Thin Film Photovoltaics
By Justin Hibbard
Thin Film Photovoltaics
• What is a thin film photovoltaic?
• Thin film voltaics are materials that have a light
absorbing thickness that is less than 10 μm.
• The thin film material can converting sunlight to
electricity.
• Thin films are predicted to be as efficient as
monocrystalline Si wafer cells with more R & D.
• Need more R & D to evolve from the laboratory
to the marketplace.
• Two competing materials are
(CIGS):Cu(In,Ga)Se2 and CdTe.
Thin Film Photovoltaics
Thin film photovoltaic devices take advantage of absorption of ir-vis-ir light by
semiconductors and converts the light to power.
What is a semiconductor?
Semiconductors are materials that require a “turn on” energy to conduct energy
≈ current.
Semiconductors share the electrons respectively with the individual atomic
nuclei and require a minimum voltage to make the electrons mobile.
Whereas metals are materials that are referred to as conductors and have
“free” electrons that respond to a small voltage.
Semiconductors are materials that prefer a Lewis/closed shell structure.
This rule may be deviated with the addition of element(s) that differ by group
number ±1,0. Which may give the material an extra or a missing (hole)
valence electron. This atomic substitution allows the material scientist to
“tune” the material by manipulation of the conduction properties of the
material.
Semiconductors are preferable because of the ability to tune the eV with the
right elemental mixture and the small amount of material needed.
Thin Film Photovoltaics
Thin films technologies have a common device/module structure: substrate, base
electrode, absorber, junction layer, top electrode, patterning steps for monolithic
integration and encapsulation but in a reverse order.
Thin film photovoltaics are cheap if the elements are not rare. They are becoming
cheaper to produce and will become cheaper with time be R & D.
Monocrystalline Si is expensive to grow. Amorphous and polycrystalline silicon
are not very efficient. Thin film CIGS cell and modules are 19.5% and 13%
efficient and CdTe cells and modules are 16.5% and 10.2% efficient. CdTe and
CIGS PV modules have the potential to reach cost effective PV-generated
electricity.
They have transitioned from the laboratory to the
market place. Pilot production/first-time manufacturing (US) ~ 25 MW. CdTe
technology ramping to 75 MW. Enjoying a flux of venture capital
funding.Transitioning from the lab to manufacturing has been much more difficult
than anticipated.
CIGS and CdTe Device Structure
CdTe
Glass
CTO/ZTO,SnO2
0.2-0.5 µm
ZnO, ITO
2500 Å
CdS
700 Å
CdS
600-2000 Å
CIGS
1-2.5 µm
CdTe
2-8 µm
Mo
0.5-1 µm
Glass,
Metal Foil,
Plastics
C-Paste
with Cu,
or Metals
CIGS
CTO/ZTO = Cd2SnO4/ZnSnOx
CIGS Deposition System
Thin Film Photovoltaics
Thin films challenges:
1)Science and Engineering support.
2)Long Term Stability.
3)In-Situ Process Diagnostics and
control.
4)Thinner CIGS and CdTe Abosrbers.
5)Need for High-Throughput , Low-Cost
Processes.
6)Inproved Voc in CdTe devices.
Thin Film CIGS Solar Cells
Efficiency
Area
(cm2)
Area
(cm2)
VOC
(V)
CIGSe 0.410 0.697
JSC
(mA/cm2)
35.1
FF
(%)
79.52
Efficiency
(%)
Comments
19.5
CIGSe/CdS/Cell
NREL, 3-stage process
CIGSe 0.402 0.67
35.1
78.78
18.5
CIGSe/ZnS (O,OH)
NREL, Nakada et al
CIGS
20.9
69.13
12.0
Cu(In,Ga)S2/CdS
Dhere, FSEC
0.409 0.83
CIAS
–
0.621
36.0
75.50
16.9
Cu(In,Al)Se2/CdS
IEC, Eg = 1.15eV
CdTe
1.03
0.845
25.9
75.51
16.5
CTO/ZTO/CdS/CdTe
NREL, CSS
CdTe
–
0.840
24.4
65.00
13.3
SnO2/Ga2O3/CdS/CdTe
IEC, VTD
CdTe
0.16
0.814
23.56 73.25
14.0
ZnO/CdS/CdTe/Metal
U. of Toledo, sputtered
Thin Film Photovoltaics
Justin Hibbard thanks the nrel.gov
webpage for pictures and
information.