomposes into Si and

H atoms. These atoms then collide with SiH4 in the 

chamber to form SiH3, and other radicals to grow an 

1 
Figure 2. Schematic of high-throughput approach

method 1: a direct comparison of two variables and

the effect of a thickness graded film. 

0

2

4

6

8

10

12

14

0 200 400 600

a-Si:H thickness () 

A

B

Thickness graded Si

a

:H

a-Si:H film. We used 20 sccm of SiH4 and at 9 mTorr. To

make thickness graded a-Si:H film, a moving shutter which

is very close to the surface of the substrate was added to

the system. The variation of the thickness can be

controlled by the speed of shutter, the deposition rate, and

deposition time. 

AMORPHOUS SI SURFACE PASSIVATION 

We show here two high-throughput approaches to

study the effects of surface treatments and thickness

variations on crystal silicon surface passivation. The first 

approach (method 1) is to apply a surface treatment such

as atomic H labeled as in A and no treatment as in B to

the part of the c-Si substrate using a mask for a fixed time

and then deposit a thickness-graded a-Si:H on the entire

sample. Figure 2 shows the schematic of method 1 

approach. In practice, it is very useful to directly compare

two different surface treatments because the treatments

are done on the same c-Si and most our daily task is to try 

to find the difference between two variables. Actually the

high-throughput system is able to compare up to ten

variables on a single substrate. This is achieved by using

a set of physical masks inside the chamber and a 

substrate manipulator. This approach is a very efficient 

tool for screening. 

Figure 3 shows the photoconductive decay lifetime 

 [7] as a function of two surface treatments and a-Si:H film

thickness. The thickness is calculated from deposition rate

on glass substrate times the deposition time. In this 

sample, the half of the c-Si substrate is exposed to the

atomic H for 3 min with a H2 flow rate of 70 sccm and 25

mTorr. A physical mask with 25 x 50 mm opening is used. 

Then, we switch to a 50 x 50 mm opening mask, and grow

a thickness graded a-Si:H. The effective minority carrier

lifetime is probed on the upper region A in figure 2 with

atomic H treatment (open squares in figure 3) and the

bottom region B in figure 2 without the treatment (solid

squares in figure 3) at various locations of different a-Si:H

thicknesses. The a-Si:H thickness varies from 55 to 600 Å.

Figure 3. Effects of the surface treatments with

(squares) and without (diamonds) atomic H on the

effective carrier lifetime as a function of a-Si:H

thickness (varied from 5 to 60 nm). 

The experiment data shows an interesting result that the

effective lifetime in general is higher without the atomic H

treatment in comparison to the one with the treatment. We

will explain this finding later. With increasing a-Si:H

thickness, the changes in lifetime is between 6.5 and 9 μs

with H treatment. However, there is an increase of lifetime

from 7 to 13 μs with increasing a-Si:H thickness without 

the atomic H treatment. The thickness at the highest

lifetime is about 400 Å. The lifetime on the bare c-Si wafer 

without any surface treatment and a-Si:H is about 6 μs.

This value serves as our baseline of a non-passivated

surface. 

Our second approach (method 2) is to apply a

continuously varied time of a surface treatment such as

atomic NH3 + H2 to the sample, rotate the sample 90°,

and then deposit a continuously thickness-graded a-Si:H

on the entire sample. This approach creates a threedimensional plot, two variables and one result, on a single

substrate and is a very powerful tool for detail process

optimization. Figure 4 shows the schematics of the

method 2. The continuous variation is achieved by a

motorized shutter, which is placed very close to the

substrate surface in the deposition system. The heated

substrate can be xyz positioned and rotate 360° relative to 

the mask. 

Figure 5 shows the effective lifetime as a function of

time in our best surface treatment with H2 + NH3 before aSi:H deposition in the x-axis and a-Si:H thickness in the yaxis. The numbers inside the matrix represent lifetimes in

μs and only selective locations on the substrate were

measured. In method 2, both variables are continuously

varied as described in the Figure 4. In Fig. 5, the thinnest

a-Si:H layer is along the bottom edge. For a short surface

treatments time (5 s), the measured lifetime shows an

increases and then slightly decrease (the left most

column) as increase of a-Si:H thickness. This result

shows a similar trend as in Figure 3 that certain thickness

of a-Si:H is needed to enhance lifetime. However, the 

2 
substrate

 2-D grading

Variable A

Variable B

Treatment time (s)

a-Si:H thickness ()

Effective carrier lifetime ( μs)

0 6 6 6

550 18 20 18 23 14

550 20 23 23 23 23

495 22 22

440 22 24 23 23 22 24

385 22 23

330 22 24 23 24 23 22

275 21 23 22 22

220 19 24 23 22 21 20

165 18 21 21 21 21

110 17 18 18 20 21 6 6

55 14 13 14 14

5 10 15 20 25 30 35 40 45 50 55 60

 

Figure 4. Schematic of high-throughput approach 

method 2. A three dimensional plot on a single 

substrate: two variables and a result. 

lifetime does not increase appreciably for a constantly

small a-Si:H thickness of 55 Å. This result points out an

important fact that the surface treatment alone is not 

adequate to passivate the interface. With the help of an aSi:H layer, lifetime after surface treatment with H2 + NH3 is

much higher than the results in Figure 3. Lifetime reached

24 μs with H2 + NH3 treatment compared to 13 μs in the

sample without atomic H treatment with 400 Å a-Si:H 

thicknesses. However, after closer examinations of the

results in Figure 5, we realize the significance of the

surface treatment in which one can achieve a high lifetime

in the c-Si with a much thinner a-Si:H. In Figure 3, the 

optimum thickness for a-Si:H is about 400 Å without a

surface treatment. However, in Figure 5, we only need

about 100 Å with a surface treatment of H2 + NH3. 

Hydrogenated amorphous silicon films content, in

general, about 10 at.% hydrogen and has a mobility

bandgap of 1.8 eV which is higher than 1.1 eV of c-Si.

When a-Si:H are deposited on c-Si as surface passivation

layers, it, in principle, forms a heterojunction because of 

the difference in bandgap just like other dielectrics such as

SiO2 and Si3N4 but at much lower temperature. The

electric field at the interface pushes back the free carriers 

back into the bulk of Si and prevents the excess carrier

loss. The H will help to passivate Si dangling bonds at the

surface to reduce the defects that act as a recombination

center for the carriers. The H inside a-Si:H can move to

the interface to further improve the surface passivation as

other advantages over traditional dielectrics. We examine

the interface between grown thin-film Si and c-Si by

transmission electron microscopy (TEM) at above surface

treatment conditions to explain the lifetime measurement

results. 

Figure 6 shows TEM images of a-Si:H and c-Si

interfaces at three surface treatments. Figure 6a is the

interface with atomic H treatment. It shows an 

Figure 5. Effective carrier lifetime as a function of aSi:H thickness (varied from 5 to 55 nm) and surface

treatment time (varied from 0 to 60 s) with H2 + NH3 . 

intermediate layer with a rough surface and epi-Si growth

with structure defects and then a-Si:H layer. This result

suggests that atomic H creates an undesired intermediate

layer before a-Si:H growth. This damaged rough surface

creates more surface defect states such that it did not

improve the effective carrier lifetime as shown in Figure 3.

This intermediate layer dominates the loss of carriers and

makes the later growth of a-Si:H useless. Figure 6b shows

the interface without atomic H treatment. It also shows an

intermediate layer but much smooth surface and a good 

epi-Si like layer appears immediately on the surface of cSi. It eventually breaks down into a-Si:H as the thickness

increases. In this case, the intermediate layer has less

effect on the carrier loss compared to the first case. The

thickness dependence of effective lifetime on a-Si:H

thickness in Figure 3 (solid squares) can be explained by

simply treating the intermediated layer as perfect c-Si. The 

actual thickness of a-Si:H shall be corrected by subtracting

from the epi-Si thickness. Figure 6c is the interface with H2

+ NH3 treatment. In this image, there is no apparent

intermediate layer! It shows a smooth and immediate

transition from c-Si to a-Si:H. The c-Si surface is well

passivated by a-Si:H and results in the best lifetime as

shown in Figure 5. We conclude that the improvement in

effective lifetime well correlates to an immediate a-Si:H 

growth on c-Si and surface treatment can be a key. 

HETEROJUNCTION SOLAR CELLS WITH SURFACE 

PASSIVATION

In a-Si:H and c-Si heterojunction solar cells process, 

we face the same challenge to grow an immediate a-Si:H

on c-Si because the above facts of growing intermediate

layer. We have reported the effect of various intermediate

layers on the performance of HJ solar cell [4]. We found

that immediate a-Si:H growth of n-type emitter yields the 

3 
Figure 6. TEM images of different c-Si and a-Si:H 

interfaces. 6a: with atomic H treatment. 6b: no atomic H 

treatment. 6c: with NH3+H2 surface treatment. All 

sample has Al cap for the TEM sample preparation. 

best cell performance. We can achieve the immediate aSi:H growth by deposited the film at lower substrate 

temperature (100°C). However, here, we show other 

approach to achieving the high-performance of cell at the

condition that the intermediate layer (epi-Si) often grows

without chemical surface treatment. 

We compared JV characteristics of two solar cells:

one with a typical NH3+H2 surface treatment and other

with an optimized treatment before forming n-type emitter.

We used a non-textured (100) p-type CZ- Si with resistivity 

of 1.2 Ω-cm and about 400 μm thick. The cell area is 1

cm2

. The preparation of n-type emitter and wafer 

properties were published elsewhere [8]. The cell result

with typical chemical treatment indicates that a direct 

transfer of the recipe from surface passivation to a-Si:H

heterojunction is not straight forward. The JV curve under

light shows an “S” shape near the open circuit voltage

(Voc). The cell basically did not work! The result suggests it 

is possible that NH3+H2 surface treatment creates a very

thin SiNx layer which is not sensitive to the lifetime

measurement but a barrier to the heterojunction. We are

able to achieve a high-performance HJ cell by re-adjusting

the mixture of NH3+H2 flow rate and process conditions. 

The cell has a better JV characteristics and an efficiency

of 13.4% with Voc greater than 0.60 V, short circuit current

density of 31.0 mA/cm2

, and fill factor of 0.72. This cell

demonstrates the importance of chemical surface

treatment although our best efficiency on the same type cSi is at 14.9%.

c-Si a-Si:H

Al

(a)

c-Si

a-Si:H

Al

epi-Si

(b)

c-Si

a-Si:H

Al

(c)

SUMMARY 

In summary, we show a detail application of the highthroughput HWCVD technique to the c-Si surface 

passivation and heterojunction solar cells study. With this

powerful tool, we quickly screen the surface treatment

variables and optimize the a-Si:H thickness. We conclude

that the best surface passivation is achieved with an

immediate growth of a-Si:H on c-Si with the aid of a 

proper chemical surface treatment. We also fabricate the

high-performance heterojunction crystalline silicon solar

cells based on the optimized surface passivation. 

ACKNOWLEDGEMENTS 

Authors like to thank E. Iwaniczko for top contact

deposition. This work was performed under DOE Contract 

#DE-AC36-99GO10337. 

REFERENCES 

1. T. Wang, Q. Wang, M. Page, and T. Ciszek, Thin 

Solid Films, 430, 261 (2003). 

2. J. Yang and S. Guha, Mat. Res. Soc. Proc, 557, 239 

(1999). 

3. H. Sakata, T. Nakai, T. Baba, M. Taguchi, S. Tsuge, 

K. Uchihashi, and S. Kiyama, Conference Record, the

28th IEEE PVSC, Anchorage, p.7 (2000). 

4. T. H. Wang, Q. Wang, E. Iwaniczko, M. R. Page, D.

H. Levi, Y. Yan, C. W. Teplin, X. Z. Wu, and H. M. 

Branz, in Proceedings of the 19th EUPVSEC 

Conference, edited by Paris, France 2004), to be 

published, (2004). 

5. Q. Wang, L. R. Tessler, H. Moutinho, B. To, J. 

Perkins, D. Han, D. Ginley, and H. M. Branz, Mat. 

Res. Soc. Proc., 762 p. A9.1. 2003. 

6. B. P. Nelson, Q. Wang, E. Iwaniczko, A. H. Mahan,

and R. S. Crandall, Mat. Res. Soc. Proc., 507 p. 927.

1998. 

7. R. K. Ahrenkiel and S. Johnston, Solar Energy

Materials and Solar Cells, 55, 59 (1998). 

8. Q. Wang, M. Page, Y. Xu, E. Iwaniczko, E. Williams,

and T. Wang, Thin Solid Films, 430, 208 (2003). 

4 
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4. TITLE AND SUBTITLE

High-Throughput Approaches to Optimization of Crystal Silicon 

Surface Passivation and Heterojunction Solar Cells 

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14. ABSTRACT (Maximum 200 Words)

We use a high-throughput (combinatorial) hot-wire chemical vapor deposition system to passivate the crystal silicon 

surface and to grow heterojunction silicon solar cells. We study the effectiveness of crystal surface treatments by atomic 

H or/and NHx radicals, followed by the growth of thin hydrogenated amorphous silicon (a-Si:H) films. Treatment and 

layer properties such as times, thicknesses, and gas mixtures can be continuously graded, creating a two-dimensional 

sample with each variable varying in one direction. This results in high-throughput optimization of the processes. 

Effective carrier lifetime is measured by photoconductive decay to evaluate the effectiveness of the surface passivation 

by surface treatments. The effective carrier lifetime increases from about 5 μs without passivation to about 24 μs with an 

optimized surface treatment and thickness a-Si:H on single-sided c-Si. Transmission electron microscopy reveals that aSi:H, a mixed phase, or epitaxial growth of thin-film Si depending on the surface treatment. Improvement in effective 

carrier lifetime correlates with an immediate a-Si:H growth on c-Si, rather than a mixed phase and epitaxial Si growth. 

We have obtained an efficiency of 13.4% on a non-textured single-sided heterojunction solar cell on p-type CZ-Si 

processed with optimized surface treatment. 

15. SUBJECT TERMS

PV; high-throughput (combinatorial); hot-wire chemical vapor deposition; heterojunction silicon solar cells; transmission 

electron microscopy (TEM); 

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