**HIGH-EFFICIENCY AMORPHOUS SILICON ALLOY BASED SOLAR CELLS AND
MODULES**

**Quarterly Technical Progress Report**

**December 1, 2003 through February 29, 2004**

**S. Guha and J. Yang**

**United Solar Ovonic Corporation**

**Troy, Michigan**

**NREL Technical Monitor: Bolko von Roedern**

**Prepared under Subcontract No. ZDJ-2-30630-19**

# **Overview**

In the last quarterly report, we showed a significant improvement of
c-Si:H solar cell performance by using an optimized hydrogen dilution
profiling. An initial active-area efficiency of 8.4% has been achieved
using a c-Si:H single-junction structure and 13.4% using an
a-Si:H/c-Si:H double-junction structure. The main improvement is in the
short circuit current density (J~sc~) and fill factor (FF). In this
quarter, we continued to optimize the hydrogen dilution profiling for
improving cell efficiency, especially for high J~sc~. We have obtained
J~sc~ over 27 mA/cm^2^ from a c-Si:H single-junction solar cell. We used
the optimized c-Si:H as the bottom cell in an a-Si:H/a-SiGe:H/c-Si:H
triple-junction structure and achieved an initial active-area efficiency
of 13.6%.

For high rate modified very high frequency (MVHF) deposition, we
continued to optimize a-Si:H/c-Si:H double-junction and
a-Si:H/a-SiGe:H/c-Si:H triple-junction solar cells. We have achieved an
initial active-area efficiency of 12.5% using an a-Si:H/c-Si:H
double-junction structure. This cell stabilized to 11.0% after prolonged
light soaking. Using an a-Si:H/a-SiGe:H/c-Si:H triple-junction
structure, we have achieved an initial active-area efficiency of 12.4%
and a stable active-area efficiency of 11.2%.

Two different projects have been carried out using the large area
deposition reactor. In the first project, we have optimized
a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cell on Al/ZnO back
reflector under production constraints. We have achieved a stable
active-area efficiency of 9.8% using a new cathode design with SiH~4~
and GeH~4~ mixture instead of Si~2~H~6~ and GeH~4~ mixture. This
efficiency is equivalent to a total area efficiency of 9.1% and ties the
previous record made with Si~2~H~6~ and GeH~4~ mixture. The second
project is to optimize the a-Si:H/c-Si:H double-junction solar cells on
large area substrate. We achieved a stable aperture-area efficiency of
9.4% with an aperture area of 460 cm^2^. An outdoor measurement of the
same module at NREL showed an efficiency of 9.2%.

The stability of c-Si:H solar cells against light soaking is an
interesting topic in real device application. Although many groups
reported that no light-induced degradation was observed in their c-Si:H
solar cells, we found that there are still light-induced degradations in
the range of 7% to 15%, depending on the material quality and device
structure (presented at the last team meeting). In this quarter, we
systematically studied the metastability of c-Si:H single-junction solar
cells under various light soaking conditions and found that absorption
in the amorphous phase is responsible for the light-induced degradation.
Since c-Si:H covers a very wide range of materials with various
microstructures such as different grain size and microcrystalline volume
fraction, it is understandable to observe different metastability
behavior in c-Si:H materials and solar cells.

# **c-Si:H solar cells made using RF glow discharge at a low rate and their application as the bottom cell in multi-junction solar cells**

##### 2.1[. c-Si:H single-junction cells with hydrogen dilution profiling]{.underline}

As presented in the last quarterly report, we have developed a hydrogen
dilution profiling technique for c-Si:H deposition where the hydrogen
dilution ratio is decreased during the intrinsic layer deposition. The
purpose of the hydrogen dilution profiling is to keep the material
structure close to the transition from the microcrystalline to amorphous
phases. Materials made under this condition normally have relatively
large fraction of amorphous component, small grains, and improved grain
boundary passivation by hydrogen. Of course, if one could find a way to
effectively passivate grain boundary defects for materials with large
grains and high crystalline volume fractions, one would not need to make
the materials close to the transition edge. As a consequence, one could
obtain high J~sc~ without significant loss in FF and open circuit
voltage (V~oc~). However, at the current stage, we believe that the
materials with small grains and compact structures are still the choice
for high efficiency c-Si:H solar cells.

The c-Si:H single-junction solar cell with an 8.4% efficiency was made
using a hydrogen dilution profiling with linearly decreasing hydrogen
dilution ratio. It was found that the cell efficiency could be improved
with the increase of the profiling rate in a certain range. A hydrogen
profiling that is too steep in its grading leads to a significant amount
of amorphous phase in the *i/p* interface region, which causes problems
in carrier transport, and hence, poor solar cell performance. In this
quarter, we continued to optimize the design of gas flow profiling. We
have obtained J~sc~ over 27 mA/cm^2^ in a c-Si:H single-junction
structure. Figure 1 shows the (a) J-V characteristics and (b) quantum
efficiency of this cell. Although the J~sc~ is higher, the V~oc~ and FF
are lower than the previous champion cell. Continued optimization is
ongoing.

##### [c-Si:H single-junction solar cells made at various substrate temperatures]{.underline}

Substrate temperature is one of the important deposition parameters to
be optimized. A high substrate temperature provides a high mobility for
radicals on the growing surface and allows the radicals to find low
energy locations to form compact structure and grains. Generally, c-Si:H
films deposited at high substrate temperatures have higher crystalline
volume fractions than those deposited at low temperatures. Too high a
substrate temperature causes low hydrogen coverage on the growing
surface and low hydrogen content in the material, resulting in a high
dangling bond density and a poor grain boundary passivation in the
material. The optimized substrate temperature for low defect material
has been found at around 250 C, similar to the optimized temperature for
a-Si:H deposition. Another crucial factor for high efficiency c-Si:H
solar cells is a low impurity density, especially oxygen. Oxygen is not
electronically active in a-Si:H, but forms donors in c-Si:H. It has been
reported that deposition at low substrate temperatures can suppress the
oxygen related donor formation in c-Si:H and allows for a relatively
high impurity level without significant deterioration of cell
performance. In this quarter, we systematically studied c-Si:H
deposition at different substrate temperatures to obtain optimum
deposition condition.

Table I lists a series of c-Si:H single-junction solar cells made under
the same conditions except at different substrate temperatures. It shows
that decreasing the substrate temperature results in an increase in
V~oc~, which could be related to the increase of the amorphous component
in the material. However, there is no clear trend in the changes of
J~sc~ and FF. The differences in the FF and J~sc~ may be due to
run-to-run variation. These results show that the range of substrate
temperature for the c-Si:H solar cell deposition is reasonably wide,
which provides a wide window for optimization of cell efficiency,
especially for multi-junction structures.

##### [Multi-junction solar cells]{.underline}

We have reported an initial active efficiency of 13.4% using an
a-Si:H/c-Si:H double-junction structure, where the c-Si:H bottom cell
was deposited with a profiled hydrogen dilution. In this quarter, we
used the same recipe for c-Si:H deposition in an a-Si:H/a-SiGe/c-Si:H
triple-junction structure and obtained an initial active area efficiency
of 13.6%. Figure 2 shows the (a) J-V characteristics and (b) quantum
efficiency of this solar cell.

Two solar cells with the highest initial efficiencies were light soaked
under 100 mW/cm^2^ of white light at 50 C for over 1000 hours. Table II
summarizes their initial and stable performance. The cells both
stabilized at 11.8%. The triple-junction cell did not show advantage
over the double-junction cell. In fact, it degraded more than the
double-junction cell. The high degradation in the a-Si:H/a-SiGe/c-Si:H
triple-junction cell is attributed to the middle cell\'s limited current
mismatching as shown in Table II. We know that the a-SiGe:H middle cell
degrades more than the a-Si:H top cell and the c-Si:H bottom cell. In
order to obtain a high stable efficiency, a bottom cell limited current
mismatching is desirable.

# 3. **MVHF glow discharge at high deposition rate**

We continue to optimize a-Si:H/c-Si:H double-junction and
a-Si:H/a-SiGe:H/c-Si:H triple-junction structures using a high rate MVHF
glow discharge. The intrinsic a-Si:H layer in the top cell and the
intrinsic c-Si:H layer in the bottom cell were deposited using a MVHF
glow discharge, but the intrinsic a-SiGe:H layer in the middle cell was
deposited using a RF glow discharge at a low rate of \~ 1 Å/s. New high
initial active area efficiencies of 12.5% and 12.4% were achieved using
the double-junction and the triple-junction structures, respectively.
Figure 3 shows the (a) J-V characteristics (b) quantum efficiency of the
triple-junction cell where a matched current density is found among the
component cells. As reported previously, the c-Si:H bottom cell in the
double junction structure showed a very good FF and less degradation
under prolonged illumination. Thus, a triple-junction cell with a c-Si:H
bottom cell limited current mismatch is desirable for a stabilized
efficiency. Based on this analysis, we optimized the cell structure by
adjusting the current mismatching. We have made an
a-Si:H/a-SiGe:H/c-Si:H triple-junction solar cell (#12468) with a strong
bottom cell limited current mismatching

and obtained a very high FF of 0.79 as listed in Table III. Although
this cell did not have the highest initial efficiency, it showed the
highest stable efficiency.

We light-soaked the a-Si:H/c-Si:H double-junction cell with the highest
initial efficiency (#12065) and three a-Si:H/a-SiGe:H/c-Si:H
triple-junction cells with different current mismatches. The stability
results are listed in Table III. The a-Si:H/c-Si:H double-junction cell
with an initial efficiency of 12.5% stabilized at 11.0%, a new record
for stable efficiency with the high rate a-Si:H/c-Si:H double-junction
solar cells.

For the a-Si:H/a-SiGe:H/c-Si:H triple-junction solar cells, the
light-induced degradation is in the range from 8% to 15%, which is
similar to the degradation in a-Si:H/c-Si:H double-junction solar cells.
Although the top cell is thinner in the triple-junction structure, the
degradation in the a-SiGe:H middle cell has contributed significantly to
the overall degradation. Similar to a-Si:H/c-Si:H double-junction cells,
the cell with a strong bottom cell limited current mismatch (#12468)
shows the lowest light induced degradation and the highest stable
efficiency of 11.2%, which is slightly higher than the best stable
efficiency of 11.0% from the a-Si:H/c-Si:H double-junction structure.
Figure 4 plots the (a) J-V characteristics and (b) quantum efficiency of
the best a-Si:H/a-SiGe:H/c-Si:H triple-junction cell.

# **Large-area deposition**

In the large-area deposition, we continued to work on two projects. One
is the optimization of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar
cells and modules on Al/ZnO back reflectors using the deposition
conditions under the constraints of a production machine. The other is
the deposition of an a-Si:H/c-Si:H double-junction solar cell on a large
area substrate.

In the last quarter, we achieved initial aperture-area efficiencies of
11.8% and 11.3% for the modules with aperture areas of 45 cm^2^ and 460
cm^2^, respectively for the double-junction solar cells. After
encapsulation, the 11.3% module\'s aperture-area efficiency became
10.6%. In this quarter, we light-soaked these large-area modules under
100 mW/cm^2^ of white light at 50 C for over 1000 hours. The results are
summarized in Table IV. A stable aperture-area efficiency of 9.4% (#
9456) has been obtained. These modules were sent to NREL for efficiency
measurements. NREL's outdoor measurement shows an efficiency of 9.2%,
and the indoor measurement under a Spire simulator shows an efficiency
of 8.8%.

For a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells with production
constraints, we reported a stable active-area efficiency of 9.4% in the
last quarter. This result was achieved with a new cathode design and
with a SiH~4~ and GeH~4~ mixture instead of a Si~2~H~6~ and GeH~4~
mixture. In this quarter, we optimized the material quality and device
structure further and achieved a stable active-area efficiency of 9.8%.
This corresponds to a stable total area efficiency of 9.1% and ties with
the previous record made with a Si~2~H~6~ and GeH~4~ mixture. Figure 5
shows the (a) J-V characteristics and (b) quantum efficiency of this
solar cell.

# 5. **Stability of c-Si:H single-junction solar cells**

The Staebler-Wronski effect has limited the development of a-Si:H solar
cells. Although significant work has been carried out in the last
twenty-five years, nor an effective way has been developed to eliminate
the light-induced degradation in a-Si:H based solar cells. As an
alternative material, c-Si:H has attracted remarkable attention. One
advantage of using c-Si:H in thin film solar cell is the better
stability against prolonged light soaking than a-Si:H and a-SiGe:H
alloys. It has generally been reported that no light-induced degradation
was observed in c-Si:H solar cells. However, the term c-Si:H represents
a wide range of materials comprised of various grain sizes and different
microcrystalline volume fractions. The highest efficiency c-Si:H solar
cells are normally deposited near the transition from microcrystalline
to the amorphous phase, which means a large amount of amorphous phase
still exists in the c-Si:H materials. Therefore, it would be logical to
assume light induced degradation in c-Si:H solar cells deposited in this
regime. Indeed, Finger and his colleagues found that c-Si:H solar cells
made with a hydrogen dilution close to the transition show light induced
degradation. The amount of degradation depends on the dilution ratio,
hence, on the microcrystalline volume fraction and grain size \[1\].

In general, c-Si:H can be considered as a two-phase material with
microcrystallites surrounded by amorphous tissue. The photons with
energy higher than the band gap of a-Si:H are largely absorbed in the
amorphous phase while the lower energy photons are absorbed in the
grains., Since the material properties inside of the grains would be
similar to crystalline silicon with no light induced defects, one may
expect no light induced defect generation in c-Si:H when it is exposed
to a red light with photon energy lower than the optical band gap of
a-Si:H. If this hypothesis is correct, there would be much less light
induced degradation in the c-Si:H bottom cell in an a-Si:H/c-Si:H
double-junction structure, because the high-energy photons are absorbed
in the a-Si:H top cell.

In this quarter, we did a systematic study of light induced degradation
of c-Si:H single-junction solar cells with various light sources. Two
sets of c-Si:H single-junction solar cells were studied. The cells in
each set were from one run of deposition on the same substrate; and
therefore, the cell performance is very similar. One set was deposited
using MVHF at a high rate with relatively low V~oc~. The other set was
deposited with RF at a low rate with relatively high V~oc~. We believe
that the RF cells have a lower microcrystalline volume fraction than the
MVHF cells. As a comparison, we also studied a set of a-SiGe:H bottom
cells. Light soaking was done at 25 C under open circuit condition. An
AM1.5 solar simulator was used as the light source. Focus lenses and
color filters were used to vary the light intensity and the spectrum.
The current density versus voltage (J-V) characteristics were measured
as a function of light soaking time under the same light as used for the
light soaking. The light soaking time was 15 hours. The J-V
characteristics in the initial state and degraded state after light
soaking were measured under an AM1.5 illumination and by quantum
efficiency measurement. Due to the difference in responses for different
wavelengths, we used the same photo-generated carrier density for
different light spectra. We have found that for solar cells with a good
FF, J~sc~ is a reasonable measure of the photo-carrier density. Thus, we
adjusted the light intensities for different spectra to produce the same
initial J~sc~ value. Here, two J~sc~ values were chosen: one is the same
as the J~sc~ under AM1.5 illumination, and the other is double the value
under AM1.5.

Figure 6 shows a comparison of the normalized maximum power, P~max~, as
a function of light soaking time for the MVHF c-Si:H single-junction
solar cell and the a-SiGe:H single-junction bottom cell, where the red
and blue lights were obtained using a 665 nm long-pass filter and a 650
nm short-pass filter. The long-pass filter only allows the photons with
energy lower than 1.86 eV, smaller than the optical band gap of a-Si:H,
to pass through and illuminates the sample. For the c-Si:H cell, the
light intensities were adjusted to produce a J~sc~44 mA/cm^2^,
approximately twice the J~sc~ under AM1.5 illumination, for the white,
red, and blue lights. For the a-SiGe:H cell, the light intensities were
adjusted to produce a J~sc~46 mA/cm^2^ with the same consideration. The
figure clearly shows that there is no detectable light-induced
degradation for the c-Si:H cell under the red light, some degradation
under the white light, and more degradation under the blue light.

Table V shows a comparison of the initial and degraded performance for
five MVHF deposited c-Si:H cells. The first two cells were light-soaked
under light intensities that produced the same J~sc~ as under AM1.5.
Cell 1 and cell 2 were under the white light and the red light,
respectively. Notice that no light induced degradation was found in cell
2 under the red

light, but it was 3.7% under the white light. To enhance the light
soaking effect, we increased the light intensities to produce 44
mA/cm^2^, which is twice the J~sc~ under AM1.5 illumination. There is
still no detectable degradation under the red light within the
experimental errors.

As discussed above, the undetectable light induced degradation under the
red light is due to very little absorption in the amorphous phase, while
the absorption in the microcrystalline phase does not produce defects.
Two mechanisms could explain the difference between the white and blue
light soaking. First, the blue light has a large portion absorbed in the
amorphous phase, which produces more defects. For the white light, the
portion of red light is mainly absorbed in the microcrystalline phase
and does not produce degradation. Second, more blue light is absorbed
nearer the *i/p* interface than in the bulk. Guha, *et al.* showed that
the *i/p* junction is the dominant junction and any changes in the
dominant junction can cause noticeable changes on the cell performance
\[2\].

Table VI lists the J-V characteristics at the initial and degraded
states for three RF deposited solar cells. The general trend is the same
as the MVHF deposited cells, i.e., the red light produces low
degradation, the white light some degradation, and the blue light more
degradation. However, unlike the MVHF cell, this cell still has 2.2%
degradation under the red light. We believe that the difference could be
caused by the different material structure. For the RF deposited cells,
the V~oc~ is larger than the MVHF deposited ones. It could indicate that
there is more amorphous component in the RF cells than in the MVHF
cells, which may partially play a role in the light-induced degradation.
Although the cut-on filter with a wavelength longer than 665 nm blocks
most of the high energy photons, there are still some photons absorbed
by the amorphous tissue, which degrades the cell performance.

Table VII summarizes the results of the a-SiGe:H bottom cells. For the
a-SiGe:H cell, the material is homogeneous, and only one optical band
gap is defined with a value around 1.4 eV. The differences among the
light soaked samples under the lights with the three spectra are mainly
due to the second mechanism (dominant junction effect)discussed above.

Overall, the light-induced degradation in c-Si:H is much less than that
in the a-SiGe:H cell even under white and blue light sources. We need to
point out that the light-soaked state is not a stabilized state since
only a 15-hour light soaking was performed.

The above experiment confirms no or very little light-induced
degradation for c-Si:H solar cells under red light with photon energy
lower than the band gap of a-Si:H. In an a-Si:H/c-Si:H double-junction
solar cell, most blue light is absorbed by the a-Si:H top cell, and the
c-Si:H bottom cell only absorbs red light with low photon energy.
Therefore, there is very little light-induced degradation in the c-Si:H
bottom cell in multi-junction structures, and a bottom cell limited
current mismatching is desirable for stable efficiency as shown above
and in the previous reports.

In summary, we have systematically studied the light-induced degradation
in c-Si:H single-junction solar cells under various light soaking
conditions. We found no detectable degradation under red light with
photon energies less than the band gap of a-Si:H. We believe that the
low energy photons are only absorbed in the microcrystalline phase,
where no light induced defect generation occurs. Under white and blue
lights, the defect generation in the amorphous phase causes certain
degradation. The fact that a larger degradation is observed under blue
light than white light is due to more absorption in the amorphous phase
and more absorption near the *i/p* interface under blue light than under
white light. These results provide an explanation that a bottom cell
limited-current mismatch in multi-junction structures with c-Si:H as the
bottom cell leads to a high stable efficiency.

# **References:**

1.  S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L.
    Houben, and M. Stutzmann, *Proceeding of 2^nd^ International
    Conference on Cat-CVD (Hot-Wire CVD) Process*, (Denver, Colorado,
    Sep. 2002), p. 202.

2.  A. Pawlikiewicz and S. Guha, *Mat. Res. Soc. Symp. Proc.* **118**,
    599 (1988).