The objective of this study is to improve the stability and efficiency of thin solar cells with emphasis on a-Si:H devices. The research project was broken down into three main phases. The first involves designing and building a UHV glow discharge system; the second involves making good quality films and eventually efficient cells; the final phase will be analytical

Herman, A. M.

Naseem, Hameed

English

NASA Technical Reports Server

YSEMI-ANNUAL REPORT
STUDY OF STAEBLER-WRONSKY DEGRADATION EFFECT IN
a-Si:H BASED P-I-N SOLAR CELLS
Principal Investigators:
A.M. Hermann, Department of Physics
H. Naseem, Department of Electrical Engineering
University of Arkansas
Fayetteville, Arkansas 72701
(K1SA-CB- 18256*4) STD3H OF £lf i IBIE8-HRGNSKY
DIGRaiAUCN Eff lCl ID A Si:E EASID P-I-N
SCIAE CEILS Semiannual Beport - (Arkansas
Oniv ) 23 F CSCI 10A
NASA-Lewis Research Center
Grant No. NAG 3-802
S68-25S70
DJDClas
01*49039
July 1988
Hameed Naseem, Principal Investigator
https://ntrs.nasa.gov/search.jsp?R=19880016586 2020-03-20T06:50:34+00:00Z
AQCNOWLEDGEMENT
This report is prepared by H. Goh. The results reported here have been
contributed by two graduate students, H. Goh and K. Ng. Dr. G. Blyholder and
Dr. Pienta of the Department of Chemistry provided assistance and usage of
the FTIR and UV-visible spectroscopy machines. The ultrahigh vacuum
deposition system built under this grant to deposit a-Si:H films utilized a
lot of existing equipments and facilities belonging to Prof. W. Brown. Much
assistance was also given by Dr. Khaliq.
CONTENTS
Page
List of Figures ii
Objective 1
Status Report 1
Full-Factorial Experiment 2
Film Diagnostics 6
1. Fourier Transform Infra-Red Spectroscopy (FTIR) 6
2. Visible Spectrophotometry 6
3. Photo- and Dark Conductivity 8
4. Film Thickness 8
5. Refractive Index 12
Summary of Results For The i-Layer a:Si-H 16
PIN Solar Cells 16
Plan of Work (June 1,1988 - September 30, 1988) 17
References 19
ii
List of Figures
Figure 1. An Example of a Curvature Effect Between Two Factors
Figure 2. FTIR Spectrum Showing Higher SiHa to SiH Ratio (Poorer Quality
Film)
Figure 3. FTIR Spectrum Showing Lower SiHz to SiH Ratio (Better Quality
Film)
Figure 4. Absorbance vs Wavelength Plot For Band Gap Calculation
Figure 5. Tauc Optical Band Gap of Sample 19
Figure 6. Tauc Optical Band Gap of Sample D3
Figure 7. Dark Conductivity Measurement
Figure 8. Light Conductivity Measurement
Figure 9. Plot of Transmittance vs Wavelength (for Thickness Determination)
1
OBJECTIVE
To improve the stability and efficiency of thin solar cells with
emphasis on a-Si:H devices. In our last semi-annual report (Dec 1987) [1],
we basically break down the project into three main phases. The first
involves designing and building an UHV glow discharge system; the second
involves making good quality films and eventually efficient cells; finally
the third will be the analytical phase.
STATUS REPORT
On May 15th, 1987 the grant was awarded. Ordering of equipments and
materials began. On August 1st, 1987 we started building the system; by the
beginning of December the entire system was in place. Leak checking
procedures followed. The system was leak checked using a Dycor residual gas
analyzer (RGA) and helium as a trace gas. At the same time, the system was
monitored for its leak rate over time.
The first 'dummy run' was made on December 9th, 1987. An aluminum foil
and a both sides polished P-type silicon wafer were used as substrates. To
our dismay, we found that the films had a lot of pinholes. Later, it was
found that the problem was due to the reflected power. From December 15th,
1987 to January 15th, 1989, we worked with GSI in Colorado to remedy this
problem.
From January 28th, 1988, we started on the full-factorial experiment
[2]. The different physical characteristics of every system and the many
factors involved, made finding the 'correct' parameter values difficult.
Thus the need to know the optimum parameters arose. Our primary goal from
this experiment was to find the optimum parameters of the built system,
2although this experiment will also furnish more information.
We believe that the bulk i-layer is the first most important problem to
be dealt with, and the full-factorial experiment should help us tackle this
problem. To the best of our knowledge, there is no published work on
optimization of this a-Si:H i-layer. This full-factorial experiment is a
structured approach based on statistical design and analysis of the
experiments. This approach will provide data on the interaction effects (if
any), the vicinity of the optimum points, and an equation on the interaction
effects.
FTIR, visible spectrophotbmetry, photo- and dark conductivity, and film
thickness measurements are then made on the samples. They provide
information on the ratio of the SiH to SiH2 bonds density, the Tauc band-
gap values, the photo-generations, density of states, etc. FTIR measurements
are made on a-Si:H deposited on both sides polished p-type (100) wafers
while the spectrophotometry, conductivity, and thickness measurements are
made on a-Si:H deposited on Corning 7059 glass. Conductivity measurements
involve contacts. Various types of contacts are investigated. They include
thermally evaporated indium, aluminum, pure silver, and brushed-on silver
paint.
Full-Factorial Experiment
Traditional experiment design has often been conducted haphazardly, with
"change one variable at a time, and keeping all the others constant",-as the
single unifying principle. This can be known as one-at-a-time type
experiments. In fact this method is not only inefficient, it is rather
incomplete in terms of information obtained. On the other hand, a formal
3experimental plan, consisting of a sequence of structured experiments (based
on statistical principle) is far more likely to produce a maximum of data,
with a minimum expenditure of effort and resources.
There are a number of varying factors involved, namely, substrate
temperature, deposition pressure, gas flow rate, RF power, feedgas
concentration, frequency, and electrode spacing. The specifications on the
feedgases are: silane 99.999% pure, diborane 99.995% pure, and phosphine
99.999% pure. No diluent is used. The frequency of the RF power is fixed at
13.56 MHz, and the electrode spacing is also fixed at 1.11" (± 0.010").
Thus, the experiment is conducted with variation of the first four
parameters: substrate temperature, pressure, gas flow rate, and RF power.
Next a choice on the ranges of the parameters is made; this will
determine the domain of our experiment:
Substrate Temperature: 230 to 250 °C
Deposition Pressure : 500 to 600 mT
Gas Flow Rate : 30 to 45 seem
RF Power : 21.6 to 30.8 mW/cm2
(3.5 to 5 W; area=162.15cra2)
The higher end of the ranges are designated with a. (+) sign, and the
lower end of the ranges are designated with a (-) sign. The experiment will
be two-level. Since there are four variables, there will be two to the power
of four or sixteen experiments to run. The values of the parameters used to
perform the experiments are (-,-,-,-), (-,-,->+), (-,-»+,-) ..... ( + ,+,+,+)
not in that order. In fact, to reduce error bias the experiments are
randomized. On the next page, there is the computational table where XI
represents the first variable, X2 the second, X3 the third, and X4 the
fourth. X1X2 represents the possible interaction of the two, while X1X2X3
represents the possible interaction of the three and so forth. These
products (interaction possibilities) are used in later computations.
Another value, chosen midway between the higher and the lower of the
ranges and designated with a (0) sign (not listed in the computing table),
is used to determine the curvature effect. Thus the (0) values for substrate
temperature, deposition pressure, gas flow rate, and RF power are 240 °C,
550 mT, 37.5 seem, and 26.2 mW/cm2 respectively. Four such experiments are
spaced evenly among the previous sixteen runs. To reduce experimental and
measurement errors, another similar set (i.e. another twenty runs) will be
repeated. Figure (1) below shows an example of a curvature effect between
two factors.
Responv Y
i
C) Mild Inn
a>
flnponicY
FACTO* A
HighB
Rwoon««Y
HighB
tO No interaction Strong i
f't- 1(») Experiment space for a two-factor experiment: (b . d) Inleraclion of A Md B: (b) no
interaction; (c) mild inunction: (d) strong interaction.
STRUCTURED APPROACH TO DESIGN OF EXPERIMENTS FOR PROCESS OPTIMIZATION,
Computing Table for Analyzing—Factorial Design* with 2 to 4 Factors
RUN
I
2
3
4
5
6
7
go
9
10
11
12
13
14
15
16
Mean Xi x^
4 4
4- - 4
4 4 4
4
4 4 -
4 - 4
4 - -
4 4 -
4 - 4
4 4 4
4 - -
4 4 .. -
4 - 4
4 4 4
4 - 4 4 - - 4 4 - 4
4 4 - 4 ' - 4 '4 - 4
4 - 4 - 4 - 4 4 - 4
4 - - - - - - - - . . 4 4 4
4 4 - - 4 - 4 4 - - 4 4
44 - - - - . 4 4 - - 4
4 - 4 - - 4 - 4 - - 4
4 - 4 4 - 4 - - 4 - 4 4
- - 4 4 44 - - - - 4
4 - 4 4 - 4 - - 4
4 - - - - 4 4 4 4 -
4 4 - - 4 4 - - 4 4
- 4 4 - - 4 4 - - 4 4
- 4 - 4 - 4 - 4 - 4 - 4
4 4 4 4 4 4 4 4 4 4 4 4
FILM DIAGNOSTICS
As mentioned before, various measurements are made.
1. Fourier Transform Infra-Red (FTIR) spectroscopy (transmittance
spectroscopy) measurements are done with a Nicolet DX 10 model FTIR machine.
The objective of this measurement is to determine both the hydrogen content
and the silicon-hydrogen bond configurations. The dihydride (SiHz) bonding
has been known to cause poor film quality. So, by finding the ratio of the
monohydride-to-dihydride, we can have a good indicator of the quality of the
film; the ratio should be as high as possible. Two absorption peaks commonly
used to determine the dihydride mode are 875 cm"1 (bending) and 2090 cm"1
(stretching). We used the peak at 2090 cm'1. The monohydride (SiH) peak at
2000 cm'1 (stretching) is used.
At first we deposited the a-Si :H on an aluminum foil, hoping to separate
the film from the foil by etching away the foil with HCL acid. If this is
possible, for sure there will be no interference from the substrate during
the FTIR measurements. This method was successfully applied on silicon
nitride film. However, our attempt on a-Si:H failed, as we believe aluminum
degrades the film. So, we resorted to using the crystalline silicon for
substrate. Figure (2) and Figure (3) compare the spectrums of the absorption
peaks. The Figure (2) indicates a poorer quality film (a-Si:H i-layer), and
Figure (3) indicates a much improved quality material.
2. Visible spectrophotometry is done on a Gary 210 spectrophotometer.
This measurement is used to determine the optical band gap (Tauc) of the
films. Wavelengths from 400 nm to 750 run are scanned. The spectrophotometer
a
a
.
•UJ
Q
5.
4!01
(N
UJ
C*
u
oose -o /.iee - eeie *a E8S2

plots the absorbance against the wavelength. A typical plot is shown in
Figure (4). The absorption coefficient a is chosen such that it is greater
than 104 cm"1 so that the solar spectrum can be absorbed [3]. Figure (4) is
interpolated, and the data is used to plot the square root of the product of
the absorption coefficient and the photon energy [/(ohu)] against the photon
energy [hu]. The intersection of that function with the x-axis gives the
optical band gap. Also, the slope of this function can be used to
characterize disorder in amorphous silicon-based alloys. Figure (5) shows a
regressed plot of sample 19 having a band gap, Eg=1.737, and Figure (6)
shows a plot of sample D3 having a band gap, Eg=1.621.
Absorbance
FIGURE 4
450 700 run
Wavelength
3. Both the photo- and dark conductivity measurements are done with the
HP-4154A semiconductor parameter analyzer. Initially the vertical geometry
is used, and the contacts are thermally evaporated aluminum and indium. We
faced surface field effect and non-ohmic problems with both metals. Then the
2.2,
» 3.
1
FIGURE 5. TftilC OPTICAL BAND GAP
SAMPLE 19 ; Eg : i.737
4
A /
/
/ **
PHOTON EHHCT, (h * nu), eU
1C
THE REGRESSION POLYNOMIAL OF LINE 1 -
(-5.660E + 00) 4- ( 3.259E+00 )-*X
THE VARIANCE - .1.317E-02.
ORIGINAU PAGE isOF; POOR QUALITY
11
D
2.2,
1
-i
1
-j
H
4
A
 H3 ' -ij -i
•Mi -j
* -IJ
.j it -i
FIGURE 6. TAUC OPTICAL BAND GAP
A SAMPLE D3 ; Eg = 1.621 eU
v 3
J4 ~iji 1
PHOTON ENERGY, ( h * n u ) , eU
THE REGRESSION POLYNOMIAL OF LINE 1 -
(-5.090E+00) + ( 3.141E+00)*X
THE VARIANCE - 3.701E-04
12
planar geometry is used for the conductivity measurements [4]. Silver paint
is used as contact. Two one-centimeter strips are painted one millimeter
apart. Copper wires are applied on top of the silver paint [5]. A voltage of
one hundred volts is swept through one of the terminals and current in the
picoamps is obtained. For the photoconductivity measurement, AMI.5 of light
is supplied by an ELH bulb. Figure (7) shows a dark measurement, and Figure
(8) shows a light measurement. Also, the ratio of the photo- over the dark
conductivity (photosensitivity) is calculated, which is a good indicator of
the quality of the film.
4. Film thickness measurement is done using a monochrometer and an
integrating sphere. Various wavelengths from 600 nm to 1100 nm is shone
through the a-Si:H film; the power is then recorded. A plot of transmitbance
against the wavelength is drawn, as shown in figure (9). The distance
between the two peaks is measured. Finally, the thickness is calculated
using thickness, d = m x Xi x Xz / 2 x R.I. x (X2 - Xi) where, n=l (next
peak), Az = peak on the right, Xi = peak on the left, and R.I.= refractive
index.
5. Again with the monochrometer and the integrating sphere, the
refractive index (R.I.) of the a-Si:H material is determined. The refractive
index is 3.5.
o o o
o o o
o • o
o o o
o
o
o
o
ORIGINAL PAGE IS.
OF POOR QUALITY!
13-
.. £ »
«t U •
n
.. c
• u
4* I
c•+ c.
A • 4»
• • c. a a 4»
•« c • o • •
C U. «« 4* 4* 41 C
• > j a a ai a> UJ
uu
o
o
iJ
o/
U-
N
u.
000
o o o
o • o
o o o
o
o
o
o
14
~52 *5
• I 4» I
•+ C. Ca m *t m
• • c a a -w
•*• c • o • •
C k. *« 4» «« 4» C
• > JCD (0 ta o>
CO
U-
b.
M
X
15
FIGURE 9. TRANSMIIIftNCE US UAUELENGIH (FOR THICMSS CAL.)
a
y
z
H
H
I
Z
X
H
Wft'OELZNGIH (DM)
16
SUMMARY OF RESULTS FOR THE i-LAYER a-Si:H
Sam Eg SiH/SiH2 Dk. OOND Lt. COND Lt/Dk D.R. (T,Pr,F,Pwr)
(A/sec)
D2
D3
D4
D5
D6
D7
D8
D9
D10
Dll
D12
D13
D14
D15
D16
D17
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
.6357
.7057
.6464
.7149
.7057
.6214
.6255
.6642
.6900
.5248
.6810
.6242
.6709
.6620
.6395
.5565
33
4
2
2
44
12
10
13
21
4
9
22
35
11
14
7
.900
.046
.019
.362
.345
.268
.685
.616
.624
.535
.084
.931
.600
.285
.640
.450
5
1
6
2
2
1
1
3
9
3
2
1
9
1
4
1
.36E-10
. 14E-10
.02E-10
.92E-10
.28E-09
.45E-09
.79E-10
.15E-10
.35E-10
. 16E-09
.09E-09
. 58E-09
.76E-09
.94E-09
. 54E-09
.08E-09
9.
5.
3.
1.
2.
2.
8.
9.
1.
5.
3.
5.
1.
8.
1.
3.
30E-5
53E-5
45E-5
39E-5
11E-5
19E-5
16E-6
09E-5
87E-4
63E-6
45E-6
OOE-5
91E-4
30E-5
47E-4
39E-6
1
4
5
4
9
1
4
3
2
1
1
3
1
4
3
3
.74E+5
.83E+5
.73E+4
.76E+4
.24E+3
.51E+4
. 57E+4
. 14E+5
.OOE+5
.78E+3
.65E+3
. 18E+4
.95E+4
.27E+4
.24E+4
.15E+3
2.986 (- - + +)
4.029 (-+++)
2.008 (+++-)
3.171 (-++-)
2.051 (+-+-)
2.111 (-+-+)
1.857 ( )
3.148 (+--+)
2.252 (++++)
1.826 (+ + - -)
1.484 (--+-)
1.699 (+ )
2.612 (+-++)
2.986 ( +)
2.111 (++-+)
1.320 (-+--)
For above: T = temperature in °C; Pr = pressure in milli-torr
F = flow rate in seem; Pwr= Power in milli-watts/cm2
temp press flow pwr
(+) 250
(-) 230
600
500
45
30
30.8
21.6
PIN Solar Cells
From the table above, taking the light/dark conductivity ratio as the
significant indicator, we chose sample D9's parameter to fabricate some
p-i-n solar cells. We manage to obtain a high Voc of 0.845 volts in one cell
and a high Jsc of 9.05 mA/cm2 in another. These cells have average areas of
0.01767 cm2. The highest efficiency obtained so far is about three percent.
17
Presently, we are plagued with low fill factors and low current densities.
The boron concentration in the i-layer is found to be a very important
factor. By varying the pumping down time (removal of the diborane), after
the deposition of the p+ layer, we obtained drastic changes in the
efficiencies of the cells. Furthermore, the thickness of the p+ layer is
found to also have a great impact on the efficiency. Just a mere 30
Angstroms or less in the thickness of the P+ layer, can affect the result
tremendously. Also pure silver was found to give better contacts.
PLAN OF WORK (JUNE 1.1988 - SEPTEMBER 30. 1988)
We believe that the i-layer is of good solar grade, by virtue of the
good Voc obtained. It is a matter of time before we combine the pieces
together to obtain the recipe of trivial fabrication of >7 percent efficient
solar cells. Right now, we have drafted a systematic plan to approach our
goal:
1. About 2000 A' thick layers of boron-doped a-Si:H films will be
deposited using various ratios of diborane to silane flow rates (from
0.3 - 1.0 vol.%) until an optimum dark conductivity of 10"3 - 10-4 S
cm"1 is achieved.
2. Similar experiments will be repeated for the n+ layer to obtain dark
conductivity of about 10~2 S cm"1.
3. The efficiency (and Voc) of the solar cells fabricated on SnO:F coated
7059 glass will be maximize by decreasing the thickness of the p+ layer
from an initial value of 150 A". The n+-layer thickness will be kept
constant during these experiments. The chamber will be pumped down for a
fixed amount of time after the p+ deposition before starting the i-layer
18
deposition.
4. The pump down time after the p+ layer deposition will be varied to
maximized the efficiency.
5. The n+ layer thickness will then be optimized.
6. The i-layer thickness may be optimized.
For the phase three study of the degradation phenomena, the detail plan
is listed in the new proposal for the next fiscal year. Briefly, in this
phase, we will investigate whether this degradation is of extrinsic in
nature (elemental). ZnO:F as a TCO has been shown to be more stable than
ITO, in terms of being reduced to its elemental metallic nature, in the
silane plasma. Trace amount of silver will be introduced into the ZnO:F
layer and its effect studied. An economical spray pyrolysis deposition
system for ZnO:F films will be set up. Other metals such as Au, Pt, and Cu
may also be incorporated to further the study.
r19
REFERENCES
1. A.M. Hermann and H.A. Naseem, Semi -annual Report on the Study of
Steabler-Wronsky Degradation Effect in a-Si:H Based p-i-n Solar Cells,
Grant No. NAG 3-802 (Dec. 1987).
2. S. Wolf and R. N. Tauber, in "Silicon Processing for the VLSI Era: Vol.
1, Process Technology", chpt. 18, Lattice Press, 1987.
3. D.E. Carlson and C.R. Wronsky: Amorphous Silicon Solar Cells, ed. by M.H.
Brodsky (Topics in Physics: Amorphous Semiconductors, vol.36, Springer-
Verlag Berlin Heiderberg New York, 1979) p. 288.
4. Semiconductors and Semimetals, vol. 21, Hydrogenated Amorphous Silicon,
Part B, Photoconductivity, (ed. J.I. Pankove). Academic Press, 1984.
5. Private communication with Y.H. Shing, JPL.


Study of Staebler-Wronsky degradation effect in a Si:H based P-I-N solar cells

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