The chemical dynamics of the
CSS growth of CdTe
Name: Guogen Liu
Advisor: Prof. Chin
Co-advisor: Prof. Barat
Date:06/20/2010
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
1. Introduction
Principle of p-n junction solar cell
Fig 1: Band structure of doped semiconductors
Fig. 3: Principle of photovoltaic device
Fig. 2: Heterojunction band-bending
Fig. 4: CdS/CdTe solar cell
The important development of CdTe Solar cell
The evaluation of CdTe solar cell
Advantage: easy to deposite and very cheap
Disadvantage: Te is scarce and Cd is toxic, but it’s 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 300~340Kg CdTe,ρCd 8.64g / cm3 ρTe 6.25g/cm3
1MW need 130~140Kg Te. World’s 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
CdTe solar cell companies and papers
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.
2. Preparation of CdTe solar cell
The process of CdTe solar cell
The CSS growth of CdTe
CSS process
CSS deposition chamber.
The process analysis of CSS
1. Sublimation
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.
2. Depositing
CdTe begins sublimating at 470℃
under 1KPa, at ΔT = 120℃, the vapor
Psub=1/100 Psou , so the resublimation
is neglected. Keeping the increasingtemperature speed at l0 ℃/min can
guarantee the surface Tsub and get
high quality CdTe thin films
3. Model of CSS growth of CdTe
3.1. Diffusion and sublimation model
The model was validated：
400 < T < 600 ℃, 102 < P < 760 Torr, 0.87 < d < 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.
3.1. Diffusion and sublimation model
3.1.1. Diffusion-limited case
3.1.2. Sublimation-limited case
Adjustable parameters α and β are empirical
constants that adjust the model’s output to
match experimental data. α is associated with
losses during evaporation and diffusion. β is
associated with losses during condensation
including the sticking coefficient and
molecular transport. α =0.36 and β =0.035
3.1.3. Determination of diffusionvs. 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
30–40 Torr.
0 and 5mm, the growth rate is sublimationlimited;
Greater than 5mm, it is diffusion-limited.
The growth rate is constant (sublimationlimited) at P<30 Torr. Above 30 Torr, the
growth rate diminishes as the pressure
increases (diffusion-limited ).
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
4. Effect factor of CSS growth of CdTe
4.1, TSou , TSub ,ΔT=TSou - TSub
4.2, PO2 , Pmix (O 2 with inert gas: N 2 ,Ar or He)
4.3, d (space) and growth time
4.4, The CdTe source material
4.1, TSou , TSub ,ΔT=TSou - TSub
Sublimation study of CdTe single crystal on
glass substrates (ΔT = 100 ℃)
Four growth regions
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.
4.2, PO2 , Pmix (O2 with inert gas: N2 ,Ar or He)
Pressure of oxygen on grain size and composition
SEM images of CdTe films:
a) 0% O2; and b) 10% O2.
Composition of CdTe films deposited at Tso = 750 °C,
Tsub = 600 °C, Pmixt = 10 Torr, using different amounts
of oxygen in the gaseous atmosphere.
Average grain diameter for CdTe layers
deposited under different nitrogen pressures.
Series resistance values determined from dark
J–V curves for cells with CdTe layers deposited
under various pressures of nitrogen
4.3, d (space) and growth time
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.
the deposition at 7.5 × 10–5 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 < T < 1324 K
The mean free path h
Nucleation of CdTe with growth time
AFM images of islands
after various growth times
Tsub=500 °C and
Tsou= 600 °C, P=200Torr):
a) t=1 min,
b) t=5 mins.
c) t=10 mins
d) t=30 mins,
e) t=60 mins.
f) shows coalescence of
growth islands (t=30 mins).
Variation of the nearest neighbour distribution
parameter, Rn, with growth time.
Variation of a) island area, and
b) island density with growth time.
4.4,The CdTe source material
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.
5. Effect factor of device performance
Fig. 7. Average
device
performance
parameters
extracted from J–
V curves as a
function of the
background gas
pressure present
during
deposition.
Cell parameters as a function of T
Cell parameters as a function
of CdTe thickness.
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.
TSou =620℃, TSub =660℃,ΔT=60℃
PO2 =1torr, PHe =14.9torr, Pmix =15torr
d=2mm, t=3.67 minutes
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_cadmium_telluride.html
17. http://www.pv-tech.org/news/tag/cdte/
18. http://www.pdfound.com/pdf/cdte-solar-cell.html
19. NREL, DOE, University of Delaware, Apollo, CSU, CSM
Thank you