Field of the invention:-
The present invention relates to a method of providing multi-layer silicon coatings
in c-silicon solar cells.
Background and Prior art:-
Conventionally, a single-layer Silicon Nitride (SiN) antireflection coating (ARC) is
deposited on crystalline silicon solar cells with thickness and refractive index
optimally matched to achieve high conversion efficiency. This is performed by a
standard manufacturing process, plasma-enhanced chemical vapour deposition
(PECVD) using silane and ammonia precursor gases. Further improvements have
been made by employing a step-layer of SiN with graded refractive index and
thickness [l:Patent pending, 2] produced in-situ by varying the relative ratio of the
two reactant gases and process time. This results in lower reflection (R~6% at
630-nm) and consequent improvement in power output distribution of the solar
cells fabricated thereon. There have been several other attempts to reduce the
amount of light reflected from top surface and interfaces with the help of double
or multi-layer ARC'S of materials that possess successively lower refractive indices
[3-5]. Analysis of these designs reveal that by increasing the number of layer
materials with appropriate refractive index and thickness in a judicious manner,
the maximum and average reflectance from the surface can be decreased to very
low magnitudes. Design simulations in porous silicon fDSi)

films with refractive index gradient [6-7] and in silicon oxy-nitride, SiN(X)O(y) [8-9]
suggest that lower values of refractive index will be effective in antireflection
coatings.
More recently, Fred Schubert et al [10] illustrated the use of oblique angle, multi-
layer deposition of silicon dioxide (SiO2) or titanium dioxide (Ti02) [11] coatings on
Aluminium Nitride (AIN) substrates for lowering the reflection further. Oblique
angle deposition is generally employed to obtain nano-structured growth of low-
density, low-refractive index films. At large vapour incident angles, the deposited
particles create a shadow over a portion of the substrate and results in the
formation of a porous nano-scale film of lower density and, consequently, lower
refractive index than the bulk material.
It is established that vapor incident angle has profound influence on the refractive
index of such grown films. The above authors fabricated thin (nano-rod) layer
materials by controlling the deposition angle that allows creation of different
refractive index layers of the same material. This class of optical materials
possessing very low refractive index was demonstrated to possess optical index
value approaching that of air. This finding was stated to have potential for
improving performance in many photonic applications such as broadband coatings
with air ambient, omni-directional reflectors, light-emitting diodes (LED), optical
interconnects and solar cell devices.

In the past, few attempts were made to apply the multi-layer recipe in industrial
environment [4, 12] for manufacturing solar cells but no promising results have
been reported. This is confirmed by conducting a patent search at the listed sites
[13], the results of which are compiled [13.1-13.11]. The abstracts show that
several patents are published on the formation of multi-layer ARC's to enhance
light absorption but none for silicon crystalline solar cells employing silicon dioxide
over-layers.
Objects of the Invention:-
An object of the invention is to reduce surface reflection, possibly below 5%, from
SiN-coated silicon wafers.
Another object of the invention is to provide increased absorption of a wide range
of wavelengths of the solar spectrum in such silicon wafers.
Yet another object of the invention is to enhance the conversion efficiency screen-
printed industrial solar cells upto 17%.
A further object is to provide a method of providing the multi-layer coating in solar
cells during manufacturing.
The existing method of anti-reflection coating viz., a step-layer of SiN ARC is
aimed to reduce reflection levels in the range 6-10% and to achieve nearly 16-
17% conversion efficiency in solar cells. Though this is well established, it

appears that there is further scope of reduction of reflection from the absorber
layer. This emerges from reports of demonstration of very low reflection on multi-
layered SiO2 and Ti02 films on AIN substrates. Design of such a stack would return
the benefits of several reflectance minima characteristic of broadband anti-
reflection coatings. Further, a silicon dioxide multi-layer stack promises
compositional integrity by virtue of minimum lattice/thermal mismatch and good
adhesion within the parent matrix in silicon crystalline solar cells.
This led to the present invention to deposit single-layer and multi-layer silicon
dioxide coatings on SiN-coated, crystalline silicon wafer specimens. Their optical,
structural, compositional and electrical characteristics are evaluated; reduction of
surface reflection observed with such a multi-layer stack is reckoned to have
benefits of increased light-generated current when adapted to large area silicon
solar cells.
Description of the invention:-
The direct relationship between refractive index of SiO2 and surface reflection can
be exploited by forming multi-layers of graded index in a monolithic design.
In this configuration, conjoint layers possess good interfacial and optical properties
for minimum reflection criteria.

In the present invention, a standard engineering tool is used to obtain graded-
index profile of silicon dioxide film stack, monolithically, on SiN-coated silicon
wafer specimens. Optical analyses of the multi-layer stack is carried out by
measuring reflectance, refractive index and also, by optical surface imaging.
Further, the SiO2 stack is applied on prototype industry standard (125xl25-mm2)
crystalline silicon solar cells. Performance tests are performed on the solar cells to
verify the power output of the composite stack.
The method according to the present invention comprises the following steps:-
a. A homogeneous, single layer silicon dioxide (SiCb) is formed by sputter
deposition technique on polished, Si3N4-coated crystalline silicon substrates.
b. Next, 2-layer, 4-layer and 6-layer stacks of SiO2 are created; each stack
consists of two films, one formed at incident angle of -45° and other at
+75°, with respect to normal, on silicon wafer substrates mounted on
specially designed substrate holder.
c. Resultant stacks are optically characterized by ellipsometry, reflection,
surface imaging and x-ray diffractometry. Refractive index of the SiO2
stack decreases progressively as the number of layers increase, and the
value is lowest in the 6-layer configuration (1.25 at 600-nm). In a similar

fashion, relative reflectance of the multi-layer stack decreases in reference to
virgin Si3N4-coated specimens, the lowest reflection being 60% of the reference
sample for the 6-layer SiO2 stack in the low wavelength regime.
d. Suitability of 100-nm and 200-nm thick SiO2 layers for silicon solar cells is
verified by applying single layers of two different thickness on semi-
processed, 125xl25-mm2 mono-crystalline silicon wafers pre-coated with
SiN-ARC. Light absorption is significant in these stacks in comparison with
raw wafer, textured wafer and Si3N4-arc wafer.
Marginal increase in solar conversion power output is reported in prototype
solar cells using the 100-nm SiO2 over-layer.
Description of accompanying drawinas:-
The illustrations accompanying this description are as follows:
Fig. 1 shows a substrate mounting holder with provision for switching between
two vapour incident angles with respect to normal.
Fig.2 shows optical paths in various silicon surfaces, illustrated with high-index
(n1,1) and low-index (n1,2) SiO2 over-layers
Fig.3 shows normalized reflectance curves of 2, 4 and 6-layer silicon dioxide

films in wavelength region 300-800-nm; Spectrum of bare Si3N4 is used as
reference.
Fig. 4 shows photographic views of various SiO2 multi-layers on Si3N4-coated Si
substrates.
Fig. 5 shows variation of refractive index with wavelength of SiO2 film with 2, 4
and 6 layers in comparison with bare Si3N4 ARC.
Fig. 6 shows X-ray diffractogram of 6-layer stack indicating predominance of
SiO2.
Fig. 7 shows photographic views of silicon dioxide-coated mono-Si wafers
(size:125x 125-mm2):
7.a) Wafer with Si3N4 coating,
7.b) Wafer with Si3N4 + 100-nm SiO2 layer and,
7.c) Wafer with Si3IN4 + 200-nm SiO2 layer.
Fig. 8 shows reflectance curves of 125xl25-mm2 standard silicon wafers during
various stages of solar cell manufacture: Raw wafer, after chemical texturing, with
SiN-ARC and with 100-nm and 200-nm SiO2 overlayers.
The invention will now be described in an exemplary embodiment as depicted in
the accompanying drawings. There can, however, be other embodiments
covered by the following description.
Detailed Description of the Invention:-
Deposition of SiO2 layers:
First, a step-layer silicon nitride anti-reflection coating is made in a PECVD tubular
furnace on polished, mono-crystalline silicon substrate specimens (lxl cm2, CZ,
<111>, 250-µm, 1-Ocm). These specimens are then over-coated with silicon
dioxide thin films in a vacuum coating system by rf sputtering. Film thickness was
monitored by quartz crystal monitor and is approximately 50-nm. Both single-layer
and multi-layer coatings are made in separate runs at different incident angles
keeping the target-substrate distance of 6.5-cm in all cases. A SiO2 target (MAK)
was used as source and Ar gas as sputter medium.
Single-layer silicon dioxide films are deposited at vapour incident angles 30°, 45°,
60° and 75° with respect to the normal. Refractive index is measured using
Sentech SE850 Ellipsometer and reflectance with UV-VIS Spectro-photometer (GBC
Cintra 40) in the range 300-800 nm and also, in the range 650-1200 nm using FT-
IR spectrometer. Optical micrographs are used to compare surface images and X-
ray diffractogram for evaluating compositional integrity of sputtered layers.
In multi-layer deposition, the specimens are mounted at oblique angle with respect
to the normal (to the target) on a specially designed substrate holder

(1.0). The latter has provision for switching between two angular planes, from -
45° to 75° position. This results in two layers deposited at two different angles one
over the other, without requiring to move the substrates manually. Further, stacks
of 4-layers and 6-layers are formed by successive depositions. Optical evaluation is
carried out on all specimens.
Single layer silicon dioxide of 100-nm and 200-nm thickness are coated on semi-
processed (pre-deposited with SiN-ARC) solar cells of size 125x125 mm2 in an
industrial vacuum coater. Subsequently, processing steps such as metal printing,
fast firing and testing are undertaken along with standard solar cells. Relative
reflectance data on standard silicon solar wafers were measured after various
process steps using a Perkin Elmer spectrophotometer in the range 340-800 nm.
Suitability of SiO2 over layers (100-nm and 200-nm) is tested in prototypes of
industry-standard (125xl25-mm2) mono-crystalline silicon solar cells.
The reflectance data shows increased absorption for both 100-nm and 200-nm
SiO2 layers; power output with SiO2 of 100-nm thickness shows marginal increase
from normal values.
Absorption and reflection in silicon surface with special reference to
silicon dioxide over layers:
A 2-layer silicon dioxide system is illustrated schematically in Fig. 2 where
optical paths (2.1, 2.2, 2.3, 2.4) of reflection and absorption on silicon surface are

indicated at various steps. The refractive index of the first layer in contact with
silicon nitride surface is shown to be larger than the second one on top surface
close to air. The optimum combination of layer thickness and refractive index
determines the reflectance at various wavelengths.
Whereas in conventional quarter-wavelength-thick anti-reflection coatings
zero reflection occurs only at one wavelength and at normal incidence, multi-layer
ARCs with graded-refractive index exhibit broadband anti-reflection properties that
will allow a fuller range of sun spectrum to be absorbed.
Evaluation of silicon dioxide multi-layers & prototype solar cells:
Evaluation was made with 6 layers of oxide films deposited alternately at -45° and
+75°, with respect to normal, whose normalized reflectance spectrum (300-800
nm) is given in Fig. 3. At low end of the wavelength spectrum, the 6-layer stack (-
45°/750/-450/750/-45°/750) shows pronounced lowering of reflection vis-a-vis the
base value of Si3N4-coated wafer by as much as 60%.
This observation is strengthened by dark optical image of the said surface as seen
in Fig. 4. Refractive index of the 3 individual stacks, as derived from ellipsometric
data, decreases progressively in the entire wavelength spectrum as the number of
silicon dioxide layers increase (Fig. 5). For the first 2-layer configuration, n has a
value 1.5 (at 600-nm). It is lower (1.25) for the 3rd stack.

This result is consistent with the large overall light absorption in this configuration.
The x-ray diffractogram confirms the dominance of SiO2 phase in this stack (Fig.
6).
Solar cell prototypes have been fabricated using 125xl25-mm2 size Si3lM4-coated
solar-grade wafers and with SiO2 (single layer normal to substrate: 100-nm and
200-nm thick) over-layers; they are screen-printed with silver paste (cathode) grid
and aluminium back surface (anode) and the contacts sinter-fired in a fast belt
furnace.
Relative reflectance curves of 125-mm standard silicon wafers during various
stages of solar cell manufacture, (raw wafer, after chemical texturing, coated with
SiN-arc and with over-layers of 100-nm and 200-nm thickness) are illustrated in
Fig. 7. This data clearly demonstrates higher absorption in the case of wafers with
SiO2 layers.
Typical film characteristics such as thickness, refractive index, normalized
reflectance and colour are listed in Table-I. The refractive index decreases
progressively from the first to the third stack as also the total reflectance of the
layers. This result is in agreement with the requirement of low-refractive index
films at the light-incident surface (in proximity to air) for least reflectance.

The value of solar cell operating current (Iop) is marginally higher for a prototype
solar cell consisting of 100-nm SiO2 layer as evident in Table-II.
Power output distribution among large production runs will be determined after
evolving a standard coating method for oblique deposition of multi-layer silicon
dioxide stacks.
REFERENCES:

We Claim:-
1. A method of providing multi-layer silicon dioxide coatings in c-silicon solar cells
comprising the steps of:
- formation of a homogeneous single layer silicon dioxide (SiO2) by sputter
deposition technique on polished, Si3 N4 coated crystalline silicon substrates;
- creation of 2-layer, 4-layer and 6-layer stacks of SiO2/ each stack consisting of
two films, one formed at incident angle of -45° and the other at +75° with respect
to normal, on silicon wafer substrates mounted on substrate holder (1.0);
- optical characterization by ellipsometry, reflection, surface imaging and x-ray
diffractometry (6.0) and
- verification of 100-nm and 200-nm thick SiO2 layers for silicon solar cells by
applying single layers of two different thicknesses on semi-processed, 125 x 125
mm2 mono-crystalline silicon wafers pre-coated with SiN antireflection coating
(ARC),
characterized in that the solar cells with the 100-nm thick SiO2 layer yields lower
average reflection than conventional Sin antireflection layer coated solar cells (8.0)
which results in higher solar cell operating current in the range of 1-3%.

2. The method as claimed in claim 1, wherein said sputter deposition technique is
conducted in a vacuum coating system by rf sputtering with Ar gas as sputter
medium.
3. The method as claimed in claim 1, wherein the refractive index of the said stack
of SiO2 decreases progressively as the member of layer increases.
4. The method as claimed in claim 1, wherein the multi-layer deposition is done on
a substrate holder (1.0) which has provision for switching between two angular
planes, from -45° to +75° to the normal positions.

The present invention provides a method to fabricate silicon dioxide thin films, in
multi-layer configuration, sputter deposited on Si3N4-coated silicon substrates.
Oblique angle deposition is employed to obtain low-refractive index films (in 2, 4
and 6-layer design, each of layer thickness 50-nm) and reduced reflectance in
comparison with conventional silicon nitride anti-reflection coating applied to
crystalline silicon solar cells. The oxide layers promise compositional integrity by
virtue of matched lattice/thermal properties and good adhesion within the parent
silicon matrix. Dominance of SiO2 phase is confirmed by X-ray diffraction analysis.
The 6-layer stack is discernible by dark surface image and, possibly well suited as
broadband anti-reflection coating for absorbing fuller range of sun spectrum.
Specifically reported are application results with 100-nm and 200-nm thick, single
SiO2 films on 125xl25-mm2 industry-standard silicon solar cells. The findings point
to a viable engineering solution, with marginal material inputs and process time, to
achieve narrow production spread and higher operating current in screen-printed
cells. It is considered that a multi-layer silicon dioxide stack with graded-index
profile of this kind will be suitable for silicon solar cells.