Trapped charging and photovoltaic performance of Cu2S-CdS cell including heat treatment effect

Bube, R. H.

Fahrenbruch, A.

Lindquist, P. F.

English

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Progress Report No. 8
MECHANISM OF THE PHOTOVOLTAIC EFFECT IN II-VI COMPOUNDS
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
January 1 - March 31, 1970
School of Engineering
Department of Materials Science
Stanford University
Stanford, California
Grant NGL-05-020-214 S-1
Principal Investigator
Richard H. Bube, Professor
Report Prepared By:
R.H.Bube
A.Fahrenbruch
P.F.Lindquist
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Progress Report No. 8
MECHANISM OF THE PHOTOVOLTAIC EFFECT IN II-VI COMPOUNDS
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
January 1 - March 31, 1970
School of Engineering
Department of Materials Science
Stanford University
Stanford, California
Grant NGL-05-020-214 S-1
Principal Investigator
Richard H. Bube, Professor
Report Prepared By:
R.H.Bube
A.Fahrenbruch
P.F.Lindquist
A
Progress Report No. 8 	 January 1 - March 31, 1970
CONTENTS
ABSTRACT, CONCLUSIONS AND RECOMMENDATIONS
I. INTRODUCTION
	 1
II. EFFECT OF HEAT TREATMENT
	 2
III. Cu-CdS CELLS	 4
IV.- FUTURE WORK	 5
Progress Report No. 8	 January 1 - March 31, 1970
ABSTRACT, CONCLUSIONS AND RECOM311ENDATIONS
A correlation has been made between the extensive series of
measurements on non-heat-treated and heat-treated Cu 2S-CdS cells.
The results demonstrate that trapped charge in the interface region,
manifested by photocapacitance measurements, is responsible for
the enhancement and quenching effects observed in the photoresponse
of heat-treated Cu 2S-CdS hetero,junctions. This trapped charge also
affects the photoresponse of non-heat-treated ,junctions, but to a
lesser extent. There can be no question but that this trapped charge
is a vital factor in tt.e photovoltaic performance of the Cu2S-CdS
cell. Whereas before heat treatment the dependence of short-circuit
photocurrent on photocapacitance (trapped charge) is linear, the
dependence after heat treatment is exponential; these are the kinds
of dependences to be expected from a small spike in the conduction
band at the interface. Complete details will be provided in Progress
Report No. 9, the Ph.D. Thesis of Peter F. Lindquist.
Properties of Cu-CdS cells prepared by vacuum evaporation of Cu
have been investigated. Dependence of properties on etching of the
surface is noted, as well as marked differences between A- and B-face
US surfaces used for Cu evaporation.
Future work includes a correlation between the ratio of cupric to
cuprous sulfide and the photovoltaic properties of the junction, as
well as investigations of surface treatment, nature of bulk CdS
conductivity, and the phenomena involved in heat treatment.
I. INTRODUCTION
The discussions of the last two Progress Reports have dealt mainly
with the properties of non-heat-treated cells. The effects of trapped
charge at the interface of such non-heat-treated Cu 2S-CdS heterojunctions
was extensively studied by photocapacitance techniques
When such a junction is cooled to about 100 0K and illuminated with
bandgap light, a large increase in the junction capacitance is observed.
After photoexcitation a significant capacitance increment, which can be
as much as 50 percent of the capacitance before illumination, remains
indefinitely.
This photocapacitance increment can be quenched by (a) heating,
(b) applying forward bias in the dark, or (c) illuminating with
photons with energy between 1.2 and 1.8 eV. The photocapacitance
data are consistent with a model in which holes are trapped in deep
levels near the interface and decrease the width of the depletion layer.
At 105 0K the short-circuit photocurrent measured after establishing
the photocapacitance exceeds the photocurrent with zero photocapacitance
by a factor of three over the entire response range.	 Thus the effect
is small but noticeable in the non-heat-treated cell.
In the present report we summarize some of the behavior of such
a cell as it is progressively heat-treated. What is presented in this
report is only a portion of the results obtained. The next Progress
Report (No. 9) will consist of the complete PhD Thesis of Peter F.
Lindquist, in which all of these details will be included and discussed.
The present report also describes some of our initial results on
Cu-CdS junctions prepared by evaporation of Cu metal.
II. EFFECT OF HEAT TREATMENT
Previous results have demonstrated that Cu 2S-CdS siligle crystal cells
with relatively high photovoltaic efficiency can be produced without heat
treatment and that trapping of charge at the interface, revealed by
photocapacitance measurements, plays a major role in determining the
electrical and photovoltaic properties of the heterojunction. In order
to relate these results directly to the properties of commercial thin-
film Cu2S-CdS cells and to our previous investigation of the heat-
treated Cu2S-CdS system, one sample was given a series of 2 minute
heat treatments in air. The I-V characteristics, capacitance-voltage
curves, spectral response and photocapacitance behavior were measured
after each heat treatment. The sample chosen was 2-B(P+E)-1, for which
extensive data had been previously obtained. The heat-treatment
temperatures were 100 0C, 150 0C and 2000C.
The main characteristicts of the results can be summarized as
follows:
1. Light-generated I-V curves show a steady decrease in short-
circuit current and a slight decrease in open-circuit voltage with
heat treatment.
2. The anomalous cross-over of light and dark I-V curves is
brought about by heat treatment.
3. The dark current at low forward and reverse bias was increased
significantly after the 100 0C and 200 0C heat treatment, but was much
lower after the intermediate heat treatment at 150 0C. The reason for
this behavior is riot clear.
4. With heat treatment the value of capacitance at zero bias
decreases and the intercept of the 1/C 2 vs. V plot increases with
increasing heat treatment. These effects can be interpreted as the
result of a widening intrinsic intermediate layer in the space-charge
2.
region.
5. Heat treatment reduces the short-circuit current at all
wavelengths, but particularly at longer wavelengths, as has been
frequently observed previously. Transient effects in the
measurement of the short-circuit current (associated with enhancement
and quenching effects) begin to appear in the curves measured
after 1500 and 2000C heat treatments. Similar effects are observed
in the spectral response of the photovoltage.
6. Figure 1 illustrates the effect of photocapacitance on the
spectral response of the short-circuit current at 105 0K
 after the
2000C heat treatment. It is evident that the presence of the
photocapacitance in this heat-treated sample has a profound effect
on the short-circuit photocurrent, increasing it by two orders of
magnitude to nearly its value at 300 0K before heat treatment.
7. Figure 2 gives another representation of the effect of
photocapacitance on photocurrent after heat treatment. The sample
was photoexcited with various intensities of 0.7 micron light at
1050K, and the initial value of photocurrent (corresponding to the
particular value of photocapacitance) was recorded. Whereas before
heat treatment the dependence of photocurrent on photocapacitance
was linear, the dependence after heat treatment is exponential.
These are the kinds of dependences to be expected from a small
spike in the conduction band.
S. The optical quenching of photocapacitance in the heat-treated
sample (200 0C) was measured at 105 0K.
 The shape of the spectrum is
qualitatively the same as that reported previously for the non-heat-
treated sample.
These results demonstrate that trapped charge in the interface
region, manifested by photocapacitance measurements, is responsible
for the enhancement and quenching effects observed in the photoresponse
of heat-treated Cu2S-CdS heterojunetions. This trapped charge also
3.
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2- B(P+E) - I H.T. 200°C, 2 MIN
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105 °K
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Figure 1. Spectral response of short-circuit current before heat treatment,
and after heat treatment, with and without photocapacitance.
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Figure 2. Variation of short-circuit current with photocapacitance
increment in heat-treated cell.
0
0	 20	 40	 60	 80	 100
	
120
--
	 AC (PO
9.
affects the photoresponse of non-heat-treated ,junctions, but to a lesser
extent. There can be no question but that this trapped charge is a
vital factor in the photovoltaic performance of the Cu2S-CdS cell.
III. Cu-CdS CELLS
To determine the effect of different modes of junction preparation
on the total photovoltaic properties of the resulting heterojunction,
experiments have been done on Cu-CdS cells made by the vacuum evaporation
of Cu onto single crystal CdS.
	 Simhony, Williams and Willis (J.Appl.
Phys. 39, 152 (1968)) investigated the electrical behavior of Cu-
diffused insulating layers in CdS. They measured layer thickness
by making capacitance measurements after various heat treatments,
and obtained valus between 1 and 10 microns. C-V dependence
characteristic of a Schottky-like barrier was found only in non-
heat-treated samples.
Our investigation is directed toward the study of thinner layers
of insulating material such as is characteristic of the temperature
and time in dipping and heat treatment of Cu 2S-CdS cells. Cells with
rather different characteristics depending on the initial treatment
have been prepared; these can be separated into three major categories.
1. Cells with only partial diode behavior in the dark result
when the CdS surface is etched least. These cells are highly photo-
conductive but show no photovoltaic response.
2. Cells with good diode behavior but little or no photovoltaic
response show small photoconductive response compared to (1). Their
major response to white light is found in an increase in reverso
photocurrent.
3. Cells with good diode behavior and appreciable photovoltaic
{	 response (short-circuit current at least 2.5 ma/cm 2) are better for
evaporation on the B-face samples, which have a rougher surface after
etching than A-face samples.
Samples for evaporation are prepared by polishing and etching in
the same way as used for Cu 2S-CdS dipped cells. After water-rinsing
and alr-drying they are coated with 0.2-1.0 micron of 99.99+ percent Cu
at about 10-5
 Torr. Leads are attached to the Cu layer with silver-paint;
ohmic indium contact is made to the CdS. The estimated temperature
during evaporation does not exceed 100°C; the period of evaporation is
about 30 seconds.
Dark forward I-V characteristics for two cells are shown in Figure 3.
F-13, a Type 2 cell, shows Id V3
 over five orders of magnitude of the
current at room temperature. This cell shows only a small variation of
capacitance with bias voltage.
	 F-21, a Type 3 cell, shows a more
complicated dependence of I on V, which more nearly approximates the
In I oG V behavior found in good Cu 2S-CdS cells; capacitance is strongly
dependent on bias voltage for this cell.
Typical I-V characteristics of Type 1, 2, anu 3 cells are given
In Figure A.
Figure 5 illustrates a signX scant difference between A-face and
B-face Cu-CdS cells. The A-face cells show much higher resistance in
the dark and strong cross-over in the light; the B-face cells show
no cross-over. This difference may be related to the different
surface produced by etching A- and B-faces, as mentioned above.
IV. FUTURE WORK
Since measurements are incomplete at this time, data on evaporated
Cu-CdS cells will be reported and interpreted in a later report. These
cells will also be heat-treated to determine the change in their
properties as the depletion width in the US increases.
Principal areas of interest for future work are the following.
1. Compositon of the CuxS layer formed by the dipping and the
----------	 —
5.
2 CELL)
10'
16-2
ICT,
Id-,
6
F
1
	 I	 I	 I	 I
105 1
10 2	 10
	 I	 10;.
V, volts
Figure 3. Details of I-V characteristics of a Type 2 and a Type 3
evaporated Cu-CdS cell.
Type 1
(F-10)
Type 2
(F-17)
Figure 4. Charactk-ristie I-V curves in li.„-ht and dark, for Type 1, 2,
and 3 Cu-CdS cells. Scale: 0.2 V/div. 0.03 m3/div.
Type 3
(F-21)
B-Face
(F-22)
A-Face
(F-23)
P.
Figure 5. Ch racteristic I-V curves in light and dark, for A-face
and 13-face evaporated Cu-CdS cells. Scale: 0.2 V/div.;
0.05 ma/div.
6.
correlation with the photovoltaic properties of the heterojunction formed.
A variety of chemical and optical methods are being considered for the
determination of the Cu'/Cu" ratio in the dipping solution as well
as of the value of x in the Cu S-CdS cells.
x
2. The effect of surface treatment of the CdS on the photovoltaic
properties of the heterojunction formed. Distinctions between
chemical and physical effects on the surface will be sought.
3. The effect of the source of conductivity in the CdS on the
photovoltaic properties of the heterojunction formed. Measurements
to date have been made with non-stoichiometric high-conductivity
CdS crystals. A crystal of CdS with In impurity is currently being
grown. Properties of Cu2S-CdS ,junctions formed on CdS crystals
with a pre-diffused thin insulating Cu-rich layer may also be
investigated.
4. Phenomena involved in heat treatment. Further detailed
investigation of the nature of the phenomena involved in heat treatment
is planned.


Mechanism of the photovoltaic effect in II-VI compounds Progress report, 1 Jan. - 31 Mar. 1970

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