![](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

Jan. 28, 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
Oct/1/03-Dec/31/03 on the NREL Subcontract No. ADJ-1-30630-09 with the
University of North Carolina at Chapel Hill. We report Raman and PL
study on the relationship between the material properties and device
performance of hydrogenated nanocrystalline silicon solar cells made by
USOC.

Sincerely,

Daxing Han

cc: Carolyn Lopez (NREL)

Donna Braxton (UNC)

**[Quarterly Technical Status Report]{.underline}** Oct/1/03-Dec/31/03

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, J.R. Weinberg-Wolf,
E.C.T. Harley, graduate students; and Keda Wang, co-P.I.

Submitted: Jan. 28, 2004

Several techniques have been used to make µc-Si:H solar cells \[1-6\].
Understanding the correlation between the absorber layer properties and
the solar cell performance is one of the important issues for
optimization of the techniques. We have used Raman and PL to
characterize the microstructure and electronic density of states for
µc-Si:H materials and the intrinsic layer of the solar cells \[7-12\].
Here, we report the relationship between the material properties and
device performance of nc-Si:H solar cells made using conventional RF,
modified VHF and microwave glow discharge. From now, we use the term of
nc-Si:H instead of µc-Si:H because the silicon grain size is in nano
dimension in our studies. The nc-Si:H cell preparing conditions and cell
performance are listed in Table I. Each sample was about 1 cm x 1 cm and
indium-tin-oxide (ITO) dotes of area 0.25 cm^2^ were deposited on the
*p* layer as the top contact.

Table I. Sample preparing conditions, cell behavior and c-Si volume
fraction in the i-layer

+-------+-------+--------+------+-----+------------------------------+
| Depos | S     | Thi    | Rate | fc  | Jsc Voc effi. Ambient        |
| ition | ample | ckness |      |     | stability                    |
| m     | ID#   |        | (    | (%) |                              |
| ethod |       | (µm)   | A/s) |     |                              |
+-------+-------+--------+------+-----+------------------------------+
| VHF   | **11  | **\    | *    | **  | **High efficiency**          |
|       | 667** | ~1.1** | *3** | 40. |                              |
|       |       |        |      | 4** |                              |
+-------+-------+--------+------+-----+------------------------------+
|       | 11538 | 1.13   | 3    | 7   | High Jsc, stable             |
|       |       |        |      | 7.8 |                              |
+-------+-------+--------+------+-----+------------------------------+
|       | 10973 | 0.86   | 3    | 7   | High Jsc, stable             |
|       |       |        |      | 4.8 |                              |
+-------+-------+--------+------+-----+------------------------------+
|       | 10976 | 1.06   | 3    | 79  | High Jsc, unstable           |
+-------+-------+--------+------+-----+------------------------------+
|       | 11545 | 1.22   | 3    | 7   | Low Jsc                      |
|       |       |        |      | 9.4 |                              |
+-------+-------+--------+------+-----+------------------------------+
| RF    | 11486 | 0.71   | 12   | 7   | Low Jsc, unstable            |
|       |       |        |      | 2.1 |                              |
+-------+-------+--------+------+-----+------------------------------+
|       | **14  | **\    | *    | **  | **High efficiency**          |
|       | 085** | ~1.0** | *1** | 53. |                              |
|       |       |        |      | 4** |                              |
+-------+-------+--------+------+-----+------------------------------+
| µ     | 13917 | 1.06   | 1    | 8   | High Jsc, stable             |
| -wave |       |        |      | 0.6 |                              |
+-------+-------+--------+------+-----+------------------------------+
|       | 7073  | 0.84   | 2    | 3   | Low Jsc, stable              |
|       |       |        | 0-30 | 7.6 |                              |
+-------+-------+--------+------+-----+------------------------------+

fc, and fg.b.represent crystalline volume fraction, grain boundary
(g.b.) fraction , respectively.

Raman measurements were made directly on solar cells using 514.5-nm line
from an argon-ion laser under ambient conditions. The penetration depth
of the 514.5-nm light is \~60 nm for a-Si:H and is larger for nc-Si:H.
fc was deduced using\[7,8\]

fc =(Ic + Igb)/\[Ic + Igb +y(L)Ia\], (1)

where Ic + Igb and Ia are integrated intensities of the peaks associated
with c-Si, g.b., and a-Si:H components, respectively. y(L) decreases
from 1 to 0.8 when the grain size (L) grows from 3 nm to 10 nm. We
obtained a volume fraction of 37.6%-80.6% and the results are listed in
Table I. The Raman TO mode for the nine cells is shown in Fig. 1. In
general, the experimental results show that a high fc in the intrinsic
nc-Si:H leads to a high Jsc and low Voc.

![](media/image2.wmf){width="3.236111111111111in"
height="7.861111111111111in"}

Fig. 1 Raman spectra for the nine solar cells Fig. 2 PL spectra at 80 K
excited by 632 nm listed in Table I. (solid) and 442 nm (dashed) laser
beams.

PL spectra were measured in a temperature range of 80-300 K using 632.8-
and 442-nm laser line excitations to probe the electronic properties in
the bulk of the intrinsic nc-Si:H layer and the top layer near the *i/p*
interface, respectively. The PL spectra at 80 K for the nc-Si:H cells
are shown in Fig. 2. The solid and dashed lines represent the PL excited
using laser wavelengths of 632.8- and 442-nm, respectively. Most of the
cells show two separated PL peaks at 80 K. The broad peak at \~1.4 eV
originates from the tail-to-tail radiative recombination in the a-Si:H
region, the narrow peak at ≤ 0.89 eV is the nc-Si characteristic peak
from the tail-to-tail radiative recombination in the c-Si g.b. regions
\[7,9-11\]. The PL efficiency can be described by

yPL = Pr/(Pr + Pnr) = 1/(1+Pnr/Pr) (2)

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 yPL. One exception in
Fig. 2 is cell #7073 which was made by µW at a very high deposition
rate. In this cell, the low energy narrow PL peak does not appear either
in the bulk or the top layer. According to Eq. (2), this could be due to
not enough radiative recombination centers available but defects in the
g.b. regions. Furthermore, the width of the band tail, kTo, can be
obtained from PL intensity, 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 \[11\]. TL is deduced from the
experimental data by

yPL = yo exp(-T/TL). (3)

![](media/image4.wmf){width="3.138888888888889in"
height="3.736111111111111in"}

Fig. 3 The intensity fraction of the PL low Fig. 4 The same data from
Fig. 3 as a

Energy peak using 632 () and 442 nm ( ) function of the i-layer
thickness for nc-Si

laser excitations for the nine solar cells. cells prepared using VHF
technique.

For the rest of the cells, the nc-Si peak intensity enhanced and the
peak energy redshift when using 442-nm laser instead of the 632-nm laser
excitation as seen in Fig. 2. Assuming the PL efficiency is the same in
both the g.b. and the a-Si regions, PLc/(PLa+PLc) at 80 K gives a
measure of crystallinity \[13\]. As shown in Fig. 3, for most nc- Si:H
cells the fraction of PLc/(PLa+PLc) increase for 442-nm excitation from
that for 632.8 nm excitation, indicating an increase of crystallinity
along the growth direction. However, for the two cells with hydrogen
dilution profiling by reducing hydrogen dilution with growth time, the
fraction decreases from the bulk to the *i/p* interface. We noticed that
the low crystallinity in the *i/p* interface is related to the best
performance cells of #11667 and #14058. Furthermore, the relative
density of defects in the g.b. regions could be obtained from the nc-Si
PL fraction of PLc/(PLc+PLa) in comparison with fg.b.. PLc/(PLc+PLa) is
governed by the volume fraction, fg.b, and the PLc when the quality of
a-Si:H matrix (or PLa) had no changes. In Fig. 4, we replot the same
data in Fig. 3 as a function of the i-layer thickness for the five VHF
cells. The correlated fg.b. from Table I is also shown. One can see that
as the thickness increased, the fg.b slowly and gradually increased from
31% to 37%. Whereas, the PLc/(PLc+PLa) curves show two regimes: first a
rapid increase from 18% to 87%, then a slow increase from 87 to 98% in
the *i/p* interface layer; and first a rapid increase from 1% to 33% and
then a decrease from 33% to 18% in the bulk. The different behavior
between fg.b. and PLc/(PLc+PLa) is attributed to the PL efficiency; in
other words, the defect density \[14\]. For the cells with thickness
from 0.7 to \~1 µm, the average defect density in both the *i/p*
interface layer and the bulk were getting lower when the i-layer
thickness was thicker; and for thickness \>1 µm, there was not much
improvement in the *i/p* interface layer and it got worse in the bulk.
The latter case could be due to larger microvoids with more dangling
bonds in the g.b. regions. Whatever, Fig.4 indicates the i-layer
thickness at \~1 µm correlating to a low g.b. defect density that is
consistent with the conclusion of the optimized device design \[6\].

Fig. 5 The slope of the low energy PL band intensity vs. temperature
from the top layer () and the bulk () for the nine cells.

![](media/image6.wmf){width="3.0833333333333335in" height="3.5in"}We
found the slope of the high energy peak (for the a-Si:H matrix) TL \~
221 in all of the nine cells in the bulk. In the top layer TL \~ 171 for
the two H-profiling cells as well as the µW cell; there was no reliable
data to determine TL for the rest of the six cells because the PL signal
from the a-Si:H matrix was 2-3 orders of magnitude lower than from the
bulk. TL for the g.b. regions are shown in Fig. 5. This data can be
categorized into two types. In the cells with high Jsc, TL is smaller
than 22 K and changes less than one degree from the bulk to the top
layer. In contrast, in the cells with low Jsc, TL is larger (24-37 K)
and changes between 3 to 8 degrees when going from the bulk to the top
layer. According to Eq. (3), the band tail width kTo is proportional to
TL; hence, the data in Fig. 5 imply that the better ordered and
uniformly grown g.b. regions are related to high Jsc. In contrast, less
ordered and non-uniformly grown g.b. regions result in a lower Jsc.

SUMMARY

Raman results indicate that the high efficiency cells had fc of 40-50%.
Generally, a high fc leads to a high Jsc and low Voc. PL excited by 632-
and 442-nm laser beams indicate that the crystallinity increased along
the growth direction. The top layers of the two H-profiling nc-Si:H
cells, however, are dominantly a-Si:H; and these cells have the highest
efficiency, regardless of the preparation technique. For the same group
of cells, a comparison of the g.b. volume fraction fg.b. and relative
luminescence efficiency PLc/(PLa+PLc) gaves the relative defect density
in the g.b. regions. The results indicated that an i-layer thickness of
\~1 µm corresponds to a low defect g.b. material, and this is consistent
with the conclusions drawn from optimized device design. The band tail
width in the g.b. region was deduced from the PL intensity temperature
dependence, and indicates that a vertical structural uniformity is
related to a high Jsc.

In summary, the volume fraction, the bandtail width, and the relative
defect density of the g.b. regions can be obtained using Raman and PL
spectroscopies. In addition to reducing crystallinity in the *i/p*
interface layer, improving the quality of the g.b. region is important
for improving the nc-Si:H solar cell performance.

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13. The PL signal origins from the a-Si:H matrix and the g.b. regiones,
    hence the PLc/(PLc+PLa) correlates to the fraction of fg.b.=
    Ig.b./(Ig.b.+ Ia) that changes parallel with fc (see table I).