NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 59 of 144
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lime silicate glass was used as the substrate. A 1 pm thick
molybdenum (Mo) layer was deposited by sputtering, and
this serves as the back electrode. The CuInGaSe2 films
were grown by coevaporation of the elements in Se
atmosphere. First, an (InGa)2Se3 layer was deposited at
3000C, and this is followed by evaporation of Cu and Se
at 55000 to make the film composition slightly Cu rich.
The composition was restored to a slightly Cu deficient one
by the addition of (InGa)2Se3 again. Film composition was
determined by electron probe microanalysis. P-type single
crystals of CuInSe2 were used for some studies, and these
were grown by gradient freeze method. They were
mechanically polished to a mirrorlike finish. To fabricate
devices, a 50-nm thick CdS layer was deposited by CBD in
about 4-5 minutes as the bath temperature was ramped
from 4000 to 7000 . The bath composition was as
follows: 0.0015 M CdSO4, 1.5 M NH4OH, and 0.15 M
thiourea A ZnO top electrode was deposited by r.f
sputtering in two layers. First, a 50 mm ZnO film was
deposited in Ar/O2 ambient to yield a high resistivity layer.
A 350-nm Al-doped ZnO was then deposited to serve as
the conductive top electrode. Ni/Al metal grids were
deposited on the ZnO for current collection. Current-
voltage characteristics of the devices were measured under
AM 1.5 Global spectrum adjusted for 1000 W/m2
illumination. Secondary ion mass spectrometry (SIMS)
was used to determine the distribution of the elements in
the films and crystals. A low beam energy of 5 kV was
chosen to minimize the effect of sputter mixing, and the
sampling rate was high to ensure high depth resolution.
The analyzed area was about 60 pm diameter. (M+Cs)+
were used to minimize matrix and surface ion yield
variations. This also maximized the Cd signal.
We used partial electrolyte (PE) solutions to study the
effect of the CBD reactants separately. For example, a Cd
PE solution contains the cadmium salt and ammonium
hydroxide in the same molar proportions as the CBD bath,
described above, and is designated Cd PE. This allowed us
to subject the absorbers to Cd and ammonium hydroxide
only without the influence of thiourea. Absorbers were
-0.2 0.0 0.2
subjected to the Cd PE treatments for different times and
temperatures. Similar baths were also constituted for the
case of Zn. Following the solution treatments, a standard
bilayer ZnO was sputter deposited to complete the devices.
We have also attempted to react Zn compounds in
vapor or solid state. The idea is to diffuse Zn into the
surface region and dope it n-type. ZnCl2 dispersed in
methanol was applied to the surface of the CIGS films, and
they were baked at 2000C for 1-2 h in air. Following this
step, the reaction products were etched in 10%-50% HCI.
This ensured removal of residual chloride and hydroxide
phases, and rendered the surfaces free of contaminants.
Bilayer ZnO was again sputter deposited for device
First, we describe the results from Cd or Zn PE
treatments of CuInSe2 films at 850C for 10 min. The
window layer is standard bilayer ZnO described above, and
it has not been specially designed to optimize the
perfonnance of devices without CdS. Fig. 1 shows the
current-voltage (I-V) curves for three devices made from
the same CuInSe2 film. The corresponding photovoltaic
parameters are shown in Table 1.
The CdS device efficiency is 12.8%, which compares
very favorably with the best devices reported to date. A
comparison of its parameters with those of the Cd- and Zn-
Table 1: Light I-V parameters of CuInSe2 devices with
CdS, Cd PE and Zn PE treatments. Total area, 0.43 cm2.
After MgF2 antireflection coating.
ZnO/ CdS ZnO/Zn PE ZnO/Cd PE
400 600 800 1000 1200
Fig. 1: Light I-V parameters of CuInSe2 devices with CdS
Cd PE and Zn PE treatments. All have two layer ZnO
window. Total area, 0.43 cm2. Before AR coating.
Fig. 2: Spectral response of the CuInSe2 devices shown i
Fig. 1. Measured before AR coating.
20 ---e- ZnO/ZnPE/CIS
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc707815/m1/59/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.