CdS growth - NJIT Physics Department

Report
CdS growth methods
Name: Guogen Liu
Advisor: Prof. Chin
Co-advisor: Prof. Barat
Date: 08/02/2010
Outline:
1. Introduction of CdS
2. CdS growth methods
3. CdS Improvement methods
4. Conclusion
1. Introduction of CdS
Cadmium sulfide is a chemical compound with the formula CdS. It is
yellow in colour and is a n-type for CdTe and CIGS solar cell.
CdS in CdTe and CIGS Solar cell
2. CdS growth methods
2.1, Chemical bath deposition (CBD)
CBD CdS films: 550 ml 10M DI water, 8 ml aqueous cadmium acetate
solution (0033M), and 47 ml aqueous ammonium acetate (10M). This
solution is heated to 363K with the tin-oxide-coated glass substrates
placed vertically into the bath at 353K to allow for thermal equilibrium.
Thiourea (0067M) is dispensed into the Cd-containing bath at 2 ml
every 5 min for a total of 8 ml. The substrates are immersed in this bath
for 37 min upon which about 80 nm of CdS is deposited. Film thickness
is varied by varying deposition time.
The effect of temperature to CBD CdS
Three steps:
(i) nucleation,
(ii) coalescence
(iii) vertical growth.
Influence of CdS thickness
CdTe1-xSx=F(d)
P(111) and grain size of CdTe as function of dCdS
CdS function: window layer; CdTe1xSx mixed-crystal layer;
for the crystallinity of the CdTe layer.
Effect of CdS thickness and S/Cd ratio
80nm CdS is good
The optimal conditions for CBD–CdS:
75 ℃ with S(thio)/Cd(CdCl2) = 6
CdS window layers with a large sulfur
vacancy band (at 1.8 eV) More sulfur
vacancies decrease the minority carrier
recombination at the CdS–CdTe
interface and at the grain boundaries
Influence of ITO surface
CdS deposited on two types ITO: (1) an
insulating ITO overcoat (2) an ion-irradiated.
CdS on surface treated ITO had a better
transmission(<550nm), whereas CdS on nontreated ITO had better transmission(>550nm)
2.2, CdS by Sputtering
low cost soda-lime glass as substrate
Grain size of the CdS film after treatment
depends strongly on the SnO2 film thicknes.
Grain size(Na) of CdS decrease(less Na) with
increase of SnO2 thickness(block Na diffuse
from glass)
The efficience is strongly dependent on the
interaction among the different layers. CdS mixes
very easily with CdTe at high deposition
temperature of 500 ℃ and this can remove the
mismatch and the interface energy states. This is
due to the presence of a smooth transition through
a mixed compound CdS1−XTeX.
Result: Voc=858mv and Jsc=23mAcm−2 with a
fill factor (ff) of 74%, efficiency of 14.6%.
Area:0.64 cm2
2.3, CdS growth by CSS
CSS CdS: Tsubstrates= 748K , d=5 mm,Tsource=923K, in 133 kPa Helium.
The CdS powder consisted of pulverized 99999% CdS crystallites which are annealed in
H2S prior to use to partially sinter the CdS source and thus minimize source ‘spitting’.
Under these conditions, a CdS film of about 80 nm could be deposited in about 7 min.
A consists large grains and more pinholes.
B is denser and contains fewer pinholes.
Compare CBD with CSVT
S/Cd=5 by CBD and the best CdS by CSVT
CBD
Both samples has 90% above 550 nm
CSVT
CBD (top 40–50 nm) and CSS (bottom 100–300 nm) CdS
films. (grain size and structure, pinholes)
3, CdS improvement method
3.1, H2 treatment of CdS
CdS by CBD can make a very compact film, but CBD is not suitable for largescale production since it is not fast and gives a waste.
CdS by sputtering was grown in presence of fluorine by introducing in the
sputtering chamber Ar containing 3% of CHF3. Despite fluorine is a donor for
CdS, we did not see any lowering of the CdS resistivity when it was grown in
presence of fluorine.
The CSS(500 ℃, sticking of CdS by gas pressure and close to source ) allows
substrate deposition temperature much higher than that of vacuum
evaporation(<150 ℃, if >150 ℃ S will re-evaporated before combining with Cd ).
Both CdS(F) by sputtering or CdS(O) by CSS are suitable to make high
efficiency cells if they are treated for 20 min, in Ar containing 20% of H2 at 400
℃ substrate temperature because CdS(F) contains CdF2 segregated in the grain
boundaries and surface. CdF2 is useful to passivate the grain boundaries, but it
block CdS to interact with CdTe. The same happens for CdS, because CdO and/or
CdSO3 formed in the grain boundaries and surface. They can be removed by the
H2 treatment. The thickness of the CdS layer is, in both cases, 80 nm.
3.2, Nano-crystalline CdS
CdTe(10 nm) has a blue shift in the absorption with an effective band gap of 2.8
eV(Bulk CdTe, 1.44 eV at 300K); CdS (15 nm) has an effective band gap of 2.98
eV(better than Bulk CdS, 2.4ev): Nano-crystalline CdTe has potential applications
as n-type window layers in CdTe solar cell
3.3, Patterned growth
Patterned growth mask on CdS
Patterned growth (left) and planar growth
(right) of CdTe
The disadvantage of polycrystalline films:
First, the grain boundaries result in enhanced migration of dopants,
device shunting, and high recombination;
Second, inhomogeneity results in efficiency losses.
References:
1. Fabrication Procedures and Process Sensitivities for CdS/CdTe Solar Cells
2. Growth and physical properties of CdS thin films prepared by chemical bath deposition
3. Influence of CdS window layer on 2-mm thick CdS/CdTe thin film solar cells
4. Photoluminescence characteristics of CdS layers deposited in a chemical bath
5. High throughput processing of CdTe/CdS solar cells
6. Influence of ITO surface modification on the growth of CdS
7. Growth of polycrystalline CdS and CdTe thin layers for high efficiency thin film solar cells
8. CdS films prepared by the close-spaced sublimation
9. Effects of thiourea concentration on CdS thin films grown by chemical bath deposition
10.The Effect of Oxygen on Interface Microstructure Evolution in CdS/CdTe Solar Cells
11. Recent progress on CdTe/CdS thin film solar cells
12. Nano-structured CdTe, CdS and TiO2 for thin film solar cell applications
13. Ordered CdTe/CdS Arrays for High-Performance Solar Cells
Thank you

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