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

**Quarterly Technical Progress Report**

**September 1, 2003 through November 30, 2003**

**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**

# **1. Overview**

In this quarter, we have carried out research in the following areas: We
optimized hydrogenated microcrystalline silicon (μc-Si:H) solar cells
using a hydrogen profiling technique. In the last quarterly report, we
showed that the deterioration of cell performance with the increase of
cell thickness is mainly due to the increase of microcrystalline phase.
Raman and X-ray diffraction confirmed that the microcrystalline volume
fraction indeed increases with thickness significantly. Using the
hydrogen dilution profiling technique, we have shown an improvement of
cell performance. We believe that reducing hydrogen dilution during the
deposition of μc-Si:H has suppressed the microcrystalline evolution. In
this quarter, we continued to use this technique to search for high
efficiency μc-Si:H single-junction and a-Si:H/μc-Si:H double-junction
cells. The slow rate μc-Si:H single-junction cell has been improved from
the previous efficiency of 7.4% to a new record of 8.4%, and for the
a-Si:H/μc-Si:H double-junction structure from 13.0% to 13.4%.

In the last quarterly report, we showed that high pressure RF glow
discharge is an encouraging method for high rate deposition of μc-Si:H
solar cells. We have obtained an initial active-area efficiency of 12.4%
using the a-Si:H/μc-Si:H double-junction structure, where the μc-Si:H
bottom cell was deposited using high pressure RF glow discharge. In this
quarter, we have made μc-Si:H single-junction and a-Si:H/μc-Si:H
double-junction solar cells on large-area substrates. We started from
low rate μc-Si:H deposition using our 2B machine (substrate area of 36 ×
38 cm^2^). We have achieved an initial efficiency of 12.1% with an
active-area of 0.25 cm^2^ using the a-Si:H/μc-Si:H double-junction
structure. The module fabrication process has been tested on the
a-Si:H/μc-Si:H double-junction cells. Preliminary results showed an
initial aperture-area efficiency of 11.8% for an unencapsulated
a-Si:H/μc-Si:H cell with an aperture area of 45 cm^2^, and 11.3% for one
with an aperture area of 461 cm^2^. The large cell with 11.3% efficiency
became 10.6% after encapsulation. Furthermore, we have increased the
deposition rate by mainly increasing pressure and power. An initial
active-area (0.25 cm^2^) efficiency of 11.3% has been achieved with an
a-Si:H/μc-Si:H double-junction structure, where the μc-Si:H intrinsic
layer in the bottom cell was deposited for 50 minutes.

We continued the optimization of a-Si:H, a-SiGe:H and μc-Si:H solar
cells using modified very high frequency (MVHF) at high deposition
rates. In this quarter, we focused on the deposition of
a-Si:H/a-SiGe:H/μc-Si:H triple-junction cells. Last year, we reported an
initial active-area efficiency of 8.7% using this structure. The
problems in the tunnel junctions and the quality of the a-SiGe:H middle
cell were causing the low efficiency. By solving the tunnel junction
problem and improving the a-SiGe:H middle cell, we have achieved an
initial active-area efficiency of 12.2% with an a-Si:H/a-SiGe:H/μc-Si:H
triple-junction structure.

We finished light-soaking experiments on a few a-Si:H/a-SiGe:H/a-SiGe:H
triple-junction cells made using the 2B machine with constraints from
the production machine. A stable active-area efficiency of 9.4% has been
achieved by light-soaking a cell with an initial active-area efficiency
of 11.4%.

# **2. High efficiency μc-Si:H single-junction and a-Si:H/μc-Si:H double-junction solar cells**

As discussed in the last report, two possible mechanisms are responsible
for the deterioration of μc-Si:H solar cell performance with increase of
thickness\--the microcrystallite collision and the increase of
microcrystalline volume fraction. Experimental results of solar cells on
specular stainless steel and textured Ag/ZnO back reflector (BR)
substrates showed that the increase of microcrystalline volume fraction
is the main problem at the current stage. To overcome this problem, we
have developed a hydrogen profiling technique to control the
microcrystalline evolution and found that a proper reduction of hydrogen
dilution ratio during the deposition can improve the μc-Si:H cell
performance. However, the data presented in the last quarterly report
are based on an unoptimized baseline. In this quarter, we started from
an optimized baseline and used hydrogen dilution profiling to increase
the efficiency for μc-Si:H single-junction and a-Si:H/μc-Si:H
double-junction solar cells.

a-Si:H and μc-Si:H solar cells were deposited using the LINE system with
RF excitation at a low deposition rate ∼ 1 Å/s. Textured Ag/ZnO BR was
used as a substrate to enhance the light trapping effect. Current
density versus voltage characteristics (J-V) were measured under an
AM1.5 solar simulator at 25 °C. Quantum efficiency was measured from 300
nm to 1000 nm to calibrate the short-circuit current density (J~sc~).
Table I gives a summary of the J-V characteristics of μc-Si:H
single-junction solar cells made with various hydrogen dilution profiles
during the intrinsic layer deposition. The first five cells were made
using the same recipe to check the reproducibility. It shows clearly
that the fill factor (FF) has a large variation from run to run. A major
factor that affects the reproducibility is the series resistance of the
ITO. The ITO process was optimized for a-Si:H based solar cells, which
have larger open-circuit voltage (V~oc~), but lower J~sc~ than μc-Si:H
solar cell. From the table, one can easily see that the cells with
higher FF have lower series resistance. The series resistances listed in
the table were deduced from the inverse of the derivative of the light
J-V characteristics under the open circuit condition, which include the
contribution from the ITO resistance, junctions, and the intrinsic
layer. At this moment, we are not sure whether the main portion of the
series resistance is from the ITO or from the intrinsic layer. Although
the baseline data are somewhat scattered, the average efficiency is
around 6.6%. Increasing the intrinsic layer deposition time by 20% did
not increase the J~sc.~ Instead the response in the middle and long
wavelength region was reduced. This phenomenon has been observed many
times before (see our previous reports). As we showed in the last
quarterly report, the increase of microcrystalline volume fraction with
thickness is the main origin of the reduction of response in the middle
and long wavelength region and a proper hydrogen dilution profiling can
control the microcrystalline evolution. In this study, we used the same
technique of reducing the hydrogen dilution ratio during the intrinsic
layer deposition. Instead of a step profiling used before, the flow
rates of active and dilute gases were continually changed with
decreasing hydrogen dilution ratio. From Line 14578 to 14660, we
gradually increase the slope of hydrogen dilution versus time. As a
result, the cell performance has been improved significantly. The J~sc~
has been improved from around 22 mA/cm^2^ in the baseline cells to over
25 mA/cm^2^ in the best cell (Line 14660). The previous record for
μc-Si:H single-junction cell was 7.4% initial active-area efficiency.
Using the hydrogen profiling technique, we achieved a new record of
8.4%. From Fig. 1, one can see that the middle and long wavelength
response has been significantly improved in the cell with a proper
hydrogen dilution profiling. Figure 2 plots the (a) J-V characteristics
and (b) quantum efficiency of the best μc-Si:H single-junction solar
cell.

The above experimental results show that hydrogen dilution profiling
indeed improves μc-Si:H solar cell efficiency. The initial purpose of
using hydrogen dilution profiling is to control the microcrystalline
evolution. We have no direct evidence to prove that we have obtained a
uniformly distributed microcrystalline volume fraction. Daxing Han
(University of North Carolina) has measured the photoluminescence (PL)
from μc-Si:H solar cells with and without hydrogen dilution profiling.
She found that the PL peak associated with the microcrystalline phase is
higher for measurements made with a short wavelength excitation than
those with a long wavelength excitation for the sample deposited with
constant hydrogen dilution. This is consistent with the microcrystalline
volume fraction increase with thickness measured by Raman and XRD.
However, in the hydrogen dilution profiled sample, the short wavelength
excited measurement did not give the enhanced PL intensity associated
with microcrystalline phase. Instead, the microcrystalline related peak
is lower for the measurement with the short wavelength excitation than
with the long wavelength excitation, indicative of a lower
microcrystalline volume fraction at the *p/i* interface than in the bulk
of the μc-Si:H layer.

We have used the improved μc-Si:H cell in the a-Si:H/μc-Si:H
double-junction structure and achieved a new record initial active-area
efficiency of 13.45%. Figure 3 shows the (a) J-V characteristics and (b)
quantum efficiency of the best a-Si:H/μc-Si:H double-junction solar
cell.

Comparing the μc-Si:H single-junction cell to the a-Si:H/μc-Si:H
double-junction cell, we find that the FF of the a-Si:H/μc-Si:H
double-junction cell is much better than the μc-Si:H single-junction
cell, especially for a bottom cell limited current mismatch. As shown in
previous reports, the FF of a-Si:H/μc-Si:H double-junction cells under
blue light illumination is as large as 0.83, indicating a high FF from
the μc-Si:H bottom cell. However, we could hardly obtain FF over 0.7 for
μc-Si:H single-junction cells. The real reason for the difference in FF
between the μc-Si:H single-junction cell and the a-Si:H/μc-Si:H
double-junction cell is an interesting topic for us. Except for the
influence of series resistance from the ITO, we suspect that during the
ITO processing, the μc-Si:H intrinsic layer gets degraded in the μc-Si:H
single-junction cell. SAXS measurements have shown that μc-Si:H has more
microvoids than a-Si:H. The ITO is evaporated in an oxygen environment.
Therefore, oxygen may have a large chance to diffuse into the μc-Si:H
layer. In addition, the tolerance for impurity is lower for μc-Si:H
cells than for a-Si:H cells. To confirm this hypothesis, SIMS analyses
for samples with and without ITO have been arranged at NREL.

# **3. Large area deposition of a-Si:H/μc-Si:H double-junction solar cells**

Our main purpose in developing μc-Si:H cells is for the bottom cell in a
multi-junction solar cell for roll-to-roll production. There are many
unexpected problems in the transfer from small-area deposition to
large-area roll-to-roll production. Before the real transfer to a
production line, we need to test on a large-area batch machine. In this
quarter, we started to make μc-Si:H single-junction and a-Si:H/μc-Si:H
double-junction solar cells using a large-area machine (2B). We first
used a similar recipe developed using the small-area machine (LINE) with
RF glow discharge at a low rate \~ 1 Å/s. For real production, we
essentially need a high rate deposition. On small-area substrate, we
have used two methods for high rate μc-Si:H deposition, the MVHF glow
discharge and high pressure RF glow discharge. The performance is very
similar for the cells made using these two methods. For example, the
best a-Si:H/μc-Si:H double-junction cell made using MVHF is 12.5%, and
the one made using high pressure RF is 12.4%. However, we expected that
both methods would have some difficulties in regard to the uniformity on
large-area deposition. In the MVHF deposition, the non-uniform electric
field distribution over a large area cathode due to the short
electric-magnetic wavelength of the excitation is the main cause for
non-uniformity of deposition. To solve this problem, one needs special
designs for the cathode. In the high pressure RF deposition, the plasma
density normally becomes non-uniform with high plasma density at the
center of the cathode. To improve the uniformity of high pressure RF
deposition, one needs to modify the cathode and the gas flow
distribution. We have installed newly designed cathodes in our 2B
machine, which significantly improved the uniformity of both a-Si:H and
μc-Si:H depositions. We expected that using high pressure RF deposition
to achieve high rate μc-Si:H deposition with reasonable uniformity would
have less difficulties than using MVHF at the current stage. In
addition, the RF excitation is commonly used in production lines and it
is desirable from a cost-effective point of view. Based on these
considerations, we decided to try large-area μc-Si:H deposition using
high pressure RF glow discharge at high rate.

μc-Si:H single-junction and a-Si:H/μc-Si:H double junction solar cells
were deposited on 36 × 38 cm^2^ textured Ag/ZnO BR using the 2B machine.
The thickness uniformity was measured using an optical method and the
uniformity of cell performance was evaluated by measuring the small-area
cells cut from large-area cells. The small-area cells were on 4×4 cm^2^
substrates and defined by ITO dots on top of the cells with an
active-area of 0.25 cm^2^. Large area modules with areas of 45 and 461
cm^2^ were fabricated using our conventional procedure with special
consideration for the properties of the μc-Si:H cell.

## 3.1 Low rate deposition

Using the knowledge gathered during the optimization of μc-Si:H solar
cells using the small-area machine (LINE), we first optimized the
μc-Si:H single-junction solar cells and achieved reasonable cell
performance. We then incorporated the μc-Si:H cell into an
a-Si:H/μc-Si:H double-junction structure. The thickness of the top cell
was optimized to produce a desired current mismatch. Table II summarizes
the J-V characteristics of two small-area a-Si:H/μc-Si:H solar cells cut
from large-area substrates. The highest initial active-area efficiency
obtained so far is 12.1 % (sample 2B 9504) with closely matched current
densities between the top and bottom cells. Sample 9511 has large
current mismatching with a strong bottom cell limitation. Although the
initial efficiency of sample 9511 is slightly lower than that of sample
9504, the stabilized efficiency of sample 9511 should be higher than
sample 9504. Figure 4 shows the (a) J-V characteristics and (b) quantum
efficiency of sample 9504.

Uniform cell performance ensures high efficiency of the large-area solar
cells and modules. As presented in a previous report (Dec. 15,
2002-March 15, 2003), we have improved the thickness uniformity
significantly by using new cathodes. The thickness non-uniformity for
the slow rate μc-Si:H deposited with the new cathode is within 15 % in a
30 × 30 cm^2^ area. Here, we present the uniformity of the solar cell
performance over the large area substrate. Figure 5 (a) shows the
distribution of the conversion efficiency of the sample 9504 across a 22
× 22 cm^2^ area. The value at each position represents the average
efficiency of the cells with an active-area of 0.25 cm^2^ on a 4 × 4
cm^2^ piece. One can see that the cells with the highest efficiency are
located in the center area of the substrate. The cells on the edge have
lower conversion efficiency, whereas, at the corner, the cell efficiency
is the lowest. The performance non-uniformity is similar to the
thickness non-uniformity. Overall, the uniformity in the area (460
cm^2^) concerned for module fabrication is acceptable. Figure 5 (b)
shows the distribution (number of cells versus efficiency) of the
conversion efficiency for all the cells with an active-area 0.25 cm^2^.
The efficiency is in the range from 9.9% to 12.3 % with a peak position
at \~ 11.5 %.

a-Si:H/μc-Si:H double-junction modules with areas of 45 and 461 cm^2^
were fabricated. Figure 6 shows a picture of three modules. The best
aperture-area efficiency is 11.8 % for the 45 cm^2^ cell, and 11.3 % for
the 461 cm^2^ one. Figure 7 shows the J-V characteristics of the two
solar cells. Several solar cells with larger area \~ 460 cm^2^ were
encapsulated. The results are listed in Table III. After encapsulation,
the efficiency decreased by \~ 7 %, which is slightly more than the loss
in the conventional lamination for a-Si:H/a-SiGe:H/a-SiGe:H
triple-junction cells. The loss of the efficiency is mainly from the
reduction of the short-circuit current density. The highest efficiency
of the encapsulated large-area module is 10.6 %.

![](media/image19.jpeg){width="5.2875in" height="5.860416666666667in"}

## 3.2. High rate deposition

Low cost throughput in the production requires high rate deposition. In
order to make a-Si:H/μc-Si:H double-junction solar cells for production,
we need to reduce the deposition time significantly. Compared to the
current production line, we believe that the deposition time for the
μc-Si:H intrinsic layer in the bottom cell should be less than half an
hour. We used high pressure RF glow discharge to deposit μc-Si:H.
single-junction and a-Si:H/μc-Si:H double junction solar cells on the
large area Ag/ZnO BR coated stainless steel substrate. The deposition
time for the intrinsic layer of μc-Si:H cells was kept at 50 minutes at
the current stage. The growth parameters, such as RF power, pressure,
and hydrogen dilution ratio were optimized to achieve high efficiency
with an acceptable uniformity. An example of thickness uniformity is
shown in Fig. 8. Although the deposition rate is high, the uniformity is
acceptable over the large area. We have made a few runs of
a-Si:H/μc-Si:H double-junction cells with the bottom cell intrinsic
μc-Si:H deposited for 50 minutes. The highest initial active-area (0.25
cm^2^) efficiency is 11.3 % as shown in Fig. 9.

#   {#section .unnumbered}

#   {#section-1 .unnumbered}

# **4. High rate deposition of a-Si:H/a-SiGe:H/μc-Si:H triple-junction cell using MVHF glow discharge**

In previous reports, MVHF has been shown to be a powerful technique for
high quality μc-Si:H at high deposition rates. With this technique, we
have achieved an initial active-area efficiency of 12.5% using an
a-Si:H/μc-Si:H double-junction structure. We also tried to make
a-Si:H/a-SiGe:H/μc-Si:H triple-junction cells since the triple-junction
structure normally has lower light induced degradation. However, the
previous triple-junction cell did not show a high efficiency due to an
unoptimized a-SiGe:H middle cell and poor tunnel junctions. In this
quarter, we focused on the optimization for a-Si:H/a-SiGe:H/μc-Si:H
triple-junction cells by improving the middle cell performance and
optimizing the tunnel junctions. We have achieved an initial active-area
efficiency of 12.2 % with an a-Si:H/a-SiGe:H/μc-Si:H triple-junction
structure. Figure 10 shows the (a) J-V characteristic and (b) quantum
efficiency of the best a-Si:H/a-SiGe:H/μc-Si:H triple-junction.

# **5. Stability of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells**

In order to simulate the conditions of the production machine, the 2B
machine has been fitted with newly designed cathodes. For the reactive
gas in the middle and bottom cells, we have also switched from a
Si~2~H~6~ and GeH~4~ gas mixture to a SiH~4~ and GeH~4~ gas mixture. We
have achieved an initial active-area efficiency of 11.4 % for the
a-Si:H/a-SiGe:H/a-SiGe:H triple junction solar cell made under
production constraints. In this quarter, we continued to optimize the
growth recipe. In the meantime, we carried out the stability study for
the triple junction solar cells.

Two a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells with different growth
conditions were light soaked under a 100 mW/cm^2^ white light at 50 ºC
for over 1000 hours. The J-V characteristics were measured at different
times during the light soaking process. The degradation kinetics show
that the conversion efficiency is stabilized after \~ 200 hours of light
soaking. The quantum efficiency was measured at zero and 1000 hours.
Details of the cells' performance are listed in Table IV. The J-V
characteristics and quantum efficiency of sample 9423 LC1 are shown in
Fig. 11. The cell that we had previously reported as having the highest
initial efficiency (9423 LC1) reached the highest stable active-area
efficiency of 9.4%, corresponding to a total area stable efficiency of
8.7%. The light-induced degradation is \~ 18 %, which is similar to that
reported in a previous light soaking study.

#   {#section-2 .unnumbered}

# 

# **6. Summary**

We have achieved an initial active-area efficiency of 8.4% for a μc-Si:H
single-junction cell and 13.45% for an a-Si:H/μc-Si:H double-junction
cell using RF glow discharge at low rate ∼ 1 Å/s. The main improvements
resulted from the hydrogen dilution profiling during the deposition of
the intrinsic μc-Si:H layer.

We continued to optimized the μc-Si:H solar cells using MVHF at high
deposition rate and focused on the high efficiency of
a-Si:H/a-SiGe:H/μc-Si:H triple-junction cell. We achieved an initial
active-area efficiency of 12.5% using this triple-junction structure.

We have started to make a-Si:H/μc-Si:H double-junction solar cells on
large area substrate using our 2B machine and achieved a 12.1 % initial
active area (0.25 cm^2^) cell efficiency, a 11.8 % initial aperture-area
mini-module (45 cm^2^) efficiency, and a 11.3 % initial aperture-area
(461 cm^2^) large module efficiency. The highest efficiency for the
encapsulated module with area of 460 cm^2^ is 10.6 %. At high deposition
rate using high pressure RF glow discharge, we have deposited μc-Si:H
solar cells with reasonably good uniformity on large area substrate.
Preliminary results show an initial active-area efficiency of 11.3% in
an a-Si:H/μc-Si:H double-junction solar cell, where the bottom cell
intrinsic layer deposition time was 50 minutes.

A stability study on the high rate a-Si:H/a-SiGe:H/a-SiGe:H
triple-junction cell has been carried out. A stable active-area
efficiency of 9.4% has been achieved.