A' Bonucci, S' Rondena, G'Longoni SAES Getters R

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A. Bonucci, S. Rondena, G.LongoniSAES Getters
RD Labs, Lainate (Milano)

• Moisture as main degradation cause in
  photovoltaic technology

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Outline

• Main photovoltaic technologies
• Energy Payback time
• Effect of sealing on the EPT
• Photovoltaic cell performances
• Main moisture degradation effect
• Moisture degradation of CIS/CIGS
• Moisture degradation in a-Si
• Moisture and oxygen in OSC
• Humidity effect on DSSC

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Main photovoltaic technologies

• First Generation
• Single crystal silicon wafers (c-Si)
• Second Generation
• Amorphous silicon (a-Si)
• Polycrystalline silicon (poly-Si)
• Cadmium telluride (CdTe)
• Copper indium gallium diselenide (CIGS) alloy
• Third Generation
• Polymer solar cells
• Dye sensitized solar cell (DSSC)

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Efficiency
Gerald Gourdin, Solar Cell Technology Current
State of the Art, Introduction to Green
Chemistry, Fall 2007
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Energy payback time

• Initial efficiency is not enough to qualify a
  photovoltaic technology.
• Environmental energy source requires that the
  produced energy in the lifetime must be higher
  than the energy used to produce the hardware
  system.

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E.A. Alsema, Environmental Aspects of Solar Cell
Modules, Sumary Report, August 1996 Report n.
96074 ISBN 90-73958-17-2
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Energy pay-back time
Specific Energy and Energy Generation
Rate relationship to EPBT. Circled data are
framed modules. Karl E. Knapp, An Empirical
Perspective on the Energy Payback Time for
Photovoltaic Modules, Solar 2000 Conference,
Madison, Wisconsin. June 16-21, 2000
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Energy payback time sealing effect
Lag time
WVTR is related to this slope
Lag time
Breakthrough time
Typical curve of pressure increasing in an
encapsulated system for permeation
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Energy pay-back time sealing effect
Extra-Energy produced by the use of an edge
sealant
Considering a degradation rate between 0.251
/yr and an EPBT without getter between 26 yr,
it is obtained a relative reduction of EPBT
between 2 20.
Lag time
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Photovoltaic cell performancesshunt and series
resistance
Series resistance effect
Shunt resistance effect
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Main moisture degradation effects

• Adhesional Degradation
• Delamination is defined as the breakdown of the
  bonds between material layers that constitute a
  module laminate. Field experience has shown that
  front-side delamination at the Glass/encapsulant
  and cell/encapsulant interfaces is more common
  than backside delamination.

• Front-side delamination
• causes optical decoupling of materials that
  transmit sunlight to the cells, resulting in
  performance degradation.
• Back-side delamination
• interrupts efficient heat dissipation and
  increases the possibility of reverse-bias cell
  heating. Higher cell operating temperatures cause
  performance degradation.

M.A. Quintana, D.L. King, F.M. Hosking, J.K.
Kratochvil, R.W. Johnson, B. R. Hansen, N.G.
Dhere and M. B. Pandit Diagnostic Analysis of
Silicon Photovoltaic Modules After 20-Year Field
Exposure, 28th IEEE PVSC, 2000, pp. 1420-1423
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Main moisture degradation effects

• Leakage current and corrosion
• Moisture induced corrosion that caused bonds
• between the grid lines and the cell to fail.
• Theres also electrical degradation of the
  encapsulant.

G. R. Mon, Reliability studies of photovoltaic
module insulation systems, Electrical
Electronics Insulation Conference, 1989 T.J.
McMahon and G.J. Jorgensen, Electrical Currents
and Adhesion of Edge-Delete Regions of
EVA-to-Glass Module Packaging, NREL/CP-520-30819
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Encapsulant degradationPVB yellowing and EVA
browning
EVA discoloration
PVB yellowing
Its caused by chemical variation of bulk and
interface for photoxidation
Q. KIM and A. SHUMKA, Solar Cells, 12 (1984) 345
- 352
G. Oreski, Sol. Energy (2009)
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Transparent Cathode Oxide degradation

• SnO2F Transparent Conductors are widely used in
  different PV technologies.
• Electrochemical corrosion at the glass-TCO
  interface was observed to result in delamination
  of the thin-film layers.
• The corrosion depends on the direction of the
  internal electric fields.
• Corrosion causes cracking that extends through
  all of the thin-film PV layers.
• Water vapor enhances the corrosion by increasing
  the conductivity of the front glass surface.
• Metallic module frames can greatly enhance the
  damage process.
• Damage has been found to occur in both a-Si and
  CdTe modules.
• A 0.78-eV thermal activation energy of the
  damage area versus time at 85 RH was found.
• The biased damp heat test with illumination
  causes damage identical to that observed in
  actual use.
• Damage rates can be dramatically increased by
  using light/dark cycles

TCO corrosion in an a-Si module after 500 h of
exposure to 1-sun, 85 RH, and 85C module
temperature.
C.R. Osterwald, Accelerated Stress Testing of
Thin-Film Modules with SnO2F Transparent
Conductors, NREL/CP-520-33567
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General degradation of c-Si and p-Si PV MODULES
IV characteristic of c-Si with the discoloration
after 10 years in the field. Some modules shows
discoloration Electron-probe microanalysis
(EPMA) indicated that the discoloration is due to
the delamination between the solar cell and EVA.
IV characteristic of p-Si module which showed
performance loss (-20) due to the reduction in
the FF caused by the increase in the Rs (series
resistance) after 10 years in the field
Kengo Morita, 3rd World Conference on
Photovoltaic Energy Conversion May 11-18, 2003
Osaka. Japan
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CIS/CIGS
Light soaking in accelerated life test!
Typical stack ZnOAl/ZnO/CdS/Cu(In,Ga)Se2/Mo/Glass

Katsumi Kushiya, IMPROVED STABILITY OF CIGS-BASED
THIN-FILM PV MODULES, Photovoltaic Energy
Conversion,2006
Johan Wennerberg , Solar Energy Materials Solar
Cells 75 (2003) 4755
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CIS/CIGS SAES-ZSW cooperation
It is found that the performance improves in
general when the cell is brought to operation
conditions. The CIS device has two different
states, the high free electron density in CdS
(HFE) and the low free electron density in CdS
(LFE). The latter state is the intrinsic state of
the device, i.e. without external excitation the
device will relax to this state. Exposure to
white light or blue or green light all can cause
the device to change from LFE state to the HFE
state. Solveig Roschier, on the 25th of January,
2002
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Amorphous Silicon

• a-Si photovoltaic technology is affected by an
  intrinsic degradation of the light, independent
  on moisture StaeblerWronski effect.
• This masks the degradation that starts to be
  relevant after some years in outdoor conditions.

E.E. van Dyk, Solar Energy Materials Solar
Cells 91 (2007) 167173
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Amorphous Silicon electrochemical degradation

• Cross-sectional SEM micrograph of part of a cell
  in the unaffected region of the module.
• Cross-sectional SEM micrograph of part of cell
  in the affected region of the module

Anodic Process Cathodic Process Precipitation
and conversion

• Dark accelerated tests cannot observe the
  phenomenon, because theres no electrical power
  (induced light) and moisture ageing in the same
  time . Cooperation between SAES and ISAAC-SUPSI.

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DSSC sensitized dye degradation
Devices (a) and (b) devices are kept in Ar-filled
dry box Device (c) is kept in atmosphere
H. Matsui, et al., Sol. Energy Mater. Sol. Cells
(2009)
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OSC the OLED analogy

• As in the OLED metallic cathodes degrades for
  moisture presence

Furthermore, dissolved oxygen reacts with
electrons in the conduction band or aids exciton
dissociation by quenching the excited state.
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OSC
Typical normalized decay parameters according to
the general Eq. (1) under AM1.5 illumination with
an incident power density of 1000Wm2 in air and
in vacuum
F.C. Krebs et al. / Solar Energy Materials
Solar Cells 86 (2005) 499516
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Conclusions

• Moisture is one of the main cause of degradation
  in PV technologies
• General issues are adhesion, leakage current,
  corrosion, encapsulant and TCO degradation
• c-Si and p-Si area affected in relevant way from
  the general issues and can lost till 10 in 10-15
  years of outdoor conditions
• CIS/CIGS are affected by specific moisture
  attack, but light soaking is needed to distiguish
  reversible from irreversible degradation
  phenomenon
• In the a-Si technology, intrinsic
  StaeblerWronski effect masks electrochemical
  degradation. Outdoor tests are needed for more
  than 1 year. Dark damp heat tests are ineffective
  to observe the phenomenon.
• DSSC shows moisture degradation of sensitized dye
• OSC exhibits moisture degradation as OLED device,
  but there is also a strong effect due to the
  oxygen dissolved in the device
• A effective packaging can decrease in relevant
  way the energy payback time

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