![](media/image1.wmf){width="0.625in" height="0.6111111111111112in"}

the university of north carolina

[at]{.smallcaps}

[chapel hill]{.smallcaps}

Department of The University of North Carolina at Chapel Hill

Physics and Astronomy CB# 3255, Phillips Hall

College of Arts and Sciences Chapel Hill, N.C. 27599-3255

Tel: (919) 962-5002

Email: daxing@physics.unc.edu Fax: (919) 962-0480

March 31, 2004

Dr. Bolko von Roedern

NREL

1617 Cole Blvd.

Golden, CO 80401

Dear Bolko,

Here is the Quarterly Technical Status Report for the period of
Jan/1/04-March/31/04 on the NREL Subcontract No. ADJ-1-30630-09 with the
University of North Carolina at Chapel Hill. We are reporting the t
buffer-layer effect on mixed-phase cells studied by micro-Raman and
Photoluminescence spectroscopy in cooperation with United Solar Ovonic
Corporation.

Sincerely,

Daxing Han

cc: Carolyn Lopez (NREL)

Donna Braxton (UNC)

**[Quarterly Technical Status Report]{.underline}** Jan/1/04-March/31/04

NREL Subcontract ADJ-1-30630-09 with the University of North Carolina at
Chapel Hill

Bolko von Roedern, NREL Technical Monitor;

Carolyn Lopez, NREL Subcontract Associate;

Daxing Han, principal investigator; Andrea Hilchey, Chris Lawyer,
graduate students; and Keda Wang, co-P.I.

Submitted: March 31, 2004

We used micro-Raman and PL spectroscopy to study the effects of an
a-Si:H buffer layer at the *i/p* interface of the mixed-phase silicon
solar cells. Mixed-phase silicon *n-i-p* solar cells were deposited
using RF glow discharge on specular ss substrates. The *i* layer is
about 250 nm thick. The top contacts are ITO dots with an active area of
0.25 cm^2^. The difference of solar cells in sample L13869 is in the
crystalline volume fraction and cell performance due to a nonuniform
deposition. In this sample, no a-Si:H buffer layer was deposited between
the mixed-phase intrinsic layer and the *p-*layer. The cells in the
second sample L13868 and third sample L13870 were deposited under the
same conditions as in the first sample except an a-Si:H buffer layer
with a thickness of 100 Å and 500 Å, respectively, was deposited between
the *p* layer and the *i* layer. The initial cell performances are
listed in Table I. In the initial state, the 100 Å thick buffer layer
blocks the shunt current and increases V~oc~ and FF for the mixed-phase
solar cells (cell 11 and cell 13), but does not show an effect on the
cell with a fully amorphous signature (cell 23). By increasing the
thickness of the buffer layer to 500 Å, the initial V~oc~ still
increased for the mixed-phase cells, but the FF decreased due to a kink
in the J-V curve caused by inadequate tunneling through the a-Si:H
buffer layer. [After light soaking, the light-induced V~oc~ increase was
observed in both the cells with no buffer layer and with a 100 Å hick
buffer layer, but was not observed in the cells with a 500 Å thick
buffer]{.underline}.

**Table I.** The initial performance of the mixed-phase solar cells with
different a-Si:H buffer layers at the *i/p* interface.

+---------+--------+----------+----------+----------+---------+-------+
| Sample  | Cell   | Buffer   | P~max~   | J~sc~    | V~oc~   | FF    |
| \#      | \#     |          |          |          |         |       |
|         |        | (Å)      | (m       | (m       | \(V\)   |       |
|         |        |          | W/cm^2^) | A/cm^2^) |         |       |
+---------+--------+----------+----------+----------+---------+-------+
| L13869  | 11     | 0        | 4.39     | 10.8     | 0.695   | 0.587 |
+---------+--------+----------+----------+----------+---------+-------+
|         | 13     | 0        | 5.47     | 10.3     | 0.852   | 0.621 |
+---------+--------+----------+----------+----------+---------+-------+
|         | 22     | 0        | 7.05     | 10.3     | 1.001   | 0.682 |
+---------+--------+----------+----------+----------+---------+-------+
| L13868  | 11     | 100      | 4.77     | 10.9     | 0.725   | 0.604 |
+---------+--------+----------+----------+----------+---------+-------+
|         | 13     | 100      | 6.08     | 10.4     | 0.905   | 0.644 |
+---------+--------+----------+----------+----------+---------+-------+
|         | 23     | 100      | 7.08     | 10.4     | 1.003   | 0.680 |
+---------+--------+----------+----------+----------+---------+-------+
| L13870  | 11     | 500      | 4.19     | 11.4     | 0.911   | 0.403 |
+---------+--------+----------+----------+----------+---------+-------+
|         | 13     | 500      | 6.83     | 11.3     | 0.970   | 0.625 |
+---------+--------+----------+----------+----------+---------+-------+
|         | 22     | 500      | 7.35     | 11.3     | 0.979   | 0.664 |
+---------+--------+----------+----------+----------+---------+-------+

Figure 1 shows the Raman spectra for the nine cells on the three
samples. The three curves from the bottom to top in each plot are
arranged with increasing microcrystallinity (decreasing Voc). One can
see that the corresponding curves in Fig. 1(b) for the samples with a
100 Å thick a-Si:H buffer layer are almost identical to those in Fig.
1(a) for the samples with no buffer layer. This is because the 514.5-nm
laser light penetrates through the 100 Å thick a-Si:H buffer layer, and
the signature of the microcrystalline grains still appears on certain
areas of the cell. However, no microcrystalline signature is observed on
the spectra of the sample with a 500 Å thick a-Si:H buffer layer since
the Raman scattering is mainly from the a-Si:H buffer layer. Concerning
the relationship between the Raman spectrum and Voc, we find that for
Voc ≥ 0.911 V, regardless of the crystallinity and the buffer layer, the
TO mode peaked at \~ 480 cm-1 with a width of 68 cm-1 but without the
peak at \~520 cm-1, indicating a typical amorphous structure. In this
case, the crystallinity in the probed region near the *i/p* interface is
below the measurement sensitivity.

![](media/image2.wmf){width="2.638888888888889in" height="3.4in"}

![](media/image3.wmf){width="2.625in" height="3.4in"}

**Figure 1.** Raman spectra for the cells on (a) L13869, (b) L13868, and
(c) L13870, corresponding to no buffer layer, 100 Å thick a-Si:H buffer
layer, and 500 Å thick a-Si:H buffer layer, respectively.

PL spectra were measured in a temperature range of 80-300 K using
632.8-nm laser line excitations to probe the electronic properties in
the bulk of the intrinsic mixed-phase layer including the a-Si:H buffer
layer. At low temperatures, the high energy PL originates from the
tail-to-tail radiative recombination in the a-Si:H region, the low
energy signal is from the c-Si grain boundary (g.b.) regions. The PL
intensity, I~PL~, is proportional to the PL efficiency as

IPL Pr/(Pr + Pnr) = 1/(1+Pnr/Pr), (1)

where Pr and Pnr are the probability for radiative- and non-radiative
transition, respectively. When the density of defects increases, Pnr
increases and Pr decreases, thereby decreasing IPL. Furthermore, the
width of the band tail, kTo, can be obtained from IPL temperature
dependence. According to the carrier thermalization mode, kTo =
kTLln(or) where TL, o and r are the slope of IPL-T, the
attempt-to-escape frequency and the carrier\'s recombination life time,
respectively . TL is deduced from the experimental data by IPL = Io
exp(-T/TL).

![](media/image5.wmf){width="4.152777777777778in"
height="3.2083333333333335in"}

Fig. 2 PL spectra at 80 K for cells 11, 13, and 22 on sample 13869 for
(a) linear and (b) semi-log plots.

![](media/image6.wmf){width="4.138888888888889in"
height="3.3194444444444446in"}

Fig. 3 PL spectra of the cells of L13869-11, L13868-11, and L13870-11
for (a) linear and (b) semi-log plots.

Now, we show the effect of crystallinity in the mixed-phase *i*-layer on
the PL lineshape. Fig. 2 shows the PL spectra at 80 K for the three
cells on sample L13869. As the crystallinity increases, the main band
becomes narrower; and the low energy (\<1.2 eV) signal increases as seen
clearly in the semi-log plot. When a portion of the microcrystalline
phase increases, the band tail of the a-Si:H matrix becomes narrower and
extra radiative recombination from the g.b. region appears. Then, we
show the effect of the a-Si:H *i/p* buffer layer on the PL lineshape.
Figure 3 plots the PL spectra of cell 11 from the three samples. These
cells are located at the same position on the substrate, but with
different thickness of the a-Si:H buffer layer (0, 100 Å, and 500Å). The
a-Si:H buffer layer makes the PL main band broad and the low energy PL
band from the g.b. regions is suppressed, which is similar to decreasing
the crystallinity as shown in Fig. 2.

![](media/image7.wmf){width="2.6666666666666665in"
height="2.888888888888889in"}The differences in the PL features among
the nine cells can also be seen from the temperature (T) dependence of
the PL intensity (I~PL~). As shown in Fig. 4 for mixed-phase cells, the
PL spectrum generally contains three components: a main band from a-Si:H
at high energy with strong temperature dependence, a g.b. band at low
energy with a weak temperature dependence, and a defect band that
appears only at T\>180 K. For the cells with no a-Si:H buffer layer
(sample L13869), with the increase of crystallinity (from cell 22 to
11), the PL intensity of the g.b. band increases, and the slope of the
I~PL~-T plot of the a-Si:H component becomes steeper. According to Eq.
(2) we obtained a PL characteristic temperature of T~L~\~20 K for cell
11, which is smaller than those from the other two cells (\~22 K). A
smaller T~L~ implies a narrower band tail. Therefore, the PL intensity
result is consistent with the PL lineshape, where cell 11 has the
narrowest bandwidth as shown in Fig. 2. For the cells with a 100 Å thick
a-Si:H buffer layer (sample L13868), the a-Si:H main-band and
defect-band for the three cells are close to each other as shown in Fig.
4 (b) due to the PL signal from the a-Si:H buffer layer. Whereas, the
g.b. band still appears in the same order as in Fig. 3(a) but with a
relatively weak intensity. The spectra in Fig. 4(c) show typical
characteristics for intrinsic a-Si:H films. Only the a-Si:H main-band
and the defect-band are observed. The g.b. band is completely suppressed
due to the PL contribution from the 500 Å thick a-Si:H buffer layer. In
addition, the PL spectra for these three cells show the common features
of the a-Si:H, including the T~L~ around 20-22K, the PL peak energy
around 1.40-1.42 eV at 80 K, and the temperature coefficient of the peak
energy around 1.5-2.0 meV/K. These data indicate a good quality a-Si:H
matrix for all the cells.

Fig. 4 PL intensities vs. temperature for the cells on (a) L13869, (b)
on L13868, and (c) on L13870, where () represents the a-Si:H main band,
() the g.b. band, and () the defect band.

After the PL was measured in the initial state (state A), *in-situ*
light soaking was done at room temperature for 4 hours using a 200
mW/cm2 white light with an infrared cut-off filter. The PL spectrum was
then measured at temperatures from 80 K to 300 K in the light soaked
state (state B). Figure 5 shows the PL spectra at 80 K before and after
light soaking for cells L13869-11, L13868-11, and L13870-11. We find
that the PL main band becomes slightly wider and the PL total intensity
also increases slightly for cell L13689-11, consistent with our previous
observation. For cell L13868-11, no detectable change was found on the
PL spectra, which could be due to the opposite effect of the a-Si:H
buffer layer and mixed-phase intrinsic layer on PL. The cell L13870-11
shows a decrease of the total PL intensity by \~40%, which is normally
observed in a-Si:H because of the light-induced defect creation.

![](media/image10.wmf){width="2.4722222222222223in"
height="2.5833333333333335in"}![](media/image12.wmf){width="2.4722222222222223in"
height="2.5833333333333335in"}

Fig. 5 PL spectra at 80 K before and after light soaking for cells (a)
L13869-11, (b) L13868-11, and (c) L13870-11.

**SUMMARY**

Similar features in both the Raman and PL spectra were observed in the
mixed-phase solar cells without and with a 100 Å-thick a-Si:H buffer
layers at the *i/p* interface. On the other hand, both the Raman and PL
results were similar to standard a-Si:H solar cells when the a-Si:H
buffer layer is 500 Å thick, where the crystalline silicon TO mode on
the Raman spectra and the g.b. component on the PL spectra completely
disappear. Light soaking results in a slight broadening in the PL main
band for the mixed-phase cell with no a-Si:H buffer layer. The
light-induced defect generation in the 500 Å-thick a-Si:H buffer layer
masked the changes in the mixed-phase intrinsic layer underneath. The
Raman and PL results are consistent with the solar cell results in the
sense that a thin a-Si:H buffer layer does not change the behavior of
the mixed-phase solar cells upon light soaking, but a thick buffer layer
does.