Silicon Wafer Solar Cells
Armin Aberle
Solar Energy Research Institute of Singapore (SERIS)
National University of Singapore (NUS)
April 2009
1
1 PV – Some background
1.
Photovoltaics (PV):
• Direct conversion of solar
energy to electrical energy
via solar cells
Advantages:
• Clean energy
• Uses an inexhaustible renewable
energy source
• Modular (from mW to TW)
• Very
y low safety
y risks
• Reliable; low maintenance cost
• Also suited for developing countries
2
The PV market
Annual PV p
A
production
n [GW]
4
• The PV market is booming
((> 30 %/a since 1999)
• Market share of silicon in 2007:
Approx 97% (!)
Si wafers ~93%,
Si thin-films ~4%
3
2
1
0
1980
1990
2000
Year
2010
Evolution of the global PV market
1 GWp / a
Mono-Si
2003
Multi Si
Multi-Si
2000
Thin film (a-Si)
Ribbon Si
1990
Graph: G. Willeke, 2006
1980
Why is silicon so dominant in PV?
‰
Si dominates the semiconductor industry (microelectronics,
displays) → Large variety of machines for industrial production
exists already.
‰
Al
Almost
t id
ideall bandgap
b d
f PV (efficiency
for
( ffi i
limit
li it = 29% att 1 sun).
)
‰
Excellent PV efficiency already realised in industry (> 22%).
‰
Good electronic and mechanical properties
properties.
‰
Abundant and non-toxic material.
‰
PV modules are long-term
g
stable ((> 20 years).
y
)
‰
Si can be made as a wafer or as a ribbon or as a thin-film on
rigid or flexible substrates.
‰
....
5
Future growth of the silicon PV industry
Aim: To lower the $/W cost of PV modules, via higher
PV efficiencies
ffi i
i and/or
d/ llower manufacturing
f t i costs
t ($/m
($/ 2).
)
Two strategies:
Larger & thinner &
cheaper Si wafers
Si thin-film
technologies
6
2 Silicon wafer solar cells
2.
PV module with Si wafers:
Module assembly
0.92 US$/W
Si wafer (mc-Si)
$
1.37 US$/W
Solar cell process
0.72 US$/W
Cost = 3-6 US$/W
Cost distribution of a PV module with
mc-Si wafers (13%, 3 US$/W)
Fantastic technology
technology, but: Need further cost reductions ($/W)
Î Major R&D efforts required
7
Structure of a simple Si wafer solar cell
Light
g beam
Front contact
Antireflection
coating
Emitter
(n-type)
Base (p-type)
Rear contact
- +
Photogeneration of electron-hole pairs in a
semiconductor
E
-
Conduction
band
hfgreen
hfred
+
+
Valence
V
l
band
Bandgap
gy
energy
Main loss mechanisms in single-bandgap
solar cells (Example: Silicon)
Powe
er density [W m-2 µm
m-1]
1600
1400
#1: P
#1
Poor usage off energy off
short-wavelength photons
1200
1000
Available
a ab e e
energy
e gy for
o PV co
conversion
e so
using a c-Si solar cell
800
600
#2: Non-absorbed photons
400
200
0
500
1000
1500
2000
Wavelength [nm]
2500
Single-bandgap p-n junction solar cells
under one-sun illumination
‰
Theoretical limit for PV efficiency of such cells: ~31% at 25ºC (W.
Shockley and H.-J. Queisser, 1960/61, calculated using
thermodynamic principles)
‰
Best such solar cell realised as yet: 26.1%
((GaAs,, 2009,, Radboud Universityy Nijmegen)
j g )
‰
Best such silicon solar cell realised as yet: 25.0%
(1999, UNSW)
Bulk recombination – a major problem in
standard industrial Si wafer cells (~250 µm
thi k)
thick)
- +
+
PV efficiency:
15 16%
15-16%
- +
Step 1 towards improved PV efficiency:
Use of a thinner wafer (~150 µm)
- +
PV efficiency:
15% (Ouch!)
- +
Step 2 towards improved PV efficiency:
Thin wafer with optimised rear surface (and
narrower front
f t fingers)
fi
)
Narrower
f
front
finger
f
- +
+ PV efficiency:
~20% ((nice!))
Reduced
contact area
Optical
mirror
Passivating
film
Highly efficient laboratory solar cells using
thin monocrystalline Si wafers
Wafer thickness 42 µm
PV efficiency 20.2%
Cell area 1.0 cm2
(Fra nhofer ISE)
(Fraunhofer
Laser-grooved buried-contact cells
‰ Invented in 1980s at UNSW
‰ Many
M
excellent
ll t properties
ti ffor cost-effective
t ff ti high-eff
hi h ff PV
‰ Lab cells up to 21%, factory cells up to 18% (BP Solar)
Narrow copper contact
SiO2 coating
lightly diffused emitter
locally diffused contact
p-type
BSF
metal back contact
16
RISE cell (Rear Interdigitated Single Evaporation)
‰ Invented at ISFH in 2005
‰ Only 1 diffusion process, only 1
metallisation process
‰ “All-back-contact” cell
Source: ISFH, 2006
‰ Holes are laser drilled through
the Si wafer to connect the front
junction with the corresponding
electrode
17
3 Summary Silicon Wafer PV
3.
‰
‰
‰
‰
‰
‰
‰
‰
‰
Si wafer PV is booming (> 25% p
p.a.)
a)
Its market share is approx 90%
3 technologies: mono, multi, ribbon
Modules are long-term stable
Good price/performance ratio
Wafers are getting thinner
Trend towards high-eff structures
Cost of modules are falling ($/Wp)
“Si wafer PV is the benchmark PV
technology and a moving target for any
competing
p
g technology”
gy
18