Bài giảng Green Energy Course Syllabus - Chapter 4: Solar Resource+PV Materials - Nguyễn Hữu Phúc

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2/18/2012 Part 1: Slide 127
Typical Amorphous Si Triple Junction
• Typical a-Si:H triple junction 
characteristics
p-type at the top to reduce distance holes 
must move (generation is strongest at the 
top of the device).
Tunnel junctions are n+/p+ contacts that 
recombine minority carriers.
The device can be translucent if the back 
material is transparent.
Stainless
Steel substrate
Metal reflector
n++ ZnO conductor
n-type contact
i, a-SiGe:H
1.45 eV gap
i, a-SiGe:H
1.63 eV gap
i, a-Si:H
1.83 eV gap
n++ ITO conductor
p-type contact
2/18/2012 Part 1: Slide 128
• Example of the response 
of a triple-junction a-Si:H 
solar cell.
• Note interference effects 
for the smooth surface.
Company Initial eff. Stable eff.
United Solar 15.2 % 13.0 %
Fuji 11.7 % 11.0 %
U. Toledo 12.5 % 10.7 %
Sharp 10.2 %
• Record small cell 
performances good.
• Modules fair Company Stable eff Module
United Solar 7.9 % 0.45 m2
Fuji 9.0 % 0.32 m2
ECD 7.8 % 0.39 m2
Sanyo 9.3 % 0.52 m2
Typical a-Si:H Triple Junction 
2/18/2012 Part 1: Slide 129
• Doped end regions :
• Doping is easier if the 
material is crystalline
• Intrinsic center:
• Energy gap is critical 
to performance.
• Alloys for band-gap 
engineering.
• Need low density of 
defect states.
a-Si:H Solar Cell Structure
p-type
Intrinsic
n-type
ZnO contact
ITO contact
Glass
2/18/2012 Part 1: Slide 130
• This is the production process for a-Si solar cells.
• Silane decomposition
• SiH4 -> a-Si:Hx + (2-x/2) H2
• Process is modified by H2 addition to the gas.
• Gas chemistry is controlled 
by the pressure regime and 
plasma conditions.
• Reactor geometries vary.
• Process parameters interact.
PECVD of a-Si
Rf or dc 
power 
source
plasma
2/18/2012 Part 1: Slide 131
• Advantages:
• Relatively simple, scalable process
• Conformal coating (good with textured contacts)
• Problem:
• Control of crystallinity, density, and electronic 
properties is difficult.
• Solution: Sputter deposition
• Sputter Si target in Ar+H2 discharge
• Allows control of crystallinity and energy gap
• Other properties do not degrade seriously.
PECVD of a-Si
2/18/2012 Part 1: Slide 132
Sputtered a-Si:H Phase 
Diagram
Increasing 
H content
100
200
300
0 1 2 3 4 5 6 7 8 9
Hydrogen Partial Pressure, PH (mTorr)2
D
ep
os
iti
on
 T
em
pe
ra
tu
re
 (°
C
)
Microcrystalline 
Si:H Et
ch
in
g
a-Si:H
a-Si:H
Crystallinity is controllable by adjusting H pressure
G. Feng. M. Kaytyar, Y.H. Yang, J.R. Abelson, and N. Maley, Proc. Spring MRS, 1992
2/18/2012 Part 1: Slide 133
• Hydrogen takes states 
from the band edge 
(weakest bonds) in Si 
and increases bond 
energy.
• This increases energy 
(mobility) gap.
a-Si:H Energy Gap Control
Energy
D
en
si
ty
 o
f S
ta
te
s
C
on
du
ct
io
n 
ba
nd
Va
le
nc
e 
ba
nd
1.9
1.8
1.7
1.6
1.5
0 10 20 30
Hydrogen Content (at. %)
M
ob
ilit
y 
(“T
au
c”
) G
ap
 (e
V
)
Increasing PH in the sputtering 
system directly controls [H].
2
C.R. Wronski et.al., 11th European PVSEC, 
Montreaux, Switzerland, Oct., 1992
2/18/2012 Part 1: Slide 134
Thin Film Silicon
• Amorphous silicon
• Issues
– Instability under light exposure 
(degrades by ~20%)
– Inefficient use of SiH4 source 
material.
– Low overall efficiency
– Already doing triple junctions, not 
much space for improvement.
2/18/2012 Part 1: Slide 135
Thin Film Devices
• Cadmium Telluride
• Basic processes easy
• Methods well understood
• Basic devices well 
understood
• Important developments at 
First Solar reported.
 easier than CuInSe2
 better than a-Si
 thin film technology
2/18/2012 Part 1: Slide 136
• Graphite Contact :
• Stability of this contact 
is a major issue
• CdTe:
• Needs to be treated 
with CdCl for best 
performance.
• CdS:
• Formed by chemical 
bath deposition.
CdTe Solar Cell Structure
CdTe
CdS
SnO2 contact
Graphite Contact
Glass
These are “superstrate” 
devices (sunlight enters 
from the glass side)
2/18/2012 Part 1: Slide 137
• Closed-space sublimation is the primary method for 
deposition of CdTe.
CdTe by CSS
600-800 °C 600 °C
Diffusion
of CdTe Substrate
CdTe
SourceS
ea
le
d 
Vo
lu
m
e
Deposition and re-evaporation rates from the substrate and source are 
controlled by relative temperatures of source and substrate.
Pump
Gas inlet
He + O2 
(1-50 Torr)
O2 increases acceptor density in the CdTe (more p-type)
For example of process, see C.S. Ferekides et.al. High efficiency CSS CdTe solar cells. Thin Solid 
Films, vol.361-362, 21 Feb. 2000, pp.520-6. 
2/18/2012 Part 1: Slide 138
• CdTe grain structure is significantly improved by 
CdCl2 treatment.
CdTe Treatment with CdCl
C
dT
e
CdCl2
CdCl2 acts as a flux, allowing an 
increase in grain size and grain 
quality.
The CdCl2 is not incorporated 
into the film.
Some oxygen may be added to 
the gas phase as well to increase 
p-type doping.
+O2
40
0°
C
2/18/2012 Part 1: Slide 139
• CdTe/CdS and CuInSe2/CdS heterojunctions 
formed by dip coating.
• Typical dip recipe:
CdS by Chemical Bath 
Deposition
Solutions:
• 0.015 M Cd salt [e.g.: CdSO4]
• 1.5 M Thiourea [SC(NH2)2]
• 30% Ammonium Hydroxide [HN4OH]
• Deionized water
• 60-80°C
• Reaction occurs spontaneously over 
several minutes. 
• Nanocrystalline Cd deposited on all 
surfaces.
W
at
er
 b
at
h
Samples
Sample 
HolderTemperature 
probe
Solution
Magnetic 
Stirrer
2/18/2012 Part 1: Slide 140
• “Top” contact 
(glass side)
CdTe Device Contacts
• “Back” contact
• Best devices use 
SnO2:F
• Surface defects 
(especially O 
vacancies) are critical 
to the contacts 
properties.
• These affect the Fermi 
energy at the interface.
• Best devices use Cu-
doped graphite.
• Cu reacts at the 
interface to form Cu2Te.
• This forms the contact.
• Instabilities are a 
problem.
2/18/2012 Part 1: Slide 141
Thin Film Devices
• Cadmium Telluride
• Issues
– Cathode contact to the 
back of the device is 
unstable
– Cd causes cancer.
– Single junction devices 
allow great improvement 
potential.
2/18/2012 Part 1: Slide 142
Thin Film Devices
• Copper-Indium Diselenide
• Hard & little understood
• Works great when done right
• Basic methods complex
• Yield is difficult to obtain
Sh
el
l S
ol
ar
 C
IS
 m
od
ul
es
2/18/2012 Part 1: Slide 143
Non-CIGS Layers:
• ZnO Top Contact
• Sputtered or by MOCVD
• One layer intrinsic
• One layer n++
• Intrinsic CdS
• Grown from solution
• Mo Back Contact
• Rf or dc magnetron sputtered
• High stress typical
Cu(In1-xGax)Se2 Solar Cells
i-CdS
Cu(In1-xGax)Se2
[p-type]
Mo contact
n++ ZnO contact
Glass
i-ZnO
2/18/2012 Part 1: Slide 144
Current Processes:
• Evaporation
• High rate
• Easy control
• Difficult to scale up
• High temperature process
• Solid Phase Reaction
• Easy to scale up
• High residual stresses
• Sequential room temperature process 
followed by a high temperature reaction
Deposition of Cu(In1-xGax)Se2
Se
2/18/2012 Part 1: Slide 145
Issues in CIGS Deposition
• Control of point defects in CIGS
• Ordered point defects modify energy gap
• Point defects control type and carrier 
concentration
• Back contact
• Selenization (solid phase reaction) causes stress 
that produces adhesion failures
• Mo produces a 0.3 eV barrier Schottky contact to 
CIGS
• Supply of Na is critical to device optimization.
2/18/2012 Part 1: Slide 146
Point Defects in CIGS
• Cu-deficient 
Cu(In,Ga)Se2 dissolves 
point defects.
• p-type
• Egap and p depend 
upon Ga content.
• b-phase: ordered 
defect compound
• n-type
• Egap~1.2 eV without Ga
900
800
700
600
500
400
300
200
100
0
a  b
a + 
Cu2Se 
(HT)
b
d
d: sphalerite
a: chalcopyrite
b: “P” chalcopyrite
a + Cu2Se (LT)
a  d
15 20 25 30
Atomic % Cu
Phase diagram from T. Haalboom et.al. Inst. Phys. Conf. Ser. 
No. 152, Proc. 11th Int. Conf. on Ternary & Multinary 
Compounds (ICTF-11) [IOP Publishing, Bristol, 1998], p. 249.
a
Te
m
pe
ra
tu
re
 (°
C
)
2/18/2012 Part 1: Slide 147
Thin Film Devices
• Cu(In,Ga)Se2
• Issues
– What limits the device performance 
is unknown.
– Limited supply of In and Ga
– Hard to make by simple methods
– Single junction devices allow great 
improvement potential.
Sh
el
l S
ol
ar
 C
IS
 m
od
ul
es
2/18/2012 Part 1: Slide 148
(Ga1-xInx)N
• Inorganic alloy that covers the entire solar spectrum for multijunction 
devices. 
• Defects and their impact are little known.
• Issues:
• Epitaxial 
devices: 
processing is 
expensive & 
difficult
• Ga & In are 
rare
Novel Concepts: Nano
• Carrier extraction: how do you get carriers 
out of a nanoparticle?
• Coulomb interaction: amplification in the 
dot -- enhances multiexcitons but enhances 
carrier loss and increases exciton energy.
• Indirect gap nano: reduces recombination 
but loses energy.
• Exciton extraction: requires two identical 
contacts with equal carrier transmission.
• Surface recombination: Almost impossible to 
eliminate.
• Exciton barriers: slow extraction.
Novel Concepts: Organics
• Exciton binding energy: Hundreds of meV 
binding energies are energy losses that are 
intrinsic.
• Diffusion length: so small that it requires 
bulk heterojunctions. Large junction areas 
produce large dark currents.
• Molecular distortions: increase 
HOMO/LUMO gap and create trap states.
• Low mobility: produces Coulomb barriers at 
contacts.
• Molecular stability: carriers are intrinsic 
reaction sites.
Novel Concepts: Photo Hydrogen
• Some proposals are to make hydrogen 
directly from sunlight in a 
photoelectrochemical process.
• Problem: compromises both electrochemical 
cell and the photovoltaic device.
• Better: design these components separately 
because voltages and currents can be 
adapted with high efficiency in a circuit.