CIGSS Thin Film Solar Cells

Year 3, Quarter 2 Report

Report no. FSEC-CR1464-04

April 26, 2004

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. 26-56-511

Year 3, Quarter 2, Report no. FSEC-CR-1464-04

######### 

######### Introduction

CIGS2/CdS thin film solar cells were routinely prepared on stainless
steel foil and molybdenum coated glass substrates. XY sweep controller
circuit has been received back from manufacturer after a minor repair.
The controller was installed and e-beam system is now fully operational.
A minor improvement has been carried out in the mechanism for holding
and rotating one complete 4" x 4" substrate. ZnO/ZnO:Al deposition by RF
magnetron sputtering and CdS deposition by chemical bath deposition
(CBD) are being carried out on regular basis. Both the scrubbers are
operating adequately. A new set-up is being fabricated for selenization
treatment using less hazardous diethylselenide as selenium source.

**[CIGS2 Thin Film Solar Cells]{.underline}**

CuIn~1-x~Ga~x~S~2~ (CIGS2) films were prepared routinely on 4"x4" size
Mo-coated glass and stainless steel. Samples were sulfurization at 475
^0^C with a dwell of 20 minutes at that temperature. Metallic precursor
CuGa/In layers were deposited by DC magnetron sputtering. Reasonably
good crystalline quality, copper-rich CIGS2 thin films were prepared by
sulfurization in a mixture of 4% H~2~S/N~2~, where nitrogen was used as
carrier gas. Near stoichiometric, copper-poor, CIGS2 thin films were
obtained by etching Cu-rich CIGS2 layers in dilute KCN. Further
deposition of CdS, ZnO/ZnO: Al, was carried out. Final stage of
depositing Ni/Al contact fingers was carried out in e-beam evaporation
system. The new mechanism facilitate deposition on one complete 4"x 4"
substrate instead of four individual pieces of 2" x 2". Complete
CIGS2/CdS thin film solar cells can now be fabricated starting from
substrate cleaning till the final stage of front contact deposition.
Work is in progress for putting together I-V and QE set-up for
characterization of solar cells.

**[Heterojunction Partner Layer, CBD CdS]{.underline}**

Heterojunction partner CdS layers were satisfactorily deposited by
chemical bath depositions on 4"x 4" CIGS2 samples. CdS deposition set-up
was modified to prevent excessive use of etching and oxidizing solution.
Instead of using a 4 liter beaker, a shallow and large trough that can
easily accommodate 4" x 4" sample is being used. The substrate with
absorber film is kept at the bottom part of trough. The amount of
solution is sufficient to completely immerse the absorber layer. The
solution is swayed manually by tilting the trough continuously from one
end to other. This ensures effective treatment and prevents excessive
use of solutions.

######### 

[Selenization Process]{.underline}

Work is in progress for putting together a set-up for selenization
process using diethylselenide (DESe), a new and less hazardous selenium
source for preparation of CuIn~1-x~Ga~x~Se~2~ (CIGS). Recently,
metalorganic selenides, such as dimethylselenide, diethylselenide
\[(C~2~H~5~)~2~Se:DESe\], and ditertialbutylselenide have attracted much
attention as alternative candidates for the Se source. DESe being liquid
at room temperature and stored in atmospheric pressure stainless-steel
container, there is less danger of widespread leakage compared with
H~2~Se which is stored in high pressure cylinders. Therefore DESe is
considered to be a promising alternative to H~2~Se for the selenization
of CIGS. Rough estimate indicates that selenization process using DESe
costs approximately the same or little bit less than using H~2~Se. Price
of DESe is \~5 times that of H~2~Se per mole (market price in February
2002). However, the partial pressure of DESe, which reflects the source
material consumption, required for the selenization process has been
found to be approximately one third or one fourth of that using H~2~Se
due to the higher decomposition rate of DESe compared to H~2~Se.
Therefore, the net price per growth run for the DESe process is
comparable to that of H~2~Se process. In addition to the material cost,
the threshold limit value (TLV) for DESe is lower than that for H~2~Se.
In summary, using DESe with inert N~2~ would be a less-hazardous process
for selenization of Cu--Ga/indium precursors in the preparation of CIGS
films.

######### Electron Beam Evaporation Source System

As mentioned in the last quarter report an engineer from manufacturer
(Thermionics) visited FSEC facility to correct the malfunctioning of
e-beam system. The system was made operational. However, it still had
minor electronic problems. Operational amplifier in the XY beam sweeper
circuit continued to burn during each deposition cycle. It was possible
to carry out deposition in the absence of sweeper circuit with metal
evaporating from only one location. This involved major risk of damaging
the instrument. To prevent the possibility of electron beam puncturing
into the body adequate amount of material was added into the liner prior
to each deposition. XY sweeper unit was sent to manufacturer for repair
work. Manufacturer had informed that the cause of continuous burning of
op-amp was passage of not properly attenuated voltage spikes in the
signal path. An inductor was added to the circuit to prevent the voltage
spikes reaching the op-amp and damaging it. The unit was received and
reinstalled. It is operating normally without problems. Therefore, it is
now possible to sweep the beam across the entire liner and effectively
distribute the heat in the evaporating material. Some depositions have
been successfully carried out without any mishaps. With the effective
working of e-beam system, it is now possible to fabricate CIGS2/CdS thin
film solar cells till the final stage of front contact deposition.

**[Testing of Shell Selenization/Sulfurization Unit]{.underline}**

Selenization/Sulfurization unit donated by Shell (former Siemens) is
being used routinely for sulfurization of two 4" x 4" and four 1.5" x 4"
metallic precursor samples in 4% H~2~S/N~2~ gas mixture in a single run.

# 

**[Graduate Teaching and Thesis Research]{.underline}**

The PI taught one Graduate course on PV Solar Energy Materials.