The deposition methods and main
factors of CdTe solar cell
Name: Guogen Liu
Advisor: Prof. Chin
Co-advisor: Prof. Barat
Date: 07/10/2010
Outline:
1. Deposition methods
2. Main Factors Affecting the efficiency
3. XRD and SEM
4. Conclusion
1. Deposition methods
1.1. Chemical vapor deposition (CVD)
1.2. Aqueous solution method
(Chemical bath deposition)
1.3. Sputtering
1.4. Thermal evaporation
1.5. Electron beam evaporation
(Physical vapor deposition)
1.6. Closed spaced sublimation (CSS)
Halogen Lamp
Substrate
CdTe
Source
Halogen Lamp
1.7. Vapour transfer deposition (VTD)(First Solar)
Abound Solar’s process
It is fully automated, continuous, and utilizes dry deposition
NREL Deposition Processes
• Conventional SnO2/CdS/CdTe
device structure (requiring a
thicker CdS layer)
• Mix “wet” and “dry”
processes
• Several heat-up and cooldown process segments
(consuming time and
increasing thermal budget)
• CTO, ZTO and CdS are
deposited on substrate at RT
by RF sputtering
• Single heat-up segment
• Crystallization of CTO, ZTO,
and CdS, and interdiffusion
occurs during the CdTe
deposition step
2. . Main factors affecting the efficiency
9) Back contact (+Cathode)
8) Anneal with Copper
7) Etch with NP or BrM
6) Anneal with CdCl2
5) CdTe absorber (3 to 8 μm)
4) CdS window ( ~0.1 μm)
3) HRT
2) TCO ( ~0.05 μm) (- Anode)
1) Glass superstrate (1000 μm)
Light
CdTe microstructure after VCC + O2
CdTe microstructure after thermal
processing and/or stress
CdTe after NP etch + Cu:dag
CdTe after thermal processing
and/or stress (VCC w/o O2)
2.1 TCO
TCO should low sheet resistance and a high
transparency. In can diffuse from ITO into
deposited layers. The use of a 100–200 nm thick
buffer layer between ITO and CdS, such as SnO2
or ZnO, can hinder the In diffusion. The buffer
layer reduces the effect of a shunt resistance
coming from pinholes in the very thin CdS layer.
2.2. CdS and buffer layer
CdS thickness optimization
• No photocurrent generated in CdS  minimize thickness to maximize
transmission to CdTe in blue region (CdS Eg=2.4eV)
• Too-thin CdS  shunting issue
• F-doped CdS exhibits higher efficiency solar cells. It could passivate the grain
boundaries and be more stable in 400 ℃ CdCl2 treatment.
Thin resistivity buffer layer
• TCO/ buffer / CdS / CdTe / electrode
• high resistivity SnO2, In2O3, ZnO, Zn2SnO4
16
2.3.CdTe
The CdTe layer should be very compact
without pin-holes and voids, uniform,
about 5 um thick, grain size about 3 um,
(111) direction(richer in Te)
Oxygen reducing pinhole density and
grain size; lessens the harmful effects of
decomposition of the front contact;
passivate donors and defects;ensures
uniform growth initiation; enhances the
p-type character of CdTe.
2.4.CdCl2 heat treatment
Methods:
• Soak CdTe film in CdCl2:MeOH, heat treat 400°C
• Heat treat 400°C in presence of CdCl2 vapor
Effects:
• CdS/CdTe interfacial mixing: CdTeyS1-y/CdSxTe1-x
• alleviates structural, electrical defects at interface
• Recrystalization and grain growth in CdTe
• Establish or increase CdTe p-type doping
• Passivation of grain boundary traps
Chloride T(ºC)
Used
CdTe
18
time
(min)
Voc
Jsc
FF
(mV) (mA/cm2) (%)
h
(%)
None
None
None
570
15.9
46.9
4.2
CdCl2
410
20
808
23.8
69.2
13.3
CdCl2
440
2
634
20.4
60.2
7.8
CdCl2
465
2
755
24.2
60.2
11.0
ZnCl2
400
15
780
23.9
59.7
11.1
2.5. back contact
Approaches for non-blocking contact
• high work function metal or degenerate semiconductor (ex: Au, Sb2Te3, graphite
paste)
• Formation of p+ doped layer on CdTe surface to promote tunnelling thru barrier
• etching to produce p+ Te-rich surface layer
• Cu doping
7059 Corning glass
• acts as p-type dopant in CdTe
CTO
ZTO
• forms Cu2Te (in conjunction with etching)
nano-CdS:O
• other dopants (ex: HgTe, graphite paste),
In contact
CdTe
Cu x Te back-contact
ITO
MgF 2
19
Ni/Al grids
2.6. Cu doping
Using a three-step process to prepare
CuxTe back contact :
1. Prepare a Te-rich layer on CdTe
back side by a chemical etch;
2. Deposit a thin Cu layer by
electron-beam evaporation;
3. Post-anneal in N2 or He at 200°C
- 300°C for 30 minutes.
Back contact
TCO/CdS/CdTe
Weak Junction
Vspv = 550 mV
TCO/Buffer/CdS/Cd
TeStrong Junction
Vspv = 800 mV
Voc = 540 mV
Voc = 815 mV
Cu/Au
(no anneal)
Voc = 490 mV
Voc = 650 mV
ZnTe:N /Ni
(no anneal)
Voc = 540 mV
Voc = 670 mV
Au
3. XRD and SEM
3.1. XRD and SEM about 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.
3.2. XRD and SEM about Tsub on grain size and composition
335 ℃
XRD of CdTe films deposited at different
substrate T in 7.5 × 10–5 mbar of air.
450 ℃
500 ℃
520 ℃ 4.2μm
Scanning electron microscopy images of
CdTe films deposited at different substrate
temperatures and 7.5 × 10–5 mbar of air.
3.3. XRD and SEM about Te doping on grain size and composition
Te doping
improves the
conductivity
of films
greatly
(a) Te content in samples by XRF.
(b) Resistivity change
crystallinity of
the films can be
improved by
appropriate Te
doping.
Figure 3. XRD of samples.
Figure 4. XRD of samples 5 and 1.
SEM images of the samples: (a) pure CdTe film deposited by rf sputtering;
(b) pure CSS CdTe film; (c) Te-doped CdTe film deposited by CSS.
(a) is under 100 nm, (b) is about 15 μm, (c ) is about 25 μm.
the electrical characteristics of Te-doped CSS CdTe films change
dramatically compared to pure CdTe films. The electrical resistivity drops
by several orders of magnitude, the carrier mobility increases by about 10–
100 times, andalso the carrier concentration increases by about 1000 times.
1, TCO( surface and section)
2, CdS(CBD) before CdCl2
3, CdS(CBD) after CdCl2
TCO surface isn’t clean completely, resulting in nonuniform CdS deposition,
potential adhesion problems, and poor device performance. CdS is too thick.
4, CdS+CdTe before CdCl2
5, CdS+CdTe after CdCl2
Non uniform CdTe layer, grain size is too small, no P-type CdTe
because of the lack of oxygen,
4. Conclusion
1, Several deposition methods, including CVD, CBD,
Sputtering, Thermal evaporation, PVD, CSS, VTD,
2, Main factors of efficiency, such as TCO, CdS and buffer layer,
CdTe, CdCl2 heat treatment,back contact,Cu doping
3, XRD and SEM of oxygen, Temperturature of subject, Te doping
on grain size and composition. SEM of Apollo sample.
4, Suggestion to Apollo sample:
a, TCO surface should be clean completely
b, Add a thin resistivity buffer layer between TCO and CdS
c, CdS is too thick and not uniform
d, Oxygen is necessary in CdTe deposition
e, Vapor CdCl2 treatment is very important
f, It is better to increase CdTe deposition temperture
g, back contact and Cu doping can affect efficiency greatly
h, Pay more attention on the interface between each layer
i, CSS reactor is better
Thank you