Name: Guogen Liu

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The chemical dynamics of the CSS growth of CdTe
Name Guogen Liu Advisor Prof.
Chin Co-advisor Prof. Barat Date06/20/2010
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Outline

• 1. Introduction
• 2. Preparation of CdTe solar cell
• 3. Model of CSS growth of CdTe
• 4. Effect factor of CSS growth of CdTe
• 5. Effect factor of device performance
• 6. Conclusion

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1. Introduction
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Principle of p-n junction solar cell
Fig 1 Band structure of doped semiconductors
Fig. 2 Heterojunction band-bending
Fig. 3 Principle of photovoltaic device
Fig. 4 CdS/CdTe solar cell
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The important development of CdTe Solar cell
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The evaluation of CdTe solar cell

• Advantage easy to deposite and very cheap
• Disadvantage Te is scarce and Cd is toxic, but
  its emission is least.
• 2009 First solar produces1.1 GW, revenue 2.1B.
  Cost 0.93/W (0.84/W in 2010)
• 1 MW of First Solar need 300340Kg CdTe,
• 1MW need 130140Kg Te. Worlds current Te can
  only support 1000GW
• Appolo hold 3000tons Te, which can afford 30GW,
  revenue 21B(conservative estimate). With the
  increase of efficiency, the reduce of Te use and
  the recycle of CdTe solar cell, the shortage of
  Te will be solve in the future.

The emission of Cd
Potential for using less Te
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CdTe solar cell companies and papers
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First Solar
First Solar processes its modules using vapour
transfer deposition (VTD), it is very similar to
CSS (closed-space sublimation). The key is that
the deposition rate of VTD is very high.
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2. Preparation of CdTe solar cell
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The process of CdTe solar cell
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The CSS growth of CdTe
CSS process
CSS deposition chamber.
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The process analysis of CSS
The physical process of the CSS is First, CdTe
source decomposes and sublimates Second, the
diffusion process of Cd and Te2 from source to
substrate Third, Cd and Te2 combine into CdTe
and CdTe resublimates if Tsub is very
high. CdTe begins sublimating at 470? under
1KPa, at ?T 120?, the vapor Psub1/100 Psou ,
so the resublimation is neglected. Keeping the
increasing-temperature speed at l0 ?/min can
guarantee the surface Tsub and get high quality
CdTe thin films
1. Sublimation
2. Depositing
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3. Model of CSS growth of CdTe
3.1. Diffusion and sublimation model
The model was validated 400 lt T lt 600 ?, 102 lt P
lt 760 Torr, 0.87 lt d lt 1mm, but could have
greater range of applicability. In general,
faster growth rates are expected at higher source
temperatures, higher difference in temperatures
(Tsource-Tsubstrate), lower pressures and lower
substrate source separation until the sublimation
limit is reached.
3.2. Diffusion and reaction model
The presence of oxygen shifts CSS from
diffusion-limited to reaction-limited growth,
primarily through source oxidation. Oxygen
enhances CdTe nucleation, reducing pinhole
density and grain size. An important beneficial
effect of oxygen is that it lessens the harmful
effects of decomposition of the front contact.
Another primary benefits of oxygen may be its
ability to passivate donors and defects if they
are present and its effect as a reactive gas that
ensures uniform growth initiation.
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3.1. Diffusion and sublimation model
3.1.1. Diffusion-limited case
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3.1.2. Sublimation-limited case
In the diffusion-limited transport region, the
growth rates have a positive exponential
dependence on Tsource and an inverse proportional
behavior to pressure. The transition between the
diffusion and sublimation regimes will occur at
pressures below 3040 Torr.
Adjustable parameters a and ß are empirical
constants that adjust the models output to match
experimental data. a is associated with losses
during evaporation and diffusion. ß is associated
with losses during condensation including the
sticking coefficient and molecular transport. a
0.36 and ß 0.035
3.1.3. Determination of diffusion- vs.
sublimation-limited case
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The growth rate is constant (sublimation-limited)
at Plt30 Torr. Above 30 Torr, the growth rate
diminishes as the pressure increases
(diffusion-limited ).
0 and 5mm, the growth rate is sublimation-limited
Greater than 5mm, it is diffusion-limited.
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3.2. Diffusion and reaction model with oxygen
Effect of PO2 and Tsou on deposition rate
Effect of PO2 and d on deposition rate
Effect of PO2 and Ptotal on deposition rate
Effect of PO2 and Tsub on deposition rate
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4. Effect factor of CSS growth of CdTe
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Sublimation study of CdTe single crystal on glass
substrates (?T 100 ?)
Four growth regions
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Fig. 4. SEM pictures of CdTe films as formed at
different Tsub. In the first regime up to about
320 C the grains grow as column, crystalline
orientation and small grains. In the second
growth regime (above 370 C), the grains are
larger and form pinholes and voids. at the
transition regime (340 C), the films are very
compact without pin-holes and voids. which is
usually used for the manufacturing of solar cells.
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Pressure of oxygen on grain size and composition
Composition of CdTe films deposited at Tso 750
C, Tsub 600 C, Pmixt 10 Torr, using
different amounts of oxygen in the gaseous
atmosphere.
SEM images of CdTe films a) 0 O2 and b) 10
O2.
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Series resistance values determined from dark JV
curves for cells with CdTe layers deposited under
various pressures of nitrogen
Average grain diameter for CdTe layers deposited
under different nitrogen pressures.
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Schematic diagram of flow patterns in CSS (a)
much of the mass is lost before reaching
the substrate (b) with reduced distance, mass
distribution is more nearly two dimensional.
Peak height versus the separation between the
substrate and baffle.
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the deposition at 7.5 105 mbar is by free
molecular transport because the mean free path is
longer than the space. The growth rate is
independent of pressure. The deposition at 6
and 2 mbar is probably diffusion limited because
the mean free path is short compared to the space
between substrate and baffle and the Cd and Te2
vapour molecules will collide several times with
nitrogen molecules before they condense on the
substrate. In the diffusion-limited transport
model, the deposition rate is an inverse function
of pressure,
912 K lt T lt 1324 K
The mean free path h
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Nucleation of CdTe with growth time
AFM images of islands after various growth
times Tsub500 C and Tsou 600 C, P200Torr)
a) t1 min, b) t5 mins. c) t10 mins d) t30
mins, e) t60 mins. f) shows coalescence of
growth islands (t30 mins).
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Variation of the nearest neighbour distribution
parameter, Rn, with growth time.
Variation of a) island area, and b) island
density with growth time.
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It was shown that the deposition rate depends on
the way the CdTe source is prepared.
source-plates led to lower rates (poor heat
transmission of plate substrate) The deposition
rate of compacting powder increases due to the
better thermal contact between powder particles.
Thickness of CdTe films obtained from different
sources as a function of the deposition time.
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5. Effect factor of device performance
Fig. 7. Average device performance parameters
extracted from JV curves as a function of the
background gas pressure present during deposition.
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Cell parameters as a function of CdTe
thickness.
Cell parameters as a function of T
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6. Conclusion

• 1. CdTe thin film solar cell will continue very
  competitive in future 30 years.
• It will drop behind because of the increase of
  Te cost and the lack of Te.
• 2. CSS is a very effective way to produce CdTe,
  because it is simplicity and
• cost- effectiveness, well-suited to
  large-scale and high efficiency. It has
• undergone some modifications for industry,
  such as CSVT, VTD.
• 3.Two models has been used to explain the effect
  factors on CSS growth of CdTe.
• 4. The following is an example of CdTe solar cell
  produced by NREL with an AM1.5 efficiency of
  15.4, The cell has a Voc of 830 mV, Jsc of 24.7
  mA/ cm2, and an FF of 74.8. Much low temperature
  has also been used in lab and industry to produce
  high efficiency CdTe solar cell.

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References 1. High efficiency CSS CdTe solar
cells 2. Preparation and Properties of CdTe
Polycrystalline Films for Solar Cells 3. The
Effect of oxygen on CdTe-absorber solar cells
Deposited by close-spaced sublimation 4. Growth
of thick CdTe films by close-space sublimation
techniqe 5. SEM characterization of CdTe growth
on CdTe(111)by close-spaced sublimation 6. CdTe
thin film solar cells Interrelation of
nucleation, structure, and performance 7.
Influence of Deposition Parameters on the
Properties of CdTe Films Deposited by CSS 8.
Control of grain size in sublimation-grown CdTe,
and the improvement in performance of devices 9.
The growth of CdTe thin film by close space
sublimation system 10.Nucleation of CdTe thin
films deposited by close-space sublimation under
a nitrogen ambient 11. Close-spaced sublimation
growth of homo- and hetero-epitaxial CdTe thick
films 12. Comparative Study of CdTe Sources Used
for Deposition of CdTe 13. Thin Films by Close
Spaced Sublimation Technique 14. PHOTOVOLTAIC
PROPERTIES OF CLOSE-SPACE SUBLIMATED CdTe SOLAR
CELLS 15. Fabrication Procedures and Process
Sensitivities for CdS/CdTe Solar Cells 16.
http//www.nrel.gov/pv/thin_film/pn_techbased_cadm
ium_telluride.html 17. http//www.pv-tech.org/new
s/tag/cdte/ 18. http//www.pdfound.com/pdf/cdte-s
olar-cell.html 19. NREL, DOE, University of
Delaware, Apollo, CSU, CSM
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Thank you
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