![](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

July 12, 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
April/01/04-June/30/04 on the NREL Subcontract No. ADJ-1-30630-09 with
the University of North Carolina at Chapel Hill. We are reporting the
micro-Raman studies of thickness dependence of µc-Si:H solar cells in
cooperation with United Solar Ovonic Corporation.

Sincerely,

Daxing Han

cc: Carolyn Lopez (NREL)

Donna Braxton (UNC)

**[Quarterly Technical Status Report]{.underline}**
April/01/04-June/30/04

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; Brittany Huie, student; Fengzheng
Liu, visiting scholar; and Keda Wang, co-P.I.

Submitted: July 7, 2004

> **Micro-Raman Studies of µc-Si:H solar cells with Varied Thickness**

Two series of µc-Si:H single-junction solar cells with thickness from
0.3-µm to 2.0-µm were studied using Micro-Raman spectroscopy. The
deposition condition of the intrinsic layers was the same for all of the
samples except the deposition time of the intrinsic layer. As listed in
Table I, one set of the samples was deposited on the textured Ag/ZnO
back reflectors (BR), and the other on specular stainless steel (SS).
X-ray diffraction (XRD) results have shown that the crystallinity
increases with the µc-Si:H film thickness. We used Micro-Raman
spectroscopy to study the effects of thickness on i-layer structures and
the electronic density of states of the solar cells.

**Table I.** Solar cell conditions

+-------------------+------------------------+------------------------+
| Sample            | i-layer thickness      | Bottom layer/substrate |
|                   |                        |                        |
|                   | (nm)                   |                        |
+-------------------+------------------------+------------------------+
| RF-12116          | 290                    | Ag/ZnO BR              |
+-------------------+------------------------+------------------------+
| RF-12120          | 490                    | Ag/ZnO BR              |
+-------------------+------------------------+------------------------+
| RF-12121          | 940                    | Ag/ZnO BR              |
+-------------------+------------------------+------------------------+
| RF-12119          | 990                    | Ag/ZnO BR              |
+-------------------+------------------------+------------------------+
| RF-12122          | 1340                   | Ag/ZnO BR              |
+-------------------+------------------------+------------------------+
| RF-12124          | 1930                   | Ag/ZnO BR              |
+-------------------+------------------------+------------------------+
|                   |                        |                        |
+-------------------+------------------------+------------------------+
| RF-12129          | 335                    | SS                     |
+-------------------+------------------------+------------------------+
| RF-12125          | 470                    | SS                     |
+-------------------+------------------------+------------------------+
| RF-12127          | 720                    | SS                     |
+-------------------+------------------------+------------------------+
| RF-12123          | 1040                   | SS                     |
+-------------------+------------------------+------------------------+
| RF-12128          | 1300                   | SS                     |
+-------------------+------------------------+------------------------+
| RF-12126          | 1970                   | SS                     |
+-------------------+------------------------+------------------------+

Micro-Raman measurements were made directly on solar cells using
514.5-nm line from an argon-ion laser under ambient conditions. The
diameter of the laser spot was 3 µm. The laser power density was one
order of magnitude lower than that of the laser-crystallization power
density. The penetration depth of the 514.5-nm light is \~60 nm for
a-Si:H and is larger for µc-Si:H. fc was deduced using \[1\]

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

where I~c~ + I~gb~ and I~a~ 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. Raman spectra were taken at four locations that are dispersed on
four quadrants for each dot.

Figure 1 shows the Raman TO modes for the cells on the textured Ag/ZnO
back reflectors with i-layer thickness from 290-nm to 1930-nm. Each
spectrum is the average of four spectra from four locations on each
solar cell. The c-Si volume fraction dispersion is ± 2-3% for the four
locations. The spectra are normalized at 520 cm^-1^ of the c-Si peak
frequency. One can see clearly that the a-Si component decreases in the
top layers of \~60 nm as the i-layer thickness increases from 290-nm to
1930-nm. The scattering signal between 530 cm^-1^ - 600 cm^-1^ is
related to the BR

![](media/image2.wmf){width="5.305555555555555in"
height="5.736111111111111in"}

Fig. 1 Raman TO modes for the cells on Ag/ZnO BR with a i-layer
thickness from 290-nm to 1930-nm. The arrows indicate the direction of
the increase of the thickness.

Figure 2 shows the Raman TO modes for the cells on SS. The results also
show that the a-Si component decreases in the top layers of \~60 nm as
the i-layer thickness increases from 335-nm to 1970-nm. Notice, the
lowest a-Si component is at 1300-nm and not at 1970-nm.

![](media/image3.wmf){width="5.305555555555555in"
height="5.736111111111111in"}

Fig. 2 Raman TO modes for the cells on ss with a i-layer thickness from
290-nm to 1930-nm. The arrows indicate the direction of the increase of
the thickness.

The Raman TO mode was deconvoluted into two Gaussian functions for
a-Si:H and intermediate components and one Lorenzian function for the
c-Si component. According to Eq. (1), the c-Si and g.b. volume
fractions, f~c~ and f~gb~ were calculated and plotted in Fig. 3 and Fig.
4 as a function of i-layer thickness for the cells on textured Ag/ZnO BR
and SS, respectively. One can see that in Fig. 3 when the thickness of
the i-layer increases, the f~c~ of the top layer increases; at above 1
µm the f~c~ gradually saturates. Meanwhile, the g.b. component decreases
slightly when i-layer thickness increases. However, as shown in Fig. 4
for the solar cells on SS substrate, when the thickness of the i-layer
increases, the f~c~ of the top layer does not increase as simply as that
shown in Fig. 3 for the solar cells on BR. The f~c~ of the top layer
reaches a maximum at above 1.3 µm and then decreases. It indicates the
effect of the substrate surface on the i-layer structures.

![](media/image4.wmf){width="3.4305555555555554in"
height="4.486111111111111in"}

Fig. 3 f~c~ and f~gb~ as a function of i-layer thickness for the cells
on textured Ag/ZnO BR.

> Fig. 4 f~c~ and f~gb~ as a function of i-layer thickness for the cells
> on SS.

**SUMMARY**

The XRD results on µc-Si films and early Raman results on µc-Si cells on
Ag/ZnO BR \[2\] showed that the f~c~ increases with the film thickness.
We have double checked the Raman data and found that although the
spectra were reproducible the values of the f~c~ were too high (60%-80%
needs to be 40%-60%) due to improper data analysis. For these two groups
of samples, two persons did the data analysis respectively. The results
were consistent for each. We found that the value of f~c~ in the top
\~60-nm layer reaches a saturation and/or decreases when the i-layer
thickness \> 1µm.

It has been found \[2\] that J~sc~ reaches a maximum with a thickness in
the range between 1 and 2 µm. Figs 3 and 4 imply a possible link between
J~sc~ and f~c~ which is that an increase of the thickness leads to an
increase of the resistance after the f~c~ value was saturated. Hence,
increasing the thickness further leads to a decrease in J~sc~.

**Reference**

1.  Daxing Han, J. D. Lorentzen, J. Weinberg-Wolf, L. E. McNeil, and Qi
    Wang, J. Appl. Phys., **94**, 5 (2003) 2930-2936.

2.  Baojie Yan, Guozhen Yue, Jeffrey Yang, Subhendu Guha, Keda Wang and
    Daxing Han, MRS *symp. Proc.* **808**, A8.5 (2004), in press.