CIGSS Thin Film Solar Cells

Year 4, Quarter 1 Report

Report no. FSEC-CR1490-04

January 14, 2005

Prepared for

National Renewable Energy Laboratory

######## 1617 Cole Boulevard

## Golden, CO 80401

## Submitted by

Neelkanth G. Dhere

Florida Solar Energy Center^®^

## 1679 Clearlake Road 

## Cocoa, FL 32922-5703

# Cocoa, FL 32922-5703 Cocoa, FL 32922-57CC

![](media/image1.wmf){width="1.2in" height="1.1027777777777779in"}

**CIGSS Thin Film Solar Cells**

###### NREL contract no. NDJ-2-30630-03, UCF/FSEC Account no. 2012 6017

Year 4, Quarter 1 Report no. FSEC-CR-1490-04

**[CIGSS thin film solar cells:]{.underline}**

CuIn~1-x~Ga~x~Se~2-y~S~y~ (CIGSS) thin films were prepared at the FSEC
photovoltaic materials lab in two steps. The first step consisted of
deposition of CuGa-In precursors using DC magnetron sputtering on
molybdenum back contact. The second step involved
selenization/sulfurization of these precursors. Selenization and
sulfurization were carried out using diethylselenide (DESe) as a
selenium source and H~2~S as sulfur source. Selenization was carried out
at 400°C for 10 minutes in diluted DESe followed by sulfurization at
475°C for 20 minutes in diluted H~2~S. CIGSS thin-film solar cells were
completed by depositing n-type CdS layer by chemical bath deposition
(CBD), ZnO/ZnO:Al window bilayer by RF magnetron sputtering and Ni-Al
front contacts by e-beam evaporation.

Composition of films was analyzed by electron probe microanalysis (EPMA)
and X-ray energy dispersive spectroscopy (XEDS). CIGSS films were
inferred to be Cu-rich or Cu-poor on the basis of sheet resistance
values. For example, values below 300 Ω taken indicated Cu-rich CIGSS
films. On the other hand, values above 5 kΩ indicated Cu-poor CIGSS
films. Composition-depth profiling was carried out by secondary ion mass
spectrometry (SIMS) and Auger electron spectroscopy (AES) in combination
with sputter-etching. Structure and morphology were studied by X-ray
diffraction (XRD) and scanning electron microscopy (SEM).

A series of experiments were carried out to deposit near stoichiometric
CIGSS thin films. The sheet resistance of the films was measured after
every selenization/sulfurization process. Several iterations were
carried out in the thickness of CuGa to achieve a slightly copper-poor
stoichiometry after selenization/sulfurization. The sheet resistance was
measured to be 9-10 KΩ/€. The thickness of the CIGSS absorber layer
measured using DEKTAK^3^ profilometer was \~1.3 μm. EPMA was carried out
at 10 kV and 20 kV. Atomic concentrations measured by EPMA at 20kV were
Cu:In:Ga:Se:S 22.38:25.69:4.13:25.68:22 showing low gallium content. XRD
pattern showed (101), (112), (200), (211), (220/204), (312), and (336)
reflections from CuIn~0.96~Ga~0.04~Se~1.08~S~0.92~ phase with a = 5.6872
Å and c = 11.3744 Å as well as reflections from molybdenum (Fig. 1).
Both EPMA and XRD analysis were carried out at National Renewable Energy
Laboratory (NREL), Golden CO. SEM micrograph showed compact grains with
very little porosity (Fig. 2). Fig. 3 shows the XEDS graph of the same
CIGSS film analyzed at 20 kV.

![](media/image2.wmf){width="3.3722222222222222in" height="2.29375in"}

Fig. 1. XRD pattern of CIGSS thin film selenized at 400°C and sulfurized
at 475°C.

![](media/image3.png){width="2.4743055555555555in"
height="1.9333333333333333in"}

Fig. 2. SEM of CIGSS thin film at x6000 magnification.

![](media/image4.png){width="2.30625in" height="1.75in"}

Fig. 3. XEDS graph of CIGSS thin film at 20 kV

SIMS and AES analysis were carried out to check the elemental
composition/depth profile of CIGSS absorber layer. AES analysis was
carried out at Major Analytical Instrumentation Center (MAIC),
Gainesville FL and SIMS analysis was carried out at AMPAC's Materials
Characterization Facility (MCF), Orlando FL. AES and SIMS depth profiles
showed increase in gallium concentration and decrease in indium
concentration towards the molybdenum back contact. Copper and sulfur
signals were constant through the bulk of CIGSS. The selenium signal was
higher than sulfur which is complemented by the EPMA results that showed
low gallium content (gallium has diffused towards the back contact) and
high selenium concentration than sulfur (S/(Se+S) = 0.46) (Fig. 4 and
Fig. 5). AES profile showed increase in sulfur concentration towards the
back contact. It also indicated presence of oxygen in the bulk of
absorber layer. This was attributed to a leak in the
selenization/sulfurization set-up. Later the leak was detected using a
mass-spectrometer leak detector. A leak found in the H~2~S line was
sealed. Further preparation of CIGSS thin-film solar cells is being
carried out.

![](media/image5.wmf){width="2.8027777777777776in" height="1.84375in"}

Fig. 4. AES depth profile of CIGSS thin film.

![](media/image6.wmf){width="2.810416666666667in"
height="1.9159722222222222in"}

Fig. 5. SIMS depth profile of CIGSS thin film.

CIGSS thin-film solar cells were completed by the deposition of CdS
using CBD technique, ZnO/ZnO:Al bilayer window by RF magnetron
sputtering and Ni-Al front contact deposition by e-beam evaporation
technique (Fig. 6)

![](media/image7.jpeg){width="1.45in" height="1.4784722222222222in"}

Fig. 6: Picture of CIGSS/CdS thin-film solar cell on molybdenum coated
glass.

**[I-V Setup:]{.underline}**

The earlier current-voltage (I-V) setup used a DAQ card (6024E) from
National Instruments. Its accuracy was 16.504 mV. The current flowing
through a 0.47 cm^2^ solar cell is typically in the range of 0.01 mA. At
the suggestion of Keith Emery (NREL) a new, more reliable and robust
setup was designed using highly accurate power supply and multimeter.

Fig. 1 shows a block diagram of the new I-V setup.

[Fig. 1: New I-V measurement setup.]{.underline}

The BOP 20-5D power supply from Kepco is capable of supplying +/- 20V
(0-5A). Its output was connected to micromanipulator probes that were in
contact with the solar cell. The 6.5 digit 34401A multimeter from
Agilent Technologies (0.0015% accuracy) was connected in series with the
solar cell to measure the resulting current. The power supply and
multimeter (both capable of GPIB communication) were connected to a
computer with an IEEE 488.2 card.

A snapshot of the I-V measurement program is shown in Fig. 2. (Note: A
diode is being tested. Fill factor is not a relevant parameter in this
case.) The desired range and step size were entered in the respective
fields. The program can now be run in steps through the required range.
The I-V plot is updated after every single measurement. After the last
measurement, the program prompts the user to save the voltage, current
and power data to an Excel sheet. Thus, the data can be saved for later
use.

The Graphical User Interface of the program was similar to the one used
earlier with the DAQ card. The Block Diagram had to be modified to a
great extent to accommodate the GPIB interface instead of the DAQ card.
The program has also been enhanced to display the Voc, Isc, Vmp, Imp and
Fill factor of the solar cell. However, the figure shows only the dark
I-V characteristic of a diode and power supplied to it.

![](media/image8.png){width="5.9944444444444445in"
height="4.495833333333334in"}

[Fig. 2: Dark I-V characteristic of a diode]{.underline}

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