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THE INFLUENCE OF GRAIN BOUNDARY DIFFUSION ON THE ELECTRO-OPTICAL PROPERTIES OF
CdTe/CdS SOLAR CELLS
D.R Levi, L.M. Woods,* D.S. Albin, TA Gessert, R.C. Reedy, and RK. Abrenhel
National Renewable Energy Laboratory (NREL)
1617 Cole Blvd., Golden, CO 80401 USA
* Colorado State University, Fort Collins, CO 80523, USA
ABSTRACT: We report on a study of the effects of diffusion of metals through polycyrstalline CdTe thin films. The
metals Ni, Pd, Cu, Cr, and Te are deposited onto the back surface of 10-pm thick CdTe/CdS device structures using
room-temperature evaporation. We found that four out of the five metals produce significant changes in the
photoluminescence (PL) of the near-junction CdTe material. These changes are explained in terms of spatial variations
of the photoexcited carrier distribution and spatial variations in the sulfur composition of the CdTeS alloy material near
the CdTeS interface. The changes in carrier distribution appear to be associated with band bending and electric fields
induced by diffusion of the metals to the CdTe/CdS interface. In addition to PL measurements, we have also utilized a
technique for detaching the CdTe film from the CdS/TCO/glass superstrate to directly access the front surface of the
CdTe absorber layer. We have used secondary ion mass spectroscopy to measure the metal diffusion profiles from this
Keywords: CdTe -1: Grain Boundary Diffusion -2: Spectroscopy -3
It is widely recognized that grain boundaries exert
significant influence on the electrical and optical properties
of polycrystalline semiconductors used in thin-film solar
cells. Grain boundaries tend to have a high concentration
of lattice defects and impurities. This can make them a
dominant factor in the electrical properties of a
polycrystalline semiconductor. Diffusion of impurities
along grain boundaries in polycrystalline thin-film.
CdTe/CdS solar cells is likely to be several ?rders of
magnitude faster than diffusion through the bulk . Rapid
diffusion along grain boundaries will have a significant
influence on the results of post-deposition processing such
as CdCl2 treatment and back contact application. Thus it is
important to further our understanding of diffusion of
impurities along grain boundaries and of how such
diffusion processes affect the electronic and optical
properties of CdTe/CdS solar cells.
2. EXPERIMENTAL CONDITIONS
Samples in this study consist of close-spaced
sublimated CdTe on 800A thick chemical-bath deposited
CdS on SnO2 / 7059 glass superstrates. The CdTe
thickness is approximately 10 pm with a grain size which
varies from approximately 1 pm at the front of the film to
approximately 5 pm at the back surface. All samples have
undergone a 4000 C post-growth vapor-CdC12 treatment.
Efficiencies for cells fabricated from these films are in the
range of 12- 14%. Evaporation at approximately 300 K is
used to deposit 500 A of the metals Ni, Pd, Cu, Cr, and Te
onto the back surface. Half of each sample is then annealed
for 30 minutes at 300* C in a flowing helium atmosphere.
Ion-beam milling is then used to remove the metal layer
from both annealed and unannealed pieces of each sample.
Annealed and imannealed pieces are then halved again.
One half of each undergoes ion-beam milling to remove 1
pm of CdTe from the back surface of the film. The other
W of each of these pairs undergoes a "lift-off" procedure
which separates the film at the CdTe/CdS interface.
Secondary Ion Mass Spectroscopy (SIMS) is then used to
measure a depth profile of the metal concentration within
Room-temperature, pulsed-excitation PL spectra are
measured at each step of the above process. Typical room-
temperature PL measurements on polycrystalline CdTe (px-
CdTe) are dominated by recombination at defects because
of the high density of defect states relative to the
photoexcited carrier density. The PL spectra described in
this study are unique in that a cavity-dumped dye laser is
used as the excitation source. The laser provides pulses 5
picoseconds in duration at a repetition rate of 1 Mhz.
Because of the extremely low duty cycle of the laser, we are
able to inject much higher photoexcited carrier densities
than are practical with a CW laser. Initial carrier densities
immediately after the laser pulse are approximately 4x1016
cm-3. At these high-injection conditions, the PL spectra are
dominated by band-to-band recombination, which we have
v -ied by studying the density-dependence of the spectra
]. Photoexcitation is through the transparent CdS window
layer at a wavelength of 600mm. The 1/e penetration depth
for this wavelength in CdTe is 0.2 pm. Numerical
modeling indicates that over 90% of the photoexcited
carriers recombine within 3000 A of the CdTe/CdS
interface, hence the term junction photoluminescence. The
measurement is conducted under open circuit conditions.
Spectral resolution is 1 mn, which corresponds to
approximately 2meV inthe wavelength range of interest
2.3 Secondary Ion Mass Spectroscopy
The SIMS measurements were carried out using a
Cameca IMS-5F instrument A beam of 02+ purified by a
mass filter was used as the source of the primary ions. The
impact energy of the primary ion beam was 5.5 keV at a
incident angle of 420 from the surface normal. The primary
current was 250 nA. An area of 200x200 pm2 was raster
scanned. Positive secondary ions generated from the
sample were accelerated normal to its surface and were
detected at 4.5 keV. Secondary ions were collected from a
60-pm2 area in the center of the raster scanned area to
minimize effects from the crater walls. High-mass
resolution techniques were utilized in the seperation of
63Cu+ from 126Te. In the sample chamber, the working
pressure was 2x1010 Torr. Secondary ions were counted
by an electron multiplier detector.
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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/m1/46/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.