CIGSS Thin Film Solar Cells

Year 3, Quarter 3 Report

Report no. FSEC-CR1473-04

June 21, 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 3, Report no. FSEC-CR-1473-04

######### 

######### Introduction

CIGS2/CdS thin film solar cells were routinely prepared on stainless
steel foil and molybdenum coated glass substrates. A new selenization
set-up using diethylselenide (DESe) as selenium source was commissioned.
Preliminary experiments for selenization were carried out. X-ray
diffraction and electron probe microanalysis have shown promising
results. Rapid thermal processing set-up is also being assembled. All
the required components have been procured. A quantum efficiency setup
was also put together. Most of the problems with software and alignment
of light path have been overcome. So far the quantum efficiency
measurements have been conducted on standard cell. Later measurements
will be carried out on cells prepared, in-house.

**[Selenization using Diethylselenide as selenium source:]{.underline}**

A selenization set-up using DESe as a source of selenium has been
designed, fabricated and installed, for use in conjunction with a
selenization/sulfurization furnace (Figure 1). DESe was selected as a
source for selenium instead of H~2~Se because of following two reasons,

1\) Toxic limit and storage pressure: H~2~Se gas is very toxic with the
time weighted average threshold limit value (TWA-TLV) of 50 parts per
billion (ppb) and must be stored at high-pressure (\~20 Kgf/cm^2^)
cylinders, while, DESe with a (TWA-TLV) of 0.2 mg/m^3^ equivalent to 250
ppb is a safer alternative to H~2~Se with a lower potential leakage
risk. Therefore, the use of DESe leads to less stringent regulations on
exhaust system.

2\) Price comparison: Rough estimates indicate that selenization process
using DESe would cost approximately same or slightly less compared to
that using H~2~Se. Price of DESe is five times that of H~2~Se per mole
(market price June 2004). However, the partial pressure of DESe, which
reflects the source material consumption, required for the selenization
process has been found to be approximately three to four times less than
that of H~2~Se, due to the higher decomposition rate of DESe compared to
H~2~Se. The actual DESe material consumption would be four to ten times
less compared to that of H~2~Se. Therefore, operational costs can be
considerably higher when hydride gases are used.

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height="3.0416666666666665in"}

Selenization was carried out on 15x10 cm^2^ glass substrates using DESe
as selenium source. A thin layer (\~100 Å) of NaF was deposited by Joule
heating on molybdenum, before the deposition of Cu-In-Ga precursors. The
precursors were selenized at 400 °C for 10 minutes using hydrogen as
carrier gas. The selenized samples were analyzed with X-ray diffraction
(XRD), electron probe microanalysis (EPMA) and scanning electron
microscopy (SEM). Figure 2 shows the XRD plot of a CIGS sample selenized
at 400 °C for 10 minutes using hydrogen as carrier gas and Table 1 gives
the EPMA data for the same sample. Figure 3 shows the XRD data of CIGS
sample selenized at 400 °C for 10 minutes and Table 2 gives the EPMA
data of the same sample.

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Table 1: EPMA data of GS4:

  ------------ ----------- ----------- ----------- ----------- -----------
  Sample ID    Cu          In          Ga          Se          Total

  GS4b         43.4325     11.9543     0.9057      43.7075     100
  ------------ ----------- ----------- ----------- ----------- -----------

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height="3.6416666666666666in"}

Table 2: EPMA data of GS4:

  ------------ ----------- ----------- ----------- ----------- -----------
  Sample ID    Cu          In          Ga          Se          Total

  GS5b         30.8883     23.9278     1.2238      43.96       99.9999
  ------------ ----------- ----------- ----------- ----------- -----------

**[Rapid thermal processing (RTP)]{.underline}**

A RTP reactor for preparation of CIGSS thin films on 4"x6"substrates has
been designed. Reaction tube and end flange for the reaction tube have
been fabricated. The infrared heaters and control units have been
procured and are currently being installed. Quotations are being sought
for making necessary electrical connections.

**[Quantum Efficiency Measurement Setup]{.underline}**

During this quarter setup for measuring the Quantum Efficiency (QE) of
photovoltaic cells was completed. Construction plans as well as control
software using Labview were obtained from Clay DeHart at NREL. The basis
of the setup was made from a wooden enclosure with removable top fixed
to a metal breadboard. The interior components were mounted on the
breadboard as shown in Figure 4. These components were then connected to
various amplifiers and finally to a computer running Labview. After
construction, a test of a standard photodiode did not yield matching
results with the photodiode's known reference QE curve. It was believed
that the light reaching the test cell was not intense enough. Hence the
source lamp was upgraded to one of higher power. The new lamp had higher
cooling requirements. A new lamp enclosure, lens holder, and fan
assembly were built to meet these requirements and a new power supply
was purchased from Kepco. Inc. After upgrading the lamp, the light
reaching the test cell still lacked sufficient intensity for a good
measurement to be obtained. The problem was attributed to component
alignment. New hardware was built and installed to allow for easier
focusing of the light beam and for better secure component placement.
The two steps listed above helped to some extent to improve intensity
but the real problem was found to be in the monochromator. Incorrect
internal settings for the monochromator had led to both a shift in the
spectrum and issues between the two internal gratings. The monochromator
settings were restored to factory values using the calibration file
obtained from CVI. This improved the intensity of the light reaching the
cell greatly.

Further tests of the reference cell produced acceptable QE curves with
the exception of several unexplained peaks that deviated from the
expected curve. The problem was attributed to second order diffractions
from the monochromator. A temporary filter was created by depositing a
0.5 micron layer of ZnTe on a glass substrate using thermal evaporation.
This filter was inserted manually at 580 nm. However, it did not remove
the undesired peaks. Later a Long Wave Pass Filter was procured from CVI
and installed. It did not remove the undesirable peaks either. The real
problem was found to be in the beam splitter transmittance values being
used by the software to compute QE. The beam splitter transmittance
curve was recalibrated for the particular setup. Further testing of the
standard cell, produced a QE curve that matched the reference curve as
shown in figure 5. Generally the measured curve is within ±0.5% of the
reference curve except for two peaks that show a deviation closer to 1%.
Towards the end of this quarter, one of the lock-in amplifier
malfunctioned and it has been shipped to the manufacturer for repair
work.

Figure 4. QE Setup

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Figure 5. QE Results

#